Full text of DOI:10.1557/JMR.2002.0306

Microstructural evolution during pyrolysis of triol-based
sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
Zhaoxia Zhou and Ian M. Reaneya)
Department of Engineering Materials, University of Sheffield, Sheffield S1 3JD, U.K.
David Hind, Steven J. Milne, Andy P. Brown, and Rik Brydson
Department of Materials, University of Leeds, Leeds LS2 9JT, U.K.
(Received 24 January 2002; accepted 28 May 2002)
Advanced analytical transmission electron microscopy has been used to investigate
microstructural evolution during pyrolysis in triol-based sol-gel thin films. At pyrolysis
temperatures up to 300 °C, the films remained amorphous; however, nanometer-sized
precipitates were observed in films heat-treated up to 400 °C for 10 min. Analytical
transmission electron microscopy indicated that the precipitates were Pb-rich, as well
as deficient in O, Ti, and Zr. Films pyrolyzed up to 500 °C for 10 min were composed
of a nanocrystalline pyrochlore phase; however, pores could be observed, situated in
the same position as the nanometer-sized precipitates at 400 °C. Face-centered cubic
Pb-rich crystallites were also present on the surface of pyrolyzed films but absent in
the fully crystallized films annealed at 650 °C. A tentative mechanism is proposed to
explain these observations.
I. INTRODUCTION
distribution of all components in the amorphous PZT,
contradicting the aim of sol-gel processing which is to
have an intimate, homogeneous mix of all elements prior
to crystallization.
Lead zirconate titanate (PZT) thin films have been
widely investigated for use as nonvolatile memories,
thermal imaging arrays, and piezoelectric micro- or
nanoactuators.1,2 Sol-gel techniques have been used ex-
tensively for the preparation of PZT thin films due to
advantages such as high purity, ease of compositional
control, low processing temperature, and low capital
cost.3,4 Sol-gel processing of PZT thin films usually in-
volves coating using a solution precursor and pyrolysis
of the dried gel at approximately 400 °C, followed by
crystallization of the layers by annealing at between 500
and 700 °C. Pyrolysis is an important and complicated
step transforming amorphous metalorganic precursors
into inorganic glassy materials, ready for crystallization.
In PZT gels, differential thermal analysis shows exother-
mic peaks between 400 and 500 °C due to combustion of
organics.5–7 It has been reported that PZT gel samples
prepared in methoxyethanol often exhibit a black color in
the temperature range 250–500 °C, resulting from the
formation of carbon residues, which may be prevented by
steam treatment of the dried gels at 300 °C.6 Some au-
thors have found elemental Pb from the reduction of Pb2+
by organic residues during pyrolysis, particularly when
samples are rapidly heated to 400 °C.8,9 However, the
formation of Pb (and PbO) destroys the homogeneous
A triol-based sol-gel route has been recently devised
by Milne et al.,7,10 which is capable of producing crack-
free single layer films of up to 500-nm thick. However,
pores and formation of second phases remain a problem
in films ജ100 nm.11 This work characterizes the micro-
structural evolution of trio-based sol-gel thin films dur-
ing pyrolysis with a view to understand the mechanisms
that lead to the formation of porosity and second phases.
Conventional transmission electron microscopy (TEM)
imaging coupled with parallel electron energy loss spec-
troscopy (PEELS), energy-filtered TEM (EFTEM),
nanoprobe energy-dispersive x-ray spectroscopy (EDX),
and high-angle annular dark-field imaging (HAADF)
was used to characterize as-deposited films heat treated
between 200 and 500 °C.
II. EXPERIMENTAL PROCEDURE
A. Preparation of thin films
The triol-based sol-gel route used to prepare thin
films for this work has been reported previously.7 The
starting reagents for a solution of PZT (53/47) were
lead(II) acetate trihydrate, titanium(IV) diisopropoxide
bis(acetylacetonate), zirconium(IV) n-propoxide,
2,4-pentanedione, and 1,1,1-tris(hydroxymethyl)ethane.
