Surface impedance measurements on coevaporated in situ grown YBa₂Cu₃O_{7 - δ} thin films at 87 GHz

In the here reported test series, we have prepared several stoichiometric YBa₂Cu₃O_{7 - δ} thin films with a thickness between 130 and 780 nm on MgO and LaAlO₃ by thermal coevaporation. Moreover, one film was grown with a copper deficiency of 6%, and two others were coated with Cu-O layers of 15 nm or 150 nm thickness. All films were deposited at a substrate temperature of about 700 °C and at a local oxygen pressure of about 10^{- 2} mbar. The surface impedance of these films was measured in a copper host cavity at 87 GHz between 4.2 K and 120 K. For the stoichiometric films, the onset of the narrow microwave transitions occured reproducibly at T_c values of about 89 K on MgO and about 92 K on LaAlO₃. The effective surface resistance at 4.2 K is similar to results of the best laser ablated films for the same substrates and comparable film thickness. As the lowest values, 15 mΩ (13 mΩ) were obtained for a 345 nm (370 nm) film on MgO (LaAlO₃). The intrinsic surface resistance is lowest for the 140 nm film on MgO, i.e. 7 mΩ at 4.2 K and 22 mΩ at 77 K. In contrast, the copper-deficient film shows a very broad microwave transition curve with a reduced T_c and much higher residual losses. The most remarkable result, however, was achieved for the films with the additional Cu-O layer, which has not enhanced the losses but seems to passivate YBa₂Cu₃O_{7 - δ} thin films against long term degradation.