  High quality, ultrathin, superconducting films are required for advanced
devices such as hot-electron bolometers, superconducting nanowire single photon
detectors, and quantum applications. Using Hybrid Physical-Chemical Vapor
Deposition (HPCVD), we show that MgB2 films as thin as 4 nm can be fabricated
on the carbon terminated 6H-SiC (0001) surface with a superconducting
transition temperature above 33K and a rms roughness of 0.7 nm. Remarkably, the
film quality is a function of the SiC surface termination, with the
C-terminated surface preferred to the Si-terminated surface. To understand the
MgB2 thin film/ SiC substrate interactions giving rise to this difference, we
characterized the interfacial structures using Rutherford backscattering
spectroscopy/channeling, electron energy loss spectroscopy, and x-ray
photoemission spectroscopy. The MgB2/SiC interface structure is complex and
different for the two terminations. Both terminations incorporate substantial
unintentional oxide layers influencing MgB2 growth and morphology, but with
different extent, intermixing and interface chemistry. In this paper, we report
measurements of transport, resistivity, and critical superconducting
temperature of MgB2/SiC that are different for the two terminations, and link
interfacial structure variations to observed differences. The result shows that
the C face of SiC is a preferred substrate for the deposition of ultrathin
superconducting MgB2 films.
