Electrode contacts on ferroelectric Pb(Zr x Ti1−x )O3 and SrBi2Ta2O9 thin films
and their influence on fatigue properties
J. J. Lee, C. L. Thio, and S. B. Desu
Citation: Journal of Applied Physics 78, 5073 (1995); doi: 10.1063/1.359737
View online: http://dx.doi.org/10.1063/1.359737
View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/78/8?ver=pdfcov
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Electrode contacts on ferroelectric Pb(Zr,Ti, -JO3 and SrBi2Ta209
thin films and their influence on fatigue properties
J. J. Lee,a) C. L. Thio, and S. B. Dew
Mrginia Polytechnic Institute and State University, Department
of
Materials Science and Engineering,
Blacksburg, Wrginia 24061-0237
(Received 1 May 1995; accepted for publication 5 July 1995)
The degradation (fatigue) of dielectric properties of ferroelectric Pb(Zr,Ti,-,)O, (PZT) and
SrB&Ta,O, thin films during cycling was investigated. PZT and SrBi2Ta,0g thin films were
fabricated by metalorganic decomposition and pulsed laser deposition, respectively. Samples with
electrodes of platinum (Pt) and ruthenium oxide (Ru02) were studied. The interfacial capacitance (if
any) at the PVPZT, RuO.$PZT, and Pt/SrBi,Ta20s, interfaces was determined from the thickness
dependence of low-field dielectric permittivity (E,.) measurements. It was observed that a low er
layer existed at the Pt/PZT interface but not at the RuO#‘ZT and Pt/SrBi2Ta,0, interfaces. In the
case of Pt/PZT, the capacitance of this inter-facial layer decreases with increasing fatigue while the
er of the bulk PZT film remains constant. This indicates that fatigue increases the interfacial layer
thickness and/or decreases interfacial layer perrnittivity, but does not change the bulk properties. For
the capacitors with Ru02/PZT/Ru0, and Pt/SrBi,Ta,Og/Pt structures, however, the 4 does not
change with ferroelectric film thickness or fatigue cycling. This implies no interfacial layer exists at
the interfaces and which can be correlated to the observed nonfatigue effect. Additionally, the
equivalent energy-band diagrams of these different capacitor structures were proposed to
complement the proposed fatigue mechanism. 0 1995 American Institute of Physics.
I. IPJTRODUCTION
Ferroelectric thin films for use in nonvolatile memories
have drawn much attention in recent years due to their
bistable nature.lm3 Among the many families of ferroelec-
tries, perovskite Pb(Zr,Ti,-,)O, or PZT thin films are the
most extensively studied. Fatigue, retention, and imprint
problems, however, limit the realization of these memories
on a commercial scale. There has been great concern and
interest in understanding the fatigue phenomena and in find-
ing a fatigue free ferroelectric material for use in non-volatile
memories. Some of the models that are pertinent to thin films
suggest space-charge layers at the interface,4 accumulation of
charged particles at the interface,5 or growth of conductive
dendrite8 to be the cause of fatigue. Regardless of the details
of the proposed mechanisms, most of the fatigue models re-
late the loss of polarization during reversals to the movement
of point defects. Recently, Desu and Yoo7 have quantitatively
proposed that defect entrapment at the ferroelectric-electrode
interfaces, which is followed by an effective one-directional
movement of defects by an internal field difference, to be the
fatigue mechanism for thin-film ferroelectrics. The most ob-
vious candidate for a mobile defect that can be trapped at
ferroelectric-electrode interfaces is the oxygen vacancy,
”
VO*
7-9 which has been identified as being responsible for the
degradation of the leakage current (dc degradation) in ca-
pacitors based on similar materials.‘* Additionally, the newly
developed fatigue-free ferroelectric thin films based on a Bi-
layered structure, e.g., SrBi,TazO, and SrBi,Nb,O,, have
“Current address: SHARP Microelectronics Technology, Inc., 5700 NW Pa-
cific Rim Blvd., Camas, WA 98607; Electronic mail: jlee@shqwa.com
been produced in many laboratories,‘*-13 and the fatigue-free
effect could be due to their low-oxygen vacancy concen-
trations.”
