Epitaxial growth of (111)-oriented BaTiO3/SrTiO3 perovskite superlattices onPt(111)/Ti/Al2O3(0001) substratesGasidit Panomsuwan, Osamu Takai, and Nagahiro Saito Citation: Applied Physics Letters 103, 112902 (2013); doi: 10.1063/1.4820780 View online: http://dx.doi.org/10.1063/1.4820780 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/103/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Infrared spectroscopy of an epitaxial BaTiO3/SrTiO3 superlattice grown on a (110) SmScO3 substrate J. Appl. Phys. 115, 184102 (2014); 10.1063/1.4875877 Compositional engineering of BaTiO3/(Ba,Sr)TiO3 ferroelectric superlattices J. Appl. Phys. 114, 104102 (2013); 10.1063/1.4820576 Enhanced microwave dielectric tunability of Ba0.5Sr0.5TiO3 thin films grown with reduced strain on DyScO3substrates by three-step technique J. Appl. Phys. 113, 044108 (2013); 10.1063/1.4789008 Dielectric properties of 001 -oriented Ba 0.6 Sr 0.4 TiO 3 thin films on polycrystalline metal tapes using biaxiallyoriented MgO ∕ γ - Al 2 O 3 buffer layers Appl. Phys. Lett. 88, 062907 (2006); 10.1063/1.2173232 Dielectric and optical properties of BaTiO 3 / SrTiO 3 and BaTiO 3 / BaZrO 3 superlattices J. Appl. Phys. 91, 2284 (2002); 10.1063/1.1433180

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Epitaxial growth of (111)-oriented BaTiO3/SrTiO3 perovskite superlattices onPt(111)/Ti/Al2O3(0001) substrates

Gasidit Panomsuwan,1,a) Osamu Takai,1 and Nagahiro Saito1,2,3

1Department of Materials, Physics and Energy Engineering, Graduate School of Engineering,Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan2EcoTopia Science Institute, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan3Green Mobility Collaborative Research Center, Nagoya University, Furo-cho, Chikusa-ku,Nagoya 464-8603, Japan

(Received 28 May 2013; accepted 19 August 2013; published online 9 September 2013)

Symmetric BaTiO3/SrTiO3 (BTO/STO) superlattices (SLs) were epitaxially grown on Pt(111)/Ti/

Al2O3(0001) substrates with various modulation periods (K¼ 4.8� 48 nm) using double ion beam

sputter deposition. The BTO/STO SLs exhibit high (111) orientation with two in-plane orientation

variants related by a 180� rotation along the [111]Pt axis. The BTO layer is under an in-plane

compressive state, whereas the STO layer is under an in-plane tensile state due to the effect of

lattice mismatch. A remarkable enhancement of dielectric constant is observed for the SL with

relatively small modulation period, which is attributed to both the interlayer biaxial strain effect and

the Maxwell-Wagner effect. VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4820780]

Oxide artificial superlattices (SLs) have been of great in-

terest in advanced electronic devices owing to their flexibility

for material design by tailoring the components and their

thicknesses. These can thus open up an opportunity to create

new functional properties, as well as a new combination

of desirable properties.1,2 In recent years, BaTiO3/SrTiO3

(BTO/STO) SLs have drawn considerable attention in the fun-

damental physics and technological applications since they

exhibit most intriguing properties, such as large dielectric con-

stant, high dielectric tunability, and strong polarization.3–7 So

far, both experimental and theoretical attempts have mainly

focused on the (001)-oriented BTO/STO SLs on various types

of substrates. In contrast, there has been very limited success

regarding the BTO/STO SLs grown along other crystallo-

graphic orientations in the literature. Considering the crystal-

lographic orientation effect on the dielectric behavior, the

perovskite films with (111) orientation have been particularly

interesting. It has been reported that the highly (111)-oriented

BaxSr1�xTiO3 and STO thin films present a large dielectric

constant and a high dielectric tunability when compared with

other crystallographic orientations or even bulk.8–11 Inspired

by these approaches, the (111)-oriented BTO/STO SLs would

be expected to contribute to the improvement of dielectric

performance in comparison with the usual (001)-oriented

BTO/STO SLs. The first study on (111)-oriented BTO/STO

SLs was reported by Nakagawara et al.12 They found a signifi-

cant enhancement of dielectric constant at a small modulation

period due to the in-plane stress effect. Over the past decades,

there has been no progress work on the (111)-oriented BTO/

STO SLs. In general, the (111)-oriented BTO and STO films

are easy to epitaxially grow on the ABO3(111) perovskite

substrates owing to their precise surface structural control.

