Non-polar AlN and GaN/AlN on r-plane sapphire

We report on the morphological, structural and optical properties of a -plane GaN grown on r -plane sapphire using an AlN buffer layer grown at high temperature via MOVPE. Growth of smooth layers has been shown with the potential for device application. Atomic Force Microscopy showed submicron features and crystallographic terraces. High resolution X-ray diffraction was used to study the crystallographic orientation of the epilayer with respect to the substrate. Anisotropies in the in-plane structure were also revealed. The comparison with samples grown using conventional two-step technology shows improved structural quality of the nonpolar GaN grown on the AlN buffer layer. Transmission electron microscope images showed that the microstructure of the a -GaN is dominated by stacking faults and threading dislocations, with density of 3.1 x 10⁵ cm^-1 and 1.2 x 10¹⁰ cm^-2, respectively. A complex microstructure was revealed at the AlN/GaN interface, where roughness originates and smoothed out away from it. The low temperature photoluminescence spectrum shows band edge emission at 355 nm and two peaks at 360 nm and 375 nm, which are related to stacking faults.