Study on the semi-insulating mechanism of undoped GaAs

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We have concentrated on the study of the mechanism of the photoquenching of EL2, a most important point defect in undoped semi-insulating GaAs. The photoquenching effect is a phenomenon in which all optical and electrical properties of EL2 are vanished by being irradiated with near-infrared lights at T<90K.The effect has been attributed to a transition of the EL2 defect into a metastable state, while some recent reports suggest contributions from other point defects than EL2. The clarification of the photoquenching effect is therefore of great importance in realizing high-quality GaAs crystals. What we found this year is as follows.(1)The photoquenching of EL2 does not occur by itself. Contributions from other defects are essential. By testing a series of samples, in which only the carbon acceptor concentration is varied, we found a strong influence of the carbon acceptor on the activation of the photoquencing effect.(2)By a quantitative analysis of the temperature dependence of the EL2 photoquenching, we found that a deep acceptor located at 70-80 meV above the valence band plays an essential role in the photoquenching effect.(3)This level, named actuator level, accounts for both the carbon-concentration and the temperature dependence of the photoquenching effect, by assuming that the actuator level triggers the photoquenching of EL2 when it is neutral and triggers the photorecovery of EL2 when it is negatively ionized.(4)By thermally-stimulated-current(TSC)measurements, we have confirmed the presence of a deep acceptor at the relevant position in the bandgap. Its charge state became neutral from the original ionized one after irradiation with the quenching light, consistent with our actuator level model.