0040-6090/02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 2 . 0 0 9 9 2 - 6
Effect of titanium substitution on film structure and ferroelectric
properties of Sr-deficient Sr
0.75Bi
2.35Ta O thin films
2 9San-Yuan Chen*, Bang-Chiang Lan, Chang-Sheng Taso
Department of Materials and Science Engineering, National Chiao-Tung University, 300 Hsinchu, Taiwan, ROC
Received 25 April 2002; received in revised form 22 September 2002; accepted 30 September 2002
Abstract
Ferroelectric films of bismuth-containing layered perovskite Sr0.75Bi2.35(Ta2yxTix)O (xs0–0.8) have been prepared using a9
metal-organic decomposition method.The effects of Ti substitution on the microstructure evolution and ferroelectric properties of Sr Bi Ta O films were investigated.A maximum remanent polarization of 2P s30.7 mCycm was obtained for the2
0.75 2.35 2 9 r
Sr Bi Ta Ti O film as compared to Sr Bi Ta O films(19.6 mCycm ) annealed at 800 8C in air.The Ti substitution2 0.75 2.35 1.8 0.2 9 0.75 2.35 2 9
for Ta leads to charge compensation for the self-produced positive Bi•Sr due to the occupation of Bi on Sr vacancies and is responsible for the increase in leakage resistance.The leakage current density as low as 2=10y8 Aycm can be obtained at an2 applied electric field of 100 kVycm.Substitution of Ti for Ta shows a positive effect on the fatigue endurance of Sr-deficient Sr0.75Bi2.35Ta O film.2 9
2002 Elsevier Science B.V. All rights reserved.
Keywords: Ferroelectric films; Layered perovskite ferroelectrics; Remanent polarization; Leakage current
1. Introduction
Layered perovskite ferroelectrics such as SrBi Nb O2 2 9 (SBN) and SrBi Ta O (SBT) are being2 2 9
widely used for applications in high-density non-volatile ferroelectric random access memory because of their excellent ferroelectric properties, characterized by lim-ited polarization fatigue and low coercive field w1,2x. They belong to the family of Aurivillius compounds with a general formula of (Bi O ) (Aq2 B
2 2 my1 m
O ) , consisting of m-perovskite units sandwiched2y 3mq1
between bismuth oxide layers. (Here A and B are the
two types of cations that enter the perovskite unit.A is Biq3, Baq2, Srq2, Pbq2, or Kq1; B is Ti4q, Taq5, Nb5q, Mo6q or W6q.) w3,4x.The ferroelectric
proper-ties, crystal structure and microstructure are remarkably influenced by the composition fluctuation.Atsuki et al.
w5x reported that the remanent polarization (P ) ofr
Sr Bi Ta Ox 2z 2 9 (0.7(x(1.0, 2.0(2z(2.6) increased
*Corresponding author.Tel.: 5731818; fax: q886-3-5724727.
E-mail address: [email protected](S.-Y. Chen).
with the decrease of the SryTa mole ratio for films
annealed at 800 8C.A maximum remanent polarization was also reported for the SBT films with 20 mol.% Sr-deficient and 10 mol.% Bi-excess composition w6,7x. The enhancement inP with decreasing Sr content fromr xs1.0 to 0.8 was attributed to the increase in grain
growth.On the other hand, Watanabe et al.w8x reported that the dependence of P on Sr content in SBN filmsr
was entirely due to the sensitivity of grain orientation to Sr content.The randomly oriented Sr-deficient SBN films have larger remanent polarization than c-axis
oriented stoichiometric SBN films.
However, although the Sr-deficient SBT films have been reported to exhibit more excellent polarization compared to stoichiometric SBT film, a partial fatigue was also observed due to the substitution of Bi for Sr sites w6,7,9x.From the viewpoint of the general formula of Aurivillius compounds, the substitution of cations for A-sites and B-sites in the SrBi Ta O is possible.Efforts2 2 9
have been made to enhance the ferroelectric properties of layered perovskite ferroelectrics by the substitution of alternative cations.For example, Nb substitution for
Fig.1.XRD patterns of Sr0.75Bi2.35(Ta2yxTix)O films annealed at 8009 8C for 0.5 h.
