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

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