The effects of substrate nitridation on the growth of nonpolar a-plane GaN on r-plane sapphire by metalorganic chemical vapor deposition

•Nonpolar a-plane GaN with substrate nitridation was directly grown on r-plane sapphire by MOCVD.•The anisotropic surface morphology evolutions for the samples with and without nitridation process were investigated by controlled growth terminations.•The reasons responsible for the effects of growth temperature of substrate nitridation on the growth of a-plane GaN were investigated in detail. The effects of substrate nitridation on the growth of nonpolar a-plane GaN directly deposited on r-plane sapphire by metalorganic chemical vapor deposition (MOCVD) were investigated. Using nitridation, high-quality a-plane GaN with flat surface was acquired. On the contrary, if the nitridation layer was removed, the epitaxial a-plane GaN exhibited deep triangular pits and poor crystalline properties. This could be attributed to the fact that uniform-distributed AlN grains were introduced by nitridation, which might act as the nucleation layer for the following a-plane GaN growth. The effects of substrate nitridation on the evolutions of anisotropic morphologies and crystalline properties were also studied by artificially interrupting the growth at different stages. The consequences suggested the nitridation layer could contribute to surface coalescence of a-plane GaN. The reasons responsible for this phenomenon were probed by Raman spectrum, and a model was proposed to explicate the effects of nitridation on the growth of a-plane GaN.