Takuya Maeda

The leakage current of ScAlN thin films grown on n ⁺ -GaN/sapphire substrates by sputtering epitaxy at growth temperatures of 420 °C, 550 °C, and 600 °C was investigated. As the growth temperature decreased, the device-to-device variation in leakage current was suppressed, and the average breakdown field increased from 3.3 to 4.8 MV cm ⁻¹ . The measured leakage current far exceeded that predicted by Fowler–Nordheim tunneling. The temperature-dependent current–voltage characteristics exhibited an exponential increase in leakage current with increasing temperature under both positive and negative bias. Arrhenius analysis revealed that the activation energy depended on the applied electric field, with values typically ranging from approximately 0.4–0.6 eV. This field dependence suggests that multiple transport processes may contribute to the leakage current.

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.

The three-dimensional (3D) stress distribution in a bulk GaN substrate with laser-induced indentations for laser slicing was characterized using stimulated Raman scattering (SRS) microscopy. Local shifts in the Raman peak corresponding to the E2H mode were detected around the indentations, indicating the induction of local stress. The spatial distribution of internal stress was successfully visualized, revealing a clear in-plane anisotropy with compressive and tensile stresses along the a- and m-axes, respectively, as well as its extension in the depth direction. SRS microscopy enables the 3D visualization of residual stress in bulk GaN substrates, facilitating the optimization of fabrication processes.

We investigated the transport properties of 2DEG in ScAlN/GaN heterostructures prepared by plasma-assisted molecular beam epitaxy. Four samples with Sc compositions of 3%–17% and ScAlN barrier thickness of 4–6.6 nm were grown on GaN/SiC template substrates. In situ reflection high energy electron diffraction patterns and atomic force microscopy images confirmed the atomically smooth surface. The atomic-resolution scanning transmission electron microscopy image demonstrated an abrupt ScAlN/GaN interface. At room temperature, the sample exhibited a sheet electron density of 2.0–3.1 × 1013 cm−2 and an electron mobility of 179–468 cm2/Vs. The sheet electron density remained nearly constant across temperatures from 2 to 400 K, indicating that the 2DEG is induced solely by the polarization effect. As the temperature decreased, the mobility increased and eventually saturated. The scattering mechanisms limiting electron mobility were analyzed, accounting for the increased effective mass due to the non-parabolicity of the conduction band at a high sheet electron density. The calculated total mobility shows excellent agreement with the experimental data, suggesting that temperature-independent interface roughness scattering is the dominant mechanism. These findings provide critical insights for understanding and improving the transport properties of 2DEG in ScAlN/GaN for HEMT applications.

The current-voltage (I-V) and capacitance-voltage (C-V) characteristics of a ScAlN thin film epitaxially grown by sputtering on an n+-GaN/sapphire substrate were investigated. The C-V characteristics showed a clockwise hysteresis during voltage sweeps from +10 V to −10 V and vice versa. When a voltage sweep delay of 10 s was applied, the hysteresis was well suppressed, indicating that it originates from carrier trapping associated with defects. The I-V characteristics exhibited a asymmetric leakage current, which decreased and became nearly symmetric under the same voltage sweep delay condition. These results highlight the importance of properly accounting for the effects of carrier trapping effect caused by the defects at or near the ScAlN/GaN interface for the characterization of ScAlN/GaN heterostructures.

We report high-density two-dimensional electron gases in ScAlN/AlGaN/AlN/GaN heterostructures grown by sputter epitaxy with an enhanced metal supply. By optimizing the growth temperature to 775 °C, we achieved atomically flat surfaces and high crystallinity. Moreover, we obtained a record-high sheet carrier density ( n _s ) of 2.24 × 10 ¹³ cm ⁻² for the sputtered ScAlN films, which is comparable to the values achieved using metal–organic chemical vapor deposition and molecular beam epitaxy. Temperature-dependent Hall effect measurements and scattering analysis using Matthiessen’s rule revealed that the high carrier density enhances interface roughness scattering, which becomes the dominant mobility-limiting mechanism at low temperatures. These results demonstrate that sputter epitaxy with an increased metal-to-nitrogen flux ratio enables high-density polarization-induced 2DEGs and provides a competitive approach for realizing high-performance ScAlN/GaN HEMT heterostructures.

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.

ScAlN is an attractive barrier material for GaN-based high electron mobility transistors (HEMTs) due to its large polarization and ferroelectricity. However, the influence of growth temperature on its structural and electrical properties remains insufficiently understood. To address this, we investigated the effects of growth temperature on the structural and electrical properties of Sc0.10Al0.90N films epitaxially grown on AlGaN/AlN/GaN heterostructures through sputtering. Atomic force microscopy and reflection high-energy electron diffraction revealed that surface morphology and crystallinity improved significantly with increasing growth temperature, with step-flow growth achieved at 750 °C. X-ray diffraction measurements confirmed coherent growth of all ScAlN layers and a decrease in the c-axis lattice constant with increasing growth temperature. Hall effect measurements demonstrated that the sheet carrier density of ScAlN/AlGaN/AlN/GaN grown at 750 °C reached 1.1 × 1013 cm−2, nearly three times higher than that of the structure without ScAlN. The sheet carrier density was lower than that calculated by one-dimensional Poisson–Schrödinger simulations using the theoretical polarization, suggesting the possible existence of negative fixed charges at the ScAlN/AlGaN regrown interface and within the ScAlN layer. These results highlight the critical role of growth temperature in sputter epitaxy of ScAlN and demonstrate the feasibility of using high-quality ScAlN as a barrier layer for GaN-based HEMTs through an industrially compatible process.