Mechanism for embedded in-plane self-assembled nanowire formation

We report a growth mechanism that produces in-plane $[1\overline{1}0]$ oriented ErSb nanowires formed during codeposition of $\mathrm{E}{\mathrm{r}}_0.3\mathrm{G}{\mathrm{a}}_0.7\mathrm{Sb}$ 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. 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 $\mathrm{GaSb}{110}$ 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.