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We simulated hydrogenated nanocrystalline silicon with molecular dynamics calculations using embedding and melt-quenching approaches. The embedding approach generates a well-defined crystallite residing in an amorphous matrix and a structure free of coordination defects. The H-distribution is inhomogeneous with an excess hydrogen density at the strained grain boundary between the nanocrystallite and the amorphous matrix. The amorphous matrix is better ordered in hydrogenated nanocrystalline-Si than in the homogenous amorphous silicon networks. Nanocrystals have been annealed at higher temperature where the nanocrystalline regions shrink in size. Nucleation and growth theory of these nanocrystals is developed. Melt-quenching simulations generate nc-Si structures which do not show a disordered grain boundary but generate a very high density of defects in the amorphous region, which cannot be annealed. The embedding approach may favor structures closer to experimental H-diluted growth conditions. Melt-quenching may better represent structures formed by laser induced recrystallization. [ABSTRACT FROM AUTHOR]

We examine different types of order measured in hydrogenated amorphous silicon (a-Si:H) and their effect on the optical absorption (band gap). We first review previous experimental work determining order on a short-range scale as probed by Raman spectroscopy, and provide, using molecular dynamics simulations, a theoretical explanation for why the band gap increases when this type of ordering is improved. We then present results on a-Si:H films deposited by hot wire chemical vapor deposition (CVD) and plasma enhanced CVD where the short-range order, from Raman spectroscopy, does not change, but order on a larger or medium-range scale does. This order is determined by measuring the width of the first x-ray diffraction peak, and was varied by depositing films at different substrate temperatures and/or different hydrogen dilutions. We find that the film band gap also increases when this type of ordering improves, and we provide a possible mechanism to explain these trends. We also suggest that much of the previous literature showing an increase in band gap with increasing film hydrogen content should be treated with caution, as these works may not have accurately deconvoluted the effects of optical adsorption due to film hydrogenation from those due to both types of lattice ordering. Finally, we argue that this same trend may apply, to a limited extent, to microcrystalline silicon. [ABSTRACT FROM AUTHOR]

Investigates the role of the atomic hydrogen by molecular dynamics simulations of the hydrogen deposition process. Analysis on backscattering events in a-Si:H and C-Si; Displacements of the silicon and hydrogen atoms of the substrate from their initial positions; Techniques used for growing a-Si:H films.

The temperature dependent H-evolution from nano-crystalline silicon is simulated with molecular dynamics. The nano-crystalline silicon model consists of a heterogeneous dispersion of nano-crystallites in an amorphous silicon matrix. An excess H-density occurs at the nano-crystallite surface. Simulations find a low temperature evolution peak at 250–400 °C, where the H-evolution occurs from the surface of the nano-crystallite. In addition there is a higher temperature peak at 700–800 °C. The two-peak feature agrees with H-evolution measurements of H-diluted a-Si:H grown near the phase boundary of crystalline growth. We find that H is released into mobile molecular and bond-centered like configurations. [Copyright &y& Elsevier]

We survey theoretical approaches to understanding the diverse metastable behavior in hydrogenated amorphous silicon. We discuss a recently developed network-rebonding model involving bonding rearrangements of silicon and hydrogen atoms. Using tight-binding molecular dynamics we find non-radiative recombination can break weak silicon bonds with low activation energies, producing dangling bond–floating bond pairs. The transient floating bonds annihilate generating local hydrogen motion and leaving behind isolated dangling bonds. Charged defects are also observed. Major experimental features of metastability including electron-spin resonance, t1/3 kinetics, dangling-bond H anti-correlation, and hysteretic annealing are explained. In the second part we focus on large metastable structural changes observed in a-Si:H. We find H atoms have a local metastability involving the flipping of the H to the backside of the Si–H bond that results in a local increase of strain and increase of dipole moments. This naturally explains the larger infrared absorption found after light soaking, and may be related to other large structural changes in the network. Directions for future research are surveyed. [Copyright &y& Elsevier]

Tight-binding molecular dynamics has been used to simulate vibrationally excited Si–H and Si–D modes. Simulations find that vibrationally excited Si–D bending modes decay much more rapidly than Si–H bending modes, resulting in SiD bonds having much higher stability than SiH bonds. This provides a viable mechanism for reduced degradation in deuterated metal–oxide–semiconductor transistors and deuterated amorphous silicon devices. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Light-induced metastable defects in a-Si:H are proposed to be silicon dangling bonds accompanied by pairs of hydrogen atoms breaking a silicon bond, forming a complex with two Si-H bonds. This supports the model of Branz. These defects are the analog of the H[sub 2][sup *] defect in c-Si and their energy correlates with the bond-angle strain. Several features of the annealing are well described by this defect complex. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Deep eutectic solvents (DES) are composite mixtures which have freezing point lesser than that of their individual components. A mixture of acetamide and lithium nitrate in the molar ratio of 78:22 is observed to form a DES with freezing point below room temperature. Here we report nanoscale dynamics in acetamide + LiNO 3 DES as studied using quasielastic neutron scattering (QENS) and molecular dynamics (MD) simulation techniques. The results of MD simulation show a transient subdiffusive motion for acetamide molecules. The acetamide molecules form a hydrogen-bond network with its neighbouring set of molecules resulting in a transient cage. The motion of the acetamide molecules is described by considering a power law memory kernel for the relaxation of the cage. The intermediate scattering function obtained from the QENS experiments of DES are described using stretched exponential relaxation function. The power law exponent describing the results of MD simulation is found to match well with the stretching exponent obtained from QENS experiments. [ABSTRACT FROM AUTHOR]