Nonequilibrium phonon effects in the energy relaxation of hot carriers in quantum wells

Abstract

The energy relaxation of photoexcited hot electrons and holes in quantum well structures has been studied extensively using time-resolved photoluminescence techniques. The energy loss rates (ELR) for both types of carrier have been systematically measured using undoped and modulation-doped structures for a wide range of well widths. It is now well established that the ELR is reduced in low dimensional structures, especially for intense photoexcitation, and this effect has been explained, at least in part, by invoking the presence of nonequilibrium phonons which are generated in the relaxation process. Our measurements show that the ELR is not a strong function of well width for either low or high excitation densities, although in the former case the electron rate is substantially lower than that for holes. For intermediate excitation densities we find a substantial increase in the ELR both in GaAs and GalnAs structures for decreasing well width. Theoretical calculations of the ELR of electrons and holes have been made using a model in which carriers confined to a single subband interact with bulk optical phonons. In wide wells the wave vector to which the carriers couple lies within the plane of confinement. For narrower wells there is an increased coupling to out-of-plane modes due to the relaxation of momentum conservation. Under these conditions the carriers then couple to a larger number of phonon modes which reduces the nonequilibrium phonon population. © 1988 SPIE.