Flow-modulation-assisted diffusion enhancement effect on the surface morphology of GaN films

The effect of flow modulation on the surface morphology of GaN thin films in the early stage of growth was investigated. GaN films were grown by using hydride vapor phase epitaxy, and the surface morphologies of the GaN films were found to be significantly modified under different flow modulation conditions. Aspect ratios of nucleated GaN islands showed a linear dependence on the duty cycle of the HCl flow rate. The decreased aspect ratio of the GaN islands due to HCl flow modulation was attributed to the enhanced Ga diffusion length because our experimental V/III ratio was lower than the typical V/III ratio for stoichiometric adsorption between Ga and N at this substrate temperature in such a way that HCl flow modulation under a constant NH₃ flow still effectively enhanced diffusion of the Ga species. In addition, from the surface morphology modification caused by an interval of gas injection between NH ₃ and HCl, we inferred that the Ga species diffused more efficiently on a bare sapphire surface than on a N-adsorbed sapphire surface.