Role of Hydrogen in the Electronic Properties of a-Si:H/c-Si Heterostructures

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. Different hydrogen contents were inserted into a-Si in different bonding configurations followed by DFT relaxation to create stable structures of a-Si:H representative of hydrogenated a-Si on crystalline Si surfaces. In contrast to typical Si heterojunctions (e.g., Si/SiO2, where the defect density is maximum at the interface), we find that the defect state density is low at the interface and maximum in the bulk of a-Si. 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 in the a-Si layer, particularly at the interface of a-Si/c-Si. The general form of the simulated defect state distribution demonstrates the passivating role of a-Si:H on c-Si substrates.