The combined effect of thermal annealing of MgO substrate and Ca substitution on the surface resistance of YBa2Cu3Oz thin films

Single-layer Y1-xCaxBa2Cu3Oz (YCBCO) thin films (x=0.00, 0.02, 0.05, and 0.10) grown on annealed as well as unannealed MgO substrates have been systematically investigated for their carrier concentration, critical current density J(c), and microwave surface resistance R, For x <= 0.05, the grain growth follows a three-dimensional-spiral growth mechanism, while for x = 0.10 we observed a mainly two-dimensional-like growth of grains. The results of Hall data reveal that the x = 0.05 film is overdoped while films with x = 0.02 and 0.10 are underdoped with respect to the x = 0.00 film. However, the Hall mobility mu(H) is highly enhanced for the x = 0.02 film. Thermal annealing of MgO substrates prior to film deposition results to an improvement in the overall superconducting properties of the film such as suppression of normal-state resistivity, enhancement of J(c), and minimization of R-s both for pure as well as Ca-substituted films. Annealing of MgO C substrates enhances the Jc value to a magnitude (i) nearly doubled for x = 0.00 films and (ii) more than an order for x = 0.02 films. Also it leads to a minimization of the R-s value to (i) more than half and (ii) nearly an order of magnitude higher, respectively, for x = 0.00 and x = 0.02 films. Furthermore, for the x = 0.02 film, below 60 K, we realized an enhanced Jc value in self- as well as in large-applied fields. For other than the Ca-2% substituted films, a suppression of Jc with a strong field dependency has been noticed. Furthermore, the R-s value of the x = 0.02 film (0.1 m Omega at 20 K, 21.9 GHz) was three times lower compared to that of the x = 0.00 film (0.35 m Omega at 20 K, 21.9 GHz). At 20 K and 21.9 GHz, the R, value for the x = 0.05 film is comparable to that of the x = 0.00 film, whereas for the x = 0.10 film it is twice that of x = 0.00. The low normal-state resistivity, enhanced mobility, high J(c), and the minimized R-s observed for x = 0.02 films firmly support the possible improvement of superconducting order parameters near the grain boundaries. Thus, the combined effect of thermal annealing of MgO prior to film deposition and substituting Y with 2% Ca may be readily applied to grow high-quality (high J(c) and low R-s) YBCO films for microwave device applications. (c) 2005 American Institute of Physics.