Full text of DOI:10.1111/j.1151-2916.2003.tb03514.x

J. Am. Ceram. Soc., 86 [9] 1560–66 (2003)
journal
Properties of Lead Zirconate Titanate Thin Films Prepared
Using a Triol Sol–Gel Route
Manoch Naksata, Rik Brydson, and Steven John Milne
Department of Materials, University of Leeds, Leeds, LS2 9JT, United Kingdom
Microstructure and phase development during the thermal
decomposition of sol–gel precursor coatings of
mirror-like surfaces. Because of faster rates of gelation, the triol
sols exhibit more favorable coating characteristics than diol
precursors, and we have found it easier to fabricate high-quality
films using the triol process.
PbZr0.53Ti0.47O3 on platinized silicon substrates have been
investigated for a triol sol–gel route. The single-layer, 0.4 ␮m
PZT films were heated from below the substrate, over the
temperature range 350–600°C, using a calibrated hot plate.
The first crystalline phase to appear was a PbPt3 intermetallic
phase at the Pt/PZT interface. Although perovskite PZT
formed at ca. 500°C, heating at higher temperatures, for
example 550°C for 30 min, was required to develop ferroelec-
tric hysteresis loops. However, the rather low value of rema-
nent polarization, Pr ؍ 11 ␮C⅐cm؊2, was consistent with
incomplete crystallization at 550°C. The values of remanent
polarization increased with increasing processing tempera-
tures, reaching 21 ␮C⅐cm؊2 for samples heated at 600°C, with
a corresponding Ec value of 57 kV⅐cm–1. Distinctive spherical
precipitates up to ca. 50 nm in size have been identified by
TEM in the lower portions of otherwise amorphous coatings,
after heating at around 350–400°C. Although their precise
composition could not be identified, they were mostly Pb-rich,
and it is speculated that they form due to reduction of some of
the lead(II) acetate starting reagent, to atomic Pb during the
early stages of thermal decomposition of the organic compo-
nents of the gel; it is possible that subsequent reactions occur
to form lead oxides or carbonates. High levels of porosity were
present in many of the fully crystallized films. The possible
reasons for this are discussed.
A feature of sol–gel derived PZT films is a characteristic
preferred orientation observed in X-ray diffraction patterns. Often
this is imparted by the underlying substrate/electrode configura-
tion; in the present work the substrate was Pt/Ti/SiO2/Si, in which
the polycrystalline platinum electrode had crystallized with a (111)
orientation in the plane of the substrate. During thermal processing
of the coated substrate, some of the original lead acetate in the
sol–gel coating is reduced to atomic lead at low temperatures, and
this then reacts with the Pt layer to form a PbPt3 intermetallic
phase, also displaying (111) orientation,7–10 before being oxidized
at higher temperatures. Because of lattice matching between PZT
and Pt, and more specifically PbPt3,11 there is a tendency for PZT
nucleation to proceed initially by heterogeneous nucleation at the
electrode interface. Because of the (111) orientation of the Pt-
based bottom electrode, the coatings preferentially nucleate with a
(111) orientation at the bottom interface, since the energy barrier
for (111) oriented nuclei is lowered by lattice matching. Other
orientations later nucleate within the bulk of the film. The relative
extent of interfacial oriented nucleation and growth, in comparison
with nucleation in the bulk of the film, is dependent on process
conditions.
Some workers have reported that a PtTix interface layer can
form, which also influences PZT film orientation.12 Furthermore,
under certain conditions it is also possible to produce strongly
(100) oriented PZT films, which may indicate that crystallization
has occurred in the absence of the PbPt3 interfacial layer.
In the present work we report on phase development, micro-
structure, and ferroelectric properties for thin films produced by
heating triol gel coatings on platinized silicon substrates using a
simple form of rapid thermal annealing in which all heating steps
are performed using a calibrated hot plate. Usually, after a first
heat treatment on a hot plate at 300–400°C, the sample would be
transferred to a furnace, or a rapid thermal annealing unit, to
complete the crystallization process. Potentially, performing all
thermal treatments on the one hot plate is a convenient means of
processing sol–gel thin films, but may produce differences in film
properties.
