Non-polar a-plane GaN epilayers were grown by metalorganic vapour phase epitaxy on r-plane sapphire substrates using three different buffer layer structures. The investigated structures consisted of GaN grown on: (A) AlN nucleation layer; (B) AlN plus Al0.25Ga0.75N intermediate layer; (C) AlN, Al0.25Ga0.75N, and GaN/Al0.25Ga0.75N superlattice. The samples were characterized by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. Structure (B) exhibited much lower overall structural defect densities than structure (A), with narrower X-ray rocking curves by a factor of 1.5 to 2. Compared to structure (B), structure (C) showed a factor of 2 lower density of screw dislocations in transmission electron microscopy although the densities of other types of defects were not significantly reduced. Type (B) and (C) structures also exhibited smoother surface morphologies than type (A).

Reduced Structural Defect Densities in a-Plane GaN Layers on r-Plane Sapphire Through Buffer Layer Engineering. R.Armitage, H.Hirayama, Y.Kondo: Physica Status Solidi C, 2007, 4[7], 2524-7