Hetero-epitaxial growth of YBCO thin films on the a-cut plane sapphire substrates

We examined the hetero-epitaxial growth of YBCO thin films on the a-cut plane sapphire substrates for the passive microwave applications, because the a-cut plane sapphire substrates have smaller in-plane anisotropy of the dielectric constant compared with that of r-cut plane sapphire substrates. The CeO₂ buffer layers and YBCO thin films were prepared by an inductive-coupled plasma sputtering method. We found that perfect in-plane alignment of the CeO₂ buffer layer could be obtained on the a-cut plane sapphire substrates. Also we could obtain the hetero-epitaxial YBCO thin films on CeO₂/a-cut plane sapphire substrates. Tc of the YBCO thin films was approximately 89 K, and the surface resistance (Rs) of the YBCO thin films was approximately 1 mΩ at 50 K and at 22 GHz, equivalently to values for YBCO thin films fabricated on the r-cut plane sapphire substrates.