Lower crystallization temperature of sol-gel PbTiO 3 on Ti/Pt-coated substrates
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Lower crystallization temperature of sol-gel PbTiO3 on TiyPt-coated substrates R. E. Avilaa) Departamento de Investigaci´on y Desarrollo, Commisi´on Chilena de Energ´ıa Nuclear, Cas. 188-D, Santiago, Chile
T. P. Velilla and P. J. Retuert Departamento de Qu´ımica, FCFM, Universidad de Chile, Santiago, Chile (Received 15 October 1998; accepted 22 February 1999)
PbTiO3 (PT) thin films have been deposited by sol-gel on PtySi, SiO2ySi, PtyTiySiO2ySi, and TiyPtyTiySiO2ySi and annealed for 45 min in the 400–670 ±C range. Analysis by x-ray diffraction (XRD) and spectroscopic ellipsometry shows that the Ti overlayer promotes early crystallization in the tetragonal perovskite phase, reducing the presence of a second phase, tentatively identified as pyrochlore, starting by 450 ±C. The refractive index and extinction coefficient sn, kd of the PT film increase rapidly with the sintering temperature in the range of 450–570 ±C and saturate by 570 ±C to values of n varying from 2.4 to 2.9, and k from 0.03 to 0.3, over the 1.65–2.95 eV range. Most of the increase of n is related to thin film densification.
Lead titanate (PbTiO3 ) (PT) and its close relative, Pb(Zrx Ti1–x )O3 (PZT), are widely studied because of their many applications in sensors, actuators, and electronic devices. In the case of thin films for memory1 and other applications, the choice of substrate largely determines the crystallinity and electronic properties of the film.2 Diffusion of Pb from PT films deposited on Si or SiO2ySi is known to lead to the formation of Pb–Si compounds. This may be avoided with the introduction of a diffusion barrier, which should be stable at the temperature of 500 to 700 ±C, necessary for sintering the PT film. One successful substrate is PtyTiySiO2ySi, where the Ti and Pt layers are typically between 100 and 200 nm thick.1 On this substrate, PT and PZT may be deposited by sputtering or sol-gel methods, among others, yielding ferroelectric, tetragonal films, after sintering in the 500–700 ±C range. As regards the substrate, grain growth and extensive defects have been reported3 in PtyTiySiO2ySi substrates after annealing in O2 at 650 ±C for 20 min. A PZT or lead lanthanum titanate film on top of Pt greatly reduces those structural modifications. The effect of thermal relaxation of stress has been observed4 in bare Pt films on ZrO2ySiO2ySi substrates, which develop hillocks between 400 and 600 ±C, but decay rapidly in the 600–700 ±C range. Aoki et al.5 report that PZT films deposited on the Ti buffer layer crystallize at lower temperature and mostly in the perovskite phase, as opposed to an alternative, unidentified, second phase which appears in films deposited directly on Pt. They suggest that the effect is to
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e-mail: [email protected] J. Mater. Res., Vol. 14, No. 6, Jun 1999
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promote early nucleation of PT at the PZTyPt interface, in the 400–500 ±C range. Inspired by these5 and other reports6,7 of depositing PZT by sol-
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