Piezoelectric photo-acoustic measurements were performed at 90K in order to investigate non-radiative recombination processes in the photo-quenching of EL2 defect levels in semi-insulating material. Time-dependent photo-acoustic spectral measurements were carried out under continuous illumination. A decrease in the photo-acoustic signals, due to an EL20 to EL2* transition, was observed during a period of illumination of about 180s; using incident photon energies of 1.0 to 1.2eV. Further illumination, for more than 600s, using the same photon energies again increased the photo-acoustic signal via a local minimum, and the signal exceeded the pre-illumination value up to saturation. A complex electron transition mechanism was used to explain the experimental results.

A.Fukuyama, T.Ikari, Y.Akashi, K.Futagami: Materials Science Forum, 1995, 196-201, 1031-6