Control of crystallization and crystal orientation of alkoxy-derived SrBi 2 Ta 2 O 9 thin films by ultraviolet irradiati
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A 650 °C annealed thin film was found to be a single phase of SrBi2Ta2O9 (SBT) and showed the c-axis orientation without ultraviolet irradiation. The crystallinity and crystal orientation of the thin films were improved by ultraviolet irradiation using an ultrahigh-pressure mercury lamp (UHPML) or a low-pressure mercury lamp (LPML) under three conditions. In particular, the crystal orientation of the 650 °C annealed thin films dramatically changed by ultraviolet irradiation using a LPML at room temperature or using an UHPML at 150 °C. The 650 °C annealed SBT thin film prepared by ultraviolet irradiation using an UHPML at 150 °C showed a remanent polarization (Pr) of 4.3 C/cm2 and a coercive electric field (Ec) of 101 kV/cm at 10 V.
I. INTRODUCTION
The thin film of a layer-structured perovskite compound SrBi2Ta2O9 (SBT) is known to show the excellent ferroelectric properties because of its specific crystal structure. By these particular properties, SBT thin film draws attention as a material for the nonvolatile ferroelectric memory applications. 1–7 The ferroelectric memory devices consist of ferroelectric thin films and silicon semiconductors accumulated circuits. During the annealing process of ferroelectric thin films, the silicon semiconductor circuit is often seriously damaged.8–10 Thus, it is imperative for the system that the processing temperature of the ferroelectric thin films be lower than the temperature used in the complementary metal-oxide semiconductor technology. Additionally, to control the crystallinity and orientation of SBT thin films is a key to improving ferroelectric properties. In particular, the highly crystallized and c-axis-oriented SBT thin films are preferred for systems in which the polarization is parallel to the in-plane direction, such as the microelectromechanical system combining the inter-digitated electrodes. On the other hand, the SBT thin films of which orientation slants from the c-axis direction, are desirable for systems in which the polarization is perpendicular to the in-plane direction, such as the ferroelectric memories. When molecules in the ground state are irradiated with ultraviolet (UV) light of appropriate wavelength, they absorb the photons and transit to the excited states that are unstable and have high reactivities. A variety of photochemical reactions occur from the excited state. Thus, the reactivities of the photochemical reactions in the exited state are different from those of the thermochemical reactions in the ground state. Therefore, more rapid and J. Mater. Res., Vol. 18, No. 4, Apr 2003
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lower temperature reactions would be attainable for the photochemical reactions compared with the thermochemical reactions. Recently, the studies on the low-temperature processing or crystallization, or densification of metal-oxide thin films by introducing an UV irradiation were actively reported.11–13 In our previous studies, we found the more simple methods using photochemical reaction for the control of th
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