Luminescence spectroscopy of InAs self-assembled quantum dots

We present selective photoluminescence and photoluminescence excitation spectroscopy on self-assembled quantum dots. These spectroscopic techniques reveal a large apparent 'Stokes' shift in In(Ga)As-Ga(Al)As self-assembled quantum dots. We demonstrate that the first two excited states observed in PLE originate from the doubly-degenerate first excited state. The ground state is not observed in the quantum dot PLE due to the sharp d-funtion-like density of states of individual dots. Our experimental data agree with a recent theoretical calculation, suggesting that both the degeneracy-lifting and broadening of energy levels are caused by the random potential of compositional, size and strain fluctuations. Due to large energy separations in these quantum dots, we also demonstrate the importance of multi-phonon relaxation processes. (C) 1997 Academic Press Limited