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Thin Solid Films 467 (2004) 104 – 111 www.elsevier.com/locate/tsf
Phase evolution of sol–gel prepared Pb(Zr0.3Ti0.7)O3 thin films deposited on IrO2/TiO2/SiO2/Si electrodes D. Van Genechten a, G. Vanhoyland a, J. D’Haen b, J. Johnson c, D.J. Wouters c, M.K. Van Bael a, H. Van den Rul a, J. Mullens a,*, L.C. Van Poucke a a
Laboratory of Inorganic and Physical Chemistry, IMO, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium b IMO-IMOMEC, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium c IMEC, B-3001 Heverlee Leuven, Belgium Received 31 July 2003; received in revised form 17 February 2004; accepted 9 March 2004 Available online 23 April 2004
Abstract We report on the microstructural analyses, by means of X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDX) and cross-section transmission electron microscopy (X-TEM), of chemically prepared Pb(Zr0.3Ti0.7)O3 thin films deposited on IrO2 substrates. The purpose of this study is to detail temperature and time dependence of the lead zirconate titanate (PZT) film microstructure on this type of conductive oxide substrate, partly through a comparison with identically processed PZT films deposited on Pt substrates. It was observed that PZT 30/70 films on IrO2 bottom electrodes, fired at temperatures up to 620 jC, are not single phase, due to extensive lead losses during the processing. The IrO2 substrate was found to be indirectly responsible for these losses. Nevertheless, good ferroelectric properties were measured ( Pr was 50 AC/cm2 for the 620 jC film). Based on the observed morphology and texture with increasing annealing time, a mechanism for phase evolution in sol – gel-derived PZT 30/70 films on IrO2 substrates is proposed. D 2004 Elsevier B.V. All rights reserved. PACS: 68.55 Keywords: PZT films; IrO2 electrode; Phase evolution; TEM
1. Introduction The potential applications of ferroelectric lead zirconate titanate (PZT) thin films in a wide range of devices, from nonvolatile semiconductor memories to sensors and actuators [1], have led to much work in trying to develop an understanding of film microstructure/property relationships. Results of such studies have shown that microstructural properties, including grain size and crystallographic orientation, surface topology and porosity, can strongly influence thin film electrical and electro-optical performance [2]. The sol – gel route has emerged as a most versatile method for preparing chemically homogeneous PZT powders as well as thin film coatings. However, one of the major limitations of the sol –gel route, as for most preparation routes for that matter, is that it does not yield the desired perovskite (Pe) phase directly after the pyrolysis of the gel, either in bulk or * Corresponding author. E-mail address: [email protected] (J. Mullens). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.03.021
in thin film form. The formation of the Pe phase is, instead, mediated by an undesirable, nonferroelectric pyrochlore (Py) phase, which is formed first after pyrolysis [3]. Several different factors, such as pyrolysis and subsequent heat-treatment temperatures [4], annealing atmosphere and Pb content [5], have been proven to influence the transformation of the Py phase into the Pe phase, and hence also the PZT’s final microstructure. In case of thin films, extensive research has been carried out studying the effects of these parameters for PZT deposited on conventional Pt bottom electrodes. From a technological point of view, however, Pt has lost its relevance: in view of a stacked cell layout, necessary for memory cell area reduction, the material does not form a barrier to oxygen diffusion to avoid oxidation of the plug material [6]. Furthermore, when a capacitor is fabricated with a PZT thin film on a Pt electrode, the lifetime and reliability of the device is limited by degradation phenomena such as fatigue and imprint [7]. A conductive oxide electrode, such as IrO2, seems to be the most attractive alternative for nonvolatile memory
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application since it acts as an oxygen donor [8], and provides a good diffusion barrier as well. The former should compensate for the electromigration of oxygen vacancies into sites in planes parallel to the electrodes, which is reported [9] to be the most probable cause for reliability problems such as fatigue. However, control over the PZT/ IrO2 microstructure remains an unsolved issue. Our study represents one of the first attempts to characterize, by means of X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive X-ray analysis (SEMEDX) and cross-section transmission electron microscopy (X-TEM) analysis, sol – gel prepared PZT thin films, deposited on IrO2 substrates and processed at low temperatures.
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diamond saw, epoxied together, mechanically thinned to 60 Am, dimple grinded to 8 Am and then FIB thinned at an angle of 5j. A Pt layer was sputter deposited on top of the PZT to avoid film damage during the final thinning process. TEM work was carried out on a Philips CM12 operated at 120 kV, equipped with an EDAX 9900 spectrometer for chemical analysis. In addition to the TEM analysis, the PZT films were characterized by scanning electron microscopy (Philips XL30 FEG) and X-ray diffraction (Siemens D5000). In order to determine whether a preferential orientation was present, integrated intensities of X-ray diffraction lines were compared with those of randomly oriented Pe PZT powders (JCPDS 50-346).
