Effect of Residual Gases on Superconducting Characteristics of Tin Films

Abstract

A special oil-free, ultra-high vacuum system has been used to deposit tin films at pressures less than 10−9 mm Hg onto room temperature substrates. These films were found to possess extremely sharp and reproducible magnetic field transitions as compared to films deposited by more conventional techniques. This resulted from breakup of the penumbra of a film deposited through a mask into electrically discontinuous islands leaving a film of uniform thickness. Specific residual gases were found to decrease the surface mobility of the tin atoms which contributed to continuous film edges and higher critical fields. For example, the critical field extrapolated to 0°K increased from 370 oe for a pure film to 490, 590, and 820 oe as the ratio of oxygen molecules to tin atoms striking the substrate increased from 0 to 3, 6, and 9% respectively. For more highly doped films the bulk characteristics were also altered, indicating the presence of oxygen in the film material. Water vapor and carbon dioxide were found also to alter the edge structure, whereas N2, H2, CH4, C3H8, C5H12, A, and CO did not. By analyzing the critical field and residual resistance data of the various films, the vacuum requirements necessary for obtaining high purity films with sharp magnetic transitions occurring at predictable field values were ascertained. An ultra-high vacuum system is not required if the partial pressure of critical gases is maintained below specified values.