Direct identification of As vacancies in GaAs using positron annihilation calibrated by scanning tunneling microscopy

We report on the identification of native vacancies in GaAs by positron annihilation with a special emphasis on As vacancy-related defects. In annealed 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 and 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 calculations of the momentum distribution employing free atomic wave functions are in good agreement with the experimental results, provided only relative changes are considered and an outward lattice relaxation is included which yields the experimental positron lifetime. These calculations also yield annihilation parameters for Ga vacancies, in good agreement with the experiment. Our results demonstrate that vacancies in both sublattices of GaAs can directly and unambiguously be identified by positron annihilation once the annihilation characteristics are calibrated by a complementary method. On the basis of this identification, the abundance of As vacancies in GaAs is discussed in terms of stoichiometry and formation energies