PRESSURE-INDUCED NEGATIVE CHARGE STATE OF THE EL2 DEFECT IN ITS METASTABLE CONFIGURATION

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 he level manifests itself in optically induced persistant changes of the electrical conductivity and DLTS signal, driven by EL2 photoquenching and extremely efficient EL2 photorecovery processes. These effects 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 existence of this level to the predictions of the EL2 theoretical models.