Carrier dynamics in low-temperature-grown ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}/\mathrm{GaAs}$ multiple quantum wells

Femtosecond pump-probe spectroscopy has been used to study ultrafast relaxation dynamics in low-temperature-grown AlGaAs/GaAs multiple quantum wells (MQW’s). It was found that the carrier lifetime of as-grown MQW’s was much longer than those that had been subjected to 30 s rapid thermal annealing. The carrier lifetimes of samples annealed to three different temperatures (600, 700, and $800°\mathrm{C})$ were measured and that annealed to $700°\mathrm{C}$ was found to be shorter than that of the other two. We also observed the quantum beats of heavy-hole and light-hole exciton in as-grown AlGaAs/GaAs MQW’s at low temperature, but not in the annealed samples. The beat frequency was 3.7 THz, corresponding to the energy difference between heavy- and light-hole excitons in the as-grown AlGaAs/GaAs MQW’s. As the temperature rose, the beats disappeared. We gave two possible reasons to explain this phenomenon. We tuned the central frequency of femtosecond Ti:sapphire laser through the resonant frequencies of heavy holes and light holes, while the phase of the quantum beats did not change.