I. Introduction
EAD ZIRCONATE TITANATE (PZT) thin films have potential
L
applications in various microelectronic systems, including
capacitors, ferroelectric memory devices, piezoelectric micro-
actuators, and pyroelectric thermal sensors. For a number of years
we have been studying sol–gel routes based on 1,3-propanediol
solutions of lead acetate, zirconium propoxide, titanium propoxide,
and 2,4-pentanedione (acetylacetone). The diol-based sol–gel
route1,2 offers the advantage of preparing thicker films than other,
more established sol–gel routes.3,4
Another sol–gel route, based on the triol 1,1,1-
tris(hydroxymethyl)ethane, CH3C(CH2OH)3, has recently been
developed
in
this
laboratory.5,6
The
new
1,1,1-
tris(hydroxymethyl)ethane, or “triol” route produces crack-free,
single-layer films up to ca. 1 ␮m in thickness, which is a value
similar to that of the diol route. However, the incidence of other
macrodefects, such as surface pores, increases with thickness,
imposing an upper limit of ca. 0.5 ␮m for producing high-quality,
II. Experimental Procedure
Sols were prepared from lead acetate [Pb(OOCCH3)2(H2O)3,
Alfa, 99ϩ%], titanium(IV) diisopropoxide bispentanedionate
[Ti(OisoPr)2(CH3COCHCOCH3)2, Alfa, abbreviated TIAA, 75
wt% in isopropyl alcohol], zirconium(IV) n-propoxide [Zr(OnPr)4,
Aldrich, 70 wt% in 1-propanol], 2,4-pentanedione [acetylacetone,
CH3COCH2COCH3, abbreviated acacH, Aldrich, 99ϩ%], and
1,1,1-tris(hydroxymethyl)ethane [CH3C(CH2OH)3, Aldrich, 99%].
First, 2.976 g of zirconium n-propoxide was mixed with 1.286
g of acetylacetone under a dry nitrogen atmosphere. The solution
was heated at ca. 90°C for 2 h. After cooling, 2.739 g of titanium
diisopropoxide bisacetylacetonate, 5.234 g of lead acetrate trihy-
drate, and 2.912 g of 1,1,1-tris(hydroxymethyl)ethane were added
to the solution. The solution was then heated at ϳ70°C for 4 h. At
B. A. Tuttle—contributing editor
Manuscript No. 187144. Received February 12, 2002; approved April 29, 2003.
Financial support for M. Naksata was provided by the Royal Thai Government.
Equipment funds were provided by the Engineering and Physical Sciences Research
Council.
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September 2003
Properties of Lead Zirconate Titanate Thin Films Prepared Using a Triol Sol–Gel Route
1561
the final stage the solution concentration was 1.21 mol⅐dmϪ3
Then for spin coating purposes the sol was diluted to 0.75
mol⅐dmϪ3 with 2-methoxyethanol; sols contained 10 mol% excess
lead acetate.
.
making good physical contact between the surface of the block and
the thermocouple.
The accuracy of each thermocouple in monitoring the temper-
ature that would be experienced by a sol–gel coating was inves-
tigated by smearing finely ground powders of two compounds,
potassium iodate (mp 560°C) and 1,1,1-tris(hydroxymethyl)ethane
(mp 200°C) onto a Pt/Ti/SiO2/Si substrate. The temperature of the
hot plate was then increased slowly from room temperature using
a heating rate of 2°C⅐minϪ1; the experimentally observed melting
temperatures, as measured by the two thermocouples, are com-
pared against the known melting temperatures13 in Table I.
There was a close similarity between known and experimentally
determined melting temperatures measured from the block ther-
mocouple. This confirmed that thermal equilibrium between the
heated block and the silicon wafer was established, and that the
block thermocouple readings would be a reliable indicator of the
temperatures experienced by the PZT gel precursor coatings
during hot-plate thermal processing steps. Hot-plate temperatures
quoted in the remaining text therefore refer to the block thermo-
couple readings.
