We report a phenomenological model to analyze the room temperature photoluminescence (PL) spectra observed from silicon nanostructures. We have explicitly incorporated the effects of localized surface states along with quantum confinement effects to obtain an analytical expression for the photoluminescence spectra. Normal as well as log-normal crystallite size distributions are considered for PL intensity calculations. Experimental PL data on a variety of nanocystalline silicon structures with directly measured crystallite size distribution have been analyzed. Our model is able to deduce size distribution parameters from PL data that agree well with the experiments.

Influence of surface states on the photoluminescence from silicon nanostructures

We use a molecular-dynamics simulation within density-functional theory to prepare realistic structures of hydrogenated amorphous silicon. The procedure consists of heating a crystalline structure of Si64H8 to 2370 K, creating a liquid and subsequently cooling it down to room temperature. The effect of the cooling rate is examined. We prepared a total of five structures which compare well to experimental data obtained by neutron-scattering experiments. Two structures do not contain any structural nor electronic defects. The other samples contain a small number of defects which are identified as dangling and floating bonds. Calculations on a bigger sample (Si216H27) show similar properties (radial distribution functions, band gap, and tail states) compared to the Si64H8 sample. Finally the vibrational density of states is calculated and compared to inelastic neutron-scattering measurements.

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First-principles study of hydrogenated amorphous silicon

A nonorthogonal tight-binding model has been developed for the system containing Si and H, where the energy functional included the contributions of both electronic and pairwise interaction between the atoms. In order to calculate the ground state structures of various clusters, energy minimization was carried out using Differential Evolution: a very recently developed biologically inspired computing technique, belonging, in general, to the family of Genetic Algorithms (GAs), but having a number of advantages over its conventional forms.

Tight-Binding Calculations of Si-H Clusters Using Genetic Algorithms and Related Techniques: Studies Using Differential Evolution

In order to investigate the sample dependence of various properties of hydrogenated amorphous silicon (a-Si:H), we have generated four samples with 216 silicon atoms and 24 hydrogen atoms using the density functional based tight binding molecular dynamics simulations. The overall structural properties of these model samples are in agreement with the previous theoretical and experimental results. While the Si-Si and Si-H pair correlation functions are independent of preparation procedure as well as initial conditions, the H-H pair correlation functions are sample dependent. The distribution of hydrogen atoms in all the samples is nonuniform and depends upon the preparation procedure as well as the initial structure from which the hydrogenated amorphous silicon sample is generated. The Si-Si bond length and Si-Si-Si bond angle distributions are nearly independent of sample preparation procedure, but Si-H bond length distributions are sample dependent. The peaks in the vibrational density of states (VDOS) at high frequencies, which are due to the Si-H bond vibrations, are in reasonable accord with the experimental results. The positions of the high frequency peaks are found to be dependent on the local environment which changes from one sample to another. While the high frequency vibrational modes related to Si-Si bond vibrations are moderately localized, the vibrational modes related to Si-H bond vibrations in a-Si:H samples are highly localized. In samples generated

Sample dependence of the structural, vibrational, and electronic properties of a − Si : H : A density-functional-based tight-binding study

