  In this paper, we explore the effect of H and its bonding configurations on
the defect state density and orbital localization of hydrogenated amorphous Si
(a-Si:H)/crystalline Si (c-Si) heterostructures using density functional theory
(DFT) studies of model interfaces between amorphous silicon (a- Si)/a-Si:H and
c-Si. To model the atomic configuration of a-Si on c-Si, melting and quenching
simulations were performed using classical molecular dynamics (MD). Different
hydrogen contents were inserted into the a-Si in different bonding
configurations followed by DFT relaxation to create the stable structures of
a-Si:H representative of hydrogenated a-Si on crystalline Si surfaces. In
contrast to crystalline heterojunctions (where the interface density is a
maximum at the interface), we find that, in the most energetically stable
configurations of H atoms, the defect state density is relatively low at the
interface and maximum at the middle of a-Si layer. Our structural analysis
shows that in these configurations, H atoms do not necessarily bond to dangling
bonds or to interface atoms. However, they are able to significantly change the
atomic structure of the heterostructure and consequently decrease the density
of defect states and orbital localization at the a-Si layer and more
significantly at the interface of a-Si/c-Si. The general form of the modeled
defect state distribution demonstrates the passivating role of a-Si:H on c-Si
substrates.
