Diagnosis of precipitate formation in pulsed-laser deposition of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ by means of in situ laser-light scattering and ex situ atomic force microscopy

We have developed a technique using laser-light scattering for monitoring and controlling precipitate formation during pulsed-laser deposition of ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (YBCO) films. By laser ablating a stoichiometric YBCO target under optimum conditions for preparing high-crystallinity YBCO films, it has been almost inevitable to have precipitates on the film surface. The dominant phases of the precipitates analyzed by μ-Auger mapping were Y-deficient compounds (CuO and BaCu${\mathrm{O}}_2$). The progress of precipitate formation at the very beginning of the film growth could be monitored in situ by measuring the intensity of the light scattered to a nonspecular direction after being impinged obliquely on the film surface. The increase of scattered light intensity was verified to result mainly from the increase of the precipitate size from the qualitative analysis of ex situ atomic force microscope images and Rayleigh scattering theory. We have proposed that the precipitates were segregated from the highly crystalline 123 phase and their formation proceeded in two steps, i.e., nucleation and growth. Then, we could expect the precipitate segregation could be reduced either by a two-step process with the aid of optical scattering data to readjust the averaged film stoichiometry or preventing the nucleation to reduce the segregated precipitates. By switching the target to an yttrium-rich (50%) pellet $\left({\mathrm{Y}}_{1.5}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}\right)$ after the deposition of 10-unit-cell-thick YBCO having precipitates, a decrease in the scattered light intensity was observed to be consistent with the reduction of precipitate. This is attributed to the compensation of the yttrium deficiency and conversion of the Y-deficient precipitates into the YBCO phase by the reaction with Y-rich ablated species. Thus the measurement of light scattering is shown to be a useful tool not only for analyzing the surface reactions, but also for suppressing precipitate formation.