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.

/pdf/first-principles-study-of-hydrogenated-amorphous-silicon-oxnjeu6bwu.pdf

First-principles study of hydrogenated amorphous silicon

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

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

The ground state structures and properties of Si3Hn (1 ≤n ≤ 6) clusters have been calculated using Car-Parrinello molecular dynamics with simulated annealing and steepest descent optimization methods. We have studied cohesive energy per particle and first excited electronic level gap of the clusters as a function of hydrogenation. Hydrogenation is done till all dangling bonds of silicon are saturated. Our results show that over coordination of hydrogen is favoured in Si3Hn clusters and the geometry of Si3 cluster does not change due to hydrogenation. Cohesive energy per particle and first excited electronic level gap study of the clusters show that Si3H6 cluster is most stable and Si3H3 cluster is most unstable among the clusters considered here.

/pdf/ground-state-structures-and-properties-of-si3hn-n-1-6-39s6myjs54.pdf

Ground state structures and properties of Si3Hn (n = 1–6) clusters

The geometries and energies of small silicon monohydride clusters (Si2H–Si10H) have been systematically investigated by density functional theory (DFT) scheme with DZP++ basis sets. Several possible geometric arrangements and electronic states have been considered for each cluster. The results on Si2H–Si4H are in good accordance with previous ab initio calculation. The geometry of ground state of Si2H is found to be a bridged C2v structure, and Si3H to be a bridged C2v, while Si4H a non-bridged Cs symmetry with 2A′ state. The non-bridged geometries of ground state of Si5H–Si10H have been found to be corresponding to C2v(2B1), C2v(2B1), C5v(2A1), Cs(2A′′) (have two types), C1 (not symmetry), and Cs(2A′), respectively. The results on Si5H, Si6H, Si8H and Si9H are different from previous calculations. Compared silicon clusters (Sin) with silicon monohydrides (SinH) clusters, the addition of a single hydrogen atom cannot cause great changes in the ground state geometries of Si2, Si3, Si4, Si7, Si9, and Si10 clusters, while in the ground state geometries of Si5, Si6 and Si8 clusters the change is great. The dissociation energies calculated indicates that Si4H, Si7H, and Si10H clusters are less stable than others.

Equilibrium geometries and electronic structure of small silicon monohydrides 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.

/pdf/structural-properties-of-amorphous-hydrogenated-silicon-gvrxgirgwc.pdf

G L Chiarotti

Papers

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