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Zr K-EDGE EXAFS STUDY OF PZT THIN FILM FORMATION FROM SOLS Related publications: I. Arčon, B. Malič,
M. Kosec, A. Kodre; Zr K-Edge EXAFS Study of PZT Thin Film Formation From Sols, Abstract We examined the local environment of zirconium atoms in the Pb(Zr
Introduction Ferroelectric thin films have been widely studied for a range of applications such as memories, capacitors, sensors or microelectromechanical devices [1,2]. In comparison to solid-state synthesis, alkoxide based sol-gel processing of multicomponent materials is recognized to yield more homogeneous products at lower processing temperatures, due to the formation of heterometallic bonding already in solution. In chemical solution deposition (CSD) of thin films the main processing steps include the synthesis of a heterometallic precursor, typically in a nonaqueous medium, the deposition of the film, and the processes occurring upon thermal treatment: drying, consolidation and crystallization of the target ferroelectric phase [3]. Understanding the structural transitions in the process of film formation from the sol would allow a better tailoring of the properties of the final product. Figure: XRD spectra (CuKa) of PZT thin films deposited on sapphire (0001) after various heat-treatments. The thermodynamically stable perovskite film on sapphire (0001) substrate crystallizes at 700
![]() ![]() In 2-methoxyethanol-based processing of Pb(Zr Figure: Ferroelectric PZT thin film deposited on platinized silicon substrate after annealing at 650°C, 15 min. (Cross-section SEM image).
In this work Zr K-edge EXAFS of the amorphous Pb(Zr
Experimental PZT sol, corresponding to the target composition of PbZr Zr K-edge EXAFS spectra of the PZT sol and thin film were recorded at the X1 station in HASYLAB at DESY (Hamburg, Germany). A Si(311) double-crystal monochromator was used with 3 eV resolution at 18 keV. Harmonics were effectively eliminated by detuning the monochromator crystal using a stabilization feedback control. Absorption spectrum of the PZT sol was measured in a standard transmission mode. The sol, concentrated to approximately 1 M solution in order to increase signal-to-noise ratio, was inserted in a liquid absorption cell with 0.5 mm lucite windows. Sample thickness of about 1mm was chosen to obtain total absorption thickness of about 2 above Zr K-edge. Empty absorption cell served for a reference spectrum measured in identical conditions. The intensity of the incident and transmitted beam was measured with Ar filled ionization cells. The standard stepping progression within [-250 eV, 1000 eV] interval relative to the Zr K-edge was adopted with an integration time of 1s/step. Zr EXAFS spectrum of the PZT film was measured in a fluorescence detection technique using a 4-channel Ge fluorescence detector. The absorption spectra were obtained as the ratio of the fluorescence detector signal and the signal of the incident photon beam from the ionization cell filled with argon at ambient pressure. The same stepping progression was adopted as in the case of the sol with an integration time of 4s/step. To improve signal to noise ratio, eighteen consecutive runs were superimposed.
Figure 1: Zr K-edge EXAFS spectra of PZT sol and amorphous thin film on (0001) sapphire: (dots) - experiment, (solid line) - EXAFS model Zirconium EXAFS spectra of PZT sol and thin film were analyzed by the University of Washington analysis programs using FEFF6 code for ab initio calculation of scattering paths [7,8]. The comparison of both spectra (Fig. 1) shows that the signal-to-noise ratio of the fluorescence EXAFS spectrum measured on the film is about an order of magnitude lower, in spite of the much longer detection time. Nevertheless, the quantitative analysis reveals that the local neighborhood of Zr atoms in the film is different from that observed in the sol. The difference can be clearly seen in Fig. 2, where the k Figure 2: The k Two shells of neighbors are discerned in the local neighborhood of Zr atoms in the sol. The fit of the EXAFS spectrum in the R range of 1.3 The local structure around Zr formed in the sol is not retained in the amorphous film. The fit of the EXAFS spectrum of the film (in the same R range of 1.3
Table 1: Parameters of the nearest coordination shells around zirconium atom in PZT sol and amorphous thin film: atomic species, average number N, distance R and Debye-Waller factor We attribute the difference between the amorphous film and the sol to the partial decomposition of organic functional groups upon heating to 350 It is conceivable that the segregation of Zr species in the film - possibly as zirconia nanoparticles within the amorphous matrix - hinders the crystallization of the perovskite phase by the formation of the transient pyrochlore-type phase, so that the crystallization of the perovskite phase is shifted to higher temperature.
Acknowlwdgmwnt Support by the Ministry of Science and Technology of the Republic of Slovenia within the National Research Program, by Internationales Buero BMBF (Germany), and by the IHP-Contract HPRI-CT-1999-00040 of the European Commision is acknowledged. L. Troeger and N. Haack of HASYLAB provided expert advice on beamline operation.
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