Structural changes in epitaxial GaN layers that had been grown onto basal-plane sapphire were studied by means of atomic force microscopy, X-ray diffraction, and transmission electron microscopy. It was found that high-temperature growth (1050 to 1080C) on optimized nucleation layers produced clear specular films. Atomic force microscopy of the as-grown surface revealed evenly spaced monatomic steps, which reflected layer-by-layer growth. These measurements also revealed a step termination density of 1.7 x 108/cm2 in 5 films. This value was in close agreement with transmission electron microscopic measurements of screw and mixed screw/edge threading dislocation densities. The measured total threading dislocation density in the 5 films was 7 x 108/cm2.

D.Kapolnek, X.H.Wu, B.Heying, S.Keller, B.P.Keller, U.K.Mishra, S.P.DenBaars, J.S.Speck: Applied Physics Letters, 1995, 67[11], 1541-3