Slow-relaxation phenomena in photoconductivity for semi-insulating GaAs

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Abstract
A new type of slow-relaxation phenomena is reported together with the conventional fatigue phenomenon in photoconductivity for undoped semi-insulating liquid-encapsulated Czochralski GaAs. First, conventional fatigue is induced by the ∼1.1-eV light irradiation (primary light). Secondary-light irradiation (∼0.8 eV) subsequently carried out is found to generate a rapid increase followed by a gradual exponential decrease in photoconductivity. The latter phenomenon shows very different characteristics from those of conventional fatigue, though the two phenomena are similar to each other. Through the detailed experimental studies, a configuration-coordinate model is presented, involving a main deep level X and its metastable excited state X* with large lattice relaxation. This phenomenon is explained by the transition of electrons which occurs in part from level X* excited by the preceding primary-light irradiation, via level X, to the conduction band as a result of secondary-light irradiation.