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Applied Surface Science 276 (2013) 229– 235

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

j ourna l ho me page: www.elsev ier .com/ locate /apsusc

tructural and optical properties of WO3 sputtered thin filmsanostructured by laser interference lithography

. Castro-Hurtadoa,∗, T. Taveraa, P. Yurritaa, N. Péreza, A. Rodriguezb,.G. Mandayoa, E. Castanoa

CEIT and Tecnun (University of Navarra), Manuel de Lardizábal 15, 20018 San Sebastián, SpainCIC microGUNE Goiru kalea 9, Polo de Innovación Garaia, 20500 Arrasate-Mondragón, Spain

a r t i c l e i n f o

rticle history:eceived 11 October 2012eceived in revised form 8 February 2013ccepted 4 March 2013vailable online 19 March 2013

eywords:

a b s t r a c t

A study of the influence of annealing temperature on the structural, morphological and optical propertiesof WO3 thin films is presented. The coatings are deposited by RF reactive magnetron sputtering and char-acterized by XRD analysis and FESEM. The XRD diagrams of the samples show a phase transition fromtetragonal to monoclinic when the annealing temperature is raised from 800 to 900 ◦C. Moreover, theincrease of the annealing temperature to 800 ◦C favors the presence of a granular structure on the surfaceof the film. A decrease in the optical energy band gap (3.65–3.5 eV and 3.5–3.05 eV for direct and indi-

O3 thin filmsanostructureaser interference lithographyptical band gap

rect transitions respectively) with annealing temperature has been measured employing Tauc’s relation.Furthermore, WO3 thin films are processed by laser interference lithography (LIL) and periodic nano-structures are obtained. The processed films are characterized by a hexagonal symmetry with a periodof 340 nm and the diameter of the nanostructured holes of 150 nm. These films show improved morpho-logical properties of interest in several applications (gas sensors, photonic crystals, etc.) independent ofthe annealing temperature.

. Introduction

Tungsten oxide (WO3) has been studied in great detail due to itsutstanding optical and electrical properties. Since the discoveryf the electrochromic (EC) effect in 1960, WO3 is considered one ofhe key materials for its application in EC devices [1–3]. WO3 suffers

crystallographic phase transition at certain temperatures whichs the origin of the thermochromism effect shown in this material.ence, it is transparent to infrared rays below a certain transition

emperature and reflects them for temperatures above it [4].In addition, WO3 is a well-established n-type semiconductor

mployed as a sensing layer for the detection of hazardous gasesuch as H2S, CO, HCHO and NOx [5–9]. As a consequence, consider-ble research on the properties of WO3 has been carried out varyinghe preparation methods [10–14]. Combining EC and gas sensingroperties, this material can be employed in gasochromic basedensing devices [15], which measure the change of the refractivendex of WO3 when it is exposed to a certain gas. Also, it has recently

een reported that combined with catalytic metals such as Pd, Pt oru, there is a color transition from transparent to dark blue whenO3 is exposed to H2 [16].

∗ Corresponding author. Tel.: +34 943 212 800; fax: +34 943 213 076.E-mail address: ichurtado@ceit.es (I. Castro-Hurtado).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.03.072

© 2013 Elsevier B.V. All rights reserved.

Regarding gas sensing applications, it is thought that the reac-tions that give rise to gas detection are mainly produced on thesurface of the material, so nanostructured thin films are moreattractive than their polycrystalline counterparts as the secondhave less porous and smoother surfaces [17,18]. In this work, LaserInterference Lithography (LIL) is employed to obtain nanostructu-red films. This technique deals with the use of interference patternsgenerated from two or more coherent beams of laser radiation forthe structuring of materials [19]. The resulting interference patternis a series of maximum and minimum intensity peaks arrangedas arrays of lines or dots. When using this radiation to interactwith materials, those areas exposed to intensities higher than theablation threshold of the material are removed, while the othersremain unaffected. This way, feature sizes down to a fraction of thelaser wavelength can be created. Two of the most important advan-tages of pulsed LIL are the non-contacting projection mode with alarge working distance and the extremely high efficiency which itprovides.

