Full text of DOI:10.1557/JMR.2002.0278

Fatigue-free La-modified Pb(Zr,Ti)O3 capacitors using a
seed layer
Santiranjan R. Shannigrahi, Sun-Hwa Lee, and Hyun M. Janga)
Department of Materials Science and Engineering and National Research Laboratory (NRL) for
Ferroelectric Phase Transitions, Pohang University of Science and Technology (POSTECH),
Pohang 790-784, Republic of Korea
(Received 16 July 2001; accepted 20 May 2002)
The development of lead zirconate titanate (PZT)-based capacitors using common Pt
electrodes has been a long-time goal of ferroelectric random access memories (FRAM).
In this work, a series of Pb1−xLax(Zr0.55Ti0.45)O3 capacitors (for 0.01 ഛ x ഛ 0.05) having
fatigue-free characteristics have been grown on Pt/ Ti/SiO2/Si substrates. Typically 2–3
mol% La-modified PZT capacitors fabricated at 580 °C by applying a PZT seed layer
exhibited fatigue-free behavior up to 6.5 × 1010 switching cycles, a low coercive field of
50–55 kV/cm, and a stable charge retention profile with time, all of which assure their
suitability for the future nonvolatile FRAM.
There is a great need to find suitable capacitors to meet
the present trend in nonvolatile ferroelectric random ac-
cess memories (FRAM) toward lower operating volt-
ages.1,2 In this challenging search to find suitable
materials, once again, we consider lead zirconate titanate
(PZT)-based perovskites as potential candidates for
FRAM1–4 and microelectromechanical systems5 applica-
tions because of their high Pr values, easy switching
characteristics, and dramatic domain refinement up to a
nanometer size, in addition to relatively low processing
temperatures and low costs. However, some serious
problems related to the reliability must be overcome be-
fore the practical implementations. These difficulties are
mainly arising from the poor fatigue resistance of PZT
after a certain number of switching cycles.
To solve the problems associated with the electrical
fatigue without using electrically leaky and expensive
oxide electrodes,6–9 in this study we have focused on the
intrinsic oxygen vacancies, which are known to play a
key role in the ferroelectric degradation of capacitors in
conjugation with film/electrode interface and domain
mobility.10,11 The concentration of oxygen vacancies can
be substantially reduced by the Pb-site vacancies (VPbЉ)
under the condition of donor-doping. This can be pre-
dicted by considering the following intrinsic defect re-
action in Pb-based perovskites: PbO ס VPbЉ + VO¨ with
Kint ס [VPbЉ][VO¨]. However, Kint increases rapidly
with temperature. Therefore, one should reduce the proc-
essing temperature to suppress the formation of oxygen
vacancies even under the donor-doping typically using
lanthanum (La). The concentrations of oxygen vacancies
that are primarily responsible for the electrical fatigue
can further be reduced by annealing La-modified PZT
films in an oxygen-rich atmosphere. Therefore, one of
the efficient ways of suppressing the formation of oxy-
gen vacancies could be low-temperature processing in an
oxygen-rich atmosphere. However, the processing tem-
perature of La-doped PZT (PLZT) is known to be sub-
stantially higher (ജ700 °C) than that of undoped PZT
film (ഛ600 °C).12
In view of these requirements, one of the feasible ap-
proaches is to insert a suitable thin seed layer between
the PLZT film and the electrode.13,14 It is well estab-
lished that the PZT-based perovskite phase is evolved
from the pyrochlore phase by nucleation and grain
growth.15 The nucleation is strongly dependent upon the
underlying layer. Therefore, a low-temperature proc-
essing can be achieved if the underlying seed layer pro-
vides favorable nucleation sites that tend to reduce the
activation energy for perovskite crystallization. Thus, we
have chosen PZT as a seed layer with an anticipation of
its three important functions because of compositional
and structural similarities between PZT and PLZT. They
are (i) lowering the processing temperature by reducing
the activation energy required for the nucleation/growth
of the perovskite phase, (ii) maximizing the preferential
growth of the PLZT film to a desired direction by suit-
ably controlling the growth direction of this template
PZT layer, and (iii) practically acting as a buffer layer
between the PLZT film and the electrode.
