No abstract

GaN directly deposited on m-sapphire by plasma-assisted molecular-beam epitaxy settles into two main crystalline orientation domains: GaN(112¯2) and GaN{101¯3}. The dominant phase is GaN(112¯2) with ⟨112¯3¯⟩GaN∥⟨0001⟩sapphire and ⟨11¯00⟩GaN∥⟨112¯0⟩sapphire in-plane epitaxial relationships. Deposition of GaN on top of an AlN(112¯2) buffer layer and growth under slightly Ga-rich conditions reduce GaN{101¯3} precipitates below the detection limits. Studies of Ga adsorption demonstrate that it is possible to stabilize up to one Ga monolayer on the GaN(112¯2) surface. The presence of this monolayer of Ga excess on the growth front reduces the (112¯2) surface energy and hence minimizes the surface roughness. Photoluminescence from two-dimensional GaN(112¯2) layers is dominated by a near-band-edge emission, which is assigned to excitons bound to stacking faults, present with a density around 3×105cm−1.