Direct Growth of a-Plane GaN on r-Plane Sapphire Substrate by Metalorganic Vapor Phase Epitaxy

We have investigated the direct growth of nonpolar a -plane GaN layers on an r -plane sapphire substrate by metalorganic vapor-phase epitaxy (MOVPE). A high-density nucleation of GaN islands was obtained on the r -plane sapphire substrate at the initial stage of the high-temperature growth without a buffer layer, which resulted in a two-dimensional (2D) growth mode. We studied the effects of V/III ratio growth conditions on the surface morphology and growth features of an a -plane GaN layer. The results showed that a high density of pits with an inverse-pyramidal shape were formed at a high V/III ratio, whereas a relatively low density of pits were formed at a low V/III ratio due to the increase in the rate of lateral growth along the c -axis direction. We successfully grew a -plane GaN layers with a flat and pit-free surface using the "two-step growth method". The method consisted of growing a first layer at a high V/III ratio and growing a second layer at a low V/III ratio. We found that the first layer plays an important role in GaN layer growth. The formation of a void-free GaN layer with sidewall facets in the first step leads to a flat and pit-free layer grown at a high rate of lateral growth along the c -axis direction in the second step.