In-plane anisotropy characteristics of GaN epilayers grown on A -face sapphire substrates

Abstract

We report on the structural in-plane anisotropy of GaN films grown on A -plane sapphire substrates by metal organic chemical vapor deposition. It is found that GaN:Si grown on A -face sapphire exhibits a strongly anisotropic wafer bending in the two orthogonal in-plane directions, with a ∼24% larger curvature along the c -axis of sapphire than along the m -axis. Using a model developed for an elastically anisotropic bilayer structure and using our curvature data, the anisotropic biaxial stresses in the two in-plane directions have been estimated as 1x -1.3 GPa and 1y -1.1 GPa along parallel and perpendicular to c -axis of sapphire, respectively. This anisotropic stress is also responsible for the distortion of the GaN hexagonal basal plane, as evidenced by x-ray diffraction measurements. The broadening of full width at half maximum of the GaN (0002) x-ray reflections varies with different azimuthal angles, correlated with the tilt of the c -axis of GaN. The in-plane epitaxial relationships between the GaN (0001) and A -face sapphire are found as a -axis of GaN aligned with c -axis of sapphire and m -axis of GaN aligned with m -axis of sapphire. © 2008 American Institute of Physics.