2. Experimental 3. Results and discussion PZT thin films were fabricated from metallo-organic solutions of dehydrated lead(II)-acetate, zirconium(IV)-npropoxide and titanium(IV)-isopropoxide dissolved in butoxyethanol [10,11]. The PZT 30/70 precursor solution had a concentration of 0.112 M, with a 16 mol% excess amount of lead. Immediately before deposition, the solution was filtered with a 0.45-Am syringe filter. The filtered solution was then deposited onto substrates at 3000 rpm during 30 s, using a Headway photoresist spinner. Each layer was dried at 200 jC for 2 min on a hot plate in air, and then given a 2-min heat treatment at 350 jC under the same conditions, to decompose residual organics. All films in this study consisted of three layers and were approximately 150 –160 nm thick after firing. Cross-section transmission electron microscopy was used to determine film thickness. Twenty-two different films were fabricated to determine phase and texture evolution as a function of time, temperature and substrate. These films were processed identically except for the final crystallization treatment. Eight films were deposited on Pt substrates, eight on IrO2 and subsequently fired at 550, 580, 600 and 620 jC, for 1 and 30 min on a hot plate in static air. Six more were deposited on the IrO2 bottom electrode and crystallized at 550 jC, for 3, 6, 10, 15, 20 and 25 min, respectively. The substrates consisted of 3535 mm (100) silicon wafer sections, onto which a 250-nm-thick SiO2 layer was thermally grown. On top of a 50-nm-thick sputter deposited adhesion layer of TiO2, a 100-nm Pt or a 50-nm IrO2 layer was sputtered to form the bottom electrode. Proper TEM sample preparation was critical for an efficient evaluation of the specimens. Plan view samples were prepared by ion milling with 5 kV Ar ions (Precision Ion Polishing System, PIPS Gatan model 691). However, when this system was used for the thinning of crosssectional samples, it systematically destroyed the PZT film. Consequently, all the X-TEM samples were prepared using a Focussed Ion Beam (FIB, Dual Ion Mill Gatan model 600), with 30 kV Ga ions. The latter implies these samples could not be used for EDX analyses. For cross-sectional views, wafer sections of 2 mm350 Am were cut using a
3.1. Phase purity, texture, nucleation and growth on Pt and IrO2 electrodes While comparing XRD and SEM results, Figs. 1 and 2, respectively, of the PZT/Pt 1 min (a) and PZT/IrO2 1 min (b) temperature evolution, one can clearly notice the influence the substrate has on film morphology, texture and phase transformation kinetics. The crystallization of a PZT film deposited on both types of substrate appears to follow the sequence: amorphous film!pyrochlore (Py)!perovskite (Pe). For PZT/Pt films, however, the conversion is completed at 550 jC 1 min and yields highly (111)-oriented single phase Pe PZT, while the PZT/IrO2 diffraction pattern shows quasi randomly oriented Pe PZT, and still contains a significant peak corresponding to the Py phase at 620 jC 1 min (Fig. 1b). Py diffraction lines are rather broad, indicating the large compositional range and fine-grained nature of this phase. The diffraction peaks of the Pe PZT phase are broader when the film is deposited on Pt. Therefore, the crystalline quality of the PZT film is improved on IrO2. According to h –2h XRD measurements on the bottom electrodes only (though also visible in Fig. 1), the Pt electrode is highly (111)-oriented and the IrO2 has a mixed but preferential (100)/(110) texture, with (100)>(110). As Pt’s lattice constant matches by 0.5% the one of PbTiO3 and by 4% the one of PZT 50/50 at conventional growth temperatures [12], a (111) texture of PZT/Pt is expected to be favoured. The observation of quasi randomly oriented Pe PZT obtained on preferentially oriented IrO2 substrates then raises questions about the possible effects of an IrO2 bottom electrode on orientation selection within PZT thin films. However that may be, after a 30-min crystallization at all of the applied temperatures (Fig. 1c), Pe (100) and (101) are present in a slight majority with respect to the other Pe/IrO2 orientations, in the following order: (100)>(101). The 1-min PZT/IrO2 XRD patterns further reveal the presence of a relatively small, transient peak at 32.5j 2h with a maximum intensity at 600 jC, possibly coming from
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Fig. 1. (a) XRD patterns of PZT/Pt films crystallized at 550, 580, 600 and 620 jC for 1 min; (b) XRD patterns of PZT/IrO2 films crystallized at 550, 580, 600 and 620 jC for 1 min; (c) XRD patterns of PZT/IrO2 films crystallized at 550, 580, 600 and 620 jC for 30 min.
a segregated, volatile PbO massicot phase that obviously produces only one reflection, namely (100). The presence of the same temporary peak at 32.5j 2h in the 30-min PZT/ IrO2 patterns (Fig. 1c) at 550 and 580 jC (and not at 600 and 620 jC) validates this possibility. PbO has been frequently reported to exhibit a tendency to volatilise, due
to its relatively high vapour pressure [13,14]. Furthermore, most authors suggest that the loss of PbO originates mainly from a segregated PbO phase, since the equilibrium vapour pressure of PbO over PbO is two orders of magnitude larger than over PT, and one over PZT [15]. Contrary to what was observed, however, the same authors also agreed on the fact that, when PbO does segregate, it stays in an amorphous phase. The temperature-dependent appearance of the Pe diffraction peaks in the X-ray diffraction patterns coincides with the growth of grains observed on SEM micrographs (Fig. 2b). Crystals that are extremely circular in the plane of the substrate and appear to grow radially with increased annealing temperature are systematically present on the IrO2 substrates; these are referred to as rosettes in this report. The circular morphology is only observed for isolated rosettes, since they grow radially until they impinge on one another, resulting in linear boundaries where they have grown together, as Fig. 2c depicts. Growth at the rosette – rosette interface appears inhibited compared to growth for the rosette – Py interface. Interestingly, as shown by the same figure, PZT/IrO2 films fired at higher temperatures differ from films fired at lower temperatures in that the volume fraction of Pe phase is larger (or stays equal when the film is already fully crystallized in the plane of the substrate) and that their Pe grain diameter is smaller: 2.5– 5 Am in the PZT/IrO2 620 jC 30 min film vs. 15– 18 Am in the PZT/IrO2 550 jC 30 min film. Since Fig. 2b shows that new rosette nuclei form as the existing ones grow, the larger volume fraction of Pe phase is attributed to both an increased nucleation and growth rate with increasing annealing temperature. It should be noted that we use the term ‘‘fully crystallized in the plane of the substrate’’: although it might seem as if the Py phase has been completely transformed into Pe (see PZT/IrO2 SEM micrographs, 580– 620 jC for 30 min, Fig. 2c) untransformed regions of Py do still exist, as shown in the X-ray diffraction patterns in Fig. 1c. The circular island morphology observed in films grown on IrO2 does not occur in films grown on Pt. Instead, a large density of Pe nuclei forms such that only a limited growth of these nuclei throughout the thickness of the film is required to produce the relatively dense grain structure seen in Fig. 2a. The nuclei all appear at the same time, and their resulting grain size is 50 –80 nm, regardless of the annealing temperature. All of the aforementioned observations indicate that the different film morphologies on Pt and IrO2 are probably related to interfacial energy differences between the electrode material and the Pe phase, assuming of course that Pe nucleation begins heterogeneously at the electrode/ film interface. As described in literature, this assumption is generally true for PZT/Pt [16], and Fig. 6, which will be discussed later on (Section 3.2.2), proves this also to be the case for PZT/IrO2. The PZT/Pt 620 jC 1 min film (Fig. 2a) differs significantly from the other PZT/Pt samples, in that it is partly comprised of micrometer-scale rosette structures,
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Fig. 2. (a) SEM micrographs of the surface of PZT/Pt films; (b) SEM micrographs showing the pyrochlore-into-perovskite conversion at the surface of the PZT/ IrO2 film, after 1 min crystallization and as a function of temperature; (c) SEM micrographs showing the variation of the resulting grain diameter as a function of temperature for PZT/IrO2, after 30 min crystallization.
