SrBi
4
Ti
4
O
15
thin films and their ferroelectric fatigue behaviors
under varying switching pulse widths and frequencies
Shan-Tao Zhang,
a)
Bin Yang, Yan-Feng Chen, and Zhi-Guo Liu
National Laboratory of Solid State Microstructures and Department of Materials Science and Engineering,
Nanjing University, Nanjing 210093, People’s Republic of China
Xiao-Bo Yin, Yuan Wang, Mu Wang, and Nai-Ben Ming
National Laboratory of Solid State Microstructures and Department of Physics, Nanjing University,
Nanjing 210093, People’s Republic of China
͑Received 17 July 2001; accepted for publication 20 November 2001͒
Polycrystalline ferroelectric SrBi
4
Ti
4
O
15
thin films were prepared on Pt-coated silicon substrates by
pulsed laser deposition. Structures of the films were analyzed by x-ray diffraction, atomic force
microscopy ͑AFM͒ and scanning electron microscopy. At an applied field of 275 kV/cm, the films
showed good hysteresis loops with remnant polarization (P
r
), saturated polarization (P
s
) and
coercive field (E
c
) of 3.11
C/cm
2
, 7.81
C/cm
2
and 68 kV/cm, respectively. At 120 kV/cm
switching pulse field, fatigue tests were carried out systematically by varying the switching
pulsewidth with a fixed duty cycle and with a fixed switching period, respectively. A gradual
increase followed by an abrupt increase of the fatigue rate was observed with the increase of the
switching pulsewidth. Field-induced defect diffusion was used to explain the results. © 2002
American Institute of Physics. ͓DOI: 10.1063/1.1435414͔
I. INTRODUCTION
In recent years ferroelectric materials have generated a
lot of interest from an application as well as from an aca-
demic point of view. Due to fast switching speed and low
operating voltage, ferroelectric thin films have been inte-
grated into silicon integrated circuits to provide high speed,
high density, high write/read endurance, radiant-hardened
nonvolatile ferroelectric random access memories
͑FeRAMs͒.
1–3
Many materials have been studied as candi-
dates for FeRAMs. Among them, Pb(Zr
x
Ti
1Ϫx
)O
3
͑PZT͒
thin films are the most promising candidates because of their
larger remnant polarization and lower processing tempera-
ture. But commercial use of PZT ferroelectric memory de-
vices has been hindered largely by fatigue, defined as the
decrease of switchable polarization with electric field
cycling in ferroelectrics.
3–5
Most recently, studies to
solve the problem of fatigue have concentrated on
replacing PZT thin films by bismuth-layered ferroelectric
thin films because of the latter’s fatigue-free properties
even with Pt electrodes.
6
Bismuth-layered ferroelectrics
belong to a family of Aurivillius phases which has a
general formula (Bi
2
O
2
)
2ϩ
(A
mϪ1
B
m
O
3m ϩ1
)
2Ϫ
, where A
ϭSr
2ϩ
,Ba
2ϩ
,Bi
3ϩ
, etc., BϭTi
4ϩ
,Ta
5ϩ
,Nb
5ϩ
, etc. and m
ϭ1,2,3,4, etc. which represents the BO
6
octahedra number
regularly interleaved by (Bi
2
O
2
)
2ϩ
layers.
7,8
SrBi
2
Ta
2
O
9
͑SBT͒, a member of that family, has been most widely
studied.
9,10
But SBT thin films have some drawbacks, for
example, low Curie temperature ͑310 °C͒,
11
which may in-
duce a drift in temperature of ferroelectric properties,
12
and a
high processing temperature ͑Ͼ800 °C͒.
6
So, it is of great
interest to investigate ferroelectric properties of SrBi
4
Ti
4
O
15
͑SBTi, for which AϭSr, Bi, BϭTi and mϭ4͒, which is
another member of the family with a relatively higher Curie
temperature ͑530 °C͒.
11
On the other hand, it has been reported that ferroelectric
fatigue is determined by the competition between domain
wall pinning and unpinning.
13,14
Charged defects have been
believed to play a detrimental role in ferroelectric fatigue
because they can be trapped at domain boundaries and thus
increase the domain pinning rate and decrease the unpinning
rate.
15,16
Since charged defects can drift under switching
fields, not only the magnitude but also the pulsewidth of
applied switching pulse fields should have a great effect on
the trap rate of charged defects. The former effect has been
widely studied,
17,18
however, the latter has not been empha-
sized systematically so far.
In this article, we report structural and ferroelectric prop-
erties of SBTi thin films prepared by pulsed laser deposition
͑PLD͒ on Pt/TiO
2
/SiO
2
/Si substrates. The fatigue depen-
dence on the pulsewidth of the applied switching pulse field
will be especially addressed.
II. EXPERIMENTAL DETAILS
SBTi pellets used as PLD targets were prepared by solid
state reaction with starting materials SrCO
3
,Bi
2
O
3
and
TiO
2
. Excess 20 mol % Bi
2
O
3
was added to compensate for
bismuth evaporation during the process. A mixture of
SrCO
3
,Bi
2
O
3
, and TiO
2
with a ratio of Sr:Bi:Tiϭ1:4.8:4
was ball milled for 12 h, preheated at 700 °C for 2 h and then
pressed into pellets. Finally the pellets were fired at 1050 °C
for2h.
SBTi thin films were fabricated at 750 °C for 15 min
using a KrF exciter laser ͑LPX205i, Lambda Physik͒ of 248
nm wavelength and 30 ns pulsewidth. In our experiments,
a͒
Electronic mail: yfchen@netra.nju.edu.cn
JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 5 1 MARCH 2002
31600021-8979/2002/91(5)/3160/5/$19.00 © 2002 American Institute of Physics