Epitaxial and highly oriented lead lanthanum titanate ($(Pb,La)TiO_3$, PLT) thin films were fabricated on various subtrates using the sol-gel method. To investigate the epitaxial growth of PLT thin films, the films were dried at 400℃ for 15 min. and crystalized at different temperature from 500℃ to 800℃ for 30min. then cooled at 2℃/min. in atmosphere. X-ray diffraction(XRD) pattern, rocking curve, pole figure, and scanning electronic microscope(SEM) were used to analyze the epitaxy of PLT films. The films fabricated on MgO(100) and sapphire(0001) substrates grow epitaxially to (100) and (111) orientations, respectively. As La concentration increases the PLT films have good orientation due to small misfit between the substrate and the PLT film by transition from tetragonal to cubic. Results of pole figures, the epitaxicy of PLT films prepared by sol-gel method were not better than that used the vacuum deposition techniques such as sputtering or CVD because the nuclei formation and growth mechanism in sol-gel amorphous films are more complicated and difficult than those of the vacuum deposition technique. Regardless of heating temperature and thickness, composition of the film, it is found that the orientation of PLT thin films fabricated on MgO(100) substrate is only a-axis orientation. This is attributed to the reduction of the compressive stress by residual tensile stress occurred during crystallization in sol-gel amorphous PLT film.
For studying of the optical properties of the highly oriented PLT thin films, the films were characterized using UV/VIS spectrophotometer, prism coupler, and atomic force microscope(AFM). PLT thin films with high La concentration have the low refractive index due to the decrease of the anisotropy of refractive indices. The propagation losses of the PLT films decrease as La concentration increases by reducing the complex effect of the surface scattering and the non-surface scattering. And though the surface roughness of the films having same composition is not affected by the film thickness, the propagation losses increase exponentially with growing the film thickness. Thus, the main mechanisms of the propagation loss with increasing the thickness are the internal scattering by the unavoidable defects and interface scattering by the multi-coating in sol-gel film rather than the surface scattering.
