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Applied Surface Science 390 (2016) 43–49

Contents lists available at ScienceDirect

Applied Surface Science

jou rn al h om ep age: www.elsev ier .com/ locate /apsusc

orphology and structural studies of WO3 films deposited on SrTiO3

y pulsed laser deposition

ossein Kalhori a,b,∗, Stephen B. Porter a, Amir Sajjad Esmaeily a, Michael Coey a,ehdi Ranjbar b, Hadi Salamati b

School of Physics and CRANN, Trinity College, Dublin 2, IrelandDepartment of Physics, Isfahan University of Technology, Isfahan 84156-8311, Iran

r t i c l e i n f o

rticle history:eceived 5 June 2016eceived in revised form 6 August 2016

a b s t r a c t

WO3 films have been grown by pulsed laser deposition on SrTiO3 (001) substrates. The effects of substratetemperature, oxygen partial pressure and energy fluence of the laser beam on the physical propertiesof the films were studied. Reflection high-energy electron diffraction (RHEED) patterns during and after

ccepted 9 August 2016vailable online 11 August 2016

eywords:O3 thin films

ulsed laser deposition

growth were used to determine the surface structure and morphology. The chemical composition andcrystalline phases were obtained by XPS and XRD respectively. AFM results showed that the roughnessand skewness of the films depend on the substrate temperature during deposition. Optimal conditionswere determined for the growth of the highly oriented films.

© 2016 Elsevier B.V. All rights reserved.

ighly oriented films

. Introduction

Transition metal oxides (TMO) with the perovskite structureave been a focus of interest recently due to their vast range oflectronic structure and physical properties [1–3]. The general for-ula for perovskite composition is ABO3, where atom A has 8 B

nd 12 O neighbours and atom B has 8 A and 6 O neighbours. WO3an be considered as a variant of the ABO3 perovskite-like struc-ure where tungsten atoms occupy the B cation sites, octahedrallyoordinated by oxygen atoms, and the A site remains unoccupiedlthough it can be filled by an alkaline ion such as Li+, Na+ or H+ forome applications such as electrochromism [4,5] or gasochromism6,7].

Lattice distortion plays an important role in the physical prop-rties of TMOs with perovskite-type structures. Between −100nd 1000 ◦C, WO3 undergoes at least four phase changes, involv-ng monoclinic, orthorhombic, hexagonal and tetragonal phases8–13]. Above 950 ◦C the structure will be a mixture of tetragonaltructure and liquid phase of WO3 as the temperature is close tohe melting point of WO3 (1470 ◦C) [14]. The stability ranges of the

arious different equilibrium phases of WO3 are listed in Table 1,long with the crystal structures and lattice parameters. In practice,he ideal cubic symmetry of tungsten trioxide is not stable due to

∗ Corresponding author at: Department of Physics, Isfahan University of Technol-gy, Isfahan, 84156-8311, Iran.

E-mail address: h.kalhori@ph.iut.ac.ir (H. Kalhori).

ttp://dx.doi.org/10.1016/j.apsusc.2016.08.052169-4332/© 2016 Elsevier B.V. All rights reserved.

the tilting of WO6 octahedra and subsequent displacement of thetungsten atom from the centre [8]. WO3 is thus an ideal model sys-tem for investigating octahedral deformation. The ability to modifyand control the octahedral connectivity has been explored in somereports [15–17]. Heteroepitaxy has been shown to be effective intuning the film/substrate interface through interfacial strain andthus dictating the crystal structure of the resulting films. For exam-ple, by changing growth temperature of epitaxial WO3 on sapphiresubstrates, films have been shown to nucleate as tetragonal, mon-oclinic, or hexagonal phases [9,15]. The orientation in RF-sputteredfilms is considerably affected by the oxygen concentration as well assubstrate material and orientation [18,19]. Post annealing of crys-talline WO3 films in oxygen or vacuum is expected to have severeeffects on their structure and stoichiometry [20].

