Investigations into Structural Electrical Superconducting and Optical Properties of Thin Tantalum Films

Abstract

Thin films of tantalum of 99 .996% purity were prepared by thermal evaporation using an electron gun in ultra high vacuum conditions. The deposition conditions were varied in order to study their influence on the microstructural, electrical and optical properties. Results are given of the effects of deposition rate and substrate temperature on the structure of films including grain sizes. Three different crystalline structures were found:- pure f.c.c., pure b.c.c. and a mixture of f.c.c. and b.c.c. phases. The effects of the various deposition parameters on electrical properties were noted; including normal resistivity (p), the variation of resistivity with temperature, the transition from the normal to the superconducting state and superconducting current densities. Results given for over 80 films show that there is a good correlation between structural features and electrical properties. Not all films exhibited superconducting properties and reasons are given for those which did not. The transition temperature from the normal to the superconducting state decreased from the bulk value of 4.48°K (for thick films) to a value of 1.4°K for a film 115Å thick. Empirical relationships between critical temperature of films (TCF)and resistivity ratio p399/P10 are given covering the range of ratios 1.2 to 6 at which value the transition temperature corresponded to the value for bulk material. Other empirical equations give the relation between TCF and p300 and TCF and the temperature coefficient of resistivity. An optical technique is described for the determination of the optical constants of cantalum with a superimposed oxide layer. The technique also gives the optical constants and thickness of the oxide layer, thus enabling the thickness of film responsible for conduction to be evaluated. Correlation between film structure and optical properties have also been found. Values of the optical constants n and k corresponding to the different crystalline phases are given. Attempts have been made to explain the reduction of TCF with reducing film thickness, either by assuming that modifications to the phonon spectrum occur through a reduction in the electron-phonon coupling constant λ or a variation of the density of states N(O) and/or an alteration of the square of the electron matrix element both of which affect λ and through λ the critical temperature.