Structural and Electrical Characterization of Rare Earth Doped Pb 0.85 La 0.15 TiO 3 Ferroelectric Thin Films
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ABSTRACT Rare earth (Gd+3 and Ce+3) substitution on La+3 sites of the sol-gel prepared Pb 0.g5Lao 5TiO 3 films is studied using x-ray diffraction, atomic force microscopy, Raman scattering, and electrical characterization techniques. With increasing content of rare earths an increase in the lattice tetragonality was evidenced from x-ray data. Raman spectra obtained form Pb 0 .85La 0 5.Ce.TiO 3 (x=0.0-0.07) and Pb0o85La0.j 5 .Gd•TiO 3 (x=0.0-0.15) films show features characteristics of PbTiO 3 perovskite. Frequency variations of the lowest soft mode as a function of the composition x and temperature corraborate the increased tetragonality in these films. The ferroelectric transition temperature, dielectric constant, and coercive field was found to increase with Gd content. The phase transition temperature and polarization values increase up to 5 at.% Ce doping while they decrease above that composition due to the reduced domain wall mobility caused by Ce precipitation. A slight increase in the surface roughness was observed with increasing rare earth content in these films. INTRODUCTION The increased use of ferroelectric thin films of PbTiO 3 (PT) and modified PT in a number of devices has lead to many experimental investigations. As a result, the effects of various factors (dopants, particle sizes, substrates, growth conditions and others) on the ferroelectric, dielectric, and phase transition properties of these films are widely discussed in the literature [1-5]. Doping in PT has been found to decrease the phase transition temperature and saturation polarization due to the reduced ferroelectric ordering [6,7]. A decrease in the tetragonality factor and phase transition temperature, changes in the degree of c-axis orientation, and the variations in dielectric and ferroelectric properties are fairly well understood with La÷3 substitution on A sites of PT lattice [8-10]. Despite their poor ferroelectric properties, La modified PT films remain promising for pyroelectric and piezoelectric applications. Rare earth doped glasses and other materials are very useful for photonic, telecommunication and optical data storage devices [11]. In this work, the effect of Ce and Gd substitution on La sites of Pb0 85Lao 15TiO3 (PLT) is investigated. The variations of phase transition temperature and ferroelectric properties are correlated with the changed lattice parameters and degree of c-axis orientation within the solubility limits of rare earths in PLT. EXPERIMENTAL Pb(CH 3COO) 2.3H20, La(N0 3)3 .6H 20, Ce(NO 3)3.6H 20, Gd(N0 3)3.6H 20, and titanium butoxide were used as precursor materials. A 0.25 M/L concentration of the sol was used to prepare the films. Films with compositions Pbo.9oLao I 5._GdTi0 .%O3 (x=0.00, 0.05, 0.10, and 0.15) and Pb0 9La0 .15_.CejTi 0 9603 (x=0.00, 0.03, 0.05, and 0.07) were spin coated on polished platinum 357 Mat. Res. Soc. Symp. Proc. Vol. 596 © 2000 Materials Research Society
at 5000 rpm for 20 sec. Each coating was dried at 400oC for 15 minutes and the film after 20 coatings was annealed at 700
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