Improvement of epitaxial GaN films grown on patterned sapphire substrate by growth mode control

Epitaxial GaN films were grown via metal-organic chemical vapor deposition (MO-CVD) on a cone-shaped patterned sapphire substrate (PSS). A 25 nm thick AlN was deposited by ex-situ sputtering as a buffer layer. The GaN films were grown under various conditions by controlling the substrate temperature (1020-1100°C) and working pressure (85-300 Torr). GaN films grown on PSS via the conventional two-step growth mode consisting of vertical (three-dimensional; 3D) growth and horizontal (two-dimensional; 2D) growth contained poly-grains on top of the cone-shaped pattern. The growth of multi-directional poly-grains on top of the cone-shaped pattern generated numerous defects even though the GaN films were grown by the epitaxial lateral overgrowth (ELO) process. In this paper, we introduce an effective method to control the growth mode of GaN on PSS during the ELO process. The GaN films grown on PSS via the optimized growth mode control showed improvement of crystal quality and surface roughness. The surface morphology and roughness of the GaN films were investigated by field-emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM) in non-contact mode, respectively. The crystal quality of the GaN films was evaluated by ω-2θ high-resolution X-ray diffraction (HR-XRD) and the cathodoluminescence (CL) was measured in the 300-800 nm wavelength range to confirm the distribution of threading dislocations.