Sols were deposited on Pt/Ti/SiO2/Si substrates (supplied by
GEC-Marconi Technology); the Pt layer was 1000 Å thick, the Ti
adhesion layer 50 Å, and the thermally grown SiO2 3600 Å. The
Pt bottom electrode was polycrystalline with a (111) orientation.
The substrates used for film deposition were cut into ϳ1 cm2
pieces, and cleaned ultrasonically with a series of cleaning
solvents: Analar grade trichloroethylene, Analar grade acetone,
and finally, 2-propanol for 3 min each. The substrates were dried
at ϳ100°C before coating.
For sol deposition, the sols were syringed though an in-line 0.2
␮m Nylon membrane filter (Pall Trinity Micro Corp., NY); all of
the substrate was covered with the sol before spin coating at a
speed of 3000 rpm, for 1 min. The coated substrates were then
routinely heat-treated at 400°C for 10 min on a custom-built hot
plate consisting of a stainless-steel cylindrical block (diameter ϭ
16 mm, length ϭ 30 mm) which was heated using internal
electrical heating elements. The block temperature was measured
by using a K-type thermocouple inserted into the steel block, and
by a contact thermocouple pressed onto the surface of the block
(Model PI8008 supplied by BDH Chemicals, U.K.); hereafter,
these are referred to as the “block thermocouple” and the “surface
contact thermocouple,” respectively. In order to avoid variations in
temperature caused by the effects of variable air flows over the
sample, the hot plate (which was located in a fume cupboard) was
shielded using a cylindrical collar, 15 cm high and 20 cm in
diameter.
After the films were annealed at 400°C for 10 min, they were
removed from the hot plate and, after the temperature was raised,
replaced for a further heat treatment of 30 min duration at various
temperatures—450°, 480°, 500°, 520°, 550°, 565°, 580°, or
600°C. Over a 30 min period, the temperature of the block was
found to fluctuate by only Ϯ3°C.
Phase development was monitored using X-ray diffraction
(Phillips APD 1700 using CuK␣ radiation); XRD peak intensities
are quoted in terms of peak heights. Scanning electron microscopy
(Hitachi S700 field emission SEM) was used to examine micro-
structural features and a Radiant Technologies (RT66a) ferroelec-
tric tester was used to measure the polarization–electric field
(P–E) response at 60 Hz for a 300 kV⅐cmϪ1 wave form. Gold dot
electrodes, 0.3 mm in diameter, were sputtered onto the film using
shadow masking; a corner of the PZT film was etched to reveal the
bottom Pt electrode. For each sample, 10 dot capacitors were
tested; average values are reported for remanent polarization, Pr,
where Pr ϭ (͉ϩPr͉ ϩ ͉ϪPr͉ )/2, and coercive field, Ec ϭ (͉ϩEc͉ ϩ
͉ϪEc͉ )/2.
The effective heating rate experienced by the sol–gel coatings
was assessed by preheating the hot plate to 600°C and observing
the time period for potassium iodate powders to melt; this
corresponded to a heating rate of ϳ100°C⅐sϪ1
.
X-ray diffraction patterns, after heating the coated substrates at
400°C for 10 min, and then for 30 min at either 400°, 500°, or
550°C, again on the hot plate, are shown in Fig. 1. After heating at
400°C, there was a sharp and intense XRD peak at 38.6° 2␪,
together with a faint, broad hump centered at 29.1° 2␪ (Fig. 1(a)).
