Nb-based A15 compound Josephson tunnel junctions fabricated using a CF4 cleaning process: II. Properties of magnetron-sputtered Nb3X(X=Al,Ge) films

Abstract

Josephson tunnel junctions with Pb counterelectrodes have been fabricated using a CF4 plasma cleaning process on A15 Nb3X(X=Al,Ge) thin films prepared by magnetron sputtering. The effects of deposition conditions on junction quality as well as superconducting parameters (gap, dc Josephson current, and magnetic penetration depth) of the films have been investigated. The junction quality is largely influenced by film deposition conditions, especially by substrate temperature. Junctions with base electrodes which are deposited on oxidized silicon (for Nb3Al) or sapphire (for Nb3Ge) radiatively heated above approximately 700 °C show high-quality I–V curves, while lower temperatures lead to increases in the excess conductance and the width of the sum-gap structure. High-quality junctions with large sum gaps (up to 4.05 mV at 4.2 K for Nb3Al/Pb and 5.05 mV for Nb3Ge/Pb) can be prepared on such films. Furthermore, systematic relationships between the superconducting parameters and the energy gaps are observed, which indicate a good homogeneity of the films. In contrast, all junctions on higher-Tc films prepared by means of self-epitaxial deposition or using a molybdenum substrate holder exhibit apparent current rise at lower voltages, indicating the coexistence of low-Tc materials. Systematic correlation between the junction quality and the residual resistivity ratio shows that such an inhomogeneity extends to the bulk of the films. Radiatively heated samples with the A15 phase composition near the boundary of the film homogeneity range exhibit favorable properties for electric applications such as large energy gaps and small magnetic penetration depths (∼150 nm). However, junctions using these films persistently exhibit nonidealities, such as a slight amount of excess conductance with onset at the Pb gap and a broadened current rise at the sum-gap in their optimized I–V curves, similar to those reported in the coevaporated films.