ultra-thin films*

Smooth and uniform epitaxial ultra-thin MgB₂ films are desired for various superconducting applications including hot-electron bolometers and single photon detectors. However, the fabrication of ultra-thin MgB₂ films using hybrid physical-chemical vapor deposition (HPCVD) is limited by the nature of film growth, which tends to first form islands which then coalescence into a continuous film, resulting in rough surface morphology. Here we compare epitaxial MgB₂ films, 5-30 nm in thickness, grown on 6H-SiC (0001) substrates with either C termination (C-face) or Si termination (Si-face) by HPCVD. We find that the MgB₂ films on C-face have less roughness and better grain connectivity than those on Si face revealed by AFM and SEM. The root-mean-square roughness of MgB₂ films is 0.4-1 nm on C-face and 2-3 nm on Si-face. The coverages of 2-nm thick MgB₂ films on C-face and Si-face deposited simultaneously is 70% and 40%, respectively.