However, the films grown on a metallic Pt electrode is much

more preferable in the viewpoint of practical application. It

has found that the films with high degree of (111) orientation

is rather difficult to achieve on the Pt(111) surface due to the

complex growth mechanism and constraints from electrostatic

and surface energy effects, resulting in either random or

preferred (100)/(110) orientations.13–15 According to the con-

text, there still remain a challenge for the growth of highly

(111)-oriented BTO/STO SLs on the Pt(111). Here, we report

the growth of (111)-oriented BTO/STO SLs on Pt(111)/Ti/

Al2O3(0001) substrates using ion beam sputter deposition

(IBSD) with double electron-cyclotron-resonance (ECR) ion

guns. The influence of modulation period on the structural,

morphological, and dielectric properties are investigated and

discussed in detail.

Symmetric BTOK/2/STOK/2 SLs were grown on Pt(111)/

Ti/Al2O3(0001) substrates with various modulation periods

(K¼ 48, 24, 12, and 4.8 nm) using IBSD with double ECR

ion guns. The thickness of the Ti buffer layer was 3 nm,

which has been reported as an optimal value to promote the

growth of (111) orientation on the Pt(111) surface.16 The

preparation details and structural properties of Pt/Ti bilayer

on Al2O3(0001) substrate are shown in Supplementary

Material (Figs. S1 and S2).21 All samples were designed to

have the same thickness of �96 nm, controlled with an equal

growth time. Both BTO and STO layers were grown at the

same rate (�0.07 A s�1). The chamber was firstly evacuated

to a background pressure of below 2� 10�6 Torr. Then Ar

gas with a constant pressure of 3� 10�4 Torr was fed into

each ECR ion gun and kept in the chamber during the growth

process. Oxygen pressure of 3� 10�4 Torr was simultane-

ously supplied close to the substrate during film growth. A

microwave power output and an accelerating voltage were

fixed at 180 V and 1.8 kV, respectively. The BTO/STO SLs

were grown at a temperature of 700 �C. After film growth,

the BTO/STO SLs were further annealed at 700 �C for

60 min and subsequently cooled to room temperature under

an oxygen pressure of 0.75 Torr.

Structural properties were characterized by X-ray diffrac-

tion (XRD, Rigaku, SmartLab) operated with a Ge(220)� 2

channel monochromator and Cu Ka radiation (k¼ 1.5418 A)

at 9 kW. Film surface morphology was observed usinga)Electronic mail: gasidit@rd.numse.nagoya-u.ac.jp

0003-6951/2013/103(11)/112902/4/$30.00 VC 2013 AIP Publishing LLC103, 112902-1

APPLIED PHYSICS LETTERS 103, 112902 (2013)

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an atomic force microscope (AFM, Seiko Instrument Inc.,

SPA-300HVþ SPI-3800N). Metal-insulator-metal (MIM)

capacitors were prepared by depositing a circle Pt top elec-

trode (5� 10�3 cm2) through a metallic mask. The dielectric

properties were measured using an LCR meter (Hikoki

3532-50).

High-resolution X-ray diffraction (HRXRD) x� 2hscans of the symmetric BTO/STO SLs are shown in Fig. 1.

Only the hhh reflections corresponding to the BTO/STO SLs

can be observed without the reflections from other oriented

planes. This result indicates that all BTO/STO SLs are pre-

dominantly (111)-oriented. The BTO/STO SLs with K¼ 48

and 24 nm are composed of two distinct reflection peaks

originating from the BTO and STO layers since their modu-

lation period are larger than the structural coherence length.

The hhhSTO reflections cannot be clearly seen due to overlap

with a strong intensity of hhhPt reflections. When the Kdecreases to 12 nm, the appearance of satellite peaks

(denoted as 61, 62,…) beside the main reflection peak

(denoted as 0) is observed. This feature is characteristic of

SL formation. However, the satellite peaks become sup-

pressed for the SL with K¼ 4.8 nm, which may suggest the

existence of interdiffusion between BTO and STO layers. In

the case of the BTO/STO SL with K¼ 4.8 nm, the d111 value

is found to be about 0.228 nm, which is larger than that of

Ba0.5Sr0.5TiO3 solid solution in bulk (0.226 nm). A lattice

expansion of d111 can be attributed to the effect of in-plane

lattice strain at the interface region of each sublayer due to a

lattice mismatch between the BTO and STO, and oxygen

deficiency in the films. The inset of Fig. 1 shows the rocking

curve x scan measured on the main 222 reflection peak of

BTO/STO SL with K¼ 4.8 nm. Full-width at half-maximum

(FWHM) is estimated to be about 0.54�, indicating good

crystallinity and high degree of (111) orientation.

