Quantitative chemical analysis of fluorite-to-perovskite transformations in (Pb,La)(Zr,Ti)O 3 PLZT thin films
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Lead loss during processing of solution-derived Pb(Zr,Ti)O3 (PZT)-based thin-films can result in the formation of a Pb-deficient, nonferroelectric fluorite phase that is detrimental to dielectric properties. It has recently been shown that this nonferroelectric fluorite phase can be converted to the desired perovskite phase by postcrystallization treatment. Here, quantitative standard-based energy-dispersive x-ray spectrometry (EDS) in a scanning transmission electron microscope (STEM) is used to study cation distribution before and after this fluorite-to-perovskite transformation. Single-phase perovskite PbZr0.53Ti0.47O3 (PZT 53/47) and Pb0.88La0.12Zr0.68Ti0.29O3 (PLZT 12/70/30) specimens that underwent this treatment were found to be chemically indistinguishable from the perovskite present in the multiphase specimens prior to the fluorite-to-perovskite transformation. Significant Zr–Ti segregation is found in PLZT 12/70/30, but not in PZT 53/47. Slight La-segregation was seen in rapidly crystallized PLZT, but not in more slowly crystallized PLZT.
I. INTRODUCTION
Solution-derived thin films fabricated from Pb-based ferroelectrics such as those in the (Pb,La)(Zr,Ti)O3 (PLZT) system are attractive for applications such as high-value integrated capacitors, piezoelectric microelectromechanical system (MEMS) devices, and ferroelectric memories.1–3 However, the Pb cation is volatile at the processing temperatures (∼600–700 °C) needed to crystallize the ferroelectric perovskite phase from the deposited amorphous layer,4–6 which makes the fabrication of stoichiometric Pb(Zr,Ti)O3 (PZT) and PLZT films difficult. Since the perovskite phase of PZT-based materials is not tolerant to A-site nonstoichiometry, Pb loss typically results in a thin layer of a Pb-deficient fluorite (or pyrochlore) second phase at the surface of the film.7–9 Pyrochlore and fluorite are closely related phases that are differentiated by the presence or absence, respectively, of ionic ordering10,11; since such ordering was not explicitly observed during this work12—and either phase would result in severely degraded electrical properties relative to the stoichiometric perovskite—the more general term fluorite will be used herein. Due to series mixing, this low-K surface layer dominates the dielectric behavior of the overall film, resulting in severely degraded properties. Various methods have been used to prevent or coma)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0353 2944
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pensate for this Pb loss and subsequent fluorite formation during the initial crystallization heat treatment. Sintered powder-derived Pb-based ferroelectrics often use excess PbO additions or burial powders.4,6 In magnetron RFsputtering of PZT, excess Pb has been deposited by cosputtering a PZT and PbO target, and varying the power to the PbO target to control PbO excess in the film.13 In PZT or PLZT materials deposited from chemical solution, techniques include heat treating the film in the presence o
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