A systematic investigation of GaAs layers grown at low temperatures was carried out by means of positron annihilation. The vacancy defects in undoped as-grown material were identified to be mainly Ga vacancies(VGa)by comparing the annihilation parameters to those of Ga vacancies in highly Si-doped GaAs. The characteristicSparameter for positron annihilation in Ga vacancies was determined to beS(VGa)=1.024(1).TheVGaconcentration increases up to1018 cm−3by decreasing the growth temperature to 200 °C. The vacancy concentration can account for the compensation ofAsGa+antisites as was previously assumed. ©1997 American Institute of Physics.