Leigh Canham Michael Sailor Nanotechnology Dept Dept of Chemistry & Biochemistry Defence Research Agency Univ of California-San Diego Worcestershire, WR14 3PS MS 0358 UNITED KINGDOM La Jolla, CA 92093-0358 44-1684-895007 619-534-8188 Kazunobu Tanaka Chuang-Chuang Tsai Joint Res Ctr for Atom Technology Global Product Operation Natl Inst Adv Interdisciplinary Res Applied Komatsu Technology Ibaraki, 305 JAPAN MS 9155 81-298-54-2701 Santa Clara, CA 95054 408-235-6847Symposium Support
*British Telecom Labs
*Hitachi Central Research Laboratory
*NEC Corporation
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
8:30 AM *F1.1SESSION F1: LIGHT EMISSION FROM NANOCRYSTALLINE SILICON
Chairs: Leigh T. Canham and Michael J. Sailor
Monday Morning, November 30, 1998
Salon E (M)
NANOSCALE SILICON LIGHT EMITTERS FOR INTERCONNECTS AND DISPLAYS: PROGRESS AND PROBLEMS. Philippe M. Fauchet , University of Rochester, Dept of Electrical and Computer Engineering, Rochester, NY.
When the size of the silicon nanocrystals drops below 10 nm, the bandgap and the luminescence energies can be tuned from the infrared to the visible spectral regions, and the quantum efficiency increases beyond 1% and can become as large as 50% at cryogenic temperatures. The amazing properties of nanoscale silicon objects have suggested that light emitting devices can be made with attractive properties for various applications, including optical interconnects and displays. Yet, after several years of intense research, there are no commercially available nanoscale silicon (nc-Si) LEDs. This talk will review the state-of-the-art in nc-Si LEDs, the physical and technological origins of the problems that have been encountered, and the approaches that have been followed to correct them. The prospects for making nc-Si LEDs that can compete in the market place will be discussed.
9:00 AM F1.2
LIGHT EMITTING MICROPATTERNS OF POROUS SEMICONDUCTORS. D. J. Lockwood , National Research Council, Inst. for Microstructural Sciences, Ottawa, CANADA; P. Schmuki, Swiss Federal Institute of Technology, Dept. of Materials Science, Lausanne, SWITZERLAND; L. E. Erickson, National Research Council, Inst. for Microstructural Sciences, Ottawa, CANADA.
We report a principle that allows writing visible light emitting semiconductor patterns of arbitrary shape down to the sub-micrometer scale. We demonstrate that porous semiconductor growth can be electrochemically initiated preferentially at surface defects created in an n-type substrate by Si++ focused ion beam (FIB) bombardment. For n-type material in the dark, the electrochemical pore formation potential (Schottky barrier breakdown voltage) is significantly lower at the implanted locations than for an unimplanted surface. This difference in the threshold voltages is exploited to achieve the selectivity of the pore formation process. Visible light emitting patterns of porous Si [1] and GaAs [2] have been created this way. At present, the size of the structures is limited only by the diameter of the writing ion beam and pattern diameters in the 50
200 nm range are possible.
1. P Schmuki, L.E. Erickson, and D.J. Lockwood, Phys. Rev. Lett. 80, 4060 (1998).
2. P. Schmuki, L.E. Erickson, D.J. Lockwood, J.W. Fraser, G. Champion, and H.J. Labb, Appl. Phys. Lett. 72, 1039 (1998).
9:15 AM F1.3
STRONGLY SUPERLINEAR LUMINESCENCE AND LARGE INDUCED ABSORPTION IN OXIDIZED POROUS SILICON FILMS. Hideki Koyama , Philippe M. Fauchet, Univ of Rochester, Dept of Electrical Engineering, Rochester, NY.
Oxidized free-standing porous silicon films have been found to exhibit two unique optical properties under relatively high cw excitation conditions: a strongly superlinear increase of the photoluminescence (PL) intensity and a very large laser-induced optical absorption. The superlinear PL appears beyond a threshold excitation intensity of
10 W/cm2 and increases very
sharply following a power law with n10. Its PL peak wavelength
is redshifted by several tens of nm, suggesting that the PL
originates from initially nonluminescent, larger-size Si
nanocrystallites. The induced absorption is linear as a function of
excitation intensity. The absorption coefficient at 633 nm increases
by a factor of 5 with an excitation Ar-laser intensity of 20
W/cm2. A maximum optical density change of 2.5 has been observed
for a sample with an initial transmittance of 20. Both the
superlinear PL and the induced absorption are fully reversible and
reproducible. These experimental results are discussed in relation to
the temperature effects on the photoexcited carriers in the Si
nanocrystallites.
9:30 AM F1.4
ENHANCING THE EXTERNAL QUANTUM EFFICIENCY OF POROUS SILICON LEDS BEYOND 1% BY A POST-ANODIZATION ELECTROCHEMICAL OXIDATION. Bernard Gelloz , Takashi Nakagawa, Nobuyoshi Koshida, Tokyo Univ of Agriculture and Technology, Div of Electronic and Information Engineering, Tokyo, JAPAN.
External quantum efficiencies (EQEs) of electroluminescent devices based on porous silicon (PS) reported to date are still below the minimum requirements for practical applications such as display devices (1 %) and optical interconnection (10 %). The maximum CW efficiencies currently reported are 0.05% and 0.18 % for devices containing a single porous layer and a porosified p+n junction structure respectively. Post-anodization anodic oxidation of PS to enhance the EQE of electroluminescence from a device based on a thin transparent indium tin oxide contact mounted on porosified n+-type silicon has been investigated. Enhancement of EQE by 3 orders of magnitude has been achieved. We report an CW EQE of 0.51%, the highest value ever obtained to date from a device containing a single PS layer. Preliminary investigations using anodic oxidation and p+n+ substrates have yielded CW EQEs in excess of the critical value of 1 %. Power efficiency of 0.08% was obtained. Anodic oxidation furthermore enhances stability of the final device. Post-anodization anodic oxidation is performed in an acidic solution under galvanostatic conditions, by monitoring the time and electroluminescence intensity during the treatment. In these conditions, oxidation preferably occurs at the surface of non-confined silicon during the first stages of the treatment. The oxidation proceeds in such a way that it mainly decreases the size of non-confined silicon in PS. The dramatic enhancement in EQE can therefore be explained by preferential reduction of leakage carrier flow through non-confined silicon. As a result, both carrier injection and localization in luminescent crystallites is enhanced. It permits precise control of interfacial properties. Anodic oxidation furthermore may be applied to every kind of PS device.
10:15 AM F1.5
OPTIMIZATION AND INTEGRATION OF ELECTRICALLY ISOLATED POROUS SILICON-BASED LIGHT EMITTING DEVICES. Karl D. Hirschman , Leonid Tsybeskov, Christopher C. Striemer, Selena Chan and Philippe M. Fauchet, Univ of Rochester, Dept of Electrical Engineering, Rochester, NY.
Previously we reported on integrated porous silicon-based LEDs with local bipolar drivers that would have possible applications in an active-matrix configuration for display or optical signal transmission. We now report further progress in device engineering and integration that has enabled the fabrication of improved light-emitting silicon devices that can be integrated with standard CMOS technology. Design of experiments methodology was used to direct device engineering experiments, providing a better understanding of how process parameters influence the resulting device performance. This strategy has resulted in the fabrication of oxide-passivated nanocrystalline silicon (OPNSi) LEDs that offer many of the required attributes of a useful silicon light-emitter. The devices exhibit electroluminescence at a bias < 5V, diode-like rectifying I-V characteristics, good stability under DC bias, and uniform emission over the LED contact area. These LEDs have been combined with a new fabrication technique which enables the formation of electrically isolated integrated LEDs in the single-crystal substrate. Each diode structure is formed in its own individual well, enabling junction-isolated devices without a common substrate electrode. Such a process is required to realize integrated LEDs which can be individually addressed in an X-Y array configuration using full-rail voltage modulation (forward/reverse bias), which should result in a significant increase the modulation frequency capability of these devices. Details of the process optimization, device integration and LED characteristics will be discussed.
10:30 AM F1.6
PHOTOLUMINESCENCE FROM SINGLE POROUS SILICON CHROMOPHORES. Mike Mason, Grace Credo, Ken Weston, Steve Buratto , University of California, Santa Barbara, Dept of Chemistry, Santa Barbara, CA.
I will discuss the results of our recent experiments where we spatially isolate and detect the luminescence from individual porous Si nanoparticles dispersed on glass at room temperature. Our experiments show a variety of phenomena not previously observed in the emission from porous Si such as a distribution of emission wavelengths, dynamic spectral shifts, resolved vibronic structure, discrete jumps in intensity, luminescence intermittency and irreversible photobleaching. We have examined the physics behind each of these phenomena and propose a new model for the emission from porous Si. In this model, the emission from porous Si originates from excitons in quantum confined Si that are strongly coupled to surface states of the Si quantum dot.
10:45 AM F1.7
ON THE ORIGIN OF LIGHT EMISSION FROM SI AND GE NANOCRYSTALS: QUANTUM CONFINEMENT, TRAPPING, AND APPLICATIONS. H. W. H. Lee , G. R. Delgado, P. A. Thielen, Lawrence Livermore National Laboratory, Livermore, CA; S. M. Kauzlarich, B. R. Taylor, C. S. Yang, and D. Mayeri, Dept of Chemistry, University of California at Davis, Davis, CA.
The observation of light emission from Si nanocrystals in 1990 initiated intense research into the nature of the phenomenon. Many theories have been proposed, but the issue remains controversial and unresolved. We will discuss our efforts to address this problem. We performed size-selective photoluminescence (PL), PL excitation, saturation, high resolution, and femtosecond spectroscopy at low and room temperatures on Si and Ge nanocrystals. HRTEM, FTIR, ESR, and NMR were also performed. We observed very similar optical behavior from Si and Ge nanocrystals. The surfaces were terminated with a variety of species including oxides and various organic alkyl groups which enabled us to differentiate between effects of the surface and from the nanocrystalline core. We observed light emission in a very specific UV-blue spectral region from both Si and Ge nanocrystals. Larger Ge particles luminescenced throughout the visible. A key result is that the optical properties can be explained by either quantum confinement or trap-mediated processes and our results differentiated between them. Size-selective spectroscopy showed that the bandgap varies as Eg + C/dn which is consistent with recent theories of quantum confinement. Saturation spectroscopy identified the various contributions of traps to the optical properties and assigned the UV-blue emission to the quantum confined bandedge, and emission at lower energies (green, red) to traps. Femtosecond spectroscopy resolved the carrier relaxation near the quantum confined bandedge and the onset of trapping and trap relaxation, confirming our saturation spectroscopy results. We will discuss the identification and characterization of these two important physical processes and show how they relate to light emission from Si and Ge nanocrystals. We will also discuss how these two processes has promoted recent advances in applying these nanocrystals to LEDs, lasers, all-optical switches, chemical catalysis, and other applications. Work at LLNL was performed under the auspices of the U.S. DOE by LLNL under contract No. W-7405-ENG-48.
11:00 AM F1.8
AUGER EFFECT AT THE ORIGIN OF THE FAST LUMINESCENT BAND OF FRESHLY ANODIZED POROUS SILICON. R. Mighaoeth, H. Maref, Laboratoire de Physique des Semiconducteurs, Faculte des Sciences de Monastir Route de l'environnement, Monastir, TUNISIA; I. Mihalcescu , J.C. Vial, Laboratoire de Spectrometrie Physique Universite, Joseph Fourier-Grenoble I, St Martin d'Heres, FRANCE; G. Panczer, Laboratoire de Physico-Chimie des Materiaux Luminescents, Univ Claude Bernard, Villeurbanne, FRANCE.
Time resolved photoluminescence measurements are performed on aged (or oxidized) and fresh porous silicon at room temperature. It is shown that the fast (ns) component has an extrinsic origin for oxidized samples while fresh samples exhibit an intrinsic behavior for the fast and slow (10µs) components. A detailed analysis is done for fresh samples. The nonlinear excitation intensity dependence for both components are correlated and described by a simple model where the Auger effect inside isolated quantum dots plays the dominent role.
11:15 AM F1.9
DEFECTS AND PHONON ASSISTED OPTICAL TRANSITIONS IN Si NANOCRYSTALS. C. Delerue , G. Allan and M. Lannoo, IEMN, Lille, FRANCE.
The optical properties of semiconductor nanocrystals are modified compared to bulk semiconductors. In the case of indirect semiconductors like silicon, the confinement breaks the selection rules so that the optical transitions become weakly allowed without the assistance of phonons. Experimentally however, phonon assisted transitions are observed in porous silicon like in bulk silicon. Therefore many groups have concluded that the radiative recombination in Si nanocrystals cannot involve defects. Here we present a detailed calculation of the phonon assisted transitions in Si nanocrystals. The electronic and the phonon spectra are fully calculated using respectively a tight binding framework and a Keating model. We show as expected that the band edge transitions are more efficient when coupled to a TO phonon. The coupling to other phonons becomes important only for very small nanocrystals. Then we examine the case of the radiative recombination in presence of an oxygen related surface defect (Si=O). We obtain a long radiative lifetime for the direct transition (without phonon) typical of an almost dipole forbidden recombination. The more efficient transition also occurs with an emission of a TO phonon exactly as in the previous case. These results show that the observation of the same phonon replica in silicon nanocrystals than in bulk silicon cannot be used to rule out the influence of defects.
11:30 AM F1.10
LUMINESCENCE STUDY OF SELF-ASSEMBLED SILICON QUANTIUM DOTS. Selichi Miyazaki , Kazutoshi Shiba, Naiki Miyoshi, Atsushi Kohno and Masataka Hirose, Dept of Electrical Engineering, Hiroshima University, Higashi-Hiroshima, JAPAN.
The temperature dependence and the temporal decay of photoluminescence (PL) from self-assembled Si quantum dots (QDs) covered with an ultrathin SiO2 layer have been investigated to discuss a possible luminescence mechanism. Hemispherical Si QDs with an areal density more than 1011 cm-2 have been spontaneously formed on SiO2/c-Si(100) and quartz substrates by the thermal decomposition of pure SiH4 at 560 - 580
C[1]. Subsequently, the dot surfaces were oxidized at
1000C in 2% O2 diluted with N2 for 90s and in
consequence were conformally covered with a 2.7nm-thick SiO2
layer. When the mean dot height is decreased from 6.3 to 1.5nm,
the optical absorption edge determined from photothermal deflection
spectroscopy is increased from 1.9 to 2.5eV and also the PL peak
energy shifts from 1.2 to 1.4 eV at room temperature[1].
A fairly weak temperature dependence of the steady state PL is
obtained for the Si QDs so prepared. For the QDs with a mean height
of 1.5nm which show a luminescence band with an FWHM of
0.3eV peaked at 1.4eV under 488 excitation, the PL
intensity is enhanced by a factor of at most 2 with decreasing
temperature to 100K and saturates at lower temperatures for an
excitation intensity lower than 20mW/cm2. For a higher excitation
intensity, the PL efficiency at temperatures lower than 70K is
significantly reduced. This is presumably because the Auger
non-radiative recombination becomes significant, being also suggested
from the fact that a decay component faster than sub-sec becomes
observable in the temporal response at 13K for 337.1 nm-pulse
excitation. The slow decay component observed in the sec-msec
region is characterized well by using a stretched exponential
function for a certain emission energy and the emission at lower
energy has shorter life time at room temperature. In addition, as the
mean dot height becomes smaller, the emission life time tends to be
shorter. With decreasing temperature, the temporal response for the
slow component becomes slower and less dependent on the emission
energy. These results imply that the radiative recombination of
photogenerated carriers through localized states is an important
pathway for the emission from the self-assembled Si QDs. Reference: [1] K. Shiba et al., Jpn. J. Appl. Phys. 36 (1997) L1279.
11:45 AM F1.11
FORMATION PROCESS OF SI NANOPARTICLES FORMED BY LASER ABLATION METHOD. Tetsuya Makimura , Taiji Mizuta, Takashi Ueda, Kouichi Murakami, Institute of Materials Science, Univ of Tsukuba, JAPAN.
Light-emitting Si nanoparticles with size of 1-10 nm can be fabricated by cooling of Si atoms that are ejected from Si targets in rare gas by laser ablation. Utilizing this pulsed and clean technique, we could fabricate nanoparticles with better-defined and higher-orderd structures. For this purpose, it is important to understand the dynamics of the formation process of the nanoparticles.
In this work, we have developed technique for observing particles from the molecules that consist of several atoms to particles with size of more than 10 nm. Because the technique is non-contact and highly sensitive, it can be applicable to a variety of materials.
Si atoms were ablated into argon gas by irradiation of a silicon target with Q-switched Nd:YAG laser light (10
). The
formed particles were irradiated with delayed OPO laser light and the
spacial distribution of the induced light emission was detected by
means of an intensified CCD camera. We found that the induced light
emission is due to highly excited atoms decomposed from the particles
formed in Ar gas, by means of space- and time-resolved spsctroscopy.
We changed the delay and obtained temporal and spatial distribution of
the particles. We directly observed a) that particles begin to be
formed 300 sec after the ablation at the central part of the
ablated atoms in 2-Torr Ar gas and b) that they grow until 1 msec.
And c) no particles was observed at less than 1 Torr. Based on these
results, a study on chemical modification of the surface of the
nanoparticles is in progress, using pulsed oxygen gas or pulsed
hydrogen gas, at the delay when the particles are
formed.
1:30 PM *F2.1SESSION F2: PROPERTIES OF NANOCRYSTALLINE SEMICONDUCTORS AND PERIODIC STRUCTURES
Chairs: Daniel Bellet and Philippe M. Fauchet
Monday Afternoon, November 30, 1998
Salon E (M)
PERSPECTIVES OF POROUS SILICON MULTILAYER TECHNOLOGY. Markus Thoenissen , Research Center Juelich, Juelich, GERMANY.
The variation of etch parameters during formation of porous silicon layers results in the formation of complex filter structures with interesting optical properties. Starting with the investigation of single layers the formation process, the optical properties, the passivation and different applications of these multilayers will be discussed. Perspectives of different applications will be shown.
2:00 PM F2.2
NANOCRYSTALLINE SILICON SUPERLATTICES: PHYSICAL PROPERTIES AND DEVICE APPLICATIONS. L. Tsybeskov , G.F. Grom, K.D. Hirshman, L. Montes, and P.M. Fauchet, Department of Electrical Engineering, University of Rocherster, Rochester, NY; T.N. Blanton, Eastman Kodak Company, Rochester, NY; J.P. McCaffrey, J.M. Baribeau, G.I. Sproulc, H.J. Labbé and D.J. Lockwood, Institute for Microstructural Sciences, National Research Council, Ottawa, CANADA.
Ordered arrays of Si nanocrystals, or nanocrystalline silicon (nc-Si) superlattices (SLs), have been fabricated by the controlled thermal recrystallization of amorphous Si/SiO2 multilayers1. The structure of the nc-Si SLs is characterized by Auger elemental microanalysis transmission electron microscopy, X-ray diffraction, and X-ray reflection. The Si nanocrystal size distribution is found to be less than 10%. The nc-Si/SiO2 interface roughness is better than 3
and remains undisturbed during high temperature
(1000C) posttreatment. The optical characterization,
including modulation reflection spectroscopy; folded acoustic mode
and polarization Raman spectroscopy FTIR, luminescence and
spectroscopic ellipsometry, clearly show the importance of the
nanocrystal orientation in terms of medium and long-range order in
nc-Si SLs. Current tunnel spectroscopy and capacitance spectroscopy
reveal a rich structure and a memory effect. The memory effect is
explained by the charging of the Si nanocrystals and can be
controlled by the nanocrystal size and a thickness of the SiO2
separating layer. The use of nc-Si SLs in electronic quantum devices
(EQD) is proposed, and an all Si, high density, non-volatile memory
is demonstrated. 1 L. Tsybeskov, K. D. Hirschman, S. P. Duttagupta, M. Zacharias, P. M. Fauchet, J. P. McCaffrey and D. J. Lockwood, Applied Phys. Lett. 72, 43 (1998).
2:15 PM F2.3
POROUS SILICON QUANTUM SUPERLATTICES. G. Lerondel , D. Midellino, G. Amato, A.M. Rossi, L. Boarino, IENGF, Torino, ITALY; A. Parisini, CNR-LAMEL, Bologna, ITALY.
Porous silicon is well known to be quite a disordered network of silicon filament with nanometric size. On the other hand, electrochemical process is well known to be able to produce with a good reliability microscopic multistructures like optical superlattice, giving rise to an alternative process for producing such systems. The aim of this work was to show that nanometric periodic PS structures can also been obtained by mean of electrochemical dissolution. According to the porous silicon morphology one can expect some connections between pore sizes and the lowest obtainable thickness.
Multilayers structures were obtained using a now conventional way. Starting from highly doped p-type substrate, an electrochemical dissolution with current density modulation was performed. The difficulty here lies in the very short dissolution times typically from 10 ms to few thousands millisecond. Consequently a first step consisted to study the etching mechanism at the beginning of the layer formation which has been found to be mainly characterised by a sublinear regime of the thickness as a function of time.
Taking into account this effect, Transmission Electron Microscopy has shown that 3D nanometric lattices can be obtained in which the dimension of crystalline i.e. diameter of the colons is equal to the layer thickness, in the order of 20 nm. Image analysis has revealed that a correlation can be found between the dimension of the vertical porosity modulation and the distance between the pores.
If such structures can be very interesting on the optical point of view (vertical quantum confinement), it has already been shown that the PS electrochemical process can be also controlled at a nanometric scale.
2:30 PM F2.4
POROSITY-INDUCED OPTICAL PHONON ENGINEERING IN III-V COMPOUNDS. Ivan M. Tiginyanu , Technical Univ, Institute of Applied Physics, Chisinau, MOLDOVA; Gert Irmer, Jochen Monecke, Technical Univ, Freiberg, GERMANY; Hans L. Hartnagel, Alexander Vogt, Technical Univ, Darmstadt, GERMANY; Claude Schwab, Jean-Jacques Grob, CNRS/PHASE, Strasbourg, FRANCE.
With regard to Si, porosity in compound semiconductors offers several potential advantages for devices. Apart from those related to the possibility of changing the chemical composition, the shift from element to compound entails a major crystallographic change. Although the overall tetrahedral sp3 bonding between atoms is retained, the centro-symmetrical lattice of the column IV element of diamond-type becomes a non-centro-symmetrical lattice of sphalerite-type for the derived II-VI and III-V compounds. This paves the way for new physical properties specific to acentricity to occur in these polar materials. For instance, Froehlich-type surface vibrational modes should occur in small structures (i.e., at high values of surface-to-volume ratio) of heteropolar semiconductors in the region between the TO and LO modes. This work presents a review of the latest achievements in manufacturing and micro-Raman scattering characterization of porous III-V layers and free-standing membranes. A technological basis was developed for electrochemical manufacturing of nanoporous layers of III-V compounds pre-implanted by 5-MeV Kr ions at low doses (109 - 1011 cm-2). The implantation was used in order to control the density of tracks initiating the dissolution. Under suitable electrochemical conditions the pores left by the dissolved material stretch perpendicular to the surface and thus leave a network-shaped nanoporous structure of average dimensions comparable to the depth of the carrier-depleted surface layer during anodization. Surface vibrations with porosity-tunable frequencies have been evidenced in GaP and GaAs. Moreover, the surface phonon frequency was found to depend upon the nature of liquid filled in the pores, i.e., upon the dielectric constant of the surrounding material. This findings open new possibilities for optical phonon engineering in III-V compounds and alloys, and for controlling the strength of the interaction between vibrational and charge carrier excitations.
3:15 PM *F2.5
RECENT PROGRESS IN THE PHYSICAL AND STRUCTURAL PROPERTIES OF POROUS SILICON. Daniel Bellet , Laboratoire de Spectrometrie Physique, Grenoble, FRANCE.
Recently porous silicon (PS) has been the subject of intensive investigation. Indeed following the demonstration in 1990 that highly porous material could emit efficient visible photoluminescence at room temperature, much efforts have been devoted to better understand the PS optical properties. However this material has also other potential applications (such as chemical and physical sensors, biomaterial...) and exhibits some interesting characteristics (a huge specific area with nearly perfect cristalline properties...).
Thus the structural properties of PS have also been investigated. The purpose of this short review is to briefly survey some recent progress related to the physical as well as structural properties of PS, focused for instance on the mechanical properties or thermal conductivity. A correlation between these properties and the requirements needed for some applications will be discussed.
3:45 PM F2.6
ULTRASOUND EMISSION FROM POROUS SILICON : EFFICIENT THERMO-ACOUSTIC FUNCTION AS A DEPLETED NANOCRYSTALLINE SYSTEM. Nobuyoshi Koshida , Takashi Nakajima, Masaru Yoshiyama, Koki Ueno, Hiroyuki Shinoda, Faculty of Technology, Tokyo University of A&T, Koganei, Tokyo, JAPAN.
The activity of porous silicon (PS) as a confined system is expected to appear in various manners. We report here that PS is useful for an ultrasonic generator owing to its efficient thermo-acoustic transmission capability.
The experimental device is composed of a patterned thin Al film electrode (30 nm thick), a PS layer, and a single-crystalline Si (c-Si) wafer. The microporous PS layer (10-50
m thick) was
formed by a conventional anodization technique. The electrical input
(1 mW/cm2) was provided to the Al electrode as a sinusoidal
current followed by Joule's heating. The emitted acoustic pressure
was measured as a function of input frequency (0.1-100 kHz) using a
microphone in a closed air space. The spacing between the PS device
and the microphone was 0.1 mm. It has been confirmed that the PS device operates as an efficient ultrasonic emitter. The output acoustic pressure amplitude was in inverse proportion to the square root of the input frequency as predicted by a theoretical analysis. The observed definite thermo-acoustic effect is due to that PS has extremely low values of thermal conductivity
and heat capacity per unit volume C,
reflecting the completely depleted electronic property of Si
nanocrystallites in PS. The product , which is a key
parameter of this device, is about 1/400 of c-Si. This enables an
efficient heat exchange between PS and air. A novel prominent function of PS is now made clear. The PS device has some advantageous features for development of functional integrated ultrasonic emitters with a sufficient output intensity in a wide band width.
4:00 PM F2.7
RESONANT TUNNELING AND CURRENT OSCILLATIONS IN Si/CaF2 MULTI QUANTUM WELLS. Androula Nassiopoulou , Vasilios Ioannou-Sougleridis, Victoria Tsakiri, NCRS Demokritos Institute of Microelectronics, Aghia Paraskevi Attikis, Athens, GREECE; S. Menard, Frank Bassani, F. Arnaud d'Avitaya, CRMC2/CNRS, Campus de luminy, Marseille, FRANCE.
We report on new important effects in vertical carrier transport in periodic nanocrystalline Si/CaF2 multiquantum wells, fabricated by Molecular Beam Epitaxy at room temperature. These effects were observed in current-voltage characteristics at room and low temperatures from structures fabricated on both n- and p-type substrates. The I-V curves showed regions of negative resistance and current oscillations at low bias voltage, as well as strong hysteresis effects. Current self-oscillations were also observed at low bias voltage. C-V measurements showed clear regions of inversion, depletion and accumulation, with a sharp transition from inversion to accumulation, indicative of an unpinned interface between silicon and multilayers. The observed effects are tentatively attributed to resonant tunneling and space charge waves under static domain formation obtained at high electric fields.
Electroluminescence was also observed, with an onset depending on the current through the structures.
The observed effects open important possibilities for applications in new silicon based devices.
4:15 PM F2.8
ATOMIC-FORCE MICROSCOPY OF SINGLE SILICON NANOCRYSTALS: MANIPULATION AND CHARGING. E.A.Boer , D.H. Santamore, H.A. Atwater, K.J. Vahala, Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena CA; M.L. Ostraat, R.C. Flagan, Department of Chemical Engineering, California Institute of Technology, Pasadena CA; L.D. Bell, Jet Propulsion Laboratory, Caltech, Pasadena CA.
Nanocrystal nonvolatile floating gate memory devices show considerable promise-initial results suggest they are fast, more reliable and consume less power than conventional floating gate memories. However, the speed of such devices is limited by a distribution of charge transit times. While not yet understood, this spread in transport properties may be the result of variations in nanocrystal size or oxide thickness, or may be related to nanocrystal interface states. To investigate this we have developed an aerosol method for the synthesis and size classification of silicon nanocrystals with
10-15 control of size in the 2-10
nm size range. We have used an atomic-force microscope in contact
mode to form 2D nanocrystal structures (e.g. lines and arrows) and
have subsequently imaged these structures in noncontact mode without
any further particle manipulation. Furthermore, we have investigated
charging of single silicon nanocrystals with a conducting AFM tip.
Other work to be discussed includes narrowing of nanocrystal size
distributions and approaches for measurement of electron transport
through single and multiple silicon particles.
4:30 PM F2.9
PRODUCTION OF SILICON NANOCRYSTALS BY THERMAL ANNEALING OF SILICON-OXYGEN AND SILICON-OXYGEN-CARBON ALLOYS: MODELS SYSTEMS FOR CHEMICAL AND STRUCTURAL RELAXATION AT Si-SiO2 AND SiC-SiO2 INTERFACES. G. Lucovsky , D. Wolfe, B. Hinds, NC State University, Dept of Physics, Raleigh, NC.
Non-crystalline thin film suboxides of Si (SiOx), and Si and C were prepared by remote plasma-enhanced chemical vapor deposition at substrate temperatures of 200 to 300
C using hydrogen
containing source gases, silane and methane. The Si-O films
displayed infrared absorptions characteristic of Si-H vibrations in
amorphous Si:H, and Si-O vibrations in suboxide bonding arrangements.
The Si-O-C alloys displayed absorptions corresponding to Si-H, Si-O
and Si-C vibrations. Anneals were performed in inert ambients, and
changes in local bonding were tracked by monitoring the intensities
and frequencies of the infrared absorptions. Formation of Si
nanocrystals was determined from Raman scattering and cross-sectional
transmission electron microscopy. Both types of suboxide alloys
showed an essentially complete loss of hydrogen when annealed to
temperatures of more than 500C. The Si-O alloys showed an
effectively complete chemical separation into Si nanocrystals and
SiO2 on annealing to about 900C. This is the same temperature
that minimizes suboxide bonding at Si-SiO2 interfaces formed by
either plasma-assisted oxidation at 300C, or by higher
temperature (800 to 900C) thermal oxidation. Silicon
crystallites did not form in the Si-O-C alloys below an annealing
temperature of 1025-1050C. Si-O-C bonding arrangements were
not present in as-deposited films, but were produced by annealing to
about 900C, and disappeared at the onset of Si nanocrystal
formation. The presence of Si-O-C bonding below 1025C is
consistent with SiC oxidation technology, wherein device-quality
interfaces are formed by thermal oxidation at temperatures in excess
of 1025C, or by annealing plasma-oxidized interfaces at
temperatures greater than 1050C. These observations are
consistent with Si-O-C interface bonding arrangements being the
origin of electronically-active interface defects at SiC- SiO2
interfaces.
4:45 PM F2.10
SYNTHESIS AND CHARACTERIZATION OF GE NANOCRYSTALS. Soumyendu Guha, Soumyendu Guha , Naval Research Laboratory, Washington DC; Tony van Buuren, Mark Wall and Lloyd L. Chase, Lawrence Livermore Laboratory, Livermore, CA.
We have synthesized Ge nanocrystals of size 4, 8, and 12 nm by ion-implanting Ge ions into SiO2 films and subsequent annealing at 830 C. X-ray, high-resolution TEM, and Raman scattering techniques were used to measure the sizes of nanocrystals. The conduction band-edge as a function of particle size was measured by near-edge x-ray absorption spectroscopy using soft x-rays. A red and blue photoluminescence were observed in these films and their origin will be discussed.
