Evidence of two kinds of acceptors in undoped semi-insulating GaAs: Positron trapping at gallium vacancies and negative ions

Positron-lifetime experiments have been performed in Zn-doped p-type and undoped semi-insulating GaAs in the temperature range 20-300 K to investigate native point defects. In p-type materials with hole concentrations of 1015-1019 cm-3, no evidence of positron trapping is observed. The temperature dependence of the positron lifetime can be explained in terms of lattice expansion associated with positron-phonon coupling. Therefore, we ascribe it to delocalized positrons. In semi-insulating GaAs, two kinds of acceptors are detected with concentrations in the range 1015-1017 cm-3: gallium vacancies and negative ions. The temperature dependence of the positron trapping at the Ga vacancy exhibits a slope break at about 130 K. A weakly bound Rydberg-like precursor state is invoked to explain this temperature dependence.