Characterization of MgO substrates for growth of epitaxial YBCO thin films

Abstract

YBCO films were grown on magnesium oxide (MgO) substrates for fabricatingstep-edge junction SQUIDs and other Josephson junction-based devices. In-plane45° grain misorientation was frequently observed in films grown on degraded orcontaminated MgO substrates. The appearance of these misoriented grains results in adecrease of the thin-film critical-current density and reduces the device yield.In this work, we investigated the chemical properties of MgO substrates withvarious surface conditions due to different substrate preparation methods andenvironmental degradation, by using x-ray photoelectron spectroscopy (XPS). The XPScharacteristics of the surface are compared before and after a thermal annealing at760 °C resembling the thin-film deposition heating cycle. The MgO substrates, after lithographicprocessing or only weeks of exposure to the laboratory environment, showed surfacedegradation characterized by the presence of hydroxyl groups, carbonate, and other possiblecarbon compounds such as bicarbonate, alcohols and carboxyl. Heating of the substrates to760 °C improves the surface quality to a certain degree with the removal of some of the abovecontaminants, but is not sufficient to recover the MgO surfaces. A final Ar ion-beam etchcleaning process at low ion energy proved to be very effective in refreshing the MgOsubstrate surface that had been degraded due to lithographic processing or storage. Filmsgrown on MgO with this pre-treatment showed perfect grain alignment and highcritical-current densities.