The role of Coulomb interaction in superconducting NbTiN thin films

We report on the superconducting properties of Nb$_{1-x}$Ti$_x$N thin films of thickness $\sim$ 10 nm, with different Ti fraction $x$ in the range $ 0 \leq x \leq 0.5$, deposited by high temperature chemical vapor deposition. In this parameter range, we observe that the superconducting critical temperature ($T_c$) increases with $x$. Our analysis, in accordance with both McMillan's and Finkelstein's theories, shows that disorder-enhanced Coulomb interaction decreases with $x$, leading to an increase of $T_c$.