Femtosecond Exciton Dynamics of II-VI Semiconductor Multiple Quantum Wells (Invited)

Abstract

Room temperature excitonic absorption peaks have been difficult to observe in II-VI semiconductors and has been attributed to strong exciton-phonon interactions. The first well-defined room temperature excitonic absorption peaks were measured in CdZnTe/ZnTe multiple quantum wells (MQWs), grown by molecular beam epitaxy on GaAs substrates. We report the first direct measurement of the relative bleaching strength of room temperature excitonic absorption by "cool" free or ionized electron-hole (e-h) pairs and "cold" excitons. In these II-VI MQWs, the exciton binding energy (E b x ≈23meV), the longitudinal-optic (LO) phonon energy (ℏωLO≈25 meV), and the thermal energy (kT ≈ 26 meV) are all comparable and thus the ionized e-h pairs are "cool" (ΔE ≈ 2 meV), in contrast to the "hot" ionized e-h pairs (ΔE ≈ 27 meV) generated in GaAs/AlGaAs MQWs Ebx≈9 meV,ℏωLO≈36 meV. Utilizing 80 fs duration pump pulses to resonantly create a distribution of "cold" excitons followed by a broad band 14 fs probe pulse to measure the induced transmission, we found that the bleaching effect of "cold" neutral excitons on the excitonic absorption was nearly twice as strong as the same density of "cool" uncorrelated e-h pairs, in contrast to theoretical estimates. We have measured a very fast exciton ionization time, τion ≈ 110 fs, which agrees well with the measured homogeneous exciton linewidth broadening, supporting a model of thermal broadening due to the ionization of excitons through LO-phonon scattering. These are the first reported room temperature experiments on femtosecond exciton dynamics in II-VI semiconductor MQWs.