Long-lived intermediate state in strain-confined quantum dots

We have studied carrier transfer to strain confined dots from a host quantum well, using selectively excited time-resolved photoluminescence spectroscopy. We find that the photoluminescence decay rate after picosecond excitation is much slower when excitation is indirect, via transfer from the quantum well, then when it is directly into the dot, even though the decay of the quantum well photoluminescence is fast. This result is inconsistent with a simple three level kinetic model of the transfer process. We propose a tentative model involving an intermediate nonradiative state, possibly the charge separated state predicted theoretically.