Epitaxy is a process by which a thin layer of one crystal is deposited in an ordered fashion onto a substrate crystal. The direct epitaxial growth of semiconductor heterostructures on top of crystalline superconductors has proved challenging. Here, however, we report the successful use of molecular beam epitaxy to grow and integrate niobium nitride (NbN)-based superconductors with the wide-bandgap family of semiconductors-silicon carbide, gallium nitride (GaN) and aluminium gallium nitride (AlGaN). We apply molecular beam epitaxy to grow an AlGaN/GaN quantum-well heterostructure directly on top of an ultrathin crystalline NbN superconductor. The resulting high-mobility, two-dimensional electron gas in the semiconductor exhibits quantum oscillations, and thus enables a semiconductor transistor-an electronic gain element-to be grown and fabricated directly on a crystalline superconductor. Using the epitaxial superconductor as the source load of the transistor, we observe in the transistor output characteristics a negative differential resistance-a feature often used in amplifiers and oscillators. Our demonstration of the direct epitaxial growth of high-quality semiconductor heterostructures and devices on crystalline nitride superconductors opens up the possibility of combining the macroscopic quantum effects of superconductors with the electronic, photonic and piezoelectric properties of the group III/nitride semiconductor family.
The nitride semiconductor materials GaN, AlN, and InN, and their alloys and heterostructures have been investigated extensively in the last 3 decades, leading to several technologically successful photonic and electronic devices. Just over the past few years, a number of "new" nitride materials have emerged with exciting photonic, electronic, and magnetic properties. Some examples are 2D and layered hBN and the III-V diamond analog cBN, the transition metal nitrides ScN, YN, and their alloys (e.g. ferroelectric ScAlN), piezomagnetic GaMnN, ferrimagnetic Mn4N, and epitaxial superconductor/semiconductorNbN/GaN heterojunctions. This article reviews the fascinating and emerging physics and science of these new nitride materials. It also discusses their potential applications in future generations of devices that take advantage of the photonic and electronic devices eco-system based on transistors, light-emitting diodes, and lasers that have already been created by the nitride semiconductors.
ScxAl1−xN (x = 0.18–0.40) thin films of ∼28 nm thickness grown on metal polar GaN substrates by molecular beam epitaxy are found to exhibit smooth morphology with less than 0.5 nm roughness and predominantly single-phase wurtzite crystal structure throughout the composition range. Measurement of the piezoelectric d33 coefficient shows a 150% increase for lattice-matched Sc0.18Al0.82N relative to pure aluminum nitride, whereas higher Sc contents exhibit lower piezoelectric coefficients. The electromechanical response of the epitaxial films correlates with the crystal quality and the presence of zinc blende inclusions, as observed by high-resolution electron microscopy. It is further found that the polarity of the epitaxial ScxAl1−xN layers is locked to the underlying substrate. The measured electromechanical properties of epitaxial ScxAl1−xN, their relation to the atomic crystal structure and defects, and its crystal polarity provide useful guidance toward the applications of this material.
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