The molecular structures, electron affinities, and dissociation energies of the Si 3 H n /S1 3 H n - (n ≤ 8) species have been examined using five hybrid and pure density functional theory (DFT) methods. The basis set used inthis work is of double-ζ plus polarization quality with additional diffuse s- and p-type functions, denoted DZP++. These methods have been carefully calibrated (Chem. Rev. 2002, 102, 231). The geometries are fully optimized with each DFT method independently. Three different types of the neutral-anion energy separations presented in this work are the adiabatic electron affinity (EA a d ), the vertical electron affinity (EA v e r t ), and the vertical detachment energy (VDE). The first Si-H dissociation energies D e (Si 3 H n →Si 3 H n - 1 +H) for the neutral Si 3 H n and D e (Si 3 H n - →Si 3 H n - 1 - +H) for the anionic Si 3 H n - species have also been reported. In the prediction of bond lengths, the BHLYP predicts the most reliable Si-Si bond lengths and the B3LYP predicts the most reliable Si-H bond lengths. The most reliable EA a d , obtained at the B3LYP and BPW91 levels of theory, are 2.34 or 2.32 eV (Si 3 ), 2.56 eV (Si 3 H), 1.73 or 1.74 eV (Si 3 H 2 ), 2.46 or 2.45 eV (Si 3 H 3 ), 1.95 or 1.93 eV (Si 3 H 4 ), 2.24 or 2.23 eV (Si 3 H 5 ), 1.41 or 1.30 eV (Si 3 H 6 ), and 2.14 eV (Si 3 H 7 ). For Si 3 H 8 , there are no reliable EA a d but there are reliable VDE. The values of VDE for Si 3 H 8 are 1.03 eV (B3LYP) or 1.10 eV (BPW91). The first dissociation energies (Si 3 H n →Si 3 H n - 1 +H) predicted by all of these methods are 2.65∼2.74 eV (Si 3 H), 2.82∼3.12 eV (Si 3 H 2 ), 2.13∼2.23 eV (Si 3 H 3 ), 2.92∼3.08 eV (Si 3 H 4 ), 2.63∼2.95 eV (Si 3 H 5 ), 3.29∼3.53 eV (Si 3 H 6 ), 2.19∼2.54 eV (Si 3 H 7 ), and 3.45∼3.62 eV (Si 3 H 8 ). For anion clusters (Si 3 H n - → Si 3 H n - 1 - +H), the dissociation energies predicted are 2.89∼2.95 eV (Si 3 H - ), 2.01∼2.27 eV (Si 3 H 2 - ), 2.83∼2.96 eV (Si 3 H 3 - ), 2.37∼2.68 eV (Si 3 H 4 - ), 2.96∼3.10 eV (Si 3 H 5 - ), 2.36∼2.74 eV (Si 3 H 6 - ), 3.03∼3.16 eV (Si 3 H 7 - ), and 1.44∼1.54 eV (Si 3 H 8 - ).

The silicon hydride clusters Si3Hn (n ≤ 8) and their anions: Structures, thermochemistry, and electron affinities

We report a systematic study of ground state structures, vibrational spectra cohesive energies and HOMO-LUMO gaps of small SinHm clusters (n=1, 2 and m=1–6) and their deuterated derivatives based on the nonorthogonal tight-binding molecular dynamics scheme. The ground state structures have been obtained by using simulated annealing. The bond lengths, bond angles and the frequencies of normal modes are found to be in good agreement with available experimental data and ab initio calculations. Our calculation of cohesive energies indicate SiH2 to be more stable than SiH3 or SiH and Si2H4, more stable than Si2H3 or Si2H5.

Ground state geometries and vibrational spectra of small hydrogenated silicon clusters using nonorthogonal tight-binding molecular dynamics

We report a systematic study of ground state structures, vibrational spectra, cohesive energies and HOMO-LUMO gaps of small SinH clusters (n=3, 10) based on the nonorthogonal tight-binding molecular dynamics scheme. The ground state structures have been obtained by using simulated annealing. In particular, we focus on how the addition of a hydrogen atom affects the ground state geometry and the stability of a Sin cluster. We find that hydrogen either enters into the surface of the cluster or occupies a position outside the cluster. In the first case, it drastically distorts the cluster, while in the latter, there is very little distortion. We find that in some cases SinH cluster has some resemblance with Sin+1 cluster. We also find that hydrogen can form bonds with more than one silicon atom. Our calculation indicates that SiH, Si3H and Si5H will be more stable and Si4H, Si6H, Si7H, Si9H and Si10H will be less stable clusters.

Geometries and vibrational spectra of small hydrogenated silicon clusters

We have studied structural properties of amorphous hydrogenated silicon usingab initio molecular dynamics simulations. A sample was generated by simulated annealing using periodic boundary conditions with a supercell containing 64 silicon and 8 hydrogen atoms. The radial pair distribution functions for Si-Si, Si-H and H-H have been studied at 300 K and are found to be in good agreement with experimental data. Our results show that hydrogen saturates the dangling bonds and reduces bond strain. We also report existence of Si-H-Si bridge sites which are likely to play an important role in understanding the light induced metastability in this material.

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Girish R. Gupte

Papers

Ground state geometries and vibrational spectra of small hydrogenated silicon clusters using nonorthogonal tight-binding molecular dynamics

Geometries and vibrational spectra of small hydrogenated silicon clusters

Structural properties of amorphous hydrogenated silicon using ab initio molecular dynamics simulations