Ta was reported to enhance the ferroelectric properties
w10,11x.The partial substitution of Sr2q by Ba3q can result in the noticeable improvement of the dielectric constant and remanent polarization w12x.An enhanced remanent polarization was also observed in the SBT film with the addition of Pb2q cation w9x.Desu et al. revealed that the thin film with 0.7SrBi Ta O –2 2 9
0.3Bi TiTaO compositions and annealed 650–750 8C3 9
exhibited higher remanent polarization and lower leak-age current density w13x.However, to author’s knowl-edge, little work has been done to investigate the role of B-site substitution in Sr-deficient SBT system.There-fore, from the viewpoint of charge compensation, tita-nium acceptor-doped SBT films were prepared on Pty
TiySiO ySi2 substrates.The effect of titanium incorporation on remanent polarization, leakage current and fatigue properties of Sr0.75Bi2.35(Ta2yxTix)O films9
will be investigated and discussed in this work.
2. Experimental
The starting materials for the metal-organic decom-position process were bismuth 2-ethylhexanoate
wBi(CH (CH ) CH(C H )COO) x, strontium 2-ethyl-3 2 3 2 5 3
hexanoate wSr(CH (CH ) CH(C H )COO) x, titanium3 2 3 2 5 2
isopropoxide wTi(OC H ) x and tantalum ethoxide3 7 4
wTa(OC H ) x with 2-ethylhexanoic acid as the solvent.2 5 5
The metal-organic precursors were mixed to form solu-tions with composisolu-tions of Sr0.75Bi2.35(Ta2yxTix)O (des-9
ignated as SBTT).Prior to film deposition, the substrate
(PtyTiySiO ySi) was cleaned in acetone and alcohol2
ultrasonic baths, and then blown dry with N gas.The2
solutions were spin-coated onto the substrate at a speed of 3000 rpm.After each coating, the as-deposited film was dried on a hot plate at a temperature approximately 350 8C to remove the solvent before application of the next coating.After the process was repeated four times, the as-deposited films were annealed at 500–850 8C in air and oxygen atmosphere for 30 min by directly placing the coated substrate into a preheated tube fur-nace.Most of the data reported here were obtained for films with four layers(ts0.37–0.42 mm).
The crystal structures of the films were analyzed by using Siemens D5000 X-ray diffraction(XRD) with Cu Ka radiation and a Ni filter.The film thickness was measured by Dektak surface profilometer.Field-emis-sion scanning electron microscopy (FESEM, Hitachi S4000) was performed to investigate the surface mor-phology of the films.Patterned top Au electrodes through a shadow mask on an area of 8.0=10y4 cm2
area were sputter-deposited onto the SBT layers to define capacitors in order to perform electrical measurements. A ferroelectric testing system (RT-66A, Radiant
Tech-nologies Inc.) operating in the virtual-ground mode was
used to obtain the remanent polarization (P )–coerciver
field (E ) hysteresis characteristics and fatigue proper-c
ties.Fatigue tests of the films were conducted using a bipolar square wave of 6 V at 1 MHz.
