ROLE OF AG IN LASER-ABLATED YBA2CU3O7-DELTA THIN-FILMS - EVIDENCE FOR GRAIN ENLARGEMENT AND ALIGNMENT

Microstructural and superconducting properties of YBa2Cu3O7-delta thin films grown in situ by pulsed laser ablation using YBa2Cu3O7-delta targets doped with 5 wt.% Ag have been studied. Ag-doped films grown on [100] LaAlO3 and [100] SrTiO3 substrates at temperatures in the range 650-700-degrees-C have shown very significant improvements over the undoped YBa2Cu3O7-delta films grown under identical conditions. A critical current density of 1.4 x 10(7) A cm-2 at 77 K has been observed on SrTiO3 substrates. Microwave measurements carried out using a microstrip resonator at the X-band have shown a low microwave surface resistance of 190 muOMEGA at 77 K and an absence of dependence on microwave power up to 13 dBm. Evidence found through scanning electron microscopic observation of Ag-doped films has indicated a substantial grain enlargement and alignment. Hence, the improved transport and microwave transmission properties of Ag-doped films have been attributed to grain enlargement and low-angle grain boundaries due to Ag. Availability of active O to the growing YBa2Cu3O7-delta lattice, due to the dissociation of Ag2O on the substrate, and the increased mobility of Ag atoms segregating out of the growing grains are presumed to be the major mechanism responsible for the enlargement and alignment of grains in Ag-doped YBa2Cu3O7-delta films.