Full text of DOI:10.1557/JMR.2000.0020

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Domain structure and electrical properties of highly textured
PbZrxTi1−xO3 thin films grown on LaNiO3-electrode-buffered
Si by metalorganic chemical vapor deposition
C.H. Lin, B.M. Yen, H.C. Kuo, and Haydn Chena)
Department of Materials Science and Engineering and Materials Research Laboratory,
University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
T.B. Wu
Department of Materials Science and Engineering, National Tsing Hua University,
Hsinchu, Taiwan, Republic of China
G.E. Stillman
Department of Electrical and Computer Engineering, University of Illinois at Urbana-Champaign,
Urbana, Illinois 61801
(Received 5 August 1998; accepted 7 October 1999)
Thin films of highly (100) textured fine-grain (lateral grain size 0.1 to 0.15 ␮m)
PbZrxTi1−xO3 (PZT) (x ס 0 to 0.7) were grown on conductive perovskite
LaNiO3-buffered platinized Si substrates by metalorganic chemical vapor deposition.
Domain configuration and crystalline orientation were studied using x-ray diffraction
and transmission electron microscopy. The predominant domain boundaries of Ti-rich
tetragonal-phase PZT and Zr-rich rhombohedral-phase PZT were found to be on the
(110) planes and (100) planes, respectively. The equilibrium domain widths were
observed and estimated numerically on the basis of transformation strain, grain size,
and domain boundary energy. The peak value of the dielectric constant was 790 near
the morphotropic boundary. Hysteresis behavior of these PZT thin films was
demonstrated. A decrease in coercive field with the increment of Zr content was
found; this variation was attributed to domain density and the multiplicity of
polarization axes. Furthermore, the low leakage current (J ഛ 5 × 10−7 A/cm2 at
V ס 4 V) was observed for all samples, and the involvement of several possible
conduction mechanisms was suggested.
I. INTRODUCTION
by trapped charge. The accumulation of oxygen vacan-
cies near the electrode/ferroelectric interfaces would in-
crease charge trapping rate and promote fatigue.6–8 To
overcome the problems of degradation, metallic oxides
like RuO2, LaSrCoO3 (LSCO), or SrRuO3 (SRO) have
been considered as promising electrodes for the metal/
insulator/metal structure because they act as a sink for
oxygen vacancies.6,9–11 Among these oxide electrodes,
perovskite electrodes are more attractive because they
offer better resistance to fatigue. It was shown that there
is no compositional dependence in fatigue characteristics
for PZT thin films grown on perovskite SrRuO3 with Zr
concentration ranging from 30% to 70% while the resis-
tance to fatigue shows evident degradation as the Zr con-
centration decreases from 50% to 30% for PZT deposited
on the RuO2 electrode.11,12 This phenomenon is sug-
gested to result from the increase of equilibrium con-
centration of oxygen vacancies with Ti content and from
the limited tolerance of RuO2 to oxygen vacancies. The
improvement of fatigue resistance by using perovskite
In recent years, the fabrication of ferroelectric capaci-
tors for high speed nonvolatile random-access memories
(NVRAMs) and for integrated micro-electromechanical
devices has attracted much attention.1,2 PbZrxTi1−xO3
(PZT) has been shown to be one of the best candidates
for applications to integrated ferroelectric devices.3,4
Metalorganic chemical vapor deposition (MOCVD), ow-
ing to its capabilities of processing large wafers with
good uniformity and step coverage, has become one
of the major techniques for fabricating ferroelectric
thin films.5
Polarization fatigue is a major reliability problem
which hinders the progress of integrated ferroelectric de-
vices. Fatigue of hysteresis behavior resulting from do-
main wall pinning is commonly recognized to be caused
a)Address all correspondence to this author.
J. Mater. Res., Vol. 15, No. 1, Jan 2000
© 2000 Materials Research Society
115

C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
oxide electrodes can be attributed to the better crystallo-
graphic compatibility of the ferroelectric/electrode inter-
face and the higher tolerance to oxygen vacancies of the
perovskite electrodes.
tures originating from different processing techniques
and different bottom electrodes. In this paper, we have
systematically varied the Zr-to-Ti stoichiometry to obtain
highly (100)-oriented tetragonal and rhombohedral PZT
thin films on LNO electrode-coated Si substrates. The
evolution of microstructure including grain morphology
and domain configuration has been investigated to reflect
their effects on the ferroelectric properties.
