ferroelastic twin structures in epitaxial wo3 thin films · 2019. 3. 8. · ferroelastic twin...

Ferroelastic twin structures in epitaxial WO3 thin filmsShinhee Yun, Chang-Su Woo, Gi-Yeop Kim, Pankaj Sharma, Jin Hong Lee, Kanghyun Chu, Jong Hyun Song,Sung-Yoon Chung, Jan Seidel, Si-Young Choi, and Chan-Ho Yang Citation: Applied Physics Letters 107, 252904 (2015); doi: 10.1063/1.4938396 View online: http://dx.doi.org/10.1063/1.4938396 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/107/25?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Epitaxial growth of high quality WO3 thin films APL Mater. 3, 096102 (2015); 10.1063/1.4930214 Structural and magnetic properties of a series of low-doped Zn 1−x Co x O thin films deposited from Zn and Cometal targets on (0001) Al 2 O 3 substrates J. Appl. Phys. 95, 7187 (2004); 10.1063/1.1667805 Dielectric properties of strained ( Ba, Sr)TiO 3 thin films epitaxially grown on Si with thin yttria-stabilized zirconiabuffer layer Appl. Phys. Lett. 78, 2542 (2001); 10.1063/1.1367309 Effect of finite domain-wall width on the domain structures of epitaxial ferroelectric and ferroelastic thin films J. Appl. Phys. 89, 1355 (2001); 10.1063/1.1332086 Model of crystal lattice strained along the preferential direction by anisotropic stress for GaAs heteroepitaxialfilms grown on vicinal Si(001) and Si(110) substrates by molecular-beam epitaxy J. Vac. Sci. Technol. A 19, 287 (2001); 10.1116/1.1323971

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Ferroelastic twin structures in epitaxial WO3 thin films

Shinhee Yun,1 Chang-Su Woo,1 Gi-Yeop Kim,2 Pankaj Sharma,3 Jin Hong Lee,1

Kanghyun Chu,1 Jong Hyun Song,4 Sung-Yoon Chung,5,6 Jan Seidel,3 Si-Young Choi,2

and Chan-Ho Yang1,6,a)1Department of Physics, KAIST, Daejeon 34141, South Korea2Korea Institute of Materials Science, Changwon 51508, South Korea3School of Materials Science and Engineering, University of New South Wales, Sydney,New South Wales 2052, Australia4Department of Physics, Chungnam National University, Daejeon 34134, South Korea5Graduate School of EEWS, KAIST, Daejeon 34141, South Korea6KAIST Institute for the NanoCentury, KAIST, Daejeon 34141, South Korea

(Received 24 October 2015; accepted 8 December 2015; published online 22 December 2015)

Tungsten trioxide is a binary oxide that has potential applications in electrochromic windows, gas

sensors, photo-catalysts, and superconductivity. Here, we analyze the crystal structure of atomically

flat epitaxial layers on YAlO3 single crystal substrates and perform nanoscale investigations of the

ferroelastic twins revealing a hierarchical structure at multiple length scales. We have found that the

finest stripe ferroelastic twin walls along pseudocubic h100i axes are associated with cooperativemosaic rotations of the monoclinic films and the larger stripe domains along pseudocubic h110i axesare created to reduce the misfit strain through a commensurate matching of an effective in-plane lat-

tice parameter between film and substrate. The typical widths of the two fine and larger stripe

domains increase with film thickness following a power law with scaling exponents of �0.6 and�0.4, respectively. We have also found that the twin structure can be readily influenced by illumina-tion with an electron beam or a tip-based mechanical compression. VC 2015 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4938396]

In recent years, much attention has been paid to twin/do-

main walls as a low dimensional mesoscopic entity exhibiting

exotic physical properties distinct from bulk properties.1–13

One of the noticeable materials to illustrate the significance of

twin walls is tungsten trioxide (WO3). Many interesting phe-

nomena at ferroelastic twin walls have been discovered such

as electronic conduction in contrast to the insulating bulk,14

superconductivity at low temperatures,15 and ionic migra-

tion.16,17 Moreover, alkali-metal doped or oxygen reduced

WO3 occupies 5d orbitals of tungsten ions as an n-type con-ductor18–22 that might further expand the versatile features of

twin wall. However, until now, the usability has been mainly

studied based on bulk crystals or nano-structures,23–27 and few

studies have been made on epitaxial films.28–31 Furthermore,

creation of the twin wall structure in an epitaxial WO3 film has

not been achieved yet. This is a clear limitation to further

advance our understanding of twin wall functionalities and

explore new possibilities in this material. Advantages of epi-

taxial films associated with controllable thickness, effects of

heteroepitaxial interface including misfit strain, tunability of

the orientation, and periodicity of domain walls can be broadly

beneficial to the manipulation of the inherent functionalities of

WO3.

