Low-temperature fabrication of pyroelectric Ba 0.8 Sr 0.2 TiO 3 thin films by a sol-gel process
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Ba 0.8Sr0.2TiO3 films were fabricated with a 0.05 M solution by a sol-gel process at temperatures between 550 and 650 °C. Analysis by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy revealed that the films annealed at 650 °C showed pure perovskite phase, tetragonal structure, and columnar grains with an average grain size of 150 nm. Electrical measurements performed on the films annealed at 650 °C showed two dielectric peaks in the dielectric constant–temperature curve, a remnant polarization of 1.4 C/cm2, a coercive field of 18.3 kV/cm, and good insulating property. The measured pyroelectric coefficient for the films annealed at 650 °C was larger than 3.1 × 10−4 C/m2K at the temperatures ranging from 10 to 26 °C and reached the maximum value of 4.1 × 10−4 C/m2K at 16 °C. The excellent pyroelectric property rendered the Ba 0.8Sr0.2TiO3 films annealed at 650 °C promising for uncooled infrared detectors and thermal imaging applications.
I. INTRODUCTION
In making devices for infrared detection and thermal imaging, uncooled pyroelectric materials, such as ferroelectrics, are an alternative to cooled semiconductors commonly used at present. Ferroelectric thin films are especially considered to have great potential since they would result in a more economical design with better performance compared with the ceramic or single-crystal detectors. The eventual challenge would be, of course, to deposit the detector as a thin film integrated into silicon based circuitry.1 For this purpose, ferroelectric barium strontium titanate (BST) has been identified as a very promising material for uncooled infrared detection applications, because its composition-dependent Curie temperature (from 30 to 400 K), makes the largest pyroelectric coefficient can be obtained at room temperature.2 Thin films of BST have been prepared by many growth techniques, such as radio-frequency magnetron sputtering, laser ablation, metalorganic chemical vapor deposition,3–5 and the sol-gel process.6–10 Compared to other deposition methods, the sol-gel process offers some advantages, such as homogeneity, stoichiometry control, and the ability to coat large and complex area substrates. However, the sol-gel-derived BST films always fail to display pronounced ferroelectric hysteresis loops and dielectric anomalies,6–10 which makes BST unsuitable for uncooled infrared detector applications. a)
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J. Mater. Res., Vol. 16, No. 3, March 2001 Downloaded: 18 Mar 2015
There are several possible causes for the disappearance or suppression of ferroelectricity of the sol-gel derived BST films: (i) The critical size for the existence of ferroelectricity of BaTiO3 is much larger than that of PbTiO3.11 (ii) The sol-gel deposition of BaTiO3 films commonly results in polycrystalline, granular films with grain diameters of lower than 70 nm due to random nucleation in the pyrolyzed gel films.10 This is in contrast to PbTiO3 films, which can e
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