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.
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We thank A.H. MacDonald for fruitful discussions, and D. Storm for facilitating SIMS measurements. For the measurements performed here, we made use of the Cornell Center for Materials Research (CCMR) Shared Facilities, which are supported through the National Science Foundation (NSF) Materials Research Science and Engineering Centers (MRSEC) program (grant DMR-1719875). The structure fabrications were realized in part at the Cornell NanoScale Facility, a member of the National Nanotechnology Coordinated Infrastructure (NNCI), which is supported by the NSF (grant ECCS-1542081), and a CCMR Superconductor Seed. D.J. and D.J.M. acknowledge funding support from the Office of Naval Research, monitored by P. Maki. D.J.M. also acknowledges device processing support from N. Green.
The authors declare no competing financial interests.
Reviewer Information Nature thanks Y. Krockenberger and the other anonymous reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a, STEM image of NbNx/AlN grown on top of a SiC substrate, showing the single-crystal nature of NbNx over a large region. The red lines have been added as a guide to show the crystallinity across the entire range measured. b, A twin boundary and a stacking fault in the MBE NbNx layer. This STEM image of NbNx/AlN on top of SiC shows a grain boundary with two cubic NbNx phases rotated across each other. The red and blue lines have been added to draw out the stacking fault seen near the twin boundary.
a, b, AFM images of epitaxial NbNx films that are 5-nm thick (a) and 35-nm thick (b), over areas of 1 × 1 μm2 and 3 × 3 μm2. RMS, root mean squared.
2θ is the angle between the incident and diffracted beams, and ω is the angle between the incident beam and the sample surface. a, The 5-nm sample. b, The 35-nm sample. There is a clear separation between the SiC and cubic NbN (first- and second-order) peaks in the 35-nm sample. But this feature is absent in the 5-nm sample, owing to the weak XDR signal intensity in such an ultrathin film.
a, Drain current (Jd) versus top-gate voltage (Vgs) transfer curves at 300 K for HEMTs grown on NbNx/SiC substrate, showing a high on/off ratio at source–drain voltages (Vds) of 0.1 V (red curve) and 3 V (blue curve). b, Jd versus Vds curves for various top-gate voltages (from bottom curve to top, −3 V, −4 V, −5 V, −6 V, −6.5 V) of GaN HEMTs at 300 K.
Extended Data Figure 5 Representative transfer curves of a HEMT structure with superconducting load.
The graph plots Jd versus Vgs (without superconductor load) and Vgs′ (with superconductor load) at 5 K, showing the phase transition of NbNx that occurs when Jd is larger than 0.1 A mm−1.
a, Summary of carrier density and mobility for different thicknesses of NbNx films, ranging from 4 nm to 100 nm. The red arrow shows that the red dots correspond to the left-hand y axis; the blue squares correspond to the right-hand y axis. b, c, Critical current density as a function of temperature for 5-nm and 35-nm NbNx films.
a, Magnetoresistance (Rxx) plotted against inverse magnetic field (1/B) before background subtraction, taken over the magnetic-field range of 10 T to 14 T. The oscillations occur at periods of 1/B—a clear indication of sharp peaks in the 2DEG density of states owing to Landau levels. The upward black arrow indicates increasing temperatures. The inset shows the 2.2 K data (light blue) with background (blue); the non-oscillating background was removed before evaluation of carrier concentration, effective mass, and scattering time. b, Landau plot of magnetoresistance relative minima plotted against inverse magnetic field. The slope of the line is ħ2πnSdH/2m*; the density and effective mass are taken from the Lifshitz–Kosevich fit to magnetoresistance oscillations.
The left-hand y axis shows the secondary-ion intensity (counts per second) of Al and Ga atoms. The right-hand y axis indicates the atomic percentages of N and Nb atoms, which are respectively 43.3% and 56.7%, indicating a N/Nb ratio (x) of 0.762 in linear (a) and log (b) scale of signal intensity. The x axis denotes the depth of the sample from the top surface.
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Yan, R., Khalsa, G., Vishwanath, S. et al. GaN/NbN epitaxial semiconductor/superconductor heterostructures. Nature 555, 183–189 (2018). https://doi.org/10.1038/nature25768
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