The effect of low-angle off-axis GaN substrate orientation on the surface morphology of Mg-doped GaN epilayers

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The effect of low-angle off-axis GaN substrate orientation on the surface morphology of Mg-doped GaN epilayers has been studied using atomic force microscopy (AFM) and transmission electron microscopy. Undoped- and magnesium-doped GaN layers were grown on (0001) GaN surfaces tilted by 0.3°, 2°, and 4° toward a ⟨11¯00⟩ direction. AFM images show the presence of pinholes associated with threading screw dislocations originating from the substrate. Mg doping causes enhanced step-flow growth with well-defined periodic steps and a tendency to cover the pinholes. In regions far from defects, atomic-resolution imaging shows the coexistence of surface regions with different atomic step densities, i.e., with slightly different inclination, that depend on the substrate tilt angle. For low tilt (0.3°), the steps involve a single basal plane (with a height = c/2). At higher tilt, the steps involve two basal planes with a tendency toward step bunching. Cathodoluminescence spectroscopic imaging has been used to correlate the electronic properties with the thin film surface morphology, showing that step bunching reduces p-type doping efficiency.