  We report ab initio investigations of hexagon-shaped, [111]/[0001] oriented
III-V semiconductor nanowires with varying crystal structure, surface
passivation, surface orientation, and diameter. Their stability is dominated by
the free surface energies of the corresponding facets which differ only weakly
from those of free surfaces. We observe a phase transition between local
zinc-blende and wurtzite geometry versus preparation conditions of the
surfaces, which is accompanied by a change in the facet orientation. The
influence of the actual III-V compound remains small. The atomic relaxation of
nanowires gives rise to smaller bond lengths in comparison to the bulk
zinc-blende structures resulting also in somewhat reduced bilayer thicknesses
parallel to the growth direction.
