Structural and ferroelectric properties of sputter-epitaxial ScAlN on GaN grown at various temperatures

ScAlN is a III-nitride ferroelectric material that has attracted considerable interest for its large remanent polarization, high thermal stability, and compatibility with GaN-based device platforms, and its properties strongly depend on growth conditions. In this study, ScAlN films were grown on Si-doped n-GaN/AlN/sapphire substrates by sputter epitaxy at growth temperatures of 420–675 °C, and their structural and ferroelectric characteristics were systematically investigated. X-ray diffraction and reciprocal space mapping revealed that the a-axis lattice constant increased, but the c-axis lattice constant simultaneously decreased, at growth temperatures above approximately 650 °C, indicating a temperature-induced modification of the ScAlN lattice. Positive-up–negative-down measurements showed a significant leakage current at temperatures above 550 °C, which prevented the saturation of the remanent polarization in the polarization–electric field characteristics. At lower growth temperatures, the films exhibited remanent polarization and coercive fields comparable to those reported for high-quality ScAlN films grown on GaN by molecular-beam epitaxy and metalorganic chemical vapor deposition. This result demonstrates that low-temperature sputter epitaxy can reproduce the intrinsic ferroelectric switching behavior of ScAlN. Thus, low-temperature sputter epitaxy effectively suppresses the leakage current and enables reliable ferroelectric switching, providing useful guidelines for optimizing ScAlN deposition processes for ferroelectric device applications.