Electron transport properties of copper films. II. Thermoelectric power

Abstract

Thermoelectric power (TEP) of as-deposited and annealed polycrystalline and epitaxially grown copper films of thickness 100-5000 Å has been measured. The TEP increases with the film thickness to reach a saturation value at about 3000 Å. The thickness dependence decreases markedly with increasing annealing temperature and also with increasing temperature of deposition. The saturation value of the TEP of as-deposited films is about 1.5 times the bulk value of copper and decreases rapidly with annealing to approach the single-crystal bulk value. In sharp contrast to the behavior of the TEP, the resistivity and its temperature coefficient and the carrier concentration of the same copper films exhibit little size effects and insignificant changes due to annealing for thicknesses larger than 1000 Å. The observed thickness dependence of the TEP of unannealed and annealed films shows no agreement with the Fuchs-Sondheimer (F-S) theory. Further, there is no correlation between the thickness dependence of the TEP and the temperature coefficient of resistivity as expected on the basis of the F-S theory. Since the thickness variation of the TEP changes markedly with annealing, the observed dependence is not a genuine mean free path effect. The observed linear dependence of the TEP on the inverse of the film thickness (as expected from the F-S theory) is totally accidental and, therefore, physically meaningless values of U= (∂lnl/∂lnE)_EF are obtained from the slopes of the lines. Arguments have been advanced to suggest that the observed thickness dependence of the TEP can be understood only in terms of the effect of a large thickness-dependent concentration of structural defects frozen in the metal films. The defect-induced changes in the TEP would be caused by the distortions in the Fermi surface and, hence, changes in the energy dependence of the mean free path or relaxation time at the Fermi surface.