LaNiO3 (LNO) is a rhombohedral perovskite (a0 ס
0.383 nm) metallic oxide with the surface resistivity as
low as 225 ␮⍀ cm for (100)-textured thin film.13 Suc-
cessful deposition of epitaxial or textured PT or PZT thin
films on LNO electrode layers by the sol-gel method,
metalorganic decomposition (MOD) process, and pulse
laser deposition (PLD) has been demonstrated by several
groups.13–16 It was reported that the nonfatigue remanent
polarization of PZT thin films deposited on LNO-coated
Si persisted up to 2 × 109 cycles.15 Nevertheless, there is
no report about the crystalline characteristics and elec-
trical properties of PZT thin films grown on LNO elec-
trode by MOCVD.
It was generally considered that PZT in the Ti-rich
tetragonal phase has lower crystallization temperature for
solution-derived thin films on Pt or RuO2 electrodes.17
However, rhombohedral phase PZT is more attractive in
memory application and pyroelectric detection due to its
lower coercive field, higher remanent polarization, and
higher pyroelectric coefficient.11 In this study, we found
that the employment of the perovskite metallic LaNiO3
layer in conjunction with MOCVD process is beneficial
to fabricating highly (100)-textured PZT thin films in
either Ti- or Zr-rich phases with good properties.
Zr-to-Ti stoichiometry has a predominant effect on the
domain configuration and, thus, the electrical properties
of PZT thin films. There are relatively few reports ad-
dressing these issues. The influence of Zr-to-Ti stoichi-
ometry on the microstructure and 90° domain assemblage
of chemical solution derived tetragonal PZT thin films
was studied by Tuttle et al.,17 who showed no evident
difference in domain width or domain population in the
randomly oriented PZT thin films. To the contrary, our
present study revealed significant differences in highly
(100) textured thin films on the basis of both x-ray
diffraction (XRD) and dark field cross-sectional trans-
mission electron microscopy (XTEM) techniques. More-
over, the dependence of dielectric and hysteresis
behavior of MOD-derived PZT on Zr-to-Ti stoichi-
ometry was studied by Klee et al.18 Foster et al.11 also
reported the variation of optical and ferroelectric prop-
erties with respective to Zr-to-Ti stoichiometry of epi-
taxially grown PZT thin films by MOCVD on (001)
SrTiO3. However, the correlation between the domain
structure and electrical properties was not emphasized in
those studies. Most studies on compositional effects were
carried out on randomly oriented PZT thin films depos-
ited on RuO2 and Pt electrodes or epitaxially grown thin
films on single-crystal oxide substrates. It is important to
understand that the discrepancies in measured electrical
properties are the direct result of different microstruc-
The direct current (dc) conducting behavior of a metal–
ferroelectric–metal (MFM) capacitor structure is another
important factor for the realization of ferroelectric de-
vices and has been intensively studied by many groups.
Conduction mechanism of perovskite ferroelectrics like
(BaSr)TiO3 (BST) or PZT thin films under different elec-
tric field strength has been described by Dietz et al. and
by Sudhama et al.19,20 It is suggested that the electrical
conduction mechanism of PZT thin films can be of
Schottky effect, Poole–Frenkel effect, or space charge
limited current effect (SCLC). In all mechanisms, the
bottom electrode plays an important role because it af-
fects not only the Schottky barrier formed at the elec-
trode/ferroelectric interface but also the crystalline
quality of the PZT overlayer. Al-Shareef et al.21 demon-
strated that PZT thin films deposited on RuO2 electrode
showed larger leakage current than on Pt electrode due to
the existence of the pyrochlore phase. The use of a hybrid
electrode RuO2/Pt was shown to effectively reduce the
leakage current. Up to now, there are still few reports
regarding the electrical conduction behavior of PZT thin
films using perovskite oxide electrodes. In this paper, the
low leakage current of PZT thin films will be demon-
strated and the involvement of several current conducting
mechanisms under different electrical field strengths will
be discussed.