3. Results and discussion
3.1. Microstructure and phase evolution
Fig.1 shows the XRD patterns of Sr0.75Bi2.35
(Ta2yxTix)O film (0(x(0.8) annealed at 800 8C for9
0.5 h. It was noted that the intensity of(1 1 5)-oriented
peak slightly increases with increasing Ti content.Above that(xs0.2), the peak intensity gradually decreases and becomes broadening, which might be attributed to poor crystallization and smaller grain size.The XRD patterns also reveal that the films are polycrystalline in nature with no evidence of secondary phases.Besides that, with increasing Ti content, the (1 1 5) reflection is slightly shifted to the high diffraction angle, indicating that the Ti ion has been dissolved into TaO octahedral6
structure and completely substituted for Ta ion.XRD studies of Sr0.75Bi2.35(Ta Ti )O films as a function1.8 0.2 9
of annealing temperature indicate that the films annealed at 500 8C are amorphous.At 600 8C, a broad diffraction peak appears approximately 2us28.58, indicating that the perovskite SBTT phase starts to develop.At 650 8C, as shown in Fig.2, the(1 1 5) diffraction peak remains broad, which implies that the film is not fully crystal-lized.With increasing the annealing temperature, at 700
8C, the (1 1 5) peak in the XRD patterns becomes
sharper and the full width at half-maximum decreases that indicates better crystallinity and an increase in grain size.However, at 850 8C, a broad diffraction peak approximately 29.58 (indicated as x-phase in Fig.2)
was found similar to Bi-deficient pyrochlore as reported by Rodriguez et al. w14x.
Fig.3 illustrates typical (FESEM) surface images
obtained from SBTT films having various Ti contents and annealed at 800 8C.It reveals that the incorporation of Ti into SBT plays an important role in both grain
Fig.2.XRD patterns of Sr0.75Bi2.35(Ta Ti )O films annealed at1.8 0.2 9 different temperature for 0.5 h.
Fig.3.SEM micrographs of Sr0.75Bi2.35(Ta2yxTix)O films with (a) xs0; (b) xs0.2; (c) xs0.4; and (d) xs0.8 compositions annealed at 8009 8C for 0.5 h.
size and grain morphology of Sr0.75Bi2.35Ta O films.2 9
The microstructure of undoped SBT(xs0) film presents a rod-like grain matrix.In contrast, with the incorpora-tion of Ti into the SBT structure, the grain size of Sr0.75Bi2.35(Ta Ti )O film is found larger than that1.8 0.2 9
of Sr0.75Bi2.35Ta O2 9 film.However, further excess Ti
(i.e. xs0.4) causes the decrease in grain size and forms
somewhat porous structure.The enhanced grain size in Sr0.75Bi2.35Ta O2 9 film with Ti addition up to 0.25 is probably related to chemical structure.The off-stoichi-ometric composition in the Sr0.75Bi2.35(Ta Ti )O film1.8 0.2 9
can be approximately considered as a solid solution of 0.75 SrBi Ta O and 0.25 Bi2 2 9 3(Ta,Ti)O with approxi-9
mately 10% excess bismuth content.Ti-based layered perovskite materials are usually expected to exhibit a lower crystallization temperature than SBT or SBN.As
reported by Hu et al.that when the SBT film was deposited on the Bi Ti O4 3 12 buffered PtyTiySiO ySi2
substrates, the SBT film can be crystallized at 650 8C, which is 50 8C lower than that for SBT films without Bi Ti O4 3 12 buffer layer because Bi Ti O4 3 12 film can be formed at lower temperature compared to SBT film
w15x.On the other hand, as excess Ti content (i.e. xs
0.4) was added, it was observed that the grain size
decreased and somewhat porous structure was formed, indicating the densification and grain growth were ham-pered with further increasing Ti content.In other words, it is impossible to consider the Sr0.75Bi2.35(Ta Ti )O1.2 0.8 9
composition as a solid solution of SrBi Ta O2 2 9 and Bi3(TaTi)O and a distorted structure was probably9
formed.
The root means square (rms) roughness of the SBTT film surfaces calculated from the atomic force micros-copy(AFM) morphologies in a 1=1 mm area is shown2
in Fig.4.Atxs0–0.3, the films exhibit smooth surface,
above that, the surface of the films becomes rough.As it was observed in SEM micrographs(Fig.3), increasing
Ti content makes it relatively easy to merge small grains into large and flat grains.
