Low Temperature Crystallization of Bismuth Layer-Structured Ferroelectric Thin Films Using Lead Titanate Sol-Gel Templat
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Low Temperature Crystallization of Bismuth Layer-Structured Ferroelectric Thin Films Using Lead Titanate Sol-Gel Templating Technique and Their Electrical Properties Junichi Karasawa, Takeshi Kijima, Eiji Natori1 and Tatsuya Shimoda F-group, Technology Platform Research Center, Seiko Epson Corporation 281 Fujimi, Fujimi-machi, Nagano-ken, 399-0293, Japan 1 NV Project, Seiko Epson Corporation ABSTRACT The crystal structure and electrical properties of lead titanate (PbTiO3: PT) sol-gel network templated bismuth layer-structured ferroelectric (BLSF) thin films were systematically investigated as a function of the doping amount of lead titanate sol-gel solution and annealing temperature. The starting solutions of lead titanate sol-gel templated BLSF were prepared by adding lead titanate sol-gel solution to BLSF solutions such as strontium bismuth tantalate (SrBi2Ta2O9: SBT), bismuth titanate (Bi4Ti3O12: BiT) and lanthanum-doped bismuth titanate ((Bi,La)4Ti3O12: BLT). These solutions were spin-coated on platinized silicon wafers and pyrolized on a hot plate, then crystallized at 550oC – 738oC by RTA (Rapid Thermal Annealing). The crystallized films with sputtered platinum top electrodes were post-annealed for electrical property measurements. In the case of SBT-PT, it was found that the added lead titanate so-gel network has no remarkable effect on lowering the BLSF (m=2) crystallization temperature but rather enhances the pyrochlore phase. In the case of BiT-PT, the bismuth layered-structure was confirmed at the temperature down to 550oC as the amount of lead titanate sol-gel network is increased. The major layered-structure, however, was not desired m=3, but unexpected m (e.g m=4 or higher). In the case of BLT-PT, lowering the BLSF (m=3) crystallization temperature down to 638oC was finally achieved within proper amount of lead titanate sol-gel network without drastic drop of ferroelectricity. A 2Pr of 32 µC/cm2 was obtained in 0.96BLT-0.04PT thin film. INTRODUCTION Bismuth layer-structured ferroelectric (BLSF) thin films such as SrBi2Ta2O9 (SBT), SrBi2(Ta,Nb)2O9 (SBTN) and (Bi,La)4Ti3O12 (BLT) are one of the most promising ferroelectric materials for use in high-density ferroelectric random access memory (FeRAM), since BLSF enables the fabrication of ultra-thin films with low voltage operation and the realization of highly reliable, lead-free FeRAM. However, a few fundamental problems originating in the nature of BLSF have been pointed out. Among them are low remnant polarization (Pr), high porosity and a high temperature crystallization process (≥700oC). The difficulty in low temperature crystallization causes serious problems when it comes to ultra high-density FeRAM having a stacked-cell structure with 3D capacitors. On the other hand, it is known that lead-containing ferroelectric thin films having a perovskite structure, such as lead titanate (PbTiO3: PT), can easily crystallize at temperatures as low as 450oC. In order to apply this advantage of lead titanate to lowering BLSF crystallization temperatur
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