Epitaxial lateral overgrowth of GaN on nano-cavity patterned sapphire substrates

Abstract

The epitaxial lateral overgrowth of GaN by metal-organic chemical vapor deposition using a nano-cavity patterned sapphire substrate (NCPSS) was investigated. The NCPSS, with a hexagonal non-close-packed nano-cavity pattern on the sapphire substrate, was fabricated by polystyrene sphere coating and size reduction by reactive ion etching, followed by deposition of alumina and thermal oxidation. The coalescence of GaN on the NCPSS was achieved by the formation of relatively large GaN islands and enhanced lateral overgrowth of the GaN islands over several nano-cavity pattern areas. The threading dislocation density (TDD) measured by cathodoluminescence measurement was significantly reduced from 2.4 × 108 cm−2 to 6.9 × 107 cm−2 by using the NCPSS. A dislocation behavior that contributes to the reduction of TDD of the GaN layer was observed by transmission electron microscopy. Raman spectroscopy revealed that the compressive stress in the GaN layer was reduced by 21% due to the embedded nano-cavities. In addition, the diffuse reflectance of GaN on the NCPSS was enhanced by 54% ∼ 62%, which is attributed to the increased probability of light extraction through effective light scattering by nano-cavities.