Using in situ wafer-curvature measurements of thin-film stress, we determine the critical thickness for strain relaxation in AlxGa1−xN∕GaN 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.
Phase separation in InGaN layers grown by metalorganic chemical vapor deposition on GaN epilayers was investigated using transmission electron microscopy. Layer thicknesses of 220 and 660nm were deposited with InN fractions ranging from 3% to 34%. At InN contents of 3%, plan-view TEM images show a homogeneous microstructure and selected area diffraction (SAD) patterns exhibit no evidence of satellite spots. InN contents of 12% result in a speckled contrast. Satellites close to the fundamental spots belonging to the wurtzite structure are present in SAD patterns and they are indicative of composition modulations lying in the (0001) growth plane. No satellites are observed along the [0001] direction, implying that phase separation is two-dimensional in nature. Samples containing InN fractions of between 22% and 28% have microstructures exhibiting much stronger contrast variations. Satellite spots in SAD patterns are further spaced from the fundamental reflections. This trend continues on increasing InN content to 34%. In addition, cross-sectional TEM images show an absence of contrast from InGaN layers with InN contents above 12%, in the vicinity of the InGaN∕GaN interface, indicating that coherency strain inhibits phase separation. Arguments are developed to rationalize these observations.
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