8:15 AM *F3.1SESSION F3: BIOLOGICAL APPLICATIONS AND SURFACE CHEMISTRY OF NANOCRYSTALLINE SEMICONDUCTORS
Chairs: A. Paul Alivisatos and Michael J. Sailor
Tuesday Morning, December 1, 1998
Salon E (M)
BIOLOGICAL TAGGING APPLICATIONS OF SEMICONDUCTOR NANOCRYSTALS. A. P. Alivisatos , Univ of California, Berkeley, Dept of Chemistry, Berkeley, CA.
Semiconductor nanocrystals exhibit strongly size dependent emission spectra, due to the quantum size effect. Further, the nanocrystals have nearly continuous excitation spectra above the threshold for absorption. As a consequence, the nanocrystals can be used as luminescent probes in biological staining experiments. The nanocrystals are in many ways superior to existing organic chromophores. Relevant applications and surface chemistry will be described in this talk.
8:45 AM F3.2
BIOINERT AND BIOACTIVE SILICON: IN-VIVO ASSESSMENT OF TISSUE COMPATIBILITY. A.P. Bowditch, K. Waters, H. Gale, E.A.M. Scott, P. Rice, Biomedical Sciences Department, DERA, Poroton Down, Wiltshire, UNITED KINGDOM. L.T. Canham , C.L. Reeves, A. Loni, T.I. Cox, DERA, Malvern, Worcestershire, UNITED KINGOM.
The semiconductor silicon has not been thoroughly assessed, nor developed to date as a potential biomaterial [1]. This is somewhat surprising, since silicon technology with its combined power of CMOS circuitry and micromachining offers medicine a broad range of intelligent miniaturised devices.. We report here an extended in-vivo study of the effects of implanting silicon structures into guinea pigs at the subcutaneous site. Histopathological examination after 1, 4, 12 and 26 weeks has been conducted and interpreted following ISO standards for biocompatibility testing.
[1] L T Canham. Adv. Mater. (1995), Vol. 7, 1033.
9:00 AM F3.3
DEVELOPMENT OF A POROUS SILICON BASED BIOSENSOR. Keiki-Pua S. Dancil , Douglas P. Greiner, Christian Gurtner, Michael J. Sailor, UCSD, Dept. of Chemistry and Biochemistry, La Jolla, CA; M.Reza Ghadiri, The Scripps Research Institue, Departments of Chemistry and Molecular Biology, and The Skaggs Institute for Chemical Biology, La Jolla, CA.
Inexpensive biosensors designed to provide rapid multi-analyte detection are highly sought after for use in drug design and disease diagnosis, as well as detection of chemical and biological warfare agents. Recently, our laboratories reported the discovery of a system which utilizes porous silicon (PS) as an immobilization matrix for biomolecules and as an optical interferometric transducer of molecular binding events. In addition to its unique optical and chemical properties, PS was chosen as the matrix material due to its high surface area. In comparison to flat surfaces, PS offers an immense increase in surface area which allows for a higher amount of immobilized receptor molecules.
The sensor is based upon changes in the refractive index of the thin PS film. When incident white light is reflected off a PS sample, Fabry-Perot fringes in its reflectometric spectrum are observed. The fringe pattern is due to the constructive and destructive interference of light being reflected off the top interface and bottom interface of the thin film. The peaks in the interference spectrum (ml) is related to the effective optical thickness (EOT) of the film by the following equation:ml=2nL where m is the spectral order, l is the wavelength, n is the refractive index, and L is the thickness of the film. Since the thickness (L) is a constant parameter, changes in the fringe pattern are directly related to changes in the refractive index of the PS matrix. Since this system is solely dependent on refractive index changes, the PS biosensor is independent of distance of analytes from the surface. Preliminary experiments with a biotinylated PS and streptavidin have been completed. An increase in the EOT is observed when streptavidin is bound to a biotinylated surface. However, when the biotinylated surface is exposed to blocked streptavidin, no binding was detected.
9:45 AM *F3.4
SURFACE CHEMISTRY OF POROUS SILICON. J.-N. Chazalviel , F. Ozanam, Laboratoire PMC, CNRS-Ecole Polytechnique, Palaiseau, FRANCE.
As-prepared porous silicon comes out covered with covalently bonded hydrogen. This hydrogen coating provides a good electronic passivation of the surface, but it exhibits limited stability, being removed by thermal desorption or converted into an oxide upon prolonged storage in air. Starting from the hydrogenated surface, an oxide layer with good electronic properties is also obtained by anodic oxidation or rapid thermal oxidation.
The hydrogenated surface may be nitridized using thermal treatments in nitrogen or ammoniac. Fast halogenation of the surface may be obtained at room temperature, but the resulting coating is rapidly converted to an oxide in the presence of moisture. Many metals have been incorporated into the pores, using either chemical or vacuum techniques.
More interestingly, organic derivatization may increase surface stability or provide chemical functionalities. The poor reactivity of the hydrogenated surface can be remedied by using various methods: thermal desorption of hydrogen, hydroxylation or halogenation of the surface, thermal or UV assisted reaction. However, most promising results have been obtained through either Lewis-acid catalyzed grafting or electrochemical activation of the surface. The latter method has been used for grafting formate, alkoxy, and recently methyl groups. In most of these methods, oxidation is present as a parallel path, and care must be taken if it is not desired. Also, 100% substitution of the hydrogens by organic groups has never been attained, due to steric hindrance problems. The electrochemical method appears especially fast, and has led to 80% substitution of the hydrogens by methyl groups, with no photoluminescence loss and a chemical stability increased by one order of magnitude.
10:15 AM F3.5
COVALENT MODIFICATION OF THE SURFACE OF POROUS AND CRYSTALLINE SILICON. Paul E. Laibinis , M.I.T., Dept of Chemical Engineering, Cambridge, MA; Namyong Y. Kim, M.I.T., Department of Chemistry, Cambridge, MA.
The hydrogen-terminated surface of porous silicon can be reacted with various nucleophilic organic reagents to form covalently attached organic films on the silicon surface. These reactions proceed without the need for electro- or photochemical input. Reagents such as alcohols, Grignard reagents, and organolithium reagents will react with the porous silicon lattice. The former reagent produces covalently attached species bonded to the surface through Si-O bonds, while the other two reagents produce Si-C attachments. Rutherford Backscattering experiments demonstrate that the reaction process derivatizes the interior of supported films of porous silicon. The use of various surface spectroscopies (including x-ray photoelectron spectroscopy and infrared spectroscopy) demonstrates the formation and stability of these films and the ability to use these reactions to derivatize the hydrogen-terminated surfaces of Si(100) and Si(111). The reactions with the silicon surface on crystalline and porous silicon allow introduction of reactive organic functionalities on the surface for subsequent structural manipulations using various chemical reactions and electrochemical processes. These solution-phase processes provide the basis for producing robust organic-porous silicon composites with tailored chemical and electronic properties as platforms for sensors and electronic/photonic devices. The talk will review our development of chemical methods to modify the surface of silicon (both porous and crystalline materials) and their extension in forming higher hierarchical structures by further manipulation of various covalently attached organic functionalities.
10:30 AM F3.6
FUNCTIONALIZATION OF POROUS SILICON SURFACES THROUGH HYDROSILYLATION REACTIONS. Jillian M. Buriak , Michael P. Stewart, Matthew J. Allen, Purdue University, Department of Chemistry, West Lafayette, IN.
Hydrosilylation of alkynes and alkenes on silicon surfaces utilizing the native Si-H termination can be smoothly and rapidly carried out (30 s to 24 h) at room temperature through hydrosilylation mediated by Lewis acid catalysts, or through thermal or photoinduction (white light). Insertion of alkynes and alkenes into surface silicon hydride bonds yields covalently bound alkenyl and alkyl groups, respectively. Different chemical functionalities can be tolerated by these hydrosilylation reactions, including ester, hydroxy, chloro, nitrile and chiral groups. Hydrophobic porous silicon surfaces demonstrate remarkable stability with respect to boiling aqueous aerated pH 1 to 10 solutions, and protect the bulk silicon from attack. Modification and tailoring of surface properties through this series of reactions induce wide variations in photoluminescent behavior of porous silicon, leading to almost complete quenching in the case of substituted and unsubstituted styrenyl termination, and minor decreases for alkyl and alkenyl functionalization. Because of the broad range of stable, modified surfaces produced using this chemistry, the work described here represents an important step towards technological applications of silicon surfaces.
10:45 AM F3.7
ENHANCEMENT IN EFFICIENCY AND STABILITY OF OXIDE-FREE BLUE EMISSION FROM POROUS SILICON BY SURFACE PASSIVATION. Hiroyuki Mizuno and Nobuyoshi Koshida, Tokyo Univ. of Agri. & Tech., Dept. of Electronic & Info. Eng., Tokyo, JAPAN.
The establishment of tuning technique of visible emission from porous silicon (PS) is very important from both physical and technological viewpoints. In previous papers[1,2], we demonstrated that the photoluminescence (PL) spectra of PS can continuously be controlled from red to blue simply by postanodization illumination method without any growth of the surface oxide. As the emission energy is increased, however, both the efficiency and the stability tends to deteriorate. We repot here that a surface termination technique of PS is very useful for improvement of the luminescence characteristics, especially in the blue band. All of the PS samples used in this study were formed on p-type Si wafers by anodization in ethanoic HF solution. Immediately after the anodization, the samples were illuminated by a tungsten lamp for 0-15 min for tuning the PL spectra from red to blue. Prepared samples were transferred into a hydrogen ambient without exposure to air in order to avoid the possible surface oxidation. After exposure for 0-12 h at room temperature, the PL characteristics were measured in comparison with the as-illuminated sample. The surface characterization was also carried out by fourier transform infrared (FTIR) absorption spectra measurements.
The major results are as follows:
Hydrogen exposure treatment produced a significant increase in efficiency and stability of the blue PL emission. Both the PL intensity and peak wavelength (430 nm) remained almost unchanged even after a continuous strong excitation in a N2 gas ambient for a long time over 1 h. In the as-illuminated sample, in contrast, the blue PL intensity was decreased by a factor of three after 1 h excitation, and correspondingly the PL wavelength showed a redshift of about 100 nm.
The relative intensity of every FTIR absorption peak related to silicon-hydrogen bonding is increased with increasing the hydrogen exposure time. Green-emissive PS samples showed a similar behavior. These results show that complete surface termination suppresses photo-induced oxidation followed by an increase in the non-radiative channels. The passivation effect becomes more apparent in the blue-emissive high-porosity sample, owing to its extremely high chemical activity.
[1] H. Mizuno, H. Koyama, and N. Koshida, Appl. Phys. Lett. 69, 3379 (1996).
[2] H. Mizuno, H. Koyama, and N. Koshida, Thin Solid Films 297, 314 (1997).
11:00 AM F3.8
MODIFICATION OF VISIBLE LIGHT EMISSION FROM SILICON NANOCRYSTALS AS A FUNCTION OF SIZE, ELECTRONIC STRUCTURE, AND SURFACE PASSIVATION. M. Wolkin-Vakrat , L. Tsybeskov, J. Jorne, P.M. Fauchet, University of Rochester, Department of Materials Science, Rochester, NY; G. Allan, C. Delerue, IEMN-ISEN, Lille, FRANCE.
Quantum confinement is the leading model to describe the visible photoluminescence (PL) in nanocrystalline silicon (Si). However, as the bandgap increases with decreasing size, the PL energy does not increase much beyond 2 eV, and only red-orange PL is achieved for samples exposed to air. Oxide-free porous Si samples with medium to ultra high porosities have been prepared by using electrochemical etching followed by photoassisted stain etching. The samples are immediately transferred to an Ar environment without being exposed to air. Red, orange, yellow, green, and blue emission as well as time resolved PL are measured. Based on these measurements, we argue that the photoluminescence is due to radiative recombination of free excitons in Si nanocrystals. Next, the samples are exposed to air and two clear trends are shown. For nanocrystallites smaller than 3 nm a fast red shift of the PL is detected. The magnitude of the red shift increases with decreasing crystallite size, and is higher than 400 meV for the ultra high porosity sample. In contrast, for nanocrystallites bigger than 3 nm there is no change in PL energy after exposure to air. We suggest that the decrease in PL energy upon exposure to air is related to recombination via an exciton trapped in a Si=O bond and we compare the experimental results with a theoretical model. In addition, PL spectra are measured at different gas environments and it is demonstrated that surface passivation plays an important role in tuning the position and intensity of the PL.
11:15 AM F3.9
EFFECT OF HYDROGEN ON PHOTOLUMINESCENCE FROM Si NANOCRYSTALS IN SiO2. S. P. Withrow , C. W. White, J. D. Budai, A. L. Meldrum, Oak Ridge National Laboratory, Oak Ridge, TN; J. Charles Barbour, Sandia National Laboratories, Albuquerque, NM; D. M. Hembree, Jr., Oak Ridge Y-12 Plant, Oak Ridge, TN.
The role of hydrogen in enhancing the photoluminescence (PL) yield observed from Si nanocrystals formed in SiO2 by ion implantation has been studied. Fused silica samples were implanted with various doses of Si at energies ranging from 100 to 400 keV and annealed one or more times under various ambients including vacuum, Ar-only, and hydrogen or deuterium containing gases (Ar+4
H2 or Ar+4D2).
A PL band centered in the near-infrared (750 nm) can be
observed after annealing at 1100C in each annealing
environment. The presence of hydrogen in the annealing ambient
enhances the PL yield considerably but has almost no effect on the PL
spectral distribution. Results are presented for annealing at
temperatures between 200C and 1100C. Depth and
concentration profiles of hydrogen have been measured in selected
samples and correlated to the PL yield. No differences in intensity
or wavelength are observed for annealing in H2 versus D2
containing gases, suggesting that the processes leading to the PL do
not directly involve the hydrogen species. Results are compared with
current models for PL which involve quantum confinement of excitons
or emission from interface states.
Oak Ridge National Laboratory is managed by Lockheed Martin Energy
Research Corp. for the U.S. Department of Energy under contract
number DE-AC05-96OR22464.
11:30 AM
F3.10
NANOSTRUCTURE OF POROUS SILICON
USING TRANSMISSION MICROSCOPY: OBSERVATION OF RESTRUCTURED
NANOCLUSTERS. Munir H. Nayfeh , Zain Yamani, Osman
Gurdal, Department of Physics, University of Illinois at Urbana,
Urbana, IL; and A.A. Al-Aql, Department of Physics, King Saud
University, Riyadh, SAUDI ARABIA.
We use cross
sectional high resolution transmission electron microscopy (XTEM) to
image the (111) atomic planes, nanoclusters, nano crystalline
islands, and the pore structure of (100) p-type porous Si. A network
of pore tracks subdivide the material into nano islands. The nano
crystallites are found throughout the porous layer and are found to
evolve such that their size, degree of order, degree of
interconnection, and visibility drop with distance from the
substrate. In the luminescent layer we find islands as small as 2 nm
that are crystalline, and islands as small as 1 nm that are fully
restructured. These restructured features may relate to a recent
quantum confinement based surface model in which the optical activity
of ultra small crystallites is produced by restructuring of the
diamond-like Si bonds into highly efficient radiative Si - Si dimer
bonds.
1:30 PM *F4.1SESSION F4: SYNTHESIS AND SPECTROSCOPY OF NANOCRYSTALLINE SEMICONDUCTORS
Chairs: Reginald M. Penner and Tony W.H. van Buuren
Tuesday Afternoon, December 1, 1998
Salon E (M)
SEMICONDUCTOR NANOCRYSTALLITES: BUILDING ARTIFICIAL ATOM STRUCTURES. Moungi G. Bawendi , MIT, Dept. of Chemistry, Cambridge, MA.
Semiconductor nanocrystallites capped by organic ligands have an atomic-like energy structure which is determined by the size of the inorganic crystalline core. We use a variety of optical methods, including single molecule fluorescence spectroscopy, to interrogate the electronic structure of semiconductor nanocrystallites of CdSe (quantum dots). The chemistry of the nanocrystallite is in large part a function of the ligand. The combination gives an artificial atom with an electronic structure and chemistry which can be tuned independently. We use this flexibility to incorporate the dots into a variety of systems.
2:00 PM F4.2
INTRABAND RELAXATION IN SEMICONDUCTOR QUANTUM DOTS. Moonsub Shim, Chris Matranga, Margaret Hines, Philippe Guyot-Sionnest , University of Chicago, Chicago, IL.
A highly debated issue in quantum dot physics is the mechanism of electronic relaxation in the strong confinement regime. Semiconductor colloid nanocrystals provide a suitable model system with the very strong confinement achievable and the flexibility afforded by chemical modification of the surface. Recently we showed that photoexcited CdSe nanocrystals exhibit a spectrally isolated and size-tunable absorption in the mid-infrared. This strong resonance is fully consistent with the expected 1Se-1Pe electronic transition.[1] The first direct visible-infrared time-resolved spectroscopy of the dynamics of the 1Pe state in such quantum dots are presented. [1] P. Guyot-Sionnest and M. A. Hines, Appl. Phys. Lett. 72, 686, 1988.
2:15 PM F4.3
POLYMERIZATION OF REACTIVE MONOMERIC NANOCRYSTALS. Yongchi Tian , Anthony Dinsmore, Syed Qadri and B. R. Ratna, Geo Centers at Naval Research Laboratory, Washington, DC.
Building up chemical entities of controlled shape on nanometer scale is an important objective of current materials chemistry. Top-down methods (e.g. laser ablation or lithography) for the nanometer fabrication can reach a minimum size of hundred nanometers regime while bottom-up chemical approach permits the preparation of nanocrystallites 1-10 nm in size. Using the monodisperse nanocrystallites as reactive monomers, fibrous or spheric macromolecules can be constructed by polymerizing primary nanocrystallites. Here we report the strategy for the preparation of Y2O3 nanofibers ( 50 nm diameter and a few micrometer length) and radially grown ZnS macromolecules (200 600 nm diameter ball). Well-defined higher order structures are developed upon thermostatically aging. The shapes of the macromolecules are correlated to their respective monomers, the primary nanocrystallites. It is anticipated that this approach should inspire synthesis of nanocrystal polymers by using primary nanoparticles as monomers.
2:30 PM F4.4
ULTRAFAST DYNAMICS OF INTER- AND INTRA-BAND TRANSITIONS IN SEMICONDUCTOR NANOCRYSTALS: IMPLICATIONS FOR QUANTUM-DOT LASERS. Victor Klimov , Christian Schwarz, Xiaoguang Yang, and Duncan McBranch Chemical Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM.
Femtosecond (fs) transient absorption (TA) and photoluminescence (PL) experiments are applied to study ultrafast energy relaxation and trapping dynamics in strongly-confined CdSe and CdS nanocrystals (NCs) prepared by different techniques. High-sensitivity TA measurements allow us to resolve complementary population/depopulation dynamics of the lowest (1S) and the first excited (1P) electron states in CdSe NCs with 1S-1P energy separation up to 16 LO phonon energies. Instead of the drastic reduction in the energy loss-rate due to a phonon bottleneck, we observe a fast sub-ps 1P-to-1S relaxation, with a rate enhanced in NCs of smaller radius, which strongly suggests that energy relaxation is mediated by nonlinear Auger-type processes. Similar energy relaxation behavior is observed in NCs with different surface treatments and in different solid-state and liquid matrices, indicating that ultrafast electron relaxation, not limited by the phonon bottleneck, is a general feature of quantum-dot systems.
To separate electron and hole relaxation paths leading to the depopulation of the lowest quantized states, we apply time-resolved PL and TA with the IR probe tuned in resonance with either electron or hole intra-band transitions. We observe extremely fast hole relaxation (sub-ps and ps time scales) in all types of samples (colloidal NCs and NC/glass composites made by high-temperature precipitation), which is likely due to efficient trapping at chalcogenide surface dangling bonds. The electron relaxation is a slower process which occurs on sub-100-ps-to-ns time scales, depending on the NC surface treatment and associated degree of passivation of metal surface ions. The strong difference in electron and hole dynamics allows us to separate conduction and valence-band contributions to IR TA, and to map the structure of electron and hole intra-band transitions.
Results of our studies have important implications for realization of NC-based quantum-dot lasers, since the lasing efficiency is crucially affected by the relationship between rates of energy relaxation, radiative recombination, and non-radiative deactivation due to surface trapping and Auger recombination.
3:15 PM *F4.5
EPITAXIAL SEMICONDUCTOR NANOCRYSTALS (CuI, CdS, InN) DERIVED FROM ELECTROCHEMICALLY DEPOSITED METAL NANOPARTICLES. Reg Penner , Sasha Gorer, HongTao Liu, Jane Ganske, John Hemminger, UCI, Irvine, CA.
A fundamentally new approach for synthesizing semiconductor nanocrystals - size-selectively - is described in this talk. Nanocrystallites (NCs) of the compound semiconductors CuI, CdS, and InN have been synthesized on the atomically smooth graphite basal plane surface using a hybrid electrochemical/chemical (henceforth E/C) method. This method involves the following steps: 1) Electrochemical deposition of metal (e.g., Cu) NCs onto an electrode surface, 2) Electrochemical oxidation of metal NCs to yield metal hydroxide (or oxide) nanoparticles, and, 3) Displacement of hydroxide (or oxygen) by an anion (e.g., I-) either in the gas phase or in the liquid phase. The dispersions of semiconductor nanocrystals generated by the E/C method possess the following characteristics: Single crystallinity, good-to-excellent size monodispersity, epitaxial alignment (with the hexagonal periodicity of the graphite(0001) surface). In addition, E/C deposited particles on graphite exhibit strong room-temperature luminescence having virtually no trap state emission, and the energy of the emitted phonons is tunable based on the crystallite diameter.
3:45 PM F4.6
STRONG PHOTOLUMINESCENCE IN THE NEAR-INFRARED FROM COLLOIDALLY-PREPARED HGTE NANOCRYSTALS. M. T. Harrison , S. V. Kershaw, M. G. Burt, BT Laboratories, Martlesham Heath, Ipswich, UK; A. L. Rogach*, H. Weller, A. Eychmuller, Institut fur Physikalische Chemie, Universitat Hamburg, Hamburg, GERMANY. *permanent address: Physico-Chemical Research Institute, Belarusian State University, Minsk, BELARUS.
We report here the first measurement of strong near-IR room temperature photoluminescence from colloidally-prepared HgTe nanoparticles. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) measurements indicate that the nanoparticles are in the cubic coloradoite phase, with a diameter of approximately 4 nm. The broad PL consists of several overlapping features between 800 and 1400 nm, representing a dramatic blue shift from bulk HgTe behaviour, which develop after synthesis yielding more dominant long-wavelength features. These novel nanoparticles may then be developed further by introducing an inorganic capping layer, such as CdTe, to improve stability and PL quantum yield. The observation of an excited state lifetime of the order of 100 ns and the demonstration of optical gain in a related CdHgTe composite quantum dot system makes this material a promising candidate for application in telecommunication systems as a broadband optical amplifier medium.
4:00 PM F4.7
ELECTRONIC STRUCTURE OF GERMANIUM NANOCRYSTALS. T. van Buuren 1, C. Bostedt2, S. Kakar3, L.L. Chase1, L.J. Terminello1; 1Lawrence Livermore National Laboratory, Livermore, CA; 2University of Hamburg, Inst. f. Exp.-Physik, GERMANY; 3UC Davis, Dept of Applied Sciences, Davis, CA
Thin films of germanium (Ge) nanocrystals with an average size of a few nanometers, have been synthesized by thermal vaporization of Ge in an Ar buffer gas. The average cluster diameter could be varied between 1 and 8nm by changing the Ar buffer gas pressure from 60 to 800 mtorr. Characterization of the nanoclusters size and shape was done by x-ray diffraction and atomic force microscopy. X-ray absorption and photoemission spectra have been used to measure the band edges of the germanium nanocrystals as a function of particle size and surface passivation. X-ray absorption in the vicinity of the germanium (Ge) L-edge in the Ge nanoclusters show a blue shift in the conduction band (CB) edge with respect to the edge of bulk Ge. We find that the blue shift is approximately 0.65 eV for Ge clusters 2.0 nm in diameter and 0.30 eV for clusters 3.5 nm in diameter. It is interesting to note that the CB shift in the Ge clusters is larger than the CB shift measured on Si clusters of the same size, prepared in a similar manner. Preliminary photoemission results show that there is also a shift in the valence band. The photoemission experiments are in progress to determine the effect of decreasing the Ge particle size on the valence band maximum. We compare these experimental results to recent photoluminescence experiments and theories predicting quantum confinement effects in the band edges of Ge nanoclusters.
This work is supported by the U.S. Department of Energy, BES-Materials Sciences, under Contract W-7405-ENG-48. C.B. acknowledges financial support from the DAAD HSP-III program.
4:15 PM F4.8
GIANT INTERNAL MAGNETIC FIELDS IN NANOCRYSTAL QUANTUM DOTS. Al L. Efros and M. Rosen, Naval Research Laboratory, Washington DC.
The interaction of electron and hole spins with those of magnetic dopants and atomic nuclei is usually negligibly weak in bulk semiconductors because it occurs via a contact potential. However, in nanosize quantum dots (QDs) this interaction, proportional to the square of the electron and hole wave functions at the position of the nucleus or magnetic ion (MI), grows dramatically as the inverse of the volume of the QD. As a result the effective magnetic field of the nuclei or of a MI acting on the electron spins is several orders of magnitude larger than in the bulk. The value of this magnetic field depends on the positions of the nuclei or MI in the QD and on the QD radius, a. The effective magnetic field of a MI acting on carriers can reach a thousand Tesla and results in the splitting of the electron and hole spin sublevels by up to 100 meV. This leads to a broadening of the optical transitions in doped QDs in no external magnetic field and to a strong PL polarization and Faraday effect when the MIs are aligned in an external magnetic field. The total magnetic field acting on the electron from the nuclei is the sum of all their contributions and depends on the degree of alignment of their spins. At high temperature the nuclear spins are disordered and the average effective magnetic field is zero. However the fluctuation of this field grows as a-3/2, and for 1.5 nm radius QDs reaches 0.25-0.7 T.
4:30 PM F4.9
EFFICIENT VISIBLE LUMINESCENCE FROM GaAs NANOCRYSTALS IN SiO2 MATRICES. Y. Kanemitsu , S. Mimura and S. Okamoto, Nara Institute of Science and Technology, Ikoma, Nara, JAPAN; K.S. Min and H.A. Atwater, California Institute of Technology, Pasadena, CA.
High-dose ion-implantation and thermal annealing technique has been shown to provide a versatile one for creating semiconductor nanocrystals in the surface region of a substrate material, since almost any ions can be implanted into any solid substrates. The nanocrystal size can be changed by controlling the ion dose, the implantation energy, and the annealing temperature. In this work, we have fabricated GaAs nanocrystals by means of Ga+ and As+ co-implantation into SiO2 matrices and have investigated photoluminescence (PL) properties of GaAs nanocrystals in SiO2 matrices by selective excitation spectroscopy. The nanocrystals were formed in the 100-nm SiO2 films on (100) crystalline Si substrates. The nanocrystals were synthesized by 3Å 1016/cm2 Ga+ and 2Å 10 16/cm 2 As+ co-implantation at 75 keV into SiO2. Subsequent annealing in vacuum resulted in precipitation of GaAs nanocrystals. Under blue laser excitation, broad PL spectra consisting of two or three bands are observed in the visible spectral region. The highest PL intensity was observed from samples passivated by deuterium implantation at a dose of 3Å 10 15/cm2 after annealed at 900
C for 10-60 min.
Hydrogen passivation of GaAs nanocrystals surface is important for
efficient luminescence. Under selective excitation at energies
within a certain band, fine structures are observed at low
temperatures. The observed fine structures depend on the
nanocrystal size. From comparison between experiments and
calculations, we concluded that the size-dependent fine structures
are attributed to radiative recombination in GaAs nanocrystals. We
have demonstrated that the size dependent optical properties are
attributed to quantum confinement states in GaAs nanocrystals and
ion implantation technique is a useful in forming compound
semiconductor nanocrystals in SiO2 and other
materials.
4:45 PM F4.10 SPECTROSCOPY OF SINGLE CdSe NANOCRYSTALLITE QUANTUM DOTS. Stephen Empedocles , Robert Neuhauser, Kentaro Shimizu and Moungi Bawendi, MIT, Dept of Chemistry, Cambridge, MA.
We use the techniques of single molecule spectroscopy to study single CdSe nanocrystallite quantum dots. Using far-field epifluorescence microscopy, we are able to image single quantum dots and obtain fluorescence spectra that are more than 50x narrower than what can be achieved using ensemble techniques. Polarization spectroscopy reveals a unique transition dipole orientation for emission in individual quantum dots. The elimination of inhomogeneous broadening reveals light driven spectral diffusion with shifts over a wide range of energies (<0.1meV to 80meV). Careful examination of single dot spectra suggests that the primary contribution to single dot lineshapes is in fact spectral diffusion rather than the inherent physics of the nanocrystallite. This indicates the strong role of the surrounding environment in the electronic structure of these nanocrystallites. Electric field studies suggest a Stark mechanism behind these spectral shifts, indicating the existence of large, fluctuating local electric fields. Single dot Stark studies also reveal that the lowest excited state possesses both polar and polarizable character, consistent with the presence of local fields around individual dots.
F5.1SESSION F5: POSTER SESSION:
MICROCRYSTALLINE AND NANOCRYSTALLINE SEMICONDUCTORS
Chairs: Jillian M. Buriak, Jean-Noel Chazalviel and Leonid Tsybeskov
Tuesday Evening, December 1, 1998
8:00 P.M.
Salons (M)
OPTICAL STUDY OF FREE-STANDING AND ON-SUBSTRATE POROUS SILICON MULTILAYER STRUCTURES FOR OPTOELECTRONIC APPLICATIONS. Selena Chan , Leonid Tsybeskov, Philippe M. Fauchet, Univ of Rochester, Dept of Electrical Engr, Rochester, NY.
Porous silicon multilayer structures are easily manufactured using a periodic current density square pulse during the electrochemical dissolution process. The difference in porosity profile, corresponding to a variation in current density, is attributed to a difference in refractive index. Manipulating the difference in refractive index, high quality optical filters can be made with a maximum reflectivity peak
100%. The next logical step to further exploit these
optical mirrors is to incorporate them into an LED device. The
benefit of adding a multilayer mirror below a luminescent film of
porous silicon, is to significantly reduce the amount of light loss
to the silicon substrate and increase the light output. However,
oxidation is required to stabilize the as-anodized porous silicon
film. This disrupts the overall index profile of the multilayer
stack, causing the peak reflectance to blue shift. This phenomena
must be quantified and accounted before device implementation. We
present a detailed study on the effects of oxidation temperature, gas
environment, and annealing time of porous silicon multilayer
structures, both free-standing and on-substrate, in a device
configuration.
F5.2
IMPROVED PL IN POROUS SILICON DUE TO THE COMPENSATION OF THE DEFECT CENTER BY ANODIC OXIDATION. Y. Show , S. Rath, S. Nozaki, and H. Morisaki, Department of Communications and Systems, The University of Electro-Communications, Tokyo, JAPAN; M. Iwase and T. Izumi, Faculty of Engineering, Tokai University, Hiratsuka-shi, Kanagawa, JAPAN.
Porous silicon (PS), prepared by anodic etching in HF solution, emits bright visible light. It is important to improve its light emission property for its application as a light emitting device. One way of improving light emission is anodic oxidation of the PS surface. In this paper, we will present the correlation between the photoluminescence (PL) and the defect structure in the anodic oxidized PS.