A 15 mol% excess of lead(II) acetate was used to com-
pensate for PbO loss by volatilization during film firing.
a)Address all correspondence to this author.
e-mail: i.m.reaney@sheffield.ac.uk
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Z. Zhou et al.: Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
Single-layer films were deposited by spin coating the
solution onto a platinized silicon substrate, giving rise to
a film cross-sectional configuration of PZT/Pt(111)/Ti/
SiO2/Si(100). The coated wet gel substrates were trans-
ferred to a hot plate and heat-treated according to the
schedules shown in Table I. The first four heat treatments
were employed to elucidate the evolution of the micro-
structure as a function of pyrolysis heating temperature,
and the last was employed as a control to show the mi-
crostructure of a typical heat treatment consisting of
separate pyrolysis and crystallization steps.
B. Characterization
X-ray diffraction (XRD) was performed on a Phillips
ADP700, using Cu K␣ radiation for phase analysis (step
scans from 20° to 60° in 2␪, at 0.02° increment). Cross-
sectional samples of the thin films for TEM observa-
tion were prepared according to Reaney.12,13 Two pieces
were fractured from a film/substrate and glued together
using an epoxy resin, with films face to face. The result-
ing package was mounted onto a disc grinder (Gatan,
model 623) and then ground and polished to give a thick-
ness of 20 ␮m. After being glued with a copper support
ring, the foils were ion-beam thinned from both sides at
an incidence angle of 15° by a Gatan duo ion-miller until
FIG. 1. XRD patterns of films with a series of heat-treatment sched-
ules: (a) 200 °C/10 min; (b) 200 °C/10 min + 300 °C/10 min;
(c) 200 °C/10 min + 300 °C/10 min + 400 °C/10 min; (d) 200 °C/
10 min + 300 °C/10 min + 400 °C/10 min + 500 °C/10 min; (e)
close to perforation, followed by final perforation using a
Gatan precision ion polishing system (PIPS). The
samples were examined using JEOL 3010 (LaB6,
300 kV), and 2010 (field emission gun, 200 kV) and
Phillips CM200 (field emission gun, 200 kV) TEMs, re-
spectively. The latter two TEMs were equipped with a
Gatan image filter (GIF, for PEELS and EFTEM) as well
as EDX detectors (Oxford Instruments). The JEOL 2010
FEGTEM is fitted with a HAADF detector, which allows
Z-contrast imaging.
400 °C/10 min + 650 °C/5 min.
amorphous, with a broad hump at approximately 29° (2␪)
[just apparent in Figs. 1(a) and 1(b)]. Films pyrolyzed at
up to 400 °C for 10 min [Fig. 1(c)] still exhibit a broad
amorphous lump, but an additional peak occurs at ap-
proximately 38.4° (2␪) attributed to Pt3Pb(111).
14,15 Close
inspection of the pattern at 31.2° (2␪) also reveals an
extremely weak intensity barely discernible from the
background. An expanded section of the spectrum is in-
set in Fig. 1 to highlight this peak which has a d-spacing
of 0.286 nm, similar to metallic Pb, d(111) ס 0.2855 nm.
Films pyrolyzed up to 500 °C for 10 min gave a further
peak previously attributed to pyrochlore12,16 [Fig. 1(d)].
In addition, the peak of Pt3Pb(111) decreased. Films py-
rolyzed at 400 °C/10 min followed by crystallization at
650 °C/5 min exhibited additional peaks typical of
perovskite PZT [Fig. 1(e)].17
III. RESULTS AND DISCUSSION
A. XRD
Figure 1 shows the XRD patterns of films prepared
using different heat treatment schedules. Consistent
throughout all patterns is a peak at approximately 40°
(2␪) from the polycrystalline Pt layer, which has a strong
(111) orientation. Films pyrolyzed at up to 300 °C were
TABLE I. Heat-treatment schedules of triol-based thin films.
