Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session V26: Light Emission and Luminescence from Silicon and Theory and Simulations |
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Sponsoring Units: DCOMP Chair: Philip Collins, University of California-Irvine Room: LACC 501B |
Thursday, March 24, 2005 11:15AM - 11:27AM |
V26.00001: Silicon quantum dots LED with external quantum efficiency of 2\% Gun Yong Sung, Kwan Sik Cho, Nae-Man Park, Tae-Youb Kim, Kyung-Hyun Kim There has been much effort to solve the inability of silicon to act as a light emitting source such as porous silicon,$^{ }$Er doped silicon, and silicon nanocrystals(nc-Si). Among these, nc-Si dispersed in SiO$_{2}$ matrix has attracted a great interest because their band gap is enlarged in comparison with bulk silicon due to quantum confinement effects. Previously, we reported that red to blue PL were observed from amorphous silicon quantum dots in silicon nitride matrix.[1,2] Therefore nc-Si in silicon nitride matrix supplies the possibility of Si-based full-color emission. We have fabricated LEDs with a transparent doping layer on nc-Si embedded in silicon nitride matrix formed by plasma-enhanced chemical vapor deposition. Under forward biased condition, orange EL with its peak wavelength at about 600nm was observed at room-temperature. The peak position of the EL is very similar to that of the PL and the emitted EL intensity is proportional to the current density passing through the device. We suggest that the observed EL is originated from electron-hole pair recombination in nc-Si. By using ITO and n-type wide bandgap semiconducting layer combination as a transparent doping layer, we obtained high external quantum efficiency greater than 2.0{\%}, which is the highest value ever reported in nc-Si based LED. [1] Nae-Man Park et al., Phys. Rev. Lett. 86, 1355 (2001) [2] Tae-Youb Kim et al., Appl. Phys. Lett. (2004), in press. [Preview Abstract] |
Thursday, March 24, 2005 11:27AM - 11:39AM |
V26.00002: Silicon Nanocrystal Field Effect Light Emitting Device (FELED) Harry Atwater, Robb Walters, George Bourianoff We have fabricated novel light emitting devices termed ``Field Effect Light Emitting Devices'' (FELEDs) incorporating silicon nanocrystals as the active optical medium. The devices consist of MOS transistors with an embedded floating gate comprised of $\sim $5E12 nanocrystals/cm$^2$ (2-4nm diameter) isolated from the channel by an $\sim $4nm tunnel oxide and isolated from the gate contact by an $\sim $8nm thick control oxide layer. The gate contact is designed to be optically transparent at the emission wavelength of the nanocrystals ($\sim $780nm). In contrast to traditional LEDs in which charges are driven into an active region by a constant current, the charges in this device are injected sequentially into the silicon nanocrystal array from the channel of the transistor by an alternating gate potential. Excitons are thus formed only at bias transitions. Time resolved electroluminescence measurements show rise times of order 100nsec and EL decay times of $\sim $30$\mu$sec for abrupt changes in gate bias (+/-6 Volts). These measurements are consistent with a charge injection model based on Coulomb-field enhanced Fowler-Nordheim tunneling. We will discuss the performance of our prototype devices, which is limited by a number of non-idealities, and comment on the prospects for efficient light emission in optimized FELED structures. [Preview Abstract] |
Thursday, March 24, 2005 11:39AM - 11:51AM |
V26.00003: Photoluminescence from silicon nanocrystals embedded in Silicon oxides Wei Pan, M.S. Carroll, R.G. Dunn, J.C. Banks, T.J. Headley Much progress has been made in highly luminescent silicon nanocrystal (SNCs). Effects of nanocrystal size and density, as well as defect passivation, are known to strongly affect the SNC optical properties. Yet their mechanisms are not completely understood. Here, we present results from photoluminescence (PL) studies. A broad PL peak, centered at $\sim 1.3$ eV and blue-shifted from the Si substrate peak at $\sim 1.1$ eV, is observed. It is attributed to emission from SNCs of diameter $\sim 5$ nm. Annealing specimen in a forming gas at 410 $^ {\circ}$C is found to change the PL intensity. Results from temperature dependent and magneto-PL studies will also be discussed.\\ The SNCs are formed by first depositing silicon rich oxides (SRO) on a [100] Si wafer, using high density plasma chemical vapor deposition. The as-deposited SRO films are then annealed in a nitrogen ambient at higher temperatures, which phase segregates the extra Si into SNCs in the oxide. The silicon content in the films was determined using Rutherford Backscattering (RBS) and nanocrystal formation was confirmed using transmission electron microscopy (TEM).\\ Sandia National Labs is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin company, for the United States Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 24, 2005 11:51AM - 12:03PM |
