Microstructure and microwave properties of YBCO thin films grown on MgO and SrTiO₃ by CVD

The microstructure and microwave properties of YBa₂Cu₃O_7−δ thin films grown on SrTiO₃ (100) and MgO (100) substrates by chemical vapor deposition have been studied. Both 100 GHz cavity measurement of surface resistance and ground plane substitution in a 5 GHz microstrip resonator show that films on SrTiO₃ have better microwave properties than those on MgO. Although there are some a-axis grains and secondary phases on the surface, a large fraction of each film on SrTiO₃ is epitaxial with its c axis normal to the substrate. The 100 GHz surface resistance of these films is less than copper for temperature ⋚ 82 K, and approaches the detection limit at 10–20 K. For the films on MgO, c-axis grains of different in-plane rotation are found together with some a-axis needle-like grains. The grain boundaries are detrimental to the microwave properties, and the resulting surface resistance at 100 GHz and 10–20 K is about 20 mΩ higher than that of films on SrTiO₃.