Control of Initial Nucleation by Reducing the V/III Ratio during the Early Stages of GaN Growth

We investigated, by atomic force microscopy (AFM), the effect of the V/III ratio on the surface morphologies of GaN epilayers during the early stage of high-temperature (HT) GaN growth. The epilayers were grown on sapphire substrates by low-pressure metal-organic chemical vapor deposition (LP-MOCVD). We observed that initial HT-GaN islands in the early stage of HT-GaN growth were larger when a lower V/III ratio was used during the low-temperature (LT) GaN-buffer-layer growth or during the early stage of HT-GaN growth (the HT-GaN-buffer growth). These larger islands were attributed to a reduced nuclei density when a lower V/III ratio was used during the LT-GaN-buffer-layer growth, or to an increased growth aspect ratio (the ratio of vertical and lateral growth rates) when a lower V/III ratio was used in the early stage of HT-GaN growth. The larger islands led to fewer defects when they coalesced, which demonstrated that the quality of GaN epilayers grown on sapphire substrates by LP-MOCVD can be improved by using a low V/III ratio during the LT or HT GaN-buffer-layer growth.