The Fabrication of Josephson Junctions with an Electron Beam

Abstract

There are numerous methods which can be used in the production of Josephson junctions in the high temperature superconductors, each with its pros and cons. In this thesis the use of a focused high energy (350keV) electron beam to damage the high temperature superconductor YBa${2}$Cu${3}$O${7-}$${\delta}$ as a method of producing Josephson junctions is discussed. The technique is possible because of the sensitivity of the YBa${2}$Cu${3}$O${7-}$${\delta}$ superconductor to the precise oxygen order and content. The displacement of the oxygen by the electron beam has a threshold of $\approx$120keV corresponding to a threshold displacement energy of the oxygen of 18eV. Above this the decrease of the T${c}$ with the electron fluence is found to be proportional to the non-ionising energy loss, or equivalently, the number of Frenkel defects produced in the YBa${2}$Cu${3}$O${7-}$${\delta}$. Heating of the sample is calculated to be negligible during the electron beam irradiation. The junctions are produced by scanning the electron beam across a patterned track of YBa${2}$Cu${3}$O${7-}$${\delta}$ at a rate which gives the appropriate fluence. Whilst the beam diameter is a few nm the damage is found to spread over a width of between 30-60nm. A line scan which produces a junction with a T${c}$ of 70 - 80K has a resistance-area product of typically 6 x 10$^{-10}$$\Omega$cm$^{2}$. This method is flexible in that it allows the production of Josephson junction by using a lower dose, or completely damaged material with a higher dose, and these damaged regions can be any shape and drawn anywhere within the limits of the movement of the electron beam. The damage produced partially anneals out when stored at room temperature, however it is believed that this instability can be overcome by overdamaging and then annealing. The Josephson junctions produced by the electron beam irradiation method have current - voltage characteristics which fit the resistively shunted junction model when measured in the small junction limit. As the temperature is reduced, an excess current appears until at low temperatures, the current voltage curves become flux-flow like. In the small junction limit the microwave power dependence is also that predicted by the resistively shunted junction model. The critical current as a function of magnetic field has a dependence which is very Fraunhofer like for all of the junctions produced so far. By using a line scan in which the fluence is varied heavy/light/heavy it is possible to produce junctions which are narrower than the track width. The period of the critical current field dependence was found to be proportional to the inverse of the product of the track and junction widths. An extension of the method used to produce the narrow junctions can be used to produce small SQUIDs which can have very low inductances. Series arrays with a few junctions were also made whose current-voltage and microwave behaviour showed that the junctions can be produced with a low spread in their critical currents.