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

Abstract

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 ${T}_{d}$ site of the broken Si-Si bond. The other H atom from the SiHHSi is also located in the energetically favorable ${T}_{d}$ 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 ${T}_{d}$ site, not the BC site. The distance between the dangling bond and the H atom in the ${T}_{d}$ site is in the range 4--5 \AA{}, in agreement with ESR data. The majority of silicon dangling bonds, both metastable and stable, exist as SiHD, with the H atom in the ${T}_{d}$ site. The microscopic process for defect creation is fairly well localized, requiring only short-range H motion, which proceeds via bond switching between neighboring ${T}_{d}$ sites. In contrast, the microscopic process for defect removal during thermal annealing involves reequilibration of H in the $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}$ 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.