Superconducting nanoelectronics using controllable Josephson junctions

This dissertation describes the fabrication, measurement and modelling for a micrometer sized direct-current superconducting quantum interference device (DC-SQID), which had its critical current controlled by a process of non-equilibrium phonon (hot-phonon) irradiation from a nanofabricated gated structure. The method of control was achieved via close proximity, normal-metal constrictions that injected hot-phonons on the Dayem bridge Josephson junctions in the DC-SQUID. A hot electron population created these hot-phonons in the control layer’s normal-metal constrictions when a bias current was applied whilst the device was immersed in liquid helium. This hot-phonon injection layer was produced from a multi-layer fabrication technique that allowed for the creation of an in-line structure; a structure fabrication through a reactive-ion etch process performed on a top-down, nano-lithographically defined constriction geometry. The controlled microSQUID device was created using an inner loop size less than a micrometer and contained two Dayem bridge Josephson junctions with a width and length of approximately 100 and 200 nanometres respectively, in a 50 nanometre thick niobium thin-film. The 70 nanometre thick chromium/titanium normal-metal constructions and the weak link Josephson junction were in thermal contact, but in electrical isolation, due to a 30 nanometre silicon dioxide dielectric layer. The device was measured at a temperature of 4.2 degrees Kelvin, and the manipulation of the critical current oscillations of the microSQUID was performed. The critical current control mechanism, utilised in this device, demonstrated a technique where the magnetic hysteresis was eliminated, and the thermal hysteresis in the current-voltage characteristics of the microSQUID was reduced. An observed characteristic of the controlled reduction of the critical current in this device, illustrated by the one-dimensional microSQUID model presented in this dissertation, was the change in the effective length of the Dayem bridge Josephson junctions. This manifested itself through the shortening of the Cooper pair coherence length in the niobium thin-film under the hot-photon irradiation. The experimental data presented in this dissertation, and its interpretation in relation to the microSQUID model, confirms that this technique, based on hot-phonon irradiation for controlling the critical current in Dayem bridge Josephson Junctions, is compatible with the Josephson effect. Therefore, my dissertation shows a feasible method for post-fabrication parameter control in superconducting circuits using Dayem bridge Josephson junctions.