We report on the growth of Al0.25Ga0.75N/GaN heterostructures grown on low dislocation density vicinal surfaces of semi-insulating c-axis GaN substrates. Atomic force microscopy (AFM), photoluminescence (PL), cathodoluminescence (CL), high-resolution x-ray diffraction (HRXRD), secondary-ion mass spectroscopy (SIMS), Hall effect, and Raman spectroscopy have been used to assess structural and electrical properties as a function of substrate offcut. Bulk GaN substrates with vicinal offcut between 0.5 degrees and 1.4 degrees are optimal with respect to surface roughness and dopant incorporation. AFM, PL, and CL show decreasing Mg incorporation with increasing offcut angle. Raman spectroscopy, used to analyze biaxial strain, confirms essentially strain-free heterostructure growth on vicinal substrates with offcut angles between 0.5 degrees and 1.4 degrees off  toward [1 (1) over bar 00]. Aluminum (Al) incorporation in the AlxGa1-x N barrier assessed by Raman vibration is in excellent agreement with trends found by HRXRD.
Physical Properties of AlGaN/GaN Heterostructures Grown on Vicinal Substrates
Grenko, J. A., Reynolds, C. L., Barlage, D. W., Johnson, M. A. L., Lappi, S. E., Ebert, C. W., ... Evans, K. R. (2010). Physical Properties of AlGaN/GaN Heterostructures Grown on Vicinal Substrates. Journal of Electronic Materials, 39(5), 504-516. https://doi.org/10.1007/s11664-010-1153-0