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 molecularbeam epitaxy of semipolar AlN(112¯2) and GaN(112¯2) films on (11¯00) m-plane sapphire. AlN deposited on m-sapphire settles into two main crystalline orientation domains, AlN(112¯2) and AlN(101¯0), whose ratio depends on the III/ V ratio. Growth under moderate nitrogen-rich conditions enables to isolate the (112¯2) orientation. The in-plane epitaxial relationships of AlN(112¯2) on m-plane sapphire are [112¯3¯]AlN║[0001]sapphire and [11¯00]AlN║[112¯0]sapphire. GaN deposited directly on m-sapphire results in (112¯2)- oriented layers with (101¯3¯)-oriented inclusions. A ~100 nm-thick AlN(112¯2) buffer imposes the (112¯2)-orientation for the GaN layer grown on top. By studying the Ga-desorption on GaN(112¯2), we conclude that these optimal growth conditions corresponds to a Ga excess of one monolayer on the GaN(112¯2) surface.