Model of hydrogenated amorphous silicon and its electronic structure

Exceptional Hydrogen Uptake in Crystalline InxGa1-xN Semiconductors

The irradiation of InN and InxGa1-xN samples with low-energy H ions results in exceptionally high hydrogen uptake in a crystalline semiconductor. This phenomenon is attributed to specific In-H complex formation. By exploiting spectral fingerprints of the In-H complexes observable in In L3-edge X-ray absorption spectroscopy, we provide direct evidence of complex formation. Density functional theory calculations assist in interpreting the X-ray absorption spectra and offer insights into the energetics of complex formation. We quantify the total amount of reversibly incorporated hydrogen in these semiconductors and discuss their strengths and weaknesses as innovative materials for hydrogen storage.

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.

Vacancies, microstructure and the moments of nuclear magnetic resonance: the case of hydrogenated amorphous silicon

Recent experiments on hydrogenated amorphous silicon using infrared absorption spectroscopy have indicated the presence of mono- and divacancies in samples for concentrations of up to 14% hydrogen. Motivated by this observation, we study the microstructure of hydrogen in two model networks of hydrogen-rich amorphous silicon with particular emphasis on the nature of the distribution (of hydrogen), the presence of defects and the characteristic features of the nuclear magnetic resonance spectra at low and high concentrations of hydrogen. Our study reveals the presence of vacancies, which are the built-in features of the model networks. The study also confirms the presence of various hydride configurations in the networks, from silicon monohydrides and dihydrides to open chain-like structures, that have been observed in the infrared and nuclear magnetic resonance experiments. The broad and the narrow line widths of the nuclear magnetic resonance spectra are calculated from a knowledge of the distribution of spins (hydrogen) in the networks.

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.