3.2. Ferroelectric properties
Ferroelectric hysteresis measurements were conducted on Sr0.75Bi2.35(Ta2yxTix)O films in MFM configuration9
at room temperature using standardized RT 66A ferro-electric test system.Fig.5 shows typical polarization versus electric field curves of Sr0.75Bi2.35(Ta Ti )O1.8 0.2 9
films recorded with y6 to q6 V excitations.The measured remanent polarization (2P ) and the coerciver
Fig.4.Rms roughness of various titanium content Sr0.75Bi2.35(Ta2yxTix)O films annealed at 800 8C for 0.5 h.9
Fig.5.P–E hysteresis loops of Sr0.75Bi2.35(Ta Ti )O films with1.8 0.2 9 800 8C heat treatment at applied voltages of 1.5, 2, 3.5, 5 and 6 V.
Fig.6.Dependence of remanent polarization on the Ti content in case of Sr0.75Bi2.35(Ta2yxTix)O films annealed at 800 8C for 0.5 h in air9 and O atmospheres at an electrical field of 150 kVycm.2
field (2E ) at an applied electric field of 150 kVycmc
are 30.7 mCycm and 75 kVycm, respectively, for the2
films annealed at 800 8C.The effect of Ti addition on the remanent polarization(2P ) of SBTT films is furtherr
illustrated in Fig.6.It was found that the SBT films with Ti substitution show much improved ferroelectric characteristics compared to Sr0.75Bi2.35Ta O films.A2 9
maximum remanent polarization was obtained with the SBTT film at xs0.2–0.25 depending on annealing
atmosphere.The enhanced ferroelectric properties in the Sr0.75Bi2.35(Ta Ti )O films prepared from precursor1.8 0.2 9
solution with Ti addition can be elucidated by the aforementioned solid solution of 0.75 SrBi Ta O and2 2 9
0.25 Bi3(Ta,Ti)O .The Bi (Ta,Ti)O9 3 9 ceramic was reported to have a Curie temperature approximately
;880 8C w16x, much higher than that (;310 8C) of
SrBi Ta O , so that the Sr2 2 9 0.75Bi2.35(Ta Ti )O film is1.8 0.2 9
expected to show good ferroelectric properties.Another possibility is the larger grain size (Fig.3) in
Sr0.75Bi2.35(Ta Ti )O1.8 0.2 9 film as compared to Sr0.75Bi2.35Ta O film.On the other hand, as the titanium2 9
addition was more than that(xs0.2), the 2P value thatr
was reduced can be attributed to poor crystallinity and decreased grain size as observed from XRD patterns and SEM microstructure.A maximum value of 2Pr
(2P s30.7 mCycm ) was obtained at the SBTT film2 r
with xs0.2 under air atmosphere. For comparison, the
effect of oxygen atmosphere on the 2P value of ther
Sr0.75Bi2.35(Ta2yxTix)O films was also shown in Fig.5.9
According to this figure, after the SBTT films was annealed in an atmosphere of O ,2 the Sr0. 75
Bi2.35(Ta1.75Ti0.25)O film exhibits a maximum remanent9
polarization (2P ) of 34.9 mCycm which is larger than2 r
that of the film annealed in air atmosphere.The higher remanent polarization in oxygen-annealed films reveals that the polarization is possibly related to defect concen-tration.The oxygen vacancy can be produced as the
tantalum is substituted by Ti acceptor but will be reduced later in the oxygen atmosphere.Therefore, a larger remanent polarization can be obtained in oxygen atmosphere.
Although the produced oxygen vacancies probably lead to the domain pinning as one can see in PZT films
w17,18x, in this work, the polarization enhancement
instead of reduction is obtained for the films with titanium doping.The unique phenomenon reveals that it is possibly related to the occupation of Bi on Sr site in the Sr-deficient SBT compositions because the substi-tution of Bi for Sr will lead to charge unbalance and produce electrons or cation vacancies.Therefore, as the Ti ions are incorporated into SBT structure, the produced oxygen vacancies, V , can be primarily used to react0••
with electrons or V0Sr to decrease the defect concentra-tion.Consequently, the enhanced 2P value in SBTTr
films withxs0–0.2 can be attributed to the contribution
from the increased grain size and the reduced defect concentration.
