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) over bar1 0 0](GaN) direction with a larger misfit had an in-plane residual stress of similar to 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 (C) 2010 Elsevier B V All rights reserved