PS samples were prepared on (100) p-type silicon substrates of a resistivity of 17-22
cm. Anodization was performed at 1
mA/cm2 for 60 min. in HF solution. Subsequent anodic oxidation
was performed at 1 mA/cm2 in HCl for 5 to 30 min. Oxidation was
confirmed by FT-IR spectroscopy. The defect structure was studied by
ESR.
The PL spectrum of the as-prepared PS exhibited a peak at 710 nm. The
Pb-center, which originates from defects in the interface
between crystalline Si and SiO2, was observed by ESR. Anodic
oxidation of the sample for 30 min. led to an increase in the PL
intensity by about 6 times and a decrease in the Pb-center intensity
by half. This result indicates that the Pb-center in PS is a
non-radiative center (emission killer center). The increase in the PL
intensity is due to oxidation of the PS surface which compensates the
Pb-center existing in the PS surface region.
F5.3
AN PROTOTYPE ULTRAVIOLET DETECTOR ON BASED-SILICON DEVICES USING POROUS SILICON THIN FILM LUMINESCENT. Limarix Peraza, Madeline Cruz, Angel Estrada, J. Avalos , Metropolitan University, Dept. of Sciences and Technology, San Juan, PR; Luis Fonseca, Oscar Resto, University of Puerto Rico, Dept. of Physics, San Juan, PR.
Using the electrical sensitivity of the silicon crystalline when is exposed to visible light, an prototype build up of thin film of porous silicon luminescent on thin bulk of silicon crystalline, was done. The orange luminescent form the porous silicon produce sensitive changes in the photoconductivity properties of the crystalline silicon, that was measured and compared in the voltage- current curves. That prototype detector is then sensitive to UV radiation and calibrates curves leave known the intensity of UV radiation. We're trying to look out for any applicability of this prototype on the biology field.
F5.4
INFLUENCE OF THE PARTICLE SIZE AND THE METHOD OF PREPARATION ON THE PROPERTIES OF MICROCRYSTALLINE FERRIC HYDROXIDE AND OF Fe2O3 OBTAINED BY DEHYDRATION. Georges Dé nès, Concordia University, Department of Chemistry and Biochemistry, Laboratory of Solid State Chemistry and Mössbauer Spectroscopy, and Laboratories for Inorganic Materials, Montreal, Quebec, CANADA; S.C.F. Au-Yeung, Chemistry Department, Chinese University, HONG KONG.
Fe2O3 is a well known semiconductor. It can be prepared by dehydration of ferric hydroxide Fe(OH)3. However, we have found that the properties of Fe2O3 depends on how Fe(OH)3 was prepared. The traditional preparation of Fe(OH)3 consists in precipitating a ferric gel by addition of a base to a ferric solution, and then freezing the gel in liquid nitrogen. Then the powder, called Fe(OH)3 gel, separates from the water upon thawing. We have discovered a hitherto unknown method of preparation, which gives directly Fe(OH)3, called Fe(OH)3 powder. The new synthetic method uses the simultaneous oxidation/hydrolysis of ferrous iron by a peroxo complex or by hydrogen peroxide, starting in neutral conditions. The pH decreases during the reaction. The precipitate is Fe(OH)3 powder and does not require freezing/thawing like Fe(OH)3 gel. Although both kinds of materials have the same chemical composition, i.e. Fe(OH)3, their properties are quite different in terms of color, thermal stability, crystallinity, particle size and magnetism. Some of the differences can be accounted for by the difference in particle size. Both show characteristics of nanoparticles, such as superparamagnetism and hyperfine magnetic field distribution. In addition, the importance of the surface relative to the bulk is much higher in Fe(OH)3 gel. The crystallinity and some of the properties of Fe2O3 can be tailored by the appropriate choice of the method of preparation of Fe(OH)3 and of the conditions of its dehydration.
F5.5
CORRELATION BETWEEN DEFECT STRUCTURES AND LIGHT EMISSION IN Si-NANOCRYSTAL DOPED SiO2 FILMS. Keisuke Sato , Yosuke Sugiyama and Tomio Izumi, Department of Electronics, Faculty of Engineering, Tokai University, Hiratsuka-shi, Kanagawa, JAPAN; Mitsuo Iwase, Department of Electrical Engineering, Faculty of Engineering, Tokai University, Hiratsuka-shi, Kanagawa, JAPAN; Yoshiyuki Show, Shinji Nozaki and Hiroshi Morisaki, Department of Communications and Systems, The University of Electro-Communications, Chofu-shi, Tokyo, JAPAN.
Lately, visible light emission from silicon nanocrystal material has become major interest because of its potential application to Si based light emitting devices. Many research workers proposed the quantum size effects for a light emission mechanism, which are caused by the confinement of electrons and holes in silicon crystals of nanometer size. However, the light emission mechanism has not completely been resolved up to now. In this paper, we have studied correlation between defect structures and light emission from Si-nanocrystal doped SiO2 films, using ESR and PL methods. Si-nanocrystal doped SiO2 films were fabricated by RF sputtering method. A sputtering target consisted of 5 mm X 5 mm Si pieces placed on a SiO2 substrate, and the number of Si pieces were 16. The samples were annealed up to 1100Åé in Ar atmosphere. We observed two kinds of ESR centers from as-grown film. One had a g-value of 2.004, which originated from Si dangling bond in a-SiOx region. The other had a g-value of 2.001, which is due to the EÅEcenter in SiO2 region. No visible light emission was observed from the as-grown film. When the film was annealed at above 900Åé, however, visible light emission was observed. Moreover, the ESR analysis revealed the presence of new three kinds of ESR centers in the annealed sample, i.e. Si dangling bond in amorphous Si (a-center: g=2.006), Si dangling bond in Si-nanocrystal/SiO2 interface (Pb-center: g=2.003) and conduction electrons in Si-nanocrystal (Pce-center: g=1.998). The PL intensity and the signal intensity of the conduction electron increased with increase of annealing temperature. These results indicate that Pce-center is associated with the emission center.
F5.6
TIGHT BINDING MODEL GW CALCULATION OF QUASIPATICLE GAPS FOR SILICON NANOCRYSTALS. C. Delerue, M. Lannoo and G. Allan, IEMN - Department ISEN, Lille, FRANCE.
The accurate prediction of the optical bandgap of semiconductor nanocrystals is an important challenge for material physicists. Classically, it requires the evaluation of two quantities: the quasiparticle gap and the excitonic correction. The quasiparticle gap is usually approximated by the one-electron gap obtained from ab initio local density or semi-empirical techniques. To check the accuracy of such predictions, one needs to go beyond these approximations using for example the so-called GW method which allows to calculate the self-energy operator. However the application of GW using ab initio wave functions defined in plane wave basis becomes extremely heavy for clusters. We thus propose a model GW formalism based on a tight binding framework. We verify that it gives correct results for the self energy corrections of the Si, Ge and diamond bulk bandgap. Due to its relative simplicity, we can apply this method to Si nanocrystals containing up to 300 atoms. We show that the self energy correction strongly increases with the confinement. We identify two contributions in the correction. The first one comes from the well-known electronic relaxation in finite systems, equivalent to an electrostatic image charge effect. The second one, which is smaller, varies linearly with the inverse dielectric constant of the crystallite. We show that the excitonic interaction also contains an image charge contribution which almost compensates the equivalent term in the self energy correction. We conclude on the accuracy of semi-empirical and ab initio calculations of the bandgap.
F5.7
EXCITATION AND LUMINESCENCE OF SIZE SELECTED NANOCLUSTERS IN POROUS SILICON. Z. Yamani, W. Howard Thompson, N. Rigakis, and M. H. Nayfeh , Department of Physics, University of Illinois, Urbana, IL.
We prepared Si that exhibits green to red rainbow photoluminescence along the sample from the meniscus to the opposite end, reflecting a gradient in the crystallite size. We measured excitation band edges of 3.75 and 3.0-3.25 eV at the meniscus and the opposite end, corresponding to
0.85 nm and 1.3 nm sizes. The
blue edge of the emission spectrum blue shifts for smaller sizes
while the red edge is somewhat insensitive to size, resulting in a
band that widens and advances towards the blue with size reduction.
In the meniscus region, for which the size distribution is the
narrowest and the average size is the smallest, the emission
bandwidth is the widest and encompasses much of the visible spectrum.
Ultra thin room temperature oxides (5-10 A) produced by H2O2
immersion is found to preserve the optical activity, while UV
enhanced ozone oxidization quenches it. The results are discussed in
terms of novel quantum confinement-engineered restructuring of the
diamond-like bonds into intrinsic radiative Si-Si dimer bonds that
may be responsible for the optical behavior. The two oxidation
effects are respectively consistent with either preservation or
breakage of the Si-Si dimer bonds. Refrences:
Z. Yamani, W. Thompson, L. Abu Hassan, and M. Nayfeh, Appl. Phys. Lett. 70, 3404 (1997); M. Nayfeh, N. Rigakis, and Z. Yamani, Phys. Rev. B 56, 2079 (1997); Z. Yamani, S. Ashhab, A. Nayfeh and M. H. Nayfeh, J. Appl. Phys. 83, 3929 (1998).; Z. Yamani, N. Rigakis, and M. H. Nayfeh, Appl. Phys Lett. 72, 2556 (1998); W. H. Thompson, Z. Yamani, L. Abuhassan, and M. Nayfeh, Appl. Phys. Lett. (August 3) 1998.
F5.8
PHOTOLUMINESCENCE
AND PHOTOACOUSTIC SPECTROSCOPY OF POROUS SILICON.
Vladimir Karavanskii , Institute of General Physics,
Russian Academy of Sciences, Moscow, RUSSIA; Alexander Obraztsov,
Physics Department of Moscow State Univ., Moscow, RUSSIA; Hideo
Okushi, Hideyuki Watanabe, Electrotechnical Laboratory, Tsukuba,
lbaraki, JAPAN.
The distinctive feature of porous Si
(por-si) is a significant shift of the fundamental absorption edge to
the shorter wavelengths, and the presence of intense luminescence in
the visible spectral range High sensitivity of por-Si layers to any
subjection makes a problem important of choice of procedure of
samples preparation and of experimental methods of investigation. As
a rule, thin layers on substrate very suitable for
electro-luminescent application, but free-standing layers are
convenient for optical transmission spectroscopy (OTS), which usually
may have differ properties from films on substrate. Really, OTS also
do not give information about non-uniformity of properties along
thickness. Earlier it was shown that photoacoustic (PA) spectroscopy
can be effective for studying of optical absorption in por-Si.
Amplitude part of PA signal gives information about non-radiative
optical absorption within thermodiffusion length in porous layer,
which can be made by selection of chopping frequency less than
optical absorption depth. Phase part of PA signal can be used to
determination of thickness and thermal properties of por-Si layers.
Comparison and analysis of spectral distribution of PA amplitude and
phase signals at different chopping frequencies in additional to
reflection and photoluminescence spectra can be used to retrieve
information about absorption spectra and homogeneity of porous layers
and localization of luminescent and non-luminescent recombination
centers in films. Several sets of por-Si samples were prepared and
investigated by photo acoustics, photoluminescence, optical
reflection and micro-Raman scattering spectroscopy. The results
obtained show that the por-Si films have similar and uniform over the
layer optical absorption in the blue-green spectral range, whereas in
the red range the optical absorption tends to increase in the layer
near substrate. The luminescent properties of por-Si vary noticeably,
and the PL is most efficient on the surface of the layer, especially
if the PL is excited by a longer-wavelength radiation (633 nm).
Taking into account that the PA signal is due to that part of the
absorbed light which is converted into the heat, one can assume that
there is also a difference in excitation spectra in the system of the
PL centers. At the same time, the recombination properties of those
centers effectively excited by the blue-green radiation are
distributed rather homogeneously over the film thickness, whereas the
centers that absorb light in the red range have relatively high
efficiency of radiative recombination (or low efficiency of
non-radiative recombination) only within a thin near-surface area.
So, in different experiments we can deal with different nature of PL
centers, which explain difficulties of interpretation of often
mutually exclusive experimental results.
F5.9
Abstract
Withdrawn.
F5.10
RECENT
ADVANCES IN THE SYNTHESIS AND PROPERTIES OF SILICON CAPPED GERMANIUM
NANOCLUSTERS. Chung-Sung Yang , Susan M. Kauzlarich,
Univ. of California-Davis, Dept of Chemistry, Davis, CA; Young-Chung
Wang, LBNL, National Center for Electron Microscopy, Berkeley, CA;
Howard W.H. Lee, Gildardo R. Delgado, LLNL, Photonic Group,
Livermore, CA.
Synthesis of silicon capped germanium
nanoclusters by a solution phase method is essential in order to gain
good control of size distribution and surface termination. Silicon
capped germanium nanoclusters are prepared by the reaction of silicon
tetrachloride with magnesium germane in tri(ethylene glycol) dimethyl
ether and terminated with various alkyl groups. High resolution TEM
has been used to establish size distribution. The off-axis electron
holography is employed to investigate the structure. The
reconstructed phase image from the hologram is able to offer a
detailed information for the radii ratio between the Ge and Si-alkyls
sphere. FT-IR, UV-vis, NMR, and photoluminescence (PL) are employed
to study the surface composition of these nanoclusters. FT-IR and NMR
spectra show that the alkyl groups are bonded directly to the silicon
surface. PL spectra of the nanoclusters shows a strong blue PL
between 360-500 nm.
F5.11
ADSORPTION
ASSISTED EXCITATION OF POROUS SILICON PHOTOLUMINESCENCE.
T.V. Torchinskaya , N.E. Korsunskaya, L.Yu. Khomenkova,
B.R. Dzhumaev, Institute of Semiconductor Physics, National Academy
of Sciences, Kyiv, UKRAINE; A. Many, Y. Goldstein, E. Savir, Racah
Institute of Physics, The Hebrew University, Jerusalem, ISRAEL.
Despite many photoluminescence (PL) studies of
porous silicon (PSi) and other silicon nanocrystal structures, the
mechanism of the PL excitation (PLE) is still not clear. We have
carried out a comprehensive investigation of the PLE mechanism in
PSi. Two different spectral bands have been observed in the PLE: one
in the visible, 400-530 nm, and the other in the UV, <400 nm.
SIMS, Auger, as well as SEM, FTIR and EPR have been used for the PSi
characterization. The influence of aging processes, chemical and ion
etching, thermal and ultrasound treatment and PSi preparation
parameters on the PL intensity was found to be quite different for
the visible and uv light excitations, indicating different excitation
mechanisms.
The existence of two PLE bands, in which the PL is strongest, as well
as other characteristics of the PL spectra to be discussed, suggest
that the PL is due partly or mostly to light absorption by complexes
adsorbed at the silicon nanoparticles or their oxide. SIMS and FTIR
data indicate that these complexes apparently consist of OH groups
and some other impurity atoms. On this basis, we conclude that the
visible PLE band originates from electron excitation from some ground
state into an energy band of localized states associated with the
adsorbed species. Possible mechanisms of the excitation transfer from
such complexes to radiative channels in PSi will be
discussed.
F5.12
GIANT
ANISOTROPY OF CONDUCTIVITY IN HYDROGENATED NANOCRYSTALLINE SILICON
THIN FILMS. A.B. Pevtsov , N.A. Feoktistov, V.G. Golubev,
Ioffe Physico-Technical Institute, St. Petersburg, RUSSIA.
Thin hydrogenated nanocrystalline silicon films are
widely used in various applications. In this work the conductivity of
doped and undoped amorphousnanocrystalline silicon thin films is
studied as a function of film thickness: a giant anisotropy of
conductivity is established. The longitudinal conductivity decreases
dramatically as the layer thickness d reduces, while the
transverse conductivity remains close to that of a heavily doped
a-Si:H.
Films of various nanocrystalline phase contents were prepared by the
standard PE CVD technique. The nanocrystallite radii
R=(40-50)Å and volume fractions were estimated
from the Raman scattering measurements. For the films of
we observed the longitudinal conductivity to drop from
10-2 to 10 as d decreases from
1500Å to 200Å. Conversely, changed less than 10 times
for the case of .
We analyzed our results in the terms of percolation conduction
through the network formed by crystallites and amorphous matrix.
In accordance with the theory [1], the percolation cluster structure
depends on d when d is smaller than the cluster correlation
length in the corresponding bulk material but still larger than the
average intercrystallite distance. Because of this mechanism
is linear in where is the
three dimensional index of the correlation length [1]. As
approximated by the latter dependence our data yield
substantially lower than the theoretical prediction .
We used our films to devise p-i-n structures
aimed at operating liquid crystal spatial light modulators. Our
measurements show these films to be promising doped layers with
a small surface leakage current in such barrier structures.
[1] A.L.Efros and B.I.Shklovskii, Electronic Properties of Doped
Semiconductors, Springer, Berlin (1982).
F5.13
PLASMONS ON
LUMINESCENT POROUS SILICON PREPARED WITH ETHANOL. O.
Resto , L.F. Fonseca, A. Ramirez-Porras, and S.Z. Weisz, Department
of Physics, University of Puerto Rico, Rio Piedras, PR; A. Many, and
Y. Goldstein, Racah Institute of Physics, The Hebrew University,
Jerusalem, ISRAEL.
We investigated the plasmon
characteristics on luminescent porous nanocrystalline silicon using
electron energy loss spectroscopy. A Physical Electronics PHI 560
ESCA/SAM system was used for the study. The samples were prepared
from p-type crystalline silicon using the conventional
electrochemical etching technique with the usual solution of HF,
ethanol and water, and critical point drying. The energy of the bulk
plasmon was measured both before and after sputter cleaning the
sample with argon-ion bombardment. We found that initially the
plasmon energy was slightly higher, 17.5 eV, but quite close to the
plasmon energy of crystalline silicon, 16.9 eV. After sputtering
for 10 min with 5 keV Ar+ ions, the plasmon energy increased to 20
eV. Exposure to the electron beam used for the measurements caused a
slow upward shift of the plasmon energy as a function of time, till a
saturation energy of 24 eV was reached. Auger spectroscopy
performed in parallel showed an increasing carbon coverage. We
prepared also samples without ethanol in the etching solution. These
samples showed consistently a plasmon energy 17 eV that did not
change upon sputtering with Ar+ ions or exposure to the electron
beam. A surface-plasmon loss at 5.5 - 6 eV was also measured.
We conclude that traces of ethanol used in the preparation of the
porous silicon serve as a source of carbon. Apparently, until
activated by Ar+ ion bombardment or exposure to the electron beam,
the ethanol is inactive and does not interact with the porous Si
surface. Upon activation, however, carbon from ethanol is adsorbed
at the surface and forms a thin film covering the silicon and
obliterating almost completely the silicon plasmon. It is our
contention that the energy loss at 24 eV is due mostly to the carbon
plasmon whose energy is 25.9 eV.
F5.14
LIGHT EMITTING
NANOSTRUCTURES IN IMPLANTED SILICON LAYERS. R.
Plugaru a, J. Piquerasb, B. Mendezb, C. Craciuna,
N. Nastasea; aInstitute of Microtechnology, Bucharest,
ROMANIA; bDpt. Fisica de Materiales, Facultad de Fisicas, Univ.
Complutense, Madrid, SPAIN.
The light emission
characteristics of silicon nanostructures are strongly dependent on
the structural properties of the crystallites. Different processes
used in order to obtain light emitting silicon structures have been
developed. Generally there are two mechanisms involved in the
nanocrystals formation: one, is related to disorder effects induced
in monocrystalline phase, by porosification or irradiation and the
second to the growth of crystallites, by deposition methods or
irradiation of amorphous phase. Ion implantation of amorphous
structures can induce the formation of a crystalline phase with light
emission properties.
The basic process of amorphous to crystalline phase transition
induced by irradiation has not been yet established. Thermal spikes
as temperature assisted processes or increase of the disorder has
been invoked for explanation. The luminescence of the implantation
induced nanocrystals depends on structural features.
It is the aim of this paper to reveal some evidences on the
mechanisms of crystalline phase formation in as-deposited amorphous
silicon layers implanted with boron ions and to correlate the
obtained microstructure with the emission properties of the samples.
To this purpose amorphous deposited silicon films were implanted with
different doses of boron ions. Some of the implanted layers were
porosified by an anodization process. Cathodoluminescence
measurements reveal the existence of blue emission in as-implanted
samples. The porosification induced changes, as the appearance of
additional emission in the red spectral range were investigated.
F5.15
THE INVERTED
MEYER-NELDEL RULE IN THE CONDUCTANCE OF NANOSTRUCTURED SILICON
FIELD-EFFECT DEVICES. R.E.I. Schropp and H. Meiling,
Utrecht University, Debye Institute, Utrecht, THE NETHERLANDS.
Thin film transistors (TFTs) offer the possibility
to study the electronic transport properties of an intrinsic
semiconductor as a function of the Fermi level position without the
introduction of dopants and/or doping related defects. Recently, we
reported on the first TFTs incorporating nanostructured silicon
deposited with the Hot-Wire Chemical Vapor Deposition technique.
These structures offer significant advantages over conventional
plasma-deposited amorphous silicon TFTs. First of all, the HW
deposited nanocrystalline silicon (nc-Si:H) TFTs do not show any
threshold voltage shift upon prolonged gate voltage stress.
Therefore, it is now possible to study the transport characteristics
at a relatively large gate voltage in a controlled fashion,
unhampered by any drift of the characteristics due to the creation of
metastable electronic defect states and/or charge trapping. Second,
the result of the field effect is that the Fermi energy moves into
the conduction band of the virtually defect-free nanocrystalline
domains in the channel region of the TFT. As the effective mobility
gap of the surrounding amorphous phase is higher than that of the
silicon crystallites, the Fermi energy is driven deep into the
band-tail distribution of the amorphous phase, a situation that could
never be achieved in purely amorphous silicon TFTs nor by heavily
doping an amorphous semiconductor. Thus, the nanostructured nature of
the silicon thin film near the gate insulator allows to shift the
Fermi level far into the tail states region of the amorphous phase.
This situation reveals for the first time the inverted Meyer-Neldel
relationship in an intrinsic semiconductor.
F5.16
VIBRATIONAL
AND ELECTRONIC PROPERTIES OF Si NANOPARTICLES. J. R. Fox
and A. A. Sirenko, Department of Physics, Penn State University,
University Park, PA.
We report on our in-situ
investigations of the vibrational and electronic properties of
isolated Si nanoparticles. Ultra-thin layers of Si have been grown by
dc magnetron sputtering in UHV on top of amorphous MgO buffer layers.
The average thickness of the Si layers ranged from submonolayer
coverage up to 10 monolayers. The coverage has been monitored by
core-level photoemission spectroscopy. The interference enhanced
Raman scattering (IERS) technique has been used to study changes in
the phonon spectra of Si nanoparticles during the crystallization
process. Marked size-dependencies in the electronic properties of the
Si quantum dots have been detected by high-resolution electron energy
loss spectroscopy (HREELS).
This work was supported by USDOE Grant DE-FG02-84ER45095 and NSF
Grant DMR-96-23315.
F5.17
X-RAY
REFLECTIVITY STUDY ON POROUS SILICON FORMATION. Virginie
Chamard , Gérard Dolino, Laboratoire de Spectrometrie Physique,
Universite J. Fourier, Saint-Martin d'Heres, FRANCE; Joel Eymery,
Departement de Recherche Fondamentale sur la Matiere Condensee,
CEA-Grenoble, FRANCE.
The room temperature
luminescence of porous silicon has raised a strong interest since its
discovery in 1990, but the formation and structure of this
nanocrystalline material are not well understood, especially in the
case of n-type silicon. For example, a two-layer structure consisting
of a macroporous under a nanoporous layer associated to crater
formation has been often observed, but not clearly explained. In
addition to the usual anodization parameters: current intensity, HF
concentration and anodization time, the illumination parameters
(wavelength, density) during the formation of n-type sample must be
considered.
Small angle (<1) X-ray reflectometry is well suited to
study the first steps of the formation of thin porous layers between
10 and 200 nm. In the specular reflection geometry (incidence angle
equals exit angle), the direction perpendicular to the sample surface
is investigated. Important information can be deduced from these
measurements: the porosity related to the critical incidence angle,
the layer thickness from the period of the interference fringes and
the surface roughness from the decrease of the intensity at large
angles.
In the off-specular geometry (fixed detector - rotating sample), the
diffuse scattering intensity is related to the in-plane correlation.
The study of Yoneda wings and diffuse intensity allows to get the
surface and interface correlation lengths.
For porous silicon, it will be shown that the surface and interface
roughness are increasing with formation time. Moreover, an unexpected
increase of the surface porosity due to chemical dissolution is also
observed on different types of porous silicon. The presence of a
crater at the top of the layer is related to the illumination during
anodization.
X-ray reflectometry is also used to get the chemical and
electrochemical photodissolution on different porous silicon types
(p- and n+).
These results will be compared to several models of porous silicon
formation.
F5.18
POROUS SILICON
FILM COMPOSITION AND PHOTOLUMINESCENCE MODIFICATIONS STIMULATED BY
TREATMENT IN HF. Viktorija B. Shevchenko, Oleksandr I. Dacenko,
Volodymyr A. Makara, Olga V. Rudenko, Oleg V. Vakulenko ,
Taras Shevchenko National Univ, Dept of Physics, Kyiv, UKRAINE;
Mykola S. Boltovets, Inst ``Orion'', Kyiv, UKRAINE.
Modifications of the anodically formed porous silicon (PS) film
composition caused by additional treatment in 48 aqueous HF as
well as further ageing in the air were investigated. A special
attention was paid to the search for correlation between the PS
composition and photoluminescent properties.
Auger spectroscopy (AS) and secondary ion mass spectroscopy (SIMS)
methods were applied for investigating the PS film chemical
composition. Infrared (IR) spectroscopy was used to study the surface
bonds of silicon with oxygen and hydrogen.
The treatment of samples in hydrofluoric acid is established to
result in a change of photoluminescence intensity (PLI) and sometimes
to shift of PL peak. When storing the samples both as-prepared and
etched in HF, a rise of PL intensity was observed. The PLI value of
HF-treated sample flattened out within about a month, whereas that of
the untreated one reached the maximum within approximately a year.
The SIMS and AS data have demonstrated that the PLI rise during
ageing is accompanied with an increase of oxygen content in PS. IR
transmission spectra have shown no bonds except Si-Si and Si-Hx
ones in the PS samples as-treated in hydrofluoric acid. As a result
of a weekís ageing of such samples in the air, the absorption bands
attributed to Si-Hx bonds have disappeared from IR transmission
spectra, whereas those associated with bonds in Si-O, Si-OH,
O3-Si-H complexes have appeared. Subsequently, only
amplification of Si-O bands was observed. Similar changes with much
smaller rate occur in the samples untreated in HF.
It is concluded that the oxide at the surface of silicon
nanostructure plays a significant role in emitting the light from
PS.
F5.19
LIGHT INDUCED
ELECTRON SPIN RESONANCE MEASUREMENTS ON MICROCRYSTALLINE SILICON
PREPARED AT DIFFERENT DEPOSITION RATES. Joachim Mueller ,
Friedhelm Finger, Peter Hapke, Heribert Wagner, Forschungszentrum
Juelich, ISI-PV, Juelich, GERMANY.
For device
applications of PECVD grown microcrystalline silicon (c-Si:H) it
is essential to combine a high deposition rate RD with good
material quality concerning the defect densities and the transport
and recombination of charge carriers. Information on this can be
obtained by dark as well as time dependent and steady-state light
induced electron spin resonance (LESR) using both white light and
infrared (IR) light. In this study we compare undoped c-Si:H
prepared at RD=2.5-4.5 Å/s and having various crystalline
volume fractions (XC) with intrinsic and (p- or n-) doped
c-Si:H samples with RD=1 Å/s and XC>90.
Interestingly, the neutral defect density does not increase for the
high-rate material, whereas differences show up in transient
experiments when the effects of IR light applied during the decay of
the conduction electron LESR signal after preceding white light
illumination are investigated. The behaviour varies concerning both
the quenching/enhancing effects of IR light and the steady-state
signal levels reached by pure IR illumination and after white/IR
light sequences. These results point to a considerable reduction of
lifetime or generation rate for IR light induced charge carriers in
the high-rate c-Si:H. The latter experiments also show
remarkable differences between n- and p-type samples. In p-type
samples IR illumination can re emit all of the electrons and holes
out of states previously filled by white light and cause both to
recombine. In n-type or intrinsic material it is suggested that part
of the light induced holes get trapped by neutral or negatively
charged defects or that excited carriers are hidden behind high
potential barriers thus inhibiting IR light induced
recombination.
F5.20
POROUS SILICON
NANOCRACKING. Gilles Lerondel, M. Bertola, G. Amato , L.
Boarino, A.M. Rossi, IENGF, Torino, ITALY; A. Parisini, CNR-LAMEL,
Bologna, ITALY.
In terms of application as emitting
or thermal isolating material, nanoporous silicon layers with very
high porosity are of great interest. Consequently few different
drying processes like supercritical, freeze and pentane drying were
developed to avoid the cracking of the porous structure. Macroscopic
or microscopic cracking can be easily detected since their are both
characterised by a loose of specular reflection on the porous silicon
surface.
We report here for a new type of cracking which was observed during a
study of freeze dried samples. Starting with a highly porous PS
sample of 4 um with a bright brawn aspect, the colour of the sample
changes after the drying procedure to become a well defined green. On
the other hand the mirror aspect of the sample was kept. This, which
results of a variation of the optical path (nd), could not be only
attributed to a change of the refractive index. In fact a study of
the reflectivity showed the sample collapse, the thickness becoming
0.6 um. Moreover an analysis of the profile of the sample shows that
its volume was reduced by a factor of 6. X ray and TEM observations
shows that such samples have lost their crystallinity becoming
completely amorphous. In situ observations shows that this
compression occurs during the sublimation or evaporation phase.
During this phase, if the sample is wetted again the initial colour
of the sample can be recovered as an effect of the elasticity of the
PS structure.
Work is still under progress to asserting if the origin of this
nanoscopic cracking can be also attributed to capillary effects, as
in the case of the well known macroscopic cracking.
F5.21
EFFECTS OF
VARIOUS HYGROGEN DILUTION RATIOS ON THE PERFORMANCE OF THIN FILM
NANOCRYSTALLINE/CRYSTALLINE SILICON SOLAR CELLS. Young J.
Song , Wayne A. Anderson, State University of New York at Buffalo,
Dept of Electrical Engineering, Amherst, NY.
Low
temperature growth of hydrogenated nanocrystalline silicon film
(nc-Si:H) by microwave ECR-CVD has been performed employing a double
dilution of silane, using a He carrier for SiH4 and its
subsequent dilution bu H2. A series of Raman spectra has shown
that a very thin (<100) nc-Si:H layer initially grown with
high H2 dilution on a glass substrate can serve as a good seed
layer for the whole film. The role of this thin layer in low
temperature junction formation has been examined by the insertion ot
the layer between the interface of both nc-Si:H(deposited with lower
H2 dilution)/c-Si and a-Si:H/c-Si heterojunction type
photovoltaic cells. This is to address the knowledge that the
device's performance is strongly influenced by the quality of the
thin film silicon/crystalline silicon interface. Various thicknesses
and H2 dilution ratios have been used to find the optimized
condition providing the best performance of the cells. The maximum
efficiency of 10.5 (Jsc=35.1mA/cm2, Voc=0.51V and
FF=0.59) has been obtained, without an AR coating, by the successive
deposition of nc-Si:H with four different H2 dilution ratios on
a crystalline silicon substrate. This is potentially a low
temperature, low cost solar cell fabrication process.
F5.22
INTEGRATION OF
MULTILAYERS IN Er-DOPED POROUS SILICON STRUCTURES FOR OPTOELECTRONIC
DEVICES. Herman A. Lopez , Selena Chan, Leonid Tsybeskov,
Philippe M. Fauchet, University of Rochester, Dept of Materials
Science and Dept of Electrical Engineering, Rochester, NY; V. P.
