Vacancy-like defects in a-Si: a first principles study

The structural and electronic properties of vacancies in a fully tetrahedral model of amorphous silicon have been investigated by ab initio calculations. A very rich behavior was observed after the atomic relaxation, from the complete rearrangement of the atoms (self-healing) to the creation of stable vacancies. We have observed that the vacancy internal relaxation volume is negative. After relaxation, the vacancy formation energies were found to be smaller than that of the crystal, and the deep gap levels either disappear or move towards the band edges.