  We report a novel growth mechanism that produces in-plane [1-10] oriented
ErSb nanowires formed during codeposition of Er0.3Ga0.7Sb via molecular beam
epitaxy (MBE). Nanowires are characterized by in-situ scanning tunneling
microscopy (STM), as well as ex-situ transmission electron microscopy (TEM) and
electron channeling contrast imaging (ECCI). We show that complexes of
macrosteps with step heights on the order of 7 nm form during nanowire growth.
The macrosteps are shown to be part of the in-plane nanowire growth process and
are directly responsible for the observed stratified distribution of in-plane
nanowires. TEM indicates that initial growth results in out-of-plane nanowires
transitioning to in-plane nanowires after a critical film thickness. A surface
energy model is put forward that shows the critical thickness is due to
minimization of the GaSb{110} surfaces formed during out-of-plane nanowire
growth. Kinetics of the transition are discussed with respect to observed
features in STM, along with the material parameters needed to achieve in-plane
nanowire growth.