Fig.7.Leakage current density of various titanium content Sr Biy 2.35(Ta2yxTix)O films at an electric field of 100 kVycm.9
Fig.8.Fatigue behavior of Sr0.75Bi2.35(Ta Ti )O1.8 0.2 9 and Sr0.75Bi2.35Ta O films under an applied voltage of 6 V at 1 MHz.2 9
3.3. Leakage current
In general, nominally undoped SBT film exhibits p-type conductivity due to Bi O2 3 volatilization during high-temperature processing w19x.Furthermore, the occupation of Bi on Sr site often occurs in the Sr-deficient SBT compositions since both radii of Sr2qand Bi3q are very close w20x, where some fraction of the positive charged Bi•Sr ions might be compensated by electrons w21x.By considering only the most probable defects, the charge neutrality can be given as follows:
•• • • y
w x w x w x w x
2V q h q Bi s2 V0 qe0 Sr Sr (1)
The charge neutrality condition can be modified through the addition of acceptor impurities to be
•• • • y
w x w x w x w x w x
2V q h q Bi s2 V0 q A90 Sr Sr Taqe (2)
Acceptor doping can result in a decrease in the concentration of ‘‘mobile’’ defects (e ) through they
various mass action relations in the system.Complete compensation by acceptor ions occurs when the acceptor concentration equals the concentration of produced extra Bi•Sr species.As shown in Fig.7, when the Sr0.75Bi2.35(Ta2yxTix)O thin films were annealed at 8009
8C in air, the leakage current density was found to
decrease with increasing Ti content to xs0.2, above
that the leakage current density becomes larger due to the produce of anion vacancies.Additionally, it was found that for the SBTT films ofx)0.3, the grain size
becomes smaller and the film surface becomes rough according to AFM analysis (Fig.4).The rough surface measurement suggests that the films have a larger leakage current density.Therefore, a minimum leakage current density of approximately 2=10y8 Aycm can2
be obtained for Sr0.75Bi2.35(Ta Ti )O film at an elec-1.8 0.2 9
tric field of 100 kVycm that is much lower than that (1=10 ) of undoped SBT film by the two orders ofy6
magnitude.The lowered leakage current was also
report-ed by Desu et al.for the films with 0.7SrBi Ta O –2 2 9
0.3Bi Ti3 (Ta1yyNby)O composition w13x.The decreased9
leakage current might be attributed to the addition of Ti acceptor ions for reducing the charge unbalance due to the occupation of Bi on the Sr sites.When the acceptor concentration is close to the concentration of the pro-duced extra Bi•Sr species, the complete charge compen-sation occurs as follows:
4q 3q •
Ti qV009qBiTa qV9 ™Ti0 qBiSr Ta Sr (3)
Furthermore, for the less Sr-deficient SBTT films, i.e. Srs0.9, Fig. 7 illustrates that the addition of acceptor
(Ti) only leads to a slight improvement in the leakage
resistance in comparison with more Sr-deficient SBT films (Srs0.75).The minimum leakage current density shows up at xs0.1 for the less Sr-deficient Sr0.9
Bi2.35(Ta2yxTix)O film.This result suggests that the9
concentration of the produced Br•Srspecies is reduced in the less Sr-deficient SBT films.This finding again indicates that an enhanced insulating characteristic was obtained by substituting Ti for Ta to reduce electron concentration in the Sr-deficient SBT films.Such a decrease is reasonable for charge compensation of Br•Sr
species.In order to mark the role of Ti acceptor in the leakage current of SBT films, another work was per-formed.Tungsten (W) donor was added into the SBT
films to substitute for Ta.On the contrary, it was observed that for the same doping amount as Ti, the addition of W donor leads to the increase of leakage current (not shown here).This phenomenon confirms that this is an acceptor effect for the reduced leakage current in the Sr-deficient SBT films.
