Optical Properties of Sol-Gel (Pb,La)TiO 3 Thin Films For Waveguide Application
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INTRODUCTION There has been interest( : in the integrated optics application of ferroelectric thin films due to their electro-optical and nonlinear optical properties [1]. Waveguides using these thin films can be attractive candidate for optical modulators which can be integrated with electronic and optoelectronic devices. Among the ferroelectric thin films, the lanthanum-doped lead zirconium titanate, (Pb, Lax)(Zr.Ti)0 3 (PLZT) thin films have been widely used in waveguide applications since it is known as transparent materials having strong electro-optic effects [2,3]. However, the composition without zirconium content (Pbi_,/uooLax)Ti1_ 0 x/400 3 (PLTx) has been known to be more adequate for waveguide applications due to its finer grain size and higher transparency than PLZT [4]. Actually, the total internal reflection (TIR) optical switch was proposed and implemented using PLT28 thin films with high quadratic electro-optic coefficient [5]. The PLT thin films have been prepared by wet chemical process such as sol-gel and metal organic decomposition (MOD) using various precursors [6,7]. However, the PLT sol-gel thin films containing large La content have been rarely studied due to the problems of solution preparation. Recently, the PLT28 sol-gel thin films were successfully fabricated using lanthanum nitrate as a La precursor [8]. In this study, the PLT sol-gel thin films on SiO 2/Si substrates were synthesized using a similar method and the optimum processing conditions were investigated to obtain quality good thin films. And their optical properties such as refractive index and transparency of the films were examined for waveguide application. EXPERIMENTAL PROCEDURE Thin Film Preparation The precursor solutions were prepared using lead acetate, lanthanum nitrate, titanium isopropoxide in 2-methoxyethanol solvent. The procedure of preparing solutions is shown in Fig. 1. La concentration in the solutions were 10mol%, 20mol%, 28mo1% and represented by PLT10, PLT20, PLT28 respectively. Also, 5mol% excess PbO was incorporated since it was found that excess PbO aids significantly in mitigating eventual PbO loss than if normal stoichiometric composition were used[l]. These solutions were spin-coated on SiO 2(10000A)/Si and glass (Coming 7059) substrates using a spinner 279 Mat. Res. Soc. Symp. Proc. Vol. 392 01995 Materials Research Society
minutes to burn out organics and nitrates, yielding about 500A film thickness per coating. Multiple coatings were performed to obtain thicker films. The films were heat treated at 520 0 C applying various heating schedule to crystallize them to single perovskite phase.
Fig. 1 Procedure of preparing PLT precursor solution Characterizations The decompostion characteristics of the gels were analyzed using DTA, TGA (Netzsch, STA 409C), FTIR(Nicolet, 520), and quadruple mass spectrometer(Spectra Lab., VS300SX). The crystalline phase and microstructure of the films were characterized using XRD (Rigaku, D/MAX-RC) and SEM (Philips 535M) respectively. The ellipsometer (Gaertner Scientifi
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