Growth and Characterization Studies of ABi 2 ta 2 o 9 (A = Ba, Sr and Ca) Ferroelectric Thin Films

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GROWTH AND CHARACTERIZATION STUDIES OF ABi2Ta2O9 (A = Ba, Sr and Ca) FERROELECTRIC THIN FILMS R. R. Das, W. Pérez, R. J. Rodríguez, P. S. Dobal, R. S. Katiyar and S.B. Krupanidhi* Department of Physics, University of Puerto Rico, San Juan PR 00931-3343 * Materials Research Center, Indian Institute of Science, Bangalore- 560012, India ABSTRACT Thin films of ferroelectric ABi2Ta2O9 bismuth-layered structure, where A = Ba, Sr and Ca, were prepared by pulsed laser deposition technique on Pt/TiO2/SiO2/Si(100) substrates. The influence of substrate temperature between 500 to 750oC, and oxygen partial pressure 100-300 mTorr, on the structural and electrical properties of the films was investigated. The films deposited above 650oC substrate temperature showed complete Aurivillius layered structure. Films annealed at 750oC for 1h in oxygen atmosphere have exhibited better electrical properties. Atomic force microscopy study of surface topography shows that the films grown at lower temperature has smaller grains and higher surface roughness. This paper discusses the pronounced influence of A-site cation substitution on the structural and ferroelectric properties with the aid of Raman spectroscopy, X-ray diffraction and electrical properties. The degradation of ferroelectric properties with Ba and Ca substitution at A-sites is attributed to the higher structural distortion caused by changing tolerance factor. A systematic proportionate variation of coercive field is attributed to electronegativity difference of A-site cations.

INTRODUCTION Ferroelectric thin films of several compositions have received major attention for their place in nonvolatile memory technology [1,2]. The reliable operation of the devices after switching for 1010-1012 electric cycles, became a major issue, as most of the compositions experienced fatigue. This prompted the community to quest for newer materials which can withstand the challenges of fatigue. Aurivillius compounds of Bi-layered series received significant attention due to their stable operation potential, beyond 1010 switching cycles [2,3]. The Bi-layered compounds assume the formula, (An-1BnO3n+1)2-(Bi2O2)2+, where A is twelvefold coordinated and B is sixfold coordinated cations, in which the lattice structure composed of nnumber of (An-1BnO3n+1)2- unit cells sandwiched between (Bi2O2)2+ slabs along pseudo-tetragonal c-axis [4]. SrBi2Ta2O9 (SBT) and SrBi2Nb2O9 (SBN) are most popularly studied compound. In the present study, the A-site cations are substituted by Ba and Ca to compare their behavior with the established SBT. Paper describes the results of phase evolution, structural and electrical properties of PLD growth films of SBT, BaBi2Ta2O9 (BBT) and CaBi2Ta2O9 (CBT), with variation of process parameters such as substrate temperature, ablation pressure and annealing temperatures. EXPERIMENTAL Polycrystalline BBT, SBT and CBT thin films were grown on Pt/TiO2/SiO2 /Si substrates by pulsed laser ablation technique using a KrF (λ = 248nm) excimer laser at a pulse repetition CC12.11.1