Fatigue is the decrease of switchable polarization with
increased number of switching cycles. It is well known that
ferroelectric PZT thin films with Pt electrodes exhibit severe
fatigue problems, while less fatigue is observed for PZT thin
films with oxide electrodes. It is believed that oxygen vacan-
cies in PZT thin films can easily exchange with the oxygen
in oxide electrodes, thereby reducing the oxygen vacancy
pileup at interfaces and reducing the fatigue. One of the
goals of this work is to provide indirect experimental evi-
dence that is consistent with the postulation of oxygen va-
cancy entrapment at ferroelectric-electrode interfaces.7v8
Ferroelectric capacitors with Pt/PZT/Pt and RuO,/PZT/RuO,
structures are fabricated. Whether the oxygen vacancies ac-
cumulated at PtfPZT and RuO#ZT interfaces during the
fatigue process is determined from the thickness dependence
of low-field dielectric measurements. l4
Consequently,
equivalent energy-band diagrams for fresh and fatigued ca-
pacitors with Pt/PZT/Pt and RuOz/PZT/RuO, structures can
be proposed to complement the observed effects.
For the Bi-layered ferroelectrics, however, less oxygen
vacancy concentration, [Vi], could be present due to the low
volatility of their oxide components, e.g., SrO, Bi203,
Ta205, Nb,O,. The Bi-layered ferroelectric crystal (e.g.,
SrBi,TazOs) can be considered to be a combination of a ma-
terial having the formula Bi,Os and a material having the
formula SrTa,O,. The latter has a tungsten bronze structure
by itself, but has a perovskite structure within the SrBi2Ta,0g
unit cell.
” Therefore, even if the Vi exist, they can be
trapped in the B&O3 layers during the course of fatigue with
no polarization loss. The equivalent energy-band diagram for
J. Appl. Phys. 78 (8), 15 October 1995 0021-8979/95/78(8)/5073/6/$6.00 0 1995 American Institute of Physics
5073
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1400, . . . , I . . , , , I I . ,
Pt/SrBi,Ta,O&‘t capacitors is also proposed for comparison
with the previous two proposed band diagrams.
II. EXPERIMENT
The test PZT samples chosen for this study were fabri-
cated by
metalorganic decomposition (MOD) on
Pt/Ti/SiO#i and RuO#iO#i substrates. All PZT samples
had the same composition: PbZr,Tit-x03 with x=0.53 and
10% excess PbO. Thin films were sintered in a preheated
furnace at 650 “C for 30 min in an 0, atmosphere. Layered
ferroelectric SrBi*Ta,O, thin films were deposited by pulsed
laser deposition (PLD) onto PUTilSiO,/Si substrates.” Films
of four to six different thicknesses were fabricated and their
thicknesses were determined by variable angle spectroscopic
ellipsometry. Samples were then loaded in sputtering systems
for deposition of top electrodes through a shadow mask. The
Pt top electrodes were deposited on SrBi,Ta,O@W’TilSiO,ISi
and PZT/Pt/TilSi02/Si samples at room temperature, while
the RuO,
top
electrodes
were
deposited on
PZT/RuO,/SiO#i samples at 200 “C.
Electrical measurements were done on discrete capaci-
tors using a probe station. Low-field dielectric properties
were carried out using a multifrequency LCR meter (HP-
9192A) with an oscillation voltage of 50 mV and frequency
of 10 kHz. The impedance spectrum analysis was performed
over the frequency range of 102-lo5 Hz. The high-field
P-E hysteresis loops were characterized on a RT66A Stan-
dardized Ferroelectric Test System. The fatigue characteriza-
tion was also carried out by the RT66A in conjunction with
an external signal generator, an HP-8116A pulse/function
generator, with cycling frequency range of OS- 1 MHz. Vari-
ous fatigue periods were carried out from 10’ to lo9 cycles.
Following each of the fatigue periods, low-field dielectric
measurement and impedance spectrum analysis were per-
formed.
