Relaxation and recombination in ultrasmall InAs quantum dots

We report a photoluminescence study of self-organized nanometer-size InAs quantum dots grown by molecular beam epitaxy on a GaAs substrate. High optical excitation has been used in order to observe emission from higher states of the quantum dots. The energy difference between adjacent states turns out to be of the order of 40-50 meV for dot diameters around 20 nm. The photoluminescence decay time at the fundamental transition is found to be of the order of 700 ps, decreasing down to 100 ps for the highest confined states. Finally, a cascade-like mechanism for the carrier relaxation in these structures is strongly suggested by the time resolved data.