EVIDENCE OF 2 KINDS OF ACCEPTORS IN UNDOPED SEMIINSULATING 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 10(15)-10(19) 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 accepters are detected with concentrations in the range 10(15)-10(17) 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.