Shell morphology and Raman spectra of epitaxial Ge-Si_xGe_(1-x) and Si-Si_xGe_(1-x) core-shell nanowires

摘要

We investigate the shell morphology and Raman spectra of epitaxial Ge-Si_xGe_(1-x) and Si-Si_xGe_(1-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 <111>, and <110> or <112> directions, respectively. A hexagonal cross-sectional morphology is observed for Ge-Si_xGe_(1-x) core-shell nanowires terminated by six {112} facets. Two distinct morphologies are observed for Si-Si_xGe_(1-x) core-shell nanowires that are either terminated by four {111} and two {100} planes associated with the <110> growth direction or four {113} and two {111} planes associated with the <112> growth direction. We show that the Raman spectra of Si- Si_xGe_(1-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 <112> core-shell nanowires compared to their (110) counterparts. We compare the Si-Si Raman mode value with calculations based on a continuum elasticity model coupled with the lattice dynamic theory.