Direct identification of As vacancies in GaAs using positron annihilation calibrated by scanning tunneling microscopy - art. no. 045203

We report on the identification of native vacancies in GaAs by positron ann ihilation with a special emphasis on As vacancy-related defects. In anneale d highly Si-doped GaAs, we observe a neutral vacancy defect with a positron lifetime tau of 280-285 ps and a high intensity of the core annihilation, in contrast to Ga vacancies which exhibit a lifetime of similar to 260 ps a nd a lower intensity of the core annihilation. This defect is identified by scanning tunneling microscopy measurements to be an As vacancy Si-Ga donor complex. We find that the same defect is also present in low it-doped GaAs , where it was earlier assigned to a neutral As vacancy. The high positron lifetime is explained by a large outward lattice relaxation. Theoretical ca lculations of the momentum distribution employing free atomic wave function s are in good agreement with the experimental results, provided only relati ve changes are considered and an outward lattice relaxation is included whi ch yields the experimental positron lifetime. These calculations also yield annihilation parameters for Ga vacancies, in good agreement with the exper iment. Our results demonstrate that vacancies in both sublattices of GaAs c an directly and unambiguously be identified by positron annihilation once t he annihilation characteristics are calibrated by a complementary method. O n the basis of this identification, the abundance of As vacancies in GaAs i s discussed in terms of stoichiometry and formation energies.