Electrical Conductance of Thin Ultrahigh-Vacuum-Evaporated Films of Tungsten on Glass

Abstract

Electrical conductance measurements have been made on thin tungsten films deposited onto Pyrex glass substrates cooled to 90°K under ultrahigh‐vacuum conditions. The variation of the conductance with temperature indicated that the films were discontinuous for average thicknesses less than about 30 Å. This structure was confirmed by electron micrographs of the films after removal from the vacuum system. Irreversible changes in the conductance were found to take place when the low‐temperature‐deposited films were annealed at 300°K. The direction of the changes depended on the average film thicknesses. Theories of conduction based on an activated tunneling process are inadequate to explain these changes, but a model based on internal thermoinic emission provides a qualitative explanation. It is suggested that the conduction current contains contributions from a space‐charge‐limited current through the substrate as well as that arising from the thermionic emission process. The experimentally observed voltage dependence of the conductance appears to confirm this suggestion. Irreversible changes in the conductance could also be brought about both by exposure of the ultrahigh‐vacuum‐deposited films to a gas pressure of 10⁻⁸ Torr and by the application of high voltages at 300°K.