YBa2Cu3O7-δ Biepitaxial and Step Edge Josephson Junctions on MgO Substrates. SQUID Amplifier

Doktorsavhandling, 1996

This thesis concerns the preparation of Josephson junctions and their applications in Superconducting Quantum Interference Devices (SQUIDs). High temperature superconductors were used for operation at 77 K, the boiling temperature of liquid nitrogen. To perform the study, high quality superconducting films had to be deposited and low noise junctions had to be prepared. This thesis treats the optimization process for the preparation of superconducting films, Josephson junctions and dc SQUIDs. The preparation of YBa2Cu3O7 (YBaCuO), high-TC superconducting films has been investigated. The optimization of the deposition parameters was performed by using the pulsed laser deposition technique. The superconducting films were prepared on (100)MgO and r-cut sapphire substrates. It has been shown that an annealing process of the MgO substrates at 950 °C, prior to the deposition of YBaCuO, improved substantially the reproducibility of the experiments. The deposition of a thin (10 nm thick) homoepitaxial MgO buffer layer improved the epitaxy of the YBaCuO thin films. Under certain deposition parameters, the application of CeO2 buffer layers improved the epitaxy of the YBaCuO layers on r-cut sapphire. Critical current density values of 1-2x106 A/cm2 have been measured at 77 K in microbridges prepared on MgO and CeO2-buffered sapphire substrates. MgO substrates have been covered with an structured photoresist mask and shortly (< 2 min) ion milled under an angle of 60 ° with respect to normal incidence. The a-b plane of the YBaCuO unit cell grows 45 ° rotated with respect to that of the MgO substrate in the area shadowed by the photoresist mask. The 45 ° misoriented region has been used for the preparation of biepitaxial junctions on MgO substrates. The biepitaxial junctions contained at least two in-series connected 45? grain boundaries. By applying longer milling times, 300- 500 nm height steps have been prepared on MgO substrates. Step edge junctions have been prepared by depositing 250 nm thick YBaCuO layers on the stepped substrates. The junctions consisted of at least two in-series, out-of-plane, grain boundaries. The spread in critical current density of the biepitaxial junctions, measured at 4.2 K, was about 4 orders of magnitude. The spread in the critical current density of junctions prepared at flat step edges (<30°) was about 3 orders of magnitude whereas at sharp step edges (= 65 °) it could be as low as a factor of 20. The yield of operating devices at 77 K was 40% at flat step edges and could be as high as 100% at sharp step edges. Biepitaxial dc SQUIDs with energy resolution values of 3x10-28 J/Hz in the white noise region and of 3x10-27 J/Hz at 1 Hz and 77 K, have been measured for a 70 pH SQUID in the best case. On the other hand, values as low as 1x10-30 J/Hz in the white noise region and 2.5x10-29 J/Hz at 1 Hz and 77 K have been measured for step edge dc SQUIDs. The best value of the energy resolution of biepitaxial dc SQUIDs was more than two orders of magnitude worse than expected under the assumption of pure Johnson noise in the junctions. Energy resolution values close to the theoretically expected values have been measured for step edge dc SQUIDs. The origin of the excess noise in the biepitaxial dc SQUIDs is possibly due to the imperfect epitaxial quality of the 45° misoriented region. Step edge dc SQUIDs are prefered for applications due to low noise properties. By applying the step edge junctions, dc SQUID magnetometers with a field resolution of 3 pT/(Hz)1/2 down to 10 Hz and 77 K have been prepared.