Vacancies near semiconductor surfaces

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Abstract
We present a calculation of the bound-state energy levels of ideal vacancies near and on a semiconductor surface. We consider the (110) surface of GaAs and also a model for more ionic semiconductors with the zinc blende crystal structure. We find that the vacancy energy levels are not greatly perturbed (by less than 0.1 eV) as the vacancy is moved toward the surface until the vacancy reaches the second layer of atoms from the surface. At this point the highest occupied level in the neutral anion vacancy moves to lower energy, and the highest-energy state occupied in the neutral cation vacancy moves to higher energy. As more ionic materials are considered, we find that the anion vacancy levels move toward and eventually into the conduction band, while the cation vacancy levels move toward and eventually into the valence band. These results suggest that the recently proposed defect model for Schottky-barrier formation is capable of accounting for experimentally observed trends in Schottky-barrier behavior.