Electronic structure of neutral and charged vacancies in Ga-related III-V compound semiconductors

Abstract

Self-consistent semiempirical tight-binding theory is used to study the neutral and charged states of the ideal vacancies in GaP and GaSb. For the neutral states, the diagonal elements of the defect potential on the first and second nearest neighbors of a vacancy are obtained by a charge-neutrality condition, while for the charged states, these matrix elements are determined self-consistently. The theory also takes the off-diagonal matrix elements corresponding to atoms adjacent to a vacancy into account. The defect energy levels and the localizations of the levels are computed using the Lanczos–Haydock recursion method. The general character of the electronic structure of vacancies in the Ga-related III-V compound semiconductors has been extracted from these calculations and from the similar calculations we performed for vacancies in GaAs. We analyze the defect states of the vacancies in terms of chemical bondings of the crystals. It is shown that, for the cation vacancies (the Ga vacancies) in the three semiconductors, both the a1 resonances at the top of the valence band and the t2 bound states in the fundamental gap consist mainly of the atomiclike p orbitals centered on the four nearest neighbors, whereas for the anion vacancies, the a1 states at the top of the valence band contain mainly the atomiclike p orbitals of the four neighboring atoms and the t2 bound states in the fundamental gap contain both the atomiclike s and the atomiclike p orbitals of the four neighboring atoms. We also find that the t2 gap levels of the charged cation (anion) vacancies in the three semiconductors are all located in the lower (upper) half of the fundamental gap. The calculated results are in agreement with available results from local density calculations and support the identifications of the vacancy defects by the positron-annihilation technique.