Computational characterization of a-Si:H/c-Si interfaces

We use ab initio molecular dynamics to generate realistic a-Si:H/c-Si interface structures with very low defect-state density by performing a high-temperature annealing. Throughout the annealing, we monitor the evolution of the structural and electronic properties. The analysis of the bonds by means of the electron localization function reveals that dangling bonds move toward the free a-Si:H surface, leaving the interface region itself completely defect free. The hydrogen follows this movement, which indicates that in the case under consideration, hydrogen passivation does not play a significant role at the interface. A configuration with a satisfactory low density of defect states is reached after annealing at 700 K. A detailed characterization of the electronic states in this configuration in terms of their energy, localization, and location reveals that, although no dangling bond states can be found near the interface, localized interface states do exist and are attributed to a potential barrier at the interface. The quantitative description of electronic localization also allows for the determination of the a-Si:H mobility gap, which, together with the c-Si band gap, yields band offsets that are in qualitative agreement with experimental observations.