Activation field of ferroelectric (Pb,La)(Zr,Ti)O
3
thin film capacitors
T. K. Song,
a)
S. Aggarwal, A. S. Prakash, B. Yang, and R. Ramesh
Department of Materials and Nuclear Engineering and Center for Superconductivity Research, University
of Maryland, College Park, Maryland 20742
͑Received 22 July 1997; accepted for publication 11 August 1997͒
We report results on the activation field and frequency dependence of the coercive voltage in
epitaxial ferroelectric thin film capacitors. Frequency dependent hysteresis loops and pulse width
dependent polarization of epitaxial La
0.5
Sr
0.5
CoO
3
/͑Pb,La͒͑Zr,Ti͒O
3
/La
0.5
Sr
0.5
CoO
3
capacitor
structures were measured as a function of La content. The coercive voltages and their frequency
dependence vary systematically with increasing La content. We show that the activation field for
polarization reversal is directly related to the c/a ratio ͑tetragonality ratio͒ of the ferroelectric layer.
A larger c/a ratio leads to a larger field to activate the motion of domain walls through the lattice.
An important consequence of a larger activation field is a stronger pulse width dependence of the
pulse switched polarization. © 1997 American Institute of Physics. ͓S0003-6951͑97͒01341-7͔
The inherent hysteretic nature of ferroelectric materials
is the fundamental property that enables nonvolatile informa-
tion storage in a ferroelectric random access memory
͑FRAM͒. For high-speed and low-voltage operation of a
FRAM device, the ferroelectric capacitor should have large
values of pulsed switched polarization (⌬P) for small write
and read pulse widths, and low switching threshold and satu-
ration voltage. For this reason, it is important to understand
the mechanism of frequency and pulse width dependence of
ferroelectric properties and to develop methods which can
improve high-speed performance of ferroelectric capacitors.
Inherent in these studies is an understanding of the micro-
scopic mechanisms by which polarization reversal occurs in
these materials.
Over the past 30–40 years, considerable insight has been
developed into the switching behavior of single crystals and
ceramic materials.
1–6
A fundamental aspect of these studies
is the correlation between the switching time, t
s
, or the rate
of switching identified by the maximum switching current,
i
max
, and the applied field, E ͑or voltage V). In general, this
has been shown to have an exponential dependence as: t
s
ϭt
0
exp(
␣
/E)ori
max
ϭi
0
exp(Ϫ
␣
/E), where
␣
is an activation
field. This equality governs the switching dynamics through
the magnitude of
␣
. Using the above relationship between
the switching time and applied field as the basis, Wieder
established the correlation between log
e
͑frequency͒ and
1/E
c
.
6
The slope of this plot gives the value of
␣
, which in
the case of single crystal barium titanate is approximately 11
kV/cm. The correlation of the experimentally determined
value of
␣
to specific microstructural parameters such as c/a
ratio ͑tetragonality ratio͒ has not been addressed. Such a cor-
relation is the central theme of this letter.
In this letter, results from the frequency dependencies of
ferroelectric hysteresis loops and pulse width dependencies
of pulse switched polarization in epitaxial La
0.5
Sr
0.5
CoO
3
/
͑Pb,La͒͑Zr,Ti͒O
3
/La
0.5
Sr
0.5
CoO
3
͑LSCO/PLZT/LSCO͒ ca-
pacitors are reported as a function of La content. In earlier
papers, we have demonstrated that La substitution leads to a
systematic decrease in the c/a ratio as well as the c-axis
lattice parameter.
7,8
LSCO/PLZT/LSCO ferroelectric capaci-
tors were deposited on Si͑100͒ with a yttria-stabilized zirco-
nia ͑YSZ͒/bismuth titanate ͑BTO͒ barrier layer composite by
pulsed laser deposition. The details on the deposition and
electrical characterization of ferroelectric properties, such as
retention or fatigue properties were described elsewhere.
9
Ferroelectric hysteresis loops were obtained with a Sawyer–
Tower circuit in the frequency range from 300 Hz to 300
kHz. The resistive leakage of the ferroelectric capacitor was
compensated with a variable resistor connected in parallel
with a 6.6 nF polystyrene sense capacitor. Pulse switching
current response was measured with switching current mea-
suring circuit.
10
The heterostructures were characterized by x-ray diffrac-
tion Bragg and
scans to evaluate the crystalline quality.
X-ray
-2
diffraction patterns of (Pb
1.0
La
0.0
͒͑Zr
0.2
Ti
0.8
͒O
3
͓PLZT͑0/20/80͔͒ and (Pb
0.9
La
0.1
͒͑Zr
0.2
Ti
0.8
͒O
3
͓PLZT͑10/
20/80͔͒ thin films are shown in Fig. 1. Epitaxial quality in
both samples was confirmed by the presence of
͕
00l
͖
types
of peaks in the Bragg scans. In-plane
scans also showed
orientation locking of the ͓100͔ and ͓010͔ directions in the
plane of the film. The x-ray diffraction patterns showed the
PLZT͑0/20/80͒ film to consist of a mixture of c axis ͑i.e.,
͓001͔ normal to substrate͒ and a axis ͓͑001͔ in-plane͒ do-
mains. From the ͑200͒ and ͑002͒ peaks of the ferroelectric
layer, the lattice constants of c and a were calculated to be
4.11 Å and 3.95 Å, respectively, for the PLZT͑0/20/80͒ film
while the PLZT͑10/20/80͒ film showed lattice constants of
cϭ 4.025 Å and aϭ3.96 Å. The c/a ratios were 1.041 for
the PLZT͑0/20/80͒ and 1.016 for the PLZT͑10/20/80͒, re-
spectively.
In Fig. 2, ferroelectric hysteresis loops from a PLZT͑0/
20/80͒ capacitor at frequencies of 1, 10, and 100 kHz are
compared with those of the PLZT͑10/20/80͒ capacitor. These
hysteresis loops demonstrate one main difference as a con-
sequence of La substitution, namely the coercive voltages are
dramatically lower with 10% La substitution. Different
maximum applied voltages were used, 10 V for PLZT͑0/20/
80͒ and 5 V for PLZT͑10/20/80͒, because V
c
of the two
capacitors were very different. The PLZT͑10/20/80͒ capaci-
tor had a V
c
ϳ1 V and hence a maximum voltage of5Vwas
chosen to saturate the capacitor. We chose a commensurate
a͒
Electronic mail: tksong@wam.umd.edu
2211Appl. Phys. Lett. 71 (15), 13 October 1997 0003-6951/97/71(15)/2211/3/$10.00 © 1997 American Institute of Physics