Transmission and scanning electron microscopy was used to examine the epitaxial lateral overgrowth of GaN onto GaN nanocolumns grown on AlN/(00•1)sapphire by means of molecular beam epitaxy. Initially, N-rich growth produced a bimodal morphology consisting of defect-free Ga-polar nanocolumns emanating from a compact highly defective N-polar layer. Under subsequent Ga-rich conditions, the nanocolumns grew laterally to produce continuous Ga-polar overlayers. Threading dislocation densities in the overlayer were in the range of 108 to 109/cm2; up to 2 orders of magnitude less than in the N-polar underlayer. It was proposed that the change in polarity was a key factor controlling the reduction in threading dislocation density.

Defect Reduction in GaN/(0001)Sapphire Films Grown by Molecular Beam Epitaxy using Nanocolumn Intermediate Layers. D.Cherns, L.Meshi, I.Griffiths, S.Khongphetsak, S.V.Novikov, N.Farley, R.P.Campion, C.T.Foxon: Applied Physics Letters, 2008, 92[12], 121902