EL2 in photoconductivity spectra of Cr-doped SI GaAs bulk crystals

Extrinsic photoconductivity measurements at 70 and 293 K in Cr-doped SI GaAs bulk crystals have been carried out in order to characterise the deep levels in this material. The photoconductivity spectra reveal two characteristic structures extended in the low-energy and the high-energy regions, respectively. It is shown that low-energy structure is connected with the Cr 2+ deep acceptor level and the high-energy one with the EL2 deep donor level. The spectrum measured after an intentionally photo-induced transfer of EL2 into its metastable state confirms the authors' interpretation of the origin of the high-energy structure. They believe that their study is the first to identify the EL2 contribution to the photoconductivity spectra of Cr-doped SI GaAs. The optical ionisation energy with respect to the conduction band and the Frank-Condon parameter of Cr 2+ centres are obtained from the experimental results.