resulting in a bimodal grain size distribution. It is clear though that these rosettes are dissimilar to the rosettes observed on the IrO2 substrates in many ways: they are filamentary, (more) porous and not as circular. Numerous reports on Pe PZT rosettes of this sort grown on Pt substrates ascribe their occurrence to the Zr-rich composition [17,18] and/or specific processing temperature profile [19,20] that was used. The latter might explain why only one PZT/Pt sample contains these rosettes. A rosette microstructure is generally not desirable in PZT films grown on Pt bottom electrodes since this often results in poor/reduced ferroelectric properties, as the decrease in (111) preferential orientation [21] for the PZT/Pt 620 jC 1 min sample (Fig. 1a) might already suggest. For the PZT/ IrO2 films on the other hand, in which rosette morphology seems to inhere, good ferroelectric response was obtained ( Pri50 AC/cm2 for the 620 jC 30 min sample). The hysteresis loops were quite leaky though, which is consistent with our findings that these films do not appear to be perovskite single phase. 3.2. Phase evolution on the IrO2 electrode Is there PbO segregation during the crystallization of the PZT/IrO2 thin films, as one could suspect from the diffraction patterns? What does the phase assemblage of the PZT/ IrO2 films look like and how does it arise? And is the IrO2 bottom electrode of any importance concerning orientation selection within the PZT thin film? For the purpose of answering these three questions, a series of PZT/IrO2 films
were prepared by crystallization for 3, 6, 10, 15, 20 and 25 min at 550 jC. Based on the results presented above, neither compositional changes nor differences in texture were observed for PZT/IrO2 films fired between 550 and 620 jC for 30 min (Fig. 1c), so we opted for the lowest of all applied temperatures so far, in order to obtain an evolution as gradual as possible. The PZT/IrO2 550 jC samples were investigated by means of SEM, plan view and cross-sectional TEM and XRD. 3.2.1. PbO segregation SEM images of the 550 jC specimens are depicted in Fig. 3, revealing the presence of white particles at the surface of the PZT film. Qualitative EDX analysis systematically showed these particles to be Pb-rich as compared to their environment. In top view, we did not establish any further compositional difference in the films: measurements on the spots marked X produced identical spectra. At the very beginning of the evolution, the particles exist only on top of the PZT matrix surrounding the perovskite rosettes; they are triangular/rectangular shaped, possibly indicating crystallinity. With increasing annealing time, 6 –10 min, they are found on top of matrix as well as rosettes, being less numerous and irregularly shaped. Notice that the rosettes in the PZT/IrO2 550 jC 6 min sample exhibit remarkable morphology; in that they have a core and an outer ring that both appear to protrude from the film surface. With further structural development, this peculiar rosette structure disappears, and so do the white Pb containing surface particles: in the final stages of the crystallization
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Fig. 4. Cross-sectional TEM micrograph showing the layer structure of an untransformed PZT/IrO2 film after 3 min at 550 jC. 1: Granular Py ‘‘seeding’’ layer, 2: quasi amorphous Py layer, 3: segregated layer.
Fig. 3. SEM micrographs of the PZT/IrO2 550 jC time evolution.
process (20 min), they are only observed at and around grain boundaries, before totally disappearing around 25 min. Fig. 4 is a cross-sectional bright field TEM image of a PZT film fired at 550 jC for 3 min. The inset of this figure shows a selected area electron diffraction (SAED) pattern, which identifies such a Pb-rich surface particle as PbO massicot. Together with this particle, there is a segregated layer present at the surface of the PZT film. Both particle and layer have the same contrast. The nature of the layer remains unrevealed: it was neither possible to perform EDX analysis (see Section 2), nor to obtain an SAED pattern (layer thickness was not sufficient for SA diaphragm). At the same time, the ex situ XRD patterns of Fig. 5 do not suggest any phase segregation after 3 min crystallization at 550 jC. They do show however that, when the PbO particles have vanished by the end of the 550 jC crystallization treatment (30 min), the transient PbO (100) reflection appears. Therefore, we propose that the segregation of lead occurs by the following mechanism: (i) segregation of an amorphous lead or lead oxide top layer (not detected by XRD); (ii) local crystallization of this layer into PbO massicot, giving rise to PbO surface particles; (iii) full crystallization into a (100)-oriented PbO layer; (iv) volatilisation of this layer, indicated by the intermediate apparition of the PbO (100) reflection in the XRD patterns.
3.2.2. Layer structure of PZT/IrO2 The relatively slow Py-into-Pe conversion for PZT/IrO2 (see Fig. 1b and c) allows for a considerable amount of Pb to segregate before the PZT film can fully convert into Pe. The 16% excess Pb in the PZT precursor solution is not sufficient to compensate for these losses. As a consequence of this process, a Pb deficient Py layer is formed in the top region of the PZT film. Since the Pe structure, as opposed to Py, can only tolerate a very limited number of A-site lead deficiencies (less than 2% at 1100 jC [5]), the Py layer is more stable and hence will not undergo any further transformation. This explains the layer structure that was observed in all the PZT/IrO2 films fired at 550 jC, shown in
Fig. 5. XRD patterns showing the 550 jC time evolution of a PZT/IrO2 film.
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Fig. 6. Cross-sectional TEM micrograph of a partially converted PZT/IrO2 film after 6 min crystallization at 550 jC.
