Effect of V/III ratio on the surface morphologies of N-polar GaN films grown on offcut sapphire substrates

We investigated the influence of the V/III ratio on the surface morphologies of N-polar GaN epilayers during the stage of high-temperature (HT) GaN growth. The epilayers were grown on 4°-offcut sapphire substrates by metal-organic vapor phase epitaxy. We observed prominent undulations on the GaN surface when a low V/III ratio was used, and ridged hillocks on the GaN surface when a high V/III ratio was used. With the V/III ratio increased from 128 to 2237, the density of ridged hillocks increased but the surface roughness was significantly reduced from 13.8 nm to 1.7 nm in 25 × 25 μm2 AFM images. The results differ from the existing reports in which a smoother N-polar GaN surface morphology was usually obtained with a lower V/III ratio. Non-uniform island formation in the initial growth of HT-GaN and enhanced step-bunching effect under a low V/III ratio were blamed for the large surface roughness of the N-polar GaN film. The increasing density of ridged hillocks on the N-polar surface grown under a high V/III ratio was attributed to the reduced diffusion rate of Ga adatoms due to excessive N. Completely hillock-free smooth N-polar GaN surfaces were achieved by further increasing the growth temperature of the GaN buffer layer.