II. EXPERIMENTAL PROCEDURE
(100)-textured LNO (0.2 ␮m) thin films were depos-
ited on Pt(0.15 ␮m)/Ti(0.05 ␮m)/SiO2(0.15 ␮m)/Si sub-
strates by radio frequency (rf) magnetron sputtering at
300 °C. The processing procedure and electrical proper-
ties of the LaNiO3/Pt/Ti multilayer electrode have been
described elsewhere.15 The subsequent PT and PZT
( 0.15 to 0.2 ␮m) thin film growth was carried out in a
low-pressure, horizontal, cold-wall quartz MOCVD re-
actor with a resistive substrate heater. The mixture of the
metalorganic precursor vapor was introduced into the
reactor through an inlet flange on one end of the reactor
system. The metalorganic sources used were lead tetra-
methyl heptanedionate, Pb(TMHD)2, zirconium tert-
butoxide, Zr(OC4H9)4, and titanium isoproproxide,
Ti(OC3H7)4. The vapor delivery used high-purity nitro-
gen as the carrier gas. PT thin films were grown at sub-
strate temperatures ranging from 450 to 600 °C. PZT thin
116
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C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
films with Zr concentration ranging from 15 to 70%
were grown at 600 °C. A systematic variation of Zr-to-Ti
stoichiometry was achieved by varying the Zr source
flow rate during growth with minor adjustment of the
flow rates of Ti and Pb. Shown in Table I are typi-
cal deposition conditions of PZT thin films on LNO
thin layers.
X-ray diffraction (XRD) scans were carried out using
a 4-circle diffractometer with Cu K␣ radiation. Cross-
sectional transmission electron microscopy (XTEM) mi-
crographs were taken using a Philips CM-12 microscope
operated at 120 kV. The chemical composition of the
PZT thin films were determined using the nanoprobe
x-ray energy dispersive spectroscopy (EDS) installed on
a TEM with electron beam converging to 30 nm. Fur-
thermore, the relative compositions of these PZT thin
films were confirmed by x-ray diffraction through the
FIG. 1. XTEM micrograph of PZT/LNO/Pt/Ti/SiO2/Si layered ca-
pacitor structure.
Bragg angle variation of PZT (200)/(002) peaks and us-
ing Pt(111) peak as a internal reference. To measure the
electrical properties, Au (2000 Å) square patterns
around 50–80 nm, and the Pt/Ti layer exhibits granular
(200 × 200 ␮m) were evaporated onto PZT thin films as
microstructure with typical grain size around 100 nm.
the top metal electrode of the metal–insulator–metal
The fine grain structures of the multilayer electrode and
structure. The whole structure was then annealed
the subsequently deposited PZT layer provide a good
at 500 °C in O2 atmosphere to improve the adhesion
microstructural uniformity for further device integration
of Au electrode and reduce possible trapped charges.
and miniaturization. The use of such a multilayer elec-
Dielectric constants were measured using a HP 4275 LCR
(inductance–capacitance–impedance) meter by perform-
trode system offers several benefits. Tseng et al.14 de-
scribed that pulse laser deposition of PLZT thin film on
ing frequency scans in the range of 10 kHz to 1 MHz
LNO/Pt/Ti multilayer electrode-coated Si showed supe-
with a oscillation voltage of 0.05 V. Dc leakage current
rior properties over the LNO single-layer electrode in
was tested using a HP 4140B pA meter; the voltage was
remanent polarization, dielectric constant, and leakage
applied to the top electrode and swept from 0 to 10 V
current. The surface resistivity of the LNO/Pt/Ti multi-
using steps of 0.01 V with 1 s waiting time for each
layer electrode used in this study is about 56.2 ␮⍀ cm,
measured using a four-point probe method. The surface
step. A Sawyer–Tower circuit was employed for meas-
uring the hysteresis behavior (P–E loop) using a 1 kHz
resistivity is appreciably lower than an LNO single-layer
sinusoidal wave.
electrode (␳ 225 ␮⍀ cm), and the lower surface resis-
tivity is owing to the parallel conductance of the Pt un-
derlayer. The low surface resistivity of the LNO/Pt/Ti
multilayer electrode reduces the distortion of measured
PZT properties in dielectric constant and remanent po-
larization. Moreover, the Pt/Ti metallic bilayer improves
the endurance of the device because the metallic layer
serves as the sink for releasing thermal stress. The prop-
erties of the subsequently MOCVD grown PZT thin
films are described in a later section.
