Kozo Makiyama

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.

N-polar GaN high-electron-mobility transistors (HEMTs) show unique transfer characteristics originating from the band structure related to the inverted HEMTs, and some scattering mechanisms are suggested theoretically. However, their experimental verification has not been sufficiently discussed yet. In this paper, we compared the experimentally obtained field-effect mobilities of N-polar and Ga-polar GaN HEMTs with each calculated scattering component using the self-consistent 1-dimensional Poisson–Schrödinger calculation, revealing the origin of the differences as the distance between the 2DEG centroid and the GaN/AlN or AlN/GaN interface. Mobility of Ga-polar GaN HEMT was superior to N-polar HEMT despite 2.5 times higher scattering center density, and the highly narrowed quantum well of N-polar GaN HEMT was the dominant factor.

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.

ScAlN has emerged as a promising material for GaN-based high electron mobility transistors (HEMTs) due to its unique piezoelectric and ferroelectric properties, which can significantly enhance electronic performance. This study investigated the epitaxial growth of ScAlN films on AlGaN/GaN HEMT structures using a sputtering method, focusing on the structural and electrical characteristics of the heterostructures. ScAlN films with varying Sc compositions (5%–20%) were grown on AlGaN/AlN/GaN/SiC template substrates. High-resolution x-ray diffraction and reciprocal space mapping revealed the coherent growth of ScAlN, and the increase in the c-axis lattice constant increases with Sc composition. Hall effect measurements of the Sc0.06Al0.94N/AlGaN/AlN/GaN HEMT structures showed an increase in sheet carrier density compared to structures prior to the growth of ScAlN, highlighting its potential for improved electrical performance. These findings underscore the value of ScAlN as a barrier layer material of GaN HEMTs, enhancing device efficiency and output power.