V26.00004: Luminescence properties of single crystalline Si quantum wells prepared by dry oxidation method and SOI wafer Young-Kyu Hong, Jae Ho Bahng, Nam Woong Song, Ja-Yong Koo We have studied on optical characteristics of single crystalline Si quantum wells fabricated from silicon on insulator (SOI) wafers. Initially 110 nm thick Si layer was reduced down to 2 nm and sandwiched between SiO$_{2}$ layers by using dry oxidation method. Thickness variation of Si quantum well was examined by transmission electron microscope (TEM). Luminescence property of these Si quantum wells was investigated by photoluminescence measurement equipped with confocal microscope and excitation source of 325 nm. Not only the thickness of Si layer, influence of SiO$_{2}$ layer thickness on luminescence property was investigated too. [Preview Abstract] |
Thursday, March 24, 2005 12:03PM - 12:15PM |
V26.00005: Si/Si:Er Multi-Nanolayers for Silicon Photonics Nguyen Quang Vinh, Tom Gregorkiewicz Si/Si:Er multi-nanolayer structures grown by sublimation MBE technique exhibit unusual optical properties which make them very interesting for photonic applications. In particular, our recent investigations have proven that a particular type of an Er-related optically active center is preferentially formed in this material. The microscopic structure of this center (labelled Er-1) is characterized by high symmetry type (orthorhombic) and comprises a single Er3+ ion most and multiple oxygen ligands, most likely in its direct surrounding. Consequently, emission from that center does not suffer from inhomogeneous broadening typical for Si:Er materials prepared by ion implantation, and the spectrum at low temperature is characterized by an ultra-narrow linewidth of $\Delta \leq$ 10 $\mu$eV. This makes Si/Si:Er multi-nanolayers attractive for Si photonics. In the contribution, we will review properties of Er- related optical centers formed in the multi-nanolayer structures as revealed by high-resolution, time-resolved photoluminescence, excitation and magneto-optical spectroscopies. In particular, the following issues will be addressed: - Details of microscopic structure of the Er-1 center. - Excitation cross-section and its variation upon excitation wavelength (band-to-band vs. subbandgap pumping) and mode (cw vs. pulsed). - The percentage of optically active Er-related centers in comparison to the total concentration of Er atoms. - Fast components appearing in the decay kinetics at high pumping rate and Auger quenching. Based on the experimental findings, potential of Si/Si:Er nanolayers for silicon photonics will be discussed. [1] N. Q. Vinh et al Phys. Rev. Lett. 90, 066401 (2003) [2] N. Q. Vinh et al Phys. Rev. B 70, 115332 (2004). [Preview Abstract] |
Thursday, March 24, 2005 12:15PM - 12:27PM |
V26.00006: Site-Selective Spectroscopy of Er-ions in Si/SiO2 films: SiO2 vs Si-nanocrystals Z. Fleischman, V. Dierolf, C. Sandmann, M White, Y. Zhao, J. Michel, M.A. Stolfi, L. Dal Negro Using site-selective combined excitation-emission spectroscopy, we have investigated various Si/SiO$_{2}$ films that have been doped with Er$^{3+}$ ions. In these films, the Er ions incorporate into many different lattice environments (sites) especially when Si-nanocrystals are present as well. This makes site-specific statements about important quantities, such as emission lifetimes and excitation efficiencies, very difficult. To circumvent this problem, we applied a site-selective excitation scheme in which the ions are excited in two steps using one or two laser sources capable of exciting two subsequent transitions. Applying this scheme under systematic variation of excitation wavelengths (around 1530nm) for the transitions from the $^{4}$I$_{15/2}$ ground state to the $^{4}$I$_{19/2}$ excited state via the $^{4}$I$_{13/2}$ state and detecting the emission (at 980 nm) from $^{4}$I$_{11/2}$ to $^{4}$I$_{15/2}$ leads to a significant line narrowing and a much clearer distinction of the emission features. This gives us the possibility to excite specific sites and study their properties individually. Comparing these results with photoluminescence measured under 488nm excitation, in which the Er-ions are excited through the excitation of the nanocrystals, allows a clear identification of sites that are related to nanocrystals and helps to identify those Er sites that are most effective in electrical excitation. [Preview Abstract] |
Thursday, March 24, 2005 12:27PM - 12:39PM |
V26.00007: Photo-induced structural transformations and non-linear photoluminescence in nanocrystalline silicon – silicon dioxide superlattices Boris Kamenev, Leonid Tsybeskov, Haim Grebel Recent reports on optical gain in Si nanocrystals have stimulated attention to linear and non-linear optical properties of these nanostructures. Usually, these measurements are performed using a nanosecond pulse of UV light with energy density up to 1J/cm$^{2}$. In this work, we study Si nanocrystal photo-induced heating by measuring Stokes and anti-Stokes components in Raman scattering and calculate sample thermal conductivity. The estimated threshold for the photo-induced structural transformations (i.e., Si nanocrystal melting) is calculated and measured to be in the range of $\sim $ 10 mJ/cm$^{2}$. We observe at least two types of photo-induced structural modifications in Si nanocrystals (e.g., nanocrystal merge and nanocrystal amorphization) and discuss their impact on the sample light-emitting properties. [Preview Abstract] |
