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Effect of Interstitial Impurities on the Residual Resistivity of Niobium Thin Films

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Author: Connell, Justin
Advisor: Outlaw, Ronald A.
Committee Members: Rice, Gary; Coleman, Randolph A.; Poutsma, John C.; Kelley, Michael
Issued Date: 2008-05-02
URI: http://hdl.handle.net/10288/484
Description: The favorable superconductive properties of niobium have led to its extensive use in superconducting radio frequency (SRF) linear accelerator cavities. In order to reduce the cost of these cavities, while at the same time developing avenues by which their properties can be enhanced, substantial effort has been directed towards developing high quality Nb/Cu thin film cavities to replace conventional bulk Nb cavities. The properties of superconducting Nb thin films are still not completely understood; however, substantial evidence exists suggesting that these properties are significantly affected by thin film purity. Thin films (~300nm) were deposited via physical vapor deposition using as-received (99.9% pure) and electrotransport purified niobium source rods. Multiple surface analysis techniques (Auger electron spectroscopy, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy) were utilized to characterize the thin film purity, while residual resistance ratios were determined for the films using the four-point-probe method at room temperature down to 10K (Tc for Nb ~ 9.3K) as a means of measuring their superconducting quality. It was found that a significant CO partial pressure inherent to the physical vapor deposition method employed obscured the gain in purity between the as-received and electrotransport purified films. However, it was clear from residual resistivity ratio measurements of an electrotransport-purified sample compared with values reported in the literature that there is a significant gain in superconductive quality when electrotransport purification is performed. Since the carbon and oxygen impurity level deposited into the thin films is very clearly an issue of related rates, future studies would require high temperature electron evaporation, such as that obtained using a high-temperature Knudsen cell, to increase the Nb deposition rate with respect to the deposition of residual impurity gases present in the system during film nucleation and growth. This would lead to improved thin film purity and thus, enhanced superconductive quality of the deposited films.
Degree: Bachelors of Science in Chemistry

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