2. Experimental

WO3 thin films are deposited on alumina substrates by RF reac-tive magnetron sputtering with a metal oxide target of 99.999%purity. The sputtering is performed under a gas pressure of5 × 10−3 mbar and the RF power is maintained at 300 W in a mixed

2 Surface Science 276 (2013) 229– 235

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tmosphere of oxygen (50%) and argon (50%). The thickness mea-ured with a KLA Tencor Profilometer for a deposition time of 1 hs 180 nm. The samples are then annealed in synthetic air for 4 hn order to stabilize their microstructure. Four different annealingreatments are performed varying the annealing temperature from00 to 900 ◦C with the aim of studying the influence of this param-ter on the structural and optical properties of WO3 thin films.

Moreover, WO3 thin films are processed by laser interferenceithography (LIL) in order to obtain periodic nanostructures. TheIL tool employed in this work is described in [19] and it uses aV laser source with a wavelength of 355 nm and pulse durationf 8 ns. The fluences used for the patterning are between 30 and0 mJ and two different configurations are tested. The first one is a-beam configuration with a theoretical resulting period of 335 nmnd the second one is a 4-beam configuration with a theoreticalesulting period of 440 nm. In all cases, a single shot is used for therocessing.

The crystalline structure of the films is characterized by X-rayiffraction (XRD) by means of a Philips XPERT MRD diffractome-er (Cu Ka1� = 1.54059 A). Furthermore, a field emission scanninglectron microscope (FESEM) JEOL model JSM-7000F is used fornalyzing their morphology and microstructure.

The reflectance and transmittance spectra of the sampleseposited onto UV-grade fused silica substrates are recorded in theavelength range 300–1000 nm using a Xenon arc lamp Oriel 6238

nd a CCD Camera Andor attached to a spectrograph Oriel MS257.hen, the optical band gap of the samples is calculated using theauc relation [20]:

h� = B(h� − Eg)m (1)

here B is constant, h� (eV) is the photon energy, Eg (eV) is theptical energy band gap and is the absorption coefficient. Thisast parameter can be obtained from [21]:

(�) = −1d

ln

(T(�)

[1 − R(�)]2

)(2)

here T is the transmittance of the samples, R the reflectance and is the thickness of the films.

The exponent m depends on the nature of the electronic transi-ions induced by the incident photons, corresponding to m = 1/2 and

= 3/2 for allowed and forbidden direct transitions respectively. Ifhe conduction band minimum and the valence band maximumccur at different points in k-space (different wave vector k), thenor crystal momentum to be conserved a phonon must participaten the process. This phonon will supply crystal momentum andnergy (Ep), which is a few hundreds electron volts [22]. Thus, theeasured energy band gap at the optical threshold in the case of

ndirect transitions will be Eg ind = Eg ± Ep while in direct transitionsg dir = Eg. Indirect transitions are represented in the Tauc relationy the value of the exponent m = 2 for allowed transitions and 3or forbidden transitions. In the case of WO3 allowed transitionsre considered [23,24], so the values ½ and 2 will be used for thealculation of the direct and indirect energy band gap respectively.

The following expression is employed for the calculation of thendex of refraction [25,26]:

(�) =(

1 + R(�)1 − R(�)

)−

√4R(�)

(1 − R(�))2− k2 (3)

here k = ˛�/4� is the extinction coefficient and � is the wave-ength of the incident photon.

. Results and discussion

In the first part of this work the experimental results concern-ng the study of the influence of annealing temperature on the

Fig. 1. XRD patterns of WO3 thin films annealed at different temperatures: 600, 700,800 and 900 ◦C.

properties of WO3 thin films are presented. The LIL nanostructuringprocess of the samples is presented in Section 4, together with ananalysis of the effect of this process on the previously mentionedcharacteristics.

