Energy levels of semiconductor surface vacancies

We present calculations of the bound state energy levels of anion vacancies near the surface of a III-V semiconductor. We consider the (110) surface of GaAs, InP, and the Ga1_xAlxAs alloy system. As the vacancy is moved toward the surface, the energy levels are only slightly perturbed until the vacancy reaches the second atomic layer from the surface. At this point, the anion vacancy levels move to lower energy. We find that there is a general trend in the vacancy energy levels with semiconductor ionicity. As the material becomes more ionic, the anion vacancy levels move to higher energy. Comparing this trend with experimentally observed Schottky barrier heights, we find a strong correlation between the position of the highest occupied level in the anion vacancy and the measured Fermi level at the surface. This result suggests that the recently proposed defect model is capable of accounting for observed trends in Schottky barrier formation.