Measurements of FIR transmissivity of electron-beam co-evaporated Nb : Si films appear to be well described by pair-breaking theory. We argue that the pair-breaking parameter cannot be fully explained by inelastic scattering only.
THE SUPERCONDUCTING PROPERTIES of disordered thin films have attracted considerable attention. We report here far infrared (FIR) transmission measurements on Nb:Si alloy films. Low temperature normal state conductivity measurements have recently been completed in this system and analyzed in terms of fluctuation, localization and electron--electron interaction effects [ 1 ]. The infrared measurements are able to give additional information about these films: they point to the existence of electron states within the superconducting energy gap, which result in additional absorption in the low energy range.
The films were prepared in an ultrahigh vacuum system containing two electron guns. They were deposited on wedge-shaped quartz crystal substrates held at 300Β°C. Details of the sample preparation and structural studies have been reported elsewhere [2]. Transmission electron microscopy and Auger spectroscopy studies show that such films are crystalline with a maximum crystallite size of ~ 10nm. The electron diffraction pattern is that of Nb with some indication of texture or preferential orientation. There is no indication of crystalline Si and thus it is likely that amorphous Si is situated in the grain boundaries.
Resistive transition temperatures were measured on companion films obtained in the same evaporation process but on separate substrates. Using photolithographic techniques, a four probe pattern was formed and the resistivity measured. The midpoint of the transition was used as a measured of To. Table 1 lists the critical temperature as well as the sheet resistance RD just above the superconducting transition for the samples used in this study. The films were typically 15 nm thick. The sample with Te = 4.5 K has a resistivity of 50#~2cm. The films are two-dimensional as far as weak localization is concerned.