III. RESULTS AND DISCUSSION
A. Structure of multilayer bottom electrode
The bottom electrode layers used in our study consist
of a LaNiO3/Pt/Ti multilayer structure deposited on ther-
mally oxidized Si wafer. Figure 1 shows the XTEM of
the whole capacitor structure used in our study. The LNO
layer shows columnar grains with average grain size
B. Effects of growth temperature on the
microstructure and electrical properties of
PbTiO3 thin films
TABLE I. Typical deposition conditions of PZT thin films.
Deposition temperature
Deposition pressure
Growth rate
600 °C
10 torr
Shown in Figs. 2(a)–2(d) are the XRD ␪–2␪ scan pro-
files of PbTiO3 (PT) thin films grown on LNO/Pt/Si at
450, 500, 550, and 600 °C, respectively. The LNO peak
was not so evident due to the close match between (100)
peaks of PT and LNO and the weaker diffracted intensity
of LNO layer. All films exhibit (100)t preferential orien-
3–5 nm/min
900 sccm
400 sccm
20–40 sccm
10–40 sccm
10–25 sccm
N2 flow
O2 flow
Pb(TMHD)2 (118 °C) flow
Zr(OC4H9)4 (35 °C) flow
Ti(OC3H7)4 (35 °C) flow
J. Mater. Res., Vol. 15, No. 1, Jan 2000
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C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
FIG. 3. Plan-view TEM micrograph showing the domain structure of
PT thin film grown at 550 °C.
1 MHz. The dielectric constants (k ס ⑀r/⑀0) of PT thin
films decrease from 293 to 155 with the reduction of
growth temperature from 600 to 450 °C. For all meas-
urements, the dielectric losses are less than 0.08 in the
measured frequency range. The hysteresis behavior of PT
thin films grown on LNO at 600 and 550 °C are shown
in Figs. 5(a) and 5(b), respectively. For 600 °C grown PT
thin films, the coercive field (Ec) is about 90 kV/cm
and the remanent polarization (Pr) is 12.5 ␮C/cm2 un-
der 400 kV/cm. For PT thin films grown at 550 °C, Ec
is around 100 kV/cm and Pr is 2.5 ␮C/cm2 under
400 kV/cm. For perovskite ferroelectric thin films, the
dielectric properties and hysteresis behavior are mainly
influenced by grain size, interfacial layer, and domain
structures. Especially, Frey et al.23 reported a drastic ef-
fect of interfacial layer on the dielectric properties of
fine-grained BaTiO3; an interfacial layer as thin as 8 Å
can alter the measured apparent dielectric and hysteresis
properties of ultrafine-grain material. Therefore, the re-
duction of dielectric constant and a relative increase of
the dielectric loss for PT thin film grown at low tempera-
ture (Ts ഛ 500 °C) can be attributed to the effect of
possible interfacial defects between grains or PT/
electrode interface. Moreover, the slim P–E loop of low-
FIG. 2. XRD ␪–2␪ scans of PT thin films grown at different tempera-
tures: (a) 450 °C, (b) 500 °C, (c) 550 °C, (d) 600 °C.
tation. However, the (002)t peak appeared as the growth
temperature (Ts) exceeded 550 °C. The c-oriented do-
main population was evaluated using the integrated in-
tensity ratio of I(002)/[I(200) + I(002)]. The resultant
values are 0.17 and 0.25 for 550 and 600 °C grown films,
respectively. The lateral grain sizes are about 0.08, 0.15,
0.2, and 0.25 ␮m for growth temperatures of 450, 500,
550, and 600 °C, respectively, as observed by XTEM.
For thin films grown below 550 °C, with no (001)t peak
observed in the XRD spectra, the predominant domain
structure are 180° domains with the c axis lying on the
substrate surface. The thin domain plate size is around
2–5 nm, as shown in a plan-view TEM micrograph in
Fig. 3. The domain size observed is smaller than the
value predicted using the model developed by Arlt
et al.,22 which will be discussed in Sec. III. C.