In this paper, we report on the crystal structure of epitax-

ial WO3 layers on a nearly matched (110)o (hereafter the

subscript “o” represents the orthorhombic index) surface of

YAlO3 substrate. We have found that atomically flat epitax-

ial films show an exotic ferroelastic twin domain arrange-

ment with a hierarchical structure at multiple length scales

made of monoclinic building blocks. These ferroelastic

domains sensitively respond to electron beam irradiation and

mechanical pressure.

In our experiments (for the details of experimental meth-

ods, see Ref. 32), WO3 thin films were grown on (110)o-ori-

ented YAlO3 substrates (orthorhombic, a¼ 5.18 Å, b¼ 5.31 Å,c¼ 7.35 Å,�

ffiffiffi

2p

apc� �ffiffiffi

2p

apc� � 2apc) by pulsed laser dep-osition. WO3 has a crystal structure similar to the ABO3 perov-

skite except for an empty A-site, which undergoes various

structural transitions with temperature variation (monoclinic

! triclinic! monoclinic! orthorhombic! tetragonal withincreasing temperature).33,34 Near room temperature, there are

two competing phases called c-WO3 and d-WO3. The c-WO3has a monoclinic crystal structure (space group P21/n,

a¼ 7.306 Å, b¼ 7.540 Å, c¼ 7.692 Å, b¼ 90.88�) with a vol-ume of �2apc� �2apc� �2apc, where apc is a pseudocubiclattice parameter (hereafter the subscript “pc” represents the

pseudocubic cell),33 exhibiting its peculiar twin structure.35

The d-WO3 is triclinic with lattice parameters (P�1,a¼ 7.309 Å, b¼ 7.522 Å, c¼ 7.678 Å, a¼ 88.81�, b¼ 90.92�,c¼ 90.93�).36

A representative surface topographic image of a 610-

nm-thick WO3 film exhibits a well aligned ferroelastic twin

structure with a herringbone pattern (Fig. 1(a)). The twin

domains have a width of �30 nm, which is more clearly rec-ognized in the AFM deflection contrast representing the

slope of height (Fig. 1(b)). Since the fast scan axis of the

AFM tip was parallel to the [001]o axis, the contrast for only

the [1�10]o-axis-parallel fine stripe domains (fine-domains) isclearly identified in the deflection image. A schematic of the

herringbone twin structure in Fig. 1(c) shows the fine stripea)E-mail: [email protected]

0003-6951/2015/107(25)/252904/5/$30.00 VC 2015 AIP Publishing LLC107, 252904-1

APPLIED PHYSICS LETTERS 107, 252904 (2015)


http://dx.doi.org/10.1063/1.4938396http://dx.doi.org/10.1063/1.4938396http://dx.doi.org/10.1063/1.4938396mailto:[email protected]://crossmark.crossref.org/dialog/?doi=10.1063/1.4938396&domain=pdf&date_stamp=2015-12-22

domains parallel to [001]o or [1�10]o axes at a length scale ofa few 10 nm. Larger stripe macro-domains, which are rotated

by 645� relative to the fine domain walls, are present at alarger length scale (a few 100 nm), and they can be classified

into two variants in terms of the aligning axes. Furthermore,

orthogonal bundles of the macro-domains, each of which

contains a single variant of stripe macro-domains, are found

at a few micron length scales. The bumpy twin walls were

observed at the boundaries between two adjacent bundles

creating the irregular shaped super-macro-domain structure

(Fig. 1(d)).

To investigate the microscopic origin for the hierarchi-

cal twin structure, we characterized the crystal structure

through x-ray h-2h scans and reciprocal space maps (RSMs)for a 200-nm-thick film. In Fig. 2(a), the h-2h scan exhibitsthe (001)pc and (002)pc diffraction peaks of the WO3 film.

The out-of-plane lattice parameter of the WO3 film was

determined to be 3.84(8) Å, which is almost the same as the

bulk c-axis pseudocubic lattice parameter of the monoclinicphase (3.84(6) Å).33 Besides, the full-width-at-half-maxi-

mum (FWHM) of the small range x-rocking curve measuredat the (001)pc WO3 peak (inset of Fig. 2(a)) was 0.061

�. Inaddition to this sharp central peak, a larger range scan

revealed the existence of two satellite peaks at off-axis

x-angles of 60.82� (see Fig. S2(b) in Ref. 32). This can beclearer in a RSM for the (002)pc reflection in the HL-plane

(Fig. 2(b)). Accordingly, we know that the ab-plane of thefilm is not exactly parallel to the substrate but tilted by v�0.82� and there are four variants in terms of the tilt direc-tions. It is also worthwhile mentioning that this mosaic tilt

angle of �0.82� detected in this thick-thickness regime(>100 nm) is gradually reduced with film thickness andeventually the tilt becomes negligible when films are thinner

than �50 nm.To further clarify the crystal structure including in-plane

lattice parameters, RSMs for two asymmetric peaks (103)pcand (113)pc were measured for the 200-nm-thick film (Figs.

