Anisotropic strain relaxation and abnormal zigzag shape planar defects in nonpolar a-GaN grown by metalorganic chemical vapor deposition

The correlation between bi-axial in-plane stress relaxation and formation mechanism of the abnormal zigzag shape prismatic stacking faults (PSFs) observed in a-GaN epilayers grown by metalorganic chemical vapor deposition was investigated using transmission electron microscopy. In a-GaN epilayers on r-plane sapphire substrates showing an anisotropic lattice mismatch, the misfit strain along the [0 0 0 1]GaN direction was mostly relaxed by the formation of basal stacking faults. On the other hand, the [1¯ 1 0 0]GaN direction with a larger misfit had an in-plane residual stress of ∼3% after the formation of the zigzag shaped PSFs and misfit dislocations. The resultant higher residual stress induced dislocation near the zigzag shaped PSFs junction and ultimately led to abnormal deviation in the junction angle of the zigzag shaped PSFs.