New Line of Solid Solution System of Oxide Ferroelectrics
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New Line of Solid Solution System of Oxide Ferroelectrics Takeshi Kijima1 and Hiroshi Ishiwara Frontier Collaborative Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan 1 Technology Platform Research Center, SEIKO EPSON Corporation, 281 Fujimi, Fujimi-machi, Nagano 399-0293, Japan
ABSTRACT Ferroelectric random access memory is one of the most promising candidates of non-volatile memories and has already been commercialized. However there still exist some problems to be solved on the ferroelectric materials. We succeeded in solving these problems by forming solid solutions between Bi2SiO5 and conventional ferroelectric materials such as Bi4Ti3O12, SrBi2Ta2O9 and Pb(Zr,Ti)O3. It was found that Bi2SiO5 enhanced crystallization of the ferroelectric materials and finally formed solid solutions with them. As a result, the crystallization temperature of the films decreased by 150 to 200 ºC, and the ferroelectric and leakage current characteristics did not degrade even in an ultra-thin film of 13 nm in thickness. On the other hand, it was also found that the ferroelectric and insulating characteristics of the BSO-added films were dramatically improved by annealing in high-pressure oxygen up to 9.9 atms.[1] Three-orders-of-magnitude improvement of the leakage current density was observed in BSO-added BLT films after annealing at 9.9 atms, while pronounced increase of the saturation polarization was observed in BSO-added SBT and PZT films. From cross-sectional TEM (Transmission electron microscopy) observation, origin of the improved characteristics was speculated to be the structural change of the films.[2]
INTRODUCTION Non-volatile ferroelectric random access memories [3] (FeRAM) have attracted considerable attention with the recent development of portable instruments such as cellular phones, laptop computers and personal digital assistants (PDA). These instruments handle a large quantity of data such as image and sound, and they require memory which is able to process large data at a high speed with low power consumption. FeRAM is considered most suitable for such requirement. Ferroelectric material retains remnant polarization even after an applied field is removed. It arises from the displacement of anions and cations in relative positions in the crystal. FeRAM utilizes this phenomenon for storing the data of "0" and "1". Device structure and the operation are the same with dynamic random access memory (DRAM) except that DRAM stores data by charge on the capacitors. Therefore, FeRAM has a large potential to substitute widely used DRAM. Currently produced FeRAM has only small capacity of 256 kbits, and has yet to be highly integrated as DRAM. This is due to the problems of the present ferroelectric materials in that the surface morphology is too rough to be used in integrated circuits which operate at a voltage lower than 2 V. It is also important to decrease the
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present crystallization temperature (higher than 600 ºC) to fabricate future high-density Fe
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