DIRECT EVIDENCE FOR A CHARGE-CONTROLLED OPTICAL QUENCHING OF EL2 CENTERS IN SEMIINSULATING GAAS

We present results of microwave photoconductivity as well as far-infra red laser magnetooptical absorption measurements obtained for LEC (liq uid 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 Zeem an absorption measured using a far-infrared laser, under the condition of BBG (below-band-gap) photoexcitation. Through these experimental o bservations, 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 ac tivation energy, DELTAE(A)=240 meV, which originates from a barrier in the conduction band related to the Franck-Condon shift of the EL2 met astable state.