Mechanisms of surface alloy segregation on faceted core-shell nanowire growth

A general two-dimensional faceted model that accounts for capillarity and deposition of an A_xB _1−x alloy is developed for the growth of the shell on a hexagonal core. With this model, the surface alloy segregation and morphological evolution in the processes of the faceted core-shell nanowire growth are studied both analytically and numerically. Mechanisms of formation of Al-rich stripes along {112} facets and Al-poor quantum dots/wires at the apices of {112} facets are identified. More specifically, it is found that diffusion tends to move the atoms from {112} facets to {110} facets. The formation of Al-rich stripes along the {112} facets is due to the large ratios of mobilities of Al atoms and Ga atoms on {112} facets, even though Al atoms diffuse slower than Ga on the {110} facets. In addition, the difference of interaction parameters in the enthalpy on different facets can also lead to lines of enhanced concentration of Al behind {112} facets. If the attachment rates of Al on the {112} facets are smaller than that on {110} facets, Al-poor dots will grow at the end of the Al-rich stripes because the growth process switches from diffusion dominant to deposition dominant when the size of the nanowire gets large. Moreover, influences of different parameters on the distribution of concentrations of the atoms in the shell are investigated in details.