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Direct Evidence for a Charge-Controlled Optical Quenching of EL2 Centers in Semi-Insulating GaAs

We present results of microwave photoconductivity as well as far-infrared laser magnetooptical absorption measurements obtained for LEC (liquid encapsulated Czochralski)-grown semi-insulating GaAs crystals. At low temperature we have observed a “photo-quenching effect” both for signals of microwave photoconductivity and for the shallow donor Zeeman absorption measured using a far-infrared laser, under the condition of BBG (below-band-gap) photoexcitation. Through these experimental observations, we conclude that the shallow donor is associated with the metastable state of the EL2 center in LEC-grown semi-insulating GaAs. From the temperature dependence of microwave photoconductivity decay with BBG pulsed photoexcitation, we can estimate the characteristic activation energy, ΔEA=240 meV, which originates from a barrier in the conduction band related to the Franck-Condon shift of the EL2 metastable state.