Point defects in GaAs studied by correlated positron lifetime, optical, and electrical measurements. II. Point defects in GaAs irradiated with fast neutrons

Positron lifetime technique is used in combination with infrared absorption, photoluminescence, and photoreflexion measurements to investigate defects created by irradiation of GaAs with fast neutrons. The integrated fluence Φ of fast neutrons ranged from 5 × 1014 to 3.8 × 1019 n°/cm2. From the dependence of the absorption coefficient α on Φ the geometrical size of the displacement cascades is estimated to r = 5 nm. The behaviour of the carrier concentration n estimated from photoreflexion experiments provides r = 20 nm. This value is characteristic of the insulating region surrounding the cascade. Almost the same value is found to be typical of the luminescence killer region and of positron trapping. Evidence of positron trapping by vacancy-type irradiation defects is given. The defects are discussed as negatively charged Ga vacancies the characteristic positron lifetime of which is estimated to τvGa = 260 to 270 ps. The vacancies disappear in an annealing stage at 500 °C. The stage is interpreted as a long-range migration of point defects resulting in a dissolution of the displacement cascades. A substage at 200 °C observed in photoluminescence experiments is discussed as recombination of vacancy-interstitial close pairs in the As sublattice which are situated outside of displacement cascades.