Faceting of local droplet-etched nanoholes in AlGaAs

Nanoholes, drilled in the (001) surface of AlGaAs by local Al droplet etching, are shown to consist of faceted inner walls. The most prominent facets of the inverted pyramidlike nano-sized etch pits are the {111}A and {1$\bar{1}$1}B surfaces, which differ in their atomic surface terminations. In the [110] direction, the {111} facets change to {112} and/or {113}, which are both stepped surfaces with (111)A terraces. Etching-temperature-dependent data indicate that this facet transition seems kinetically hindered up to etch temperatures above 660 °C, at which point the walls along [1$\bar{1}$0] have already evolved completely towards {1$\bar{1}$1}B facets. The redeposited ring material outside the nanohole develops facets with indices of (115) and higher, thereby forming relatively flat structures. The facets and their indices are unraveled by a combination of atomic force microscopy, scanning electron microscopy, and x-ray diffraction, which is performed on an ensemble as well as on a single hole using nanodiffraction. These results imply that this nanoconfined etching process can be largely understood in a vapor-liquid-solid scheme, which includes the bulk thermodynamics in the Al-Ga-As system, the surface energies of low index facets, and their etch rates and surface terminations