To confirm an in-plane orientation relationship, the

BTO/STO SLs were further investigated with in-plane pole

figure analysis. In-plane pole figures measured from the

{100} and {110} planes for the BTO/STO SL (K¼ 4.8 nm)

are displayed in Figs. 2(a) and 2(b), respectively. Two sets of

three-fold symmetry are clearly observed at a¼ 35.3� and

54.7� for the {100} and {110} pole figures, respectively.

This is evidence that the (111)-oriented BTO/STO SL has an

in-plane orientation on the Pt(111) layer with two in-plane

orientation variants (denoted as A and B) related by a 180�

in-plane rotation along the [111]Pt axis. The origin of this

twinning is due to a relatively small energy difference in the

two different stacking sequences of the (111)-oriented BTO

and STO layers on Pt(111).17,18 A schematic diagram show-

ing possible atomic arrangement at the interface between the

(111)-oriented BTO/STO SL and the Pt(111) is represented

in Fig. S4. The epitaxial orientation relationships of the

(111)-oriented BTO/STO SLs on the Pt(111) layer can be

expressed as follows:

h111iBTO k h111iSTO k h111iPt and

f�110gBTO k f�110gSTO k f�110gPt:

Grazing-incidence X-ray diffraction (GIXRD) 2hv scans

were also examined on all STO/BTO SLs along the h�110idirection in order to evaluate in-plane lattice strain, as

revealed in Fig. 2(c). The reflection peaks associated with

the BTO and STO phases are obtained from the deconvolu-

tion of measured GIXRD 2hv spectra. The stronger intensity

in the BTO reflection peak in comparison with the STO

reflection peak is due to the later growth of the BTO layer

and the small penetration depth of an incident X-ray beam.

With a decrease in the modulation period, a gradual shift of

the BTO reflection peak toward a higher 2hv is observed. On

the other hand, the STO reflection peak shifts to a lower 2hvand starts overlapping the BTO reflection peak. This can be

suggested that the d�110 of the BTO layer slightly decreases in

FIG. 1. HRXRD x� 2h scans of the (111)-oriented BTO/STO SLs with var-

ious modulation periods. The inset shows the rocking curve x scan meas-

ured on the main 222 reflection peak of BTO/STO SL (K¼ 4.8 nm).

FIG. 2. In-plane pole figures measured from (a) {100} and (b) {110} planes

of the BTO/STO SL (K¼ 4.8 nm). (c) GIXRD 2hv scans along h�110i direc-

tion of the BTO/STO SLs with various modulation periods. The vertical

dash and dot lines indicate the angle position of �110 reflections for BTO and

STO single-layer films grown on Pt(111)/Ti/Al2O3(0001) substrates at the

same conditions, respectively.

112902-2 Panomsuwan, Takai, and Saito Appl. Phys. Lett. 103, 112902 (2013)

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order to adjust to the smaller d�110 value of the STO layer

(biaxial compressive strain), whereas the d�110 of the STO

layer increases so as to adjust to the larger d�110 value of

the BTO layer (biaxial tensile strain). As demonstrated in

Fig. 4(a), the magnitude of in-plane biaxial strain (Sjj) for

both BTO and STO layers of the SLs gradually increases

with decreasing modulation period.

Figures 3(a)–3(d) illustrate the AFM topography images

of the BTO/STO SLs with various modulation periods. All

samples exhibit a highly dense microstructure without obvious

crack and pinhole. With a decrease in the modulation period,

the grain size and root-mean-square (rms) roughness become

increased. A larger grain size is formed due to the coalescence

of several small grains once the grains grew together. This

morphological evolution may be caused by the different

growth mechanism of two constituent materials or the differ-

ence in surface energy and mobility of Ba and Sr atoms.

