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

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 degrees, 2 degrees, and 4 degrees toward a < 1 (1) over bar 100 > 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 degrees), 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.