Thermally-activated charge reversibility of gallium vacancies in GaAs

The dominant charge state for the Ga vacancy in GaAs has been the subject of a long debate, with experiments proposing $-$1, $-$2 or $-$3 as the best answer. We revisit this problem using {\it ab initio} calculations to compute the effects of temperature on the Gibbs free energy of formation, and we find that the thermal dependence of the Fermi level and of the ionization levels lead to a reversal of the preferred charge state as the temperature increases. Calculating the concentrations of gallium vacancies based on these results, we reproduce two conflicting experimental measurements, showing that these can be understood from a single set of coherent LDA results when thermal effects are included.