Epitaxial growth at PZT/Ir interface
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Ken-ichi Yoshida,a) Tomoko Saitou, and Atsuhito Sawabe Department of Electrical Engineering and Electronics, Aoyama Gakuin University, Setagaya, Tokyo 157-8572, Japan (Received 17 December 1999; accepted 28 August 2000)
Surface oxidation of an iridium film and the possibility of epitaxial growth of PZT spin-coated film on sputtered iridium were investigated. The free surface on the iridium film oxidized over 400 °C with random orientation. Nevertheless, both the PZT(111)/Ir(111) and PZT(100)/Ir(100) interfaces were realized using the highly oriented iridium layer. These results suggest that PZT nucleation has priority over iridium surface oxidation.
I. INTRODUCTION
Ferroelectric thin films of Pb(Zr,Ti)03 (PZT) and SrBi2Ta209 (SBT) are promising materials for data storage capacitors in nonvolatile memories. PZT has an advantage of larger polarization over SBT but also has the disadvantage of requiring a higher saturated voltage. Lowering the PZT capacitor saturation voltage might be possible, not only by making the film thinner but also by controlling the film orientation.1 The perovskite structure intrinsically exhibits slow saturation against applied voltage since the real-life thin film is randomly oriented polycrystalline where a grain is a single crystal and has a multidomain structure. When an electric field is absent, the perovskite grain is separated into neighboring (100) and (001) domains to minimize the strain in the tetragonal crystal structure. At high electric field, the (100) domains in the field direction begin to switch active for polarization, which are inactive at the lower electric field. It is the origin of the 90° domain walls which causes high voltage saturation. The resultant polarization gradually increases with the increasing volume of the (001) domains which do not saturate until higher voltage is applied. However, in the case of (111)-oriented film, there seems to be only a single polarization vector against the electric field because the magnitudes of the (111) polarization vectors are equal to both the (100) and (001) domains. In other words, there seems to be only 180° domain walls. That is why the (111) orientation is preferable for realizing the lower voltage operation. Although the magnitude of the (111) vector is geometrically 1/√3 of the (001) polarization, it is not detrimental to the device operation because the large PZT polarization. a)
Present address: OMRON Corporation. J. Mater. Res., Vol. 15, No. 12, Dec 2000
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It is reported that lead perovskite films easily obtain relatively high preferred orientation on some substrates which have similar atomic arrangement and close interplanar spacings.2,3 The desirable orientation for the (100), (110), or (111) direction could be obtained if appropriate substrates are chosen and if those substrates have a clear surface. According to the atomic arrangement in the surface of Fig. 1, iridium is one of the promising substrate materials for the preparation of (100)- or (111)-orient
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