Molecular beam epitaxy of semipolar AlN($11\bar{2}2$) and GaN($11\bar{2}2$) on m-sapphire

Abstract

We report on the plasma-assisted molecular-beam epitaxy of semipolar $\hbox{AlN}(11\bar{2}2)$ and GaN($11\bar{2}2$) films on $(1\bar{1}00)$ m-plane sapphire. AlN deposited on m-sapphire settles into two main crystalline orientation domains, $\hbox{AlN}(11\bar{2}2)$ and $\hbox{AlN}(10\bar{1}0),$ whose ratio depends on the III/V ratio. Growth under moderate nitrogen-rich conditions enables to isolate the $(11\bar{2}2)$ orientation. The in-plane epitaxial relationships of $\hbox{AlN}(11\bar{2}2)$ on m-plane sapphire are $[11\bar{2}\bar{3}]_{\rm AlN} \vert \vert [0001]_{\rm sapphire}$ and $[1\bar{1}00]_{\rm AlN} \vert \vert [11\bar{2}0]_{\rm sapphire}.$ GaN deposited directly on m-sapphire results in ($11\bar{2}2$)-oriented layers with ($10\bar{1}\bar{3}$)-oriented inclusions. A ∼100 nm-thick AlN($11\bar{2}2$) buffer imposes the ($11\bar{2}2$)-orientation for the GaN layer grown on top. By studying the Ga-desorption on GaN($11\bar{2}2$), we conclude that these optimal growth conditions corresponds to a Ga excess of one monolayer on the GaN($11\bar{2}2$) surface.