Several methods have been presented to deposit the orientedWO3 film involving molecular beam epitaxy [21], thermal evapo-ration [22], RF magnetron sputtering [23,24] and chemical vapourdeposition [25]. Pulsed laser deposition (PLD) is also a method growthe films [26–28]. Laser parameters and deposition conditions haveimportant effects on the flim structure and morphology. For exam-ple, Lethy et al. revealed a phase transition from monoclinic W18O49to orthorhombic WO3 by increasing the oxygen partial pressureduring the growth [29]. The surface roughness, film crystallinityand grain size of the films have been shown to increase with higher

substrate temperatures [30]. In the present study, we have tried tomake a complete study of different effective growth parameters inorder to smooth WO3 films on SrTiO3 (001) substrates by the PLD

44 H. Kalhori et al. / Applied Surface Science 390 (2016) 43–49

Table 1Equilibrium phases of WO3 and their stability ranges. Note that these transition temperatures exhibit hysteresis effects and there is not full agreement in the literature.

Phase Structure Lattice parameters Stability range Reference

�-WO3 Tetragonal a = 0.525 nm, c = 0.391 nm 1010–1170 K [10]�-WO3 Orthorhombic a = 0.734 nm, b = 0.757 nm, c = 0.775 nm 600–1010 K [11]�-WO3 Monoclinic a = 0.730 nm, b = 0.754 nm, c = 0.769 nm, � = 90.88◦ 290–600 K [8]

nm, � ◦ ◦ ◦

663 nm

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2

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3

3

isTidoat

�-WO3 Triclinic a = 0.730 nm, b = 0.751 nm, c = 0.7681�-WO3 Monoclinic a = 0.5277 nm, b = 0.71555 nm, c = 0.7H-WO3 Hexagonal a = 0.730 nm, 0.780 nm

ethod. SrTiO3 was chosen as the substrate because it has an idealubic perovskite structure and a lattice parameter, 0.3905 nm thats compatible with WO3. To the best of our knowledge in spite of aew reports about the characterization of WO3/SrTiO3 films [31,32],here is no comprehensive study on the growth of WO3 films by this

ethod with regard to growth on SrTiO3 substrates. The effect ofarameters such as substrate temperature, oxygen pressure and

aser energy fluence on the physical properties of the films werenvestigated using different techniques such as XRD, XPS and AFM.

e find the optimal conditions for the deposition of smooth films.

. Materials and method

The WO3 thin films were grown on (001)-oriented 10 × 10 mm2

rTiO3 substrates supplied by Crystal GmBH. High-purity WO3owders (Sigma-Aldrich, >99.99%) were pressed (150 bar) and sin-ered (900 ◦C) for 8 h to form a target of diameter 20 mm. In ordero deposit the thin films, the PLD chamber was first evacuatedo a base vacuum of 10−6 mbar, and then pure O2 gas (99.99%)as introduced keeping the oxygen pressure constant at 60 �bar

uring the deposition. A KrF excimer laser with a pulse durationf 23 ns and a wavelength of 248 nm was used. The repetitionate was 3 Hz with a laser fluence of about 1.2 J/cm2. Thin filmsere deposited at temperatures of 300 ◦C, 400 ◦C, 500 ◦C, 600 ◦C,

00 ◦C and 800 ◦C. The second batch of samples was fabricated witharying deposition pressures while maintaining a substrate tem-erature of 600 ◦C. Three films were prepared at oxygen pressuresf 10, 20 and 60 �bar in order to see the effect of oxygen pressure.amples were also prepared under a deposition pressure of 60 �barnd a substrate temperature of 600 ◦C with a reduced laser fluen-ies of 0.6 and 0.9 J/cm2. For each deposition, 3000 laser shots wereelivered to the target. A reflection high-energy electron diffrac-ion (RHEED) gun (23 keV accelerating voltage) was used duringnd after film growth to produce surface diffraction patterns. X-rayhotoelectron spectroscopy (XPS) measurements were carried out

n ultra-high vacuum, using Al K� X-rays from a twin anode sourcend a VG Scientific CLAM2 energy analyser. The topography of theurface was examined using contact mode atomic force microscopyAFM). The X-ray diffraction (XRD) analysis was carried out using

Panalytical X’Pert Pro Cu K� radiation source (�Cu = 0.15405 nm)n the �–2� geometry. Thicknesses of the films were determined bymall angle X-ray reflectivity.

