Surface texture and interior residual stress variation induced by thickness of YBa_2Cu_3O_(7-δ) thin films

摘要

YBa_2Cu_3O_(7-δ) (YBCO) thin films with various thicknesses from 80 nm to 2000 nm are prepared on single crystal SrTiO_3 by means of pulsed laser deposition technique. While it is hard for the x-ray diffraction to observe the evolutions in epitaxial orientation and interior lattice structure, the atomic force microscope shows the degraded surface morphologies and coalesced particles which arise from more misoriented grains with increasing thin film thickness. A detailed Raman spectrum investigation reveals that the a-axis grains exist predominately at the top surface of the films as the total thickness of the film increases up to 2000 nm. It is also evident that the Raman peak corresponding to the O2+/O3- mode emerges with the redshift first and then the blueshift as the film thickness increases. These Raman shifts suggest that the tensile stress in the (a, b) plane arising from the lattice mismatch between the epitaxial film and substrate may release gradually with increasing the film thickness, while the additional tensile stress emerges due to the a-axis grains present at the top surface of thin film. It is believed that the thickness effect of YBCO thin films on the superconducting transition temperature (T_c) and critical current density (J_c) is attributed to the variation of interior residual stress and surface morphology associated with the lattice mismatch and grain orientation, respectively.