The sharp peak at a d-spacing of 2.33 Å is attributed to the PbPt3
transitory interfacial phase which forms in the reducing atmo-
sphere existing at the substrate–film interface during the decom-
position of organic components at low temperatures.8–10
The very broad peak at 29.1° 2␪ is a common feature in sol–gel
PZT films, and is usually attributed to poorly crystallized or
fine-grained intermediate phase(s) having a pyrochlore or flourite
crystal structure.14,15
The first faint perovskite PZT peaks appeared after heating a
sample at 480°C, but the broad peak around 29.1° 2␪ continued to
be present and there was still evidence of the PbPt3 phase. When
the temperature was increased to 500°C, a slight shift in d-spacing
from 2.33 Å for the 400°C sample to 2.35 Å for the 500°C sample
(and also for samples heated at higher temperatures) occurred, and
the peak profile became less sharp (Fig. 1(b)). This slight increase
in d-spacing, of what was assigned to be a 111 peak of PbPt3 in the
400°C sample, is consistent with the peak at higher process
temperatures being now principally due to 111 PZT,11 rather than
PbPt3.
The variation in the intensity of the 111 PZT peak was
expressed in terms of a parameter ␣111, where ␣111 ϭ I111/(I100 ϩ
I110 ϩ I111); the increase in (111) preferred orientation over the
temperature range 520° to 600°C is displayed graphically in Fig. 2.
The increasing relative intensity of the 111 PZT peak above 500°C
indicates the development of a progressively greater proportion of
(111) oriented PZT crystallites in films which were processed at
higher temperatures. Other perovskite X-ray peaks also became
sharper and more intense in samples heated at 550° or 600°C,
indicating improved crystallization throughout the film (Fig. 1(c)).
Cross-sectional TEM studies provided more detailed informa-
tion on phase development. Initially, samples heated at low
TEM samples were prepared in cross section via the conven-
tional “sandwich” method.18 The disks were dimpled to ca. 50 ␮m
thickness and subsequently thinned to electron transparency using
a Gatan PIPS low-angle, low-accelerating-voltage ion beam thin-
ner. These were examined in a Philips CM20 TEM/STEM oper-
ating at 200 keV, and fitted with a SATW thin window EDX
detector (Oxford Instruments) and PEELS spectrometer (Gatan).
Further investigations were performed using a Philips CM200 field
emission TEM equipped with a Gatan imaging filter, for energy
filtered imaging (EFTEM)
III. Results
Table I. Thermocouple Calibration against Known Melting
Points of 1,1,1-Tris(hydroxymethyl)ethane (THOME) and
Potassium Iodate
A commercial specialist wafer thermocouple was not available
to us; instead two conventional thermocouple configurations were
used to monitor the temperature experienced by the films during
thermal decomposition. Before commencing the film decomposi-
tion experiments, the accuracy of each thermocouple reading was
assessed. The temperature readings from the thermocouple in-
serted into the block were consistently higher by around 10% than
the equivalent readings from the surface thermocouple. The lower
reading from the surface contact thermocouple reflects problems in
Thermocouple calibration (°C)
Inserted
thermocouple
Surface
thermocouple
Substance
mp (°C)
THOME
KIO3
200
560
203
564
183
510

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Journal of the American Ceramic Society—Naksata et al.
Vol. 86, No. 9
Fig. 2. Variation in intensity of the 111 PZT peak expressed in terms of
a parameter ␣111, where ␣111 ϭ I111/(I100 ϩ I110 ϩ I111).
the underlying electrode, as shown for a film heat-treated at 600°C
in Fig. 5.
Grain sizes in the films annealed between 500° and 600°C
showed an increase from ca. 0.2 to 0.3 ␮m, with many columnar
grains growing through the full film thickness. A few pockets of
nanosize grains were present, attributed to a pyrochlore interme-
diate phase, as indicated in XRD plots.14,15 Most of these pockets
were concentrated near the surface of the film.
