Characterisation of Ferroelectric Lithium Tantalate thin Films Prepared by a Sol-Gel Process
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ABSTRACT Thin LiTaO3 films were prepared by spin coating of polymerized sol-gel precursor solution. Films have been deposited on single crystal silicon substrate, Ti/Pt or SiO 2 coated silicon substrate. Films were characterized by x-ray diffraction, dielectric and pyroelectric measurements. High Curie temperature (above 550 0 C) was assumed for LiTaO3 thin films from the temperature dependence of dielectric constant. Replacing 35% of tantalum by titanium atoms in the LiTaO3 precursor solution has resulted the thin films with Curie temperature of 330 oC. The lower Curie temperature leads to the larger pyroelectric coefficient at room-temperature, which is more than double that of the undoped LiTaO 3 thin films. The dielectric, pyroelectric, and ferroelectric properties have been compared to the single crystal LiTaO3 and ceramic Lio.9lTao.73Tio.3603 . LITaO 3 thin films are available by sol-gel process at low temperature, and their properties may possibly be controlled by varying the composition of the sol-gel precursor solution.
Introduction This work is the continuation of our efforts to develop new materials for integrated room temperature infrared detectors and for integrated electro-optical applications. The change in polarization with temperature which occurs in any polar material is known as the pyroelectric effect. As a subgroup within polar materials, ferroelectric crystals generally exhibit the strongest pyroelectric effects. Our previous studies on infrared detectors based on PbTiO 3 and PLT have shown promising resultsl. 2 . For sensitive pyroelectric detectors, LiTaO3 is also a good material due to excellent figure of merit 3. Although LiTaO 3 does not have the perovskite structure, it also forms ABO3 lattices with oxygen octahedra. On the other hand, LiTaO 3 pyroelectric sensors have a much lower microphonic sensitivity effect due to their relatively low piezoelectric coefficients (d33 =-8pC/N, d 31 =-2pC/N). This provides better performance in a vibrational or intensely acoustic environment 4. The very high Curie temperature (618 °C) provides a temperature independent sensitivity over a wide operational temperature range. Replacing 36% of tantalum by titanium atoms in LiTaO 3 ceramic has reduced the Curie temperature from 620 oC to 358 oC 5, which leads to better sensitivity as a roomtemperature infrared detector. On the other hand, LiTaO3 has been reported as a better material for optical waveguide because it is highly resistive against photorefractive damage, and also because of its large nonlinearities, and short-wavelength transparency 6. Integrated microsensor applications of pyroelectric materials require film deposition techniques to be compatible with conventional device processing. Sol-gel processes have been used to produce polycrystalline ceramic films at significantly lower temperatures than other more traditional ceramic fabrication methods 7. Compared to the other deposition methods, such as rf sputtering, electron beam 'evaporation, and MOCVD, sol-gel processing also offers the possibi
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