YBCO films were grown on magnesium oxide (MgO) substrates for fabricating
step-edge junction SQUIDs and other Josephson junction-based devices. In-plane
45°
grain misorientation was frequently observed in films grown on degraded or
contaminated MgO substrates. The appearance of these misoriented grains results in a
decrease of the thin-film critical-current density and reduces the device yield.
In this work, we investigated the chemical properties of MgO substrates with
various surface conditions due to different substrate preparation methods and
environmental degradation, by using x-ray photoelectron spectroscopy (XPS). The XPS
characteristics of the surface are compared before and after a thermal annealing at
760 °C
resembling the thin-film deposition heating cycle. The MgO substrates, after lithographic
processing or only weeks of exposure to the laboratory environment, showed surface
degradation characterized by the presence of hydroxyl groups, carbonate, and other possible
carbon compounds such as bicarbonate, alcohols and carboxyl. Heating of the substrates to
760 °C
improves the surface quality to a certain degree with the removal of some of the above
contaminants, but is not sufficient to recover the MgO surfaces. A final Ar ion-beam etch
cleaning process at low ion energy proved to be very effective in refreshing the MgO
substrate surface that had been degraded due to lithographic processing or storage. Films
grown on MgO with this pre-treatment showed perfect grain alignment and high
critical-current densities.
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