In-Situ Control of the Direction of Spontaneous Polarization in Ferroelectric thin Films by RF-Magnetron Sputtering
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ABSTRACT La-modified PbTiO 3 thin films were grown on (100)MgO by rf-magnetron sputtering. Thin films were prepared by "two step method" in which the different Pb/Ti ratios during the growth were used for the first step(O-4nm of thickness) and the subsequent second step(>4nm). The Pb/Ti ratios of the first step were selected from 1.2 to 1.5. The Pb/Ti ratio of the second step, on the other hand, was fixed at 1.4. Strong dependency of the direction of the spontaneous polarization(Ps) to the Pb/Ti ratio of the first step was observed. It is concluded that the direction of the Ps in the films is determined by the atomic species of the first layer of the films.
INTRODUCTION Ferroelectric materials have many useful properties, such as piezoelectricity, pyroelectricity, ferroelectricity, opto-electric property etc. and are widely used in various kinds of transducers. Recently, progress in the processing technology of thin films makes it possible to prepare high quality ferroelectric thin films and micro-devices using ferroelectrics [1]. However, usually ferroelectrics have a domain structure in as-grown state and we do not use most of the properties of ferroelectrics. To utilize them, a polarizing treatment applying a high voltage to ferroelectrics at high temperature is required to polarize the Ps in ferroelectrics. The polarizing treatment sometimes leads to a break-down of the thin film. It is difficult to control the direction of the Ps at any position in the film using the conventional method. Therefore, an in-situ process which can control the orientation of the Ps in the films is strongly required. The establishment of such in-situ process is essential to develop a smart material. A "self-polarized" thin film was reported for the Pb-based ferroelectric thin film prepared by rf-magnetron sputtering[2,3]. However, the direction of Ps was not controlled. The domain orientation of the surface of LiNbO 3 is controlled by proton-exchange method[4]. This method only controls the direction of Ps at the surface. In this paper, we report the in-situ control of the direction of Ps in Pb-based ferroelectric thin films by rf-magnetron sputtering. We found that the precise control of the composition at the initial stage of the film growth effectively controls the direction of the Ps in the film. 41 Mat. Res. Soc. Symp. Proc. Vol. 459 ©1997 Materials Research Society
EXPERIMENTAL
Lanthanum-modified PbTiO 3 (PLT) thin films were grown on the (100)MgO single crystal substrates by planar rf-magnetron sputtering using an epitaxial condition. Typical sputtering conditions are shown in Table I. In this study, we used a "two step growth" consisting of the first step (film thickness of 0-4nm) and the second step of the growth( >4nm). In each step, we use different Pb/Ti ratios: Pb/Ti ratios of the first step were chosen between 1.2 and 1.5. On the other hand, Pb/Ti ratio of the second step was fixed at 1.4. Pb/Ti ratio was adjusted by controlling the input power during the sputtering. The films thus obtained were examined by X-ray d
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