Pressure-induced negative charge state of theEL2 defect in its metastable configuration

Abstract

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.

Energy Levels and Electrical Activity of Dislocation Electron States in GaAs

“…An acceptor level associated with the metastable configuration of EL2 has been recently discovered by Baj et al [14]. This level is normally degenerate with the conduction band of GaAs but under a hydrostatic pressure exceeding 0.2 GPa it enters the bandgap.…”

Orientation of Metastable EL2 under Uniaxial Stress

“…It was found moreover that orientation of EL2* centers depended on the method of the EL2-photoquenching [2] which confirmed the attribution of EL2 metastability to the transformation AsG a ↔ VGaΑsi [3,4]. The key role in the thermal recovery of EL2 in n-type GaAs is played by the acceptor level of EL2* ([EL2*]-/0) [5,6] which is orientationally degenerate and split under [111] stress into two components: the triplet ([EL2*]T' -/ 0 ) a n d t h e singlet ([EL2*]s -/ 0 ) s u b l e v e l s . W e p e r f o r m e d t h e e l e c t r i c a l c o n d u c t i v i t y a n d H a l l effect measurements of n-type GaAs in order to investigate the electrical properties of these levels under [111] uniaxial stress.…”

“…We performed the numerical analysis of the obtained stress-dependences of electron concentration at T = 30 K assuming the energy of the unsplit [EL2*] -/0 level (Ε0 = 14 meV) from Ref. [5]. The best fit to experimental points measured after EL2-photoquenching performed under high stress, when only the [EL2*]T -/ 0 s u b l e v e l i s p r e s e n t ( a l l E L 2 * c e n t e r s a r e o r i e n t e d a s l a n t t h e a x i s o f [10].…”

Stabilization of the Distorted Configuration of the EL2 Defect Induced by the Free Electron Capture in GaAsP

“…[1] and references therein). Since the metastable defect (EL2*) can capture an additional electron (acceptor level of EL2* -see [2]), some energy can be gained in this process. If only this gain were sufficiently large, the distorted configuration of the EL2 defect could be stabilized.…”