Effect of microstructure of YBa2Cu3Ox films on power handling capability studied with microstrip resonators

Microwave properties of two types of YBa2Cu3 Ox (YBCO) films with different microstructure have been studied at 4.2 K and 6 GHz. One type of film was grown at a constant substrate temperature of 705 °C during the film growth (constant-growth films). The other type of film ws prepared with two-stage process: initial growth at a lower temperature of 630 °C and subsequent higher-temperature deposition at 705 °C (two-stage-growth films). Although both types of film have very similar intrinsic YBCO material properties such as c-axis lattice constant, Tc, and unloaded quality factor Qu at low power levels, quite different microwave properties are observed at high power levels. The two-stage-growth films with small grains less than 100 nm show fairly small power dependence of Qu and resonance frequency on input power up to +10 dBm. On the other hand, the constant-growth films with large grains (∼200 nm) show significant changes in these properties, accompanied by an appearance of asymmetric resonance curves. These results show that the power handling capability of granular YBCO films is mainly affected by the extrinsic film microstructure and that the quality of the grain boundaries causes the difference in power dependence.