3.4. Fatigue behavior
Fatigue endurance was tested with 1 MHz bipolar pulses at 6 V.Fig.8 shows the fatigue behavior of SBT films annealed at 800 8C for 0.5 h. It was observed that
a partial loss of 2P after fatigue was observed for ther
Sr-deficient Sr0.75Bi2.35Ta O2 9 film without Ti doping. The percentage of the remanent polarization after 1010
cycles was approximately 87% of the initial value, which is consistent with the literature results w6,7x.On the other hand, as Ti was added to substitute for Ta in the SBT films, i.e. Sr0.75Bi2.35(Ta Ti )O , the film shows1.8 0.2 9
no decay in the polarization change up to approximately 10 switching cycles but the decay in8 P was observed
r
to be approximately 9% of the initial value after 1010
cycles.This indicates that the incorporation of limited titanium into the layered perovskite structure does not cause appreciable fatigue degradation.Kang et al. revealed that SrBi Ta O and Bi TiTaO films grown at2 2 9 3 9
the similar deposition conditions showed different fatigue behaviors.The difference should come from the oxygen stability in the perovskite layer w22x.However, Desu et al.reported that thin films of 0.8SrBi Ta O –2 2 9
0.2Bi TiTaO solid solution exhibit good fatigue char-3 9
acteristics and the decay in remanent polarization was observed to be less than 5% of initial value after 1010
switching cycles w23x.If attention was paid to the atomic ratio of Sr0.75Bi2.35(Ta Ti )O , it was found that both1.8 0.2 9
films of Sr0.75Bi2.35(Ta Ti )O1.8 0.2 9 (this work) and
0.8SrBi Ta O –0.2Bi TiTaO2 2 9 3 9 (Desu et al.) have similar
compositions.Although the oxygen vacancy will be produced as the Ta was substituted by Ti, it does not cause further degradation in fatigue behavior of Sr0.75Bi2.35(Ta Ti )O film.1.8 0.2 9
There are several proposals that attempt to explain the fatigue behavior of ferroelectric thin films especially for PZT w24–26x.These proposals are all related to defects, which may be trapped at domain boundaries to lower their mobility.However, there have been different views of the species of the defects and the location of the trapping sites.Oxygen vacancy and electronyhole
injection have been suggested as possible mechanisms for the fatigue of PZT films w25,26x.In our case, for Sr-deficient Sr0.75Bi2.35Ta O film, it was considered that2 9
the electron could be produced by substituting Bi for Sr vacancy since both radii of Sr (1.16 A) and Bi (1.02˚ A˚) are very close.The electron can be produced by substituting Bi for Sr vacancy.According to the fatigue model proposed by Warren et al. w26x, the released electronic charge trapped at domain walls can inhibit the domain motion and lead to the partial suppression of the switchable polarization in Sr0.75Bi2.35Ta O film2 9
that is in agreement with our fatigue data shown in Fig. 8.However, the released electron is possibly compen-sated from the oxygen vacancies produced from the substitution of Ti for Ta.Consequently, the addition of Ti into SBT structure shows a positive effect on fatigue behavior of Sr0.75Bi2.35Ta O film.2 9
4. Conclusions
1. With the incorporation of Ti into Sr0.75Bi2.35Ta O2 9
films, the grain growth can be further enhanced. However, excess Ti content causes the poor crystal-lization and reduces the grain size.
2. Ferroelectric properties of Sr0.75Bi2.35Ta O films have2 9
been significantly enhanced with titanium doping up to xs0.2–0.25 (i.e., Sr0.75Bi2.35Ta Ti O1.8 0.2 9).The
measured remanent polarization(2P ) values increas-r
es from 19.6 to 30.7 mCycm at an electrical field of2
150 kVycm.
3. The substitution of Ti ions for Ta can lead to an increase in leakage resistance and the leakage current density is as lower as 2=10y8 Aycm at an applied2
electric field of 100 kVycm.
4. Substitution of Ti for Ta shows a positive effect on the fatigue endurance of Sr0.75Bi2.35Ta O film.2 9
Acknowledgments
The authors gratefully acknowledge the National Sci-ence Council of the Republic of China for its financial support under Contract No.NSC-89-2218-E-009-049.
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