Fig. 6 for a sample fired for 6 min. Pe nucleation starts heterogeneously at the bottom electrode. The resulting grains first grow vertically, until they reach the Pb deficient Py layer, and then radially until they impinge on one another, which is a possible indication for the poor— beneficial—influence of the IrO2 substrate on Pe nucleation and growth. The presence of the star shaped contrasts in the SEM images of Fig. 3 might be another indication for this poor influence, since they show that the rosettes develop a substantial amount of internal strain during their growth. This strain probably causes the grains to break up into
I ratioð100Þ ¼
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segments: in the final stages of the film evolution microcracks were observed that coincide with the star shaped strain contrasts. Moreover, because of the built up internal strain, several grains tear off the bottom electrode creating voids, as is the case in the image of Fig. 6. These cracks and voids, together with the earlier mentioned phase impurities, are responsible for the relatively leaky hysteresis loops, recorded for all of the PZT/IrO2 films. One can further see that the PZT film in Fig. 6 consists of two different layers of Py structures. The first one, which was labelled the Py seeding layer, has a granular texture and is crystalline. By SAED on this layer, six diffraction rings corresponding to the oxygen deficient pyrochlore structure Pb2(Zr,Ti)2O6 (JCPDS 26-142) were detected. This is the layer that is able to fully convert into perovskite. The second layer is quasi-amorphous and yields a diffuse diffraction pattern that we could not identify. However, based on our XRD results, this has to be a Py layer as well, which cannot transform into Pe. 3.2.3. Pe texture evolution and influence of the IrO2 electrode It should be clear from previous results that there probably is no good lattice matching between the IrO2 (100)/(110) electrode and Pe 30/70 PZT. Since perovskite nucleation starts at the electrode-film interface though, it was verified if Pe (100) and Pe (101) are the principal orientations during the entire film evolution. In order to do so, the integrated peak intensity ratios of the five main Pe orientations were calculated (Fig. 5), with respect to each other and to their intensities in randomly oriented PZT powders. The intensity ratio for Pe (100) for example, was calculated using the equation:
Ið100Þm =½Ið100Þm þ Ið001Þm þ Ið101Þm þ Ið110Þm þ Ið111Þm Ið100Þpdf =½Ið100Þpdf þ Ið001Þpdf þ Ið101Þpdf þ Ið110Þpdf þ Ið111Þpdf
with Im the measured peak intensity and Ipdf the intensity in randomly oriented PZT powder (JCPDS 50-346). The peak intensity ratios were plotted as a function of crystallization time (Fig. 7). Only Pe (100) is a majority orientation during the entire film evolution, but this effect cannot simply be ascribed to a good match with the IrO2 electrode since PZT is known to exhibit a (100) self-texture, even in powders [19 – 22]. It is further impossible to explain the increase respectively decrease of the Pe (101) and (111) peak intensity ratios with increasing time, on the basis of a lattice match between an accordingly oriented Pe PZT 30/70 unit cell and IrO2 (100)/(110). A study by Meyers and Chapin [17] has shown that rosettes can nucleate from an equiaxed seed grain from which emanate radially lamellae of Pe PZT with different crystallographic orientations. Analogously, the IrO2 electrode could be able to nucleate a (hkl) textured Pe seed,
and still no preferred film orientation would be expected, in accordance with our XRD results. The rosettes that were obtained however do not have a central seed grain.
Fig. 7. Texture evolution of the 550 jC PZT/IrO2 film. An intensity ratio >1 indicates a preferential (‘‘majority’’) orientation, =1 is randomly oriented.
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They are single crystals of Pe PZT, as only a very slight arcing of the electron diffraction spots is observed in the SAED pattern taken (Fig. 8). SAED was carried out on several rosettes and no distinct crystallographic orientation was observed. It is believed that the arcing of the diffraction spots is caused by the internal strain present in the rosettes. This strain divides the rosettes into segments slightly disoriented from one another, as indicated by the presence of the bending contours (Fig. 8). At a tilting angle of 30j for this particular rosette, these diffraction contrasts indicate the position of the segment boundaries, where, later on in the evolution, microcracks will form, as mentioned before in this report. One can further notice that the rosettes have very sharp boundaries. This remarkable appearance of rosette morphology may be a consequence of the diffusion profile during Pe growth, which can be interpreted in two different ways. Firstly, unlike for instance MOD precursors, sol – gel precursors have definite polymeric structures whose ligands remain even at relatively high temperatures. This could severely restrict the mobility of cations with respect to concentration redistribution. As a result, a sharp boundary is maintained during growth (a concentration gradient favours the formation of an undulated interface, giving rise to a fuzzy appearance of the boundary). Secondly, conversion into Pe from another crystalline phase such as Py would require dissolution of the Py nanocrystallites prior to incorporation as part of the growing Pe single crystal. This also could lead to a porous and fuzzy appearance of the boundary. However, since Pe conversion takes place from the oxygen deficient Py phase Pb2(Zr,Ti)2O6 which has a stoichiometry nearly identical to Pe (=2Pb(Zr,Ti)O3), this
transformation does not require diffusion but merely a reordering of the cations within a unit cell.
4. Summary and conclusion In this report, the crystallization of perovskite 30/70 PZT thin films deposited on IrO2/TiO2/SiO2/Si electrodes was studied by means of XRD, SEM and TEM, partly through a comparison with identically processed PZT films deposited on Pt/TiO2/SiO2/Si substrates. It has been shown that the rather low nucleation density for the PZT/IrO2 system increases with increasing annealing temperature. The circular grains, referred to as rosettes, are imperfect single crystals of perovskite PZT that nucleate heterogeneously at the bottom electrode. They develop a considerable amount of internal strain during their evolution, which causes voids and cracking in the final stages of the film development. None of the PZT/IrO2 films, annealed at temperatures ranging from 550 to 620 jC, has a distinct preferential orientation: only a slight (100) texture was observed. These and other observations indicate that the (100) preferentially oriented IrO2 electrode does not match the perovskite 30/70 PZT lattice, and that it has a rather unfavourable influence on the PZT’s crystallization kinetics and morphology. At the end of the crystallization treatment, the PZT film is phase impure. It consists of (i) a perovskite layer, (ii) a probably lead-deficient pyrochlore layer and (iii) a top layer that first crystallizes locally and then fully into PbO massicot. Because of the slow pyrochlore-into-perovskite conversion, diffusion of Pb or PbO to the film surface is substantial. As a consequence, the PZT film is partly lead deficient and hence not able to fully transform into perovskite PZT. These phase impurities, together with the earlier mentioned cracks and voids, are believed to be responsible for the relatively leaky hysteresis loops that were recorded for all of the PZT/IrO2 films. Nevertheless, Pr values up to 50 AC/cm2 were measured. In order to avoid the presence of residual pyrochlore, it seems necessary to accelerate perovskite nucleation and growth with respect to diffusion of Pb (or PbO). In this perspective crystallization treatments at temperatures above 620 jC could be useful, for an increase in the volume fraction of perovskite PZT and a decrease of grain diameter was observed with increasing annealing temperature. Smaller grains are also expected to have less internal stress, which will minimize the possibility of cracking.
Acknowledgements
Fig. 8. Plan view bright-field TEM micrograph and selected area electron diffraction pattern (B=[001], 30j tilt) of the perovskite rosette.
Dr. M.K. Van Bael and Dr. G. Vanhoyland are postdoctoral fellows of the Fund for Scientific Research Flanders, Belgium (FWO). This research is partially financed by ‘Het Vlaamse Gewest-IWT’. The authors wish
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to thank H. Vander Meeren, P. Van Marcke and H. Pellaerts for technical assistance.
