Enhanced crystalline quality of non-polar a-plane AlGaN epitaxial film grown with Al-composition-graded AlGaN intermediate layer

The non-polar a -AlGaN epitaxial film was successfully grown on the semi-polar r- sapphire substrate by metal-organic chemical vapor deposition technique. An Al-composition-graded Al x Ga 1−x N (x = 0.0 to 1.0) intermediate layer with varying film thickness from 260 to 695 nm was deposited between the high-temperature AlN layer and the non-polar a- AlGaN epitaxial film to enhance the morphological and crystalline quality. The non-polar a -AlGaN epitaxial films were investigated by using atomic force microscopy (AFM), high-resolution x-ray diffraction, photoluminescence (PL) spectroscopy and the Hall effect measurement techniques. The characterisation results indicate substantial improvements in surface morphology and crystalline quality for the non-polar a- AlGaN epitaxial film grown by adding an Al-composition-graded AlGaN intermediate layer. The surface roughness measured from AFM and the defect-related emission (yellow band) relative to the near-band-edge emission from PL spectra were decreased significantly by optimizing the layer thickness of the Al-composition-graded AlGaN layer. A relatively low background carrier concentration down to −4.4 ×  10 17 cm −3 was achieved from Hall effect measurement for the non-polar a- AlGaN epitaxial film.