Role of Ag in laser-ablated YBa 2Cu 3O 7−δ thin films : Evidence for grain enlargement and alignment

Abstract

Microstructural and superconducting properties of YBa 2Cu 3O 7−δ thin films grown in situ by pulsed laser ablation using YBa 2Cu 3O 7−δ targets doped with 5 wt.% Ag have been studied. Ag-doped films grown on 〈100〉 LaAlO 3 and 〈100〉 SrTiO 3 substrates at temperatures in the range 650–700°C have shown very significant improvements over the undoped YBa 2Cu 3O 7−δ films grown under identical conditions. A critical current density of 1.4×10 7 A cm -2 at 77 K has been observed on SrTiO 3 substrates. Microwave measurements carried out using a microstrip resonator at the X-band have shown a low microwave surface resistance of 190 μΩ 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 brain boundaries due to Ag. Availability of active O to the growing YBa 2Cu 3O 7−δ lattice, due to the dissociation of Ag 2O on the substrate, and the increased mobility of Ag atoms segragating out of the growing grains are presumed to be the major mechanism responsible for the enlargement and alignment of grains in Ag-doped YBa 2Cu 3O 7−δ films.