Characterization of BaTiO 3 Thin Films Deposited by Pulsed-Laser Ablation
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CHARACTERIZATION OF BaTiO 3 THIN FILMS DEPOSITED BY PULSED-LASER ABLATION CHRISTOPHER SCARFONE, M. GRANT NORTON, C. BARRY CARTER, JIAN LI AND JAMES W. MAYER Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853. ABSTRACT Thin films of barium titanate (BaTiO3) have been deposited by pulsed-laser ablation onto (001)-oriented MgO substrates. The films were epitactic as evidenced by both x-ray diffraction and ion-channeling techniques. The film surface appeared smooth and contained a low density of particulates. This latter feature is believed to be due to the formation of target pellets having a very high density. INTRODUCTION Barium titanate (BaTiO3) is the prototype ferroelectric material and there have been numerous proposals for its use in electronic components, for example, capacitors and infra-red detectors [1]. BaTiO 3 has a Curie point of-130'C, above this temperature the unit cell is cubic and the phase is not ferroelectric. Below the Curie point the unit cell is distorted to the tetragonal form with a dipole along the c-direction. In addition to exhibiting ferroelectric properties the tetragonal form of BaTiO 3 has a large electro-optic coefficient and is therefore of interest in various nonlinear optical applications. A particular example may be as an optical switch [1]. For many of the proposed applications, the use of heteroepitactic thin films of BaTiO3 would be a desirable alternative to the use of either single-crystals which are difficult and expensive to prepare or to bulk polycrystalline material which can suffer from, among other things, a varying degree of porosity which will affect the dielectric-constant measurement. For applications as optical waveguides it is necessary that the refractive index of the thin film is larger than that of the substrate. For the combination described in this paper this requirement is met. The refractive indices of MgO and BaTiO3 are 1.74 and 2.40, respectively. A further requirement for application in optoelectronic devices is that the film surface must be smooth. A major drawback in the use of the pulsed-laser ablation technique for the preparation of thin films can be the formation of large (up to 1pgm) particulates on the film surface, this is referred to as splashing [2]. The splashing effect can be reduced by using a lower laser-power density or by adjusting the deposition geometry [e.g. 3]. However, the formation of particulates cannot be entirely avoided. A further factor that is important in determining the number of these surface particulates is the nature of the target pellet. Targets that have a low density, a high degree of porosity, would be expected to be more susceptible to particulate formation as the rapid expansion of any gas trapped in these pores may lead to the ejection of material from the surface or near surface region of the pellet. In this study, the pellet density was optimized by a systematic process of determining the density as a function of sintering conditions. Recent studies of the early stages of the h
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