Abstract
We successfully reduced the microwave surface resistance R s of YBa 2Cu 3O y (YBCO) deposited on MgO substrates by covering the films with an overdoped Y 1− x Ca x Ba 2Cu 3O y (Ca-doped YBCO) layer. The YBCO films deposited by laser ablation on MgO substrates had in-plane misaligned c-axis-oriented. These misaligned grains cause large angle grain boundaries, which would increase R s. For the R s reduction due to Ca-doping, the optimum thickness of the Ca-doped YBCO layer was about 50 nm, and we did not observe the reduction of R s for films on LaAlO 3 substrates with perfectly in-plane aligned grains. From these experimental results, we concluded that the selective diffusion of Ca ions to the grain boundaries of the YBCO layer is the origin of the low R s. Moreover, films that were annealed in oxygen for 60 h showed lower R s than that of short-time-annealed (for 1 h after deposition) films. These simple methods to reduce R s of YBCO films would be applicable to the fabrication of microwave passive devices.
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