Exciton fine structures and energy transfer in single InGaAs quantum-dot molecules

We present a spectroscopic study of single quantum-dot molecules (QDMs) formed by two closely stacked In_0.5Ga_0.5As/GaAs layers. It was found that the interdot coupling and directional energy transfer between the two adjacent dots can be controlled by temperature tuning. Direct and indirect excitons, as well as charged excitons in single QDMs were classified and identified by excitation-power dependent, excitation-energy dependent and polarization-resolved micro-photoluminescence measurements. With the increasing temperature, the direct-exciton intensity decreases while the indi-rect-exciton intensity increases. A rate equation model considering phonon mediated processes has been developed. The directional energy transfer in QDMs is explained in terms of the phonon-assisted tunnelling of hole between the two adjacent dots.