Penetration depth study of very thin superconducting Nb films

Using a low-frequency two-coil technique, we measure the magnetic penetration depth lambda(T) of superconducting Nb films with thicknesses 20 A <= d <= 228 A sputtered onto oxidized Si substrates. We find a phenomenological dependence of T-c on d, T-c/8.5 K approximate to tanh(d/70 A) for films thinner than 250 A. lambda(-2)(T)/lambda(-2)(0) is well fitted by weak-coupling dirty-limit theory with a weak-coupling gap, Delta(0)=1.8k(B)T(c). lambda(-2)(0) agrees with dirty-limit theory, given the experimental values of transition temperature T-c and residual resistivity rho(0). These results indicate that the suppression of T-c is due to mechanisms that weaken the effective pairing interaction and not due to pair breaking interactions.