The room-temperature dielectric constant of the

BTO/STO SLs at 105 Hz is found to increase with decreasing

modulation period from 496 for K¼ 48 nm to 927 for

K¼ 4.8 nm, as revealed in Fig. 4(b). Our result shows a simi-

lar tendency to the previous studies by Nakagawara et al.and Tabata et al.,3,12 which the biaxial interlayer strain plays

a dominant role in enhancing dielectric constant. However,

apart from the biaxial strain effect, Catalan et al. reported

that the origin of dielectric enhancement in the multilayer

and SLs with fine modulation period has been described in

term of interfacial polarization or Maxwell-Wagner (M-W)

effect.19,20 To confirm the dominance of M-W effect on the

dielectric properties, the real (e0) and imaginary (e00) permit-

tivity for all SLs were carried out as a function of frequency

(102 Hz� f� 106 Hz), as shown in Figs. 5(a) and 5(b),

respectively. A significant increase in the e0 and e00 are found

at low-frequency region, especially for the SLs with

K� 12 nm. Such strong frequency dispersion at low frequen-

cies implies that the conduction mechanism may play a dom-

inant role in the dielectric properties of the SLs with small

modulation period. The M-W series capacitor model was fit-

ted to the imaginary permittivity (e00) of the SL as a function

of frequency using following relation:19,20

e00ðxÞ ¼ 1

xC0ðRi þ RbÞþ ðe0 � e1Þ

1þ x2s2;

where x is the angular frequency, RiþRb is the total resist-

ance of the dielectric layers, s is relaxation time of the entire

SL, C0 is geometric factor, and e0 and e1 are the dielectric

constants at zero and infinite frequencies, respectively. The

subscript i and b refer to interfacial-like and bulk-like layers,

respectively. As demonstrated in Fig. 5(c), the e00 calculated

from M-W model can fit very well with the experimental

FIG. 3. AFM topography images (1� 1 lm2) of the BTO/STO SLs with var-

ious modulation periods.

FIG. 4. (a) In-plane biaxial strain generated on BTO and STO layers and (b)

dielectric constant of BTO/STO SLs measured at 105 Hz plotted against

modulation period.

FIG. 5. Frequency dependence of (a) real (e0) and (b) imaginary (e00) permit-

tivity at room temperature of the BTO/STO SLs. (c) The fitting of Maxwell-

Wagner model to the measured e00 values of STO/BTO SL (K¼ 4.8 nm) as a

function of frequency.

112902-3 Panomsuwan, Takai, and Saito Appl. Phys. Lett. 103, 112902 (2013)

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data for the SL with K¼ 4.8 nm at low frequencies.

However, at high frequencies (>104 Hz) where space charge

polarization is absent, the e00 values are deviated from the M-

W model. The inconsistency between the experimental data

and the M-W model at low frequencies is observed for the

SLs with K¼ 24 and 48 nm. The onset of M-W effect at

small modulation period indicates that the defects are formed

at the interface between the constituent layers of SL. Such

interfacial defect is believed to be caused by a much higher

density of oxygen vacancies and atomic interdiffusion at the

interface, which thus lead to an increase in electrical conduc-

tivity and carrier mobility. According to obtained result, it

can be suggested that the enhancement of dielectric constant

at small modulation period is mainly attributed to the M-W

effect. On the other hand, the interlayer strain effect seems

to be a nominal role in influencing dielectric properties when

compared with the M-W effect.

In conclusion, BTO/STO SLs with desired modulation

periods were grown on Pt(111)/Ti/Al2O3(0001) substrates

by double IBSD. The X-ray structural analysis reveals that

the SLs are highly (111)-oriented with two in-plane orienta-

tion variants related by a 180� rotation along the [111]Pt

axis. Moreover, the BTO and STO layers in the SLs are

found to be under in-plane biaxial compressive and tensile

strains, respectively. The magnitude of the in-plane biaxial

strain in two constituent layers of SLs increases as the mod-

ulation period decreases. The enhancement of dielectric

constant observed at small modulation period can be well

described by the M-W effect. For the interlayer strain

effect, it seems to play a nominal role when compared with

the M-W effect. The results presented here can be the

guideline for the latter experimental research in controlling

the growth and structure of (111)-oriented perovskite super-

lattices on metallic Pt electrode with desirable dielectric

properties.

This work was partially supported by the global COE

for Education and Research of Micro-Nano Mechatronics of

Nagoya University. The authors would like to thank Assoc.

Professor Nobuyuki Zettsu for providing LCR meter for the

dielectric measurement.

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preparation of epitaxial Pt(111)/Ti on Al2O3(0001) substrate, structural

properties of epitaxial Pt(111) on Al2O3(0001), and schematic diagram

showing atomic arrangements of BTO/STO SL on Pt(111) and

Al2O3(0001).

112902-4 Panomsuwan, Takai, and Saito Appl. Phys. Lett. 103, 112902 (2013)

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