Epitaxial orientation of III-nitrides grown on $R$-plane sapphire by metal-organic-vapor-phase epitaxy

The absolute epitaxial relationships between III-nitride films and $R$-plane sapphire are investigated using convergent beam electron diffraction. The resulting relationships are found to be$[1−100]III-N‖[11−20]sapphire$ and $[0001]III-N‖[−1101]sapphire$⁠, respectively. The O–Al bonds in the sapphire surface are antiparallel to the N-III bonds in the III-N films. High-resolution transmission electron microscopy shows that the AlN nucleation layer is asymmetrically strained. Along its $c$ direction, the AlN layer is in extension whereas it is relaxed perpendicular to $c$⁠.