Influence of nitridation process on characteristics of N-polar AlGaN films grown by MOCVD

The N-polar AlGaN epi-layers with an Al composition of ~10% were grown on (0001)-oriented c-plane sapphire substrates by metal-organic chemical vapor deposition (MOCVD) technology. Special attention was paid on the nitridation process in the epitaxial growth of N-polar AlGaN films. The optical microscope (OM) measurement results demonstrated that the size of hexagonal defects on the surface of N-polar AlGaN epi-layer decreased dramatically with our improved nitridation process. Furthermore, the structural, electrical, and optical properties of N-polar AlGaN epi-layers were characterized extensively by means of X-ray rocking curves (XRCs), Hall effect, and photoluminescence (PL) spectroscopy. It was found that the defects-related blue-band (BB) emission was greatly suppressed and the threading dislocation (TD) density was significantly reduced by the optimized nitridation process. These characterization results reveal that the reformed nitridation process plays a vital role in the improvement in the crystalline quality as well as the electrical and optical properties of the N-polar AlGaN epi-layers.