Preparation and Characterization of (Bi,La) 4 Ti 3 O 12 Films by the Sol-Gel Technique
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Preparation and Characterization of (Bi,La)4Ti3O12 Films by the Sol-Gel Technique Eisuke Tokumitsu, Takeaki Isobe, Takeshi Kijima1 and Hiroshi Ishiwara1 Precision and Intelligence Laboratory, 1 Frontier Collaborative Research Center Tokyo Institute of Technology 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan ABSTRACT (BiLa)4Ti3O12 (BLT) films have been fabricated by the sol-gel technique and characterized. It is shown that the partially replacing Bi with La can promote the crystallization, hence can reduce the crystallization temperature of the film. It is also demonstrated that crystalline orientations of sol-gel derived ferroelectric BLT films can be controlled by the growth conditions.
INTRODUCTION Recently, ferroelectric materials have attracted much attention for non-volatile memory applications. There are two types in ferroelectric memories (FeRAMs) : one is conventional capacitor-type FeRAM [1] and the other is transistor-type FeRAM using ferroelectric-gate FETs [2-3] where the ferroelectric thin film is used as a gate insulator. Since the memory cell of the so called 1T1C type FeRAMs consists of one MOSFET and one ferroelectric capacitor and we have to reduce the size of these devices for high-density implementation, a large remanent polarization Pr of the ferroelectric films is desirable. On the other hand, transistor-type FeRAMs, the remanent polarization of the ferroelectric film as a gate insulator should match the charge which can control the channel conductivity of FETs [4,5]. The induced charge per unit area by the gate insulator, Q/S, can be calculated as Q/S=εE, where ε and E indicate dielectric constant of the gate insulator and applied electric field, respectively. For example, the maximum charge induced by SiO2 is 3.5 µC/cm2, if we assume that the maximum electric field (breakdown field) is 10 MV/cm and that the relative dielectric constant of SiO2 is 3.9. On the other hand, the remanent polarization of well-known ferroelectric materials, Pb(Zr,Ti)O3 (PZT) and SrBi2Ta2O9 (SBT) films are from 30 to 50 µC/cm2 and about 10 µC/cm2, respectively. These values are much larger than the maximum charge which can be induced by SiO2. This means that even if we have a good ferroelectric film with large remanent polarization, we cannot use the entire polarization to control the channel conductivity of metal-oxide-semiconductor (MOS)-FETs but can use only a small part of the polarization, namely a small minor loop. In such a case, the obtainable memory window, which is given by 2 VC, will become small, because the coercive voltage, VC, of the minor loop is smaller than that of the saturated P-E hysteresis loop. Hence, small or moderate remanent polarization is required for the ferroelectric films in transistor-type FeRAM applications. CC13.9.1
In this work, we have characterized (Ba,La)4Ti3O12 (BLT) films [6] prepared by the sol-gel technique. From the analogy of Bi4Ti3O12 [7,8], the BLT may have large anisotropy in electrical properties. Hence, the control of crystalline orientation is important to ob
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