3.1. Non-structured WO3 thin films

3.1.1. Crystalline structureTemperature plays a fundamental role in the crystallization of

WO3. As reported in Ref. [4], several allotropic modifications exist,such as monoclinic, orthorhombic and tetragonal. The idealizedstructure of stoichiometric tungsten trioxide is a ReO3-type, wherethe metal atoms are placed in the corners of a cubic structure andsurrounded by oxygen atoms, which form a corner-sharing octahe-dron (atomic positions Re (0, 0, 0) and O (1/2, 0, 0)). In the case WO3,the small W metal atoms tend to be displaced from their ideal cen-ter positions by the influence of temperature and small amounts ofimpurities, causing an increase in the energy gap [1]. Table 1 showsthe crystalline behavior of samples deposited by different methodssuch as screen printing [27], sputtering [28] and vacuum evapo-ration [29]. Moreover, the influence of substrate temperature andannealing temperature is also reported. As it is shown, the increaseof the substrate temperature during deposition is on the side offorming a more symmetric crystal structure.

The XRD patterns obtained for the films prepared in this studyare presented in Fig. 1. In accordance with Akl et al. [11], WO3 filmssputtered on unheated alumina substrates show an amorphousstructure, as no diffraction peaks are found. The peaks indicatedwith arrows in the characteristics of the samples annealed at tem-peratures ranging from 600 to 800 ◦C correspond to the tetragonalphase (ICDD No. 01-089-4481), while the remaining peaks belongto Al2O3. This phase is described by the spatial group P4/nmm (cellparameters a = 5.276 A, c = 7.846 A). However, as observed in Fig. 1,there is a transition from tetragonal to monoclinic phase (ICDD No.01-072-0677) when the samples are annealed at 900 ◦C. This tran-sition related to a temperature increase has also been reported byother authors such as Solis et al. [27] and Hoel et al. [29]. A clearevidence of this phase transition is presented in the detail of thediffractogram in Fig. 1 obtained in the range of 21–24◦. The XRDpatterns of the samples annealed at lower temperatures show onlytwo peaks with a preferential (1 1 0) orientation whereas the sam-

◦

ples annealed at 900 C show four peaks corresponding to the (1 1 1)(0 0 2) (0 2 0) and (2 0 0) directions and a preferential orientation inthe (0 0 2) direction. The average grain size calculated using theScherrer formula for the most intense peaks is 30–34 nm for the

I. Castro-Hurtado et al. / Applied Surface Science 276 (2013) 229– 235 231

Table 1Crystalline structure depending on preparation technique and conditions.

Deposition technique Tsubstrate Annealing temperature Crystalline structure Grain size References

Evaporation/condensation chamber Tevaporation = 1100 ◦C 200 < T < 600 ◦C Tetragonal 6 nm [27]Screen printing – 300 < T < 800 ◦C Tetragonal and monoclinic (P21/n) 40 nmDC sputtering (W target) Room temperature – Amorphous – [28]RF sputtering (W target) Room temperature – Amorphous – [31]

500 ◦C – Orthorhombic –

RF sputtering (W target) floating regime Room temperature 400 ◦C Monoclinic (Pc) 11 nm [7]RF sputtering (W target) 260 ◦C – Tetragonal 100–200 nm [10]Evaporation/condensation chamber Tevaporation = 1100 ◦C T > 500 ◦C Transition from tetragonal to

monoclinic (P21/n)23 nm [29]

RF sputtering (W target) 200 ◦C – Amorphous – [9]300–600 ◦C Monoclinic (P21/n) –

RF sputtering (W target) Room temperature – Amorphous – [13]200 < Ts< 300 ◦C – Orthorhombic 11Ts > 300 ◦C – Orthorhombic and monoclinic 13

RF sputtering (W target) 300 ◦C – Monoclinic (P21/n) ∼ 50 nm [39]RF sputtering (WO3 target) Room temperature – Amorphous – [12]

350 ◦C Hexagonal & Monoclinic (Pc) -

RF sputtering (W target) 300 ◦C 450 ◦C Orthorhombic – [40]Hydrothermal synthesis – – Hexagonal and monoclinic – [6]

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omparison of the crystalline structure of samples obtained by different methods.

onoclinic phase and 10–20 nm for the samples crystallized in aetragonal phase.

