Ferroelectric property of epitaxial Bi 4 Ti 3 O 12 films prepared by metalorganic chemical vapor deposition
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Keisuke Saito Application Laboratory, Analytical Department, Philips Japan, Ltd., 35-1, Sagamiono 7-chome, Sagamihara-shi 228-0803, Japan (Received 7 August 2000; accepted 25 October 2000)
The orientation dependence of the ferroelectricity of epitaxially grown Bi4Ti3O12 thin films was investigated. The (001)-, (118)-, and (104)-oriented Bi4Ti3O12 films were epitaxially grown on (100)cCaRuO3//(100)SrTiO3, (110)cSrRuO3//(110)SrTiO3, and (111)cSrRuO3//(111)SrTiO3 substrates, respectively, by metalorganic chemical vapor deposition. Ferroelectric property with different magnitude was observed for (001)and (118)-oriented films but for (104)-oriented film due to its large leakage current. The remanent polarization and the coercive field were 1.5 C/cm2 and 15 kV/cm, 16.5 C/cm2 and 132 kV/cm for the (001)- and (118)-oriented thin films, respectively. The spontaneous polarization (PS) was 4.0 C/cm2 and 27.0 C/cm2 for (001)- and (118)-oriented films, respectively. This was different from the result of SrBi2Ta2O9 in that the ferroelectricity was not observed for (001)-oriented one, and was in good agreement with the estimation from the crystal structure. The estimated PS values along the c and a axes of Bi4Ti3O12 were 4.0 and 48.4 C/cm2, respectively, and agreed well with the reported values for the single crystal. Furthermore, both films showed good fatigue endurance after 7.8 × 1010 switching cycles measured with 500 kHz rectangular pulses.
I. INTRODUCTION
II. EXPERIMENTAL PROCEDURES
Ferroelectric random-access memory (FRAM) has been investigated because of its nonvolatile character. SrBi2Ta2O9 (SBT) is an attractive material because of its good fatigue resistance.1 We already reported results using the epitaxially grown SBT films that showed the spontaneous polarization (PS) value along the a axis of SBT was about 22 C/cm2.2,3 The large PS is advantageous in reducing the ferroelectric capacitor area; i.e., the memory cell area of a FRAM. However, this value is not enough for the application to high-density FRAM. On the other hand, Bi4Ti3O12 (BIT) was reported to have a larger PS value than that of SBT in bulk form: 50 C/cm2 along the a axis.4–6 However, there are no reports for the estimation of the PS value of BIT films along the a axis using epitaxially grown films. In the present study, we first prepared epitaxially grown BIT films with c axis and non-c-axis orientations by metalorganic chemical vapor deposition (MOCVD) and investigated the orientation dependence of the ferroelectric property of BIT film. On the basis of these results, we estimated the ferroelectricity along the a- and c-crystal axes.
BIT films were deposited by MOCVD using an apparatus having a vertical cold-wall-type reaction chamber. BIT films were prepared by MOCVD from Bi(CH3)3– Ti(O 䡠 i-C3H7)4 (TRI Chemical Laboratory Inc., Yamanashiken, Japan) O2 system.7 The (001)-oriented films were deposited at 800 °C, and the (118)- and (104)-oriented films were deposited at 880 °C. We deposited BIT films at relatively high deposition temperature t
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