Full text of DOI:10.1557/JMR.2003.0240

Suppression of pyrochlore phase formation in Sr0.7Bi2.4Ta2O9
thin films using nanoseed layer
Se-Yeon Jung, Seung-Joon Hwang, and Yun-Mo Sunga)
Department of Materials Science & Engineering, Daejin University, Pochun-koon,
Kyunggi-do 487-711, Korea
(Received 17 March 2003; accepted 12 May 2003)
A Sr0.7Bi2.4Ta2O9 (SBT) seed layer approximately 40 nm thick was formed on
Pt/Ti/SiO2/Si substrates, and SBT thin films with the same chemical composition were
deposited on the substrates with and without seed layer using sol-gel and spin coating
methods. The influence of seed layer on the phase formation characteristics of SBT
thin films was investigated using x-ray diffraction and scanning electron microscopy
analyses. Formation of pyrochlore as well as Aurivillius phase was observed in both
the unseeded and seeded SBT films heated at 740 °C. However, it was revealed that
Aurivillius phase formation was enhanced in seeded SBT thin films and pyrochlore
phase formation was highly suppressed. In this study, two possible mechanisms for the
suppression of pyrochlore phase formation were proposed from the perspectives of
activation energy difference between Aurivillius and pyrochlore phase formation, and
Bi-ion diffusion to pyrochlore phase.
Among ferroelectric materials strontium bismuth tan-
talate (SrBi2Ta2O9; SBT) has been intensively studied
for nonvolatile ferroelectric random access memory
applications due to its relatively low voltage operation,
low leakage current, fast switching, and fatigue-free
properties with Pt bottom electrodes.1–3 SBT has the Au-
rivillius structure, composed of alternating one Bi2O2
layer and two SrTa2O7 perovskite layers along
c axis.4,5 However, the pyrochlore phase, defined as a
bismuth-deficient and nonferroelectric phase, has been
often observed during the processing of SBT thin
films.6,7 Refinement carried out by Rodriguez et al.6 in-
dicates that the stoichiometry for the pyrochlore com-
improved ferroelectric properties due to lattice distor-
tion.12 For the preparation of Sr0.7Bi2.4Ta2O9 (SBT) sols,
Sr-isoproxide (Aldrich Chemical, Milwaukee, WI),
Bi-tri-amyloxide, and Ta-penta-ethoxide (High-Purity
Chemical Co., Osaka, Japan) were used as the starting
chemicals, and all of these alkoxides were handled in a
glove box under dried Ar atmosphere. In the SBT sol
preparation, 2-methoxyethanol was used as the solvent.
Sr-isopropoxide was dissolved in 2-methoxyethanol by
refluxing it at 130 °C followed by distillation. Bi-
amyloxide was added to the above solution followed
by refluxing it at 60 °C and distillation at 110 °C, so that
a Sr–Bi double alkoxide solution was formed. Ta-
ethoxide was dissolved in 2-methoxyethanol by refluxing
at 130 °C. SBT sol was prepared by refluxing this Ta
alkoxide with Sr–Bi double alkoxide solution. SBT sol
with a concentration of 0.05 M was used for the spin
coating experiments.
pound in the SBT system is Sr0.2(Sr0.5Bi0.7)Ta2O6.75
.
Seeding has been used as a highly effective way to
enhance the kinetics of a specific crystalline phase for-
mation in a material for many years.8–11 In this study
very thin SBT seed layer (40 nm) was introduced be-
tween Pt bottom electrode and SBT thin film and the role
of the seed layer on the phase formation characteristics of
SBT thin films was studied. Two possible mechanisms
were proposed for the suppression of pyrochlore forma-
tion in SBT thin films grown on seed layers.
The SBT seed layer was prepared by one-time spin
coating (Headway Research Inc., Garland, TX) on Pt/Ti/
SiO2/Si substrates at a spin speed of 3000 rpm for 30 s.
The seed layer was crystallized at 740 °C for 1 h in air
atmosphere. Then SBT sol was coated on the crystallized
seed layer for a total of nine times, and between each
coating step drying at 150 °C and pyrolysis at 420 °C
were performed to remove organic matter. The final SBT
films were annealed at 740 °C for 160 min in air atmos-
phere. SBT films without seed layer were also fabricated
using the same route for the comparison to the films with
seed layer. The flow chart showing the fabrication pro-
cedure of SBT thin films with seed layer is presented in
The composition of SBT selected was Sr0.7Bi2.4Ta2O9,
since it has been known that SBT with an off-stoichiometric
composition (Sr1−xBi2+yTa2O9) shows significantly
a)Address all correspondence to this author.
e-mail: ymsung@daejin.ac.kr
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Fig. 1. X-ray diffraction (XRD; Rigaku Ultima 2000,
Tokyo, Japan) was performed for the phase analysis
of these thin films using a precision thin film attach-
ment. The incidence beam angle was fixed as 3° and
2-theta scans with a 0.02° step and 5 s duration were
conducted. The morphological feature of each film was
investigated and compared using scanning electron
microscopy (SEM; Philips XL-30 E-SEM, Eindhoven,
The Netherlands).
Figure 2 shows the SEM plan and cross-sectional
views of SBT seed layer crystallized at 740 °C for 1 h.