Shown in Fig. 4 are the dielectric constant and loss
tangent measurements of PT thin films grown at different
temperatures in the frequency range from 10 kHz to
118
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C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
FIG. 5. Hysteresis behaviors of PT thin films grown at (a) 600 °C and
FIG. 4. Dielectric constant and loss measurements of PT thin films
(b) 550 °C.
grown at different temperatures.
for PZT thin films with Zr content 15%, 30%, and 45%,
respectively, showing a continuing increase of c-oriented
temperature-grown films (Ts ഛ 550 °C) is due to the lack
domain population with increasing Zr concentration.
of switchable 90° domain assemblage, as described in the
The grain size and domain configuration variance of
previous paragraph.
both tetragonal and rhombohedral PZT thin films were
studied using XTEM and shown in Figs. 7(a)–7(c). Dark-
C. Domain structures of PZT thin films with
respect to Zr-to-Ti stoichiometry
field TEM micrographs were taken close to (100) pole.
For tetragonal phase PZT (i.e., Zr/Ti ס 30/70 and 45/55),
The Zr-to-Ti ratio influences the microstructure of
PZT thin films in several ways including the preferential
orientation, the domain population, and the domain
widths. Shown in Fig. 6 are the XRD ␪–2␪ spectra of
highly textured PZT thin films with various Zr-to-Ti ra-
tios. In the tetragonal phase, the preferred crystalline ori-
entation changes gradually from highly (100) to (001)
with increasing Zr content. The transition of the pref-
erential orientation can be due to the lattice constant
variation. The PZT with 45% Zr content showed a com-
paratively broader Bragg peak located at around 45° and
was deconvoluted into (200)t and (002)t peaks. The c-
oriented domain population could then be obtained for
PZT with different Zr content. The relative integrated
intensities I(002)/[I(200) + I(002)] are 0.25, 0.35, and 0.5
the polarization is along one of the six 001 direction
c
and a/c 90 degree domain walls are lying on the (011)c
plane. Thus, the 45° inclined domain boundaries are
clearly observed, in Figs. 7(a) and 7(b). On the other
hand, rhombohedral phase PZT (Zr/Ti ס 70/30) has its
polarization axis along the (111)c orientation, and the
energetically favorable domain boundaries can be on ei-
ther the (110)c or (100)c planes for epitaxial (100)c ori-
ented rhombohedral PZT thin film, as described by
Streiffer et al.24 In the current study, it was found that the
domain boundaries of the rhombohedral phase PZT thin
films lie mostly along the (100)c plane and are perpen-
dicular to the film surface, as seen in Fig. 7(c). The cause
to the dominance of (100)c type domain wall will be
illustrated in a later section.
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C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
of transformation strain from cubic to tetragonal phase
due to constraint imposed by grain boundaries. The strain
term ⑀ is defined as [(c/a) − 1]/2 for tetragonal phase
ferroelectrics. According to Eq. (1), it can be found that
the larger the c/a ratio is, the smaller is the domain width.
The c/a ratio of PZT decreases from 1.06 to 1.025 with
increasing Zr content from 0% to 45%. The domain sizes
of tetragonal PZT (30/70) and PZT (45/55) were calcu-
lated to be around 10 and 18 nm, respectively, using Eq.
(1). These values are consistent with the domain size
observed by XTEM [i.e., 10 nm for PZT (30/70) in
Fig. 7(a) and 20 nm for PZT (45/55) in Fig. 7(b)]. It is
noted that Arlt’s model predicts the equilibrium domain
width under the grain boundary constraint. From the
XTEM observation, domains with enlarged width were
found near the film surface [e.g., Figs. 7(a) and 7(b)]
where strain relief could easily occur during the para-
electric-to-ferroelectric phase transition.
In Sec. III. B, the domain structure of lower tempera-
ture (T ഛ 550 °C) grown PbTiO3 thin films was de-
scribed. The domain structure was observed to be
composed of primarily 180° domains with domain size
ranging from 2 to 5 nm, which is smaller than the pre-
dicted value ( 10 nm) using Arlt’s analytical method.