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Phase evolution of sol–gel prepared Pb(Zr0.3Ti0.7)O3 thin films deposited on IrO2/TiO2/SiO2/Si electrodes D. Van Genechten a, G. Vanhoyland a, J. D’Haen b, J. Johnson c, D.J. Wouters c, M.K. Van Bael a, H. Van den Rul a, J. Mullens a,*, L.C. Van Poucke a a
Laboratory of Inorganic and Physical Chemistry, IMO, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium b IMO-IMOMEC, Limburgs Universitair Centrum, B-3590 Diepenbeek, Belgium c IMEC, B-3001 Heverlee Leuven, Belgium Received 31 July 2003; received in revised form 17 February 2004; accepted 9 March 2004 Available online 23 April 2004
Abstract We report on the microstructural analyses, by means of X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive X-ray analysis (SEM-EDX) and cross-section transmission electron microscopy (X-TEM), of chemically prepared Pb(Zr0.3Ti0.7)O3 thin films deposited on IrO2 substrates. The purpose of this study is to detail temperature and time dependence of the lead zirconate titanate (PZT) film microstructure on this type of conductive oxide substrate, partly through a comparison with identically processed PZT films deposited on Pt substrates. It was observed that PZT 30/70 films on IrO2 bottom electrodes, fired at temperatures up to 620 jC, are not single phase, due to extensive lead losses during the processing. The IrO2 substrate was found to be indirectly responsible for these losses. Nevertheless, good ferroelectric properties were measured ( Pr was 50 AC/cm2 for the 620 jC film). Based on the observed morphology and texture with increasing annealing time, a mechanism for phase evolution in sol – gel-derived PZT 30/70 films on IrO2 substrates is proposed. D 2004 Elsevier B.V. All rights reserved. PACS: 68.55 Keywords: PZT films; IrO2 electrode; Phase evolution; TEM
1. Introduction The potential applications of ferroelectric lead zirconate titanate (PZT) thin films in a wide range of devices, from nonvolatile semiconductor memories to sensors and actuators [1], have led to much work in trying to develop an understanding of film microstructure/property relationships. Results of such studies have shown that microstructural properties, including grain size and crystallographic orientation, surface topology and porosity, can strongly influence thin film electrical and electro-optical performance [2]. The sol – gel route has emerged as a most versatile method for preparing chemically homogeneous PZT powders as well as thin film coatings. However, one of the major limitations of the sol –gel route, as for most preparation routes for that matter, is that it does not yield the desired perovskite (Pe) phase directly after the pyrolysis of the gel, either in bulk or * Corresponding author. E-mail address: [email protected] (J. Mullens). 0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2004.03.021
in thin film form. The formation of the Pe phase is, instead, mediated by an undesirable, nonferroelectric pyrochlore (Py) phase, which is formed first after pyrolysis [3]. Several different factors, such as pyrolysis and subsequent heat-treatment temperatures [4], annealing atmosphere and Pb content [5], have been proven to influence the transformation of the Py phase into the Pe phase, and hence also the PZT’s final microstructure. In case of thin films, extensive research has been carried out studying the effects of these parameters for PZT deposited on conventional Pt bottom electrodes. From a technological point of view, however, Pt has lost its relevance: in view of a stacked cell layout, necessary for memory cell area reduction, the material does not form a barrier to oxygen diffusion to avoid oxidation of the plug material [6]. Furthermore, when a capacitor is fabricated with a PZT thin film on a Pt electrode, the lifetime and reliability of the device is limited by degradation phenomena such as fatigue and imprint [7]. A conductive oxide electrode, such as IrO2, seems to be the most attractive alternative for nonvolatile memory
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application since it acts as an oxygen donor [8], and provides a good diffusion barrier as well. The former should compensate for the electromigration of oxygen vacancies into sites in planes parallel to the electrodes, which is reported [9] to be the most probable cause for reliability problems such as fatigue. However, control over the PZT/ IrO2 microstructure remains an unsolved issue. Our study represents one of the first attempts to characterize, by means of X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive X-ray analysis (SEMEDX) and cross-section transmission electron microscopy (X-TEM) analysis, sol – gel prepared PZT thin films, deposited on IrO2 substrates and processed at low temperatures.
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diamond saw, epoxied together, mechanically thinned to 60 Am, dimple grinded to 8 Am and then FIB thinned at an angle of 5j. A Pt layer was sputter deposited on top of the PZT to avoid film damage during the final thinning process. TEM work was carried out on a Philips CM12 operated at 120 kV, equipped with an EDAX 9900 spectrometer for chemical analysis. In addition to the TEM analysis, the PZT films were characterized by scanning electron microscopy (Philips XL30 FEG) and X-ray diffraction (Siemens D5000). In order to determine whether a preferential orientation was present, integrated intensities of X-ray diffraction lines were compared with those of randomly oriented Pe PZT powders (JCPDS 50-346).
