Surface superconductivity as the primary cause of broadening of superconducting transition in Nb-films

We study the origin of broadening of superconducting transition in sputtered Nb films. From simultaneous tunneling and transport measurements we conclude that the upper critical field Hc2 always corresponds to the bottom of transition R~0, while the top R~Rn occurs close to the critical field for destruction of surface superconductivity Hc3 ~ 1.7 Hc2. The two-dimensional nature of superconductivity at H>Hc2 is confirmed by cusp-like angular dependence of magnetoresistance. Our data indicates that surface superconductivity is remarkably robust even in disordered polycrystalline films and, surprisingly, even in perpendicular magnetic fields.