Epitaxial growth of GaN films on atomically stepped (0001) lithium-niobate (LiNbO₃) substrates

The structural properties of GaN films grown epitaxially on atomically-flat lithium niobate (LiNbO₃) substrates were investigated. Prior to GaN film growth, high-temperature treatment of the as-received LiNbO₃ removed surface damage and produced atomically stepped surfaces with an average roughness of ~0.11 nm. The micro-steps were nearly parallel and periodic over the entire substrate surface. The step terrace width was ~212 nm, and the average step height was ~0.25 nm. GaN thin films were grown on flat surfaces with AlN buffer layers and GaN buffer layers grown at low temperature by using molecular beam epitaxy. Atomic force microscopy measurements showed a typical Ga-face GaN surface with spiral hillocks. The average roughness of the GaN film was ~0.56 nm. X-ray diffraction (XRD) measurements indicated that the GaN (0001) plane was parallel to the (0001) plane of the LiNbO₃ substrate. The full width at half-maximum of the XRD rocking curve for GaN (0002) was ~122.14 arcsec, which is comparable to that of highquality GaN films grown on other common substrates. Using transmission electron microscopy, we observed that the GaN epitaxial layers grown on LiNbO₃ had a crystalline relationship of (0001) GaN//(0001) LiNbO₃ with [10-10] GaN//[11-20] LiNbO₃. The successful growth of an epitaxial wide-band-gap GaN film on nonlinear optical LiNbO₃ is promising for the development of integrated multi-functional optoelectric devices on ferroelectrics.