  Many disordered superconducting films exhibit smeared tunneling spectra with
evident in-gap states. We demonstrated that the tunneling density of states in
ultrathin MoC films is gapless and can be described by the Dynes version of the
BCS density of states with a strong broadening parameter Gamma accounting for
the suppression of coherence peaks and increased in-gap states. The thinner the
film, the lower the Tc and the superconducting energy gap Delta and the larger
the Gamma. MoC films of 3 nm thickness deposited simultaneously on silicon and
sapphire substrates reveal very similar scalar disorder, evidenced by the equal
sheet resistance, but exhibit different superconducting characteristics of Tc,
Delta and Gamma, suggesting that pair breaking responsible for the dissipation
channel and the suppression of superconductivity originates on the
film-substrate interface. It indicates that sapphire is a stronger pair
breaker. Interface pair breaking can be operative in other cases as well.
