Positron lifetime experiments were performed on Zn-doped p-type and undoped semi-insulating material at temperatures which ranged from 20 to 300K. In the case of p-type materials with hole concentrations of 1015 to 1019/cm3, no sign of positron trapping was detected. The temperature dependence of the positron lifetime could be explained in terms of a lattice expansion which was associated with positron-phonon coupling, and was attributed to delocalized positrons. In the case of semi-insulating material, 2 types of acceptor were detected, with concentrations that ranged from 1015 to 1017/cm3. These acceptors were Ga vacancies and negative ions. The temperature dependence of positron trapping at the Ga vacancy exhibited a break in slope at about 130K. A weakly bound Rydberg-like precursor state was used to explain this temperature dependence.
C.Le Berre, C.Corbel, K.Saarinen, S.Kuisma, P.Hautojärvi, R.Fornari: Physical Review B, 1995, 52, 8112-20