Shape evolution of InAs quantum dots during overgrowth

Abstract

The effects of strain and thickness of an In x Ga 1− x As ( x=0−0.2) cap layer grown at low temperature on large low-growth-rate InAs quantum dots (QDs) are systematically studied by atomic force microscopy. The dot height drastically reduces and the dot shape transforms into an elongated ridge-valley structure at the early stage of GaAs overgrowth, while the dots tend to preserve their shape during InGaAs capping. The effects of elastic energy and surface energy included in the surface chemical potential can qualitatively explain the observed surface evolution. The increase of the elastic energy of the QDs and surface energy of the cap drives the In atoms away from the InAs QDs to the GaAs surface. The In atom migration away from the InAs QDs and the unfavorable growth of GaAs on top of the QDs results in a collapse of the InAs QDs and a valley formation at the beginning of the GaAs overgrowth. In case of the In x Ga 1− x As cap layer, an increased In content decreases the elastic energy and also the cap layer surface energy, resulting in a reduction of the chemical potential gradient and consequently a preservation of the QD shape. At high overgrowth temperature, the material transport from the dots to the cap surface is less pronounced due to increased In–Ga intermixing.