Effect of r-plane (1-102) sapphire off-cut angle on the anisotropic strain in nonpolar Si-doped a-plane (11-20) GaN

We studied the growth and the characteristics of nonpolar Si-doped a-plane GaN grown on r-plane sapphire substrates with different off-cut angles which were changed in the range of -0.2° ~ +0.4°. Samples grown by using -0.2° and +0.2° off-cut angles showed triangular pit-free and smooth surfaces, which resulted from enhanced lateral growth owing to the epitaxial films having a Ga face. On the other hand, the sample grown by using +0.4° off-cut angles revealed a high density of pits and low crystalline quality due to a high density of dislocations. The strain determined by using calculations with the lattice parameters also showed a dependence on the off-cut angles. We expect r-plane sapphire with off-cut angles in the range of -0.2° ~ +0.2° to be very effective for improving the crystalline quality and the surface morphology of a-plane GaN.