The relation between the band gap and the anisotropic nature of hydrogenated amorphous silicon

The network and nature of hydrogenated amorphous silicon (a-Si:H) are conventionally interpreted in terms of a continuous random network (CRN) of Si-Si bonds, weak Si-Si, Si-H bond and dangling bonds. A CRN requires that the smallest anisotropic features like dangling bonds and bonded hydrogen are randomly distributed and reside as isolated configurations in the network. However, in recent years more and more theoretical and experimental evidence have been found that both the isolated dangling bond and the isolated hydrogen are not present in the a-Si:H network. To the contrary, all studies come to the conclusion that the real nature of the a-Si:H is to contain more local structural order than expected from a CRN. These insights offer new opportunities to revisit the origin of several properties of a-Si:H, which are up to now explained within the framework of the CRN model. In this contribution we will discuss that many diagnostics like nuclear magnetic resonance, positron annihilation, small angle x-ray spectroscopy, density analysis and infrared spectroscopy on a-Si:H consistently demonstrate that a-Si:H exhibits an anisotropic network. In dense disordered networks the hydrogen predominantly resides in hydrogenated divacancies, whereas for less dense networks the hydrogen predominantly resides in poly-vacancies up to nanosized voids. We will discuss that hydrogenated divacancies in a disordered network contribute to the amorphous nature of a-Si:H and its electronic structure like the band gap, gap tails and the defect gap states.