While the above structural information is fairly typical of
sol–gel PZT films, we have observed unexpected microstructural
features in films heated at low temperatures, for which there is
little published information. Spherical particles, up to ca. 50 nm in
diameter, consistently appeared in samples heated at low temper-
atures, for example 350°C for 30 min. These were located in the
lower portions of the otherwise amorphous ca. 0.4 ␮m thick films
(Fig. 6). Some pores of similar morphology to the particles were
also present; these may have formed due to elimination of some of
the precipitates during TEM sample preparation. At higher mag-
nifications, and from SAED patterns, the precipitates were shown
to be polycrystalline, exhibiting lattice spacings of around 0.3 nm
(Fig. 7). There was also evidence of discrete single crystallites
(Fig. 7(a)); presumably single crystallites coalesced to form the
larger, approximately spherical polycrystallites. Athough it was
difficult to obtain accurate information on chemical and physical
structure because of the small particle dimensions, scanning TEM
and EFTEM indicated that many of the spherical polycrystalline
particles were lead-rich. Other authors have reported precipitates
similar in form to those described above; they suggested that the
precipitates were PbO,16 thought to arise directly from excess lead
acetate added to the starting sols.
Fully crystallized films exhibited high levels of porosity, with
pore sizes ranging up to ca. 50 nm. A porous region of a film
heated at 600°C, showing both inter- and intragranular pores, is
highlighted in Fig. 8. Possible causes of this porosity are discussed
below.
For samples heated on the hot plate at 520°C for 30 min, the
polarization–electric field (P–E) loop was a broad ellipse, consis-
tent with a lossy dielectric.17 The XRD patterns and TEM had
indicated the presence of perovskite PZT at Ն500°C, together with
the pyrochlore phase; therefore the nonferroelectric P–E response
of the 520°C sample probably reflects the dominant influence of
intermediate nonferroelectric phase(s), and any amorphous mate-
rial in the film. The small grain size of the perovskite phase at
these low temperatures could also suppress domain switching and
prevent P–E hysteresis.
Fig. 1. X-ray diffraction plots for films heated at 400°C for 10 min
followed by heating for 30 min at (a) 400°, (b) 500, and (c) 550°C.
temperatures were examined, confirming the formation of a
crystalline intermetallic phase at the bottom Pt electrode. For
example, Fig. 3(a) shows the bottom interfacial region of a sample
heated at 400°C for 30 min, indicating the interfacial layer to be
crystalline (Fig. 3(b)), with a depth of ϳ25 nm. A similar thickness
was observed for a sample heat-treated at 350°C. From spot
analysis by STEM/EDX, the composition of the layer was found to
be PbPt3, which is in agreement with previous studies.9 There were
local variations in the thickness of this layer, including regions,
particularly at columnar grain boundaries in the Pt electrode,
where short circuit diffusion pathways for Pt gave rise to convex
hemispherical irregularities, as shown to the left of the arrow in
Fig. 3(a).
Because the interfacial phase becomes unstable in oxidizing
conditions,10 the amount of PbPt3 is a function of process
temperature and time.8 Consequently the average thickness of the
PbPt3 layer in our films decreased for process temperatures above
400°C. A very thin interfacial layer was still evident in the sample
heated at 600°C (Fig. 4); however, because the layer was now only
around 5 nm in thickness, it was not possible to confirm its
composition. No discrete interface layer was observed in a film
heat-treated at 700°C.
It was apparent from both bright-field diffraction and phase
contrast lattice imaging of samples heated at 500°C or higher that
immediately above the bottom electrode, columnar grains of the
perovskite phase were closely aligned with the (111) orientation of

September 2003
Properties of Lead Zirconate Titanate Thin Films Prepared Using a Triol Sol–Gel Route
1563
Fig. 5. TEM bright-field image of a PZT film after heating on a hot plate
at 400°C for 10 min followed by a treatment at 600°C for 30 min showing
͗111͘ lattice fringes near the bottom electrode.
Heating a film at 550°C for 30 min resulted in the onset of
ferroelectricity (Fig. 9(a)), with Pr ϭ 11 ␮C⅐cmϪ2, and Ec ϭ 66
kV cm–1. Increasing the temperature to 600°C produced an
increase in measured polarization values, Pr reaching 21 ␮C⅐cmϪ2
for the 600°C sample (Fig. 9(b)). This was accompanied by a slight
decrease in coercive field, from 66 to ca. 55 kV⅐cm–1. The
reduction in coercive field may in part be due to an increase in
grain size; TEM data showed that the estimated average grain size
increased from around 0.2 ␮m in the 550°C sample to 0.3 ␮m in
the 600°C sample. Progressive changes in values of remanent
polarization and coercive field are plotted in Fig. 10.
