Rotation of in-plane structural anisotropy at the interface of an a-plane InN/GaN heterostructure grown by MOCVD on r-plane sapphire

We have epitaxially grown InN thin films on c- and r-plane sapphire substrates by metal–organic chemical vapor deposition (MOCVD) using GaN buffer. Their structural and optical comparisons revealed the high quality of the a-plane InN/GaN heterostructure on r-plane sapphire. Remarkable in-plane structural anisotropy, which manifests itself as additional rocking curve (RC) broadening, has been observed as usual by high-resolution X-ray diffraction (HRXRD) for both the GaN buffer and the InN overlayer; however, we observed a 90°-rotation of the anisotropy, i.e., from the M-shape for the GaN buffer to the W-shape for the InN overlayer, in the a-plane InN/GaN heterostructure. This anisotropy rotation, a hitherto unknown behavior in monolithic growth of nonpolar III-nitrides, has a strong correlation with the crystallographic tilts between the individual epilayers and the r-plane sapphire substrate in the InN/GaN heterostructure. These observations, together with the direction of the unintentional miscut (∼0.09°) of the r-plane sapphire substrate, provide evidence for the effect of island coalescence on lattice tilt in epitaxial growth with large lattice mismatches.