2. Experimental 3. Results and discussion PZT thin films were fabricated from metallo-organic solutions of dehydrated lead(II)-acetate, zirconium(IV)-npropoxide and titanium(IV)-isopropoxide dissolved in butoxyethanol [10,11]. The PZT 30/70 precursor solution had a concentration of 0.112 M, with a 16 mol% excess amount of lead. Immediately before deposition, the solution was filtered with a 0.45-Am syringe filter. The filtered solution was then deposited onto substrates at 3000 rpm during 30 s, using a Headway photoresist spinner. Each layer was dried at 200 jC for 2 min on a hot plate in air, and then given a 2-min heat treatment at 350 jC under the same conditions, to decompose residual organics. All films in this study consisted of three layers and were approximately 150 –160 nm thick after firing. Cross-section transmission electron microscopy was used to determine film thickness. Twenty-two different films were fabricated to determine phase and texture evolution as a function of time, temperature and substrate. These films were processed identically except for the final crystallization treatment. Eight films were deposited on Pt substrates, eight on IrO2 and subsequently fired at 550, 580, 600 and 620 jC, for 1 and 30 min on a hot plate in static air. Six more were deposited on the IrO2 bottom electrode and crystallized at 550 jC, for 3, 6, 10, 15, 20 and 25 min, respectively. The substrates consisted of 3535 mm (100) silicon wafer sections, onto which a 250-nm-thick SiO2 layer was thermally grown. On top of a 50-nm-thick sputter deposited adhesion layer of TiO2, a 100-nm Pt or a 50-nm IrO2 layer was sputtered to form the bottom electrode. Proper TEM sample preparation was critical for an efficient evaluation of the specimens. Plan view samples were prepared by ion milling with 5 kV Ar ions (Precision Ion Polishing System, PIPS Gatan model 691). However, when this system was used for the thinning of crosssectional samples, it systematically destroyed the PZT film. Consequently, all the X-TEM samples were prepared using a Focussed Ion Beam (FIB, Dual Ion Mill Gatan model 600), with 30 kV Ga ions. The latter implies these samples could not be used for EDX analyses. For cross-sectional views, wafer sections of 2 mm350 Am were cut using a
3.1. Phase purity, texture, nucleation and growth on Pt and IrO2 electrodes While comparing XRD and SEM results, Figs. 1 and 2, respectively, of the PZT/Pt 1 min (a) and PZT/IrO2 1 min (b) temperature evolution, one can clearly notice the influence the substrate has on film morphology, texture and phase transformation kinetics. The crystallization of a PZT film deposited on both types of substrate appears to follow the sequence: amorphous film!pyrochlore (Py)!perovskite (Pe). For PZT/Pt films, however, the conversion is completed at 550 jC 1 min and yields highly (111)-oriented single phase Pe PZT, while the PZT/IrO2 diffraction pattern shows quasi randomly oriented Pe PZT, and still contains a significant peak corresponding to the Py phase at 620 jC 1 min (Fig. 1b). Py diffraction lines are rather broad, indicating the large compositional range and fine-grained nature of this phase. The diffraction peaks of the Pe PZT phase are broader when the film is deposited on Pt. Therefore, the crystalline quality of the PZT film is improved on IrO2. According to h –2h XRD measurements on the bottom electrodes only (though also visible in Fig. 1), the Pt electrode is highly (111)-oriented and the IrO2 has a mixed but preferential (100)/(110) texture, with (100)>(110). As Pt’s lattice constant matches by 0.5% the one of PbTiO3 and by 4% the one of PZT 50/50 at conventional growth temperatures [12], a (111) texture of PZT/Pt is expected to be favoured. The observation of quasi randomly oriented Pe PZT obtained on preferentially oriented IrO2 substrates then raises questions about the possible effects of an IrO2 bottom electrode on orientation selection within PZT thin films. However that may be, after a 30-min crystallization at all of the applied temperatures (Fig. 1c), Pe (100) and (101) are present in a slight majority with respect to the other Pe/IrO2 orientations, in the following order: (100)>(101). The 1-min PZT/IrO2 XRD patterns further reveal the presence of a relatively small, transient peak at 32.5j 2h with a maximum intensity at 600 jC, possibly coming from
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Fig. 1. (a) XRD patterns of PZT/Pt films crystallized at 550, 580, 600 and 620 jC for 1 min; (b) XRD patterns of PZT/IrO2 films crystallized at 550, 580, 600 and 620 jC for 1 min; (c) XRD patterns of PZT/IrO2 films crystallized at 550, 580, 600 and 620 jC for 30 min.
a segregated, volatile PbO massicot phase that obviously produces only one reflection, namely (100). The presence of the same temporary peak at 32.5j 2h in the 30-min PZT/ IrO2 patterns (Fig. 1c) at 550 and 580 jC (and not at 600 and 620 jC) validates this possibility. PbO has been frequently reported to exhibit a tendency to volatilise, due
to its relatively high vapour pressure [13,14]. Furthermore, most authors suggest that the loss of PbO originates mainly from a segregated PbO phase, since the equilibrium vapour pressure of PbO over PbO is two orders of magnitude larger than over PT, and one over PZT [15]. Contrary to what was observed, however, the same authors also agreed on the fact that, when PbO does segregate, it stays in an amorphous phase. The temperature-dependent appearance of the Pe diffraction peaks in the X-ray diffraction patterns coincides with the growth of grains observed on SEM micrographs (Fig. 2b). Crystals that are extremely circular in the plane of the substrate and appear to grow radially with increased annealing temperature are systematically present on the IrO2 substrates; these are referred to as rosettes in this report. The circular morphology is only observed for isolated rosettes, since they grow radially until they impinge on one another, resulting in linear boundaries where they have grown together, as Fig. 2c depicts. Growth at the rosette – rosette interface appears inhibited compared to growth for the rosette – Py interface. Interestingly, as shown by the same figure, PZT/IrO2 films fired at higher temperatures differ from films fired at lower temperatures in that the volume fraction of Pe phase is larger (or stays equal when the film is already fully crystallized in the plane of the substrate) and that their Pe grain diameter is smaller: 2.5– 5 Am in the PZT/IrO2 620 jC 30 min film vs. 15– 18 Am in the PZT/IrO2 550 jC 30 min film. Since Fig. 2b shows that new rosette nuclei form as the existing ones grow, the larger volume fraction of Pe phase is attributed to both an increased nucleation and growth rate with increasing annealing temperature. It should be noted that we use the term ‘‘fully crystallized in the plane of the substrate’’: although it might seem as if the Py phase has been completely transformed into Pe (see PZT/IrO2 SEM micrographs, 580– 620 jC for 30 min, Fig. 2c) untransformed regions of Py do still exist, as shown in the X-ray diffraction patterns in Fig. 1c. The circular island morphology observed in films grown on IrO2 does not occur in films grown on Pt. Instead, a large density of Pe nuclei forms such that only a limited growth of these nuclei throughout the thickness of the film is required to produce the relatively dense grain structure seen in Fig. 2a. The nuclei all appear at the same time, and their resulting grain size is 50 –80 nm, regardless of the annealing temperature. All of the aforementioned observations indicate that the different film morphologies on Pt and IrO2 are probably related to interfacial energy differences between the electrode material and the Pe phase, assuming of course that Pe nucleation begins heterogeneously at the electrode/ film interface. As described in literature, this assumption is generally true for PZT/Pt [16], and Fig. 6, which will be discussed later on (Section 3.2.2), proves this also to be the case for PZT/IrO2. The PZT/Pt 620 jC 1 min film (Fig. 2a) differs significantly from the other PZT/Pt samples, in that it is partly comprised of micrometer-scale rosette structures,
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Fig. 2. (a) SEM micrographs of the surface of PZT/Pt films; (b) SEM micrographs showing the pyrochlore-into-perovskite conversion at the surface of the PZT/ IrO2 film, after 1 min crystallization and as a function of temperature; (c) SEM micrographs showing the variation of the resulting grain diameter as a function of temperature for PZT/IrO2, after 30 min crystallization.
