Ternary
III–V alloys of tunable bandgap are a foundation
for engineering advanced optoelectronic devices based on quantum-confined
structures including quantum wells, nanowires, and dots. In this context,
core–shell nanowires provide useful geometric degrees of freedom
in heterostructure design, but alloy segregation is frequently observed
in epitaxial shells even in the absence of interface strain. High-resolution
scanning transmission electron microscopy and laser-assisted atom
probe tomography were used to investigate the driving forces of segregation
in nonplanar GaAs–AlGaAs core–shell nanowires. Growth-temperature-dependent
studies of Al-rich regions growing on radial {112} nanofacets suggest
that facet-dependent bonding preferences drive the enrichment, rather
than kinetically limited diffusion. Observations of the distinct interface
faceting when pure AlAs is grown on GaAs confirm the preferential
bonding of Al on {112} facets over {110} facets, explaining the decomposition
behavior. Furthermore, three-dimensional composition profiles generated
by atom probe tomography reveal the presence of Al-rich nanorings
perpendicular to the growth direction; correlated electron microscopy
shows that short zincblende insertions in a nanowire segment with
predominantly wurtzite structure are enriched in Al, demonstrating
that crystal phase engineering can be used to modulate composition.
The findings suggest strategies to limit alloy decomposition and promote
new geometries of quantum confined structures