Bondarenko, Belarusian State University of Informatics and
Radioelectronics, Minsk, BELARUS.
The importance to
fiber optics and the possibility of single and chip-to-chip photonic
interconnect communication in silicon is the driving force behind the
interest in 1.5 m emission from erbium (Er3+) doped porous
silicon (PSi). Oxidized PSi is an ideal host because it provides the
necessary environment to activate the erbium ions. Er3+ is
incorporated into PSi by a simple electro-infiltration technique in
which the Er3+ concentrations are larger than 1019
cm-3. After the Er-doped PSi is thermally oxidized at 900-1150
C, it exhibits a narrow (FWHM 15 meV) and intense
room temperature photoluminescence at 1.5 m. PSi
multilayers with very high reflectivity (R 100 ) in the 1.5
m range have been incorporated into Er-doped PSi structures
resulting in an enhancement of photoluminescence. Multilayers are
also incorporated into our previously reported Er-doped PSi LED
structure. Preparation, properties, and advantages of the multilayer
structures as well as electroluminescence from Er-doped PSi devices
will be discussed.
F5.23
SURFACE
RELAXATION EFFECTS ON THE ELECTRONIC PROPERTIES OF POROUS SILICON.
E. Vázquez, M.R. Beltran , and
J.Tagüeña-Martinez, C. Wang, and L. E. Sansores.
Instituto de Investigaciones en Materiales, UNAM, MEXICO
Centro de
Investigación en Energía, UNAM, Temixco, MEXICO.
The calculation of the electronic band structure of nanocrystalline
semiconductors can be achieved by two possible approaches: first
principle methods and semi-empirical or tight-binding calculations.
The second approach is less computationally intense; however, now a
days, with the increasing computing power it is possible to obtain
ab-initio results for big systems of atoms. In particular, the
local density functional formalisms (LDA) have proved to be accurate
in describing the total energies, geometry and the electronic
properties of materials. On the other hand, there is an agreement
that although quantum confinement is fundamental for the properties
of porous silicon (por-Si), its enourmous surface plays also an
important role. For example, surface treatments modify its optical
intensity response, color and luminescent efficiency. Nevertheless,
most theoretical treatments choose a fixed geometry on the surface
and ignore the relaxation effects. In this work, we simulate por-Si
with a periodic crystalline silicon lattice where columnar holes have
been introduced and saturated with hydrogen atoms. Our purpose is to
analyze the effect of surface relaxation on the electronic band
structure and compare it with the results obtained previously with a
fixed geometry. We consider an LDA-GGA aproximation on a 32 atom
supercell considering plane waves as basis set and a real space grid
with a cutoff of 540 Ry.
This work has been partially supported by projects: CRAY-UNAM-008697,
DGAPA-IN101797 and IN103797, CONACyT-25455-E and
4229-E.
F5.24
ROLE OF THE
OXIDATION PROCESS ON POROUS SILICON PHOTOLUMINESCENCE AND ITS
EXCITATION. Tatiana V. Torchinskaya , Nadezhda E.
Korsunskaya, Larisa Yu. Khomenkova,Berdishukur R. Dzhumaev, Institute
of Semiconductor Physics, National Academy of Sciences, Kiev,
UKRAINE; Yehuda Goldstein, Esther Savir, Avraam Many, Racah
Institute of Physics, Hebrew University, Jerusalem, ISRAEL; S.M.
Prokes, Naval Research Laboratory, Washington, DC.
In spite of numerous investigations of the influence of the
oxidation process on the photoluminescence (PL) of porous silicon
(PSi), it is still unclear. One viewpoint is that the basic role of
oxygen is the passivation of dangling bonds which are non-radiative
recombination centers. However, there is significant evidence in the
literature which attributes the PL to defects in the silicon oxide.
Taking into account that PS preparation process is accompanied by
the reaction of silicon with oxygen, we investigated the effect of
preparation conditions on the oxide composition, measured by X-ray
photoelectron emission spectroscopy (XPS), and PL spectra. In
parallel, the number of dangling bonds was measured using EPR. The
native oxidation process during aging and artificial oxidation in dry
oxygen and in NaCl electrolyte were studied as well. We have carried
out a complex investigation of PSi PL and PL excitation (PLE) spectra
as well as electroluminescence (EL) spectra. Two PLE spectral bands:
visible (400-520 nm) and ultraviolet (<400 nm) (UV), were
observed. During aging and oxidation in dry oxygen the PL band
intensity and peak position at UV excitation change in the same way.
The analysis of the PL intensity and EPR show that PL intensity
changes could not be solely explained in terms of the dangling bond
densities. The correlation of the amount of the Si suboxide, measured
by XPS, with the intensity of the UV PL excitation leads us to
conclude that light absorption occurs not only into the Si wires,
but also into Si suboxides. Some correlation between PL peak
position and oxide composition has also been observed. This fact and
results of PL and EL spectra investigation before and after
additional oxidation in the electrolyte may be explained by the
variation of oxide composition.
F5.25
CHARACTERIZATION
OF nc-Si/a-SiO2 SUPERLATTICES SUBJECTED TO THERMAL TREATMENT.
G. Grom , L. Tsybeskov, K.D. Hirschman and P.M.
Fauchet, Department of Electrical Engineering, University of
Rochester, Rochester, NY; T.N. Blanton, Eastman Kodak Company,
Rochester, NY; J.P. McCaffrey, J.-M. Baribeau, G.I. Sproule, H.J.
Labbe and D.J. Lockwood, Institute for Microstructural Sciences,
National Research Council, Ottawa, CANADA.
The morphology of
nc-Si/a-SiO2 superlattices (SLs) is studied using X-ray
diffraction, small angle X-ray reflection, polarization Raman
spectroscopy, transmission electron microscopy and atomic force
microscopy. It is demonstrated that high temperature annealing (up
to 1100C) and oxidation in O2/H2O ambient do not
destroy the SL structure, which retains its periodicity and
abruptness of interfaces. It is found that oxidation at high
temperatures reduces the defect density in nc-Si/a-SiO2 SL and
induces the lateral coalescence of Si nanocrystallites (NCs). The
size distribution, shapes, packing density and crystallographic
orientations of the Si NCs are studied as functions of oxidation
time. A thermodynamic model is proposed which explains the growth
dynamics of Si NCs during post-recrystallization thermal treatment.
Photoinduced intersubband absorption is studied to verify the quantum
confinement regime in Si NCs and feasibility of using the structure
as an infrared photodetector.
F5.26
PREPARATION
AND OPTICAL PROPERTIES OF FREE-STANDING POROUS SILICON FILMS WITH
HIGH POROSITIES. Dong-Sheng Xu, Guo-Lin Guo , Lin-Lin
Gui, You-Qi Tang, Peking Univ, Institute of Physical Chemistry,
Beijing, CHINA; Bei-Ri Zhang, Guo- Gang Qin, Peking Univ, Dept of
Physics, Beijing, CHINA.
We have prepared
non-collapsed free-standing porous silicon (PS) films with various
porosities even higher than 90% using the electrochemical etching
electropolishing, chemical dissolving and supercritical drying
method. The results of studies on optical absorption,
photoluminescence (PL) and Raman scattering from free-standing PS
films with porosities in a range of 41% to 94% are presented. A
blue shift of the transmission curves and an increase of the PL
intensity with enhancing porosity have been observed. For the PS
samples with porosities above 80%, (ahv)1/2 can no longer be fitted
by a linear function, but even though for the PS film with porosity
of 94% the absorption date do not show that the nanometer Si
particles in the PS have direct energy gap. We have observed that in
the photon energy region of the PL spectra the optical absorption
spectra are always weak. And the overlap in photon energy between the
absorption spectrum and the PL spectrum decreases with increasing
porosity. Meanwhile, A experimental result that the average diameter
of Si crystallites in the PS film decreases with increasing porosity
has been obtained by Raman scattering measurement. The fact that no
notable dependence of the PL peak energy of the PS film upon
decreasing the sizes of Si crystallites or upon increasing porosity
is contrary to the prediction of the quantum confinement model for
the PL of PS.
F5.27
GROWTH OF Si
NANO-WHISKERS ON A H-TERMINATED Si{111} SURFACE. N.
Ozaki , Y. Ohno, S. Takeda, M. Hirata, Dept of Phys, Graduate School
of Sci, Osaka Univ, Toyonaka, Osaka, JAPAN.
We have
grown Si nano-whiskers on a Si surface. Unlike the whiskers
previously grown via vapor-liquid-solid (VLS) mechanism, the
minimum diameter of the crystalline whiskers in our study, i. e.
2.8nm reaches the critical value at which the effect of quantum
confinement probably comes out. The nano-whiskers are much longer
than a micrometer. We have found that many whiskers grow
epitaxially or non-epitaxially on the substrate along the novel
direction of <112> as well as the <111> direction.
In our growth procedure, we first deposited gold on a H-terminated
Si surface and prepared the molten catalysts of Au and Si
at 773K. Under the flow of high pressure of silane gas, we have
succeeded in producing the nano-whiskers without any structural
defects. We present the details of growth condition and discuss the
growth mechanism of the nano-whiskers extending along the <112>
direction.
F5.28
ROOM
TEMPERATURE VISIBLE ELECTROLUMINESCENCE FROM SILICON NANO-CRYSTALS.
Nenad Lalic and Jan Linnros, Royal Inst of Technology,
Dept of Electronics, Stockholm, SWEDEN.
Light
emitting diodes (LED), based on silicon nano-crystals formed by
implantation of Si into SiO2 and high-temperatue annealing, have
been manufactured and evaluated. Implantation doses were between 3
1016 cm-2 and 3 1017 cm-2,
while the oxide thicknesses were between 120 and 500 .
Thin oxide layers, in combination with a highly doped polycrystalline
Si layer, were necessary to enhance carrier injection into the
nano-crystals. Visible electroluminescence (EL) has been observed
already at 8 V at room temperature. Continous operation in air
ambient was possible for the diodes with oxide thicknessses of 120
and 180 . No fatigue of the EL has been observed. The
dependence of EL on implantation dose and oxide thickness was
analyzed and the quantum efficiency of the diodes will be presented.
Spectral and time-resolved EL properties were compared to those of
the photoluminescence (PL). Similar stretched-exponential decay
shapes were found in both cases. Similar decay time constants of
several microseconds for both EL and PL support the hypothesis of
nano-crystal luminescence and rule out the avalanche emission as a
light origin.
F5.29
ABSENCE OF
CARRIER HOPPING IN POROUS SILICON. I. Mihalcescu , J.C.
Vial and R. Romestain Laboratoire de SpectromÈtrie Physique,
UniversitÈ Joseph Fourier-Grenoble I, Saint Martin d'HËres, FRANCE.
It is often presumed that the stretched exponential
decay of photoluminescence (PL) in porous silicon is a consequence of
a variable range hopping of photoexcited carriers between the
localized states of the three dimensional silicon sponge structure.
In this paper, however, we show unambiguously that carrier hopping in
porous silicon is absent in the microsecond time range, from ambient
temperature up to 450K. We demonstrate this by comparing resonantly
and non resonantly excited PL decays. The invariance of the decay
shape is interpreted in the light of different carrier recombination
models.
F5.30
PHASE
SEGREGATION IN SIPOS: FORMATION OF Si NANOCRYSTALS. Anna
Vilà, Joan R. Morante, Barcelona Univ, Dept of Electronics,
Barcelona, SPAIN; Brigitte Caussat, Polytechnic National Institute,
ENSIGC, Toulouse, FRANCE; Patrick Barathieu, Emmanuel Scheid,
Laboratory of System Analysis and Architecture, Toulouse, FRANCE.
Semi-insulating Polycrystalline Silicon (SIPOS) is a
substoichiometric silicon oxide (SiOx). Recently, different
applications such as electrical passivation, CMOS integrated
circuits, heterojunction transistors and solar cell applications have
been reported. Its properties vary between those of polycrystalline
silicon (x=0) and stoichiometric silicon oxide (x=2) depending on the
used technological parameters. Thus, the appropriate choice of growth
conditions is crucial to accurate control its structural, electrical
and optical properties in order to assure reproducibility and hence
device performance. However, as these layers are out of equilibrium,
thermal treatments required for the applications can induce phase
segregation which modify the layer properties.
In this work, a characterization of SIPOS layers deposited on
SiO2 is presented, as a function of annealing conditions (time
and temperature), in order to better understand the processes
involved in segregation of silicon nanocrystals. Correlation between
optical and XPS measurements allows to know the average starting
composition of this amorphous material. After annealing, the phase
segregation takes place and the SiO2 phase concentration is
increased, whereas the ratio between suboxides changes and the
remaining silicon precipitates forming nanocrystals near the
SIPOS-SIO2 interface. Dimensions and distance between clusters
depends on annealing conditions. It is found to have influence on the
optical properties of the resulting material. Likewise, modifications
of the electrical characteristics of these layers are also found due
to the nanocrystal appearance.
F5.31
FORMATION AND
LUMINESCENT PROPERTIES OF POROUS SILICON DOPED WITH ERBIUM BY
ELECTROCHEMICAL PROCEDURE. Leonid Dolgyi, Svetlana Volchek, Nikolai
Kazuchits, Valentina Yakovtseva, Vitaly Bondarenko, Vladimir
Petrovich, Belarussian State University of Informatics and
Radioelectronics, Dept of Microelectronics, Minsk, BELARUS; Galina
Grom, Herman Lopez, Leonid Tsybeskov, Philippe Fauchet,
University of Rochester, Dept of Electrical Engineering, Rochester,
NY.
The present work is concerned with Er-doped
porous silicon (PS). The characteristic feature of the work is that
PS doping has been realized by an electrochemical procedure followed
by a high temperature annealing. 5-m thick PS layers of 40-60%
porosity were formed on p-type Si of 0.3 Ohm×cm resistivity. Er
incorporation was performed via the cathode polarization of PS in the
electrolytic bath containing Er(NO3)3. High temperature
treatment in the oxidizing medium at 500-1150ƒC was utilized to
provide either partial or total oxidation of PS:Er layers. IR
spectroscopy, SIMS and X-ray microanalysis were used to study a
chemical composition of the samples. The photoluminescence (PL) and
photoluminescence excitation (PLE) spectra were investigated. After
the partial oxidation (the temperature range of 500-900ƒC), sharp
Er-related photoluminescence at 1.54 m was observed. A high
temperature treatment at the temperature above 1000ƒC caused a
significant increase in the Er-related radiant intensity. The
temperature quenching of the 1.54 m emission did not exceed 20%
in the temperature range from 77 to 300 K. Resonant features were
observed in PLE spectra of totally oxidized PS. Five peaks at 440,
520, 650, 800, and 980 nm were revealed. The strongest excitation
occurred at 440-450 nm. The mechanism of luminescence is discussed.
The excitation of different Er3+ energy levels, cross-relaxation
interactions and emission due to the 4I13/2-
4I15/2 transition are considered. Application of the
Er-doped PS for LEDs and integrated optical waveguides is
presented.
F5.32
MEAN-FIELD
DIELECTRIC FUNCTION MODELING OF POROUS SILICON CARBIDE USING A
STATISTICAL APPROACH. Jonathan E. Spanier and Irving P.
Herman, Columbia Radiation Laboratory and the Department of Applied
Physics, Columbia University, New York, NY.
The IR
reflectance spectrum of porous silicon carbide (PSC) has a
reststrahlen band that is different than that of bulk SiC. Effective
medium theories, such as the Maxwell-Garnett (MG) model are only
moderately successful in representing the topology and describing the
dielectric function of PSC. We have previously shown that the
Looyenga-Landau-Lifshitz mean-field model produces features in the IR
spectrum not predicted by the MG theory. We now report on the
application of other effective medium theories, including the Bergman
statistical representation. These refined models lead to improved
agreement between modeled and measured reflectance spectra. The
origin of features in the reflection spectra will be discussed within
the framework of both topological and nontopological effects. The
possibility of predicting other physical properties of PSC and
similar materials using this approach is also discussed.
Work supported by the Joint Services Electronics Program No.
DAA-G55-97-1-0166 and Kulite Semiconductor Products, Inc., Leonia,
NJ.
F5.33
ELECTRO-POLYMERIZATION
IN POROUS SILICON FILM. R.K. Soni , L.F. Fonseca, O.
Resto, A. Guadalupe, and S.Z. Weisz, Physics Department, University
of Puerto Rico, San Juan, PUERTO RICO.
Porous
silicon films were created by electrochemical anodization of n-type
epitaxial layer, resistivity 4.6 Ohm-cm grown on a p-type substrate
under 100 Watts/cm2 illumination. The films show strong orange-red
photoluminescence and wide vertical holes of 2-3 micron size when seen
by SEM. Semi-transparent conducting polypyrrol films were deposited
by electrochemical polymerization with current density of 2 mA/cm2 for
different electro-polymerization times. Micrograph pictures taken
after polymerization show the evolution of the polymer synthesis
process. The polymer films impregnate into the wide holes and grow
sideways suggesting strong current distribution on the walls. The
impregnation of the polymer film due to sideways growth provides a
useful mean to fabricate stable contact for electroluminescence
operation from porous silicon.
F5.34
ORIGIN OF THE
NEAR VISIBLE PHOTOLUMINESCENCE BAND FROM SILICON SUBOXIDE THIN
FILMS. F. Wang , D.M. Wolfe and G. Lucovsky, Departments
of Physics, Materials Science and Engineering, Electrical and
Computer Engineering, North Carolina State University, Raleigh, NC.
Silicon subxide (SiOx, x<2) thin films might be
promising material for optoelectronics application because they
exhibit stable photoluminescence ranging from infrared to blue.
However, no concensus has been reached concerning the origin of its
near visible luminescence band. In this work a series of silicon
suboxide thin films with different oxygen content were prepared by
remote plasma enhanced chemical deposition, followed by 90 second
rapid thermal annealing (RTA) at temperatures from 500 to
1100C in sequence at an interval of 50C. The samples
were characterized by photoluminescence, Raman scattering, FTIR
spectroscopy, optical transmission and high resolution transmission
electron microscopy. The main results are (a) the PL spectra shifts
from infrared towards visible with increasing oxygen content, (b) as
RTA temperature increases, the PL intensity increases substantially
in high oxygen content films, but in low oxygen content films the PL
was completely quenched after 700C RTA, (c) from Raman, FTIR,
TEM and XPS data it is found that amorphous silicon clusters are
embedded in suboxide matrix in as-deposited films. After RTA the size
of amorphous silicon clusters grows due to silicon precipitation from
suboxide phase and are crystallized. However, crystallization
temperature increases with increasing oxygen content, e.g., oxygen
free films are crystallized after 750C RTA, but no
crystallization occurs in high oxygen content films after
1000C RTA. Based on these results it is concluded that the
near visible PL band is originated from amorphous silicon
nanoclusters due to quantum confinement.
F5.35
Abstact
Withdrawn.
F5.36
LONG
LUMINESCENCE LIFETIME OF 1.54 m Er3+ LUMINESCENCE FROM
ERBIUM DOPED SILICON RICH SILICON OXIDE AND ITS ORIGIN. Se-Young
Seo, Jung H. Shin and Choochon Lee, Department of
Physics, Korea Advanced Institute of Science and Technology (KAIST),
Yusung-gu, SOUTH KOREA.
The photoluminescent
properties of erbium doped silicon rich silicon oxide (SRSO) is
investigated. SRSO with 10 and 1 at. excess silicon and 0.4 at.
erbium is deposited by electron cyclotron resonance plasma
enhanced chemical vapor deposition of SiH4 and O2 with
concurrent sputtering of erbium and subsequent rapid thermal anneal
at 900C to precipitate silicon nanoclusters. In both cases,
strong 1.54 m luminescence due to 4I13/2
4I15/2 Er3+ 4f transition is observed, excited via
energy transfer from carrier recombination in silicon nanoclusters to
Er 4f shells. The luminescent lifetimes are found to be 4 and 7 msec,
which is longer than that reported from Er in any semiconducting host
material, and comparable to that of Er doped SiO2 and
Al2O3. The dependence of the erbium luminescent lifetime on
pump power and on background illumination shows that de-excitation of
excited erbium atoms by auger excitation of free carriers is very
inefficient in SRSO by a factor of about 500 when compared to that of
crystalline silicon. This we identify as a significant factor in
explaining the origin of the long luminescent lifetime. Due to the
long lifetime, erbium doped SRSO can be pumped to high power without
showing saturation. The possibility of using such erbium doped SRSO
as a novel waveguide material will be discussed.
F5.37
INVESTIGATION
OF BLUE LUMINESCENCE FROM Si AND Ge NANOCRYSTALS.
Gildardo R. Delgado , Howard W. H. Lee, Peter Thielen,
Lawrence Livermore National Laboratory, Livermore, CA; Susan
Kauzlarich, Boyd R. Taylor, Chung-Sung Yang and Daniel Mayeri,
Department of Chemistry, University of California, Davis, CA.
We report on optical studies of monodispersed Si and
Ge nanocrystals derived from a distinctive synthetic route which
provides control of the size and the surface termination. We have
achieved the fabrication of nanocrystals that are surface terminated
with various organic groups via reaction with alkyl-Li or
alkyl-grignard reagents. The size of the particles are in a regime
where quantum confinement plays a significant role. Size-selective
photoluminescence (PL) and photoluminescence excitation spectroscopy
on these nanocrystals with a distribution of particle sizes showed
monotonic spectral shifts as the excitation or detection wavelength
was changed. Tunable PL ranging from 350 to 460 nm and from 364 to
600 nm for the appropriate particle sizes for Si and Ge nanocrystals
respectively, was achieved. These spectral shifts vary as d-1.4,
where d is the nanocrystal diameter. These shifts, along with
experimental absorption spectra and PL decays, agree with recent
tight binding and pseudopotential calculations for small nanocrystals
emitting in a specific UV-blue spectral region. In particular, we
present results of detailed optical investigation of the blue
emission from Si and Ge nanocrystals. In case of Si, the optical
results from nanocrystals terminated with organic groups are similar
to monodispersed nanocrystals processed from porous silicon which are
terminated with SiO2. Despite the different fabrication techniques,
the nanocrystalline systems produce nearly identical optical results,
thus providing strong evidence that quantum confinement (QC) is
responsible for this specific emission. Based on our optical results,
we present strong evidence for QC obtained from various samples of Si
and Ge nanocrystals processed and terminated differently. Our optical
analysis and comparison with theoretical models indicate that the
observed PL arises from the nanocrystalline core. These results
clearly demonstrate that efficient PL results from QC effects where
the predominant features are size and the shape of nanostructures.
Work at LLNL performed under the auspices of the USDoE under contract
number W-7405-ENG-48
F5.38
FEMTOSECOND
SPECTROSCOPY OF Si AND Ge NANOCRYSTALS. P.A. Thielen ,
H.W. H. Lee, G.R. Delgado, and J.D. Cooke, Lawrence Livermore
National Laboratory, Livermore, CA.
The observation
of efficient and visible light emission from Si and Ge nanocrystals
of the appropriate sizes has initiated an enormous amount of research
into their optical and electronic properties. In spite of this, the
nature of the luminescence has remained controversial and
unresolved. To directly address this issue, we performed femtosecond
pump-probe spectroscopy on Si and Ge nanocrystals using single and
multicolor techniques. We report on the ultrafast dynamics of the
relaxation of the initially excited state of the nanocrystalline
core. In addition, we observed the onset of trapping and trap
relaxation from these nanocrystals. Power dependent pump-probe
results show evidence for multiple carrier relaxation and
carrier-carrier interaction near the bandedge. Our results uphold
our earlier model of the luminescence in which the frequently
observed low energy luminescence (red, green) results not from direct
recombination processes, as typically believed, but rather from trap
emission, while a specific blue luminescence exhibit dynamics more
representative of direct recombination processes. We will present a
general model of the excitation and relaxation pathways for these
nanocrystals and discuss this in terms of the nature of the light
emission.
Work at LLNL was performed under the auspices of the U.S. DOE by LLNL
under contract No. W-7405-ENG-48.
F5.39
NONLINEAR
OPTICAL PROPERTIES OF SEMICONDUCTOR QUANTUM DOTS. H.W.H.
Lee , G.R. Delgado, P.A. Thielen, J.D. Cooke, Lawrence Livermore
National Laboratory, Livermore, CA; S.M. Kauzlarich and B.R. Taylor,
Dept of Chemistry, University of California at Davis, Davis, CA.
We report on the characterization of the dispersion
of the complex third order nonlinear optical properties of various
semiconductor quantum dots (Si, Ge, ZnSe, and GaN) using Z-scan and
Antiresonant Ring Interferometric Nonlinear Spectroscopy (ARINS)
techniques. The samples are composite structures consisting of
quantum dots incorporated into polymer thin films and in organic
colloidal suspensions. The dependence of the nonlinear optical
properties on particle size, surface termination, number density, and
matrix was observed and offers a convenient method to control or
tailor these properties. The mechanism for the nonlinearity will be
discussed. The temporal response of the nonliearity was also
measured with femtosecond Kerr gate and multi-wave mixing experiments
and show response times ranging from nanoseconds to femtoseconds.
The implications of these results for nonlinear optical applications
such as all-optical switching is very promising and will be
discusssed.
Work at LLNL was performed under the auspices of the U.S. DOE by LLNL
under contract No. W-7405-ENG-48.
F5.40
LIGHT-PROMOTED
HYDROSILYLATION ON POROUS SILICON: PHOTOPATTERNING AND LITHOGRAPHY OF
FINE ALKYL AND ALKENYL FEATURES. Michael Stewart ,
Jillian Buriak, Purdue University, Department of Chemistry, West
Lafayette, IN.
The immediate use of porous silicon
(por-Si) in integrated circuit fabrication has been delayed by the
fragility and reactivity of freshly etched, hydrogen passivated
material. A method is described that enables high levels of alkyl
and alkenyl passivation. An ordinary tungsten light source is used
to activate the surface towards nucleophilic attack. Terminal
alkenes and alkynes are hydrosilylated by the activated surface. The
hydrophobic surface is resistant to corrosion by boiling acidic and
alkaline solutions and preserves most of the photoluminescence
intensity. Regiospecific hydrosilylation is possible using focused
patterns of light on the por-Si surface. The photopatterned regions
are able to survive alkaline treatment while unfunctionalized regions
are dissolved, making possible the in situ manufacture of bright
por-Si LEDs on crystalline Si. Features on the order of 40 microns
have been made using an inexpensive and simple photopatterning
apparatus.
F5.41
ROLE OF
OXIDATION PROCESS ON POROUS SILICON PHOTOLUMINESCENCE AND ITS
EXCITATION. Tatiana V. Torchinskaya , Nadezhda E.
Korsunskaya, Larisa Yu.Khomenkova, Berdishukur R. Dzhumaev, Institute
of Semiconductor Physics, National Academy of Sciences, Kyiv,
UKRAINE; Avraham Many, Yehuda Goldstein, Esther Savir, Racah
Institute of Physics, The Hebrew University, Jerusalem, ISRAEL;
Sharka M. Prokes, Naval Research Laboratory, Washington, DC.
In spite of numerous investigations of the
influence of oxidation process on the photoluminescence (PL)
of porous silicon (PSi) is not clear yet. One popular viewpoint
is that the basic role of the oxygen is the passivation of
dangling bonds which are non-radiative recombination centers.
However there are a lot of papers which attribute the PL to
defects in silicon oxide. Taking into account that PS preparation
process is accompanied by the reaction of silicon with
oxygen, we investigated the effect of preparation regimes on
the oxide composition, measured by X-ray photoelectron
emission spectroscopy (XPS), and PL spectra. In parallel the
number of dangling bond was measured using EPR. The native
oxidation process during aging and artificial oxidation in dry
oxygen and in NaCl electrolyte were studied as well. We have
carried out a complex investigations of PSi PL and PL excitation
(PLE) spectra as well as electroluminescence (EL) spectra. Two
PLE spectral bands: visible (400-520 nm) and ultraviolet (<400 nm)
(UV), were observed. During aging and oxidation in dry oxygen the
PL band intensity and peak position at UV excitation change in the
same way. The analysis of PL intensity and EPR spectra
variations under above mentioned treatments show that PL intensity
changes could not be explained by dangling bond number variation
only. The correlation of the Si suboxide quantity, measured by
XPS methods, with the intensity of the UV PL excitation gives the
leads us to conclude that this light absorption occurs not only
into Si wires, but also into Si suboxides. Some correlation
between PL peak position and oxide composition have been
observed. This fact and results of PL and EL spectra
investigation before and after additional oxidation in
electrolyte may be explained by the variation of oxide
composition.
F5.42
CONFORMAL
SELF-ASSEMBLY OF Si/SiOX NANOCOMPOSITE THIN FILM.
Thomas Phely-Bobin , and Fotios Papadimitrakopoulos,
Department of Chemistry, Polymer Science Program, Nanomaterials
Optoelectronics Laboratory, Institute of Materials Science,
University of Connecticut, Storrs, CT.
In recent
years much attention has been focused on nanosized silicon.
Recently, our laboratory has reported a high energy nanomilling
process that leads to the fabrication of stable colloidal suspension
of nanosized silicon (with particle size in the range of 7 to 40 nm
and a size distribution of about 25%). More recently, we reported a
sonication based oxidation process that yields SiOx coated
nanoparticles, which spontaneous adsorb onto different substrates
such as glass, gold or mica. We presently explain the mechanism of
this spontaneous adsorption based on a careful control of the pH of
the ethanolic suspension. The reversal of the ionic sphere on
nanoparticle surface from negative to slightly positive at pH's of
4.8 and lower, allows the metastable colloid to self-assemble back as
a network of SiOx. The absorption characteristics and the
refractive index of these assemblies vary considerably with respect
to inter-particle distance controlled by the SiOx network. Upon
treatment with buffered HF, these assemblies agglomerate to form Si
networks with extremely high refractive indices.
F5.43
LUMINESCENCE
STOKES SHIFTS AND LOCALIZED STATES IN SURFACE-OXIDIZED Si
NANOCRYSTALS. Y. Kanemitsu and S. Okamoto, Nara
Institute of Science and Technology, Ikoma, JAPAN.
Efficient visible photoluminescence (PL) has been observed in
low-dimensional Si nanostructures such as nanocrystals, and quantum
wells. Si/SiO2 nanocrystals and wells show broad PL in the red
spectral region at room temperature. These broad PL spectra suggest
that Si/SiO2 quantum structures are inhomogeneous systems and
the inhomogeneously broaden PL spectrum will be due to the
fluctuation of the well size, strains and structural variations
between c-Si well and SiO2 barrier layers. In the study of
inhomogeneous materials, selective excitation spectroscopy is a
powerful method to extract intrinsic properties from inhomogeneously
broadened spectra. In this work, we discuss luminescence mechanisms
and electronic structure of surface states in Si/ SiO2
nanostructures.
For selective excitation at energies within broad PL band, TO-phonon
related structures are observed in Si nanocrystals, like bulk c-Si.
Since c-Si is an indirect gap semiconductor, the absorption and
emission of momentum-conserving phonons are needed for the light
absorption and emission processes. Moreover, there is a small energy
difference in the TO-phonon assisted luminescence between Si
nanocrystals and bulk c-Si (the luminescence Stokes shift). With a
decrease of the nanocrystal size, the luminescence Stokes shift
increases. Moreover, the phonon-related structures are sensitive to
the surface structure of Si nanocrystals. In oxidized Si
nanocrystals, the luminescence Stokes shift is large and TO-phonon
structures are unclear, compared with the case of H-terminated Si
nanocrystals. The shallow band-edge and deep localized states are
formed in surface-oxidized Si nanocrystals.
