Effect of heat treatment on formation of sol-gel (Pb, La)TiO 3 films for optical application
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Effect of heat treatment on formation of sol-gel (Pb, La)TiO3 films for optical application Junmo Koo, Sung-Uk Kim, Dae Sung Yoon, Kwangsoo No, and Byeong-Soo Bae Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Taejon 305-701, Korea (Received 26 May 1996; accepted 9 August 1996)
Lead lanthanum titanate [(Pb, La)TiO3 ] sol-gel films have been prepared to investigate the effect of heat treatment on the fabrication of uniform and crack-free thick films by applying different heating schedules. The surface morphology as well as the optical properties such as refractive index, optical transmission, and optical propagation loss of the films was examined, depending on the film thickness. Because the slower and longer heating is enough to remove the organic and nitrate residues and diminish the thermal shock while heating the films, slower and longer heating can produce the uniform and crack-free thick films having higher refractive index as well as lower optical propagation loss. Also, the drying and heating of the films on a hot plate in every coating resulted in ˚ without any defects and microcracks. the fabrication of thick films having above 8000 A This film presented the highest refractive index as well as the lowest optical propagation loss which grows exponentially with increasing the film thickness due to the scattering of defects in the film.
I. INTRODUCTION
Given the increasingly important role of integrated optics, thin film materials that are transparent and show good optically nonlinear properties are in large demand. Among the ferroelectric thin films, the lanthanum-doped lead zirconium titanate, (Pb, La)(Zr, Ti)O3 (PLZT), thin films have been widely used in waveguide applications.1,2 The advantages of La doping as a chemical modifier are known in several aspects: in reducing light scattering resulting from multiple refraction at the boundaries of the randomly oriented grains by reducing the distortion of the oxygen octahedral unit cell, in producing an extensive series of homogeneous solid solution compositions without any secondary phases by virtue of its high solubility in the PZT perovskite structure, in enhancing the densification process by producing a significant number of lattice vacancies, and in controlling the grain growth behavior through promoting a highly uniform microstructure. However, when an amorphous PLZT film prepared by using the sol-gel method is heattreated, perovskite phase and undesired pyrochlore phase are simultaneously formed. The ferroelectric PLZT thin film should have higher perovskite content in order to exhibit better electro-optic properties. The composition without zirconium content [(Pb1–x/100 Lax )Ti1–x/400 O3 is called PLTx] has been known to be more adequate for waveguide applications due to its finer grain size and higher transparency than PLZT.3 Actually, the total internal reflection (TIR) optical switch was proposed and implemented using PLT28 thin films with
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