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

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 deposite d 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 Ga N on an optimized buffer layer of 20 nm thickness was N-face (-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 (nitr ided sapphire, annealed buffer layer or GaN substrate) at the deposition of GaN epitaxial layer. The higher V/III ratio enhanced the nucleation densit y, and reduced the size of hexagonal facets. The nuclei, forming the favora ble hexagonal facets of wurtzite GaN, should grow laterally along the {10(1 ) over bar 0} directions to cover a room among the facets until coalescence . After coalescence, -c GaN growth on a flat hexagonal facet results in a p yramidal hexagonal facet. The growth mode for -c GaN has been discussed wit h respect to surface structure and migration length of adsorbing precursors , in comparison with Ga-face (+c) GaN. (C) 2000 American Institute of Physi cs. [S0021-8979(00)02114-9].