Large area deposition of YBa₂Cu₃O₇ films by thermal co-evaporation

Reactive evaporation techniques cannot be simply scaled up for larger YBCO films because the zone of enhanced O₂ pressure in front of the substrate will also increase in height and is impenetrable for the metal vapours. We have circumvented this problem by separating the evaporation and oxidation zones. Smooth films without any precipitates on a 10×10 μm² scale could be fabricated on 30×30 mm² MgO substrates. The homogeneity of the stoichiometry and of the thickness was 2%. Good normal state electrical properties have been achieved: ρ(300 K)/ρ(100 K) = 3.0 and ρ(100 K) = 40 μΩcm. The superconducting transition temperature was typically 87 K with a variation of ΔT_c ≤ 0.5 K over the sample area while J_c at 77 K varied between 1 and 6·10⁶ A/cm². The microwave surface resistance at 77 K was found to be R_eff = 60 mΩ at 87 GHz. In addition, we deposited YBCO films on 3 inch LaAlO₃ wafers with T_c = 86 K ± 1 K and tested the feasibility of deposition of YBCO films on 4 inch silicon wafers with a YSZ buffer.