The main purpose of this study is to demonstrate fa-
tigue-free behavior of PZT-based capacitors with a series
of different La contents. The composition of the ferro-
electric layer can be summarized as Pb1−xLax(Zr0.55Ti0.45)O3
a)Address all correspondence to this author.
e-mail: hmjang@postech.ac.kr
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with x between 0.01 and 0.05 (i.e., PLZT with 1 to
films are all above 0.96, indicating a remarkably strong
[111] orientation.16 Contrary to these, the PLZT film with-
out the seed layer exhibited a random orientation with the
presence of an amorphous phase. However, this amor-
phous phase gradually disappeared and the degree of
[111] preferential orientation of the PLZT layer in-
creased with the annealing temperature up to 640 °C.
This suggests that the observed random orientation of the
PLZT layer at 580 °C in the absence of the PZT seed
layer is closely related to the higher phase-formation
temperature of perovskite PLZT, as discussed previously
(i.e., ഛ600 °C for PZT films versus ജ700 °C for PLZT
film).12
The room-temperature P–E hysteresis loops of PLZT/
PZT capacitors with a variety of the La contents are
shown in Fig. 2. A typical area of the top Pt electrode was
10−4 cm2, and measurements of ferroelectric properties
were performed using a RT6000S ferroelectric tester. All
of these capacitors fabricated on the bottom Pt electrode
exhibited fatigue-free behavior up to, at least, 5 × 1010
switching cycles. As summarized in the inset, the rema-
nent polarization (2Pr) decreases gradually with increas-
ing La content but the coercive field (Ec) first decreases
for the La content up to 3 mol% and then increases
slightly above this critical level. Considering the impor-
tance of fatigue-free capacitors with low operating volt-
ages, we will hereafter focus on the PLZT/PZT capacitor
having 3 mol% of La modification.
5 mol% of La). For the structural and compositional
similarities between the seed layer and the active PLZT
layer, we have employed the PZT layer having the com-
position of Pb(Zr0.55Ti0.45)O3 as a seed layer on (111)
Pt/Ti/SiO2/Si substrate.
The PZT and PLZT precursors were synthesized on
the basis of the above chemical formulas using lead ac-
etate trihydrate, zirconium propoxide, titanium isopro-
poxide, and lanthanum acetate hydrate in acetic acid
solution. The prepared sols were diluted with alcohol and
water to a concentration of 0.25 mol/l and used for the
coating. The sols were spun coated at 4000 rpm for 20 s
to form uniform gel films on (111) Pt/Ti/SiO2/Si sub-
strates. For the removal of organics, the deposited films
were directly inserted into a furnace, preheated to a tem-
perature of 500 to 650 °C for 0.5 h. The coating and
firing process was repeated several times. Both the firing
and annealing operations were performed in flowing
oxygen ambient. The film thickness, as estimated by the
cross-sectional field-emission scanning electron micros-
copy, was slightly less than 300 nm.
Figure 1 shows the XRD ␪–2␪ scan results (M18XCE
diffractometer, MAC Science) of the annealed PLZT films
with and without the PZT seed layer. The PLZT films de-
posited on the PZT seed layer show a highly [111]-
oriented preferential growth. If one takes the lattice
match between the chosen PZT and PLZT compositions
into consideration, the preferential growth along the
[111] direction seems reasonable. The relative peak in-
tensities, I(111)/{I(111) + I(100)}], of the seeded PLZT
Figure 3 compares P–E hysteresis loops of the Pt/
PLZT/PZT/Pt capacitor at three different applied volt-
ages. The capacitor is characterized by well-saturated
P–E curves. The inset of Fig. 3 summarizes the variation
of 2Pr and Ec with varying applied voltage. 2Pr of the
FIG. 2. Polarization hysteresis loops of PLZT/PZT capacitors meas-
ured at 25 °C. The inset shows variations of 2Pr and Ec with the La
content at three different voltages.
