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In this paper, three types of strain‐relaxation interlayers were employed to compensate the lattice mismatch‐induced strain, in order to improve the crystalline quality of semi‐polar (11‐22) AlGaN epi‐layers, grown on m‐plane (10‐10) sapphire substrates by metal‐organic chemical vapor deposition (MOCVD). The semi‐polar (11‐22) AlGaN samples were characterized with ultraviolet‐visible absorption spectroscopy, X‐ray diffraction (XRD), and atomic force microscope (AFM) measurements. The characterization results showed that the in‐plane compressive strain was effectively relaxed by the insertion of the strain‐relaxation interlayers, leading to a significant improvement in the crystalline quality of the semi‐polar AlGaN epi‐layers. In particular, the semi‐polar AlGaN sample with an Al‐composition‐gradually‐varied AlGaN interlayer demonstrated a superior crystalline quality and the best surface morphology among all the samples, as shown by the remarkably decreased full width at half maximum (FWHM) of XRD peak, the density of point defects, and the surface roughness.

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