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"-10' cm, 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 10"-10' cm '. 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.
We have performed positron Lifetime experiments an semi-insulating (SI) GaAs in darkness at 20 K, before and after illumination with light bf photon energy 1.32 eV. EL2 COncentratiON have been measured by infrared absorption (mn). After illumination, positrons detect a metastable vacancy whose trapping rate correlates with the total EL2 concentration. We conclude that this vacancy is included in the metastable configuration of EL2 (ELZ*). This correlation allows us to propose a value of (3.0+0.3) xIOL6 s-' for the positron trapping coefficient at EU' at 20 K.
We show that positron trapping at negative As vacancies revealed under illumination in bulk semi-insulating GaAs correlates with a form of near-band-edge absorption known as reverse contrast (RC). We conclude that it is the ionization of As vacancies to their negative charge state that gives rise to RC absorption.
We show that two different types of photoquenching effect take place under low temperature illumination of lightly n-type bulk GaAs. Both phenomena result in an increase in positron trapping at vacancies. The first associated with a decrease in EL2 absorption, is produced with light of 1.1 μm wavelength and recovers near 100 K. While little photoquenching related to EL2 is observed after illumination close to the band edge (0.83 μm), persistent increases in Hall voltage and positron lifetime accompanied by a decrease in near band-edge absorption are observed. These latter phenomena recover at 50 K.
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