Films
Heat-treatment schedules
B. TEM
a
b
c
d
200 °C/10 min
200 °C/10 min + 300 °C/10 min
200 °C/10 min + 300 °C/10 min + 400 °C/10 min
200 °C/10 min + 300 °C/10 min + 400 °C/10 min + 500 °C/
10 min
1. Microstructural evolution
a. Film interior
Figure 2 shows representative TEM images and cor-
responding selected area electron diffraction patterns
from cross sections of films with different pyrolysis
e
400 °C/10 min + 650 °C/5 min
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Z. Zhou et al.: Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
heating schedules. From Fig. 2, it is evident that the films
decrease in thickness as pyrolysis heating temperature
and time are increased. A profile of film thickness as a
function of each successive pyrolysis temperature is il-
lustrated in Fig. 3. The thickness of the crystallized films
is included for comparison. A 7% shrinkage occurred
between 200 °C/10 min and 300 °C/10 min. However,
most shrinkage took place in the ranges of 300 ꢀ 400 °C
and 400 ꢀ 500 °C, 53% and 38% of the total, respec-
tively. Little difference in film thickness was detected
between films pyrolyzed up to 500 °C and the films crys-
tallized at 650 °C.
PZT is amorphous. Figure 2(b) shows a typical TEM
image from a film pyrolyzed at 200 °C/10 min + 300 °C/
10 min + 400 °C/10 min. Spheres (approximately 10 nm)
features are situated mainly at the lower half of the film
toward the Pt bottom electrode. Most spheres manifested
dark contrast, but near the film surface, some exhibiting
bright contrast could be found. Electron diffraction pat-
terns [inset of Fig. 2(b)] revealed that the films were still
mainly amorphous, but additional discrete diffraction
spots were observed close to the first ring, indicating the
presence of some crystallites. Figure 2(c) shows a typical
image of films with pyrolyzed at 200 °C/10 min +
300 °C/10 min + 400 °C/10 min + 500 °C/10 min. Films
displayed a similar microstructure as in the previous film
shown in Fig. 2(b), but the contrast of the spheres was
reversed from dark to bright. Electron diffraction patterns
Figure 2(a) shows a bright-field TEM image of films
heat-treated for 200 °C/10 min + 300 °C/10 min. The
film microstructure was featureless, and the associated
selected area diffraction pattern (inset) confirms that the
FIG. 2. Representative TEM images and corresponding selected area electron diffraction patterns from cross sections of films with different
pyrolysis heating schedules: (a) 200 °C/10 min + 300 °C/10 min; (b) 200 °C/10 min + 300 °C/10 min + 400 °C/10 min; (c) 200 °C/10 min +
300 °C/10 min + 400 °C/10 min + 500 °C/10 min; (d) 400 °C/10 min + 650 °C/5 min.
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Z. Zhou et al.: Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
[inset of Fig. 2(c)] indicated that the film matrix was now
composed of nanocrystalline pyrochlore phase in agree-
ment with XRD data. Figure 2(d) shows a TEM cross-
sectional image of a crystallized film (400 °C/10 min +
650 °C/5 min), which was composed mainly of a perov-
skite phase, as illustrated by the inset [100]PZT selected
area electron diffraction pattern. However, pockets of
nanocrystalline pyrochlore phase were also present near
the film surface together with pores 10–40 nm in diam-
eter situated in the middle of the films at PZT grain
boundaries.
b. Film surface
Further examination of Fig. 2 reveals that some films
have faceted particles present on their surface, depending
on the pyrolysis condition. At 300 °C, only a few crystals
are found at the surface, but coincident with the maxi-
mum shrinkage, faceted crystals are present in samples
heat-treated at 400 °C. The crystals remain up to 500 °C
but are absent in the samples heat-treated at 650 °C.
From Fig. 2 it is evident that a complex series of micro-
structural and geometric changes occurs in the films as a
function of increasing pyrolysis temperature. Before the
mechanism of crystallization may be fully understood,
the origins of the microstructural features must be
identified.
2. Characterization of microstructural features
a. Spheres in film interior
FIG. 3. Profile of film thickness as a function of pyrolysis tempera-
ture: (a) 200 °C/10 min + 300 °C/10 min; (b) 200 °C/10 min + 300 °C/
10 min + 400 °C/10 min; (c) 200 °C/10 min + 300 °C/10 min +
400 °C/10 min + 500 °C/10 min; (d) 400 °C/10 min + 650 °C/5 min.