111. RESULTS AND DISCUSSION
A. PffPZVPt
The thickness dependence of the dielectric permittivity
of the capacitors with the Pt/PZT/Pt structure is illustrated in
Fig. 1. The effective dielectric permittivity (6,) increases
from 680 (0.11 pm) to 1100 (0.73 ,um) in fresh samples. It is
also demonstrated in Fig. 1 that er of PZT capacitors de-
creases as cycling proceeds. Similar thickness dependence of
er has been reported for PMN, BaTiO,, and PLZT thin films
prepared by various techniques. The proposed mechanisms
to interpret this behavior include the formation of Schottky
barriers at the interfacesI and changes in grain size or den-
sity effects.t7”*
Recently, it has been demonstrated by Lee
and Deyr4’t9
that the formation of a low er Schottky deple-
tion layer at the Pt/PLT contact lowers the effective er for
thin PLT films. The Schottky depletion layer exhibited a
lower er due to the dielectric saturation and piezoelectric
compression.
Similar to the Pt/PLT contact, the Schottky behavior of
the Pt/PZT contact has also been observed.20 The equivalent
energy-band diagram of a Pt/PZTlPt capacitor is schemati-
cally illustrated in Fig. 2(a). The space-charge concentration
L
_ !=-tlPzTlR
-0-O
-9-l
-. lo6
-v-10’
-
. . ..- I#
-0,. 109
200 ’ ’ * ’ a ’ . ’ ’ ’ a 3
0.0 0.2
0.4 0.6
0.8
Thickness @m)
PIG. 1. Thickness and fatigue cycles dependence of dielectric permittivity
for capacitors with Pt/PZT/Pt structure.
due to the oxygen vacancy (i.e., [Vi]) at the surface and the
inner ferroelectric region for PZT thin films have been esti-
mated to be 5 X 10” and 1 X 1018 cme3, respectively.2’ There-
fore, the band bending associated with Schottky contacts ex-
hibited a higher slope near the contacts and a lower slope
near the bulk of the PZT film. Note that the depletion regions
extend right into the interior of the PZT thin film, a result of
the high er of the PZT, or of the total space charge being too
low. Because the depletion regions do not effectively screen
the interior from the surface, the energy from the lowest
point of the bottom of the conduction band to the Fermi level
[i.e., qV,, in Fig. 2(a)] is greater than (q t,bj - qx), where q t,bi
and q,y are the work function and electron affinity of PZT
film, respectively.
According to Fig. 2(a), the measured capacitance (C,)
of Pt/PZT/Pt capacitor is composed of three capacitors in
series: a capacitor representing the bulk material (C,) and
two representing the interfaces (Ci). The capacitance value
of the sample can be formulated as follows:
(a)
Top
Bottom
(b)
FKL 2. Proposed energy-band diagrams of capacitors with Pt/PZTiPt struc-
ture at states of (a) fresh and (b) after fatigue. The dash line in (b) is for the
same capacitor with fresh state.
5074 J. Appl. Phys., Vol. 78, No. 8, 15 October 1995
Lee, Thio, and Desu
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0.2 0.4 0.6 0.8
Thickness (fhm)
FIG. 3. The fatigue cycle and thickness dependence of l/C, for Pt/PZT/F’t
capacitors. The parallel lines indicate that the g does not change with fa-
tigue cycles.
1 1 2
-=
c, c,‘c,’
(1)
dn,
dB
2di
-=
VOA
-+-
EBM
ei eOA ’
(2)
where d, and er are total sample thickness and dielectric
constant respectively; d, and es are bulk thickness and di-
electric constant respectively; di and Ei are interfacial layer
thickness and dielectric constant respectively; 6 is the per-
mittivity of free space; and A is the sample area. The inter-
facial layer thickness (di) has been estimated to be -200
A ‘9*22 which is much smaller than the film thickness (d,).
Therefore, dB ==d, , and E@ (2) can be approximated by
1 d, 2
c,=-+-
EBEOA Ci *
(3)
Thus, a plot of l/C, vs d,, yields l/Ci from the y-axis in-
tercept and bulk thin-film permittivity 6s from the slope.
