Anisotropy of tensile stresses and cracking in nonbasal plane AlxGa1−xN/GaN heterostructures

Al x Ga 1 − x N films grown on nonpolar m {11¯00} and {112¯2} semipolar orientations of freestanding GaN substrates were investigated over a range of stress states (x≤0.17). Cracking on the (0001) plane was observed beyond a critical thickness in the {11¯00} oriented films, while no cracking was observed for {112¯2} films. Theoretical analysis of tensile stresses in AlxGa1−xN for the relevant planes revealed that anisotropy of in-plane biaxial stress for the nonpolar {11¯00} planes results in the highest normal stresses on the c-planes, consistent with experimental observations. Shear stresses are significant in the semipolar case, suggesting that misfit dislocation formation provides an alternative mechanism for stress relief.