IV. Discussion
The combined results of XRD and TEM are indicative of the
extended growth of a (111) preferentially oriented PZT layer
which originates during the earliest stages of crystallization from
the bottom electrode. As stated in the Introduction, there is a close
lattice match between Pt and PZT(53/47), but the match between
PZT and the PbPt3 interfacial layer, identified here by TEM, is
closer, with a difference in d111 spacing of ϳ1% as opposed to
ϳ4% for Pt;11 there are also structural similarities between the
Fig. 3. (a) TEM bright-field image of a PZT film after heating on a hot
plate at 400°C for 30 min; (b) corresponding selected area electron
diffraction pattern from the interfacial layer.
Fig. 4. TEM bright-field image of a PZT film after heating on a hot plate
at 400°C for 10 min followed by a treatment at 600°C for 30 min; the arrow
highlights a narrow 5 nm interfacial layer.
Fig. 6. TEM micrograph of a sample heated at 350°C for 30 min,
showing spherical particles in an amorphous matrix; pores are indicated by
arrows.

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Journal of the American Ceramic Society—Naksata et al.
Vol. 86, No. 9
Fig. 8. TEM micrograph of a sample heated at 400°C for 10 min and
600°C for 30 min showing (a) intergranular and (b) intragranular porosity.
higher (111) preferred orientation in films heated from below the
substrate could suggest that this pseudo unidirectional heating
drives the formation and growth of crystallites off the bottom
electrode more effectively than for samples heated uniformly in a
tube furnace. In furnace samples, a greater proportion of crystal-
lization in the film would occur from nucleation in the bulk, or at
the top surface of the film which is not subject to the orientating
influence of the bottom interface, thereby reducing measured (111)
orientation.
However, as well as differences in the direction from which the
coatings were heated, the rates of heating were also different.
Heating rates were faster on the hot plate, ϳ100°C⅐s–1, compared
with 2°C⅐s–1 for furnace firing. Hence different stress levels may
exist during crystallization which could also affect the extent of
preferred orientation.
Differences were also obseved in the P–E response. The
hysteresis loops of the hot-plate samples were broader than those
of corresponding furnace samples; hence the Ec values for hot-
plate samples were significantly higher, e.g., 57 kV⅐cm–1 com-
pared with 40 kV⅐cm–1 for furnace samples heated at 600°C.18,19
The broader P–E loops, and higher Ec values, may be due to
greater residual stresses in the more rapidly unidirectionally heated
hot-plate samples, but the grain size was also smaller in hot-plate
samples—both of which could restrict domain mobility and
ferroelectric switching.
Fig. 7. (a) TEM micrograph of a sample heated at 350°C for 30 min. (b)
Selected area electron diffraction pattern obtained from the polycrystalline
particle.
The high levels of porosity in the films, made either on the hot
plate or in a furnce, is unsatisfactory for any proposed device
applications. Clearly the further development of the triol sol–gel
route requires that porosity be eliminated. It is noted that it is
possible to prepare high-density 0.1 ␮m films by the triol route,
and the films have a microstructure similar to 0.1 ␮m films
prepared using an established acetic acid/methoxyethanol route18
(0.1 ␮m is the maximum crack-limiting single-layer thickness of
established routes). The future challenge is to produce dense single
films Ͼ0.1 ␮m from the triol route. It is probable that porosity is
linked to greater difficulties in eliminating organic decomposition
vapors from the thicker single-layer sol–gel films. Hence more
gradual low-temperature heating (instead of one step at 400°C)
should be investigated in the future, to minimize structural
disruption during organic decomposition, and thereby reduce
porosity levels.