F5.44 THERMODYNAMICS
AND KINETICS OF CRYSTALLINE SILICON CLUSTERS EMBEDDED IN THE
MELT. P. Keblinski , Materials Science Division,
Argonne National Laboratory, Argonne, IL, and Forschungszentrum
Karlsruhe, Karlsruhe, GERMANY.
Molecular-dynamics
simulations and the Stillinger-Weber three-body potential are used to
study the growth and stability of silicon clusters of diameters from 2
to 5 nm embedded in the melt. Our simulations show that the melting
temperature of such nano-clusters is lower than the bulk melting
temperature by an amount proportional to the inverse of the cluster
size. However, the kinetics of the growth and melting of such small
Si clusters is essentially the same as that of homoepitaxial growth.
We show that, as it is observed in homoepitaxial silicon growth, the
mobility of the highly-curved crystal-liquid interface is controlled
by diffusion in the melt, and is characterized by the same activation
energy. The optimal growth conditions are subsequently used to
synthesize fully-dense nanocrystalline silicon microstructures with a
grain size up to 8 nm obtained by growth from the melt into which
randomly oriented nucleus were inserted. We
gratefully acknowledges support from the Alexander von Humboldt
Foundation. This work was also supported by the U.S. Department of
Energy BES-Materials Science under Contract No. W-31-109-Eng-38.
8:30 AM *F6.1SESSION F6: SYNTHESIS AND PROPERTIES OF
MICROCRYSTALLINE AND NANOCRYSTALLINE
SEMICONDUCTORS
Chairs: Robert W. Collins and Chuang-Chuang Tsai
Wednesday Morning, December 2, 1998
Salon E (M)
NANOSTRUCTURED PRECURSORS FOR SEMICONDUCTORS, BATTERY MATERIALS AND CONTACTS. David S. Ginley 1, Douglas Schulz1, Calvin Curtis1 and Fred Tepper2; 1National Renewable Energy Laboratory, Golden, CO; 2Argonide Corp., Sanford, FL.
We will survey some of the applications of nanoparticullate inks as precursors to electronic materials. Nanoparticle based precursors are amenable to incorporation into stable colloidal inks which can subsequently be applied by low cost technique and potentially with high resolution similar to ink jet printing. Because of their size, nanoparticullate inks have demonstrated unique reactivity leading to improved materials and materials with unique properties compared to conventional particulate or vacuum growth approaches. We will report on the use of simple metathesis reactions to produce CdTe and CuIn(Ga)Se2 nanoparticles and their subsequent reactivity when applied in a spray deposition process. We have also investigated the application of Al and Ag nanoparticullate inks for contacts to Si. We have demonstrate that significantly improved reactivity and consequently electronic properties can be obtained by producing particles with clean surfaces or by surface derivitization of the particles to prevent surface oxide formation. Recent work especially in the Li battery area has shown that materials demonstrating order on length scales between fully crystalline materials and amorphous materials can have enhanced properties over either of the end points. We have investigated the use of nanoparticullate VOx for Li battery cathodes.
This research was supported by the U.S. Department of Energy National PV Program and the U.S. Department of Energy, Office of Energy Research; Chemical Sciences and Materials Sciences Divisions under contract #DE-AC36-83CH10093.
9:00 AM F6.2
STUDY OF ELECTRICAL PROPERTIES OF Ge-NANOCRYSTAL- LINE FILMS PREPARED BY CLUSTER-BEAM EVAPORATION TECHNIQUE. Souri Banerjee , H.Ono, S.Nozaki and H.Morisaki, Department of Communications and Systems, The University of Electro-Communications, Tokyo, JAPAN.
Recently, a lot of interest has been focussed on the optical properties of Si and Ge nanostructures in order to understand the mechanism of visible light emission from them. Since the nanostructures have unique properties in abundance, we felt, another aspect of the problem, namely, the study of their electrical properties would need an exposure which may lead to new device application.
Here, we present electrical characterisation of Ge-nanocrystalline films prepared by cluster-beam evaporation technique. The films of thicknesses 40nm or less, have been deposited at room temperature (Ge-RT) and at liquid nitrogen temperatures (Ge-LNT). It was reported that Ge-RT did not contain oxide, while Ge-LNT was easily oxidized [1]. So in Ge-LNT, the nanostructures are covered with an oxide layer resulting into an higher petential barrier between the particles. The I-V characteristics have been measured along the vertical direction of the film. For such measurements Au is deposited on the Si substrate prior to film deposition. Then a small dot (1 to 2mm.dia) of Au is again deposited on the Ge film. The room-temperature I-V measurements of the Ge-RT films have clearly indicated two distinct operating regions. Below a critical bias (30mV), the films exhibit Coulomb staircase in the I-V plot where Ge nanoparticles are expected to behave as Coulomb islands, essential for the operation of a single electron device. But above that critical bias, there is a Zener type breakdown resulting into a very high field emission current. Furthermore, the modifications in the resistivity of the films by electron- beam irradiation have also been investigated with an aim for possible direct-beam patterning. Currently, similar investigations on Ge-LNT films are underway. Due to difference in morphology in Ge-LNT films, as mentioned above, a much prominent Coulomb blockade is expected.
[1] Sato et al. Appl.Phys.Lett. Vol.66, 3176 (1995).
9:15 AM F6.3
LOW TEMPERATURE ECR PLASMA ASSISTED MOCVD MICROCRYSTALLINE AND AMORPHOUS GAN DEPOSITION AND CHARACTERIZATION FOR ELECTRONIC DEVICES. Z. Hassan, M.E. Kordesch , Department of Physics and Astronomy, Ohio University, Athens, OH; W.M. Jadwisienzak, H.J. Lozykowski, Department of Electrical Engineering and Computer Science, Ohio University, Athens, OH; W. Halverson and P.C. Colter, Spire Corporation, Bedford, MA.
GaN films have been deposited over a range of temperatures from 325K to 925K by ECR plasma MOCVD on silicon (111) and (100), sapphire and quartz using triethylgallium and molecular nitrogen or ammonia as reagents. Growth rates of 2 microns/hr are achieved on temperature-controlled substrates (total reactor pressure 0.5 mTorr, 250 watts at 2.45 GHz).
Films deposited at 475, 875 and 925 K on sapphire show the GaN(0002) diffraction peak and sharp photoluminescence lines (at 10 K) between 370 and 400 nm and broad emission at 530-550 nm. Broad photoluminescence at 390 nm is observed from GaN/Si(111). Films deposited at 325 and 375 K show no evidence of a crystalline phase or GaN(0002) diffraction peak. The films are smooth and optically transparent. A broad photoluminescence peak at 520 nm, with a fwhm of about 150 nm is also observed (at 10K). The optical bandgap is measured to be about 3.3 eV. All of these films show a GaN LO phonon mode at 736 cm-1. IR spectra indicate some hydrocarbon impurities in the low temperature films.
Prototype devices (Schottky barrier diodes) have been made from MOCVD GaN and amorphous GaN. Recent calculations suggest that amorphous GaN may be a useful electronic material. The physical properties of the low temperature amorphous and microcrystalline GaN films will be compared to GaN films grown at higher temperatures and using a commercial MOCVD reactor. This work was supported by the Ballistic Missile Defense Organization grant N00014-96-1-0782 and SBIR Phase II Contract 33615-96-C-2622 from the Air Force Research Laboratory.
9:30 AM F6.4
NANOSTRUCTURED ARRAYS FORMED BY FINELY FOCUSED ION BEAMS. R.A. Zuhr , J.D. Budai, P.G. Datskos, C.M. Egert, A. Meldrum, K.A. Thomas, C.W. White, Oak Ridge National Laboratory, Oak Ridge, TN; L.C. Feldman, Vanderbilt University, Nashville, TN ; M. Strobel, K.-H. Heinig, Research Center Rossendorf, Dresden, GERMANY.
It has been shown that amorphous, polycrystalline, and single crystal nanoparticles can be formed in a variety of substrates by ion implantation under prescribed conditions combined with subsequent thermal annealing. Typical sizes of these particles range from a few nanometers to several hundred nanometers in diameter. They may be metals or semiconductors and can be made of either single or multiple elements, including compound semiconductors, by using sequential implantation techniques. Such composite materials frequently exhibit unique optical properties and may find applications in optical devices. One problem of composite materials formed in this way is that the particle size is generally not uniform. In addition, the particles are randomly distributed throughout the implanted region. The purpose of this work is to make uniformly spaced lattices of colloidal particles of more uniform size by using a finely focused ion beam (FEI, FIB200) to implant ions only into a microscopic region (typically 60 nm diameter) at each point of a two-dimensional array. Calculations using a kinetic Monte Carlo code indicate that such localized implants should form a localized concentration of colloids that, under proper conditions, may Ostwald ripen into a single large colloid at each lattice site. Under these ideal circumstances the colloids formed would be of nearly uniform size because of the identical particle dose implanted at each spot, and in addition they would be uniformly arranged on a two-dimensional lattice. Such a composite would exhibit greatly improved optical characteristics. Initial work is being done with a 60 nm 30 keV Ga beam implanted into Si and SiO2 substrates. Results of particle formation as a function of implant and annealing conditions by Rutherford backscattering, X-ray analysis, and both scanning and transmission electron microscopy will be presented and discussed. Oak Ridge National Laboratory is managed by Lockheed Martin Energy Research Corp. for the U.S. Department of Energy under contract number DE-AC05-96OR22464.
9:45 AM F6.5
SYNTHESIS AND CHARACTERIZATION OF LAMELLAR NANOCRYSTALS OF MANGANESE OXIDE AND THEIR SELF-ASSEMBLY INTO ORGANIZED STRUCTURES. Stephanie L. Brock , Oscar Giraldo, Manuel Marquez, Jaya Nair, University of Connecticut, Department of Chemistry, Storrs, CT; Volker Urban, Pappannan Thiyagarajan, Intense Pulsed Neutron Source, Argonne National Lab, Argonne, IL; Steven L. Suib, University of Connecticut, Department of Chemistry, Chemical Engineering, and Institute of Materials Science, Storrs, CT.
Manganese oxides containing mixed valent manganese are a class of semiconducting oxides with unique properties which have led to their investigation as battery materials and as both oxidation and photo-oxidation catalysts. These materials are typically produced by precipitation techniques which yield large particles, usually on the order of microns. Due to the high surface area, nanoparticles of manganese oxide would be expected to yield enhanced catalytic properties as well as interesting photoelectronic properties due to quantum confinement effects. However, because of the strong tendency to precipitate, there are very few routes which permit the formation of stable solutions of nanocrystalline manganese oxide.
We have developed a simple synthetic route for the production of nanocrystalline manganese oxides as aqueous/alcohol sols. The synthesis employs tetraalkylammonium cations (tetramethylammonium, tetraethylammonium, tetrapropylammonium, or tetrabutylammonium) which stabilize the nanocrystals, preventing their rapid assembly into large particles. The sols are prepared by reaction of tetraalkylammonium permanganate with a solution of alcohol (2-butanol or ethanol) and water and the average manganese oxidation state of the product sol, determined by titration methods, is 3.7-3.8. UV/visible data on the sols reveal an absorbance maxima at ca 300 nm, considerably blue shifted from the bulk value (425 nm), indicative of quantum confinement in this system. X-ray diffraction measurements of the sols dripped onto a glass slide and allowed to dry reveal layered structures with interlayer spacings of 12-19
, due to
the incorporation of tetraalkylammonium cations between the layers.
The crystallite dimension in the direction perpendicular to layers is
130-300 . In addition to UV/visible and X-ray diffraction,
T.E.M. and small angle neutron scattering data will be presented and
the assembly of the particles to form ordered arrays on flat
surfaces, will be described.
10:30 AM F6.6
OPTICAL PROPERTIES OF FULLERENE-LIKE MS2 (M=Mo,W): ABSORPTION, RAMAN AND RESONANCE RAMAN MEASUREMENTS. Gitti L. Frey and Reshef Tenne, Department of Materials and Interfaces, Weizmann Institute, Rehovot, ISRAEL; Manyalibo J. Matthews and Mildred S. Dresselhaus, Department of Physics, Massachusetts Institute of Technology, Cambridge, MA; Gene Dresselhaus, Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology, Cambridge, MA.
Recently, mg quantities of MS2 (M=Mo,W) fullerene-like nanoparticles (generically denoted inorganic fullerene-like material - IF) and nanotubes, were reproducibly obtained. Optical absorption measurements show that the semiconductivity of MoS2 and WS2 is preserved in the IF and nanotube structures. However, a decrease (red shift) in the exciton energies of the IF compared to the 2H bulk is found. In addition, a 1D quantum localization, in the direction perpendicular to the layers (
), is found for IF with less than 5 MS2 layers
resulting in a blue shift of the excitons. A systematic study of the
effect of the IF size and the number of atomic layers reveals that
the position of the excitons is dependent on the number of IF layers
(n) rather than on the particle size.
Raman and resonance Raman measurements of IF-MoS2 have been
studied at various temperatures. Since the exciton energy is
dependent on the number of IF layers, the resonant behavior is
studied by measuring different IF samples rather than using different
exciting lasers. Under resonance conditions, new peaks are observed
which are assigned to disorder-induced zone edge phonons and second
order scattering processes. The coupling of the first order
E12g and A1g modes to the A exciton is studied by dynamic
band calculations.
10:45 AM F6.7
OPTICALLY DETECTED MAGNETIC RESONANCE AND PUMP-PROBE PHOTOLUMINESCENCE MEASUREMENTS OF CdS/HgS/CdS NANOPARTICLES. E. Lifshitz , H. Porteanu and A. Glozman, Technion, Israel Institute of Technology, Department of Chemistry and Solid State Institute, ISRAEL; H. Weller and A. Eychmüller, Universität Hamburg, Institut für Physikalische Chemie, Hamburg, GERMANY.
The onion-like CdS/HgS/CdS nanoparticles consist of a CdS core, epitaxially covered by one or two monolayers of HgS and additional external monolayer of CdS. The energy band-gap difference of the semiconductor components, in this system (Eg(CdS)bulk=2.5 eV, Eg(HgS)bulk=0.4 eV), generates an electronic quantum well structure. The present abstract describes our efforts to identify the influence of the interfaces in CdS/HgS/CdS on the localization of the photogenerated carriers and the magneto-optical properties of the materials. These properties were investigated by the utilization of optically detected magnetic resonance (ODMR) and pump-probe photoluminescence (PL) spectroscopy. A PL spectrum of CdS/HgS/CdS nanoparticles, excited by a single laser beam with energy above the band-gap, consists of a dominant exciton band and additional weak band. The dominant band corresponds to an exciton located at the HgS layer, while the weak band, more-likely, corresponds to trapped carriers at interface sites. However, pumping the sample with a variable intensity 610 nm beam and probing the luminescence with a weak modulated 457.9 nm beam, caused quenching of the interface band and substantial enhancement of the exciton band. The latter may be associated with resonance saturation of the interface trapping sites.
The ODMR spectra of the studied materials consist of three major resonance signals. Examination of these spectra at different laser excitation powers and microwave modulation frequencies, have shown strong correlation between two signals, while the third has shown independent behavior. The theoretical simulation of the resonance events and their corresponding line-shape was calculated from a model of local structure using a phenomenological spin Hamiltonian that includes the Zeeman, zero-field splitting and exchange interactions. These simulations suggest that the independent signal corresponds to an exciton with exchange coupling that is greater than the Zeeman interaction. While, the remaining pair of signals in the spectra corresponds to a weakly interacting trapped electrons and holes. The latter process required the consideration of anisotropy, supporting the above assumption that some of the photogenerated carriers are trapped at the interface between the core and the epitaxial shells.
11:00 AM F6.8
PHOTOCONDUCTIVITY OF CdSe QUANTUM DOT SOLIDS. C.A. Leatherdale , C.R. Kagan, W.K. Woo, M.G. Bawendi, Massachusetts Institute of Technology, Dept. of Chemistry, Cambridge, MA.
Three dimensional arrays of semiconductor nanocrystallites have potential as a novel electronic material provided their tunable electronic states can be made the basis set for the new solid. We study photoconductivity in close-packed arrays of CdSe quantum dots. A variety of surface ligands and core-shell structures are used to tune the inter-dot spacing and degree of surface passivation. We find that the absolute charge generation efficiency is surprisingly insensitive to the size of the quantum dot and the size and structure of the capping ligand. This result suggests that in poorly passivated quantum dots, charge extraction from localized surface traps rather than quantum dot core electronic states dominates the photoconductive properties.
11:15 AM F6.9
OPTICAL AND ELECTRICAL PROPERTIES OF CdTe NANOCYSTAL QUANTUM DOTS PASSIVATED IN AMORPHOUS TiO2 THIN FILM MATRIX. A.C. Rastogi, Shailesh Sharma and S. Kohli.
A number of elemental, Si and Ge and compound CdTe, CdS and CdSe semiconductor nanocrystallites embedded in amorphous SiO2 or Al2O3 thin films or in bulk glasses have been investigated in the past due to strong nonlinear optical properties displayed by the structure. The inert matrix having high resistivity provides barrier for spatially confining the carriers in the nanocrystals thus showing quantum effects in certain size domain for the crystallites, usually less than 20 nm. We have prepared CdTe nanocrystal quantum dots sequestered in TiO2 thin film matrix by R.F. sputtering technique from a composite CdTe/TiO2 target. Structural studies by electron microscopy showed the CdTe nanocrystallite formation is nucleation controlled. The size , distribution and orientation of the crystallite is strongly affected by the concurrent growth behavior of TiO2 and modified by the sputtering parameters. The optical gap derived from the spectroscopic dependence of the absorption coefficient showed blue shift concurrent with the CdTe nanocrystal size reduction due to quantum size effects. The shifts neither follow the strong nor weak confinement regimes which have been explained on the basis of anisotropic growth of CdTe nancrystals. Additional studies of substrate temperature and variation in dispersion of crystallites showed that the later affected the band gap beside the reduction in transmittivity. It was observed that TiO2, in addition to serving as an ideal passivator and provider of barrier for carrier confinement for observing the quantum effects, also shows O2 vacancy dependent conductivity modulation. This offers a unique mean to investigate the electrical coupling and tunneling behavior of carriers between the CdTe nanocrystallites. The changes in the electrical conduction and field dependent current behaviour of the CdTe quantum dots: TiO2 composite film reduced by H2 annealing have been investigated and reported in this work.
11:30 AM F6.10
DIELECTRIC DISPERSION STUDIES OF SEMICONDUCTOR NANOCRYSTAL COLLOIDS: EVIDENCE FOR GROUND STATE DIPOLE MOMENT. Moonsub Shim , Margaret A. Hines, Philippe Guyot-Sionnest, University of Chicago, James Franck Institute, Chicago, IL.
We have recently discovered that CdSe nanocrystal colloids have a large dipole moment. This has an important implication on the description of the electronic structure, selection rules, recombination, and Stark effect.[1] Due to the intrinsic polar nature of wurtzite structure, a permanent dipole moment of CdSe nanocrystals may be expected. However, dielectric dispersion studies show that the dipole moment of cubic zinc blende ZnSe nanocrystals is similar to that of CdSe nanocrystals. This dipole moment may be a generic attribute of nanoparticles with polar bonds and irregular shapes and may significantly affect the understanding of these materials. 1. S.A.Blanton et al, Phys. Rev. Lett. 79, 5, 865.
11:45 AM F6.11
OPTICAL PROPERTIES OF SELF-ORGANIZED InGaAs/GaAs QUANTUM DOTS IN FIELD-EFFECT STRUCTURES. A.Babinski , A.Wysmolek, T.Tomaszewicz, J.M.Baranowski, Institute of Experimental Physics, Warsaw University,Warszawa, POLAND; C. Lobo, C.Jagadish, Electronic Materials Engineering Dept, RSPhysSE, ANU, Canberra, AUSTRALIA; R. Leon, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA.
One of the most successful approaches to the quantum dots (QDs) investigation is the use of field-effect structures. The QDís electron occupancy can be controlled in such structures by applied bias and the electrical and optical properties of QDs charged with a few electrons can be investigated. In the present communication we report on the photoluminescence (PL) and electroreflectance (ER) measurements on the self-organized InGaAs/GaAs QDs in the field-effect structure. Structures investigated have been grown by a low pressure Metalorganic Vapor Phase Epitaxy (MOVPE). A feature below the energy gap of GaAs has been found in the ER spectrum at room temperature, which has been attributed to the optical transition within the QDs. A systematic redshift of this feature with electric field within the structure has been found. It has been found that the PL signal from the QDs in the field-effect structure strongly depends on the bias voltage. The PL signal from the QDs can be completely quenched in the reversely biased structure both at T=300K and at T=4.2K. It is proposed that this dependence results from the QD's electron occupancy changes driven by the bias. In reversely biased structure, due to a long thermalization time the photogenerated electrons are swept out of the QDs by the electric field before the radiative recombination. Steady electron occupancy of the QDs in positively biased structure enables the radiative recombination in the QDs. The electrically modulated PL (e-m PL), using the bias dependence of PL signal is proposed as a tool for QDs investigation. The e-m PL spectral characteristics obtained using the e-m PL is analysed and disscussed.
1:30 PM *F7.1SESSION F7: OXIDE AND CHALCOGENIDE SEMICONDUCTORS
Chairs: Vicki L. Colvin and Peter C. Searson
Wednesday Afternoon, December 2, 1998
Salon E (M)
EFFICIENT PHOTOVOLTAIC CONVERSION BY MESOPOROUS JUNCTIONS. Michael Grätzel , Institute of Photonics and Interfaces Swiss Federal Institute of Technology, Lausanne, SWITZERLAND.
Nanocrystalline semiconductor films are constituted by a network of mesoscopic oxide or chalcogenide particles, such as TiO2, ZnO. Nb2O5, WO3, Ta2O5 or CdS and CdSe, which are interconnected to allow for electronic conduction to take place. The pores between the particles are filled with a semiconducting or a conducting medium, such as a p-type semiconductor, a hole transmitter or an electrolyte, forming a junction of extremely large contact area. In this fashion, the negatively and positively charged contact of the electric cell become interdigitated on a length scale as minute as a few nanometers, leading to a whole series of intriguing properties for which a realm of important applications can already be foreseen. The unique optical and electronic features of such junctions may be exploited to realize opto-electronic devices exhibiting outstanding performance including super-capacitors, sensors, photovoltaic injection cells, luminescent diodes and electrochromic displays, as well as photochromic switches and intercalation batteries. The lecture focusses on recent advacement in the area of the dye sensitized nanocrystalline solar cell.
2:00 PM F7.2
THE SYNTHESIS AND PROPERTIES OF NANOCRYSTALLINE TiO2 PREPARED FROM NON-HYDROLYTIC REACTIONS. Tim Trentler, Arti Agrawal, Jane Bertone, Joel Cizeron and Vicki Colvin , Dept. of Chemistry, Rice University, Houston TX.
Nanocrystalline TiO2 if of great interest not only for its applications to solar cells but also for its use as a precursor in ceramic processing of titania materials. Nearly all chemical approaches to the synthesis of this material have relied on hydrolysis which produces materials with hydrated surfaces and some internal hydroxyls. The presence of hydration in nanocrystalline titania may enhance sintering at higher temperatures or alter the electrical properties of nanocrystalline films. In order to evaluate the role of water in these materials, we have developed reactions for the production of anatase TiO2 which proceed by entirely non-hydrolytic routes in dry organic solvents. High resolution transmission electron microscopy and powder x-ray diffraction show evidence of well formed nanocrystals; depending on reaction conditions domain sizes ranging from 5-15 nanometers can be synthesized. Electron microscopy and x-ray diffraction studies of these dehydrated anatase nanocrystals indicate that sintering occurs at a much higher temperature than nanocrystalline anatase made hydrothermally. The anatase to rutile phase transition also occurs at a higher temperature in these systems and shows a dependence on grain size. These results emphasize the importance of sintering and grain growth in the processing of ceramic materials, and suggest strong size dependent effects are important in determining the stability of nanocrystalline oxides.
2:15 PM F7.3
DYNAMICS OF LITHIUM INTERCALATION IN ANATASE TITANIUM DIOXIDE. Roel van de Krol , Albert Goossens, Joop Schoonman, Delft Univ of Technology, Dept of Inorganic Chemistry, Delft, THE NETHERLANDS; Eric Meulenkamp, Philips Research Laboratories, Eindhoven, THE NETHERLANDS.
Nanocrystalline TiO2 is a potential candidate as an electrode material in rechargeable lithium-ion batteries. To elucidate the mechanism of lithium insertion, thin smooth films of anatase TiO2 are used as a model system. The well-defined surface area and known potential distribution in this type of film allow accurate quantitative interpretation of the experiments. The anatase TiO2 electrodes are prepared by electron-beam evaporation; the lithium ions are supplied by a liquid electrolyte. Galvanostatic discharge curves show a constant potential plateau, indicating a two-phase equilibrium. Previous reports indeed showed a reversible phase transition from anatase TiO2 to anatase Li0.5TiO2 upon intercalation. However, from the amount of charge needed for intercalation, it is found that in smooth films only a small fraction of the TiO2 is converted to Li0.5TiO2. In contrast, in-situ XRD of lithium intercalation in nanocrystalline TiO2 electrodes indicate that 100
of the TiO2 is (reversibly) converted to
Li0.5TiO2.
Potential dependent capacitance measurements show that after the
extraction of lithium ions a small region at the surface of the
electrode has a much higher donor density (1019 cm-3) than
the bulk of the electrode (1017 cm-3). This indicates the
presence of irreversibly trapped lithium ions in the region where
intercalation has taken place. The extent of this region depends on
the intercalation potential, values of 7 nm and 17 nm are found at
-1.0 and -1.2 V vs. SCE, respectively. The amount of charge needed
for intercalation accurately agrees with the charge needed to convert
this region from TiO2 to Li0.5TiO2.
Accordingly, capacitance techniques provide a convenient in-situ tool
to examine the lithium intercalation process. The dynamics of the
TiO2 Li0.5TiO2 phase front as it runs into the
TiO2 can now be investigated.
2:30 PM F7.4
ELECTRICAL CONDUCTIVITY OF NANOCRYSTALLINE TITANIUM DIOXIDE CERAMICS. P. Knauth , CNRS-University of Aix-Marseille, Marseille, FRANCE; H. L. Tuller, Massachusetts Institute of Technology, Center for Materials Science and Engineering, Cambridge, MA.
Nanocrystalline titanium dioxide has attracted considerable attention in recent years. Photoelectrochemical cells with a respectable photovoltaic conversion efficiency are obtained using nanosized titanium dioxide particles and TiO2 is widely used for photocatalysis, e.g. for water treatment by oxidation of dissolved organics. Engineering of nanocomposites, such as Pt/TiO2, may offer attractive improvements on photocatalytic processes by increasing surface reactivity while suppressing charge carrier recombination. Furthermore, titanium dioxide is used for gas sensors and the sensitivity and response time of sensors may be significantly improved by use of fine-grained materials. In this study, we report the first systematic examination of the electrical and defect properties of nanocrystalline titanium dioxide, which are of fundamental relevance for the above applications. The electrical properties of dense nanocrystalline titanium dioxide ceramics with an average grain size of 35 nm were investigated by impedance spectroscopy and compared with a coarsened material with micrometer size grains. The nanocrystalline ceramics show an uncommon domain of ionic conductivity at high oxygen pressures and a steep increase of electronic conductivity at low oxygen pressures. The ionic conductivity, unusual for titanium dioxide, may be related to the high density of grain boundaries, which are fast diffusion paths at reduced temperatures, and to the space charge regions adjacent to grain boundaries. The steep oxygen pressure dependence of conductivity at low oxygen pressure and the low enthalpy of reduction of nanocrystalline titanium dioxide may be related to reduced defect formation energies at interface sites. The increased sensitivity at low oxygen pressures is certainly of interest for oxygen sensor applications.
3:15 PM F7.5
CHARGE SEPARATION VERSUS CARRIER TRAPPING IN SEMICONDUCTOR NANOCRYSTALS. Travis Green , Clemens Burda, Stephan Link, Tina Masciangioli, Victor Volkov, Janet Petroski, and Mostafa El-Sayed, Georgia Institute of Technology, Department of Chemistry and Biochemistry, Atlanta, GA.
We have measured the dynamics of semiconductor nanocrystals following the generation of an electron-hole pair from ultra-fast laser pulses. Femtosecond transient absorption, picosecond photon counting, and nanosecond flash photolysis spectroscopies were used to characterize the relaxation of charge carriers through trapped states. Electron-hole donors and acceptors were attached to the surface of the nanocrystals in order to isolate the charge carriers and determine their trapping processes. In addition, the electron-hole transfer rates between the semiconductor nanocrystals and attached receptors / donors were measured. These studies provide insight into the potential for charge separation versus carrier trapping for semiconductor nanocrystals.
3:30 PM F7.6
NOVEL ELECTRONIC CONDUCTANCE CO2 SENSORS BASED ON NANOCRYSTALLINE SEMICONDUCTORS. Marie-Isabelle Baraton, LMCTS, ESA 6015 CNRS, Faculty of Sciences, Limoges, FRANCE; Lhadi Merhari , Ceramec, Limoges, FRANCE; Patrick Keller, Karina Sweiacker, Jörg-Uwe Meyer, Fraunhofer Institute for Biomedical Engineering, Sensorsystems/Microsystems Dept., Sankt Ingbert, GERMANY.
Low-cost portable gas sensors capable of selectively detecting CO2 in the 100 ppm-10% range in air are of great interest in areas as diverse as agriculture, air conditioning or exhaust gas monitoring. We have recently demonstrated that screen-printed sensors using a mixture of sub-20 nm instead of micron-sized BaTiO3/CuO/additives powder exhibit a much higher sensitivity (up to one order of magnitude) to CO2. In this paper, we focus on the surface chemistry of nano-BaTiO3/CuO during O2 and CO2 adsorptions. We show by Fourier transform infrared (FTIR) spectrometry, thus without using any electrodes, that the nano BaTiO3/CuO/additives mixture behaves like a p-type semiconductor at the operating temperature. The variations of the electrical conductivity of the nanomaterial during CO2 exposures in air are then followed in situ by FTIR spectrometry and prove to be dependent on the surrounding oxygen pressure. These infrared measurements are then correlated to the electrical measurements performed on the nano- BaTiO3/CuO based sensor. Preliminary electrical response modelling shows a good agreement with the surface barrier layer theory.
This research is supported by the European Commission in the framework of the Brite-Euramill program (contract BRPR-CT95-0002).
3:45 PM F7.7
THE GROWTH KINETICS OF NANOCRYSTALLINE ZINC OXIDE PARTICLES FROM COLLOIDAL SUSPENSIONS. Eva M. Wong, John E. Bonevich*, and Peter C. Searson , The Johns Hopkins Univ, Dept of Materials Science and Engineering, Baltimore, MD; *National Institute of Standards and Technology, Metallurgy Division, Gaithersburg, MD.
Colloidal chemistry techniques were used to synthesize ZnO particles in the nanometer size regime. The particle aging kinetics were determined by monitoring the optical band edge absorption and using the effective mass model to approximate the particle size as a function of time. We show that the growth kinetics of the ZnO particles follow the Lifshitz, Slyozov, Wagner theory for Ostwald ripening. In this model, the higher curvature and hence chemical potential of smaller particles provides a driving force for dissolution. The larger particles continue to grow by diffusion limited transport of species dissolved in solution.