Since Arlt’s equation was derived on the basis of the
consideration of strain energy and surface energy of do-
main boundaries, while the major factor for controlling
the size of 180° domains is the electrostatic energy, the
deviation of the calculated and actual domain sizes was
observed.
For the rhombohedral phase PZT (70/30), despite the
difference of polarization axis and domain boundary
plane as compared with the tetragonal counterparts, the
domain width is around 30 nm, similar to that found in
the tetragonal phase PZT. The dominance of (100) type
domain boundaries in the rhombohedral phase PZT can
be understood by following Arlt’s approach. From
Eq. (1), the determining factors of equilibrium domain
FIG. 6. XRD ␪–2␪ scans of PZT thin films with different Zr-to-Ti
width are mainly the surface energy of domain bound-
stoichiometries: (a) Zr/Ti ס 15/85, (b) Zr/Ti ס 30/70, (c) Zr/Ti ס
aries and the constraint strain energy given by grain
boundaries due to the paraelectric to ferroelectric phase
transition. The dominance of (100) type domain bound-
aries can be attributed to the less strain imposed from
grain boundaries.
The surface morphology of PZT thin films is also in-
fluenced by the domain structure, as shown in the XTEM
micrographs. For PZT (30/70) thin film, Fig. 7(a), the
surface of a specific grain is 45° inclined from the sur-
face normal. PZT (45/55) shows a moderate roughness,
Fig. 7(b). On the other hand, the surface of the rhombo-
hedral phase PZT (70/30) is relatively flat except near the
grain boundary area, Fig. 7(c). The difference in surface
morphologies of PZT thin films with respect to their Zr
to Ti stoichiometry is attributed to the strain relief as
described in the previous paragraph along with the pres-
45/55, (d) Zr/Ti ס 60/40, (e) Zr/Ti ס 70/30.
Arlt et al.22 have developed a model to describe the
relationship among 90° domain width, grain size, and
tetragonality (c/a ratio) of a ferroelectric crystal. The
expression is shown as below:
32␲␴G
w =
,
(1)
ͱ
E⑀2
where w is the equilibrium domain width, ␴ ס 3 × 10−3 J/m2
for PbTiO3 is the surface energy per unit area of domain
boundary,25 G is the grain size, E (C11 ס 1.2 × 1011 N/m2
for PbTiO3) is Young’s modulus, and ⑀ is the magnitude
120
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C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
FIG. 7. Dark-field XTEM micrographs of PZT thin films: (a) Zr/Ti ס 30/70, (b) Zr/Ti ס 45/55, (c) Zr/Ti ס 70/30. (The domain boundaries are
indicated by arrows.)
ence of different type of domain boundaries. Similar ef-
fects have also been theoretically analyzed by Streiffer
et al. for PZT thin films grown on SrTiO3.24
D. Ferroelectric properties of PZT thin films
Shown in Fig. 8 are the dielectric constants (k ס ⑀r/⑀0)
of PZT thin films, which increase from 293 for PbTiO3 to
790 for PZT (45/55). The dielectric losses are less than
0.1 in the measuring frequency range. Dielectric con-
stants close to the morphotropic boundary are lower than
those values reported by Klee et al.18 but are close to the
values reported by Foster et al.11 The major factor af-
fecting the dielectric constants is the film orientation.
The solution-derived PZT thin films in the study of Klee
et al.18 were of (111)/(100) mixed orientations, while the
MOCVD-derived PZT films in the study of Foster
et al.11 and the present study were predominantly (001)
or (100) oriented. PZT at morphotropic boundary con-
sists of both tetragonal and rhombohedral phases with
their polarization axes along 100 c and 111 . The flex-
c
ibility of polarization vectors responding to electric field
for the (111)/(100) mixed oriented films could give
higher dielectric constant values than (100)- or (001)-
oriented films.
FIG. 8. Dielectric constant of PZT thin films with various Zr-to-Ti
stoichiometries.
