Shell morphology and Raman spectra of epitaxial Ge-SixGe1-x and Si-SixGe1-x core-shell nanowires

摘要

We investigate the shell morphology and Raman spectra of epitaxial Ge SixGe1-x and SixSixGe1-x core shell nanowire heterostructures grown using a combination of a vapor liquid solid (VLS) growth mechanism for the core, followed by in-situ epitaxial shell growth using ultra-high vacuum chemical vapor deposition. Cross-sectional transmission electron microscopy reveals that the VLS growth yields cylindrical Ge, and Si nanowire cores grown along the , and or directions, respectively. A hexagonal cross-sectional morphology is observed for Ge-SixGe1-x core-shell nanowires terminated by six {112} facets. Two distinct morphologies are observed for Si-SixGe1-x core-shell nanowires that are either terminated by four {111} and two {100} planes associated with the growth direction or four {113} and two {111} planes associated with the growth direction. We show that the Raman spectra of Si-SixGe1-x are correlated with the shell morphology thanks to epitaxial growth-induced strain, with the core Si-Si mode showing a larger red shift in core-shell nanowires compared to their counterparts. We compare the Si-Si Raman mode value with calculations based on a continuum elasticity model coupled with the lattice dynamic theory. Published by AIP Publishing.