Interfacial structure of semipolar AlN grown on m -plane sapphire by MBE

Abstract

The interfacial properties of epilayers grown on m-plane sapphire by plasma-assisted molecular-beam epitaxy (PAMBE) were investigated in the context of achieving high-quality semipolar GaN/AlN heterostructures with reduced internal polarization. Epilayers grown under metal-rich conditions with and without nitridation pre-treatment of the sapphire surface were compared using transmission electron microscopy (TEM). In addition to the dominant semipolar orientation, the films exhibited an interfacial zone comprising AlN nanocrystals with the m-plane orientation. The size of the nonpolar nanocrystals increased significantly with sapphire nitridation. A good epitaxial relationship with the substrate was attained by both the semipolar as well as the nonpolar AlN. However, the interfacial structure was perturbed by interfacial roughness and pit formation. The surface nanofacets were correlated to the increased introduction of nanocrystals with m-plane orientation.