The response of WO3 based gas sensors is influenced by the crys-alline structure of the films. As an example, in Ref. [27] it is reportedhat the films crystallized in the tetragonal phase show sensitivityoward H2S at room temperature. Moreover, in Ref. [7] a compari-on of the gas sensing properties of hexagonal and monoclinic WO3as been performed.

.1.2. Morphological propertiesThe effect of the annealing temperature on the morphology

f the films is shown in Fig. 2. There is an observable difference

Fig. 2. FESEM micrographs of samples an

between samples annealed at 600 and 700 ◦C and those annealedat higher temperatures. Samples annealed at lower temperatureshave a granular structure but covered by a very smooth fine layer onthe top which does not appear in the samples annealed at 800 ◦C. Onthe other hand, samples annealed at 900 ◦C have a special morphol-ogy (see Fig. 2d) as a non-uniform but continuous film is obtained.These films present a partial dewetting which could lead to the for-mation of droplets if the temperature was increased. The difference

in morphology is explained taking into account the thermal prop-erties of bulk WO3. The melting temperature (which diminishesat the nanoscale) is at 1470 ◦C but its sublimation begins at 800 ◦C[4,30].

nealed at 600, 700, 800 and 900 ◦C.

232 I. Castro-Hurtado et al. / Applied Surface Science 276 (2013) 229– 235

Fig. 3. Transmittance spectra of 180 nm-thick WO3 films as deposited and annealeda

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Fig. 4. (a) Dependence of (˛h�)2 on the incident photon energy (h�) for the WO3

diminishment of the effective refractive index of the samples.Optical characterization serves as a qualitative method to

determine the stoichiometry of the samples, which has influenceon their electro and gasochromic performance. In Ref. [37], the

t 600, 700 and 800 ◦C.

.1.3. Optical propertiesFig. 3 shows the transmittance spectra of WO3 samples as

eposited and annealed at 600, 700 and 800 ◦C. The transmittanceecreases sharply for the wavelengths comprised between 400 and50 nm due to a fundamental absorption edge as reported in [13].

The sputtering process is performed under a partial pressure of0% of O2 and semi-transparent films are obtained in all cases. It is inccordance to [31] where a color change from blue to transparent iseported when the oxygen flow ratio is changed from 20 to 30%. Thisoloration effect is related to the oxidation of substoichiometric

O3-x films.The optical energy band gap is determined employing the Tauc

elation (Eq. (1)), extrapolating to = 0 the straight line in the linearegion from the (˛h�)−m vs h� plot [32]. Though many literatureeports indicate only the indirect band gap of WO3 [33], there arelso theoretical predictions for the existence of a direct band gap34]. In fact, a satisfactory fit is obtained for both direct (m = 1/2)nd indirect (m = 2) allowed transitions as shown in Fig. 4a and b,espectively.

Fig. 5 shows that the mean energy band gap decreases from.65 to 3.5 eV for direct transitions (Eg dir, m = 1/2) and from 3.5 to.05 eV for indirect transitions (Eg indir, m = 2) when as-depositedamples are compared to the samples annealed at 800 ◦C. Therror bars represent the sample-to-sample variations, calculatedrom 4 different samples. A decrease in the energy band gap from.37 to 3.05 eV is also reported in Ref. [13] for samples depositedt room temperature and at a substrate temperature of 500 ◦Cespectively. Moreover, in Ref. [35] a monotonic drop of Eg withncreasing annealing temperature is attributed to crystallizationffects because a change in the structure of the crystals from a fine-rained hexagonal to large-grained monoclinic is produced. In Ref.36] a decrease in Eg from 3.42 to 3.24 eV is observed when the sub-trate temperature is changed from room temperature to 350 ◦C. Its known that in WO3−x films containing oxygen vacancies, theres a structural distortion in the WO6 octahedra which leads to anncrease of W W distance and changes in W O splitting [1]. Theand gap is sensitive to W O bond length and as a consequenceigher optical band gap are measured in amorphous films. Hence,g increases with the presence of oxygen vacancies. In addition,he grain size of the samples is increased with substrate tempera-ure, which causes a narrowing of the band gap. In Ref. [36], filmseposited at a substrate temperature of 250 ◦C have grain sizes of0 nm and exhibit an Eg of 3.39 eV whereas in the samples depositedt 350 ◦C and grain sizes of 165 nm energy of 3.24 eV has been cal-

