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.