Critical Thickness for Onset of Plastic Relaxation in (1122) and (2021) Semipolar AlGaN Heterostructures

Stress relaxation via misfit dislocation formation by slip in lattice mismatched semipolar wurtzite GaN alloys is viable due to the presence of resolved shear stresses on the basal (0001) plane. The critical thicknesses h c for misfit dislocation formation in AlGaN films grown by molecular-beam epitaxy on two semipolar orientations of GaN, (2021) and (1122), are investigated using high-resolution X-ray diffraction, confirmed with transmission electron microscopy, and compared with Matthews–Blakeslee calculations of theoretical h c as a function of alloy composition. The experimentally observed and calculated critical thickness values are larger for the (2021) oriented films, consistent with the lower resolved shear stresses compared with the (1122) orientation.