Influence of deposition parameters on superconductivity and surface morphology of NdBa sub(2)Cu sub(3)O sub(7- delta ) films grown by pulsed laser deposition with a single crystal target

摘要

High-quality NdBa sub(2)Cu sub(3)O sub(7- delta ) (NBCO) thin films have been successfully grown by on-axis pulsed laser deposition (PLD) using a large single crystal NBCO target. The influence of deposition parameters, including the substrate temperature, oxygen partial pressure, laser energy density, and the target-substrate distance on the quality of NBCO thin films was investigated in detail. The superconducting properties, microstructure, and the surface morphology of NBCO films as functions of deposition parameters were examined by transport and dc superconducting quantum interference device (SQUID) measurements, X-ray diffraction (XRD), high-resolution scanning electron microscopy (SEM), and atomic force microscopy (AFM). A significant reduction in the number of large droplet density caused by laser-target interaction was achieved when an NBCO single crystal was used as the target. However, the density of the small-sized precipitates and out-growths (approximately 0.2 mu m) was strongly dependent on the target-substrate distance. By optimizing the deposition conditions, especially the target-substrate distance, the formation of misoriented surface particles and out-growths could be eliminated. Hence, smooth NBCO films with T sub(c0) identical with 92.5 K and J sub(c) identical with 4x10 super(6) A/cm super(2) at 77 K were obtained. It was found that the optimum substrate temperature and oxygen pressure were the same as the values corresponding to a sintered target. However, the optimum target-substrate distance corresponding to the single crystal target was larger than the value referring to the sintered target. The NBCO films deposited from the single crystal target possessed better homogeneity and higher reproducibility. The mechanism of the formation of precipitates and out-growths were discussed in terms of laser-target interaction, the laser plasma expansion dynamics, and the film growth dynamics. Experimental results demonstrate that smooth NBCO films with T sub(c0) greater than or equal to 90 K and J sub(c)>10 super(6) A/cm super(2) at 77 K can be reproducibly grown by PLD with a single crystal target.