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Large tensile-strained BaTiO3 films grown on alattice-mismatched La-doped BaSnO3 bottom
electrode†Lizhikun Gong,a Ko Marunouchi,a Akira Chikamatsu,
Hiromichi Ohta c and Tsukasa Katayama *cd

b

Perovskite BaTiO3 has been widely studied and utilized in various applications owing to its high permittivity,
ferroelectricity, and stability. However, its low ferroelectric-paraelectric phase transition temperature (TC,
120 °C) limits its application. The TC can be increased by applying an epitaxial strain provided by a latticemismatched substrate. However, applying large tensile strain on BaTiO3 is difficult, especially when abottom electrode is present. In this study, we successfully fabricated large tensile-strained BaTiO3 films
using La-doped BaSnO3 bottom electrodes. A tensile strain of 2% was achieved, which is three times larger
than that previously reported for BaTiO3 films grown on bottom electrodes. By adjusting the thickness of
the BaTiO3 layer between 20 and 300 nm, tensile strain can be varied within the range of 0.6–2%.
Received 1st March 2024,
Accepted 23rd April 2024
DOI: 10.1039/d4ce00197d

Remarkably, the TC of the obtained films exceeds 400 °C. In addition, although it was considered thattensile-strained BaTiO3 films have ferroelectric polarization in the in-plane direction, the 0.6% tensilestrained film showed ferroelectric polarization in the out-of-plane direction. This finding reveals that theferroelectric polarization direction was slightly tilted away from the predominant in-plane direction of the

rsc.li/crystengcomm

film.

1. Introduction
Perovskite BaTiO3 is one of the most promising ferroelectricmaterials owing to its desirable properties such as highpermittivity (εr), tunability of εr, piezoelectricity, andstability.1,2 It is widely used in electronic components such as
multilayer ceramic capacitors and microwave devices.3–7 The
properties of BaTiO3 can be controlled by varying the strain
from the substrate. For instance, in epitaxial BaTiO3 filmgrown coherently on the DyScO3 substrate, the remnant
polarization value of the ferroelectric phase is significantly
higher (70 μC cm−2) compared to that of the bulk (24 μC
cm−2).8 Furthermore, BaTiO3 films demonstrate a 500 °C
increase in ferroelectric transition temperature (TC) comparedwith the bulk (120 °C).8 In addition, a flexoelectric effectoccurs, causing the alignment of the polarization direction inaGraduate School of Information Science and Technology, Hokkaido University,
N14W9, Sapporo 060-0814, Japanb
Department of Chemistry, Faculty of Science, Ochanomizu University, 2-1-1
Otsuka, Bunkyo, Tokyo 112-8610, Japanc
Research Institute for Electronic Science, Hokkaido University, N20W10, Sapporo
001-0020, Japan. E-mail: katayama@es.hokudai.ac.jpdJST-PRESTO, Kawaguchi, Saitama 332-0012, Japan† Electronic supplementary information (ESI) available: See ESI for the details of
AFM images of the BaTiO3 films. See DOI: https://doi.org/10.1039/d4ce00197d

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the out-of-plane direction without the application of anelectric field.8–10
The magnitude of substrate-induced strain can becontrolled by selecting the substrates, bottom electrodes, andfilm thickness. For instance, when BaTiO3 films were
coherently grown on SrTiO3 (a = 3.905 Å) or GdScO3 (a =3.965 Å) substrates, compressive strains of 2 and 0.7% were
induced in the films, respectively,8,11 whereas bulk BaTiO3
exhibited a tetragonal perovskite structure with a = 3.992 andc = 4.036 Å. Such application of compressive strain increases
in remnant polarization (Pr), which is associated with anincrease in the c/a ratio.8–12 Conversely, the application oftensile strain on BaTiO3 films has also been investigatedusing substrates with large lattice constants such as MgO (a= 4.213 Å) and MgAl2O4 (a/2 = 4.042 Å).13–18 The films showed
an enhancement in TC (200–220 °C) and ferroelectricity along
the in-plane direction.17,18 However, these substrates are
insulators, and it is difficult to prepare bottom electrodes
while maintaining a high tensile strain on the BaTiO3 layer.
In fact, in previously prepared BaTiO3 films with bottom
electrodes, tensile strain was limited to 0.57%.19,20 To achievea large tensile strain in the BaTiO3 film, we investigated Ladoped BaSnO3 (LBSO, hereafter) epitaxial films as electrodes
because they have a cubic perovskite structure with a = 4.116
Å, which is 3% longer compared to BaTiO3 and exhibits
rather high electrical conductivity over 103 S cm−1 at room

