Structure of a-Si:H from Harris-functional molecular dynamics

Nanoscale structure of microvoids in a-Si:H: a first-principles study

In this paper, we have studied the shape, size, and number density of atomic microvoids in hydrogenated amorphous silicon (a-Si:H). By jointly employing experimental infrared data and ab initio simulations, we propose a simple and effective hydrogenation scheme, which is capable of producing large atomistic models of a-Si:H for studying microvoids. Our results suggest that hydrogen atoms in the networks are distributed in sparse (or isolated) and clustered environments. For a-Si:H models with 9-14 at.% hydrogen, we find approximately 3-4 at.% of total hydrogen atoms are distributed in the isolated phase. The density of the clustered phase is found to be between 6-12 at.%, which appears to depend on the amount of hydrogen in the network. The calculation of radii of gyration of atomic microvoids shows that the diameter of the microvoids is distributed from 6 Å to 12 Å. A few hydrogen molecules have also been observed to form inside the microvoids in our study, the concentration of which is about 1 at.% relative to silicon atoms. A comparison of our results with those from small-angle x-ray scattering (SAXS), infrared (IR) absorption, nuclear magnetic resonance (NMR) and calorimetric studies are presented.

A study of hydrogen microstructure in amorphous silicon via inversion of nuclear magnetic resonance spectra

We present an inverse approach for studying hydrogen microstructure in amorphous silicon. The approach consists of generating a prior distribution (of spins/hydrogen) by inverting experimental nuclear magnetic resonance (NMR) data, which is subsequently superimposed on a network of amorphous silicon. The resulting network is then relaxed using a total-energy functional to obtain a stable, low-energy configuration such that the initial spin distribution is minimally perturbed. The efficacy of this approach is demonstrated by generating model configurations that not only have the correct NMR spectra but also satisfy simultaneously experimental structural, electronic and vibrational properties of hydrogenated amorphous silicon.

New Approaches to the Computer Simulation of Amorphous Alloys: A Review

In this work we review our new methods to computer generate amorphous atomic topologies of several binary alloys: SiH, SiN, CN; binary systems based on group IV elements like SiC; the GeSe₂ chalcogenide; aluminum-based systems: AlN and AlSi, and the CuZr amorphous alloy. We use an ab initio approach based on density functionals and computationally thermally-randomized periodically-continued cells with at least 108 atoms. The computational thermal process to generate the amorphous alloys is the undermelt-quench approach, or one of its variants, that consists in linearly heating the samples to just below their melting (or liquidus) temperatures, and then linearly cooling them afterwards. These processes are carried out from initial crystalline conditions using short and long time steps. We find that a step four-times the default time step is adequate for most of the simulations. Radial distribution functions (partial and total) are calculated and compared whenever possible with experimental results, and the agreement is very good. For some materials we report studies of the effect of the topological disorder on their electronic and vibrational densities of states and on their optical properties.

Materials modeling by design: applications to amorphous solids

In this paper, we review a host of methods used to model amorphous materials. We particularly describe methods which impose constraints on the models to ensure that the final model meets a priori requirements (on structure, topology, chemical order, etc). In particular, we review work based on quench from the melt simulations, the 'decorate and relax' method, which is shown to be a reliable scheme for forming models of certain binary glasses. A 'building block' approach is also suggested and yields a pleading model for GeSe(1.5). We also report on the nature of vulcanization in an Se network cross-linked by As, and indicate how introducing H into an a-Si network develops into a-Si:H. We also discuss explicitly constrained methods including reverse Monte Carlo (RMC) and a novel method called 'Experimentally Constrained Molecular Relaxation'. The latter merges the power of ab initio simulation with the ability to impose external information associated with RMC.