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The development of high performance wide-bandgap AlGaN channel transistors with high current densities and reduced Ohmic losses necessitates extremely highly doped, high Al content AlGaN epilayers for regrown source/drain contact regions. In this work, we demonstrate the achievement of semi-metallic conductivity in silicon (Si) doped N-polar Al0.6Ga0.4N grown on C-face 4H-SiC substrates by molecular beam epitaxy. Under optimized conditions, the AlGaN epilayer shows smooth surface morphology and a narrow photoluminescence spectral linewidth, without the presence of any secondary peaks. A favorable growth window is identified wherein the free electron concentration reaches as high as ∼1.8 × 1020 cm−3 as obtained from Hall measurements, with a high mobility of 34 cm2/V·s, leading to a room temperature resistivity of only 1 mΩ·cm. Temperature-dependent Hall measurements show that the electron concentration, mobility, and sheet resistance do not depend on temperature, clearly indicating dopant Mott transition to a semi-metallic state, wherein the activation energy (Ea) falls to 0 meV at this high value of Si doping for the AlGaN films. This achievement of semi-metallic conductivity in Si doped N-polar high Al content AlGaN is instrumental for advancing ultrawide bandgap electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Rutile compounds have exotic functional properties that can be applied for various electronic applications; however, the limited availability of epitaxial substrates has restricted the study of rutile thin films to a limited range of lattice parameters. Here, rutile GeO2 is demonstrated as a new rutile substrate with lattice parameters of a = 4.398 Å and c = 2.863 Å. Rutile GeO2 single crystals up to 4 mm in size are grown by the flux method. X-ray diffraction reveals high crystallinity with a rocking curve having a full width half-maximum of 0.0572°. After mechanical polishing, a surface roughness of less than 0.1 nm was obtained, and reflection high-energy electron diffraction shows a crystalline surface. Finally, epitaxial growth of (110)-oriented TiO2 thin films on GeO2 substrates was demonstrated using molecular beam epitaxy. Templated by rutile GeO2 substrates, our findings open the possibility of stabilizing new rutile thin films and strain states for the tuning of physical properties. [ABSTRACT FROM AUTHOR]

Ultrawide-band-gap (UWBG) semiconductors are promising for fast, compact, and energy-efficient power-electronics devices. Their wider band gaps result in higher breakdown electric fields that enable high-power switching with a lower energy loss. Yet, the leading UWBG semiconductors suffer from intrinsic materials' limitations with regard to their doping asymmetry that impedes their adoption in CMOS technology. Improvements in the ambipolar doping of UWBG materials will enable a wider range of applications in power electronics as well as deep-UV optoelectronics. These advances can be accomplished through theoretical insights on the limitations of current UWBG materials coupled with the computational prediction and experimental demonstration of alternative UWBG semiconductor materials with improved doping and transport properties. As an example, we discuss the case of rutile GeO2 (r-GeO2), a water-insoluble GeO2 polytype, which is theoretically predicted to combine an ultra-wide gap with ambipolar dopability, high carrier mobilities, and a higher thermal conductivity than β-Ga2O3. The subsequent realization of single-crystalline r-GeO2 thin films by molecular beam epitaxy provides the opportunity to realize r-GeO2 for electronic applications. Future efforts toward the predictive discovery and design of new UWBG semiconductors include advances in first-principles theory and high-performance computing software, as well as the demonstration of controlled doping in high-quality thin films with lower dislocation densities and optimized film properties. [ABSTRACT FROM AUTHOR]

We report on the demonstration of ferroelectricity in ScxAl1-xN grown by molecular beam epitaxy on GaN templates. Distinct polarization switching is unambiguously observed for ScxAl1-xN films with Sc contents in the range of 0.14–0.36. Sc0.20Al0.80N, which is nearly lattice-matched with GaN, exhibiting a coercive field of ∼ 4.2 MV/cm at 10 kHz and a remnant polarization of ∼135 μC/cm2. After electrical poling, Sc0.20Al0.80N presents a polarization retention time beyond 105 s. No obvious fatigue behavior can be found with up to 3 × 105 switching cycles. The work reported here is more than a technical achievement. The realization of ferroelectric single-crystalline III–V semiconductors by molecular beam epitaxy promises a thickness scaling into the nanometer regime and makes it possible to integrate high-performance ferroelectric functionality with well-established semiconductor platforms for a broad range of electronic, optoelectronic, and photonic device applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • A type-I band alignment was determined for Y 0.07 Al 0.93 N/GaN heterojunction. • Demonstration of YAlN-based HEMT with remarkable carrier transport properties. • Identification of key factors impacting temperature-dependent transport properties. Incorporating rare earth element scandium (Sc) into III-nitride wurtzite lattice offers remarkable non-oxide ferroelectrics, tunable spontaneous polarization coefficients, and significantly enhanced linear and nonlinear optical properties. Yttrium (Y), a Sc substitute, is also capable of similar properties when alloyed with III-nitride materials, but its potential has remained largely unexplored. In this study, we present the first experimental investigation into the energy band gap alignment and transport properties of a YAlN based high-electron-mobility transistor (HEMT) structure. Our measurements revealed a type-I band alignment for the Y 0.07 Al 0.93 N/GaN heterojunction, with a small valence band offset of −0.1 eV and conduction band offset of 2.2 eV. We then developed a novel Y 0.07 Al 0.93 N/AlN/GaN heterojunction-based HEMT and evaluated its transport properties. A mobility of 1000 cm2/Vs, a sheet electron concentration of 4.2 × 1013 cm2, and a sheet resistance of 148 Ω/□ were measured at room temperature, which are among the best values reported for rare earth element-containing nitride epitaxial heterostructures. We have further performed temperature-dependent Hall measurements and identified the dominant factors affecting transport properties at different temperature ranges. The excellent electrical characteristics and compatibility with mainstream semiconductor technology represent a significant step forward in the development of next-generation multifunctional HEMTs. [ABSTRACT FROM AUTHOR]