Thursday, March 24, 2005 12:39PM - 12:51PM |
V26.00008: Growth and optical properties of silicon nanowires grown by vapor phase epitaxy Sun Gon Jun, Mark S. Miller, Justin Jackson We report on the growth and properties of silicon nanowires. The nanowhiskers were grown by vapor phase epitaxy (VPE) on Si (111) and (100) surfaces using gold as a catalyst, relying upon a vapor-liquid- solid mechanism. Most of the results we present here are for atmospheric pressure growth using either silicon tetrachloride or dilute silane in hydrogen, varying the temperature, concentration, and flow rate. The Au catalysts were created either by depositing thin Au layers of on Si, which break up into nano-scale droplets upon heating, or by electron beam lithography. The size and shape of these droplets play a large role in determining the resulting nanowire morphology. The silicon nanowires, observed by transmission electron microscopy and scanning electron microscopy, exhibit growth defects that include bending and kinking. The wire sizes ranged from 20 to 300 nm, with lengths from 100 nm to 20 $\mu$m, depending on catalyst size and growth conditions. TEM lattice images show the wire growth direction depends on wire size, with smaller wires growing in the [110] direction. Increasing the temperature leads to a broader distribution of wire widths and a faster growth rate. Optical properties include photoluminescence spectra, which show a strong peak near and below the silicon band edge, which may be explained by the axial strain. [Preview Abstract] |
Thursday, March 24, 2005 12:51PM - 1:03PM |
V26.00009: Parametric Study of the Thermal Conductivity in Binary Solids John Lyver, Estela Blaisten-Barojas Simulated binary systems of atoms interacting through Lennard-Jones potentials have been studied to determine the thermal conductivity of solid samples as a function of variable relative concentration, mass ratio, hard-core atomic diameter, and attractive well depth. In this study, we use an isobaric Monte Carlo simulation to prepare a simulated sample at near-zero pressure and then use an isoenergy Molecular Dynamics simulation to reach equilibrium. Once at equilibrium, the dynamic Green-Kubo approach is taken to calculate the heat current time-dependent autocorrelation function and determine the thermal conductivity of the sample. Our results indicate that the thermal conductivity has a strong dependence on the investigated relative parameters and relative concentrations of the two species. Discrepancies with the inverse temperature dependence at low temperatures have shown to be significant. [Preview Abstract] |
Thursday, March 24, 2005 1:03PM - 1:15PM |
V26.00010: The Effect of Solvent and Precursor Functionalization on the Fractal Dimension and Physical Properties of Sol-Gel derived Silica Sudin Bhattacharya, John Kieffer We have used molecular dynamics simulations based on a recently developed three-body charge-transfer potential to simulate sol-gel condensation reactions that produce nano-porous silica gel structures. The effect of solvent molecules on the fractal dimension of the gels prior to, and after, supercritical drying is investigated. With pure silicic acid monomer precursors, the sol-gel aggregation process produces fractal 3-D network structures. However, with silicic acid dimers and dimers functionalized with cyclohexyl groups, closed cage-like structures are observed prior to further polymerization into a network. We examine how this structural variation affects the fractal dimension and structure-property relationships of the system. [Preview Abstract] |
Thursday, March 24, 2005 1:15PM - 1:27PM |
V26.00011: Simulation of the liquid crystalline behavior of cuboidal colloidal particles Bettina John, Abraham Stroock, Fernando Escobedo The effect of the shape of colloidal particles on their liquid- crystalline bulk-phase behavior was investigated in this work. The particles considered have a cuboidal shape with axes lengths a = b $\ne $ c and experience purely repulsive (hard-core) interactions. These particles exhibit marked self-assembly properties because of their flat facets and square edges. Monte Carlo simulations at constant osmotic pressure were used to map out the lyotropic phase diagram of such hard particles. For cubical particles (i.e., with c=a), a liquid-crystalline phase known as cubatic phase forms at intermediate concentrations. Such a mesophase exhibits orientational ordering along three axes (cubatic order) but significant translational disorder. The isotropic-to-cubatic phase transition was found to be first order and be driven by the interplay of orientational and packing entropy. The onset and stability of the cubatic phase was little affected by the roughness of the cuboid surfaces and by moderate size polydispersity. The phase diagrams were also mapped out for cuboidal particles with aspects ratios ranging from c:a=1:3 to c:a=8:1. As expected, the phase behavior of long cuboidal rods approaches that of spherocylinders of similar aspect ratios; e.g., nematic and smectic phases were observed for c:a=8:1 while only the smectic phase is observed for c:a=5:1. The cubatic phase was prominent at c:a in the range of 1 to 4. [Preview Abstract] |