(111) atom/anion arrangements in each structure. Hence the (111)
oriented PbPt3 layer is considered to be the principal reason for
(111) PZT film preferred orientation. The interface layer lowers
the nucleation energy of (111) oriented PZT crystallites which
therefore grow preferentially from the bottom electrode. This,
together with subsequent nonoriented nucleation within the bulk of
the film, gives an overall (111) preferred orientation in XRD
patterns.
In comparison to films made previously by the same triol
sol–gel processing route, but for which the final heat treatment
took place in a tube furnace, the present hot-plate samples
exhibited a greater level of (111) orientation. For example,
samples heated at 600°C in a tube furnace18,19 showed an
orientation parameter ␣111 ϭ 0.5, compared with ␣111 ϭ 0.8 for
the hot-plate samples. Although the time scales over which a
temperature gradient exists across the film must be very short, the
Cross-sectional TEM revealed approximately spherical particles
or aggregates in triol-derived films. These were first observed in a

September 2003
Properties of Lead Zirconate Titanate Thin Films Prepared Using a Triol Sol–Gel Route
1565
Fig. 9. Polarization–electric field hysteresis loops for PZT films heated
at 400°C for 10 min followed by 30 min at (a) 550° and (b) 600°C.
Fig. 10. Variation of Pr and Ec as a function of process temperature.
matrix, it was not possible to identify the structure and composi-
tion of this low-temperature phase. However, it is probable that
even though the precipitates could originate from atomic Pb,
oxidation and carbonation reactions could later produce lead
oxides and carbonates. The origins and composition of these
precipitates are under further investigation, as is their link to film
porosity; the results of a detailed TEM investigation in conjunction
with colleagues at the University of Sheffield will be reported in
due course.20 It is conceivable that as thermolysis proceeds,
outward diffusion from the precipitates occurs, leaving residual
pores that at least in part contribute to porosity in the final PZT
films. Some Zr-rich precipitates have also been identified which
could signify segregation of a Zr-rich PZT phase,18 but again the
reasons for this are unclear.
sample annealed at 350°C for 30 min and were dispersed in an
amorphous matrix; predominantly the precipitates were located in
the lower portions of the film. The Pb-rich phase identified in
many of the precipitates probably originates from chemical reduc-
tion of some of the (excess) lead(II) acetate starting reagent. The
fact that they are not found near the surface of the films is
consistent with a formation mechanism linked to chemical reduc-
tion. Diffusion distances for the escape of gaseous gel decompo-
sition products increase as a function of film thickness; there will
also be slower rates of inward diffusion of oxygen to the lower
portions of the film. Hence the partial pressure of oxygen near the
base of the 0.4 ␮m films is presumably lower than in a 0.1 ␮m film
heated in the same manner. It is well known that reduction of
Pb(II) to atomic Pb occurs at the bottom Pt interface in sol–gel
PZT films, and that interdiffusion produces a layer of PbPt3.
However, in our thicker 0.4 ␮m single layers, lower partial
pressures of oxygen at the base of the film could be promoting a
greater incidence of Pb(II) reduction so that in addition to
interfacial PbPt3 there is a segregation and nucleation of atomic Pb
within the film. These then coalesce to form the polycrystalline
spherical particles in Fig. 7. A similar reduction reaction has been
proposed to explain the presence of Pb and PbO, identified by
XRD, in PZT powders formed by pyrolysis of various metallo-
organic precursors after heating at 400–500°C,21 and we also have
have obtained XRD evidence of Pb, formed by reduction of lead
acetate, as an intermediate phase in the formation of PZT powders
by spray pyrolysis of sol–gel precursor solutions.22
V. Conclusions
Crystallization to perovskite PZT in the 0.4 ␮m single-layer
films started at around 500°C, but to obtain a ferroelectric P–E
response, an annealing temperature of 550°C was required. How-
ever, improved ferroelectric properties were obtained in films
heated at 600°C.