4:00 PM F7.8
GIANT SPLITTING OF EXCITON SPIN SUBLEVELS IN MN-DOPED CDS QUANTUM DOTS. D. M. Hofmann , B.K. Meyer, I. Physics Institute, Justus-Liebig-University Giessen, Giessen, GERMANY; G. Counio, T. Gacoin, J. P. Boilot, Laboratoire de Physique de la Matiere Condensee, CNRS UMR 7643, Ecole Polytechnique, Palaiseau, FRANCE; A.I. Ekimov, A.F. Ioffe Physical-Technical Institute, St. Petersburg, RUSSIA; Al. Efros, M. Rosen, Nanostructure Optics Section, Naval Research Laboratory, Washington DC.
Mn doped CdS quantum dots dispersed in sol-gel silica matrices have been studied by optical and magnetic resonance experiments. The Mn concentration is controlled by the precipitation process and in these samples varied from about one to a few ions per quantum dot. Photoluminescence and Photoluminescence-excitation spectra show that excitation of the quantum size levels of the host CdS nanocrystals results in strong 2.1 eV luminescence from the known Mn2+ internal transition. Electron paramagnetic resonance (9 GHz) shows the Mn2+ spectra with a hyperfine coupling constant of Aiso = 6.9 mT. It is typical for Mn2+ located on tetrahedral sites in the CdS lattice. Both observations give strong evidence that Mn is present inside the dots.The Mn2+ has a drastic effect on the optical and magnetooptical properties of the CdS nanocrystal. In an external magnetic field of 1.5 T, a difference of as much as 20 meV in the absorption edges for right and left circularly polarized light is seen in doped nanocrystals. Optically detected magnetic resonance shows that this effect is caused by the alignment of spins of the paramagnetic Mn2+ ions (electron spin S = 5/2) in the external magnetic field. The magnitude of this effect is described in terms of a giant splitting of the exciton spin sublevels in the effective magnetic field created by the Mn ions.
4:15 PM F7.9
A NOVEL ROUTE TO PREPARE Mn-DOPED ZnS NANOCRYSTALS. M. Azad Malik, Paul O'Brien, Neerish Revaprasadu , South Kensington, Imperial College, London, UK.
ZnS is used as a phosphor and in electroluminescent devices. The electroluminescence properties of this material can be considerably enhanced by doping with Mn2+. TOPO (tri-
-octylphosphineoxide) capped ZnS:Mn nanoparticles
were prepared by injecting a hot solution of (diethyldithiocabamato)zinc(II) and manganese dichloride
(1:0.05) in TOP (tri--octylphosphine) into preheated TOPO at
250C. The nucleation of the Q-dots is driven by the
decomposition of the precursor. The particles have been characterized
by EDAX, UV/Vis, TEM, ICP and PL methods. A broad photoluminescence
emission at 540 nm is typical of Mn-doped ZnS. To our knowledge this
is the first one-pot synthesis of Mn-doped ZnS by using a stable
single molecule precursor. 4:30 PM F7.10
METAL-CHALCOGENIDE SELF-ASSEMBLY: ANGSTROM SCALE ASSEMBLY. Geoffrey F. Strouse , Jean-Jacques Gaumet, University of California Santa Barbara, Dept of Chemistry, Santa Barbara, CA.
The assembly of sub-nanometer molecular components into flexible, 3-dimensional arrays opens new routes into the design and preparation of nano-scale electronic materials. Electronic coupling between components of an array can be applied to novel electronic architectures. We have successfully prepared a series of mono-disperse, nano-scale metallo-sulfur, -seleno semiconductors that are readily modified at both the metal-chalcogenide core and surface capping. These materials have discrete edge and face providing finite control over both molecular size and surface modification. We will discuss the unique advantage of using metal-cluster materials for nano-assembly and the application of bio-inspired self-assembly for the preparation of a 3-dimmensional structure with evidence for electronic coupling between components. Correlation of the optical properties, NMR, and mass spectrometry will be discussed.
4:45 PM F7.11 IN SITU DIAGNOSTICS OF NANOMATERIAL SYNTHESIS BY LASER ABLATION: TIME-RESOLVED PHOTOLUMINESCENCE SPECTRA AND IMAGING OF GAS-SUSPENDED NANOPARTICLES DEPOSITED FOR THIN FILMS. D.B. Geohegan , A.A. Puretzky, S.J. Pennycook, and A. Meldrum, Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN; G. Duscher, MPI für Metallforschung, Institut für Werkstoffwissenschaft, Stuttgart, GERMANY.
The gas phase synthesis of nanocrystals and nanowires by laser ablation of solid targets into background gases has been hampered by a lack of knowledge of the spatial and temporal scales for clustering and nanoparticle growth. Using gated-ICCD photography of Rayleigh scattering and in situ photoluminescence from gas-suspended silicon nanoparticles, the dynamics of nanoparticle formation, transport, aggregation and deposition for thin films have recently been revealed.1 The first in situ time-resolved photoluminescence spectra of gas-suspended nanoparticles were also acquired and used to maximize the luminescence of film precursors in the gas phase, prior to deposition .2 Gentle gas-phase oxidation of gas-suspended 1-10 nm Si nanoparticles yielded three in situ photoluminescence bands (1.8, 2.6, 3.2 eV) similar to oxidized porous silicon, but with a pronounced vibronic structure.2 Structureless photoluminescence bands were reproduced in the films (2.0, 2.6, 3.2 eV) only after standared annealing. These in situ diagnostic techniques have now been extended to study the formation and transport of nanocrystals of Ge, ZnO, Y2O3, MgO, and other materials. Using the time-resolved diagnostics, particles unambiguously formed in the gas phase were collected on TEM grids. The particles were analyzed by bright field and Z-contrast TEM, as well as high resolution EELS. Individual nanoparticles are compositionally profiled by acquiring multiple EELS spectra while translating the 0.27 nm electron beam. This research was sponsored by the Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp., for the U.S. Department of Energy, under contract DE-AC05-96OR22464.
1 D.B. Geohegan, A.A. Puretzky, G. Duscher, S.J. Pennycook, Appl. Phys. Lett. 72, 2987 (1998).
2 D.B. Geohegan, A.A. Puretzky, G. Duscher, S.J. Pennycook, Appl. Phys. Lett. (est. July 27, 1998).
F8.1SESSION F8: POSTER SESSION:
MICROCRYSTALLINE AND NANOCRYSTALLINE SEMICONDUCTORS
Chairs: Robert W. Collins, Philippe M. Fauchet and Toshiyuki Sameshima
Wednesday Evening, December 2, 1998
8:00 P.M.
Salons (M)
LOW TEMPERATURE DEPOSITION OF POLYCRYSTALLINE SILICON THIN FILMS PREPARED BY HOT WIRE CELL METHOD. Mitsuru Ichikawa , Jun Takeshita, Akira Yamada, Makoto Konagai, Tokyo Institute of Technology, Dept of Electrical and Electronic Engineering, Tokyo, JAPAN.
Hot wire (HW) cell method has been newly developed and successfully applied to grow polycrystalline Si films at a low temperature with a relatively high growth rate. It was demonstrated in our previous experimental and theoretical works that the radical flux (SiH3/H) on the growing surface was a key factor to grow high quality amorphous and crystal silicon. Thus, we propose HW-cell method as a new deposition process, and we present some preliminary results on the structural characterization of the films. In HW-cell method, the reactant gases are decomposed on the heated tungsten filament by catalysis and pyrolysis as well. The filament temperature, measured by pyrometer, ranged from 1700 to 2000
C. The substrate temperature varied from 100 to
400C. The total pressure varied from 0.015 to 0.2Torr. The
films were deposited from SiH4 on Corning 7059 glass substrates.
The polycrystalline (or micro-crystalline) Si thin films can be grown
at low temperatures of 100-400C with deposition rate of
0.7-1.0nm/s by using SiH4 at a total pressure of 0.1Torr. The film
crystallinity was changed from polycrystalline to amorphous with
decreasing the total pressure. This phenomena was well explained with
our chemical reaction model, that is, SiH2 radical whose
reaction-rate with SiH4 is higher than SiH3 radical deactivates
during the travel from HW to substrate and SiH3 radical can
preferentially impinge upon the surface at a high pressure region.
The X-ray analysis clearly showed that the films grown at the
filament temperature of 1700C have a very strong (110)
preferential orientation. The films consist of large grains as well
as small grains, and it was found from cross-section SEM and TEM that
the films have columnar structure. These results suggested that the
HW-cell method will be a promising candidate to grow device-grade
polycrystalline Si films for photovoltaic application.
F8.2
ENHANCEMENT OF THE AMORPHOUS TO MICROCRYSTALLINE PHASE TRANSITION IN SILICON FILMS DEPOSITED BY SiF4-H2-He PLASMAS. G. Cicala , M. Losurdo, P. Capezzuto, G. Bruno, Plasma Chemistry Research Center, MITER-CNR, Bari, ITALY; T. Ligonzo, L. Schiavulli, Dip. Fisica, Univ. Bari, INFM Bari, Italy; C. Minarini, ENEA, C.R. Portici, Na, ITALY; M.C. Rossi, Dip. Ingegneria Elettronica, Univ. Roma, Roma, ITALY.
Since the demonstration that microcrystalline silicon films exhibit good properties for optoelectronic applications, there has been a renewed interest on the growth of such silicon films by plasma enhanced chemical vapor deposition (PECVD). Recently, several methods have been reported for the preparation of thin films of microcrystalline silicon embedded in a-Si:H and, among these, the hydrogen dilution of silane is considered the conventional method to be used as reference. However, the main subject of these investigations was on the enhancement of the amorphous-to-microcrystalline phase transition through the selective etching mechanism of the amorphous phase by H-atoms. In the present paper, the feasibility of the SiF4-H2-He plasma system in promoting the growth of microcrystalline silicon films is demonstrated. The addition of He favours the amorphous-to-microcrystalline transition, and pure microcrystalline films are deposited at temperature and r.f. power as low as 120
C and 15 watt, respectively. Based on excited species
emission data, by optical emission spectroscopy (OES), and on growth
rate values, by laser reflectance interferometry (LRI), selectivity
is shown to results from the competition between F-atoms etching and
H-atoms assisted deposition.
The results obtained by grazing incidence X-ray diffraction (XRD),
microRaman and spectroscopic ellipsometry (SE), which sometimes are
not easily correlated, have been used to deduce grain size (100-300
nm) and microcrystalline fraction (50-100%). Furthermore, the
samples have been investigated by Fourier transform infrared
spectroscopy (FTIR), for the hydrogen content, and by Constant
Photocurrent Measurement (CPM) and temperature dependence of
conductivity, for the electrical transport
properties.
F8.3
LOCAL STRUCTURE AND Er3+ EMISSION FROM PSEUDO-AMORPHOUS GaN:Er THIN FILMS. S.B. Aldabergenova , M. Albrecht, H.P. Strunk, Univ Erlangen-Nürnberg, Inst für Werkstoffwissenschaften, Lehrstuhl für Mikrocharakterisierung, Erlangen, GERMANY; A.A. Andreev, A. F. Ioffe Physical-Technical Inst, St.-Petersburg, RUSSIA; C. Inglefield, J. Viner, I. Ermakov, P.C. Taylor, Univ of Utah, Dept of Physics, Salt Lake City, UT.
We report on dominant Er3+ luminescence at room temperature in nominally amorphous GaN:Er. The films are deposited by DC magnetron co-sputtering of Ga targets with additional pellets of metallic Er and are post-growth annealed at temperatures up to 800
C. Raman spectra of as deposited
material are characterized by a broad band centered at 440 cm-1
and a sharp peak at 508 cm-1 and show GaN:Er to be a two-phase
material consisting of a dominant amorphous phase and nanocrystalline
inclusions. Annealing the samples at a temperature threshold of
750C enhances the intensity of the Er3+ luminescence
at 1.535 m by factor of about 25. Raman and high resolution
transmission electron microscopy measurements of the structure of
these annealed films show a corresponding increase of the volume
fraction of GaN nanocrystallites. These small crystallites can
contribute to the strong enhancement and sharpening of the Er3+
photoluminescence and photoluminescence excitation spectra in
a-GaN:Er after annealing.
F8.4
STRUCTURE AND OXYGEN SENSING PROPERTIES OF TiO2 THIN FILMS. F. Edelman1, A. Rothshild1, Y. Komem1, B. Mikhelashvili2, A. Chack3, M.Z. Atashbar4, W. Wlodarski4, and F. Cosandey 5; 1Technion-Israel Institute of Technology, Materials Engineering Department, Haifa, ISRAEL; 2Technion-Israel Institute of Technology, Electrical Engineering Department, Haifa, ISRAEL; 3Technion-Israel Institute of Technology, Solid State Institute, Haifa, ISRAEL; 4Royal Melbourne Institute of Technology, Department of Communication and Electronic Engineering, Melbourne, AUSTRALIA; 5Department of Ceramics and Materials Engineering, Rutgers University, Piscataway, NJ.
Increasing awareness of well human being to environmental issues has attracted many scientific researches in the field of gas sensing. Titanium dioxide (TiO2) thin films due to the high refractive index, good chemical stability and large dielectric constant are suitable for many application from optical coating to microelectronic devices. The change of its conductivity due to gas exposure has made it a good candidate for gas sensing applications. This work mainly concerns with the use of TiO2 thin films as an oxygen gas sensor. In this study, the effect of structure and microstructure on the properties of the film with respect to the oxygen sensing has been investigated.
The films with the thickness of
0.2 m, were deposited on
oxidized Si substrate by e-gun sputtering (EGS) from TiO2 target,
or by reactive ion sputtering (RIS) from Ti target. The EGS-TiO2
films were amorphous in the as-deposited state, transforming to
microcrystalline anatase structure as a result of vacuum annealing
and remaining stable in this modification up to 800C. The
RIS- TiO2 films in the as-deposited state have a nanocrystalline
structure composed of mixtures of anatase, rutile and amorphous
phases. These films were stable up to 400C and then
transformed to rutile at T>500C, keeping their
nanocrystalline morphology. We demonstrate the dependence of the
electrical conductivity of the TiO2 films to oxygen exposure. It
was found that the EGS films have faster response/recovery cycle and
lower response temperature limit compare to RIS-films. A detailed
analysis of these results on the oxygen sensitivity of the film has
been discussed.
F8.5
DECHANNEALING STUDY OF NANOCRYSTALLINE SI:H LAYERS PRODUCED BY HYDROGEN IRRADIATION OF SILICON CRYSTALS. V.P.Popov , A.K. Gutakovsky, I.V. Antonova, K.S. Zhuravlev, G.P. Pokhil, I.I. Morosov.
In our work the study of Si-H synthesis was carried out at constant and pulse beams with currents up to 10 mkA*cm2 and 100 mA*cm2 respectively. Irradiation of silicon by light ions (H, He) at beam currents less than 1 mkA*cm2 generates predominantly point defects. The RBS, channeling of He ions and HREM where used for study the implanted silicon, identification of the structural defects and the details of nanoclusters. Pulse implantation of hydrogen ions allowed us to create the layers, containing hydrogen as well as the defects in high concentrations at hydrogen content higher than 15%. We find in samples after moderate annealing in the range 200-500oC strong energy dependence of dechanneling connected with formation of nanocrystallites having mutual small angle misorientation. Their sizes in the range 2-4 nm were estimated on the basis of the suggested dechanneling model and HREM measurements. Correlation between visible photoluminescence (PL) and sizes of nanocrystallites in hydrogenated n-Si:H is observed.
F8.6
A STUDY ON THE ENHANCEMENT OF CONDUCTIVITY AND TRANCEMITTANCE OF ZINC OXIDE THIN FILMS BY HYDROGEN ADDITION. Youn Seon Kang , Hae Yeol Kim, Jai Young Lee, Korea Advanced Institute of Science and Technology, Dept of Materials Science and Technology, Taejon, KOREA.
Highly conductive and transparent hydrogen doped ZnO thin films were prepared on glass substrates by using rf reactive magnetron sputtering from zinc target. The reactive sputtering gas was composed of argon, oxygen and 0
24 hydrogen. The effects
of a hydrogen on the structural, electrical and optical properties
of ZnO thin films were investigated in detail. There was a close
relationship between electro-optical properties and structural
properties. Using X-ray diffraction analysis, we could find that the
c-axis was highly oriented perpendicular to the substrate in the
range of 08 hydrogen, but the {0002} preferred
orientation was degraded and the direction of grain growth was
changed from {0002} to {1010}{1011}{1120} etc. as hydrogen
content increased above 8. And cross-sectional SEM images
showed that colummar structure was well developed in ZnO thin films
to some limit of hydrogen content (8 hydrogen) but, higher than
the limit, secondary nucleation occurred and grain boundary density
sharply increased. These increase of grain boundary density caused
the electro-optical properties of ZnO thin films to degrade because
grain boundaries act as trap sites lowering the mobility of free
carriers. In addition, we heat treated the as-deposited hydrogen
doped zinc oxide films for ionization of hydrogen atoms and
recrystallization of the films. After heat treatment, c-axis
orientation of ZnO films, which are evaluated by full width at half
maximun FWHM of XRD peak, was enhanced and grain boundary density
decreased. As a result, the resistivity was lowered to the value of
10-3 cm and the transmittance was improved to about
88 in visible wavelength region on optimum condition (8
hydrogen addition, 500oC, 1hr heat treatment in vacuum furnace),
which was the highest value yet reported for ZnO films doped with
hydrogen. F8.7
OPTICAL ANALYSIS OF PLASMA ENHANCED CRYSTALLIZATION OF AMORPHOUS SILICON FILMS. Laurent Montè s, Leonid Tsybeskov, Philippe M. Fauchet, Dept. of Electrical Engineering, University of Rochester, Rochester, NY; Kiran Pangal, James C. Sturm, Sigurd Wagner, Dept. of Electrical Engineering, Princeton University, Princeton, NJ.
Low-temperature crystallization of a-Si is important for display and Silicon-On-Insulator (SOI) technologies. We present optical characterization (Raman scattering and photoluminescence) of H2 and O2 plasma enhanced crystallization of a-Si:H films. H2 plasma treatment is shown to be the most efficient, leading to larger grain sizes, and both H2 and O2 plasma lead to visible photoluminescence (PL). Recently, the PL of re-crystallized a-Si films has been explained in terms of quantum confinement. The mean size of the crystallites in our re-crystallized films is determined by Raman scattering for different treatments parameters. No correlation between size and the photon energy of the visible emission is found. However, we can clearly distinguish between the PL from purely amorphous and re-crystallized a-Si:H films : Their PL temperature dependence and spectra are very different. The origin of the visible PL in re-crystallized thin Si films is discussed.
F8.8
AN LED BASED ON ANODIZED POLYCRYSTALLINE SILICON ON TRANSPARENT SUBSTRATE. C. Striemer , S. Chan, H.A. Lopez, K.D. Hirschman, H. Koyama, Q. Zhu, L. Tsybeskov, and P.M. Fauchet, Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY; N. Kalkhoran, G. N. Kim, Spire Corp., Bedford, MA.
LEDs based on anodized polycrystalline silicon (poly-Si) on a transparent (glass or quartz) substrate have been fabricated utilizing electrochemical etching of poly-Si deposited by low-pressure chemical vapor deposition (LPCVD) and standard microelectronic processes including ion implantation and metallization. The photoluminescence (PL) in as anodized poly-Si (poly-PSi) is very similar to the PL in anodized single-crystal Si. The PL peak is near 750 nm and the PL is unstable in an ambient environment under 458 nm laser excitation with an intensity of < 100 mW/cm2. Raman scattering in poly-PSi shows a signature of Si nanocrystals. The device consists of an ITO-layer on transparent substrate, poly-PSi with a metal contact deposited on a cap layer (poly-Si or polymer). The cap layer is introduced to prevent metal electrodiffusion and device breakdown. Light-emission clearly visible by naked eye has been achieved under voltage below 10 V and current of
10 mA/cm2. The poly-PSi LED transient characteristics,
stability and potential application for displays will be
discussed.
F8.9
EFFECT OF HYDROGEN PLASMA TREATMENTS AT VERY HIGH FREQUENCY ON p-TYPE AMORPHOUS AND MICROCRYSTALLINE SILICON FILMS. R. Rizzoli, C. Summonte, CNR-Lamel, Bologna, ITALY; R. Pinghini, E. Centurioni , F. Zignani, A. Desalvo, Dipartimento di Chimica Applicata e Scienza dei Materiali, Bologna University, Bologna, ITALY.
It is known that the layer-by-layer technique is employed to produce thin p-type microcrystalline silicon layers at 13.56 MHz. Yet, the details of the etching/deposition process have not been completely described. An accurate control of the etch/deposition process is of great importance if the engineering of very thin layers is needed. In this paper, very high frequency (100 MHz) plasma enhanced chemical vapour deposition (PECVD) was used to deposit p-type microcrystalline silicon. Subsequent deposition and hydrogen plasma treatments have been performed. The samples have been ex-situ electrically, optically and structurally characterized. The etching rate as a function of the amorphous or microcrystalline substrate character is also reported. The results have shown that the competition of several processes occurs, namely deposition, hydrogen abstraction, and chemical etching of the substrate. In particular, very efficient chemical etching is observed to occur if the samples undergo a pure H2 plasma. The etching rate depends on RF power density, pressure and hydrogen gas flow. Plasma power as low as 21 mW/cm2 and H2 gas flow as low as 2 sccm were observed to produce an efficient etching, yet, slow enough to ensure a good control of the process. Basing on the separation of the effects occurring in the different plasma condition, a model is proposed, for thin p-type microcrystalline silicon deposition.
F8.10
MICROCRYSTALLINE SILICON THIN FILMS DEPOSITED BY LOW TEMPERATURE MAGNETRON SPUTTERING: OPTICAL AND ELECTRICAL CHARACTERIZATIONS. J.E. Gerbi , J.R. Abelson, University of Illinois at Urbana-Champaign, Urbana, IL; Ben Amor, G. Ecole Polytechnique, FRANCE.
Hydrogenated and deuterated microcrystalline silicon (uc-Si:H, uc-Si:D) thin films are of interest for macro-electronic technologies: they can serve as optical absorber or doped contact layers in solar cells, or as the nucleation layer in the direct deposition of polycrystalline silicon on glass for thin film transistors. In addition, these films may be valuable for further optoelectronics applications, as similar films have demonstrated unique optical and electronic properties including luminescence. The grain size, shape, and defect density significantly modify the electronic properties of uc-Si; therefore, it is highly desirable to control the film microstructure through the growth process. We previously showed that DC reactive magnetron sputtering (RMS) of a Si target produces uc-Si:H (uc-Si:D) films when sufficient H2 (D2) is added to the Ar working gas, and that the substitution of D2 for H2 lowers the partial pressure at which the microcrystalline regime is entered, producing films with more crystalline character as seen by ellipsometry.
We will observe, for films grown at various partial pressures of H2 or D2, the microstructure-dependent electronic structure using real time spectroscopic ellipsometry. We report post-deposition TEM, spectrophotometry, Raman spectroscopy, and electrical characterizations including photoluminescence. To explain the implantation-related isotope effect, we present binary collision (TRIM) simulations of the energy distribution, range, and recoil behaviors of the various H vs. D fluxes impinging on the target and substrate, and their resultant dynamic concentrations in the film.
F8.11
MICROSTRUCTURE AND SIZE DISTRIBUTION OF COMPOUND SEMICONDUCTOR NANOCRYSTALS SYNTHESIZED BY ION IMPLANTATION. A. Meldrum , I.A. Anderson, C.W. White, L.A. Boatner, R.A. Zuhr, Oak Ridge National Laboratory, Oak Ridge, TN; D.O. Henderson, M. Wu, A. Ueda, R. Mu, Fisk University, Dept. of Physics, Nashville, TN.
Ion implantation is a versatile technique by which compound semiconductor nanocrystal precipitates may be synthesized in a wide variety of host materials. The component elements that form the compound of interest are implanted sequentially into the host and the compound then precipitates during thermal annealing. Using this technique, we have synthesized compound semiconductor nanocrystal precipitates of ZnS, CdS, PbS, and CdSe in a fused silica matrix. The resulting microstructures and size distributions were investigated by cross-sectional transmission electron microscopy. Several unusual microstructures were observed, including a band of relatively large nanocrystals at the end of the implant profile for ZnS and CdSe, polycrystalline agglomerates of new phases such as gamma zinc silicate, and the formation of central light-contrast features inside CdS nanocrystals. The observed size banding of the ZnS and CdSe nanocrystals is attributed to enhanced diffusion in the implanted region. Electron energy-loss spectroscopy of the light-contrast feature inside the CdS nanocrystals is consistent with the presence of a void. While each of these microstructures is of fundamental interest, such structures are not desirable for potential device applications for which a uniform, monodispersed array of nanocrystals is required. For this reason, additional samples were prepared by varying the implant temperature, implant order, ion fluence, and annealing temperature and atmosphere. These experiments demonstrate that the most important parameters for controlling the size distribution are the ion fluence and annealing temperature. Those specimens with a low concentration of implanted ions or annealed at lower temperatures had significantly narrower size distributions and did not contain the rich variety of unusual microstructures observed in the high-fluence, high-temperature samples. These observations demonstrate the microstructural complexity found in compound nanocrystal precipitates formed by ion-implantation techniques and suggest avenues by which the size distributions may be restricted.
F8.12
PREPARATION AND OPTICAL ABSORPTION OF GaAs NANOCRYSTALS EMBEDDED IN SiO2 THIN FILMS. Hao Wang , Ruiqin Ding, Youwei Zhang, Huidong Yang, Caiguo Zhang, Wuyi Univ, Dept of Mathematics and Physics, Jiangmen, Guangdong, PR CHINA.
GaAs nanocrystals embedded in SiO2 thin films have been prepared on optical silica plates and silicon wafers by radio frequency magnetron cosputtering technique and post-annealing at 573K in vacuum. X-ray photoelectron spectra and Raman spectroscopy strongly suggest the existence of GaAs nanocrystals being 3nm in average size dispersed in SiO2 thin films. From Raman spectroscopy, a red shift and broadening of the Raman peak have been observed with respect to that of the bulk GaAs crystals. Optical absorption spectra of the GaAs nanocrystals have been studied. In comparison with that of the bulk GaAs crystals, the nanocrystalline GaAs show discrete electron level as well as a large blue shift of the absorption edge. The quantum size effect and the dielectric confinement are suggested to discuss the observed experimental details.
F8.13
GROWTH OF BORON CARBIDE NANOWIRES AND NANOCRYSTAL ARRAYS BY PECVD. D. N. McIlroy , Daqing Zhang, University of Idaho, Dept of Physics, Moscow, ID; Yongjun Geng, M. Grant Norton, Washington State University, School of Mechanical and Materials Engineering, Pullman, WA; J.H. Streiff, Jeanne L. McHale, B. Broocks, University of Idaho, Dept of Chemistry, Moscow, ID; Gelsomina De Stasio, Institut de Physique Appliquee, Ecole Polytechnique Federale, Lausanne, SWITZERLAND.
Using the technique of plasma enhanced chemical vapor deposition we have been able to grow single crystal boron carbide nanowires and quantum dot/quantum wire arrays. The nanowires have an average diameter of 30 nm and lengths in excess of 13um. The quantum dots have an average size of 50 nm. Transmission electron microscopy reveals that the tips of the nanowires are capped with Fe. This indicates that nanowire growth is through an Fe mediated vapor-liquid-solid growth mode. The Fe-B eutectic phase reduces the deposition temperature of boron carbide from approximately 1500 C to 1100 C. While large diameter whiskers of boron carbide on the order of a micron have been observed, we believe that this is the first observation of boron carbide nanowires and self assembled boron carbide quantum dots. By taking advantage of this Fe mediated growth process we have been able to preferentially deposit boron carbide by selectively depositing Fe prior to boron carbide growth. We have examined the electronic structure of these nanowires using Raman spectroscopy and NEXAFS. Two new excitation modes have been observed for these boron carbide nanowires which have not previously been observed for large single crystals of boron carbide. We believe that these modes are specific to the nanowires and are a consequence of new degrees of freedom that are normally forbidden in larger samples. The NEXAFS measurements reveal that the electronic structure is equivalent to that of bulk samples of boron carbide.
F8.14
AND PROPERTIES OF SILVER- AND COPPER-CHALCOGENIDE NANOCLUSTERS. A. Eichhoefer , D. Fenske, N. Zhu, Institut fuer Anorganische Chemie der Universitaet Karlsruhe, GERMANY.
Unusual properties of semiconductor nanoparticles compared to bulk materials are of considerable interest in current research. For the interpretation of measured effects it is important to have a detailed information about the particle surface and the structure of the particles itself. In this context single crystal X-ray crystallography which we use to investigate crystalline metal chalcogenide cluster compounds provides the best understanding of structural details.
In the first part of the poster we will present structures of some new silver selenide nanoclusters as [Ag30Se_8(SetBu)_14(PnPr_3)8], [Ag_90Se_38(Se_^tBu)_14(PEt_3)_22], [Ag_114Se_34(Se^nBu)_46(P^tBu_3)_14], [Ag_112Se_32(Se^nBu)_48(P^tBu_3)_12] and [Ag_172Se_40(Se^nBu)_92(dppp)_4
NANOPARTICLES. M. Azad
Malik , Paul O'Brien, Neerish Revaprasadu, Department of Chemistry,
Imperial College, South Kensington, London, UK. Nanoparticles of CuSe (8 nm, dia) or CuInSe2 (10 nm, dia) capped with TOPO (tri-
-octylphosphineoxide) and close to
monodispersed have been synthesized using a novel single source
route. The CuSe nanoparticles were synthesized by injecting a
solution of (methyl--hexyldiselenocarbamato)copper(II) in TOP (tri--octylphosphine) into preheated TOPO at 250C and
CuInSe2 nanoparticles by injecting solutions of InCl3 and CuCl
into TOPO at 100C, followed by the injection of TOPSe
(1.0M) at 250C. The particles have been characterized by
using several techniques including EDAX, XRD, UV/Vis, PL, ICP and
TEM. CuSe showed a blue shift of 0.89 eV and CuInSe2 of 2.0 eV
in relation to the bulk material. The absorption spectrum of
CuInSe2 also displays an excitonic peak at 348 nm, the emission
maximum at 444 nm in the photoluminescence spectrum of CuInSe2, is
red shifted in relation to the excitonic peak.
F8.17
THE PREPARATION AND CHARACTERIZATION OF NANOCRYSTALLINE INDIUM TIN OXIDE FILMS. J. Aikens , H.W. Sarkas, R.W. Brotzman, Jr., Nanophase Technologies Corporation, Burr Ridge, IL.
Nanocrystalline ITO powder is prepared by gas phase condensation (GPC) and used to produce conductive optical coatings. Materials produced by GPC are commercially available at reasonable cost and bridge the gap between basic science and product development. Nanocrystalline ITO films are transparent (>95%), conductive, and offer distinct device processing advantages. The oxidation-state and surface chemistry of ITO may be tailored for particular applications and offers a flexible, affordable alternative to competitive films. The preparation and characterization of nanocrystalline ITO films and device applications will be discussed.
F8.18
MBE GROWTH AND PHOTOLUMINESCENCE STUDIES OF SELF-ASSEMBLED InSb QUANTUM DOTS ON GaSb (001). Anwen Liu , Chih-Hsiang LIn, B.H. Yang, and S.S. Pei, SVEC, Univ. of Houston, TX; S. Lee and S S. Perry, Department of Chemistry, Houston, TX.
Since the finding that the Stranski-Krastanov (SK) or layer-plus-island growth mode could produce coherent islands under appropriate conditions, there have been considerable efforts in the growth and characterization of self-assembled semiconductor quantum dots (QDs). Most previous work has focused on InAs/GaAs QD systems. Study of the self-assembled InSb QDs on GaSb has been limited, so did the staggered window type-II QD heterostructures. InSb has the lowest band gap energy in the binary compounds which makes it very interesting for the mid-infrared devices. Additionally, InSb has the largest lattice constant in the III-V binary compounds which makes it very nature to fabricate InSb QD device. The lattice mismatch in the InSb/GaSb system (6.3%) is similar to the InAs/GaAs case.
