The microstructures of gas-source molecular beam epitaxial GaN films which had been deposited onto (00•1) sapphire substrates, using NH3 as the N source, were studied by means of transmission electron microscopy. In the case of a nucleation layer which was deposited at 500C, high-quality material with only dislocations (5 x 109/cm2) in the volume of the film were obtained. The resultant structural quality was poor in the case of a nucleation layer which had been deposited at 550C. Inversion domains and {11•0} prismatic defects were observed. Due to the high density of defects in the nucleation layer, especially the 500C one, the existence of vertical planar defects could not be verified. A tentative explanation was proposed for the difference in microstructure of the epilayers. In the case of the 500C nucleation layer, extensive recrystallization occurred during annealing; leading to a phase transformation from cubic to hexagonal and to the growth and reorientation of pre-existing misoriented micro-grains. In the case of the 550C nucleation layer, the recrystallization process was less extensive because larger grains with an hexagonal structure already existed in the layer. The continuous hexagonal layer which covered the substrate underwent a structural transformation. If it were assumed that the vertical planar defects were already present in the nucleation layer, marked recrystallization (as in the case of 500C nucleation layers) could lead to the reorganisation of the structure and thus to the disappearance of vertical planar defects, whereas they survived in epilayers which had been grown on a 550C nucleation layer.

Effect of the Nucleation Layer Deposition Temperature on the Nature of Defects in GSMBE GaN Films. P.Vennéguès, N.Grandjean, J.Massies, F.Semond: Journal of Crystal Growth, 1999, 201-202, 423-8