2(c) and 2(d)). To interpret the RSMs, we first consider a

model of pseudocubic unit cell imposed by a monoclinic dis-

tortion similar to the monoclinic unit cell of bulk WO3 (M0in Fig. 2(e)). Given a monoclinic angle b and four-foldtwins, RSMs for {H0L}pc or {K0L}pc reflections of M0 result

in three split peaks (red crosses in Fig. 2(c)). Two of them

((H0L)pc and (-H0L)pc) are shifted toward �L or þL direc-tions by an equal k-space distance away from the expected

pseudo-tetragonal position deduced from (00 L)pc peaks; the

FIG. 1. AFM images of a 610-nm-

thick WO3 film on (110)o YAlO3 sub-

strate. (a) The topographic image and

(b) tip deflection image representing

the slope of height. (c) Schematic of

the herringbone twin structure consist-

ing of macro- and fine-domains. (d)

Large scale unflatten AFM image

showing super-macro-domains sur-

rounded by bumpy boundaries. Terrace

structure with a single unit cell step

height along ½110�subpc is seen superim-posed on the twin wall structure.

FIG. 2. X-ray diffraction study on a 200-nm-thick WO3 film on (110)oYAlO3 substrate. (a) A h-2h scan along [00L]pc showing (001)pc and (002)pcWO3 peaks. The inset exhibits a x-rocking curve measured at the (001)pcWO3 peak. (b) RSM for (002)pc reflection. (c) RSM for (103)pc reflection.

(d) RSM for (113)pc reflection. The reciprocal lattice unit (r.l.u.) was defined

to be the reciprocal value of d-spacing of the YAlO3 (110)o planes. (e) Aschematic displaying the monoclinic unit cell of WO3 rotated from M0 to M

by the mosaic tilt angle around the local b-axis. Four-fold twin structure wasconsidered when the peak indices were assigned in the RSMs.

252904-2 Yun et al. Appl. Phys. Lett. 107, 252904 (2015)


in-plane H position of them gives the reciprocal of the a-axislattice parameter and the extent of splitting is related to the

monoclinic angle b. In addition, the third peak detected atthe expected L without any shift corresponds to {0KL}pcreflections. The in-plane k-space position of the peak gives

information of b-axis lattice parameter. To ensure the modelof unit cell, we would also check the peak positions of

{HHL}pc reflections. If the model is correct, the peaks of

{HHL}pc should be split into two variant peaks and the

amount of split along L-axis should harmonize with the

aforementioned L-split between (H0L)pc and (�H0L)pcpeaks.

With this in mind, we next extract the lattice parameters

of the WO3 thin film from the measured RSMs. Different

from the usual peak split tendency, both the RSMs in this case

seemed to produce the peak split along the horizontal H-axis

rather than the L-axis. It arose from the fact that the normal of

ab-plane of film was tilted by 60.82� with respect to the nor-mal of substrate creating four-fold mosaic domains. Thus, the

experimental results, which at first seemed to be different

from the expectation, can now be understood by introducing

mosaic tilts into the interpretation. The (103)pc and (�103)pcpeaks were rotated around the K-axis by �0.82� and þ0.82�,respectively. Being traced along powder arcs to recover the

case without the mosaic tilt, the original k-space positions

could be found and marked by red crosses on the maps (Fig.

2). With the monoclinic structural model, we could determine

the pseudocubic lattice parameters of the WO3 thin film to be

a¼ 3.65(6) Å, b¼ 3.75(9) Å, c¼ 3.84(8) Å, and b¼ 89.1(8)�.We note that the deviation of monoclinic angle (b) from 90�

has a very similar value to the tilt angle (v� 0.82�). In fact,the appearance of split peaks coincidentally along the horizon-

tal axis stems from the following situation. As described in

the schematic (Fig. 2(e)), the monoclinic unit cell of the WO3film is rotated around the local b-axis so that the c-direction isperpendicular to substrate. This happens because sharing the

same c-axis of neighboring fine domains is favorable for mini-mizing the elastic deformation at the twin walls. It was also

