InAs-InP coupled quantum dot systems

We have studied the alternate stacking of quantum dot (QD) layers of different materials, more specifically, the coupling of InAs and InP. InAs QDs grown on a GaAs buffer layer were buried by a GaAs spacer layer, on top of which InP islands were grown. The microstructure of such a system and the InP islands morphology and density were studied as a function of the GaAs spacer thickness. For a spacer thickness of 2 nm, a rough surface comprised by a mound density equivalent to the buried InAs QD density could be inferred from XTEM, HRTEM and AFM experiments. XTEM and HRTEM showed InP islands nucleated at both, mound tops and in between mounds. Only those islands nucleated at mound tops were over an InAs QD. On the other hand, a 5 nm GaAs spacer is enough to obtain a relatively flat surface and InP islands which are defect-free and vertically aligned to buried InAs QDs. AFM scans of the InP QDs showed that their size dispersion reached a minimum for the 5 nm GaAs spacer but their size increased monotonically with the spacer thickness. We found, for the InP islands, that their size uniformity improved and their size has decreased when compared to the one obtained for single InP layers, being strongly dependent on the GaAs spacer thickness.