SELF-OSCILLATIONS AT PHOTOINDUCED IMPURITY BREAKDOWN IN GAAS

The effects of microwave (MW) irradiation oil photoexcited free and donor-bound electrons are studied in semi-insulating, bulk GaAs in the temperature range of 2-20 K. The main observations are as follows: an abrupt increase in the photoinduced microwave absorption (PMA), a concurrent decrease of the donor-acceptor pair photoluminescence and self-oscillations in the PMA when the incident MW power exceeds a threshold value. The PMA self-oscillation frequency varies with photoexcitation intensity and incident MW power in the range of 0.5-3 kHz. These nonlinear phenomena are observed under spatially uniform MW irradiation and photoexcitation, without any electrical contacts. A model is developed for the dependence of the free and donor-bound electron densities on the microwave power and photoexcitation intensity under the conditions of donor impact ionization (breakdown) by the MW heated free electrons. A linear dependence of the electron temperature on the free-electron density is assumed. The self-oscillation frequency is determined by the remote donor-acceptor recombination rate. The model-calculated steady state and self-oscillations of the free-electron density agree well with the observed PMA behavior. It is thus shown that spatially uniform self-oscillations are an intrinsic property of the photoinduced impurity breakdown.