Optically active higher-order bands were investigated at low temperatures in fast-neutron irradiated float-zone material by using a Fourier transform infra-red spectrometer with optical excitation. It was found that the photo-excitation process exhibited an exponential time dependence, and that the decay was logarithmic; with a time-constant of 105s at 7K. The saturation value of the absorption coefficients depended upon the logarithm of the illumination intensity. These characteristics were associated with the slow relaxation of photo-excited carriers which originated mainly from defect clusters. The relaxation behavior of photo-excited carriers was analyzed by using a macroscopic barrier model. This gave good agreement with the observations.

Y.Shi, F.M.Wu, Y.D.Zheng, M.Suezawa, M.Imai, K.Sumino: Journal of Physics - Condensed Matter, 1994, 6[41], 8645-53