resulting in a bimodal grain size distribution. It is clear though that these rosettes are dissimilar to the rosettes observed on the IrO2 substrates in many ways: they are filamentary, (more) porous and not as circular. Numerous reports on Pe PZT rosettes of this sort grown on Pt substrates ascribe their occurrence to the Zr-rich composition [17,18] and/or specific processing temperature profile [19,20] that was used. The latter might explain why only one PZT/Pt sample contains these rosettes. A rosette microstructure is generally not desirable in PZT films grown on Pt bottom electrodes since this often results in poor/reduced ferroelectric properties, as the decrease in (111) preferential orientation [21] for the PZT/Pt 620 jC 1 min sample (Fig. 1a) might already suggest. For the PZT/ IrO2 films on the other hand, in which rosette morphology seems to inhere, good ferroelectric response was obtained ( Pri50 AC/cm2 for the 620 jC 30 min sample). The hysteresis loops were quite leaky though, which is consistent with our findings that these films do not appear to be perovskite single phase. 3.2. Phase evolution on the IrO2 electrode Is there PbO segregation during the crystallization of the PZT/IrO2 thin films, as one could suspect from the diffraction patterns? What does the phase assemblage of the PZT/ IrO2 films look like and how does it arise? And is the IrO2 bottom electrode of any importance concerning orientation selection within the PZT thin film? For the purpose of answering these three questions, a series of PZT/IrO2 films
were prepared by crystallization for 3, 6, 10, 15, 20 and 25 min at 550 jC. Based on the results presented above, neither compositional changes nor differences in texture were observed for PZT/IrO2 films fired between 550 and 620 jC for 30 min (Fig. 1c), so we opted for the lowest of all applied temperatures so far, in order to obtain an evolution as gradual as possible. The PZT/IrO2 550 jC samples were investigated by means of SEM, plan view and cross-sectional TEM and XRD. 3.2.1. PbO segregation SEM images of the 550 jC specimens are depicted in Fig. 3, revealing the presence of white particles at the surface of the PZT film. Qualitative EDX analysis systematically showed these particles to be Pb-rich as compared to their environment. In top view, we did not establish any further compositional difference in the films: measurements on the spots marked X produced identical spectra. At the very beginning of the evolution, the particles exist only on top of the PZT matrix surrounding the perovskite rosettes; they are triangular/rectangular shaped, possibly indicating crystallinity. With increasing annealing time, 6 –10 min, they are found on top of matrix as well as rosettes, being less numerous and irregularly shaped. Notice that the rosettes in the PZT/IrO2 550 jC 6 min sample exhibit remarkable morphology; in that they have a core and an outer ring that both appear to protrude from the film surface. With further structural development, this peculiar rosette structure disappears, and so do the white Pb containing surface particles: in the final stages of the crystallization
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Fig. 4. Cross-sectional TEM micrograph showing the layer structure of an untransformed PZT/IrO2 film after 3 min at 550 jC. 1: Granular Py ‘‘seeding’’ layer, 2: quasi amorphous Py layer, 3: segregated layer.
Fig. 3. SEM micrographs of the PZT/IrO2 550 jC time evolution.
process (20 min), they are only observed at and around grain boundaries, before totally disappearing around 25 min. Fig. 4 is a cross-sectional bright field TEM image of a PZT film fired at 550 jC for 3 min. The inset of this figure shows a selected area electron diffraction (SAED) pattern, which identifies such a Pb-rich surface particle as PbO massicot. Together with this particle, there is a segregated layer present at the surface of the PZT film. Both particle and layer have the same contrast. The nature of the layer remains unrevealed: it was neither possible to perform EDX analysis (see Section 2), nor to obtain an SAED pattern (layer thickness was not sufficient for SA diaphragm). At the same time, the ex situ XRD patterns of Fig. 5 do not suggest any phase segregation after 3 min crystallization at 550 jC. They do show however that, when the PbO particles have vanished by the end of the 550 jC crystallization treatment (30 min), the transient PbO (100) reflection appears. Therefore, we propose that the segregation of lead occurs by the following mechanism: (i) segregation of an amorphous lead or lead oxide top layer (not detected by XRD); (ii) local crystallization of this layer into PbO massicot, giving rise to PbO surface particles; (iii) full crystallization into a (100)-oriented PbO layer; (iv) volatilisation of this layer, indicated by the intermediate apparition of the PbO (100) reflection in the XRD patterns.
3.2.2. Layer structure of PZT/IrO2 The relatively slow Py-into-Pe conversion for PZT/IrO2 (see Fig. 1b and c) allows for a considerable amount of Pb to segregate before the PZT film can fully convert into Pe. The 16% excess Pb in the PZT precursor solution is not sufficient to compensate for these losses. As a consequence of this process, a Pb deficient Py layer is formed in the top region of the PZT film. Since the Pe structure, as opposed to Py, can only tolerate a very limited number of A-site lead deficiencies (less than 2% at 1100 jC [5]), the Py layer is more stable and hence will not undergo any further transformation. This explains the layer structure that was observed in all the PZT/IrO2 films fired at 550 jC, shown in
Fig. 5. XRD patterns showing the 550 jC time evolution of a PZT/IrO2 film.
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Fig. 6. Cross-sectional TEM micrograph of a partially converted PZT/IrO2 film after 6 min crystallization at 550 jC.
