Growth mode and surface morphology of a GaN film deposited along the N-face polar direction on c-plane sapphire substrate

Abstract

The dependence of polar direction of GaN film on growth conditions has been investigated by changing either the group-V/group-III ratio (V/III ratio) in supplying the source gas or the deposition rate. GaN films were deposited on a nitrided sapphire by two-step metalorganic chemical vapor deposition. The surface morphology changed from flat hexagonal to pyramidal hexagonal facet with the increase of V/III ratio. However, the polar direction of GaN on an optimized buffer layer of 20 nm thickness was N-face $\mathrm{(-c)}$ polarity, independent of both the V/III ratio and the deposition rate. The polarity of the GaN epitaxtial layer can be determined by that of an interface (nitrided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation density, and reduced the size of hexagonal facets. The nuclei, forming the favorable hexagonal facets of wurtzite GaN, should grow laterally along the $\text{{101̄0}}$ directions to cover a room among the facets until coalescence. After coalescence, $\mathrm{-c}$ GaN growth on a flat hexagonal facet results in a pyramidal hexagonal facet. The growth mode for $\mathrm{-c}$ GaN has been discussed with respect to surface structure and migration length of adsorbing precursors, in comparison with Ga-face $\mathrm{(+c)}$ GaN.