Dynamics of photo-excited carriers in self-assembled quantum dots

Abstract

Carrier dynamics in InAs/GaAs self-assembled quantum dots have been studied by using time-resolved photoluminescence experiment. We have studied a series of doped quantum dot structures by looking at the role of the experimental conditions, such as the laser excitation intensity, the crystal temperature and the intersublevel energy, on the carrier relaxation time. For all samples, we have found two distinct relaxation regimes. At a crystal temperature of 77K, a rise time of the quantum dot emission signal of a few tens of ps has been measured under low photocarrier densities. This rise time decreases significantly, down to few ps, as the laser intensity increases. These results show that carrier-carrier scattering processes play a significant role at high photo-excited carrier densities. Under the low-excitation regime, the dot emission rise time depends on the dopant type, on the doping level on the degree of intermixing and on the temperature. Our results obtained on structures having a relatively low density of dots indicate that transport processes (diffusion and localization at the InAs/GaAs interfaces) limit the dot capture efficiency at low temperatures. The experimental conditions and the dot structural parameters that give rise to ultrafast capture and intra-dot relaxation times are discussed.