Preparation and Characterization of Ba and Nb Substituted SrBi 2 Ta 2 O 9 Compounds

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PREPARATION AND CHARACTERIZATION OF Ba AND Nb SUBSTITUTED SrBi2Ta2O9 COMPOUNDS Rasmi R. Das, P. S. Dobal, A. Dixit, W. Perez, M.S. Tomar*, R. E. Melgarejo* and Ram S. Katiyar Department of Physics, University of Puerto Rico, San Juan PR 00931-3343 * Department of Physics, University of Puerto Rico, Mayaguez PR 00681

ABSTRACT Bi-layered ferroelectric compounds are considered most promising for non-volatile memory applications due to their high fatigue endurance. We have prepared SrBi2Ta2O9 powders with Ba (A sites) and Nb (B sites) substitutions using a novel solution based route. The powders were pressed and sintered at 1050oC to obtain high quality targets. Thin films were prepared using these ceramic targets on Pt/TiO2/SiO2/Si substrates using pulsed laser deposition (PLD) technique. The effects of growth conditions on phase formation as well as structural and electrical properties in films are studied. Initial results on films show good hysteretic characteristics. Though phase formation begins at much lower temperature, these films crystallize in a complete layered perovskite phase when prepared at 700oC. Optical phonon modes in these materials exhibit systematic variations with changing compositions. The changes in the Raman spectra are explained in terms of Ba and Nb substitutions at A and B sites, respectively. The temperature dependence of Raman spectra exhibits the substitution induced changes in the transition temperatures of these materials.

INTRODUCTION Due to recent advances in thin film growth technology, the interest in investigating ferroelectric materials has increased manyfold because of their potential applications in memory devices, sensors and actuators, IR detectors, transducers, microwave devices and others [1-3]. Lead zirconate titanate (PZT) is one of the most promising materials for integrated ferroelectric devices, especially for memory applications [4]. Because of imprint and fatal fatigue failure of these ferroelectrics, a new series of compounds so called Bi-layered perovskites, in particular SrBi2Ta2O9 (SBT), were proposed by Scott et al [3]. The low leakage current, low operating voltage, fast switching speed and negligible fatigue up to 1012 electric cycles are the advantages of layered perovskite for memory applications [5]. The lattice structure of these family of compounds is described by a general formula (Am-1BmO3m+1)2-(Bi2O2)2+, where A is mono, di, trivalent cation and B is quatro or pentavalent cation and ‘m’ is the number of perovskite unitcells sandwiched between two bismuth oxide slabs [6]. In case of SBT, m=2, which shows displacive transition from the symmetry I4/mmm to A21am. In this transition, the Sr ion at the Asite of the pseudoperovskite layer moves towards O-Ta-O octahedral chain, which is the origin of ferroelectricity [7,8]. In this work, we reported the preparation of A and B-site substituted SBT thin films by PLD technique. The effect of replacement of A-site Sr with different Ba concentrations and BCC5.6.1

site Ta by Nb on structural and electrical propert