Superconducting Tunneling in Single-Crystal and Polycrystal Films of Aluminum

Abstract

The energy gap ($2\ensuremath{\Delta}$) 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 $2\ensuremath{\Delta}$ and ${T}_{c}$ were changed. Empirically, it was found that ${T}_{c}$ was increased by 3.2% and $2\ensuremath{\Delta}$ 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.