YBA2CU3O7 FILMS EPITAXIALLY GROWN ON MGO, LAALO3, SRLAALO4 AND AL2O3 SUBSTRATES - STRUCTURAL AND SUPERCONDUCTING PROPERTIES IN CORRELATION WITH THE MICROWAVE SURFACE-RESISTANCE AND THE FAR-INFRARED TRANSMITTANCE

YBa2Cu3O7 epitaxial films have been grown by laser ablation on MgO, La AlO3, SrLaAlO4 and Al2O3 substrates. Films on LaAlO3 have excellent st ructural, superconducting and microwave properties, reflected by surfa ce-resistance (R(S)) values systematically in the range 0.5-1 m Omega (10 GHz, 77 K). On MgO substrates, such low R, (< 0.50 m Omega) values can be reached on both fully ''normal-0 degrees'' and ''45 degrees'' in-plane oriented YBa2Cu3O7 films, whereas in mixed (0 degrees-45 degr ees) films the presence of the high-angle grain boundaries leads to a surface resistance larger than 100 m Omega. On SrLaAlO4 and bare Al2O3 substrates, despite good superconducting properties, obtained for sap phire due to the specific substrate surface treatment, the structural quality and surface resistance are moderate. The latter results are in complete agreement with the correlation we have demonstrated between structural quality and surface resistance as well as far-infrared tran smittance. We also point out the importance of a R(S) determination as an intrinsic superconducting parameter: the absence of a direct corre lation with low-frequency measurements shows that the latter are not s ufficient to fully characterize the superconducting thin films.