Microscopic mechanisms for creation and removal of metastable dangling bonds in hydrogenated amorphous silicon

We present a microscopic model for metastable Si dangling-bond defect creation in hydrogenated amorphous silicon, which is applicable to both light-induced defect creation in solar cells ~Staebler-Wronski effect! and bias-stress-induced defect creation in thin-film transistors. Light or gate bias causes electron-hole pairs or electrons, respectively, to be localized on short, weak Si-Si bonds, which then break. A hydrogen atom, from a neighboring, doubly hydrogenated weak Si-Si bond ~SiHHSi! moves to the Td site of the broken Si-Si bond. The other H atom from the SiHHSi is also located in the energetically favorable Td site. Overall, the reaction produces two SiHD defects. Each SiHD defect is an intimate Si dangling bond and Si-H bond, where the H atom is in the Td site, not the BC site. The distance between the dangling bond and the H atom in the Td site is in the range 4–5 Å, in agreement with ESR data. The majority of silicon dangling bonds, both metastable and stable, exist as SiHD, with the H atom in the Td site. The microscopic process for defect creation is fairly well localized, requiring only short-range H motion, which proceeds via bond switching between neighboring Td sites. In contrast, the microscopic process for defect removal during thermal annealing involves reequilibration of H in the a-Si:H network and is a global process involving a large fraction of H atoms. The rate-limiting step for this process is Si-H bond breaking from SiHHSi sites, which accounts for the maximum activation energy of 1.5 eV. We present a revised hydrogen density of states diagram, in line with this process.