Etude et réalisation de jonctions Josephson en nitrure de niobium à barrière semi-métallique en TaxN ; application aux circuits logiques micro-ondes à impulsions quantiques RSFQ

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 TaxN barrier. The study of sputtered TaxN thin films as a function of deposition parameters showed a good reproducibility of the film properties. The NbN/TaxN/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/cm2. 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/TaxN/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).
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Theses
Superconductivity [cond-mat.supr-con]. Université Joseph-Fourier - Grenoble I, 2007. French


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Contributor : Romano Setzu <>
Submitted on : Thursday, December 6, 2007 - 4:18:45 PM
Last modification on : Friday, April 24, 2015 - 1:13:56 AM

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Romano Setzu. Etude et réalisation de jonctions Josephson en nitrure de niobium à barrière semi-métallique en TaxN ; application aux circuits logiques micro-ondes à impulsions quantiques RSFQ. Superconductivity [cond-mat.supr-con]. Université Joseph-Fourier - Grenoble I, 2007. French. <tel-00194521>

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