Diffuse Phase Transition in Epitaxial BaTiO3 Thin Films
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The thickness dependence of the dielectric properties of epitaxial BaTiO3 thin films was investigated for thicknesses ranging from 15 to 320 nm. The films were deposited by low-pressure metalorganic chemical vapor deposition on (100) MgO substrates. The relative dielectric permittivity and the loss tangent values decreased with decreasing thickness. High-temperature dielectric measurements showed that with decreasing film thickness, the ferroelectric-to-paraelectric transition temperature decreased, the relative dielectric permittivity decreased, and the phase transition was diffuse. The c/a ratio also decreased with decreasing film thickness. The observed behavior for epitaxial films of BaTiO3 was attributed to the presence of strain in the films.
I. INTRODUCTION
Extensive studies have been carried out on thin film ferroelectric materials in recent years due to their applicability to microelectronics and integrated optics. Potential applications include nonvolatile and dynamic random-access memories, electro-optic devices and electro-optic switches. Many of these applications require their dielectric properties to be comparable to bulk material. Films, however, possess properties that differ from those of the bulk. The dielectric properties of ferroelectric thin films often depend on thickness. For thin PZT films, it has been demonstrated that with decrease in the film thickness, there is (i) reduction in the permittivity and the remanent polarization; (ii) an increase in the value of the loss tangent and the coercive field, and (iii) the diffuseness of the phase transition.1,2 The thickness dependence of dielectric properties has also been studied for barium titanate, barium strontium titanate,3 PNZT,4 and PbZrO35 thin films. While room-temperature permittivity of ceramic BaTiO3 has been reported to be higher than 3000,6,7 the thin films have been found to possess permittivity much lower than that of the bulk.8,9 The permittivity decreases with decreasing film thickness.10 Polycrystalline BaTiO3 films prepared on Pt (111)/SiO2/ Si (100) substrates at 650 °C by the sol-gel method, have shown the relative dielectric permittivity and the remanent polarization to increase and also the coercive field to decrease with increasing film thickness.11 The loss
a)
Present address: MRCAT, Advanced Photon Source, Sector 10, Bldg. 433, Room B003, Argonne National Lab, Argonne, IL 60439. J. Mater. Res., Vol. 17, No. 3, Mar 2002
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tangent, however, has been observed to be independent of the film thickness. These changes in the properties have been attributed to an increase in the grain size and decrease in the defect concentration, which takes place with increasing film thickness. The structural and electrical properties of epitaxial BaTiO3 thin films on platinized MgO have been studied by Yano et al.10 For thickness less than 10 nm, the films showed an increase in the c/a ratio. For films with thickness higher than 50 nm, the lattice parameter a showed an elongation. The
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