. Results and discussion

.1. Effect of temperature

For the WO3 films grown on STO at substrate temperatures rang-ng from 300 to 800 ◦C, RHEED patterns were recorded for cleanubstrates and only intermittently during and after the film growth.he RHEED patterns for a clean STO (001) along the [100] direction

s shown in Fig. 1a. The RHEED beam was directed along the [100]

irection throughout the deposition process. After about 10 minf WO3 deposition, the RHEED patterns were found to be sharpnd streaky. Patterns corresponding to films prepared at tempera-ures of 400 ◦C and 600 ◦C along their [100] azimuthal direction are

= 88.81 , � = 90.985 , � = 90.985 230–290 K [8], � = 91.76◦ <230 K [12]

– [13]

shown in Fig. 1b and c. The streaks in both patterns consist of someregular spots indicating island growth, but either the terraces of theislands are large enough to give rise to the coherent surface diffrac-tion, especially at 600 ◦C (which gives rise smaller RHEED spots), orthe islands have merged into continuous films. The RHEED patternalong the [100] direction of the film is aligned with that of the sub-strate, indicating the epitaxial relationship ((001) WO3 || (001) STOand [100] WO3 || [100] STO) and a fairly smooth film surface.

The surface morphology of thin films studied by AFM is shownin Fig. 2a–f. There is evidence of granular nanoscale particles withdifferent sizes which increase with the substrate temperature. TheRMS roughness calculated from a 1 × 1 �m2 area shown in Fig. 3ais less than 2 nm for all films deposited at temperatures less than700 ◦C which are ∼100 nm thick, but it increases to 21 nm for thetemperature of 800 ◦C. Another function that gives important infor-mation about the symmetry of height distribution in the filmsis skewness. Skewness is the third moment of profile amplitudedefined as follows:

Rsk = 1NRRMS

N∑

i=1

[zi − z]3 (1)

Here RRMS is the roughness, N is the number of data points andzi and z are the height of each point and the average height of thesurface respectively. Skewness is a measure of the lack of symme-try at the surface. The measured skewness for films with an areaof 1 �m2 shown in Fig. 3a reveals that the skewness decreases onincreasing the temperature up to 700 ◦C, crossing the zero between500 ◦C and 600 ◦C. The positive values of skewness for films grownat temperatures of 300–500 ◦C and negative values for the filmsof 600 ◦C and 700 ◦C show that the surfaces comprise spikes anddents, respectively. The abundance of holes in samples grown at600 ◦C and 700 ◦C indicates that the islands are large enough togive rise to a surface with some valleys between them which cor-relates with the RHEED pattern obtained at 600 ◦C. With a largeroughness and grain size, the morphology at 800 ◦C is completelygranular with large-size particles. This is probably due to the localmelting of species on the structure, which negates the substrateeffects, leading to the island growth on the STO surface.

XRD patterns of thin films are shown on a logarithmic intensityscale in Fig. 3b. The �–2� scan of the STO blank substrate shows the(001) plane reflection at 2� = 22.80◦. In the film of 300 ◦C just thispeak appears and no more pattern relative to the crystallizationof WO3 is observed. It is clear that the STO (001) peak appears tobecome broader in WO3 films deposited at higher substrate tem-peratures, of up to 500 ◦C. The widening of the (001) peak is theresult of progress in the crystallization of the films at higher tem-peratures. Nucleation of crystalline phases at lower temperaturesis likely driven by tensile strain due to lattice mismatch with theSTO substrate. For films grown at 600 ◦C and 700 ◦C three distinctpeaks appear at 23.1◦, 23.7◦ and 24.2◦, which are signatures of themonoclinic WO3 phase (JCPS Card No. 075-2072) with a = 0.727 nm,

b = 0.750 nm, c = 0.382 nm and = 89.93◦, and are assigned to (001),(020) and (200) planes respectively, indicating that the film is grow-ing in a polycrystalline mode. This structure is similar to that seenpreviously in the growth of WO3 on STO [9] where the monoclinic

H. Kalhori et al. / Applied Surface Science 390 (2016) 43–49 45

Fig. 1. RHEED patterns taken along the [100] azimuthal direction for the following: (a) SrTiO3 (001), (b) WO3 deposited with a substrate temperature of 400 ◦C (c) and 600 ◦C.