found that the (103)pc and (�103)pc peaks subject to the mosaicrotation were of diffuse shape along the horizontal reciprocal

axis in accordance with the vertical twin walls separated by

the short width of fine domains (a few 10 nm). On these

grounds, we can conclude that the origin of the fine twin

domains is due to the cooperative mosaic tilt of monoclinic

unit cells, thereby producing þa and �a fine domains. On theother hand, the a-axis lattice parameter was slightly smallerthan that of the substrate, while the b-axis lattice parameterwas larger. A recurring appearance of a-axis and b-axisdomains, i.e., the macro-domain structure, is necessary to

effectively minimize interfacial misfit strain energy with the

substrate offering a nearly matched interface on average.

To investigate the cross sectional area of a WO3 film, a

transmission electron microscopy (TEM) study was per-

formed for a 70-nm-thick WO3 film (Fig. 3). In the weak-

beam-dark-field (WBDF) TEM image of the as-prepared

sample with a zone axis along [001]o (Fig. 3(a)), strained-

pillar structures were recognized through the contrast

depending on local strain. The widths of pillars were meas-

ured to be less than 10 nm. Although these dense twin walls

can be attributed to the relatively thin thickness of the film as

compared to the film used for the AFM image, one should be

cautious to conclude that the measured width in TEM repre-

sents the fine-domain width of the film accurately. The TEM

specimen was fabricated in a way that the foil thickness

along the zone axis was very thin for electron transmission,

which would lead to a different mechanical boundary condi-

tion from that of the as-grown film. In reality, the twin struc-

ture observed at the initial stage in the film was easily

relaxed by illumination with a weak electron beam with a

flux of 14 pA/cm2 (Fig. 3(b)). The illumination gradually

removed twin walls and associated individual fine domains

to form remaining larger domains. The susceptibility of the

twin structure to external perturbation leads us to conjecture

that the twin walls are primarily involved in large strains and

strain gradients being less relevant to interfacial reconstruc-

tion or/and defect accumulation in this thin film. The insta-

bility of twin walls in TEM specimen did not allow careful

high-resolution TEM studies on the twin walls, but it was

possible to examine the relaxed phase of WO3 regarding the

epitaxial coherency and interfacial structure between film

and substrate. Figure 3(c) shows a Z-contrast high-angle-an-

nular-dark-field scanning TEM (HAADF-STEM) image for

an interfacial region. It was observed that a WO2 sub-layer

was formed right on the YO layer of the substrate and W

atoms occupied only the B-sites of the perovskite keeping

the A-site empty. Figure 3(d) shows a selected-area-electron-

diffraction (SAED) pattern for the interfacial region. The red

spots can be indexed to the diffraction peaks of WO3 and the

green spots to the ones of YAlO3. The detection of exact

square reciprocal cell of WO3 indicates the monoclinic

ca-plane in this relaxed film was parallel to the zone axis.Misalignment between the reciprocal cells of YAlO3 and

WO3 by �1.5� along the horizontal reciprocal axis indicates

FIG. 3. TEM study for a 70-nm-thick WO3 film. (a) The WBDF TEM image

of the as-prepared sample showing a highly strained fine domain structure.

(b) The fine domain structure easily relaxed by illumination of a weak elec-

tron beam with a flux of 14 pA/cm2. (c) The HAADF-STEM image of the

interface of WO3 film and YAlO3 substrate. (d) A SAED pattern for an inter-

facial region. The dashed red (green) line box represents the pseudocubic

reciprocal unit cell of WO3 (the reciprocal unit cell of YAlO3). The scale

bars indicate 20 nm.



that the pseudocubic c-axis of substrate makes an angle of�1.5� with that of the film, which is expected in the ortho-rhombic substrate.

Next, the thickness dependence of the fine- and macro-

domain widths was examined using multiple samples with dif-

ferent thicknesses. As examples, in-plane piezoresponse force

microscopy (PFM) images of four representative samples with

thicknesses of 42 nm, 81 nm, 202 nm and �400 nm are shownin Figs. 4(a)–4(d). Remarkably, macro-domains were visible

in the PFM images enabling us to evaluate their macro-

domain widths by Fourier-transforming the PFM images

(insets of Figs. 4(a)–4(d)).37 Because the monoclinic phase of

WO3 with a space group of P21/n is centrosymmetric, the sig-

nificant enhancement of piezoresponse on either side of

macro-domain walls is unexpected. It is most likely due to the

fact that the twin wall areas are subject to large deformation

against the nearby macro-domain lowering local symmetry.