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temperature.21–27 In this study, we investigated the crystal
structures and ferroelectric properties of tensile-strained
BaTiO3 films grown on 50 nm-thick LBSO bottom electrodes
prepared on SrTiO3 (001) (STO) substrates. By adjusting thefilm thickness, the tensile strain values can be varied
between 0.6 and 2%. The TC of the 0.6 and 1.3%-tensilestrained films were ∼400 and ∼500 °C, respectively, which issignificantly higher than that of the bulk BaTiO3 (120 °C). In
case of the polarization versus electric field (P–E) curves,
ferroelectric behavior did not appear along the out-of-plane
direction for the 1.4–2%-tensile strained films probably
because the ferroelectric polarization appears in the in-plane
direction. On the contrary, the 0.6%-tensile-strained film also
exhibited ferroelectric polarization along the out-of-plane
direction, suggesting that the ferroelectric polarization
direction was slightly tilted away from the in-plane direction.

2. Experimental
Epitaxial films of BaTiO3 were fabricated on LBSO (2% Ladoped, La0.02Ba0.98SnO3)-buffered SrTiO3 (001) substrates
using a pulsed laser deposition (PLD) technique.24 During
the deposition, the substrate temperature, laser energy, andoxygen partial pressure were maintained at 850 °C, 0.5 J cm−2
pulse−1, and 0.5 Pa for the BaTiO3 layer and 750 °C, 2 J cm−2
pulse−1, and 20 Pa for the LBSO layer, respectively. The typical
thickness of the LBSO layer is 50 nm.
The crystal structures of the obtained films were evaluatedusing high-resolution X-ray diffraction (XRD, ATX-G, Rigaku
Co.) with Cu Kα1 radiation and variable-temperature XRD(D8 Discover, Bruker AXS GmbH) with Cu Kα1 and Kα2radiation. The film thicknesses were determined by analyzing
the Pendellösung fringes around the Bragg diffraction peaks
and X-ray reflectivity. For the calculation of strain, we
employed the formula: strain = afilm/abulk − 1, which iscommonly used in previous literature. The surface
morphology was observed by atomic force microscopy (AFM,
Nanocute, Hitachi High-tech). The ferroelectric properties ofthe films were measured using a ferroelectric tester(Multiferroic II, Radiant Inc.). In the measurement, an 80
nm-thick Pt electrode with a diameter of 100 μm was used as
the top electrode, whereas the LBSO layer was used as thebottom electrode.

Fig. 1 Out-of-plane XRD patterns of the BaTiO3 films with varied
thickness grown on LBSO/STO substrates. The asterisk indicates the(002) STO substrate peak.

lower qz/2π side with increasing tBTO. As the tBTO increases
from 20 to 300 nm, the c-axis length of the BaTiO3 layer
increases from 3.979 to 4.006 Å. The c-axis lengths wereshorter than those of bulk BaTiO3 (c = 4.036 Å), indicating
the presence of tensile strain within the films. Fig. S1† shows
the film surface morphologies. The root mean squareroughness values are smaller than 0.3 nm, confirming theatomically flat surface of the films.
Fig. 2 shows the reciprocal space mappings (RSMs) aroundthe (103) diffraction spot for the BaTiO3 film with varied
thicknesses. In case of the LBSO layer, the in-plane a-axis
length (4.08 Å) is shorter than the out-of-plane c-axis length(4.125 Å) owing to the compressive strain from the STO

3. Results and discussion
Fig. 1 shows the out-of-plane XRD patterns of the BaTiO3
films as functions of the BaTiO3 thickness (tBTO). The films
were grown on the LBSO bottom electrodes prepared on STO
substrates. The range of tBTO is set to 20–300 nm. The (002)diffraction peaks of LBSO and BaTiO3 appear at qz/2π = 4.85
and 5.00–5.02 nm−1, respectively. In addition, clear
Pendellösung fringes were observed around the (002)diffraction peaks of LBSO, indicating the highly (00l)orientation of the LBSO bottom electrode layer. On thecontrary, the (002) diffraction peaks of BaTiO3 shift toward a

2766 | CrystEngComm, 2024, 26, 2765–2769

Fig. 2 Reciprocal space mappings for the BaTiO3 film around the(103) diffraction spots. tBTO = (a) 20, (b) 50, (c) 100, and (d) 300 nm.