F bar ana

pTcsI2twiopap

dofitaft

3

ttt

ig. 2. AFM images of the films corresponding to constant oxygen pressure of 60 �nd (f) 800 ◦C.

hase was found to nucleate at a growth temperature of 500 ◦C.he film grown at 600 ◦C is single-phase monoclinic including therystallites grown in three different directions. Fig. 3c shows thechematic of these three directions on the surface of STO (001).n the film grown at 700 ◦C, a weak broad peak also appears at9.9◦ in addition to these three peaks. This peak is attributed tohe (810) plane of tetragonal W5O14 (JCPS Card No. 041-0745)ith a = 2.333 nm and c = 0.379 nm. The equilibrium phase of WO3

s tetragonal in the range of 700–900 ◦C [9,33], which is why webserve this phase at higher temperatures. The film at 800 ◦C dis-lays completely the patterns of tetragonal W5O14 structure. Thus,

phase transition from monoclinic to tetragonal occurs at this tem-erature range for the WO3 films grown by PLD.

Fig. 3d shows the X-ray reflectivity (XRR) curves of the films atifferent temperatures. The best fitting curves were found usingptimised values of the thickness, density and roughness of eachlm, which are tabulated in Table 2. The density and thickness of

he deposited films are about 6 g/cm3 and 100 nm respectively forll the films. The roughness of the films determined by fitting wasound to increase with temperature, which is in agreement withhe AFM results.

.2. Effect of oxygen pressure

The chemical composition of the films was highly dependent onhe oxygen partial pressure in the chamber. For example, at 10 �bar,he surface was dark and reflective, indicating the metallic nature ofhe surface. The vacancies in films with different oxygen pressures

d deposition temperatures of (a) 300 ◦C, (b) 400 ◦C, (c) 500 ◦C, (d) 600 ◦C, (e) 700 ◦C

were characterised by XPS analysis, which is shown in Fig. 4a–c.The XPS data for films grown under an oxygen partial pressure of60 �bar (Fig. 4a) can be deconvoluted with a doublet consisting ofW4f7/2 and W4f5/2 peaks located at 35.6 eV and 37.7 eV respectively.According to the literature, these peaks are attributed to the fully-oxidised tungsten ions (W6+) [34,35], with less than 1% W5+ whichis a negligible amount in our measurements. The stoichiometry ofthe films calculated from the weight ratio of oxygen to tungstenwas determined to be WO3.0. By decreasing the oxygen pressureto 20 �bar, a shoulder is observed at lower binding energies. As isshown in Fig. 4b, the W4f spectrum is fitted with three doubletsof 35.6 and 37.7 eV related to W4f7/2 and W4f5/2 of W6+, 34.3 and36.4 eV related to W5+ and 33.3 and 35.4 related to W4+. The chem-ical composition of the film was determined to be WO2.75 whichshows a clear change in structure due to the lowering of the oxy-gen partial pressure. In the film prepared with the lowest partialoxygen pressure 10 �bar (Fig. 4c), the W4f peak is fitted with fourdoublets corresponding to the different tungsten ions of W6+, W5+

and W4+ and also W0 (peaks located at 30.8 and 32.9 for W4f7/2 andW4f5/2 respectively). The composition of the film calculated fromthe XPS result is determined to be WO1.91.

The morphology of the films is a function of the pressure of theoxygen in the chamber. Fig. 4d–f shows the AFM micrographs offilms grown under different oxygen pressures at a constant sub-

strate temperature of 600 ◦C. Some typical line scans of the surfaceare shown in the inset of each figure. The 10 �bar film had a metallicappearance and a smooth surface with no trace of granular par-ticles. Increasing the oxygen pressure leads to the formation of

46 H. Kalhori et al. / Applied Surface Science 390 (2016) 43–49

Fig. 3. (a) The calculated RMS roughness and skewness from the AFM images, (b) XRD �–2� scans of STO (001) and WO3 films on STO (001) with different substratetemperatures, (c) schematic of the growth of different WO3 unit cells and their relative planes on the surface of (001) STO, and (d) X-ray reflectivity of WO3 films on SrTiO3

(001) with different substrate temperatures.

Table 2Refined parameters for different films determined from x-ray reflectivity results.

