Fabrication and DC Characterisation of SIS Junctions for Applications in Sub-/Millimetre Astronomy

  • Jakob Wenninger

Student thesis: Master of Science by Research

Abstract

Observations in sub-/millimetre astronomy rely on superconductor/insulator/superconductor (SIS) mixers to convert the frequency of a signal from an astronomical source to a lower, processable frequency. The frequency conversion relies on the non-linearity of the DC current--voltage (IV) characteristic of an SIS junction, the heart of each SIS mixer. The aim of this work was to fabricate SIS junctions and characterise their direct current (DC) properties. \par The fabrication is obtained by means of thin-film deposition technology and photolithography. SIS junctions with aluminium oxide (AlO\textsubscript{x}) insulator barriers have been fabricated in the facilities of the Observatoire de Paris in the past. After a break of fabricating SIS devices, the existing recipe for Nb/AlO\textsubscript{x}/NbN junctions required adoption to changes in the equipment. An alternative insulating layer, aluminium nitride (AlN\textsubscript{x}), was also investigated, since the AlO\textsubscript{x} insulator barrier has a minimum manufacturable thickness limiting the maximum current through the SIS junction. A detailed literature review was carried out on the fabrication of AlN\textsubscript{x} junctions. Considering different techniques, plasma nitrification of Al is closer investigated under guidance of fabrication parameters from literature.\par The fabricated junctions are tested for DC IV characteristics by dipping the chips into a liquid He bath at 4\,K. The characteristic DC IV response of an SIS junction has a large resistance below the gap voltage and a normal resistance beyond the gap voltage. At the gap voltage, the current raises rapidly in the so called non-linearity of the junction. A considerable fraction of fabricated junctions shows no SIS DC IV response, meaning that there were issues with the control of the fabrication conditions which impacted the reproducibility of the fabrication process. Three batches of in total seven chips with AlO\textsubscript{x} insulator barriers and one chip with an AlN\textsubscript{x} insulator barrier showed SIS DC IV responses. However, the measured critical current density through the junction in both cases is well below the intrinsic limit of the AlO\textsubscript{x} barrier material, which requires in turn adoption of the recipe to decrease the barrier thickness. This is achieved by decreasing the exposure time of the Al layer to O\textsubscript{2} gas or N\textsubscript{2} plasma during the fabrication of AlO\textsubscript{x} or AlN\textsubscript{x}, respectively. The gap voltage was found to be $V_\text{gap,\,avg} = (2.70 \pm 0.05)\,\text{mV}$. The measured gap voltage is also below the expected value for a junction with Nb and NbN as the superconducting electrodes. In fact, the gap voltage resembles more a junction with Nb on both electrodes of the SIS junction. This implies issues with the nitrification of the Nb during the NbN deposition.\par
Date of Award1 Aug 2020
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorLucio Piccirillo (Supervisor) & Anna Scaife (Supervisor)

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