High Remanent Polarization in Laser Ablated SrBi 2 Nb 2 O 9 Thin Films
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High Remanent Polarization in Laser Ablated SrBi2 Nb2 O9 Thin Films Rasmi R. Das, P. Bhattacharya, W. Pérez, and Ram S. Katiyar Physics Department, University of Puerto Rico, San Juan, Puerto Rico 00931-3343
ABSTRACT Pulsed-laser-deposition technique was used to grow SrBi2 Nb2 O9 (SBN) thin films on platinized silicon substrates. The effect of annealing temperature and film thicknesses on the structural and electrical properties has been studied. The average grain size and rms surface roughness was found to increase with increasing annealing temperature. The degree of orientation along the (200) direction was increased with the film thicknesses. The remanent polarization was found to be increased with the film thicknesses and was attributed to the selftexturing characteristics of SBN films. Thin films with higher thickness (~570 nm) exhibited high value of remanent polarization (~38 µC/cm2 ) with coercive field of 185 kV/cm. There was a reduction of coercive field with the film thickness. The dielectric constant was observed to be independent of the film thickness. The increase in loss tangent with increasing film thicknesses was attributed to the reduction of dielectric breakdown strength of the films. The SBN thin films showed minimal fatigue characteristics and suitable material for memory devices. INTRODUCTION Ferroelectric random access memory (FRAM) has drawn global attention due to its nonvolatile nature. Bismuth layered ferroelectrics, in particular SrBi2 Ta2 O9 (SBT), is a prime candidate material for FRAM devices [1,2]. There is a constant surge to enhance the ferroelectric properties of bismuth-layered perovskites without sacrificing the fatigue and leakage current characteristics. SBT has been modified by various doping and substitutions at Sr- and Ta-sites for further improvement of its remanent polarization (> 10µC/cm2 ) [3,4]. In our earlier report, we have substituted Ba and Ca at Sr-site and increased the value of remanent polarization of SBT thin films [5]. A large number of reports are intended to fabricate device quality bismuth layered thin films on platinum and Pt-coated silicon substrates [5,6]. SrBi2 Nb2 O9 (SBN) has the similar crystal structure and have higher curie temperature (~440 o C) than for SBT (~335 o C) [7]. The biaxial strain of SBN was reported to be < 0.002, which is lower than SBT and lead based perovskites [8]. The displacement of Nb ion inside the octahedral NbO6 is larger than Ta inside TaO 6 octahedra [9], which is expected to show higher value of remanent polarization. Watanabe et al [10]. have reported the remanent polarization of 12.5 µC/cm2 in the case of randomly oriented SBN thin films. More recently, there has been a series of report on textured and epitaxial SBN thin films by Schlom [11,12] and co-workers and accordingly SBN thin films exhibited remanent polarization of ~11.4 µC/cm2 on Pt-coated silicon with SrRuO3 buffer layer. Lettieri et al [12] have reported the growth of oriented SBN thin films along (103) direction which exhibited remanent polarizatio
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