Chemical structure evolution and orientation selection in sol-gel-prepared ferroelectric Pb(Zr,Ti)O 3 thin films
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G. Maes Chemistry Department, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium (Received 20 November 2000; accepted 30 May 2001)
We studied in detail the chemical structure evolution of Pb(Zr1−x,Tix)O3 (PZT) thin films on Pt electrodes during the initial thermal steps of their preparation using an alkoxide-based sol-gel process. Absorption-reflection Fourier transform infrared spectroscopy (AR-FTIRS) was used to monitor chemical reactions occurring in the films on a real temperature scale. We demonstrate that the chemical state of the pyrolyzed film strongly depends on pyrolysis conditions and can have a large effect on the orientation selection in the film. First, residual acetate groups, resulting from incomplete decomposition of the Pb acetate precursor, strengthen the (111) PZT texture component after crystallization. Second, OH bonds, which are seen to remain in the film after pyrolysis under specific conditions, are seen to strengthen the intensity of the PZT(100) reflection. Possible mechanisms behind these observations are discussed.
I. INTRODUCTION
Sol-gel processing is an established technique to prepare high-quality Pb(Zr1−x,Tix)O3 (PZT) ferroelectric thin films. Their recent successful integration on silicon for nonvolatile memory applications1 has emphasized the need to tailor sol-gel methods for process compatibility (lower crystallization temperature) and to fit device requirements (smaller film thickness). The development of methods for orientation control is especially important in view of obtaining (111)-oriented films, which feature superior ferroelectric switching properties. A great variety of factors, including precursor type and homogeneity,2–5 the Pb excess,6 type and heat treatment of the bottom electrode,7–9 as well as the temperatures and heating rates applied for pyrolysis and crystallization10 have been shown to affect orientation selection in sol-gel-prepared PZT films. A variety of models exist to explain these dependencies.10–14 For instance, Brooks et al. have speculated that variations in the valence of Pb are responsible for the observed dependence of orientation on pyrolysis temperature.10 Huang et al.13 proposed that oxidation reactions of the precursor are responsible for the reduction of Pb and the subsequent formation of Pt3Pb intermetallic, thought to serve as a template
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Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 16, No. 9, Sep 2001
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for PZT(111) growth. Schwartz et al.14 have shown that variations in the crystallization temperature, as a result of variations in precursor chemistry, can drastically alter the nucleation mechanism and hence the properties of sol-gel-prepared PZT films. While these models, in fact, suggest the importance of chemical changes in the films, their limited applicability beyond the specific precursor chemistry studied illustrates that chemical effects are strongly precursor dependent. Hence, a detailed insight into
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