Figure 3 illustrates such plots for capacitors with different
fatigue periods along with their corresponding Ed. Note that
if d, was known and l/C, vs dB were plotted, the slopes of
these plots should be same as in Fig. 3. Hence, Fig. 3 can
distinguish the factors of inter-facial capacitance (CJ and
bulk dielectric constant (eg) for each period of fatigue cy-
cling. The almost parallel lines among all the fatigue periods
implied that es was constant and Ci was decreasing as the
fatigue progressed. By rearranging the data, Fig. 4 illustrated
the change ratio of es and Ci along the fatigue periods. In
summary, more capacitor fatigue resulted in a wider deple-
tion width and/or a smaller dielectric permittivity at the in-
terface but the bulk dielectric permittivity remained constant.
A quantitative fatigue mechanism has been developed by
Desu and Yoo7 based on defect entrapment at ferroelectric-
electrode interfaces. These defects can drift to interfaces due
to the resultant internal field difference. The most probable
candidate for the mobile defect is the oxygen vacancy
Initial
10' 10s
10'
109
Number of Cycles
FIG. 4. The change ratios of Ci and ES on Pt/PZT/Pt capacitors after various
fatigue periods.
(Vi). Note that the oxygen vacancy concentration for the
fatigued PZT capacitor could be higher than that of the fresh
sample, and that the mechanism for the generation of Vi was
proposed by Mihara et ~1.~~ Interestingly, the observed data
(Ed constant, Ci decreasing) in this study also indicates that a
higher concentration of Vi was introduced at the Pt/PZT
interfaces as the fatigue progressed. The equivalent energy-
band diagram for the fatigued PZT capacitors with the struc-
ture Pt/PZT/Pt can then be represented as in Fig. 2(b). The
higher concentration and higher total amount of Vi induce a
deeper band bending and a higher effective interfacial layer
thickness (di), respectively, at the Pt/PZT interfaces. The
former results in a higher interfacial electric field and conse-
quently a lower E, at the interfaces. These two effects (i.e.,
low 6r and high di) keep lowering the interfacial capacitance
(CJ and effective er of the PZT capacitor as the fatigue
progresses. Note that the internal electric field [i.e., slope of
the conduction band in Fig. 2(b)] in the bulk PZT thin film is
diminished as the Vi moves toward the interfaces, explain-
ing the slight increase in eB with increasing fatigue period
(Fig. 4).
The P-E hysteresis loops for a Pt/PZT/Pt capacitor
measured under various conditions are shown in Fig. 5. A
symmetric P-E hysteresis loop with
P,-21
,&/cm2 and
EC-37 kV/cm was observed in the fresh PZT capacitor with
an applied voltage of 5 V (loop A in Fig. 5). This capacitor
was then subjected to a fatigue test (+5 V, 1 MHz) with
4X lo7 cycles which yielded a diminished P, (loop B in Fig.
5). The P, of the fatigued capacitor, however, can be re-
gained with an applied voltage of 14 V (loop C in Fig. 5).
This experiment indicates that domains within the bulk of a
fatigued PZT thin film can be activated by a higher applied
voltage. Alternatively, the physical properties of the bulk of
PZT thin film are not altered by the fatigue cycling. The
lowering of P, at lower applied voltages of the fatigued ca-
pacitor could be due to either domains locked by the surface
space charges or a smaller fraction of the applied voltage
seen by the bulk of PZT thin films. This result is consistent
with the unchanged 6s for fatigued capacitors.
Furthermore, we can represent the PZT capacitor as an
equivalent RC circuit shown in the closed caption of Fig. 6.
J. Appl. Phys., Vol. 78, No. 8, 15 October 1995 Lee, Thio, and Dew
5075
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0.33 pm
40
Am: 2.1X104 cm’
htlgw: +/- .5v, 1
MHZ,
L-a-
B 20
3
s 0
5
= B -20
-5 0 5 10 15
Voltage (V)
PIG. 5. The P-E hysteresis loops for a Pt/PZT/Pt capacitor measured under
various conditions. Loop A: fresh capacitor with an applied voltage of 5 V
Loop B: fatigued capacitor with an applied voltage of 5 V. Loop C: fatigued
capacitor with an applied voltage of 14 V.