Figure 4 illustrates four typical high-resolution electron
micrographs (HREM) from the spheres in films pyro-
lyzed up to 400 °C. Spheres with dark contrast yielded
FIG. 4. HREM micrographs of spheres with (a)–(c) dark and (d) bright contrast.
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Z. Zhou et al.: Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
lattice images [Figs. 4(a)–4(c)], defining them as crys-
talline, whereas all those with bright contrast appeared
amorphous [Fig. 4(d)]. Furthermore, the average diam-
eter of the bright spheres (approximately 8 nm) was
smaller than the dark (approximately 10 nm). Most of the
dark spheres were composed of a number of individual
crystallites, Figs. 4(b) and 4(c), which contained two or
three different lattice spacing and orientations. The d-
spacings of the crystallites shown on the HREM images
were measured and calibrated with respect to a Si(111).
Typically, values between 0.30 and 0.32 nm were found.
Figure 5(a) presents a typical bright-field TEM image
showing a film cross section containing a mixture of
spheres with both bright and dark contrast. The density/
thickness map [Fig. 5(b)] reveals that spheres with dark
contrast, e.g., A, were denser than those with bright con-
trast, e.g., B. Elemental maps [Figs. 4(c)–4(e)] suggested
all the spheres were O-, Ti-, and Zr-deficient with respect
to the matrix. However, a Pb map could not be obtained
because Pb has a delayed major edge at a high energy
loss (Pb M4,5 2484 eV).18,19 Instead, EDX analysis with
an electron beam probe size of 1 nm was performed on
the spheres. EDX spectra of the spheres with dark con-
trast, the surrounding matrix, and the adjacent spheres
with bright contrast are shown in Figs. 6(a)–6(c), respec-
tively. All the spectra were acquired under the same con-
ditions. Spheres with bright contrast gave spectra with a
poor signal/noise ratio in regions nominally of the same
thickness (50–100 nm). To quantitatively compare the
spectra, the relative integrated intensity for each peak
was calculated as I/IZr + ITi) by assuming that the sum of
Zr and Ti content would stay roughly the same, since
together they constitute a fully occupied B-site of the
ABO3 formula unit. The comparison is shown in Table II.
Because the spheres were approximately 10-nm diameter
and located in a matrix approximately 50-nm thick, it
was estimated that the electron beam broadening would
be approximately 30 nm when passing through the
FIG. 6. EDX spectra of (a) the spheres in dark contrast, (b) the sur-
rounding matrix, and (c) the adjacent spheres in bright contrast.
TABLE II. Comparison of relative integrated intensity of peaks of Pb,
Ti, and Zr in EDX spectra (Fig. 6).
Relative intensity
Location
IPb/(IZr + ITi)
IZr /(IZr + ITi)
ITi/(IZr + ITi)
FIG. 5. (a) Bright-field TEM image of the films showing cross sec-
tions containing a mixture of spheres in both dark and bright contrast.
Dark spheres
Bright spheres
Matrix
2.5
0.5
0.7
0.7
0.7
0.7
0.3
0.3
0.3
(b) The corresponding thickness map, where (c–e) are O-K, Ti-L2,3
,
and Zr-L2,3 energy loss maps, respectively.
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Z. Zhou et al.: Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
specimen.20 It was anticipated therefore that the acquired
EDS spectra were composed of a contribution from both
the targeted precipitates and the matrix. Despite this con-
tribution, however, spheres with dark contrast were al-
ways Pb-rich, whereas the matrix and bright spheres
were correspondingly Pb-deficient. It is well known that
prolonged exposure of PZT to the electron beam (par-
ticularly a high-energy focused beam) results in PbO
volatilization.21 The observation of a Pb-deficient matrix
is perhaps therefore not surprising. However, the large
Pb concentration in the dark spheres coupled with the
matrix contribution strongly indicates that they are Pb-
rich. In addition, they are Zr-, Ti, and O-deficient, ex-
cluding therefore any possibility they are the same
composition as the matrix. The spheres with bright con-
trast are less dense, give EDX spectra with a poor signal/
noise in regions of nominally the same average thickness,
and have a composition quantitatively similar to the ma-
trix. It is therefore concluded the bright spheres are pores.
b. Surface crystals
A high-magnification image of particles on the surface
of a sample heat-treated to 400 °C is shown in Fig. 7(a).