The hysteresis behavior of PZT thin films grown on
LNO is shown in Figs. 9(a)–9(d). All hysteresis loops
show saturation at 200 kV/cm applied field. It is worth
noting that saturated hysteresis loops can only be ob-
served for thin films annealed at 500 °C for 30 min after
top electrode deposition. Unannealed PZT thin films gen-
erally show unsaturated, slanted hysteresis loops with
evident shift along electric field axis. It was suggested by
Klee et al.18 and later by Lee et al.26 that the deformation
of hysteresis loop is associated with the internal field due
to trapped charges such as lattice vacancies or impurity
atom located at grain boundaries or domain walls. Under
suitable annealing condition, those trapped charges can
be removed and domain motion can be more easily
activated.
J. Mater. Res., Vol. 15, No. 1, Jan 2000
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C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
FIG. 9. Hysteresis behaviors of PZT thin films with various Zr-to-Ti stoichiometries: (a) Zr/Ti ס 30/70, (b) Zr/Ti ס 45/55, (c) Zr/Ti ס 60/40,
(d) Zr/Ti ס 70/30.
The remanent polarizations (Pr) of the PZT thin films
are around 13 and 12.5 ␮C/cm2 for tetragonal phase PZT
with 30 and 45 at.% Zr content, respectively. Rhombo-
hedral phase PZT shows lower Pr values, which are 10
and 8 ␮C/cm2 for 60% and 70% Zr content PZT, respec-
tively. Factors affecting the remanent polarization value
include grain size, preferential orientation, and film
stress. Fine-grain PZT thin films generally show smaller
Pr values because of the higher density of grain bound-
aries, which would trap space charges and hinder the
domain motion. However, the effect of grain size might
not be the dominating factor that affects the Pr value in
the present study. It was reported by Klee et al.18 that
solution-derived (001)/(111) mixed oriented PZT thin
films with lateral grain sizes around 0.1–0.2 ␮m display
Pr values exceeding 20 ␮C/cm2. The most important fac-
tor can be the preferential orientation. For tetragonal PZT
thin films, they were highly (100)t oriented and exhibit
lower Pr values when compared with the reported Pr
values of highly (001)t-oriented epitaxial films. For
rhombohedral PZT, the lower Pr values can be attributed
oriented remanent polarization value on 001 c axes. The
third factor concerns the differential thermal expansion
stress. It was illustrated by Tuttle et al.27 that the Pr
values of (111)-oriented PZT thin films deposited on
Pt/Si are different from those deposited on Pt/sapphire.
Detailed studies of film stress versus temperature re-
vealed that PZT thin films deposited on platinized Si
sustained larger tensile thermal stress than other plati-
nized oxide single-crystal substrates like MgO or sap-
phire and generally showed slanted hysteresis loop.
For PZT thin films, the coercive field (Ec) decreases as
the Zr concentration increases. The Ec values decrease
from 80 to 40 kV/cm. Similar studies have been reported
by Klee et al.18 for PZT thin films deposited on Pt elec-
trode which are also consistent with the results reported
by Foster et al.11 for PZT thin films epitaxially grown on
SrRuO3/SrTiO3 substrates. It is speculated that the varia-
tion of Ec values is probably due to the domain boundary
density variation with respect to the Zr-to-Ti ratio for
PZT thin films in their tetragonal phase. PZT thin films
with higher Ti content contain more domain boundaries
due to the smaller equilibrium domain width and tetrago-
nality (higher transformation stress), thus resulting in
to its 111 polarization axes. The measured remanent
c
polarization value is the projection of the actual 111 -
c
122
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C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
higher coercive field. The lower Ec value found in the
rhombohedral PZT as compared with the tetragonal
lustrated Schottky barrier limited current. Yoo et al.28
suggested the dominance of the Schottky effect in the
high-field region. Sudhama et al.20 demonstrated the
conducting behavior in different electric field strength
and suggested the ohmiclike behavior in low field while
the high-field current conducting might involve Schottky
effect, Poole–Frenkel effect, and space charge limited
current. For all specimens used in this study, the micro-
structure has been confirmed by XTEM to be composed
of highly oriented textured grain; thus, the contribution
of grain boundaries lying parallel to the plane of elec-
trode is negligible.
phase can be attributed to the multiplicity of eight 111
c
polarization axes of the rhombohedral phase as compared
with the six 100 axes of tetragonal phase. Moreover,
t
the component of the normal electric field for aligning of
111 -oriented dipole can be smaller than the electric
c
field required for altering 100 dipole to 001 orienta-
t
t
tion for these highly (100)t textured thin films.