Temp (◦C) Press (�bar) Energy ( J cm−2) Thickness (nm) Roughness (nm) Density (gr cm−3)

300 60 1.2 105.5 6.1 5.9400 60 1.2 95.5 7.2 6500 60 1.2 103.5 6.9 6.25600 60 1.2 115 9.9 6.05700 60 1.2 90 18.8 7.15800 60 1.2 74 32 6.55

99

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600 20 1.2

600 10 1.2

600 60 0.6

ranular particles with approximate sizes of 100 nm and 200 nmorresponding to pressures of 20 and 60 �bar, respectively. Linecans again indicate that the roughness is maximum in the high-st oxygen pressure. This result is comparable with the observedncrease in roughness of TiO2 films with the increase of oxygenartial pressure from 5–mTorr to 90–mTorr [36].

Fig. 5 shows the XRD and RHEED patterns of the films grownnder different oxygen pressures. The film grown at 10 �bar showso peaks in the range of 23–25◦ that are characteristic of mono-linic or tetragonal WO3, but instead, it has a small peak at 36.0◦.his corresponds to the (261) plane of the orthorhombic phase ofO2.625 (Card No. 01-081-1172), which clearly reveals the oxygen

acancy described by XPS results for this film. The RHEED patternas not clearly visible. The out-of-plane �–2� scan for the 20 �barlm contains a single peak at 2� = 23.23◦ along with the sharperubstrate peaks. Since the composition of the film determined byPS revealed an oxygen vacancy for this film, the XRD pattern is best

atched with monoclinic WO2.92 (Card No. 00-030-1387) with the

attice parameters of a = 1.193 nm, b = 0.382 nm, c = 5.972 nm and = 98.3◦ and the mentioned peak corresponds to (010) planes. This

9.01 6.25 7.1 6.75

8.7 5.92

observation strongly suggests (010) plane epitaxy between WO3films and (001) STO. This result is like the structure of the filmsgrown on quartz substrates by Lethy et al. [29] in similar oxygenpressures as both of them are monoclinic single phase. Accordingto these results, the lattice mismatch between (010) plane of thefilm and (001) plane of the substrate is determined to be 2.9%. Theincrease in the crystallinity of the film with the oxygen pressureis confirmed by the RHEED pattern of this film by the appear-ance of spotty streaks. In the film grown under an oxygen pressureof 60 �bar three peaks appear at 23.2◦, 23.6◦ and 24.2◦. As dis-cussed previously, the peaks can be attributed to a polycrystallinemonoclinic WO3 phase. Increasing the oxygen pressure leads toan increase in the deposition rate and thus the monoclinic phasecannot grow as a single-crystalline film. XRR results for differentoxygen pressures with the refined parameters are shown in Table 2.The calculated densities show that the film grown under a low pres-sure of 10 �bar, has the highest density, but its thickness is much

smaller than those grown at higher pressures. The RMS rough-ness increases with oxygen pressure during the synthesis. These

H. Kalhori et al. / Applied Surface Science 390 (2016) 43–49 47

Fig. 4. XPS of the films prepared with the oxygen pressure of (a) 60 �bar, (b) 20 �bar, and20 �bar, and (f) 10 �bar, line scan profiles determined from the AFM images correspondin10 �bar.

Fig. 5. XRD patterns of the films prepared with different oxygen pressures, the insetshows both the XRD of a selected area and RHEED patterns of these films.

(c) 10 �bar, AFM of the films prepared with the oxygen pressure of (d) 60 �bar, (e)g to the films prepared with the oxygen pressure of (g) 60 �bar, (h) 20 �bar and (i)

results are in good agreement with the AFM results showing thatthe 10 �bar film was smoother than the other films.

