The nucleation, growth and orientation of lead zirconate titanate thin films prepared ..... [6,26], After annealing for 300s, Fig.4d, a strongly columnar perovskite.
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Orientation of Rapid Thermally Annealed Lead Zirconate Titanate Thin Films on (111) Pt Substrate
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Keith G. Brooks, Ian M. Reaney, Radosveta Klissurska, Y. Huang, L.ABursill* and N. Setter
Laboratoire de Ce'ramique, Departement de Materiaux, Ecole Polytechnique Federate de Lausanne, Lausanne, CH-1015, Switzerland * On leavefromSchool of Physics, The University of Melbourne, ParkviUe, 3052, Vic. Australia Abstract The nucleation, growth and orientation of lead zirconate titanate thin films prepared from organometallic precursor solutions by spin coating on (HI) oriented platinum substrates and crystallized by rapid thermal annealing was investigated. The effects of pyrolysis temperature, post-pyrolysis thermal treatments, excess lead addition, and Nb dopant substitution are reported. The use of post pyrolysis oxygen anneals at temperatures in the regime of 350-450°C was found to strongly effect the kinetics of subsequent amorphous-pyrochloreperovskite crystallization by rapid thermal annealing. The use of such post pyrolysis anneals allowed films of reproducible microstructure and textures (both (100) and (111)) to be prepared by rapid thermal annealing. It is proposed that such anneals and pyrolysis temperature effect the oxygen concentration/average Pb valence in the amorphous films prior to annealing. Such changes in Pb valence state then effect the stability of the transient pyrochlore phase and thus the kinetics of perovskite crystallization. Nb dopant was also found to influence the crystallization kinetics.
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1. Introduction The crystallization of and hence resultant microstructure, texture and ferroelectric properties of perovskite lead zirconate titanate (PZT) thin films are known to depend upon numerous parameters l -8- ^ l . Published techniques for the synthesis of PZT thin films can be divided into two categories: those which use in-situ crystallization (i.e., crystallization during deposition) and those which involve post deposition crystallization of a pre-existing amorphous layer. MOCVD and physical deposition at elevated temperatures fall into the former category 1 > 1. For in-situ crystallization, O2 partial pressure is a critical process control parameter. The second category includes most chemical and low temperature physical deposition techniques. Fox and Knipanidhi demonstrated that the oxygen content of as-sputtered lead lanthanum titanate films had a profound effect on the transformation to perovskite during subsequent annealing. Oxygen deficient films fully transformed to perovskite, whereas pyrochlore formed when an excess of oxygen was present. This was shown to be related to Pb valence state, and independent of Pb content 1 1. The effects of annealing atmosphere (i.e., PO2) on perovskite crystallization from an amorphous phase has also been reported. Other thermal treatments, pre- and post- crystallization, are known to influence the nucleation, microstructure and texture and electrical properties of PZT films i h An investigation of the critical parameters for post deposition crystallization of amorphous films derived from spin casting of sol-gel solutions is presented below. The chemical and thermal stability, crystallographic structure and quality of the substrate material(s) also play key roles in the preparation of high quality PZT thin films [1.2,6,7], The most frequently reported substrate material system to date is Pt/Ti/Si02/Si. The deposition conditions and post depositica annealing of the Pt are known to effect its microstructure, density and orientation l » ]. The degree of orientation of the Pt affects the microstructure of the PZT thin films deposited on it. Other factors for which an effect on the crystallization behavior of perovskite thin films has been reported include thermal processing parameters (temperature, heating rates, time and atmosphere) [e.g.i,6,9,lO ll,12] film stoichiometry or more specifically Pb content [1,6,11,13,14] and dopant additions e
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Considering the specific case of films prepared by the chemical techniques of sol-gel and MOD, other parameters are also significant. Film synthesis is sensitive to the precursors used (solubility, reactivity and molecular weight) ( -g18,19,20], the conditions of pre-hydrolysis and condensation in the case of sol-gel processes (acid or base catalysis)! *)* * ] and combustion characteristics of the precursor material (thermal stability, organic content) during pyrolysis. These factors impact on the nano-scale homogeneity of the amorphous film prior to crystallization. In previous work, we have observed that PZT films of random orientation, or strong (111) or (100) textures could be prepared on Ti/Pt bilayer metallizations by rapid thermal annealing (RTA) I^l. Previous transmission electron microscopy (TEM) crystallization investigations of free standing sol-gel derived PZT films on Pt TEM grids showed that the P02 and heating rate during the annealing process strongly effected the crystallization temperature and resultant phases obtained (perovskite, pyrochlore) l l . In this study, we have investigated the critical parameters controlling the nucleation, growth, and resultant microstructure of PZT films prepared by sol-gel processing on Pt/Ti/Si02/Si substrates. Specifically, the effects of pyrolysis temperature, low temperature (
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Fig. 4 TEM transverse section micrographs of 350°C pyrolyzed PZTfilmsRTA annealed at 600°C for a) Is, b) 2s c) 5s and d) 300s. Inset in a) and b) are electron diffraction patterns from the respective films (compare to Fig. 3).
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Fig. 5 XRD patterns for 420°C pyrolyzed films RTA annealed at 600°C for Is, 2s and 300s, employing heating rates of 150°C/s, 80°C/s and 80°C/s, respectively.
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Fig. 9 Schematic representations of the changes in grain size and structure which occur as a function of increasing pyrolysis temperature for samples RTA amiealed at 60O°C/5m; a) 350°C (compare Fig. 4d), b)380°C (compare Fig.8) and c) 430°C (compare Fig. 10) pyrolysis temperatures.
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Fig. 10 SEM micrograph of the surface of a (100) textured film prepared by pyrolysis at 430°C, followed by 600°C/60s RTA anneal.
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Fig. 11 TEM transverse section of 350°C pyrolyzed film annealed in Ar at 480°C for 5m, showing occurrence of PbPtx.
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Fig. 12 X-ray diffraction patterns of films pyrolyzed at 350°C a) treated in fonning gas atmosphere for 2m at 460°C and b) after subsequent treatment in O2, 460°C for 30m, showing the fomation of PbPt and subsequent reoxidation of the reacted Pb. x
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Fig. 13 X-ray diffraction patterns of PZT films pyrolyzed at 370°C and a) RTA annealed; 600°C/lm, b) treated in 02 at 37075m then RTA annealed; 600°C/lm, and c) treated in O2 at 370°C/30m then RTA annealed; 600°C/lm in air (note scale changes at 28 and 34° 26).
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Fig. 17 X-ray diffraction pattern of randomly oriented PZT thin film RTA annealed at 600°C/30s.
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