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, V-Ga, according to their positron lifetime and ann ihilation momentum distribution. The charge state of the vacancies is neutr al. This is ascribed to the presence of positively charged As-Ga(+) antisit e defects in vicinity to the vacancies. Theoretical calculations of the ann ihilation parameters show that this assignment is consistent with the data. The density of V-Ga is related to the growth stoichiometry in LT-GaAs, i.e ., it increases with the As/Ga beam equivalent pressure (BEP) and saturates at 2x10(18) cm(-3) for a BEP greater than or equal to 20 and a low growth temperature of 200 degrees C. Annealing at 600 degrees C removes V-Ga. Inst ead, larger vacancy agglomerates with a size of approximately four vacancie s are found. It will be shown that these vacancy clusters are associated wi th the As precipitates formed during annealing. (C) 2000 American Institute of Physics. [S0021- 8979(00)02812-7].