Temporal Behavior of the Energy Relaxation of Hot Carriers in ZnO Single Crystals

Abstract

Energy relaxation of optically-generated hot carriers in ZnO is explored theoretically as a function of time. Cooling curves are extracted from the energy loss rates obtained by using the Kogan formula extended to a degenerate electron-hole hot plasma at low lattice temperatures. The energy losses of the carriers by various carrier-phonon interactions are taken into account. The screening of the carrier-phonon interactions is included within the random phase approximation. We observed an initial slow energy relaxation process followed by a rapid relaxation at both high and low carrier temperatures. The dominant relaxation process at the earlier cooling stage in the temperature range 10 2 K Tc 10 3 K is the excitations of optical phonons. The energy relaxation in the low carrier temperature range (Tc 10 2 K) is determined mainly by acoustic phonon scattering. The initial relaxation time of ZnO is found to be on the order of femtoseconds. The relaxation time is increased by a factor of ten if carrier screening eects are taken into account, irrespective of the initial carrier temperature.