Coulomb blockade effects in anodised niobium nanostructures

Abstract

Niobium thin film nanostructures were fabricated using electron beam lithography and a liftoff process involving a four layer resist system. Wires having micrometres in length and below 200 nm in width were deposited on an insulating substrate and subsequently thinned by anodisation. The resistance of the wires was monitored in situ and could be trimmed by controlling the anodisation voltage and time. The method appears appropriate for the fabrication of very small resistors for use as biasing resistors for single electron devices. Transport properties of these resistors were measured at millikelvin temperatures. They had nonlinear current-voltage characteristics around zero bias and showed a transistion from superconducting to insulating behaviour. Analysis of the offset voltage showed that a Coulomb blockade set in when the sheet resistance of the films exceeded the superconducting quantum resistance approx 6.45 kOhm. Samples in a single electron transistor-like geometry with two variable thickness weak links were made by combined angular evaporation of niobium and anodisation. Overlapping gate electrodes were deposited on these samples. The drain-source current-voltage characteristics could be modulated by the gate voltage, giving a response typical for a system of multiple ultrasmall tunnel junctions. The thesis contains a five page appendix with lithography recipes.