Here, we will report self-assembled InSb QDs grown on GaSb (001) substrates by molecular beam epitaxy. Atomic force microscopy was used to examine surface morphology as a function of growth temperature and monolayer coverage. Optimum Stranski-Krastanov growth condition was found to deliver relatively uniform size and distributions of QDs. Deposition of 3.6 monolayers of InSb on GaSb substrate at 410
C produced a surface with /cm2 dots with an
average height of 14 nm and a diameter of 50 nm. Growth interruption
and post-growth annealing have greatly changed the size and
distribution of QDs. Photoluminescence study of QDs will also be
presented.
F8.19
DETERMINING
NANOHETEROGENEITY USING COINCIDENCE MEASUREMENTS IN HIGHLY CHARGED
ION BASED TIME-OF-FLIGHT SECONDARY ION MASS SPECTROMETRY.
Alex V. Hamza , Thomas Schenkel, Alan V. Barnes, and
Dieter H. Schneider, University of California, Lawrence Livermore
National Laboratory, Livermore, CA.
The high (>1)
secondary ion yield per incident primary ion, which is produced when
slow, highly charged ions impinge on a surface, affords the
collection of time-of-flight secondary ion mass spectra in
coincidence mode. In coincidence mode a spectrum may be acquired in
which a particular secondary ion is required for each recorded
primary ion event. Since the primary highly charged ion induces the
emission of secondary ions from a localized area of 20 nm, the
coincidence measurement insures that secondary ion emission be from
within 20 nm of the coincident secondary ion emission. Hence
localized (20nm scale) chemical information is obtained. An example
of the power of this technique with highly charged ions is presented
for a tungsten/SiO2 patterned silicon sample wafer. Details of the
wafer processing steps can be discerned from the coincidence
spectra. By this coincidence method trace impurities can be
associated with tungsten features.
This work was performed under the auspices of the U. S. Department of
Energy at Lawrence Livermore National Laboratory under contract
number W-7405-ENG-48.
F8.20
SYNTHESIS OF
NANOCRYSTALLINE GALLIUM NITRIDE BY MECHANOCHEMICAL REACTION. S. Cai,
Dept of Electrical and Electronic Engineering, University of Western
Australia, Nedlands, WA, AUSTRALIA; T. Tsuzuki, Special Research
Center for Advanced Mineral and Materials Processing, University of
Western Australia, Nedlands, WA, AUSTRALIA; T.A. Fisher,
Dept of Electrical and Electronic Engineering, University of Western
Australia, Nedlands, WA, AUSTRALIA; P.G. McCormick, Special Research
Center for Advanced Mineral and Materials Processing, University of
Western Australia, Nedlands, WA, AUSTRALIA.
GaN has
attracted considerable recent interest owing to potentially important
optoelectronic applications in the blue/UV region. Much of the
recent activity is focussed on devices and material fabricated from
epitaxially grown GaN. GaN powders are also important due to their
potential for blue large area displays and investigation of quantum
size effects. In this paper nanocrystalline GaN has been fabricated
using a simple mechanochemical reaction (high energy ball milling)
through the solid state displacement reaction of gallium oxide with
magnesium nitride. The reagents were sealed in a hardened steel vial
with steel balls under argon atmosphere and were milled at room
temperature for 6 hours, forming gallium nitride and magnesium
oxide. The as-milled powder was annealed for several hours and then
the by-product removed through washing with hydrochloric acid and
deionized, deoxygenated water. The phase analysis of powders by
x-ray diffraction showed that gallium nitride particles form with the
wurtzite structure. The average crystal size of gallium nitride was
determined by x-ray diffraction to be 7 nm and 12 nm for samples
annealed at 500C and 800C respectively,
suggesting that particle size increases with annealing temperature.
Transmission electron microscopy measurements of the samples annealed
at 800C showed that separated particles of GaN formed
during milling with particle sizes of 20nm. Optical absorption
measurements showed a weak bandgap onset at 360nm. Optical
measurements will be reported. Thus we have shown that GaN
nanocrystalline structures can be formed using a simple
mechanochemical milling technique. This technique has the potential
to be used for producing large quantities of GaN.
F8.21
A ROBUST
EXCITON POLARITON IN A QUANTUM WELL WAVEGUIDE. Masataka
Shirai , Kazuhiko Hosomi, Tomoyoshi Mishima, Toshio Katsuyama,
Central Research Laboratory, Hitachi Ltd, Tokyo, JAPAN.
In nanometer-scale semiconductor structures, an
exciton interacts strongly with a photon due to the quantum
confinement effect. This strong interaction leads to the
stabilization of a quasi-particle called exciton polariton. Such a
polariton is expected to be applied in optical devices in which the
phase change of the polariton propagating along a quantum well
waveguide is utilized [1]. In this work, we investigated the
temperature dependence of the refractive index change under an
electric field and discussed the contribution of the polariton. We
found that the magnitude of the refractive index change was larger
than the expected value from the line width of the absorption
spectrum at low temperature.
The sample used here was a quantum well waveguide composed of a GaAs
quantum well (7.5 nm thick) sandwiched between GaAlAs core layers
(1.8 m). We measured the phase change by using a Mach-Zehnder
interferometer. The magnitude of phase change was obtained from the
shift of the output fringe pattern, and it was converted to
refractive index change. The temperature was varied from 6 to 300 K.
Our experimental results showed that the refractive index change
remained high up to 40 K, where its value was three times larger than
that at 300 K. This result indicates that the damping factor of the
polariton is 0.3 meV, which is smaller than the line width (5 meV) of
the exciton absorption spectrum. This result means that in the
quantum well waveguide polariton can be more stable than that of the
bulk polariton. Accordingly, polariton can be utilized in the
development of small and efficient optical devices.
This work was performed under the management of FED as apart of the
MITI R&D Program Quantum Functional Devices Project supported by
NEDO.
[1] T. Katsuyama, K. Ogawa, J. Appl. Phys. 75, 7607
(1994)
F8.22
ENHANCED
PHOTOLUMINESCENCE FOR ZnS NANOCRYSTALS DOPED WITH Mn2+ CLOSE TO
CARBOXYLIC GROUPS AND/OR S2- VACANCIES. Tetsuhiko
Isobe , Takahiro Igarashi, Miho Konishi, Mamoru Senna, Keio Univ.,
Dept Applied Chemistry, Hiyoshi, Yokohama, JAPAN.
When the particle size of ZnS doped with Mn2+ (ZnS:Mn) decreases
from micrometer to nanometer, the sextet EPR signal with g = 2.0013
and the hyperfine coupling constants, A = 9.0mT, is
observed together with the sextet signal with g = 2.0024 and
A = 6.9mT. This shows two different states of Mn2+
ions in nanocrystals. The symmetry of Mn2+ crystal field is
lower and the spin-lattice interaction is higher with S2-
coordination number lower than 4, e.g., at the near surface, as
confirmed by the zero-filed splitting constant and the peak width,
respectively. The symmetry of the Mn2+ crystal field in bulk
also becomes lower with decreasing the particle size because of
larger fraction of near-surface Mn2+. These results could
explain effective energy transfer between ZnS and Mn2+ and a
higher probability of Mn2+ d-d transition in the
photoluminescence (PL) processes of ZnS:Mn nanocrystals at 580nm.
When ZnS:Mn nanocrystals are modified with carboxylic acids, the PL
intensity due to Mn2+ d-d transition increases. The COO-
groups are excited by the same energy as ZnS to exhibit PL at 440nm.
On the other hand, when the S/Zn ratio decreases, the increase in the
PL intensity due to the Mn2+ ions are also observed together
with the appearance of PL at 440nm due to S2- vacancies. When
ZnS:Mn nanocrystals are irradiated by a UV light of 350nm, the
increase in the PL intensity is observed regardless of the
modification by carboxylic groups. This is likely to be attributed
to the increase in the near-surface vacancies by UV irradiation. All
phenomena are characteristic of nanocrystals. Since the probability
on the existence of Mn2+ ions at the near-surface is higher for
nanocrystals, the strong interaction between Mn2+ and COO-
or S- vacancies is expected.
F8.23
ORGANIZED
POLYMER FILMS CONTAINING SEMICONDUCTOR QUANTUM DOTS FOR MESOSCOPIC
ELECTRON DEVICES. Yu.N. Savin , O.N. Bezkrovnaya, Yu.V.
Kamensky, A.V. Tolmachev. Institute for Single Crystals National
Academy of Science of Ukraine, Kharkov, UKRAINE.
The
optical and electrotransport properties of semiconductor
nanoparticles in dielectric matrixes draw attentiion now because
quantum dimension effects can be observed here and such structures
can be used in photonics and nanoelectronics. The organized molecular
assembles such as polymer Langmuir-Blodgett (LB) films evoke
particular interest due to possibility to obtain the ordering
molecular structures controlled in size.
In this communication the results of investigation of nucleation and
growth PbS and Ag2S nanoparticle processes in polyvinnylbutiral
(PVB) and polyamidoacid (PAA) are reported. LB films containing PbS
or Ag2S nanoparticles was obtain by two stage. At first,
Langmuir monolayer of PAA or PVB was formed at water subphase
containing Pb(NO3) or Ag(NO3) salts and then was transfered
on the quartz, gold and CaF2 substrates. At the second stage LB
films was exposed in thiourea water solution to obtain lead sulphide
and silver sulphide nanoparticles.
The nucleazation and growth kinetics of nanoparticles depending on
parameters determinating the thermodynamics of polymer monolayer
(pH, concentration of Pb, Ag and S ions, temperature, surface
preasure) as well duration of exposion are studied by IR spectroscopy
and STM. Elemental composition of LB films is studied by XPS. By UV
spectroscopy the shift of longwave edge of LB film absorbtion spectra
are founded. This displacement is due to Q-state PbS and Ag2S
nanoparticlts. The effects of nanoparticle size on the absorbtion
spectra polymer LB films are discussed.
F8.24
CHEMICAL
COMPOSITION OF SEMICONDUCTOR NANOPARTICLES IN GLASS. G.
D. Lian , P. D. Persans, T. M. Hayes, M. Stukowski, M. Frederick,
Physics Department, Rensselaer Polytechnic Institute, Troy, NY.
Precipitation of semiconductor nanoparticles in
glass produces useful optical materials that are robust in
environments that are chemically or thermally hostile to particles
alone. Control of particle composition gives us much greater
latitude than control of size alone in the design of new systems.
In many commercial preparations, small amounts of semiconductor
precursors, for example Cd, S, or Se, are added to major glass
components (SiO2, B2O3...) and melted at
1400C. The composition of the resulting crystallites
is determined by chemical equilibrium and kinetic effects at the
precipitation temperature and will certainly not correspond to the
composition of the starting material. In this paper we combine and
compare several techniques including optical absorption, resonant
Raman scattering, x-ray absorption and x-ray fluorescence
spectroscopies, x-ray diffraction, and electron microscopy to deduce
the effects of glass composition and preparation, and heat treatment
on particle composition.
F8.25
PHOTOLUMINESCENCE
OF ZINC OXIDE QUANTUM PARTICLE THIN FILMS. Eva M. Wong
and Peter C. Searson, The Johns Hopkins University, Dept of Materials
Science and Engineering, Baltimore, MD.
Quantum
size ZnO particles were prepared by colloidal synthesis from zinc
acetate in 2-propanol. The particle size was dependent on aging time
and temperature and was monitored by optical absorption
spectroscopy. Phosphor films were fabricated by electrophoretic
deposition of ZnO quantum particles from these stable suspensions.
Constant current techniques were used to prepare the thin films thus
eliminating the limited deposition rates associated with constant
voltage techniques. All the films exhibited a blue shift relative to
the characteristic green emission associated with bulk ZnO. This
blue shift in the visible photoluminescence was related to the
enlargement of the band gap as demonstrated by a linear dependence of
the energy of the visible photoluminescence peak on the energy of the
band to band transition. These results show that the visible
emission involves the valence band and a deep level state, consistent
with the oxygen vacancy model.
F8.26
ALIGNED
DISORIENTATION OF MOSAIC BLOCKS, INDUCTED OF PBS NANOPARTICLES IN
LANGMUIR-BLODGETT FILMS OF LEAD STEARATE. I.A.
Shneiderman ,Yu.N. Savin, A.G. Fedorov, A.V. Tolmachev, Institute for
Single Crystals of National Academy of Sciences of Ukraine, Kharkov,
UKRAINE.
The processes of formation and controlled
growth of metallic and semiconductor nanoparticles and monolayers in
dielectric matrixes draw attention because obtained structures
manifest quantum dimensional effects and may be used in photonics and
nanoelectronics.
Langmuir-Blodgett lead stearate multilayers on silicon substrates
were exposed in H2S under pressure 10 Torr for obtaining
nanoparticles of PbS. The structure of these objects before and after
treatment was examined by means of small-angle X-ray diffraction and
double crystal X-ray spectrometry. The multilayer films consist of 20
layers and have an Y-type stacking with the period of 4.89 nm in
initial state. Changes of multilayer period, interface roughness and
disordering of mosaic blocks depending on treatment time were
detected after processing. It was shown that the block structure
appears in LB multilayers on silicon and quartz substrates with the
block dimensions of 60*60*100 nm. Period of the multilayer
increases on 2 at the exposition of LB multilayers during 8
hours, r.m.s. roughness of the lead containing layers is increased on
0.12 nm. Aligned disorientation of all blocks in relation to the
surface normal with maximum inclination of 16 after t=8 h.
treatment was detected by X-ray diffraction.
The formation of PbS compound after H2S exposition was found by
XPS. The broad absorbtion line was observed in optical spectra. The
longwave absorbtion edge was shifted on 1.4 eV in blue region
respecting to the bulk PbS. This displacement is due to Q-state PbS
nanoparticles.
The phenomenon model, based on the relaxation of internal mechanical
stress originating due to formation of PbS nanoparticles at the
boundaries of blocks is discussed.
F8.27
MICROSTRUCTURE
AND SENSING PROPERTIES OF CRYOSOL DERIVED NANOCRYSTALLINE TIN
DIOXIDE. S.M. Kudryavtseva , S.V. Kalinin, L.I. Kheifets,
A.M. Gaskov and A.A. Vertegel, Inorganic Chemistry Division, Dept.
Chemistry, Moscow State University, Moscow, RUSSIA.
Tin dioxide is a prominent material for semiconductor gas sensors and
catalysts. The best sensing properties are reported for the samples
with the grain size less or equal to 6 nm. Thus, development of
reliable synthetic route for the preparation of nanocrystalline tin
dioxide with controlled microstructure is the problem of considerable
practical interest.
In the present research we applied the newly-developed cryosol
technique for preparation of bulk and thick film nanocrystalline tin
dioxide gas sensors. The essence of the synthetic route employed is
cationic-resin treatment of sodium stannate solutions resulting in
stable sols of stannic acid. Freeze drying of thus prepared colloid
solutions yielded nanocrystalline tin dioxide powders. Bulk samples
were obtained by pressing the freeze-dried precursor with further
annealing. The films were prepared by screen-printing technique. The
samples were characterized by means of XRD, X-ray diffraction
broadening analysis, BET, and resistivity measurements. Study of the
sensing properties showed that sensitivity to H2S of the
crysol-derived pellets is several times better than that of the
samples obtained by traditional precipitation route. On the contrary,
thick films prepared by cryosol method are almost insensitive to
H2S.
The SEM studies revealed that bulk samples prepared by cryosol method
consist of very thin (about 10 nm thick) platelets, while the samples
obtained by traditional routes are made up of tree-dimensional
aggregates. This feature of microstructure is probably the cause of
better sensing properties. At the same time, the two-dimensional film
microstructure is destroyed during thermal drying of the
screen-printed tapes, and the thick films possess worse sensing
properties.
F8.28
CHARACTERIZATION
OF LASER ABLATED GERMANIUM NANOCLUSTERS. F. Shen, Y. Zhang,
S. Vijayalakshmi and H. Grebel Optical Waveguide
Laboratory, New Jersey Institute Of Technology, Newark, NJ.
Films containing Ge nanoclusters were prepared by
laser ablation technique. Radiation from a KrF excimer laser at
=248 nm was focused on a crystalline Ge (100, n-type, 1E16
cm-3) target. The ablated material was collected onto a glass
substrate placed 3 cm from the target. The films were composed of
micron sized droplets which by themselves were made of
nanocrystallites. In addition, smaller nanosclusters were dispersed
in between the droplets. Atomic Force Microscopy was used for the
morphological characterization of the films. The crystallinity of the
nanoclusters was measured by X-ray diffraction. Crystallinity and
size distribution of the nanoclusters was also measured by Raman
scattering. The cluster size was estimated from the observed shift in
the Raman line as comapred to the spectra of a crystalline germanium.
Z-scan technique was used to measure the nonlinear optical properties
of these films. Strong correlation was observed between the large
nonlinear optical constants and the nanocluster size. For example, we
measured nonlinear refractive index values as high as 1e-7 esu at
=532 nm. Effect of annealing on the optical properties and
photoluminescence spectra will also be discussed. Comparison between
these results and those found for Si nanoclusters[1] will be given.
[1] S. Vijayalakshmi, F. Shen and H. Grebel, Appl. Phys.
Letts, 71, 3332 (1997).
F8.29
CHEMICAL
SOLUTION DEPOSITED COPPER-DOPED QUANTUM DOT FILMS.
Nirmala Chandrasekharan , Gary Hodes, Department of
Materials and Interfaces, Weizmann Institute of Science, Rehovot,
ISRAEL.
The energy level structure of semiconductor
quatum dots changes gradually from the band structure of the bulk
semiconductor to an atomic-like structure as the quantum dot size
decreases. This results in an increase in the effective optical band
gap as a function of crystal size. Apart from this inherent
flexibility in tailoring quantum dot properties, it is of interest to
see how doping affects these properties further. For quantum dots in
the size range investigated by us, a single dopant atom would
drastically change the properties of the dots and therefore doping
might be expected to affect the semiconductor properties in a
different manner than it does for bulk semiconductors.
We have studied Cu-doped CdSe quantum dots, deposited as films by
chemical solution deposition and compared their properties to those
of undoped CdSe quantum dot films. The CdSe quantum dot films were
deposited on either glass or SnO2 - conducting glass from
aqueous solutions containing CdSO4 , potassium nitrilotriacetate
(KNTA) (as complexing agent) and Na2SeSO3. The Cu-doped
films were prepared by adding 0.8 mM CuCl in KNTA (1% of the Cd
concentration) to the deposited solution. The films were ca. 70nm
thick.
The deposited films contained ca. 5% Cu (relative to Cd) for the
small crystal sizes (see below) and the Cu concentration decreased
for larger crystals. The crystal size was very temperature dependent
and varied from 3 nm (at 5 C) - slightly smaller than the
equivalent non-doped films (3.5 nm) to 8nm at 80 C (larger
than the 6 nm undoped films ). The size distribution of the doped
films were also larger than the undoped ones. The optical absorption
spectra of the films reflected the quantum size effects with smaller
crystal size resulting in an increased blue spectral shift. In
addition, the Cu-doped samples exhibited a very pronounced tail in
the onset region, which was absent in the undoped samples. This tail
was present also in the photoluminescence (PL) spectra, as a very
pronounced extension of the PL signal to lower energies as well as in
liquid junction photocurrent spectra. Treatment with aqueous KCN
solution, which removed the Cu species from the small crystal samples
(as confirmed by SIMS and XPS analyses), also removed this
sub-bandgap component in all the spectra for the small crystal sizes,
and to a lesser extent for the larger ones.
While we do not know for sure the location of the Cu species, the
fact that it is readily removed by the KCN treatment suggests that it
is located at the surface of the individual quantum dots for small
(3-5 nm) crystals. In support of this assumption, XPS shows te
presence of Se-O species, presumably surface oxidized, in the
low-temperature undoped films, but none in the doped ones. This
suggests that Cu takes the place of these surface Se-O sites. XPS
also indicates that the Cu is predominantly in the form of univalent
copper, although we cannot rule out the presence of divalent copper
at this stage.
F8.30
STRUCTURAL
PROPERTIES OF SINTERED LEAD SULFIDE NANOPARTICLES. Bernd
Rellinghaus , F. Einar Kruis, Kornelius Nielsch, Heinz
Fissan, and Eberhard F. Wassermann, Experimental
Low-Temperature Physics, Gerhard-Mercator University, Duisburg,
GERMANY. Process and Aerosol Measurement Technology,
Gerhard-Mercator University, Duisburg, GERMANY.
We
report on the results of structural investigations on PbS
nanoparticles generated by means of an aerosol route at room
temperature [1]. The primary particles are exposed to different
sintering temperatures, TS, in the gas phase and subsequently
deposited onto substrates. The resulting sintered PbS nanoparticles
exhibit almost monodisperse size distributions with mean particle
diameters ranging from 3 to 18 nm depending on the primary particle
sizes and the sintering temperatures, respectively. The structure of
these nanoparticles is examined utilizing transmission electron
micoscopy (TEM) and X-ray diffraction (XRD). When the sintering
temperature is increased to roughly 400C, the particles
start to show significant facetting, thus indicating the formation of
a crystalline phase. From XRD and electron diffraction experiments
the crystal structure is determined to be of the NaCl type (cubic B1
phase), and the lattice constant is identical to that of bulk PbS.
Further increasing TS leads to the formation of monocrystalline
particles. This is due to the fact that the particle sizes as
measured with the TEM are in good agreement with the crystal sizes as
determined from the width of the Bragg peaks observed in the XRD
investigations.
[1] F. Einar Kruis, Kornelius Nielsch, Heinz Fissan, Bernd
Rellinghaus, and Eberhard F. Wassermann, Preparation of
size-classified PbS nanoparticles in the gas phase, accepted for
publication in Appl. Phys. Lett. (1998).
F8.31
SURFACE
RELAXATION IN CDSE NANOCRYSTALS. K. Leung and K. B.
Whaley, Department of Chemistry, University of California at
Berkeley, Berkeley, CA.
Total energy minimization is
applied to semi-empirical tight-binding CdSe nanocrystal models with
diameter up to 31 Å. The crystallites have wurtzite lattice
structure and ligand-terminated surfaces. It is found that the
qualitative features of the surface relaxation in nanocrystals and in
infinite surfaces are similar. With surface relaxation incorporated,
the onsets of absorption are determined by interior-to-interior
transitions with little contribution from surface-localized states.
The fine structure and exchange splittings derived from these
clusters are in agreement with experimental measurements. The
effects of the surface termination on the exciton fine structure are
analyzed.
F8.32
OPTICAL PROBES
OF NUCLEATION OF SEMICONDUCTOR NANOPARTICLES IN GLASS. P.
D. Persans , T. M. Hayes, Physics Department, Rensselaer Polytechnic
Institute, Troy, NY.
Direct tools such as x-ray
diffraction and transmission electron microscopy are not useful for
the observation of nanoparticles embedded in a solid matrix such as
glass. The energy shift of optical transitions in semiconductor
nanoparticles can be used to indicate the density and size
distribution of such particles. We have studied the shape and
position of the lowest energy optical absorption peak in borosilicate
glasses doped to produce CdS, ZnxCd1-xS, and
CdSxSe1-x nanoparticles. We will discuss and give examples
of the optical indicators for homogeneous nucleation, heterogeneous
nucleation, and ripening.
F8.33
PREPARATION
AND OPTICAL PROPERTIES OF SILVER-COATED CdS NANOPARTICLES.
J. W. Haus , Z. Yuan, S. Wei, N. Hetherington, P. Stathos,
G. D. Lian, P. D. Persans, Physics Department, Rensselaer Polytechnic
Institute, Troy, NY.
Metal-coated semiconductor
nanoparticles can exhibit unique optical properties due to local
field enhancement. The plasmon resonance of the metal shell can be
tuned over a wide wavelength range by changing the thickness of the
shell relative to the radius of the semiconductor core. When the
metal shell plasmon resonance coincides with the lowest excited state
of the semiconductor core, optical bistability has been predicted.
We are able to shift the plasmon resonance of silver on 10 nm CdS
particles from 400 nm to 550 nm by varying silver concentration and
reduction process. We report on the results of cw, nanosecond, and
picosecond pulse z-scan spectroscopy to elucidate the origin of the
observed optical nonlinearities.
F8.34
THEORETICAL
INVESTIGATION OF EFFECTIVE QUANTUM DOTS INDUCED BY STRAIN IN
SEMICONDUCTOR WIRES. Kenji Shiraishi , Masao Nagase,
Seiji Horiguchi, and Hiroyuki Kageshima, NTT Basic Research
Laboratories, Atsugi-shi, Kanagawa, JAPAN.
Quantum
dots fabricated in semiconductors have been intensively studied
during this decade. It has been the fabrication technique of small
dot-shaped structures that is the crucial for the formation of
quantum dots. In this contribution, we theoretically propose the
introduction of strain in a quantum wire structure as a new method
for forming quantum dots, instead of actually synthesizing dot-shaped
structures.
It has been reported that oxidation of a Si nano-structure generates
a huge amount of stress: The compressive stress at the Si/SiO2
interface can amount to about 25000 atm., when a 10-nm diameter Si
wire is fabricated by a self-limiting process of oxidation [1]. This
compressive stress results in a two-dimensional compressive strain of
about 1% perpendicular to the wire direction when the stress is
uniformly applied to the Si wire. We studied the effect of this
strain applied to a small part of a Si quantum wire by using
first-principles band structure calculations. The calculated results
indicate that the band gap of the Si crystal under the
two-dimensional compressive strain decreases by about 130 meV
compared with the bulk value. Accordingly, introduction of strain to
a small part of a Si wire results in the formation of an effective
quantum dot. It has been reported that pattern dependent oxidation
(PADOX) can lead to the local oxidation of Si nanostructures [2].
Consequently, the local oxidation by applying PADOX to Si wire
structures is the promising technique to generate enough stress to
convert a small part of Si into an effective quantum dot.
[1] H. I. Liu et al., J. Vac. Sci. Technol. B11, 2533(1993).
[2] Y. Takahashi, H. Namatsu, K. Kurihara, K. Iwadate, M. Nagase,
K. Murase, and M. Tabe, IEEE Trans. Electron. Devices 43,
1213(1996).
F8.35
MODIFICATION
OF SEMICONDUCTOR NANOPARTICLES VIA S-SITES. Nicholas A.
Kotov , Tong Ni, Margaret Eastman, Oklahoma State University,
Department of Chemistry, Stillwater, OK.
Effect of
species adsorbed on the surface of semiconductor nanoparticles on
their optical properties represents both fundamental and practical
importance. So far only metal atoms located on the surface of
nanoparticles were used for surface modification and stabilization.
Typically, organic molecules attached to metal centers by covalent or
coordination bonds affected the emission of nanoparticles by either
enhancing or quenching the excitonic (near band gap) and/or trapped
carrier emission bands, while virtually no change of the position and
intensity of UV absorption bands of semiconductor cores could be
seen. The surface sulfur atoms of CdS, CdTe and Bi2S3 nanoparticles
(NP) of 2-5 nm in diameter were accessed by using an organometallic
complex of copper with labile inorganic ligand in the coordination
sphere. Nuclear magnetic resonance (NMR), electron spin resonance
(ESR), infrared (FT-IR) and fluorescence spectroscopy (Fl) indicated
that metal ion is directly connected to the S-sites on the
nanoparticle surface while retaining its original organic ligands
with pi-systems. No change in the diameter of the semiconductor
core could be observed by HR-TEM. S-modification of NP resulted in
appearance of new bands in the near UV region of the spectrum and
splitting of the trapped carriers emission band. The change in
photophysical properties of nanoparticles is discussed in terms of
interaction of electronic levels of NP and pi-orbitals of organic
ligands.
F8.36
CHARACTERIZATION
OF ZnSe NANOPARTICLES PREPARED USING ULTRASONIC RADIATION METHOD.
Jianfeng Xu , Wei Ji, Sing-hai Tang, National Univ of
Singapore, Dept of Physics, SINGAPORE; Wei Huang, Institute of
Materials Research and Engineering, SINGAPORE.
ZnSe
nanoparticles have attracted much attention in recent years due to
their potential for applications in areas such as nonlinear optical
devices and fast optical switches. They can be prepared in structured
media such as films, or non-structured media, i.e. without a
supporting matrix. For nanoparticles in non-structured media it is
difficult to maintain isolation without using surfactant. Because
most reported ZnSe nanoparticles are prepared and stabilized by
capping or passivation with organic materials, the influence of the
stabilizer on ZnSe nanoparticles could not be avoided. In this work,
we present a new method for preparing ZnSe nanoparticles without any
stabilizer, which makes it possible for us to study the real
properties, especially the surface character, of ZnSe nanoparticles.
First, Zn nanoparticles were prepared by the inert-gas evaporation
technique with induction heating. These Zn particles were mixed with
sodium selenide solution under ultrasonic radiation. Due to the high
reactivity of Zn nanoparticles, ZnSe particles were successfully
prepared under ultrasonic radiation. X-ray diffraction revealed that
the ZnSe particles prepared have a zincblende structure. Electron
microscopy showed that the particles are in the range of a few to tens
nanometers depending on the experimental conditions. They were also
characterized by X-ray photoelectron spectrometry, Raman and infrared
techniques.
F8.37
ELECTRON BEAM
EXCITED PLASMA CVD FOR SILICON GROWTH. Kazuhiko Okitsu ,
Mitsuru Imaizumi, Koji Yamaguchi, Masafumi Yamaguchi, Tamio Hara,
Toyota Technological Inst., Nagoya, JAPAN; Tadashi Ito, Toyota
Central R&D Labs., Inc., Aichi, JAPAN; Masahiko Ban, Masakuni Tokai,
Kawasaki Heavy Industries Inc., Chiba, JAPAN; Kazuhiko Kawamura,
Chubu Electric Power Co., Inc., Nagoya, JAPAN.
Electron beam excited plasma(EBEP) CVD is one of the novel methods of
CVD. For the case of conventional P-CVD, the dissolution rate is low
and hydrogen dilution is usually necessary to grow microcrystalline
silicon layer. On the other hand, EBEP-CVD has a capability of making
it from 100% SiH4 without dilution. In this paper, Si growth
using the new deposition method EBEP-CVD has been proposed.
The plasma is generated by the collision of electrons accelerated by
about 100V. Because the gas molecules have high ionization efficiency
in this region, they could be decomposed in high rate. In our
equipment, the plasma size was full of the chamber (about 60 cm in
diameter). The substrates used were Si wafers and the oxidized
SiO2 layer. The growth was investigated under various
conditions: the flow rate of source gas, the chamber pressure, the
substrate temperature, the electron accelerating voltage, the
electron beam current and the position of the substrate.
The grown layers were analyzed by Raman scattering, photoluminescence
spectroscopy(PL), and so on. Most of all samples have Raman peak near
520cm-1, which is due to the crystalline. The grain sizes of all
crystalline samples were about 10nm.
In PL, the peaks which are similar to those of the proton/electron
irradiated Si have been observed in some samples. They are thought to
be caused by the plasma damage. As another feature of this plasma, it
is formed independent to the electric or magnetic field. We can avoid
acceleration to the substrate by electric and magnetic power, then
EBEP-CVD is expected to grow a high quality layer with a poor damage.
Then we have also investigated the plasma damage with pure H2
plasma in order to decrease plasma damage.
F8.38
A II-VI
SEMICONDUCTOR NANOSTRUCTURE BASED SCINTILLATOR FOR HIGH PERFORMANCE
GAMMA-PHOTON IMAGING. S.P. McGinnis , B. Das, West
Virginia University, Department of Computer Science and Electrical
Engineering, Morgantown, WV.
