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.

Date of Award	1973
Original language	English