An Investigation into some Properties of Thin and Ultra Thin Niobium Films

Abstract

Thin films of niobium were prepared by the thermal evaporation of 99.99% pure niobium. The electrical properties of the films were measured before the films were removed from the deposition chamber. The effects on the electrical properties of the various deposition parameters of vacuum, substrate temperature and deposition rate were noted. A series of films, ranging in thickness from 80 Å to 5475 Å, was prepared by depositing niobium in a vacuum of 5 x 1078 torr at rates of about 250 Å mi-1 onto substrates held at 500°K. After removal from the vacuum the superconducting current densities, the transition temperature and the normal resistivity of each film was determined. An optical technique is described for the determination of the oxide thickness grown on the film after exposure to an atmospheric environment. Film thickness was measured by an electrical method and by means of an interferometer. Good agreement was obtained between both methods providing a correction was made for the oxide layer. The resistivity of the films increased as the thickness decreased, with the resistivity ratio p 300/P 10 Varying from 1.16 for a film 45 R thick to 5.75 for a film 5440 Å thick. The transition temperature decreased from the bulk value 9.22°K to a minimum of 3.0°K for a film 45 Å thick. These results can be explained by assuming that modifications to the phonon spectrum in these films are responsible for the observed decrease in the transition temperatures. By applying the McMillan equation, with the suggested modifications to the phonon spectrum, to the films prepared in this work, values are derived for transition temperature and grain size which are in close agreement with the experimental results.