Influence of Sapphire Substrate Orientation on the van der Waals Epitaxy of III-Nitrides on 2D Hexagonal Boron Nitride: Implication for Optoelectronic Devices

The van der Waals (vdW) epitaxy of three-dimensional (3D) device structures on two-dimensional (2D) layers is particularly interesting for III-nitrides because it may relax lattice matching and thermal mismatch requirements and can allow convenient lift-off of epilayers and optoelectronic devices. In this article, we report the vdW epitaxy of 3D GaN/AlGaN on 2D h-BN grown on a-, c-, and m-plane sapphire substrates via metal-organic chemical vapor phase epitaxy. First, we study 2D h-BN layers grown on a-, c-, and m-plane sapphire to demonstrate the effect of the substrate on h-BN growth and h-BN alignment. We find that hBN can align itself to its preferred c-axis with a slight misorientation on the m-plane sapphire substrate. However, the differences in crystallographic orientation, thermal expansion coefficient, and surface energy of differently oriented sapphire substrates strongly influence the surface morphology (good for a- and c-planes) and the adhesion of h-BN layers (lift-off only possible for the c-plane). Second, the vdW growth of 3D GaN/AlGaN on 2D h-BN grown on a-, c-, and m-planes of sapphire was investigated. High-resolution X-ray diffraction (HR-XRD) 2 theta-omega scan and selected area electron diffraction pattern were used to demonstrate the misorientation of GaN/AlGaN grown on 2D h-BN/m-plane sapphire compared to polar GaN grown on 2D h-BN/a- and c-plane sapphire. It was found that the morphology and crystalline quality of GaN/AlGaN are directly affected by the 2D h-BN layers. These results provide initial insight into the impact of substrate orientation, thereby acting as a guide for the potential design of III-nitride/h-BN vdW epitaxy seeking to use nonpolar or semipolar planes of sapphire for optoelectronic devices such as light-emitting diodes (LEDs), high-power electronics, and detectors.