Slow relaxation of excited states in strain-induced quantum dots

We have studied photoluminescence from GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{-}\mathit{x}}$As strain-induced quantum dots in a magnetic field. These dots have high radiative efficiency and long (∼ns) luminescent decay times. At low excitation intensities, corresponding to average carrier densities of less than one electron-hole pair per dot, excited-state (‘‘hot’’) luminescence due to slow interstate relaxation is observed. At intermediate intensities, where there are several electron-hole pairs per dot, the hot luminescence disappears, showing that the relaxation rate has increased. However, the excited-state emission reemerges at high excitation when the ground state is saturated. The interstate relaxation rate in the quantum dots under low excitation is at least two orders smaller than that of the host quantum well. The reduced rate is attributed to the discrete density of states in a quantum dot, which inhibits single-phonon emission because the excitons are spatially too large to couple to phonons with the required energy. When there are several electron-hole pairs per dot, carrier-carrier interaction accelerates relaxation. The magnetic field is used to separate the quantum dot states and allows us to probe how their relaxation depends on energy. We find that there is a strong increase in the relaxation rate when the sublevel energy exceeds about 20 meV. © 1996 The American Physical Society.