Direct evidence for a charge-controlled dipolar structure of the EL2 complex center in semi-insulating GaAs

Abstract

EL2 center is a key defect controlling the electrical and optical properties of semi-insulating GaAs. Controversial hypotheses were proposed to establish its quantum scheme and to relate it to possible chemical or structural origins. Very recently a model was proposed that included a shallow donor impurity and a lattice defect; these two charged centers are associated in a dipolar structure corresponding to two different possible configurations depending on the charge state of the dipole. This should explain unexpected phenomena such as the persistent photocapacitance, photoconductivity, and photoluminescence quenching. Here we bring some direct experimental evidence supporting and completing the proposed theoretical model. The two distinct energy-level schemes related to the two different configurations are selectively revealed in electrical thermally-stimulated conductivity experiments and this conclusively confirms the model.