Effective microwave surface resistance of gold-contacted YBa2Cu3O7-x thin films

The microwave surface resistance (R-s) of YBa2Cu3O7-x (YBCO) films sputtered simultaneously on both sides of CeO2 coated sapphire wafers of 3 in. diameter was measured using the disk resonator technique at a frequency of 1.92 GHz. By deposition of Au layers of various thicknesses on the unpatterned YBCO side of the disk resonator, we studied the effective R-s of the Au-contacted YBCO films. Although the Au layer was not directly exposed to the microwave power, it dramatically increased the effective R-s of the YBCO film. For example, R-s(77 K) = 16.5 mu Ohm of a 300-nm-thick YBCO film increased to 85 mu Ohm by the deposition of a 0.1-mu m-thick Au layer. The increase of the Au thickness to 1.2 mu m resulted in a further enhancement of the effective R-s up to 560 mu Ohm. We explain this effect in terms of the impedance transformation model. According to this model the effective R-s of the Au/YBCO bilayer decreases with increasing YBCO film thickness. However, Au layers with thicknesses above 1 mu m considerably enhance the effective R-s even for thick YBCO films (700-800 nm). A higher quality of the YBCO films (in terms of shorter London penetration depths) reduces the effect of a Au layer, while a low electrical resistivity of this layer leads to a further increase of the effective R-s. (C) 1999 American Institute of Physics. [S0003-6951(99)01752-0].