Abstract
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 Δxy 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 Δz valleys in the tensile Si above the island's apex. The relative importance of the Δxy 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 Δz 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 nextnano3 package with the experimentally obtained island shape and composition as input parameters.
Highlights
Growth and structural investigationsSiGe islands were grown by solid source molecular beam epitaxy (MBE) in the Stranski–Krastanow growth mode
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
A series of molecular beam epitaxy (MBE)-grown SiGe islands with intentionally varied Ge flux to realize different Ge gradients along the growth direction was investigated with respect to their structural and optical properties
Summary
SiGe islands were grown by solid source molecular beam epitaxy (MBE) in the Stranski–Krastanow growth mode. The experimentally observed small lateral island distance (2 nm) and, strongly overlapping strain fields were included –HH transitions), whereas for the red arrows a large (100 nm) lateral island spacing and non-overlapping strain fields were assumed The Ge profile was varied in a fitting loop to obtain the best correspondence between simulation and experiment (figure 2) This way the Ge and strain distribution are determined. It turned out to be crucial in the simulations to take into account the close proximity of the islands, due to which their individual strain fields overlap This was accomplished by restricting the FEM modelling.
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