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substrate. The (103) diffraction spot of BaTiO3 was observed
as a single spot without splitting, indicating that the BaTiO3layer had a tetragonal structure. With increasing tBTO, thediffraction spot of BaTiO3 shifts toward the higher qz/2π side.
The a-axis lengths of the BaTiO3 films are in the range of
4.01–4.07 Å, which is shorter than that of the LBSO layer,
whereas it is longer than that of bulk BaTiO3. These results
confirmed the presence of tensile strain in the BaTiO3 films.
Fig. 3(a) shows the lattice constants of the BaTiO3 films asa function of tBTO. Remarkably, the c-axis was shorter than
the a-axis length, in contrast to the behavior observed forbulk BaTiO3. With increasing tBTO, the a (c) values of thefilms monotonically increased (decreased) and approachedthose of the bulk single crystal. Fig. 3(b) shows the tensile
strain values of the BaTiO3 films. At tBTO = 20 nm, the tensile
strain value is as high as 2.0%, which is much higher thanthe previous report on epitaxially grown BaTiO3 films on
MgO substrate with the bottom electrode layer (<0.57%).19,20
The tensile strain value decreases with increasing tBTO andreaches 0.6% at tBTO = 300 nm. The decrease in the tensile
strain with increasing tBTO was attributed to the relaxation of
the in-plane lattice with increasing distance from thesubstrate because of the generation of dislocations in thefilm.28,29
The BaTiO3 films were deposited using PLD at a substratetemperature (Ts) of 850 °C and an oxygen partial pressure(PO2) of 0.5 Pa, with a laser energy of 0.5 J cm−2 pulse−1.
Notably, PLD conditions are known to influence the latticestrain and properties of BaTiO3 films. For instance, the c/avalues decrease with increasing Ts for BaTiO3 films grown on
multilayer-buffered Si substrates.30 Additionally, it has beenreported that oxygen vacancies are introduced at low PO2,

while high PO2 results in locally incoherent grain boundaries
in BaTiO3 films.31 Moreover, high laser energy has beenshown to increase the growth-induced defect structures andcreate defect dipoles in BaTiO3 films.9 Hence, in this study,
the influence of lattice strain due to defects induced by PLD
conditions may affect the properties alongside the effects ofstrain provided by the substrate.
Fig. 4(a) illustrates the temperature (T) dependence of theout-of-plane XRD patterns of the BaTiO3 film with tBTO of 300
nm. To facilitate an understanding of the temperaturedependent changes in the BaTiO3 peak position, thehorizontal axis is normalized by the SrTiO3 peak position,
where 2θSTO and 2θBTO represent the 2θ values of the SrTiO3(002) and BaTiO3 (002) diffraction peaks, respectively. The
XRD equipment used for temperature dependence differsfrom that used at room temperature (refer to Fig. 1–3, see
experimental section). For tBTO = 100 and 300 nm, the curvesof 2θBTO – 2θSTO versus T exhibit kinks at ∼500 and ∼400 °C,
respectively (Fig. 4(b)). Since bulk SrTiO3 is known not toundergo phase transitions in its crystal structure, theobserved kinks are attributed to changes in the lattice
constants of the BaTiO3 films. Such a change in latticeFig. 3 (a) Lattice constants and (b) tensile strain values for the BaTiO3
films as a function of tBTO. Figure (a) also includes the a- and c-axis
lengths of bulk BaTiO3.8

Fig. 4 (a) Temperature dependence of the out-of-plane XRD patterns
for the BaTiO3 film with tBTO = 300 nm. (b) The 2θBTO – 2θSTO values asa function of T for the BaTiO3 films with varied thickness.

