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We present the results of electrical-conductivity and low-temperature deep-level transient spectroscopy (DLTS) measurements performed with use of monochromatic light at hydrostatic pressures up to 0.7 GPa on n-type GaAs samples containing the EL2 defect. Based on our results we give clear experimental evidence that there exists an acceptorlike level of the metastable EL2 configuration. Without pressure this level is resonant with the conduction band and therefore unoccupied, but under pressure it enters the energy gap capturing free electrons and leading to the negative charge state of the metastable EL2. The electrical activity of the level manifests itself in optically induced persistent changes of the electrical conductivity and DLTS signal, driven by EL2 photoquenching and extremely efficient EL2 photorecovery processes. These eff'ects vanish at the temperature at which EL2 thermally recovers its normal configuration. We exclude the "negative-U" case for the level and determine its energetic position and the pressure shift. We claim that the negative charge state of the metastable EL2 is responsible for the optical accessibility of the metastable configuration. Finally, we refer the experimentally confirmed existence of this level to the predictions of the EL2 theoretical models.

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%e present experimental results showing that at hydrostatic pressure exceeding approximately 0.3 GPa the EL2 metastable state becomes optically active in both n-type and semi-insulating GaAs, and the pressure-induced optical recovery (PIOR) can be easily observed. PIOR is practically full and extremely efficient (with an efficiency comparable with that of the photoquenching process). The proper choice of the energy of the illuminating light allows reversible cycles of photoquenching and subsequent optical recovery of the characteristic intra-EL2 absorption at 5 K. PIOR is a few orders of magnitude more efficient than pure optical recovery processes already reported for atmospheric pressure.It is well known that in GaAs the EL2 defect in its neutral charge state manifests itself in the photoionization background starting from 0.77 eV and the charactertistic

It is shown with correlated magnetic resonance and electrical measurements that the PIn antisite is the prevailing defect in InP grown by molecular-beam epitaxy at low temperature. The first ionization level of the PIn antisite is resonant with the conduction band, which makes the material n-type conducting due to autoionization of the PIn antisite.

We present an experimental study of the EL2-defect thermal recovery in n-type GaAs under hydrostatic pressure up to 1.2 GPa. The most characteristic experimental result is the nonmonotonous pressure dependence of the temperature at which the EL2 defect thermally recovers. We solve numerically the equations describing the recovery process, and we prove that in order to explain our experimental data it is absolutely necessary to take into account the existence of the recently discovered acceptorlike level of the metastable EL2. Moreover, we show that the most straightforward explanation of all our experimental results should assume that in n-type GaAs the recovery process always proceeds via the negative-charge state of the metastable EL2. Since our explanation of the recovery process does not require the recovery rate to be directly proportional to the free-electron concentration, the problem of the so-called Auger-like thermal recovery of the EL2 defect in n-type GaAs seems to be finally gone.