Study on growing thick AlGaN layer on c-plane sapphire substrate and free-standing GaN substrate

In this study, the thick AlGaN epilayers have been grown on the c-plane sapphire substrate and the free-standing GaN substrate using low-temperature AlN nucleation layers by low-pressure metal-organic chemical vapor deposition (LPMOCVD). High resolution X-ray diffraction (HRXRD), atom force microscopy (AFM), scanning electron microscopy (SEM), photoluminescence (PL) and Raman scattering measurements have been employed to study the crystal quality, threading dislocation density, surface morphology, optical properties and phonon properties of thick AlGaN epifilms. The results indicate that AlGaN epifilms crystal quality can be improved greatly when grown on the free-standing GaN substrate. We calculated the threading dislocation density and found that thick AlGaN epifilm grown on the free-standing GaN substrate is much lower in total threading dislocation density than that grown on the sapphire substrate, although the surface morphology is rougher than that of sapphire substrate.