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.
Positron lifetime measurements are used in combination with optical (infrared absorption, photo‐luminescence, and photoreflexion) and electrical (Hall and resistivity) investigations to study native point defects and their complexes in as‐grown GaAs crystals. In undoped n‐type GaAs which contains 2 × 1016 cm−3 EL2° centres strong positron trapping by a vacancy‐type defect is detected. The point defect is discussed as a vacancy in the As sublattice appearing in a neutral or negative charge state, V As0,−. In heavily doped GaAs dopant‐vacancy complexes of the type AV As+ and DV Ga− are identified from optical measurements in p‐type and n‐type crystals, respectively. Positrons which are repelled by AV As+ are evidently trapped by acceptor‐type donor‐Ga vacancy complexes in n‐type GaAs. The decomposition of the lifetime spectra provides characteristic positron lifetimes of 295 ps and 260 ps for the V As0,− and DV Ga− defects, respectively. A specific positron trapping rate of 6 × 10−8 cm3 s−1 (3 × 1014 s−1) is estimated from the comparison of electrical and positron lifetime parameters and by identifying the compensating centres in heavily doped n‐type GaAs with DV Ga− defects. The number of grown‐in vacancies is discussed. Temperature dependent studies indicate a change in the nature of the dominating positron trap at low temperatures. Annealing experiments are discussed as a compensation of vacancies around 500 °C probably by in‐diffusion of acceptor‐type impurities.
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