FIG. 1. XRD patterns of PLZT and PLZT/PZT films fabricated on
Pt/Ti/SiO2/Si substrates and annealed at 580 °C for 0.5 h.
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capacitor was 30 ␮C/cm2 at an applied voltage of 5 V
The retention profile was fairly independent with time
up to 104 s, as shown in Fig. 5. The normalized retaining
charge of the PLZT capacitor was estimated by compar-
ing these values with 2Pr, and it was approximately 70%.
Compared with the retention characteristics of the Pt/
PZT/Pt capacitor (approximately 45%),20 this is a re-
markable improvement. It is known that oxygen vacancy
and increased to 48 ␮C/cm2 at 10 V. These values are
substantially higher than those of the fatigue-free layered
perovskites, SrBi2Ta2O9 (SBT)17 and Bi3.25La0.75Ti3O12
(BLT),18,19 which range between 6 and 28 ␮C/cm2 at
10 V. The low coercive field of 50 kV/cm makes this
capacitor especially desirable for the application to low-
voltage-driving devices.
Figure 4(a) shows the P–E hysteresis loops of the
PLZT capacitor (3 mol% La) before and after the elec-
trical fatigue test at a frequency of 1 MHz. As presented,
the capacitor is characterized by a well-saturated P–E
curve even after being subjected to 6.5 × 1010 switching
cycles. The P–E curve taken after the fatigue test (filled
circles) does not show any noticeable asymmetric behav-
ior caused by imprinting failures. The fatigue-free char-
acteristics of the PLZT capacitor (3 mol% La) are
summarized in Fig. 4(b). The capacitor shows little
change both in the switching polarization (Psw) and in the
nonswitching polarization (Pns) up to 6.5 × 1010 switch-
ing cycles. As shown in Fig. 4(b), the values of the non-
volatile charge [i.e., (+Psw) − (+Pns) or (−Psw) − (−Pns)]
are approximately 22 ␮C/cm2 and remain essentially
constant throughout the switching cycles. Thus, the PLZT
capacitor has a sufficient sensing margin for the identifica-
tion of logical state throughout the read/write cycles. The
fatigue-free behavior has also been observed for other
switching voltages up to 10 V. The values of the non-
volatile charge at a lower frequency (e.g., at 1 KHz) were
essentially the same as those measured at 1 MHz (approxi-
mately 22 ␮C/cm2).
FIG. 4. Results of the fatigue test at 1 MHz: (a) room-temperature
P–E hysteresis loops of the Pt/PLZT/PZT/Pt/Ti/SiO2/Si capacitor
(3 mol% La) at an applied voltage of 5 V before and after being
subjected to 6.5 × 1010 read/write switching cycles; (b) variation of
+Psw, +Pns, −Psw, and −Pns versus number of cycles at a switching
voltage of 5 V. The corresponding films were annealed at 580 °C for
0.5 h.
FIG. 3. Polarization hysteresis loops of the PLZT/PZT capacitor
(3 mol% La) on a Pt/Ti/SiO2/Si substrate at three different applied
voltages.
FIG. 5. Retention profile of the Pt/PLZT/PZT/Pt/Ti/SiO2/Si capaci-
tor (3 mol% La) measured at 25 °C.
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is one of the primary causes of the retention failure.21
Thus, the observed stable retention characteristics seem
to be closely related to the reduced concentration of oxy-
gen vacancies in the present PLZT capacitors. The leak-
age current density is another important parameter for
device applications. It was as low as 5 nA/cm2 at an
applied field of 150 kV/cm (approximately 5 V).
We have reported innovative ferroelectric properties
of the PLZT capacitors prepared using a PZT seed layer.
The fatigue-free behavior and the stable retention profile,
along with the low coercive field, observed in the PLZT
capacitor (3 mol% La) potentially make it a promising ca-
pacitor for the future nonvolatile FRAM.
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ACKNOWLEDGMENT
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This work was supported by the KISTEP of Korea
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