Kazuhisa Ikeda

The discovery of ferroelectric ScAlN sparked a search for new polar wurtzite alloys; however, incorporating transition metals remains a formidable challenge. Here, we demonstrate the synthesis of wurtzite‐phase NbAlN, a polar transition‐metal nitride alloy that is highly compatible with GaN‐based heterostructures. Despite the strong metallic character of elemental Nb, we achieved coherent epitaxial NbAlN films that preserve the wurtzite crystal structure and metal polarity of the underlying GaN, as directly visualized by annular bright‐field scanning transmission electron microscopy. Nb incorporation induces a monotonic lattice expansion and produces coherent Nb‐enriched nanoscale regions that are accommodated within the continuous wurtzite framework. Functionally, NbAlN barriers induce a substantial enhancement in the two‐dimensional electron gas density at the AlN/GaN heterointerface, consistent with polarization‐related carrier modulation by the metal‐polar NbAlN barrier. This study demonstrates that d ‐electron transition metals can be integrated into polar nitride systems without compromising their functional asymmetry, making NbAlN a promising building block for polar nitride heterostructures and carrier‐density‐engineered GaN electronics.

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.

ScAlN has attracted considerable attention as a promising barrier material in GaN-based high-electron-mobility transistors. However, few studies have been conducted on the effect of thermal annealing on the structural, electrical, and transport properties of ScAlN barrier layer sputter-deposited on an AlGaN/AlN/GaN heterostructure. Therefore, in this study, ScAlN barrier layers were grown by sputter epitaxy on templates prepared via metalorganic vapor phase deposition. Post-deposition annealing was conducted at 750 °C, which preserved the wurtzite structure of the ScAlN films and maintained coherent interfaces without interdiffusion or phase change. Hall effect measurements revealed that annealing substantially increased the sheet carrier concentration (∼8.0×1012cm−2) and electron mobility exceeding 1000 cm2 V−1 s−1 of the heterostructure. The mobility was limited by acoustic deformation potential, polar optical phonon, and interface roughness scattering. Notably, the increased roughness scattering at higher Sc content might be due to local compositional fluctuations. These results demonstrate that thermal annealing effectively enhances the transport properties of ScAlN barrier layers sputter-deposited on GaN-based heterostructures, offering a promising route for fabricating high-performance and scalable GaN-based electronic devices.

Epitaxial polarity inversion of N-polar (−c) GaN to Ga-polar (+c) GaN was demonstrated by combining metalorganic vapor phase epitaxy (MOVPE) and surface oxidation processes. Annealing –c-GaN films at 900 °C in oxygen and the introduction of a low-temperature GaN buffer layer resulted in successful polarity inversion. Annealing time impacted the formation of voids at the regrowth interface due to the etching of the oxide film and the GaN growth mode. These results represent a promising approach to pseudomorphic epitaxial polarity inversion of GaN for novel devices that utilize both polarities, such as high-efficiency wavelength conversion devices.

GaN polarity inversion from Ga-polar (+c) to N-polar (−c) was demonstrated by combining multiple metalorganic vapor phase epitaxy (MOVPE) and surface oxidation and nitridation processes. Annealing the +c-GaN film at 900 °C caused surface roughening, and β-Ga₂O₃ was formed when the annealing time exceeded 240 min. Subsequent nitridation at 870 °C and GaN regrowth by MOVPE achieved −c-GaN growth on the +c-GaN film. Competition between +c-GaN regrowth and oxide film decomposition led to the formation of voids near the regrowth interface. We demonstrate a new potential for the epitaxial growth of –c-GaN films, enabling the utilization of high-quality +c-GaN films.

Photon‐pair generation based on optical parametric down‐conversion has attracted for the application as a light source for quantum information. Highly efficient wavelength‐conversion devices require a polarity‐inversion structure when using nitride semiconductors. A transverse quasi‐phase‐matching (QPM) polarity‐inverted GaN bilayer channel waveguide device is suitable for efficient wavelength conversion. This study designed a cross‐section device to satisfy the modal dispersion phase‐matching condition between the TM₀₂ mode pump light and the TM₀₀ mode signal/idler light. Moreover, an AlN oxidation interlayer fabricates the Ga‐polar/N‐polar (+c/−c) GaN layers via metalorganic vapor‐phase epitaxy (MOVPE). A 145 nm thick film layer with a macro‐step‐free surface is grown by optimizing the −c‐GaN growth conditions and reducing the substrate off‐angle to 0.2°. Next, the AlN layer is oxidized in an electric furnace and MOVPE is used to regrow a 1500 nm thick +c‐GaN layer. A macrosteps‐free surface can be achieved by reducing the off‐angle to 0.2° and optimizing the −c‐GaN growth conditions to avoid hillock formation. These results pave the way for improving the efficiency of GaN transverse QPM wavelength‐conversion devices.