Superconducting Tunneling in Single-Crystal and Polycrystal Films of Aluminum

The energy gap ($2\Delta$) and transition temperature ($T_c$) of aluminum films in Al-insulator-$M$ tunnel junctions was measured. The orientation and crystallite size of the aluminum film was changed for different tunnel junctions by changing the substrate and/or substrate temperature at evaporation. The second metal ($M$) was either aluminum or indium. It was found that the strain on the aluminum film due to differential thermal expansion could account for most of the observed variation in $T_c$. If $M$ was indium, both $2\Delta$ and $T_c$ were changed. Empirically, it was found that $T_c$ was increased by 3.2% and $2\Delta$ was decreased by 3.3% owing to the indium film. This effect is not understood. The energy gaps of the aluminum films were $3.64k{T}_c$ for tunneling in the [100] direction, $3.52k{T}_c$ for tunneling in "isotropic" material, and $3.41k{T}_c$ for tunneling in the [111] direction. This anisotropy is in approximate agreement with theoretical predictions.