Fig. 6 for a sample fired for 6 min. Pe nucleation starts heterogeneously at the bottom electrode. The resulting grains first grow vertically, until they reach the Pb deficient Py layer, and then radially until they impinge on one another, which is a possible indication for the poor— beneficial—influence of the IrO2 substrate on Pe nucleation and growth. The presence of the star shaped contrasts in the SEM images of Fig. 3 might be another indication for this poor influence, since they show that the rosettes develop a substantial amount of internal strain during their growth. This strain probably causes the grains to break up into
I ratioð100Þ ¼
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segments: in the final stages of the film evolution microcracks were observed that coincide with the star shaped strain contrasts. Moreover, because of the built up internal strain, several grains tear off the bottom electrode creating voids, as is the case in the image of Fig. 6. These cracks and voids, together with the earlier mentioned phase impurities, are responsible for the relatively leaky hysteresis loops, recorded for all of the PZT/IrO2 films. One can further see that the PZT film in Fig. 6 consists of two different layers of Py structures. The first one, which was labelled the Py seeding layer, has a granular texture and is crystalline. By SAED on this layer, six diffraction rings corresponding to the oxygen deficient pyrochlore structure Pb2(Zr,Ti)2O6 (JCPDS 26-142) were detected. This is the layer that is able to fully convert into perovskite. The second layer is quasi-amorphous and yields a diffuse diffraction pattern that we could not identify. However, based on our XRD results, this has to be a Py layer as well, which cannot transform into Pe. 3.2.3. Pe texture evolution and influence of the IrO2 electrode It should be clear from previous results that there probably is no good lattice matching between the IrO2 (100)/(110) electrode and Pe 30/70 PZT. Since perovskite nucleation starts at the electrode-film interface though, it was verified if Pe (100) and Pe (101) are the principal orientations during the entire film evolution. In order to do so, the integrated peak intensity ratios of the five main Pe orientations were calculated (Fig. 5), with respect to each other and to their intensities in randomly oriented PZT powders. The intensity ratio for Pe (100) for example, was calculated using the equation:
Ið100Þm =½Ið100Þm þ Ið001Þm þ Ið101Þm þ Ið110Þm þ Ið111Þm Ið100Þpdf =½Ið100Þpdf þ Ið001Þpdf þ Ið101Þpdf þ Ið110Þpdf þ Ið111Þpdf
with Im the measured peak intensity and Ipdf the intensity in randomly oriented PZT powder (JCPDS 50-346). The peak intensity ratios were plotted as a function of crystallization time (Fig. 7). Only Pe (100) is a majority orientation during the entire film evolution, but this effect cannot simply be ascribed to a good match with the IrO2 electrode since PZT is known to exhibit a (100) self-texture, even in powders [19 – 22]. It is further impossible to explain the increase respectively decrease of the Pe (101) and (111) peak intensity ratios with increasing time, on the basis of a lattice match between an accordingly oriented Pe PZT 30/70 unit cell and IrO2 (100)/(110). A study by Meyers and Chapin [17] has shown that rosettes can nucleate from an equiaxed seed grain from which emanate radially lamellae of Pe PZT with different crystallographic orientations. Analogously, the IrO2 electrode could be able to nucleate a (hkl) textured Pe seed,
and still no preferred film orientation would be expected, in accordance with our XRD results. The rosettes that were obtained however do not have a central seed grain.
Fig. 7. Texture evolution of the 550 jC PZT/IrO2 film. An intensity ratio >1 indicates a preferential (‘‘majority’’) orientation, =1 is randomly oriented.
110
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They are single crystals of Pe PZT, as only a very slight arcing of the electron diffraction spots is observed in the SAED pattern taken (Fig. 8). SAED was carried out on several rosettes and no distinct crystallographic orientation was observed. It is believed that the arcing of the diffraction spots is caused by the internal strain present in the rosettes. This strain divides the rosettes into segments slightly disoriented from one another, as indicated by the presence of the bending contours (Fig. 8). At a tilting angle of 30j for this particular rosette, these diffraction contrasts indicate the position of the segment boundaries, where, later on in the evolution, microcracks will form, as mentioned before in this report. One can further notice that the rosettes have very sharp boundaries. This remarkable appearance of rosette morphology may be a consequence of the diffusion profile during Pe growth, which can be interpreted in two different ways. Firstly, unlike for instance MOD precursors, sol – gel precursors have definite polymeric structures whose ligands remain even at relatively high temperatures. This could severely restrict the mobility of cations with respect to concentration redistribution. As a result, a sharp boundary is maintained during growth (a concentration gradient favours the formation of an undulated interface, giving rise to a fuzzy appearance of the boundary). Secondly, conversion into Pe from another crystalline phase such as Py would require dissolution of the Py nanocrystallites prior to incorporation as part of the growing Pe single crystal. This also could lead to a porous and fuzzy appearance of the boundary. However, since Pe conversion takes place from the oxygen deficient Py phase Pb2(Zr,Ti)2O6 which has a stoichiometry nearly identical to Pe (=2Pb(Zr,Ti)O3), this
transformation does not require diffusion but merely a reordering of the cations within a unit cell.
4. Summary and conclusion In this report, the crystallization of perovskite 30/70 PZT thin films deposited on IrO2/TiO2/SiO2/Si electrodes was studied by means of XRD, SEM and TEM, partly through a comparison with identically processed PZT films deposited on Pt/TiO2/SiO2/Si substrates. It has been shown that the rather low nucleation density for the PZT/IrO2 system increases with increasing annealing temperature. The circular grains, referred to as rosettes, are imperfect single crystals of perovskite PZT that nucleate heterogeneously at the bottom electrode. They develop a considerable amount of internal strain during their evolution, which causes voids and cracking in the final stages of the film development. None of the PZT/IrO2 films, annealed at temperatures ranging from 550 to 620 jC, has a distinct preferential orientation: only a slight (100) texture was observed. These and other observations indicate that the (100) preferentially oriented IrO2 electrode does not match the perovskite 30/70 PZT lattice, and that it has a rather unfavourable influence on the PZT’s crystallization kinetics and morphology. At the end of the crystallization treatment, the PZT film is phase impure. It consists of (i) a perovskite layer, (ii) a probably lead-deficient pyrochlore layer and (iii) a top layer that first crystallizes locally and then fully into PbO massicot. Because of the slow pyrochlore-into-perovskite conversion, diffusion of Pb or PbO to the film surface is substantial. As a consequence, the PZT film is partly lead deficient and hence not able to fully transform into perovskite PZT. These phase impurities, together with the earlier mentioned cracks and voids, are believed to be responsible for the relatively leaky hysteresis loops that were recorded for all of the PZT/IrO2 films. Nevertheless, Pr values up to 50 AC/cm2 were measured. In order to avoid the presence of residual pyrochlore, it seems necessary to accelerate perovskite nucleation and growth with respect to diffusion of Pb (or PbO). In this perspective crystallization treatments at temperatures above 620 jC could be useful, for an increase in the volume fraction of perovskite PZT and a decrease of grain diameter was observed with increasing annealing temperature. Smaller grains are also expected to have less internal stress, which will minimize the possibility of cracking.
Acknowledgements
Fig. 8. Plan view bright-field TEM micrograph and selected area electron diffraction pattern (B=[001], 30j tilt) of the perovskite rosette.
Dr. M.K. Van Bael and Dr. G. Vanhoyland are postdoctoral fellows of the Fund for Scientific Research Flanders, Belgium (FWO). This research is partially financed by ‘Het Vlaamse Gewest-IWT’. The authors wish
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to thank H. Vander Meeren, P. Van Marcke and H. Pellaerts for technical assistance.
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