They are faceted, and EDX analysis [Fig. 7(b)] suggests
they are Pb-rich. The carbon peak may be an artifact of
the analysis arising from hydrocarbon contamination in the
microscope. Wu et al.,22 using Rutherford back-scattered
spectrometry (RBS), detected an enrichment of Pb at the
surface of as-deposited thin films prepared by a sol-gel
route, which disappeared after annealing at 550 °C/
30 min. However, they failed to identify the Pb-rich
phase. Figure 8 shows selected area diffraction patterns
FIG. 8. Selected area diffraction patterns from the faceted particles
growing on the film surface. All patterns could be indexed according
to a fcc lattice with a ס 0.61 nm: (a) [100]; (b) [101]; (c) [103]; (d)
[211]; (e) [114] zone.
FIG. 7. TEM images of (a) particles on the surface of the films with
(b) the accompanying EDX spectrum.
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Z. Zhou et al.: Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
from the particles. All diffraction patterns could be in-
dexed according to a face-centered cubic lattice with
a ס 0.61 nm. Furthermore, tilting experiments revealed
that the angles between Figs. 8(a) and 8(b) ([001] and
[103]) and between Figs. 8(c) and 8(d) ([103] and [101])
are 18° and 27°, respectively, as predicted from an fcc
structure. These data however do not correspond to any
known Pb-containing phase.
parallel to a [110] zone with (111) fringes dominant in
the image. Note how the Pt3Pb layer and the underlying
Pt have coincident fringes, suggesting an approximate
cube//cube relationship for the fcc lattices. The distortion
away from a perfect cube//cube relationship is due to
slight difference in the lattice parameter between Pt3Pb
and Pt. The presence of a Pt3Pb phase indicates that
strong reducing conditions apply at 400 °C, sufficient to
form metallic Pb.
c. Pt/PZT interface
d. High-angle annular dark-field imaging
The Pt/PZT interface was also characterized. Figure 9
shows a HREM image of an interfacial layer between the
Pt electrode and amorphous PZT from a sample heat-
treated up to 400 °C. EDX analysis (approximately 1-nm
probe size) of the Pt layer, the intermediate phase, and
PZT layer qualitatively identified the phase as being of
stoichiometry Pt3Pb, as reported by Huang et al.14,15 and
Kaewchinda et al.23 The HREM image is approximately
Films heat-treated up to 400 °C were also examined
using a high-angle annular dark-field detector. Figure 10
shows a cross section of the entire film together with the
substrate recorded with (a) bright-field and (b) dark-field
detectors. The precipitates on the surface and in the bulk
of the film appear bright, revealing that they contain
heavy atoms. This confirms the energy-filtered elemental
maps and EDX analysis in the previous sections (a) and
(b), which indicated that they were both Pb-rich. In ad-
dition, there is a bright layer between PZT and Pt con-
sistent with the presence of a Pt3Pb layer. Surprisingly,
the Ti adhesion layer also exhibits bright contrast, which
suggests that back-diffusion of Pb has occurred.
C. Crystallization mechanism
A tentative mechanism for the crystallization of the
triol-based sol-gel films is schematically illustrated in
Fig. 11. Films pyrolyzed up to 300 °C are mainly amor-
phous (stage I), but at 400 °C, decomposition of the or-
ganic occurs and Pb-rich precipitates are nucleated in the
film interior (stage II). Under the prevailing reducing
conditions, confirmed by the presence of Pt3Pb at the
Pt/PZT interface, the Pb-rich precipitates are most likely
to be metallic. In addition, a broad faint peak at 31.2° in
2␪ in XRD patterns is present which could be attributed
to Pb(111) 0.2855 nm. However, lattice images demon-
strate that the precipitates are composed of multiple crys-
tallites and that the d-spacings are closer to PbO
(0.31 nm) than Pb. It is proposed therefore that TEM
foils originally contained metallic Pb but that oxidation
FIG. 9. HREM image of interfacial layer between the PZT and Pt
substrate showing a Pt3Pb interphase.