The P–E hysteresis loops were broader than for corresponding
samples prepared in earlier studies using furnace annealing. The
implication was that stress levels were higher in hot-plate samples
due to the unidirectional nature of the heating process, and faster
heating rates. Grain sizes and the levels of (111) preferred
orientation also differed between the two heating methods which
could have an impact on domain switching behavior.
Unfortunately because of the small dimensions of the particles,
and the “contamination” of the EDX spectra by the surrounding

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Journal of the American Ceramic Society—Naksata et al.
Vol. 86, No. 9
6S. J. Milne, R. Kurchania, J. D. Kennedy, M. Naksata, S. Arscott, D. Kaewchinda,
N. Sriprang, and R. E Miles. “Triol Sol-Gel Route for Preparing PZT Thin Films,”
Ceram. Trans., 95, 95–101 (1999).
At lower temperatures a PbPt3 interfacial layer was identified in
TEM film cross sections. The thickness of this layer was tempera-
ture–time dependent, which is consistent with previous studies. A
coherent (111) oriented PZT layer was observed to grow outward
from the (111) PbPt3 layer, confirming the importance of the
intermetallic phase on PZT preferred orientation.
Within the lower portion of the films, spherical precipitates, up
to ca. 50 nm in diameter, were observed in TEM micrographs after
treatment at 350°C. These were polycrystalline and mostly Pb-
rich, although Zr-rich regions were also identified by STEM/EDX.
It is thought that these features could contribute to the high
porosity in the fully decomposed PZT films. However, porosity
may simply be caused by overly rapid heating rates during the
expulsion of organic decomposition vapors, and therefore further
research into the origins of porosity is required.
7R. Lbibb, R. Castanet, and A. Rais, “Thermodynamic Investigation of Pb–Pt
Binary Alloys,” J. Alloys Compd., 302, 155–58 (2000).
8S. Y. Chen and I-W. Chen, “Temperature–Time Texture Transition of PZT Thin
Films: I, Role of Lead-Rich Intermediate Phases,” J. Am. Ceram. Soc., 77, 2332–44
(1994).
9Z. Huang, Q. Zang, and R. W. Whatmore, “Low Temperature Crystallization of
Lead Zirconate Titanate Thin Films by a Sol-Gel Method,” J. Appl. Phys., 85 [10]
7355–61 (1999).
10Y. L. Tu and S. J. Milne, “Characterisation of Single-Layer PZT Films Prepared
from an Air Stable Sol-Gel Route,” J. Mater. Res., 10, 3222–31 (1995).
11Powder Diffraction File, International Centre for Diffraction Data, Newtown
Square, PA: Pt 4-802; PbPtx 6-574; PZT 33-784.
12T. Tani, Z. Xu, and D. A. Payne, “Preferred Orientations for Sol-Gel Derived
PLZT Thin Layers,” Mater. Res. Soc. Symp. Proc., 310, 269–74 (1993).
13D. R. Lide (Ed.), Handbook of Physics and Chemistry, 74th ed. CRC Press, Boca
Raton, FL, 1993–94.
14C. D. A. Lakeman, Z. Xu, and D. A. Payne, “On the Evolution of Structure and
Composition in Sol-Gel-Derived Lead Zirconate Titanate Thin Layers,” J. Mater.
Res., 10, 2042 (1995).
Acknowledgments
15A. P. Wilkinson, J. S. Speck, A. K. Cheetham, S. Natarajan, and J. M. Thomas,
“In Situ Diffraction Study of Crystallisation Kinetics in PZT,” Chem. Mater., 6, 750
(1994).
We wish to thank I. M. Reaney and Z. Zhou, University of Sheffield, and J.
Kemprasit for useful discussions on the TEM data.
16S. X. Wang, L. M. Wang, and C. L. Lin, Microscopy and Microanalysis, Vol. 6
(Suppl. 2: Proceedings); pp. 458–59. Springer, Heidelberg, Germany, 2000.
17B. Jaffe, W. R. Cook, and H. Jaffe, Piezoelectric Ceramics. Academic Press, New
York, 1971.
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