Positron annihilation spectroscopy of native vacancies in as-grown GaAs

Native vacancy defects are studied in as-grown GaAs materials by positron lifetime measurements. Direct evidence of native monovacancy-type defects is found in as-grown n-type GaAs (Te, Sn or Si) but not in as-grown semi-insulating In or Cr doped GaAs or semi-insulating undoped GaAs or p-type GaAs(Zn). It is shown that the positron trapping and annihilation in the native defects is strongly dependent on the position of the Fermi level in as-grown n-GaAs. It is concluded that the configurations of the native monovacancy defects in GaAs (Te or Sn) change with the position of the Fermi level. Two Fermi level controlled transitions are found: one is located between 0.02 eV and 0.05 eV, the other is about 0.12 eV below the conduction band. It is proposed that the two transitions correspond to two charge state transitions of the arsenic vacancy