In situ measurements of the critical thickness for strain relaxation in AlGaN∕GaN heterostructures

Using in situ wafer-curvature measurements of thin-film stress, we determine the critical thickness for strain relaxation in ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}∕\mathrm{Ga}\mathrm{N}$ heterostructures with $0.14⩽x⩽1$ . The surface morphology of selected films is examined by atomic force microscopy. Comparison of these measurements with critical-thickness models for brittle fracture and dislocation glide suggests that the onset of strain relaxation occurs by surface fracture for all compositions. Misfit-dislocations follow initial fracture, with slip-system selection occurring under the influence of composition-dependent changes in surface morphology.