The film seems to be broken up to wormlike crystals and
clusters of very tiny crystals due to very small thickness.
From the morphology perspective the wormlike crystals
and tiny crystals are believed to be Aurivillius and py-
rochlore phase, respectively.7,11 The cross-sectional view
images reveal that the averaged thickness of the seed
layer is approximately 40 nm.
FIG. 2. SEM images of (a) plan and (b) cross-sectional views of SBT seed
layer formed on Pt/Ti/SiO2/Si substrates and heated at 740 °C for
1 h. Here, A and P denote Aurivillius and pyrochlore crystals, respectively.
FIG. 3. XRD patterns of unseeded and seeded SBT films heated at
740 °C for 160 min. Here, A105, A200, and P222 denote reflections
from Aurivillius (105), Aurivillius (200), and pyrochlore (222),
respectively.
FIG. 1. A flow diagram showing the preparation procedure of
SBT thin films with seed layer.
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Figure 3 shows XRD patterns of unseeded and seeded
the Aurivillius and pyrochlore phases, they suppress the
formation of pyrochlore phase selectively, while enhanc-
ing the formation of the Aurivillius phase.
SBT films heat treated at 740 °C for 160 min. Although
the Aurivillius (105) peak was observed in both the
films the Aurivillius (105) intensity of the seeded one was
much higher than that of the unseeded one and the full
width at half-maximum value of seeded ones was lower,
which implies well-developed Aurivillius (105) crystals
in the seeded ones. Also, pyrochlore (222) peaks with
low intensity were observed at the right shoulder of SBT
(105) peaks. It was found that the relative integrated peak
intensity of pyrochlore (222) to Aurivillius (105) was
higher in unseeded films. This difference certainly im-
plies the pyrochlore volume fraction is higher in un-
seeded films.
Figure 4 shows SEM plan and cross-sectional views
of unseeded and seeded SBT films heated at 740 °C for
160 min. Although both films show the formation of
typical wormlike Aurivillius crystals, the seeded ones
show large and well-developed grain morphology, which
corresponds well to the XRD results showing the higher
Aurivillius peak intensity for seeded ones in Fig. 3. Also,
the formation of clusters of fine pyrochlore crystals be-
tween Aurivillius crystals was observed in both films.
Using image analysis techniques the averaged pyrochlore
volume fraction values were determined as approxi-
mately 0.39 and approximately 0.12, for unseeded and
seeded SBT films, respectively, which also matches well
to the XRD results presenting the difference in the rela-
tive XRD integrated peak intensity of pyrochlore. Thus,
it is apparent that although the seed layers contain both
The mechanism of pyrochlore suppression was con-
sidered from two different perspectives. First, the acti-
vation energy for nucleation and crystal growth for
pyrochlore crystals would be higher than that for Auri-
villius, and thus kinetics of Aurivillius phase formation is
much faster. The epitaxial growth of Aurivillius crystals
from the interface of Aurivillis crystals in seed layer
would be more favorable than that of pyrochlore crystals,
possibly due to the low activation energy barrier. Second,
the suppression of pyrochlore phase formation in seeded
SBT films may occur due to the diffusion of Bi ions
from SBT film to seed layer during annealing at 740 °C.
In the case of seeded film a Bi concentration gradient
exists between the seed layer containing Bi-deficient py-
rochlore phase and the SBT film. Thus, diffusion of Bi
ions from film to seed layer can occur during annealing
processes and some pyrochlore crystals in the seed layer
will be transformed to Aurivillius crystals to some de-
gree. Therefore, the number of pyrochlore crystals in
seed layer would be reduced and thus the formation of
pyrochlore crystals in SBT thin films could be sup-
pressed. The rapid transformation of pyrochlore crystals
to Aurivillius would be possible due to their very fine
crystal size. However, some remaining pyrochlore crys-
tals would enhance the formation of pyrochlore crystal
growth in SBT film. Because the volume of SBT film
is more than nine times the seed layer, the amount of Bi
FIG. 4. SEM images of plan and cross-sectional views of (a,b) unseeded and (c,d) seeded SBT films coated on seed layer/Pt/Ti/SiO2/Si substrates
and heated at 740 °C for 160 min. Here, A and P denote Aurivillius and pyrochlore crystals, respectively.
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consumption in the SBT film for this diffusion reaction
would be negligible. These two proposed mechanisms
of pyrochlore suppression in seeded SBT thin films are
schematically presented in Fig. 5. To identify which
mechanism plays a major role in the suppression of py-
rochlore, the activation energy for pyrochlore phase for-
mation and the pyrochlore amount dependency on the
seed layer thickness are under investigation and will be
reported soon.
In conclusion, the introduction of SBT seed layer be-
tween Pt bottom electrode and SBT thin film greatly
suppressed the formation of pyrochlore phase in SBT
thin films, while enhancing the formation of ferroelectric
Aurivillius phase. Thus, one can not only enhance Au-
rivillius phase formation kinetics but also improve fer-
rolectric properties of SBT thin films using this seed
layer concept.
ACKNOWLEDGMENT
The authors wish to thank Mr. J-Y. Park in RFC at
Daejin University for his SEM work.
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FIG. 5. Proposed mechanisms for pyrochlore suppression in seeded
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