Slow-relaxation phenomena in photoconductivity for semi-insulating GaAs. II. Distribution among samples

Photoelectric fatigue and spike photoconductivity (SPC) are systematically studied in twenty-three pieces of undoped semi-insulating liquid-encapsulated Czochralski GaAs crystal (six ingots, four suppliers). Distributions among samples for photosensitivity, SPC-decay time constant, SPC spectrum, and oscillatory behavior are discussed. Through these studies, the metastable state X* (some kind of excited state of the main deep center X present in GaAs) is found to be classified into two states X*a and X*f , according to whether the electronic transition X*→X due to irradiation with the photon energy near 0.80 eV is allowed or forbidden, respectively. State X*a is responsible for the SPC phenomenon, while both states X*a and X*f are responsible for photoelectric fatigue. Moreover, the concentration of state X*a is estimated to be about 10−3 times that of state X*. These results suggest that there exist various configurations of metastable states.