Thursday, March 24, 2005 1:27PM - 1:39PM |
V26.00012: Theoretical and Computational Studies of Three-dimensional Phase Separation K.G. Wang, M.E. Glicksman, K. Rajan The diffusive interactions occurring within a population of precipitates dispersed throughout a contiguous matrix may be described on the basis of a diffusion screening length. Screening theory predicts as functions of the dispersoid volume fraction the changes in diffusion screening length, maximum particle size, coarsening rate, and the scaled particle-size distribution. Furthermore, by considering fluctuations observed in the growth rates of individual particles, we report on developments of a stochastic theory of phase separation. Also, particle-size distributions and the maximum particle radii predicted as a function of time from theory and simulations are shown to agree well with experimental results obtained from measurements performed on Al$_{3}$Li precipitates in binary Al-Li alloys. Lastly, we calculated the spatial correlation function of these microstructures. We revealed through data analysis the relationship between the critical length scale for diffusion-mediated coarsening and spatial correlations in the microstructure. [Preview Abstract] |
Thursday, March 24, 2005 1:39PM - 1:51PM |
V26.00013: Orbital ordering and volume collapse in cerium G\"{o}khan Esirgen, Andrew K. McMahan, Richard T. Scalettar Realistic all-orbital many-body calculations are performed for cerium, covering a large volume range. $16$-orbital cerium model Hamiltonians are obtained from the local-density approximation and solved using the fluctuation-exchange approximation. It is found that the thermodynamic behavior of cerium is coupled to orbital-ordering transitions of its $f$ electrons. Different arrangements of $f$ electrons occur for the $\alpha$- and $\gamma$-cerium; moreover, metastable solutions also exist. In addition magnetic and spectral properties are analyzed. [Preview Abstract] |
Thursday, March 24, 2005 1:51PM - 2:03PM |
V26.00014: Interpreting magnetic resonance experiments from first principles Chris Pickard, Jonathan Yates, Si\^an Joyce, Francesco Mauri A method for predicting the results of magnetic resonance (NMR/EPR) experiments will be presented. It is based on the plane wave pseudopotential method, within density functional theory. Using a modified (gauge including) version of the projector augmented wave method, all-electron accuracy can be achieved. I will describe how the method is being adopted by the experimental community to assist in the extraction of the maximum amount of information from their magnetic resonance experiments. The range of applicability of the method will be emphasized, by reference to successful collaborative applications: porphyrin molecules, boron carbides, complex zeolite silicates, zircon, sodium and calcium silicate glasses and hydrogen bonded molecular crystals. [Preview Abstract] |
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V26.00015: A new method to fitting the I-V characteristics of field emission arrays Ming-Chieh Lin In recent years, the wide applications of vacuum electronic devices call for a well development of field emission cathodes. Field emission arrays (FEAs) are a good candidate for the use as a field emitter. The emission property of an emitter is characterized by the I-V curves. The field emission could be described by the well-known Fowler-Nordheim (FN) equation. The FN plots are widely employed to fit the experimental data. With the advancement in the fabrication technology, the emission current density of a FEA achieves even higher and higher values than ever before. In recent experiments of FEAs, the FN plots show that the results are apart from the FN fits. In this work, we propose a new method to fitting the I-V characteristics of the FEAs that may be operated at high current density. The FN equation is modified with an effective work function. The effective work function characterizes the surface properties of the field emission array. A self-consistent FN equation including the space charge effects of the field emission electrons is demonstrated to be a good fit to the I-V characteristics of FEAs. The theoretical results show good agreement in comparisons with some experimental results. [Preview Abstract] |
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