Film structure and enhanced superconductivity in evaporated aluminum films

The increase in the superconducting transition temperature of evaporated aluminum films is correlated with the following structural properties of the films: the average grain size, the average lattice constant, the film thickness, and the presence or absence of a thin oxide layer surrounding each grain in the film. It is found that for all the films studied, the transition temperature $T_c$ is a function only of the average grain size $a$ with $\ln \left(T_c\right)\propto a^{-0.82\mathrm{}}$. This approximate $\frac{1}{a}$ dependence is in agreement with several theories which indicate that the increased transition temperatures result from changes in the phonon spectrum at the surface of small grains. Even though we have determined that there is a thin oxide layer surrounding each grain in several films, an electron-exciton-electron enhancement mechanism that has been proposed is not observed. However, the presence of the oxide may influence the magnitude of the enhancement.