Nature of the Stranski-Krastanow Transition during Epitaxy of InGaAs on GaAs

We report first quantitative measurements by energy-selected imaging in a transmission electron microscope of In segregation within an uncapped islanded ${\mathrm{In}}_{0.25}{\mathrm{Ga}}_{0.75}\mathrm{As}$ layer grown epitaxially on GaAs. This layer has the lowest In concentration at which islanding occurs and, then, only after a flat $\sim 3\mathrm{nm}$ alloy layer has been formed. In buildup by segregation at the surface of this initial flat layer is considered the driving force for islanding and, importantly, the segregation process introduces the characteristic delay seen before the Stranski-Krastanow transition. We observe strong inhomogeneous In enrichment within the islands (up to $x\left(\mathrm{In}\right)\approx 0.6$ at the apex) and a simultaneous In depletion in the remaining flat layer.