Thesis presented November 12, 2007
Abstract: This thesis research brought to the development and optimization of SNS (Superconductor-Normal Metal-Superconductor) Josephson junctions with NbN electrodes and a high resistivity Ta
xN barrier. The study of sputtered Ta
xN thin films as a function of deposition parameters showed a good reproducibility of the film properties. The NbN/Ta
xN/NbN trilayers have the expected critical temperature (16K). The junctions showed a clear dependence of the
RnIc product as a function of the partial nitrogen pressure during deposition; the
RnIc is the product between the junction critical current and its normal resistance, indicating the upper limit Josephson frequency. We have also obtained some very large
RnIc products, up to 3.74mV at 4.2K for critical current densities
Jc of about 15kA/cm
2. Junctions show the expected Josephson behaviors, respectively Fraunhofer diffraction and Shapiro steps, up to 14K. Moreover junctions appear to be self-shunted. The junctions temperature dependence of
Jc has been fitted by using the long SNS junction model in the dirty limit, which gives a normal metal coherence length of about 3.8nm at 4.2K. We have finally studied a multilayer fabrication process, including a common ground plane and bias resistors, suitable for RSFQ (Rapid Single Flux Quantum) logic basic circuits. To conclude we have been able to show the performance superiority of NbN/Ta
xN/NbN junctions over the actual niobium junctions, as well as their interest for realizing compact RSFQ logic circuits. In fact these junctions do not need external shunt-resistors as the Nb junctions, and can operate at clock frequencies above 150 GHz up to 10K (against 50GHz at 4.2K for the Nb technology).
Keywords: Josephson junctions, metal-insulator transition
On-line thesis.