FIG. 10. HAADF images of entire film and substrate (heat-treated up to 400 °C) recorded with (a) bright-field and (b) dark-field detectors.
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Z. Zhou et al.: Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
FIG. 11. Schematic representation of the microstructural evolution during the early stage of pyrolysis.
occurs almost instantly on exposure to the atmosphere.
This premise would explain the results from the electron
energy filtered elemental maps, which suggests they are
O-deficient with respect to the matrix. It also explains the
occurrence of multiple crystallites in the precipitates and
the presence of d-spacings closer to PbO than Pb.
Any metallic Pb at 400 °C not bound in the Pt3Pb layer
would gradually oxidize. The rate of oxidation would
depend on temperature and distance from the film sur-
face. Pb expelled from the film is likely therefore to be in
the form of an oxide, which results in highly faceted
Pb-rich precipitates growing on the film surface. At
500 °C (stage III), pyrolysis is complete and the film is
no longer under reducing conditions. The metallic Pb in
the dark precipitates has completely oxidized by the out-
ward diffusion of Pb2+ ions, resulting in pores. In addi-
tion, the matrix is no longer organically modified and is
comparatively rigid. The pores therefore remain embed-
ded in the matrix and are residual upon crystallization to
perovskite. A combination of inhomogeneity and coales-
cence of these pores results in the final film microstruc-
ture (stage IV).
amorphous state, collapsing of the pores may occur
coupled with homogenization of the Pb2+ throughout the
film. The mechanism is however tentative; further work
is required to fully understand the origins of the porosity.
IV. CONCLUSIONS
Microstructural evolution during pyrolysis (200–
500 °C) in triol-based sol-gel thin films has been inves-
tigated by XRD and TEM. At pyrolysis temperatures up
to 300 °C, the films remained amorphous; however,
nanometer-sized precipitates were observed in films
heat-treated up to 400 °C for 10 min. Analytical trans-
mission electron microscopy indicated that the precipi-
tates were Pb-rich, as well as deficient in O, Ti, and Zr.
Pt3Pb forms during pyrolysis confirming that reducing
conditions are present. Films pyrolyzed up to 500 °C for
10 min were composed of a nanocrystalline pyrochlore
phase. However, pores could be observed, situated in the
same position as the nanometer-sized precipitates at
400 °C. Face-centered cubic Pb-rich crystallites were
also present on the surface of pyrolyzed films. The ex-
istence of porosity in triol-based films may arise from
Pb-rich precipitates that form during pyrolysis. Inhomo-
geneity of the pyrolyzed microstructure is responsible for
inhomogeneity in the crystallized film. Homogenization
of the film while in amorphous state by pyrolysis at
400 °C for longer times should improve the final crys-
tallized microstructure.
Two main predictions may be drawn from this mecha-
nism: (i) Any pores residual after pyrolysis will remain in
the final microstructure. (ii) The Pb/Pb2+ distribution will
be inhomogeneous. Therefore, it is suggested that longer
and more controlled pyrolysis around 400 °C will im-
prove the film microstructure by allowing the slow reoxi-
dation of Pb ꢀ Pb2+. If this occurs in a semirigid
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Z. Zhou et al.: Microstructural evolution during pyrolysis of triol-based sol-gel single-layer Pb(Zr0.53Ti0.47)O3 thin films
10. S. Arscott, R.E. Miles, and S.J. Milne, Ferroelectrics, 228, 61
ACKNOWLEDGMENTS
(1999).
This work was supported by the Engineering and
Physical Science Research Council (EPSRC) of the U.K.
by Grant GR/M33501/101. The authors acknowledge
Dr. I.C. Ross, Department of Engineering Materials,
and Dr. T-S. Wang and Mr. A. Walker, Department
of Electronic and Electrical Engineering, University of
Sheffield, for discussions and technical assistance.
11. Z. Zhou, I.M. Reaney, D. Hind, J. Khemprist, and S.J. Milne,
J. Am. Ceram. Soc (submitted for publication).
12. I.M. Reaney, Microelectron. Eng. 29, 277 (1995).
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15. Z. Huang, Q. Zhang, and R.W. Whatmore, J. Appl. Phy. 86, 1662
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