Epitaxial growth of Ge-Si$_{x}$Ge$_{1-x}$ core-shell nanowire heterostructures with tunable shell content

Core-shell nanowire heterostructures are an interesting testbed for band engineering at the nanoscale. Here we present the growth of germanium (Ge) -- silicon-germanium (Si$_{x}$Ge$_{1-x})$ epitaxial core-shell nanowire (NW) heterostructures, with tunable Si and Ge shell content. The Ge NWs were grown using the Au-catalyzed vapor-liquid-solid (VLS) growth mechanism. Subsequently, the Si$_{x}$Ge$_{1-x}$ shells are grown \textit{in-situ}, conformal onto the Ge NW using ultra-high-vacuum chemical vapor deposition. We use transmission electron microscopy to confirm that both the core and shell are single crystal, and cross-sectional scanning transmission electron microscopy energy dispersive x-ray spectroscopy to determine the shell thickness and content. Our data show that the Si and Ge shell content can be tuned depending on the SiH$_{4}$ and GeH$_{4}$ partial pressures during the shell growth, effectively enabling band engineered core-shell nanowire heterostructures.