High performance
gamma-photon imaging for medical, scientific, and security
applications requires the development of new scintillator materials
with high sensitivity, response speed, spatial and spectral
resolution, and good material properties. However, the past
discovery of inorganic scintillatror materials has been characterized
as an ad hoc experimental approach which did not allow for the
engineering of an optimum material [1]. In response to this need, we
have developed a scintillator consisting of a dense periodic array of
doped II-VI semiconductor quantum wires. The semiconductor quantum
wires are electrochemically synthesized in a pre-formed anodized
aluminum template using a thin-film based technique we have
previously developed. This technique provides precise control over
the quantum wire size and composition, and can be used to directly
integrate the scintillator material with semiconductor
photodetectors. The size and composition of the quantum wires is
determined for each specific application using a model of the
scintillation process in a quantum confined system. We will present
the results of this model along with a preliminary experimental
characterization of the scintillator.
[1] P.A. Rodnyi, Physical Processes in Inorganic
Scintillators , CRC Press, 1997.
F8.39
NANOPARTICLE-BASED
CONTACTS TO CdTe. Douglas L. Schulz , Doug Rose, Rosine
Ribelin, Kim M. Jones, Calvin Curtis, Pete Sheldon, Tim Gessert,
David Ginley, National Renewable Energy Laboratory, Golden, CO.
Cu-Hg-Te nanoparticles have been used as a component
of a graphite-based contact to CdTe in solar cells. In this
approach, Cu0.04Hg0.96Te nanoparticles are synthesized by
reaction of the metal iodides with sodium telluride in dry, degassed
methanol at low temperature. The as-synthesized black colloid was
characterized by x-ray diffraction with broad reflections from HgTe
(PDF#32-665; Coloradoite) phase observed. Transmission electron
microscopy (TEM) showed the product consists of micron-sized
agglomerates of 10-20 nm particles. TEM energy dispersive
spectroscopy indicated the presence of Hg, Cu, and Te with minimal C
and O while TEM selected area diffraction revealed the particles are
polycrystalline.
The Cu0.04Hg0.96Te nanoparticles were mixed with graphite
paste and employed as the back contact to several CdTe solar cells.
The back contact was fabricated as follows: first, a nitric
acid:phosphoric acid etch was performed on a CdCl2-treated
CdTe/CdS/SnO2-/glass superstrate composite; second, the
nanoparticle/electrodag paste was applied; third, this contact was
annealed under helium for 20 to 40 min at temperatures from 220 to
320C; finally, silver paste was applied and a final anneal
in air at 100C for 1 h was performed. The solar cells
produced using this nanoparticle-based dag exhibited good electrical
properties as evidenced by standard IV characterization. To date,
the best device exhibited the following metrics: Voc = 834 mV;
Jsc = 20.7 mA/cm2; FF = 63.0 %; Rseries = 6.3
; Rshunt = 1570 ; and efficiency = 10.9 %. The
effects of variable annealing time/temp and dag loading upon device
characteristics will be discussed.
This research was supported by the U.S. Department of Energy National
PV Program and the U.S. Department of Energy, Office of Energy
Research; Chemical Sciences and Materials Sciences Divisions under
contract #DE-AC36-83CH10093.
F8.40
METAL
CHALCOGENIDE CLUSTERS. CORRELATION BETWEEN MASS SPECTROMETRY AND
OPTICAL PROPERTIES. Jean-Jacques Gaumet , Geoffrey F.
Strouse, University of California Santa Barbara, Dept of Chemistry,
Santa Barbara, CA.
Clusters composed of discrete
numbers of metal (Zn, Cd) and chalcogenide (S, Se) atoms allow the
investigation of the onset of semiconducting properties in nano-scale
II-VI materials. This class of clusters, based on an adamantoid
cage, forms an architectural subunit of the wurtzite lattice.
Although the chemistry of such structures have been investigated,
surprisingly, the mass spectrometric properties of such clusters are
not well-known. Mass spectrometry techniques are useful in analysis
of the dimension of clusters, by observing the ionized fragments. We
have carried out a careful mass-spectroscopic analysis on a series of
mono-disperse metal-chalcogenide clusters in which the substitution
of the core and surface are carefully controlled. Analysis of the
optical properties with respect to the structure of the cluster
suggest the importance of surface derivatization on the lowest
optical properties and electronic gap in these
materials.
F8.41
PRESSURE
INDUCED STRUCTURAL TRANSFORMATION IN CRYSTALLINE AND NANOCLUSTER
ASSEMBLED GaAs. Sanjay Kodiyalam , Alok Chatterjee,
1Ingvar Ebbsjo, Rajiv K. Kalia, Hideaki Kikuchi, Aiichiro Nakano,
2Jose P. Rino, Priya Vashishta, Concurrent Computing Laboratory
for Materials Simulations, Dept. of Physics & Astronomy, Dept. of
Computer Science, Louisiana State Univ.; 1Studsvik Neutron
Research Laboratory, SWEDEN; 2Universidade Federal de Sao Carlos,
BRAZIL.
Structural correlations in nanophase GaAs
are studied using multimillion atom molecular dynamics simulations
based on space-time multiresolution algorithms. Structural
transformations in bulk and nanophase GaAs under pressure are also
studied. The interatomic potential including both two- and three-body
terms is validated using neutron scattering data. Work supported by
NSF, DOE, AFOSR, USC-LSU MURI, ARO, PRF, NSF USA-Japan International
Grant, and Louisiana LEQSF.
F8.42
PROPERTIES OF
IMPURITIES IN SPHERICAL QUANTUM DOTS. Gerardo J.
Vazquez , Juan Adrian Reyes, Marcelo del Castillo-Mussot, Instituto
de Fisica, UNAM, Mexico City, MEXICO; Harold N. Spector, Illinois
Institute of Technology, IL.
We calculate, in
analogy with hydrogen atom, a number of quantum properties of
impurities center in spherical dots taking into account a barrier
potential at the boundary of the spherical dot and the difference
between the screening parameters inside and outside the dot. We
employ a shooting method to solve numerically the resulting
one-dimensional radial Schroedinger equation.
F8.43
GERMANIUM
NANOSTRUCTURES FABRICATED BY PULSED LASER DEPOSITION.
K.M.Hassan , A.K.Sharma, J.Narayan,J.F.Muth1 and
R.M.Kolbas2 Department of Materials Science and Engineering, NSF
Center for Advanced Materials Processing and Smart Structures,North
Carolina State University, Raleigh, NC; 1,2 Department of
Electrical and Computer Engineering, North Carolina State University,
Raleigh, NC.
Quantum confined nanostructures of
semiconductors such as Ge and Si are being actively studied due to
interesting optical and electronic transport properties. Fabrication
of these nanocrystalline structures has been done using MBE at higher
substrate temperatures. Pulsed laser deposition (PLD) is an
attractive technique for growing novel structures at lower substrate
temperature compared to MBE and other techniques.
We,therefore,fabricated Ge nanostructures buried in the matrix of
polycrystalline-AlN grown on Si(111) by pulsed laser deposition at
lower substrate temperatures than that used in all previous studies.
The characterization of these structures was performed using HRTEM
and Raman spectroscopy. HRTEM observations show that the Ge islands
are single crystal with a pyramidal shape.The average size of Ge
islands was determined to be 10-15nm,considerably smaller than that
produced by other techniques.The Raman spectrum reveals a peak
downward shift,upto 295cm-1, of the Ge-Ge mode caused by phonon
confinement in the Ge-dots. Photoluminescence studies have shown
interesting dependence on the size of Ge nanocrystallite. The
importance of PLD in fabricating novel nanostructures is discussed.
F8.44
STRUCTURE AND
SURFACE MORPHOLOGY OF CdSe NANOCRYSTALS: DEVIATION FROM 1 TO 1
STOICHIOMETRY. Jason M. Taylor, Tadd Kippeny, Vanderbilt University,
Dept. of Chemistry, Nashville, TN; Johnathan C. Bennett, Mengbing
Huang, L.C. Feldman, Vanderbilt University, Dept. of Physics,
Nashville, TN; Sandra J. Rosenthal , Vanderbilt
University, Dept. of Chemistry, Nashville, TN.
CdSe
nanocrystals have served as the prototypical quantum dot system.
Nevertheless, the complete molecular structure of the nanocrystals
has not been precisely determined owing to the difficulty of probing
the nanocrystal surface. We have applied Rutherford Backscattering
(RBS) to this problem. CdSe nanocrystals were prepared in the
standard fashion by the pyrolysis of dimethyl cadmium and selenium
in trioctylphosphine oxide (TOPO).1 The
nanocrystals were carefully washed to remove excess TOPO. For this
study the diameter of the nanocrystals is tuned from 20-32 as
verified by transmission electron microscopy. RBS was performed on
thin films (2-4 monolayers) of TOPO capped nanocrystals dispersed on
graphite substrates. A thick (600nm) CdSe film was used as a
standard for these measurements and indicated the correct 1:1
stoichiometry.
The RBS results indicate the currect picture of wurtzite
crystallinity with 1:1 stoichiometry pertains only to the interior of
the nanocrystal. For all sizes studied thus far we find the
nanocrystals are cadmium rich with an average Cd to Se stoichiometry
of 1.24 0.04 to 1. The sensitivity of the RBS technique also
allows the determination of the TOPO surface coverage. We find an
average coverage of 33%, consistent with previous studies.2 The
magnitude of the deviation in the stoichiometry is consistent with a
cadmium rich surface, however size variation suggests a more complex
picture. A cadmium rich surface could originate from the
stabilization provided to surface Cd atoms by the TOPO passivating
ligands. We are exploring this hypothesis and the possibility of
tuning the surface morphology by preparing the nanocrystals in
trioctylphosphine (TOP), a selenium passivating ligand.3
1. X. Peng, J. Wickham, and A.P. Alivisatos, J. Am. Chem.
Soc. 120, 5343 (1998).
2. J.E.B. Katari, V.L. Colvin, and A.P. Alivisatos, J.
Phys. Chem 98, 4109 (1994).
3. L.R. Becerra, C.B. Murray, R.G. Griffin, and M.G. Bawendi
H. Chem. Phys. 100, 3297 (1994).
F8.45
IMPURITY
STATES IN AN ASYMMETRIC QUANTUM WIRE. Juan Adrian Reyes ,
Marcelo del Castillo-Mussot, Carlos Ignacio Mendoza, Instituto de
Fisica, UNAM, Mexico City, MEXICO.
We calculate
eigenenergies and eigenfunctions of an impurity at the boundary of
two quantum wires. We solve the corresponding one-dimensional
Schroedinger equation with an asymmetric Coulomb potential in the
momentum space.
F8.46
LIGHT EMITTING
DIODES BASED ON CdSe NANOCRYSTALS / POLY(P-PHENYLENE)
HETEROSTRUCTURES. W.K. Woo , M.G. Bawendi, Dept. of
Chemistry, Massachusetts Institute of Technology, Cambridge, MA;
Jason Pinto, Michael F. Rubner, Dept. of Materials Science and
Engineering, Massachusetts Institute of Technology, Cambridge, MA;
Michael Ramey, John R. Reynolds, Dept. of Chemistry, Center for
Macromolecular Science and Engineering, Univ of Florida, Gainesville,
FL.
Light emitting diodes (LEDs) with inorganic
semiconductor CdSe nanocrystals as the active material are
fabricated. Semiconducting polymer [poly(p-phenylene) or PPP] and
conducting polymer [polyaniline or PAN] layers are introduced by
means of a layer-by-layer sequential adsorption technique to
facilitate the hole injection from the ITO to the nanocrystal layer.
By changing the nanocrystal size, the devices can emit from green to
red with low operating voltages. The device efficiency and
electroluminescence (EL) spectrum show dependence on the thickness of
the polymer film and the nanocrystal layer, as well as on the applied
voltage.
F8.47
SOLUTION
SYNTHESIS OF ALKYL-STABILIZED GERMANIUM NANOCRYSTALS FROM THE ZINTL
SALTS SODIUM GERMANIDE AND MAGNESIUM GERMANIDE. Susan M. Kauzlarich,
Boyd R. Taylor , University of California-Davis, Davis,
CA; Howard W. H. Lee, Gildardo R. Delgado, J. Diane Cooke, Lawrence
Livermore National Laboratory, Livermore, CA.
The
reactions of the Zintl salts NaGe and Mg2Ge have been shown to
make Ge nanocrystals of high quality with a narrow size
distribution. The reaction takes place in refluxing glyme solvents
at ambient pressure, and requires no further treatment to produce
crystalline nanoparticles. The nanocrystals are stabilized by
reaction with RLi or RMgX. The alkyl-stabilized nanocrystals form
clear colloidal suspensions in nonpolar solvents for R = methyl,
butyl, or octyl. Nanocrystals stabilized with longer alkyl chains
form better colloidal suspensions. The colloidal suspensions formed
by this method were characterized by Fourier transform infrared
(FTIR) spectroscopy, high-resolution transmission electron microscopy
(HRTEM), ultraviolet-visible (UV-vis) absorption spectroscopy,
size-selected photoluminescence (SSPL) and size-selected
photoluminescence excitation (SSPLE) spectroscopies. FTIR indicates
the presence of alkyl groups on the surface of the Ge nanocrystals.
HRTEM shows the presence of crystalline nanoparticles with the same
structure as bulk Ge. UV-vis spectra show that the nanocrystals
absorb strongly in the blue and UV, at higher energies and
intensities than bulk Ge. SSPL and SSPLE spectra agree with the
predictions of quantum confinement models.
F8.48
FEMTOSECOND
INTERFACIAL ELECTRON DYNAMICS OF SEMICONDUCTOR NANOPARTICLES.
Clemens Burda , Travis C. Green, Stephan Link, Mostafa A.
El-Sayed, Laser Dynamics Laboratory, Georgia Institute of Technology,
Atlanta, GA.
The charge carrier dynamics of
semiconductor nanoparticles were studied by means of fs pump probe
spectroscopy on the electron transfer from the photoexcited
nanoparticles to bound electron acceptors. Our new studies on CdX/EA
system (X = S, Se) show that electron-dynamics is rate determining
for the bleach recovery. In contrast to the CdS/MV2+ system, where
the bleach recovery is due to hole trapping, in the CdSe/EA systems
electron back transfer was found to be responsible for the recovery
of the bleach. This could be confirmed by directly monitoring the
formation and decay of the intermediate radical ions. These results
are discussed in terms of the respective proposed mechanism for the
excited state relaxation.
8:15 AM *F9.1SESSION F9: MICROCRYSTALLINE AND POLYCRYSTALLINE SEMICONDUCTORS
Chairs: David S Ginley and Kazunobu Tanaka
Thursday Morning, December 3, 1998
Salon E (M)
STRUCTURAL AND ELECTRONIC PROPERTIES OF LASER CRYSTALLIZED SILICON FILMS. Toshiyuki Sameshima , Tokyo A&T University, Tokyo, JAPAN.
This paper will discuss electrical and optical properties of pulsed-laser crystallized silicon films, which have been widely investigated for a variety of applications in many devices, for example, polycrystalline silicon thin film transistors (poly-Si TFTs) and thin film solar cells. Pulsed laser heating causes the rapid phase change from amorphous to crystalline state. Crystalline state dominates for laser energy just above the crystalline threshold energy density. And it strongly depends on laser heating conditions of energy densities and pulse numbers. The average grain size and the carrier mobility increase as the laser energy density increases. There is however another threshold whose energy density is much higher than that of the crystalline threshold. Above the second threshold, reductions of the average grain size and the carrier mobility occur. A high carrier mobility (>500 cm2/Vs) has been already demonstrated by poly-Si TFTs fabricated in laser crystallized films. Reduction of the density of defect states and grain boundary properties will be discussed. Moreover, this paper will include post annealing in order to reduce the density of defect states and increase carrier mobility at low temperature.
8:45 AM F9.2
NUCLEATION PROCESSES IN Si CVD ON ULTRATHIN SiO2 LAYERS. Tetsuji Yasuda , Doo-Sup Hwang, Kazuyuki Ikuta, Satoshi Yamasaki, and Kazunobu Tanaka, Joint Research Center for Atom Technology (JRCAT), Tsukuba, JAPAN.
Vapor-phase preparation of microcrystals and microcrystalline films commonly involves nucleation processes on substrate surfaces. For instance, Si CVD on SiO2 begins with formation of Si nuclei followed by island growth. Nucleation centers postulated in such a case are defects, impurities, strained bonds, and so on. However, identity of the nucleation sites is hardly clear. We investigate nucleation densities in Si CVD on SiO2-covered Si wafers, focusing on the effects of the oxide-preparation methods. Different oxidation methods result in different defect densities and network structures of the oxide layers. Oxide dependence of the nucleation processes therefore provides insights into the identity of the nucleation centers. Ultrathin SiO2 layers (0.4-2 nm) were prepared by oxidizing H-terminated or reconstructed Si surfaces. Oxidation methods employed were thermal oxidation at 775
C, and UV-ozone and plasma
oxidation both at room temperature. The as-oxidized samples were
transferred in vacuo to a Si UHV-CVD chamber, which ensures that the
nucleation processes were not affected by surface contamination. Si
deposition was carried out using Si2H6 at 580C.
Our experiments changing the Si2H6 pressure showed that the
SiO2 surfaces indeed have a finite number of preferred
nucleation sites per unit area. Among the three oxidation methods
investigated, lowest nucleation density (1 109
cm-2) is obtained on the thermal oxide. Nucleation density on
the UV-O3 oxide is about twice as high as that on thermal oxide.
Plasma oxide shows nucleation density of 5 109
cm-2 which is independent of both the surface orientation and
termination of the initial Si surfaces. These results suggest that
strained bonds and ion damages in the oxide layers, at least partly,
account for the observed nucleation density. This study, partly
supported by NEDO, was carried out at JRCAT under the joint research
agreement between NAIR and ATP.
9:00 AM F9.3
NANOMETER-SCALE Si-SELECTIVE EPITAXIAL GROWTH USING ULTRATHIN SiO2 MASK. Noriyuki Miyata , Heiji Watanabe, and Masakazu Ichikawa, Joint Research Center for Atom Technology, Angstrom Technology Partnership (JRCAT-ATP), Ibaraki, JAPAN.
We performed Si-selective epitaxial growth (Si-SEG) on Si windows in an ultrathin SiO2 mask formed by electron-beam-induced selective thermal decomposition (EB-STD)[1] to fabricate Si nano wires (<20 nm in width). The SiO2 mask layer, about 0.3 nm thick, was grown on a clean Si(001)-2
1
surface under Torr O2 at 650C. A
clean 15-nm-wide Si(001) 21 linear window was formed in the
SiO2 layer by a 30-keV focused electron beam (EB) irradiation and
successive SiO2 thermal decomposition at the EB irradiated areas
under ultrahigh vacuum. Si-SEG on the windows was performed using
molecular beam epitaxy with a Si2H2 gas source at
650C. Scanning reflection electron microscopy, scanning
electron microscopy (SEM), and micro-probe reflection high-energy
electron diffraction (RHEED) were used to examine the Si window and
the Si-SEG layer. The six-layer Si growth (one layer = 0.136 nm)
was performed on a 15-nm-wide Si window. Under these growth
conditions, Auger electron spectroscopy confirmed that the Si was
not deposited on the oxide layer. The 21 RHEED pattern was
observed on the Si window even after the Si-SEG. SEM images showed
that the Si-SEG layer grown on the window was thicker than the
SiO2 layer. Our results demonstrate that Si-SEG is possible on a
10-nm-scale Si window formed by EB-STD. [This work, partly
supported by NEDO, was performed at JRCAT under the joint research
agreement between NAIR and ATP.] [1] S. Fujita, S. Maruno, H.
Watanabe, and M. Ichikawa, Appl. Phys. Lett. 69, 638
(1996).
9:15 AM F9.4
GaAs MICRO CRYSTAL GROWTH ON As-TERMINATED Si SURFACE BY LOW ENERGY FOCUSED ION BEAM. Toyohiro Chikyow and Nobuyuki Koguchi, National Research Institute for Metals, Tsukuba, Ibaraki, JAPAN.
GaAs micro crystals were grown periodically on As-terminated Si (001) surface by sequential supply of Ga and As molecular beam, followed by low energy focused ion beam irradiation. Recently periodical micro crystal growth on semiconductor surfaces has been attempted for practical device applications, such as quantum dot array structures or photonic crystals. We have been proposing a sophisticated fabrication method, ``Droplet Epitaxy'', where GaAs micro crystals were grown from Ga droplets without damages caused by ion etching, and have been reporting successful results. In order to apply this method to fabricate array structures, initially periodical nucleation sites are formed on a chemically stable As-terminated Si (001) surface by low energy focused ion beam. Subsequently Ga molecule are supplied to the surface to form Ga droplets on the sites. Finally As molecules are supplied to grow GaAs micro crystals. As-termination was carried out on a n-type Si (001) surface by direct current heating in As molecule atmosphere in a MBE chamber. A focused Ga ion beams in 100 eV was irradiated to the surface in another UHV chamber to remove As layers periodically Ga and As molecules were applied to the surface sequentially at 400 C by MBE. On the surface periodical Ga droplets 200nm in size were observed. Ga atoms were thought to migrate on the stable As-terminated Si surface and be trapped at the Si-bared region caused by ion beam. By As molecule supply, GaAs micro crystals were found to grow from the Ga droplets. From the obtained results, arrays of various size GaAs micro crystal was found to be potentially possible on the As-terminated Si (001) surface, in a combination with low energy focused ion beam system and MBE.
9:30 AM F9.5
CONTROLLED NUCLEATION OF MICROCRYSTALLINE SILICON FILMS ON AMORPHOUS SILICON FOR APPLICATIONS IN a-Si:H SOLAR CELLS. Joohyun Koh, Yeeheng Lee, C.R. Wronski, R.W. Collins , Penn State University, Center for Thin Films Devices, Unversity Park, PA; H. Fujiwara, Electrotechnical Laboratory, Tsukuba City, Ibaraki-ken, JAPAN.
We have exploited the high sensitivity of real time spectroscopic ellipsometry (RTSE) to study delayed and immediate nucleation of microcrystalline silicon films (
c-Si:H) on amorphous silicon
(a-Si:H) surfaces. The c-Si:H is prepared by rf plasma-enhanced
chemical vapor deposition from SiH4 at different H2 dilution
levels, R=[H2]/[SiH4]. For depositions with 15R80 on
untreated a-Si:H surfaces, initial film growth occurs in the
amorphous phase. Upon continued growth, however, a thickness onset
dc for crystallite formation is found that decreases with
increasing R. For example, delayed microcrystal nucleation during
film growth occurs near dc1000 for R=15 and
dc100 for R=50. In a-Si:H solar cell applications,
two thicknesses are of interest (i) 200 , which is the
thickness of both doped layers and Voc-enhancing p/i interface
layers, and (ii) 0.5 m, which is the thickness; of the
bulk i-layers. We find that the optimum stabilized a-Si:H p-i-n solar
cell performance is obtained in an i-layer growth process that use;
the maximum possible R while avoiding the transition to
microcrystallinity versus i-layer thickness. Using this strategy,
RTSE has been applied successfully for solar cell optimization. For a
single step i-layer, R=10 provides the best cell stabilized
performance as it avoids the transition to crystallinity in 0.5
m thick layers. For a two step i-layer, an initial 200
with R=40 that avoids the transition in thin layers, followed by a
bulk layer with R=10, lead to a significant improvement in
performance over the single step i-layer. Finally, we find that
additions of B3(CH3)3 or BF3 doping gases to the
H2/SiH4 mixture increase dc. For example, addition of 1%
B(CH3)3 to the gas phase increases dc from <50 to
>200 for R=200. Immediate nucleation of p-type single-phase
c-Si:H for use in n-i-p solar cells can be obtained only if the
initial a-Si:H i-layer is exposed to an atomic H pretreatment that
generates nanocrystal seeds on the i-layer surface.
10:15 AM F9.6
PREPARATION AND CHARACTERIZATION OF MICROCRYSTALLINE SILICON/CRYSTALLINE SILICON SOLAR CELLS. Klaus Lips , Lothar Elstner, Walther Fuhs, Stefan Gall, Ngo Duong Sinh, Hahn-Meitner-Institut, Abt. Photovoltaik, Berlin, GERMANY.
We have deposited thin boron and phosphorous doped microcrystalline silicon (
c-Si) layers by
electron cyclotron resonance chemical vapor deposition (ECRCVD, 2.45
GHz, 1000 W) on crystalline silicon wafers and on quartz substrates
at T=325 C. The c-Si layers were characterized by
Raman spectroscopy, reflectivity, Hall effect, ESR, and SEM. The thin
films have crystalline volume fractions of 65-75% and a conductivity
varying from = 10-100 (cm)-1. The solar
cells were prepared on polished (100) orientated p- and n-type CZ
wafers with diffused back surface field. The c-Si emitter layer
thickness was varied between 70nm and 160nm. A mesa etching was
performed to define the area of the cell. Aluminum was used for the
front and back contact metallization and a single anti-reflex coating
of silicon nitride was finally deposited on the top surface. The
cells were characterized by spectral response and by IV and
frequency-modulated capacitance voltage (MCV) measurements in the
temperature range 120 K < T < 400 K. The maximum AM1.5
efficiencies are above 14% for 0.16 cm2 cells and above 13%
for
4 cm2 cells. However, we find that the short circuit current
decreases with decreasing temperature depending on the electronic
properties of the emitter layer. This is not observed for cells with
diffused emitters. We will discuss to what extend this behavior is
linked with the transport properties of the c-Si emitter and/or
to recombination at the interface.
10:30 AM F9.7
PROBING THE ELEMENTARY SURFACE REACTIONS OF HYDROGENATED SILICON PECVD BY IN-SITU ESR. Satoshi Yamasaki , Claus Malten, Takehide Umeda, Junich Isoya, and Kazunobu Tanaka, JRCAT, Tsukuba, Ibaraki, JAPAN.
Microcrystalline silicon can be grown by the PECVD (plasma enhanced chemical vapor deposition) of high hydrogen dilution silane plasma and hydrogen plasma treatment. Atomic hydrogen is believed to play a crucial role in the surface reactions. We use in-situ ESR (electron-spin-resonance) to show the nature of surface reactions during hydrogen and Ar plasma treatments of hydrogenated silicon films using a remote plasma. Dynamic changes of the Si dangling-bond signal intensity were observed during and after the plasma treatment. Hydrogen has two main roles; the saturation of Si dangling bonds to form Si-H bonds and the abstraction of H from Si-H bonds to form Si dangling bonds. We show the existence of a surface region with a surprisingly high spin density of the order of 1013 cm-2 only during the hydrogen plasma treatments. This is one order of magnitude larger than the surface dangling bond density after the plasma is turned off. This means that the main role of hydrogen atoms is to create dangling bonds by breaking Si-Si and/or Si-H bonds rather than termination of dangling bonds to form Si-H bonds. To clarify the hydrogen reactivity in detail, in-situ ESR measurements of Ar plasma treatments were also performed. Although Ar plasma creates a high spin density region like the hydrogen plasma treatment, the decay after turning off the plasma is much faster than for the hydrogen plasma. This clearly points out the difference in the surface microchemical reactivity between the hydrogen atoms and excited-states Ar atoms.
10:45 AM F9.8
Abstract Withdrawn.
11:00 AM F9.9
EVAPORATED POLYCRYSTALLINE GERMANIUM FOR NEAR INFRARED PHOTODETECTION. L. Colace , G. Masini, F. Galluzzi, G. Assanto, Department of Electronic Engineering, Terza University of Rome, Rome, ITALY.
In the last two decades much effort has been devoted to the fabrication of thin epitaxial Ge and SiGe films on silicon substrates for potential applications of group IV band gap engineering in the near infrared. Amorphous hydrogenated Ge films have been investigated as well in thin film transistors and multijunction thin film solar cells. The low cost of the fabrication process and its low thermal budget is suitable for high quality layers on low cost substrates and for large area production. However the bandgap of about 1.0 eV is not suitable for near infrared photodetection. In this work we present a low cost near infrared photodetector based on a polycrystalline Ge film thermally evaporated on a silicon substrate. The device operation extends down to 1.5micron. In this contribution, we demonstrate that, by proper choice of deposition conditions and device structure, a responsivity of 16mA/W and a response speed of a few nanoseconds can be achieved at the wavelength of 1.3micron. In addition, we describe the operation of a 16 pixel linear detector array, with dot pitch of about 100 micron. We will report on the complete optoelectronic characterization of the fabricated devices, including current-voltage characteristics, photocurrent spectra and speed response. The results demonstrate that the proposed approach is promising for the fabrication of 1.3 -1.55 micron near infrared photodetectors integrated on silicon chips.
11:15 AM F9.10
MIRRORLESS UV LASERS IN ZnO POLYCRYSTALLINE FILMS AND POWDER. Hui Cao , H.C. Ong, J.Y. Dai, Y.G. Zhao, X. Lui, J.Y. Wu and R.P.H. Chang, Materials Research Center, Northwestern University, Evanston, IL.
We report the first observation of UV lasing ZnO powder and ZnO polycrystalline films grown on amorphous fused silica substrates. In the absence of any fabricated mirrors, laser action occurs in the closed loops formed by multiple optical scattering of light. High-quality ZnO-Zn phosphor powder was deposited onto ITO coated glass substrates using the electrophoretic process. The average particle diameter is l00 nm The scattering mean free path is characterized to be close to the emission wavelength (385 nm). Thus closed loop paths for light could be formed by multiple scattering of light. The samples were optically pumped to a frequency-tripled mode-locked Nd:YAG laser (355 nm, l0 Hz repetition rate, l5 ps pulse width). The pump beam was focused to a spot or a stripe about 50 to l00 microns on the powder surface. When the pump power exceeds a threshold, suddenly sharp peaks emerge in the emission spectra. As the pump power increases further, more sharp peaks appear. Above the threshold the total emission intensity increases much more rapidly with the pump power. This indicates that laser action has occurred in the ZnO powder. We have observed similar lasing behavior in ZnO polycrystalline films deposited on amorphous fused silica substrates by laser ablation. TEM images illustrate the polycrystalline grain structure of the ZnO films. The grains appear in irregular shapes, and their sizes vary from 50 nm to 150 nm. The coherent back scattering measurement shows that the ZnO films are strong scattering media. We have imaged the excitation area using a microscope objective and UV sensitive CCD camera. Above the lasing threshold, we observed the random ring cavities in the films. This gives a direct proof that ``mirrorless lasers'' have occurred in these highly disordered films.
11:30 AM F9.11 ELECTRONIC PROPERTIES OF INDIVIDUAL GRAIN BOUNDARIES IN ZnO AND SrTiO3 BICRYSTALS AND POLYCRYSTALS. Bryan D. Huey , Dawn A. Bonnell, University of Pennsylvania, Dept. of Materials Science and Engineering, Philadelphia, PA.
An understanding of the effects of nanometer scale defects is becoming increasingly important as critical dimensions shrink for semiconducting applications. These effects are especially relevant for hybrid device structures that incorporate semiconducting oxides, usually with low carrier concentrations. In this study, an atomic force microscope (AFM) is used for surface potential mapping in the vicinity of interfaces while in-situ lateral biases are applied through microfabricated metal contacts. Accordingly, the electronic properties of individual grain boundaries have been studied for semiconducting ZnO polycrystals. The results indicate a variation in grain-boundary potential barriers as a function of local chemical content. For SrTiO3 bicrystals, this variation has been observed to exist at the nanoscale. The voltage dependence of grain boundary electronic properties has been directly measured as well, allowing a comparison of the local and macroscopic interface density of states (obtained from the voltage dependence of both current and capacitance). Surface potential scanning probe microscopy has thus been utilized to observe electronic properties previously inaccessible on such a small scale.