Moreover, films under the mosaic tilt are expected to create a

large strain gradient to reconcile with flat substrates under-

neath, leading to the involvement of flexoelectric polariza-

tions.8–10 It has been found that an oxygen deficient sheet

layer can be a non-centrosymmetric tetragonal phase.38 A

more rigorous mechanism of the unusual piezoresponse and

microscopic atomic arrangements at the twin walls and

interfaces still remain unexplored.

The double logarithmic plot of domain widths versus

film thickness was carefully obtained (Fig. 4(e)). The linear

slope in the log-log plot determines the scaling exponent to

be 0.60 (60.01) for the macro-domain. Similarly, the valueof the fine-domain is determined to be 0.42 (60.05). Thesescaling exponents significantly deviate from the 0.5 expected

in the typical Landau-Lifschitz-Kittel law.39 Similar devia-

tion has been reported in the ferroelectric domains of multi-

ferroic BiFeO3; irregular domain walls characterized by a

roughness exponent and a fractal Hausdorff dimension ex-

hibit the domain size scaling with an exponent �0.6.40,41 Itwill be an interesting topic to investigate the fractal-like

behavior in this hierarchical twin structure in future studies.

Finally, we address the rearrangement of the macro-

domains by the AFM tip-based mechanical force. For an as-

grown 53-nm-thick WO3 film, in-plane PFM image was

captured to visualize (at nominally low loading force of

approximately 50–100 nN) the initial ferroelastic domain

microstructure as shown by Figure 5(a). Thereafter, within

the initially visualized region (shown by dashed-line frame

in Figs. 5(a) and 5(b)), a contact mode scan with a grounded

AFM-tip exerting a comparatively high-loading force of

approximately 2.5 lN was performed. All fast scan axes arealong [1�10]o including the compressive scan. In Fig. 5(a),super-macro-domain boundaries are indicated by white

solid lines in the selected area (the dashed box). After scan-

ning the dashed-line region with a compressive force of

2.5 lN, the in-plane PFM image (Fig. 5(b)) was capturedunder the same conditions as before, and allows a direct

FIG. 4. Thickness-dependent domain widths. (a)–(d) In-plane PFM images

at the selected WO3 thicknesses. The interval between neighboring bright

(or dark) contrasts corresponds to the macro-domain width. It has a tendency

to be wider with increasing film thickness. Bright (dark) contrast represents

in-plane piezoresponse pointing to the positive (negative) vertical direction.

Tip orientations during measurements are depicted at the right corner. All

the scale bars indicate 500 nm. (Inset) The Fourier transform of the in-plane

PFM image. All the Fourier transform images have the same size and the

unit of tick labels in (d) is lm�1. (e) The double logarithmic plot of themacro-domain (m) and fine-domain (f) widths versus the film thickness.Because of the AFM resolution limit, fine-domain widths of four samples

were obtained through AFM images (solid blue circles), and the others from

the horizontal satellite peaks observed in the H-scans of x-ray diffraction

(open blue circles).

FIG. 5. Domain reorientation induced by the AFM tip-based nano-mechani-

cal force. (a) In-plane PFM image in the as-grown state of a 53-nm-thick

WO3 film. The orientation of AFM tip during the experiment is expressed

by the tip cartoon. Fast scan axis of the tip was along [1�10]o and the slowscan direction was [001]o. Dashed-line box represents the area where the

scan with a loading force of 2.5 lN was performed. White solid lines aresuper-macro-domain boundaries. As shown in the (b) in-plane PFM images,

macro-domains were rearranged by the mechanical force. All the white scale

bars indicate 500 nm.



comparison with initially observed domain-microstructure.

Within the area where a high mechanical pressure was

applied, the super-macro-domains and their boundaries

were displaced, which means mechanical-force induced

rearrangements of macro-domains (Fig. 5(b)). Because the

irregular-shaped bundles do not fit with each other exactly,

some local domains and their boundaries near the edges of

the super-macro-domains could be highly strained. The

more unstable boundaries are likely to be rearranged toward

the stable state.

In summary, the comprehensive structural study of epi-

taxial WO3 thin films on YAlO3 substrates provided useful

insights into their inter-relevance among the monoclinic cell,

four-fold mosaic tilt, misfit strain, and the emergence of a

ferroelastic hierarchical twin structure (including fine-,

macro-, super-macro-domains).

This work was supported by the National Research

Foundation of Korea Grant funded by the Korean

Government (Contract Nos. NRF-2014R1A2A2A01005979

and NRF-2013S1A2A2035418) and the Global Frontier

Hybrid Interface Materials of the NRF of Korea funded by

the Korea Government (2013M3A6B1078872).

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