Self-oscillations at photoinduced impurity breakdown in GaAs.

The effects of microwave {MW) irradiation on 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.