Carrier Transfer in Closely Stacked GaAs/AlGaAs Quantum Dots Grown by Using Droplet Epitaxy

Abstract

We investigate the carrier transfer in stacked droplet epitaxially grown GaAs quantum dots (QDs) in experiments and calculations. While in the Stranski-Krastanov growth mode, QDs align due to stain propagation, droplet epitaxy QDs pose a difficulty for achieving coupled stacked QDs due to their random positioning. We demonstrate that carrier transfer is possible in such structures by designing their size and areal density. We achieve a significant geometrical overlap between stacked QDs by employing an areal density of 3.9×1010 dots/cm2 and an average QD diameter of 45.5 nm. A clear redshift in the position of the photoluminescence peak is observed when the separation layer’s thickness is reduced from 16 nm to 2.5 nm. Theoretical calculations of the electronic states of the stacked QDs with varying degrees of misalignment confirm that this red-shift is mainly caused by a lowering of the ground state energy due to coupling. To separately analyze the effect of vertical carrier transfer between QDs, we investigate samples with two layers of stacked QDs of different sizes. We demonstrates in photoluminescence experiments that carriers readily transfer to the larger QD when the barrier thickness is reduced to a degree where tunneling is possible.