Hydrogen above Saturation at Silicon Vacancies: H-Pair Reservoirs and Metastability Sites

We propose that hydrogen-passivated multivacancies which appear to be fully saturated with H can actually capture additional H in electrically inactive sites. In silicon, first-principles total energy calculations show that splitting an (m≥2) multivacancy into a mono- and an (m-1) vacancy provides a low-strain pairing site for H, 0.4eV per H lower than any known bulk pairing site. This monovacancy ejection mechanism is an excellent candidate for the H reservoir found both in crystalline and amorphous Si. A distinct H pairing on the fully saturated m vacancies, by forming an internal surface Si-Si dimer, provides the final state of light-induced metastable degradation of hydrogenated amorphous silicon