Planar tunneling spectroscopy of thin-film Nb, Nb/normal metal and Nb/semiconductor structures

The electrical characteristics of thin film niobium, and tunnel junctions formed between thin-film niobium and both normal metals and semiconductors have been investigated. A thickness dependence of the superconducting critical temperature Tc and the superconducting gap parameter Δ have been observed for thin film superconductor/normal metal structures. Although many studies exist, the underlying mechanism is not fully understood. The niobium-semiconductor system offers new opportunity to investigate superconductor/normal metal interfaces and holds promise for microelectronics applications. Studies of the electrical transport across the interface between the two materials are presented. The thickness dependence study was performed on high-quality, DC-magnetron sputter deposited Nb on single-crystal sapphire. The DC resistivity was measured as a function of ternperature to evaluate the film quality and determine Tc. The critical temperature was found to monotonically decrease with decreasing thickness, consistent with existing theory. Additionally, the superconducting gap 2Δ was measured as a function of film thickness for Nb/Al2O 3/Ag tunnel junctions. This value was found to also decrease monotonically with decreasing thickness. Furthermore, the first observation of an increase in the ratio of 2Δ(0)/kBTc with decreasing thickness is presented. The superconductor-semiconductor study was performed on Nb-InGaAs bilayers. The Schottky barrier formed at the interface between these two materials was utilized as a tunneling barrier. A method was developed to fabricate small area junctions. The tunneling conductance as a function of semiconductor properties and sample temperature have been found to be consistent with results reported in the literature.U of I OnlyThesi