Connecting Composition-Driven Faceting with Facet-Driven Composition Modulation in GaAs–AlGaAs Core–Shell Nanowires

Abstract

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.

Segregation scheme of indium in AlGaInAs nanowire shells

“…It is possible to distinguish the GaAs core by the absence of In and Al. In the shell, radial segregation of Al along the ridges between two facets of the hexagonal NW core is visible as the three Al-rich stripes in the reconstructed NW volume, in agreement with previous works [15,16,18,19,29,30]. In addition to the Al-rich planes, APT reveals that the shell distribution of the other group-III atoms is not perfectly random throughout the NW shell.…”

“…The competition between In and Al to stick on the {112} nanofacets strongly favors Al on the B-polar surfaces: this mechanism may generate a net flux of In atoms towards the surroundings of the {112}B Al-rich planes. The literature on GaAs-AlGaAs core-shell NWs also reports that the {112}B nanofacets are systematically wider than the {112}A ones [18,30]; concomitantly, the surface-curvature gradient from the {110} to the {112} facets, depends on the polarity of the {112} nanofacet [30]. The polarity-dependent width and curvature of the NW surface may facilitate the In incorporation in proximity of the B-polar directions by directly acting of the local NW surface energy or by influencing the strain relaxation.…”

“…Previous reports show that the composition of an AlGaAs shell deposited around a GaAs NW exhibits nanoscale fluctutations [15,16,[27][28][29][30]. This phenomenon depends on the different surface mobility of Ga and Al during growth and the presence of surface-potential gradients (μ) on the NW sidewalls.…”

“…The ridges between two of these facets are {112} nanofacets with alternating A-and B-polar (Ga-and As-terminated) surfaces [18,19,30]. {110} and {112} facets have different and species-dependent sticking coefficients [30]. In addition, the increased surface curvature at the ridges increases the local surface energy and affects the adatom mobility.…”

Tuning adatom mobility and nanoscale segregation by twin formation and polytypism

“…It is often observed that the sidewalls of twinned NWs are composed of alternated {111}Aand {111}B-type facets rather than the flat {110} type [57]. In our case, like in [50], our measurements cannot prove that the NW facets become atomically rougher in presence of twins and stacking faults. For simplicity, we neglected the surface roughness in the discussion of the second mechanism of segregation enhancement.…”