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The Stranski–Krastanow growth of SiGe islands by deposition of SiGe alloys instead of pure Ge allows us to control both the Ge concentration and gradient in the islands. In contrast to the commonly found increasing Ge content with island height, growth conditions for islands with nearly constant and even decreasing Ge profile along the growth direction were found. Atomic force microscopy, transmission electron microscopy and high-resolution x-ray diffraction were employed to determine the islands' size, shape, lateral distance and Ge composition. Efficient photoluminescence is emitted from these islands. We show that for islands with higher Ge contents at the bottom than at the apex, transitions between heavy holes and electron $Δ_{xy}$ states in the compressive Si regions around the island's circumference dominate the photoluminescence spectra instead of the usually observed recombination between heavy holes and electrons in the $Δ_z$ valleys in the tensile Si above the island's apex. The relative importance of the $Δ_{xy}$ transitions is enhanced for lateral island distances less than 10 nm, where overlapping strain fields of neighbouring islands increase the compressive strain in the Si region between them. At intense photoexcitation, recombinations between electrons in the $Δ_z$ valleys and light holes within the islands appear in the photoluminescence spectra. These so far, for SiGe islands, unobserved transitions were identified by a quantitative modelling of the band structure within the islands and in the surrounding Si matrix based on full 3D simulations using the nextnano$^3$ package with the experimentally obtained island shape and composition as input parameters

Combined structural and photoluminescence study of SiGe islands on Si substrates: comparison with realistic energy level calculations