By using impedance spectroscopic techniques, different RC
components in the equivalent circuit of any dielectric me-
dium can be separately identified. The total impedance of a
simple RC circuit can be expressed as Z= R -jX. The resis-
tance (R) and reactance (X) of the impedance (Z) for this
equivalent RC circuit can be derived from following equa-
tions:
L
R=
2Ri
RB
1 + o’R;C; + 1 + w’R;C; ’
x=
20R~Ci
wR&,
1 + w’R;Cf + 1 + w’R;C; ’
(4b)
where R, and R, are the resistance of interfacial layer and
bulk, respectively; and w is the applied frequency. Note that
the reactance X also equals (- ~/WC,). Therefore, the effects
of the various RC components can be separately identified
by looking at the low-frequency peaks in the impedance
spectra (plotting -X vs w) if the various C’s have signifi-
cantly different values from one another.
The impedance spectrum of the fatigued Pt/PZT/Pt ca-
pacitor is illustrated in Fig. 6. The impedance spectrum re-
1 10
Frequency (kHz)
FIG. 6. Impedance spectrums of the Pt/PZT/Pt capacitor after various fa-
tigue periods.
1300 .II.,III.,~~~.,111.,..1.
(3
2
1200 1
.I
i
l
l
‘5
l .
l
i
$i
1100 -
RUO~/PZT/RUO*
Arsa: 2.1XiO~ cm.2
0
Fmq: 10 kHz
VG50mV
loo0 ~,,~‘~~(~‘~~,,‘~~~~“*~~
0.0
0.1 0.2 0.3 0.4
0.5
Thickness (pm)
1400
, 1 3 8 . I 7
3
- 0.4
2
ET
i
z!
RU02/PZT/RUOZ
ii 1200 -
Thickness: 0.33 pm
Area:2.1X104cm2
3
8
Fatigue: +/- 5V. 1 MHz
Freq: 10 ltHz
V~50mV
1100
’ 3 ’ 3 4 8 ’
’
InhI
10’ lo3 lo5
10’
log
Number of Cycles
FIG. 7. (a) Thickness and (b) fatigue-cycle independence of
l
, on
Ru02/PZTiRu02 capacitors.
vealed low-frequency peaks which were beyond the scope of
the analyzer. These peaks were the effects of RC resonance
at the interface of Pt/PZT, since we assumed that CB had a
significantly lower value compared with Ci in Eq. (4). As the
fatigue cycles progressed, the resonance frequencies gradu-
ally increased (right shifted) due to the reduction of the RC
constant at the interface. Although the shift implied the re-
duction of either Ci or Ri, it was consistent with the above
discussion that Ci decreased gradually with fatigue due to
the increasing of depletion-layer thickness and decreasing of
depletion-layer permittivity.
B. RuO,/PZT/RuO,
For the capacitor with the structure Ru02/PZT/Ru02,
however, values of er are essentially constant (-1200) for
different capacitor thicknesses [Fig. 7(a)]. Note that the .+ of
the Pt/PZT/Pt capacitor presented in Fig. 4 is also -1200.
Additionally, er of Ru02/PZT/Ru02 capacitors does not de-
grade with increasing fatigue cycles [Fig. 7(b)]. These obser-
vations imply that there is no interfacial layer between RuO,
and PZT. The absence of depletion layers at RuO,/PZT in-
terfaces and the ability of RuO, electrodes to consume V, by
increasing their nonstoichiometry prevent the accumulation
of Vi at RuO,/PZT interfaces during fatigue cycling.
The equivalent energy-band diagram of a capacitor with
the Ru02/PZT/Ru0, structure is illustrated in Fig. 8(a).
Since the contact between RuO, and PZT is of the Schottky
typcz3
band bending should be observed at the two inter-
faces. The Schottky barrier height
(qq5Bn)
at the RuO,/PZT
contact, however, is much lower than that at the Pt/PZT con-
tact. Therefore, the depletion regions are very short and can
effectively screen the interior from the surface. The effect of
5076
J. Appl. Phys., Vol. 78, No. 8, 15 October 1995
Lee, Thio, and Desu
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