3.3. Effect of laser fluence

The fluence of the pulsed laser beam has an important effect onthe morphology and the structure of films deposited by PLD [37].Up to now, all the films discussed were deposited with a laser flu-ence of 1.2 J cm−2. Two further films were deposited with fluenciesof 0.6 and 0.9 J cm−2 at a temperature of 600 ◦C under an oxygenpressure of 60 �bar in order to have an idea of the effect of the flu-ence on the flim structure. AFM results of different films shown inFig. 6a–c reveals that decrease in the energy fluency from 1.2 J cm−2

to 0.9 J cm−2 lead to a drop in the roughness and the grain sizesof the film. No significant change is observed in the morphologyby further decreasing the energy fluence to 0.6 J cm−2. It is appar-ent that the lower energy flims are much smoother than the filmgrowth at 1.2 J cm−2, but the island growth mode is found in allcases. A change in the growth of WO3 films from Volmer-Webermode to a different mode is difficult to achieve [23,38]. Fig. 6d com-pares the XRD of the films deposited with different energy fluencies.Three peaks appear in the range of 23–25◦ with different inten-sities, indicating polycrystalline monoclinic WO3. It can be saidthat the peaks corresponding to the (100) plane decrease and the

peaks corresponding to the (002) and (020) planes increase whenthe energy fluency of laser decreases from 1.2 J cm−2 to 0.6 J cm−2.These results show a dependence of the orientation of the planeson the fluence of the laser beam.

48 H. Kalhori et al. / Applied Surface Science 390 (2016) 43–49

F 0.9 J cb ned byt �bar

oopgntsotws

cdbaTwsopeWmastfi

4

dApr

ig. 6. AFM of the films prepared with the oxygen pressure of (a) 1.2 J cm−2 �bar, (b)y keeping the other parameters constant and (e) electron beam intensity determihe film deposited with the substrate temperature of 600 ◦C, oxygen pressure of 60

Fig. 6e shows the RHEED diffracted beam intensity as a functionf time during the growth of WO3 film with the energy fluencef 0.6 J cm−2. During the initial phases of deposition, the RHEEDattern consists of bright spots, which indicates the Volmer-Weberrowth mode. In other words, at first WO3 particles agglomerate toucleate and form islands on the STO substrate surface. After 2 min,he patterns become streakier, and the spotted patterns disappearlowly. The intensity of reflected electron beam is fixed after 3 minf the starting the deposition. The RHEED patterns correspondingo the fixed spot intensities (after 3 min), were completely streakyithout any spots in the patterns, indicating the production of a

mooth WO3 surface.So far, the results show that at higher oxygen pressures, poly-

rystalline phases of WO3 such as monoclinic and tetragonal areeposited on the STO substrates. It seems that the misfit strainetween the film and substrate increases with oxygen pressure,nd thus it doesn’t allow WO3 films to grow epitaxially on the STO.hese results are clearly seen in the report of Moulik et al. [31]here polycrystalline WO3 structures are deposited on STO sub-

trates using similar deposition conditions like temperature andxygen pressure. On the other hand, as was shown, lower oxygenressures lead to oxygen vacancies in the WO3 structure. How-ver, we have shown that it is possible to grow highly-oriented

O3 films with the Volmer-Weber growth mode on STO by the PLDethod. The optimal conditions to obtain the most oriented films

re a substrate temperature of 600 ◦C and an oxygen partial pres-ure of 60 �bar. Lower energy fluencies of the laser also improvehe film orientation, due to the lower rate of the growth of WO3lms.

. Conclusions

WO3 films with thickness between 60 and 100 nm were

eposited on (001) SrTiO3 substrates using pulsed laser deposition.

phase transition was observed on increasing the substrate tem-erature above 700 ◦C from a monoclinic to a tetragonal phase. Theoughness of the films increases with substrate temperature and

[

[

m−2, and (c) 0.6 J cm−2. (d) XRD of the films prepared with different energy fluencies RHEED system as well as RHEED patterns in different stages of the deposition for

and the energy fluency of 0.6 J cm−2.

laser fluence. We found that only the films grown under an oxygenpressure of 20 �bar were epitaxial with an orientation of [010] per-pendicular to the (001) plane of the substrate. The interfacial effectsare key in achieving epitaxial growth under the aforementionedconditions, with the exception of films grown under low pressureswhere oxygen vacancies are unavoidable. The optimal conditionsto achieve highly oriented WO3 films are a substrate temperature of600 ◦C, an oxygen partial pressure of 60 �bar and energy fluenciesof the laser beam as low as 0.6 J cm−2.

Acknowledgement

This work was partly supported by Science Foundation Irelandfrom Grant 13/ERC/I2561.

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