E. Dc current-voltage characteristics
Shown in Fig. 10 is the leakage current measurement
(log J versus V1/2 plot) of the PZT thin films. For all the
measurements, a voltage was applied to the top elec-
trodes and swept from zero to positive voltage (0 to
10 V). The sweep step was 0.01 V with a waiting time of
1 s/step. Two distinct and nearly linear regions with a
transition occurring near 4 V (voltage1/2 ס 2 V1/2) were
observed for all curves. The leakage current of perovskite
ferroelectric thin films has been intensively stud-
ied.19,20,21 The low-field (voltage1/2 < 2 V1/2) conduction
with low current density (J ഛ 5 × 10−7 A/cm2) is gen-
erally recognized as a contribution from mobile charge in
the ferroelectric film. For the high-field region (volt-
age1/2 > 2 V1/2), the conducting mechanism may be due
to the Schottky effect, Poole–Frenkel effect, or space
charge limited current. Dietz et al.19 characterized the
leakage current of (Ba,Sr)TiO3 (BST) thin films and il-
The relationship between current density (J) and elec-
tric field (E) due to the Schottky effect can be expressed
as the following:19,20
ր
2
−q⌽0 + ␤SE1
J = AT2 exp
,
(2)
ͩ
ͪ
s
KBT
where A is the Richardson–Dushman constant, T is the
absolute temperature, q is the electronic charge, ⌽0 is the
Schottky barrier height, KB is the Boltzman constant, and
␤S is a constant. From Eq. (2), log(J) has a linear relation
with V1/2 if the conducting mechanism is Schottky effect
dominated and is generally in agreement with our
observation.
Although the actual mechanism controlling the I–V
behavior of PZT thin films can be even more compli-
cated, we might be able to describe the I–V behavior of
these PZT thin films with the following approach. In the
low-field region (voltage < 4 V; region I in Fig. 10), the
I–V curve of the PZT thin films capacitor exhibits ohmic-
like behavior contributed by mobile charges and leakage
current density is typically lower than 5 × 10−7 A/cm2. In
higher field (4 V < voltage < 6.5 V; region II in Fig. 10),
the log(J) versus V1/2 plot shows a linear region and
indicates the dominance of Schottky effect. For high-
field strength (voltage > 6.5 V), the deviation from lin-
earity in log(J) versus V1/2 plot indicates a complex
mechanism involving Schottky effect, Poole–Frenkel ef-
fect, and space charge limited current; furthermore, the
effect of series resistance from contact or probe tip may
also contribute to the nonlinearity.
IV. CONCLUSIONS
Highly textured PT thin films have been grown on an
LNO electrode over a large temperature range (450–
600 °C) using MOCVD techniques. The optimal growth
temperature for requisite electrical properties was found
to be 600 °C. TEM observation showed that the 90° do-
main started to form only for growth temperature higher
than 550 °C. The predominant domain structure is of the
180° type with their polar axis lying on the substrate
surface for PT thin films grown at lower temperatures.
FIG. 10. log(J) versus V1/2 plot of PZT thin films. The two distinct
nearly linear regions are indicated by dashed lines.
J. Mater. Res., Vol. 15, No. 1, Jan 2000
123

C.H. Lin et al.: Domain structure and electrical properties of highly textured PZT thin films grown on LaNiO3-electrode-buffered Si
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in the device operating voltage range was found. The
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ACKNOWLEDGMENTS
C.H.L., B.M.Y., H.C.K., T.B.W., and H.C. dedicate
this article to Prof. Gregogry E. Stillman, who passed
away in July 1999. We are deeply saddened by his pass-
ing and owed our gratitude to his scholarship, leadership,
and friendship. Prof. Stillman has been an outstanding
educator and researcher. His legacy will never be forgot-
ten by us. This work is supported by the United States
Department of Energy under the Contract No. DEFG02-
96ER45439 through the Frederick Seitz Materials Re-
search Laboratory, University of Illinois at Urbana-
Champaign (UIUC). The use of facilities in the Center
for Microanalysis of Materials at UIUC is deeply appre-
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