Study on the semi-insulating mechanism of undoped GaAs

我们主要研究了非掺杂半绝缘GaAs光猝灭的机理。光猝灭效应是指EL2的所有光学和电学性质在90K的近红外光照射下消失的现象。这种效应被归因于EL2缺陷向亚稳态的转变，而最近的一些报道表明，EL2以外的其他点缺陷也起到了作用。因此，光猝灭效应的研究对于实现高质量的GaAs光猝灭晶体具有重要意义。我们今年的发现如下：(1)EL2的光猝灭不是自己发生的。其他缺陷的贡献是必不可少的。通过测试一系列只改变碳受体浓度的样品，我们发现碳受体对光猝灭效应的激活有很强的影响。(2)通过对EL2光猝灭的温度依赖关系的定量分析，我们发现位于价带上方70-80 meV的深受主在光猝灭效应中起着重要的作用。(3)这个能级称为驱动能级，它同时解释了光猝灭效应的碳浓度和温度依赖关系。通过假设激励器能级在中性时触发EL2的光猝灭，在负电离时触发EL2的光恢复。(4)通过thermally-stimulated-current(TSC)measurements，实验，我们证实在带隙中的相应位置存在深度受主。经猝灭光照射后，其电荷态由原来的电离态变为中性，这与我们的致动器能级模型一致。

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.