Defect identification in GaAs grown at low temperatures by positron annihilation

We use positron annihilation to study vacancy defects in GaAs grown at low temperatures (LT–GaAs). The vacancies in as-grown LT–GaAs can be identified to be Ga monovacancies, VGa, according to their positron lifetime and annihilation momentum distribution. The charge state of the vacancies is neutral. This is ascribed to the presence of positively charged AsGa+ antisite defects in vicinity to the vacancies. Theoretical calculations of the annihilation parameters show that this assignment is consistent with the data. The density of VGa is related to the growth stoichiometry in LT–GaAs, i.e., it increases with the As/Ga beam equivalent pressure (BEP) and saturates at 2×1018 cm−3 for a BEP⩾20 and a low growth temperature of 200 °C. Annealing at 600 °C removes VGa. Instead, larger vacancy agglomerates with a size of approximately four vacancies are found. It will be shown that these vacancy clusters are associated with the As precipitates formed during annealing.