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there is spatial freedom for Ti ions along the c-axis direction,
leading to spontaneous polarization along the c-axis. In ourcase, the BaTiO3 film exhibits a tetragonal structure (a = b ≠c) with the a-axis length longer than the c-axis length.
Consequently, the location of the spatial region available forTi ion displacement differs from that of the bulk material,
resulting in a change in the polarization direction.
In contrast, at tBTO = 300 nm, a ferroelectric hysteresis
loop was observed, accompanied by polarization-switchingcurrent peaks. The Pr value in the out-of-plane direction was
3 μC cm−2. This suggests that the ferroelectric polarization
direction was slightly tilted away from the in-plane direction
at tBTO = 300 nm because of the weaker tensile strain (0.6%)
compared to other films (1.3–2%).

4. Conclusions

Fig. 5 P–E and I–E curves of the BaTiO3 films with thickness of (a) 20,(b) 50, (c) 100, and (d) 300 nm.

constants has also been observed in bulk and film BaTiO3
owing to a transition from the ferroelectric tetragonal phase(P4mm) to the paraelectric cubic phase (Pm3m).8 Thus, theobserved kinks in the films with tBTO = 100 and 300 nm were
derived from ferroelectric-to-paraelectric phase transitions.
Choi et al. predicted the TC values of BaTiO3 film under
biaxial in-plane strain based on thermodynamic analysis;8
the predicted TC was in the ranges of 300–500 °C and 400–
800 °C when 0.6 and 1.3%-tensile strains were applied to the
BaTiO3 film, respectively. These predicted TC values are
consistent with our results: TC for the tBTO = 100 and 300 nm
films with a tensile strain of 0.6 and 1.3% were ∼400 and∼500 °C, respectively. On the other hand, no obvious kinks
were observed in the 2θBTO – 2θSTO values versus T curves for
the BaTiO3 films with tBTO = 20 and 50 nm below 600 °C.
Fig. 5 shows the polarization versus electric field (P–E)
curve for BaTiO3 films with tBTO = 20, 50, 100, and 300 nm.
The measurements were conducted at 25 °C and a frequencyof 10 kHz. At tBTO = 20 nm and 50 nm, no ferroelectric
hysteresis loops were observed in the out-of-plane direction.
This is probably because the ferroelectric polarization wascompletely along the in-plane direction, as observed in apreviously reported BaTiO3 film under tensile strain.12,17 It isnoted that bulk BaTiO3 with a perovskite structure exhibits
tetragonal, orthorhombic, and rhombohedral symmetries
below the ferroelectric transition temperature. While thepolarization of the tetragonal phase aligns along the c-axis,
that of the orthorhombic and rhombohedral phases alignalong the [110] and [111] directions, respectively.32 The
direction of polarization is primarily determined by theavailability of spatial regions where Ti ions can undergo
displacement. For instance, in tetragonal BaTiO3 with c > a,2768 | CrystEngComm, 2024, 26, 2765–2769

To achieve high tensile strain on the BaTiO3 film with thebottom electrode, we fabricated BaTiO3 films on a LBSO
bottom electrode. The tensile strain values increase from 0.6
to 2% with decreasing thickness from 300 to 20 nm. The TC
values of the 0.6 and 1.3%-tensile strained films are ∼400
and ∼500 °C, respectively, which are much higher than thoseof the bulk (120 °C) and previously reported tensile-strained
films (220–220 °C). For the 1.4–2% tensile-strained sheet, theferroelectric hysteresis loops were not observed in the out-ofplane direction, probably because the ferroelectric
polarization aligns in the in-plane direction owing to thetensile strain. On the contrary, the 0.6%-tensile strained film
displayed ferroelectric behavior along the out-of-plane
direction, suggesting that the ferroelectric polarization
direction was slightly tilted away from the in-plane direction.

Conflicts of interest
The authors declare no competing financial interest.

Acknowledgements
This work was supported by JST, PRESTO Grant Number
JPMJPR21Q3, Japan, JSPS KAKENHI (20H02614 (TK)), and the
Kao Foundation for Arts and Sciences (T.K.). H.O. was
supported by a Grant-in-Aid for Scientific Research on
Innovative Areas (19H05791) from JSPS. L.G. was supported
by China Scholarships Council 202008050264.

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