Grain Growth Behavior of Bismuth Titanate Thin Film on Metallic Silicon Substrates
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ABSTRACTS Bismuth titanate (Bi 4 Ti 3 Oi 2 ) thin films were produced from acetate precursors by spin coating deposition followed by crystallization using rapid thermal processing (RTP).. Crystallographic orientation was characterized by x-ray diffraction, and indicated that it varied with the number of coating layers for the films deposited on single crystal silicon wafers. Cross sectional samples were prepared and studied by Transmission Electron Microscopy (TEM). The results showed that grains randomly nucleated at the interface where a thin amorphous silica layer was naturally formed. Grains nucleated in the [020] direction or near this direction showed a faster growth rate than that in the other directions, and eventually dominant in determining the grain orientation of the film. Preferential orientation in the [020] direction observed by TEM is identical to the XRD results for the multilayer thin films. The thickness and grain size of thin film were also investigated.
I. INTRODUCTION Bismuth titanate (Bi 4 Ti 3 O02 , abbreviated BIT) is one of few important ferroelectric (FE) materials. Single crystal of BIT is a layer-structure compound and has a very high Curie temperature (Tc=6750 C). Above the Curie temperature, it possesses a pseudo-tetragonal structure proposed by Aurivillius[1l as shown in Figure 1. As temperature decreases below Tc,
spontaneous polarization in the a-direction results in increasing the lattice distortion (b/a)[2], the structure becomes monoclinic with a=5.41 1 A, b=5.448A, c=32.83 A at room temperature reported by Cummins and Cross[ 3]. However, x-ray and neutron diffraction data are consistent with a polar orthorhombic structure[4 ]. The BIT has been studied because of its attractive ferroelectricity and electro-optical One is in the a-2 behavior. There are two spontaneous polarizations in a single BIT crystal. 2 direction and another is in the c-direction with magnitude of 50±5 ILC/cm and 4.0±0.1 ýLC/cm respectively. They can be reversed independently. Switching of the Psc (in c-direction) can change the birefringence and tilt the optical indicatrix of BIT in the a-c plane by approximately 100 which is attractive for electro-optic device. In addition, the large spontaneous polarization in the a-direction makes BIT potentially useful for applications requiring large value of switching charge. Upon the difficulty in fabrication of single crystal BIT, preferentially oriented BIT ceramics have received a great deal of attention in order to utilize its properties. There have been several reports of BIT thin film fabrications over past few years[ 5],[6],[ 7 ]. However, only few papers reported on making BIT thin film by wet chemical techniques. Recently, the results from our research indicated that the acetate-derived[ 81 BIT thin films produced by multilayer deposition technique showed significant crystallographic orientation in some directions as shown in Figure 2. The variation of x-ray diffraction intensity indicates that grain orientation of the thin film changed with the number of
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