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