ulated. In this case, the major drop on the optical energy bandap is obtained for the samples annealed at 800 ◦C, which presentifferent morphology and crystallization clearly evident in Fig. 2.

180 nm-thick samples as deposited and annealed at 600, 700 and 800 ◦C. (b) Depend-ence of (˛h�)1/2 on the incident photon energy (h�) for 180 nm-thick WO3 samplesas deposited and annealed at 600, 700 and 800 ◦C.

The spectral distribution of the refractive index n (Eq. (3))measured with � in the range of 350–750 nm shows a minimumnear 460 nm in annealed samples and at 500 nm in as-depositedfilms. The refractive index (at � = 550 nm) increases from 1.77 inas-deposited samples to 2.76 for samples annealed at 700 ◦C butdiminishes to 2.35 for samples annealed at 800 ◦C. These resultsare in accordance with the values reported in Ref. [36], where n (at550 nm) increases from 1.92 for samples deposited at room temper-ature to 1.99 for samples deposited at a substrate temperature of350 ◦C. The increase in n is associated to a higher density of the filmsas annealing temperature is raised. However, samples annealed at800 ◦C have a higher porosity as shown in Fig. 2 which results in a

Fig. 5. Energy band gap of WO3 180 nm-thick samples as deposited and annealedat 600, 700 and 800 ◦C.

I. Castro-Hurtado et al. / Applied Surface Science 276 (2013) 229– 235 233

F a) Simo am coa

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which appear about 600 nm. These peaks suggest the existenceof weak photonic band gaps after the processing. These bandgaps appear in increasing wavelengths with increasing annealing

ig. 6. Simulated patterns and FESEM micrographs of LIL processed WO3 samples: (f 35 mJ and (b) processed sample annealed at 600 ◦C. (c) Simulation of process: 4-bennealed at 900 ◦C.

nfluence of annealing treatment on the stoichiometry and ECroperties of thermally evaporated WO3−x nanowires is studied.amples annealed at 500 ◦C (O/W atomic ratio of 2.86) showeeper color and transmittance difference from bleached toolored state than as-deposited ones (O/W ratio of 2.6). Thisifference is attributed to the presence of more W6+ active species

n more stoichiometric films. Regarding gasochromic applications,amamoto et al. [15] observed that the coloring rate and degree ofoloration of sputtered WO3 films under H2 exposure is affectedy the annealing process. In their work gasochromic coloration

s determined by the change in transmittance of the films at = 645 nm due to exposure to 1% of H2 in argon gas. A reduction onhe gasochormic performance is measured comparing sputteredlms deposited at a substrate temperature of 200 ◦C, and thoseost-annealed at 500 ◦C. In their case, an O/W ratio of 3 is measuredor all samples, but as-deposited films show an amorphous struc-ure while samples annealed at 500 ◦C crystallize in a monoclinichase. Thus, electro and gasochromic properties of the films can beontrolled with the annealing treatment which provides oxidationnd crystallization of the samples. In this work, samples withigh stoichiometry are achieved in all cases, as shown in the highransmittance spectra of the samples. Moreover, crystallizationncreases with annealing temperature as shown in Fig. 1. As aonsequence, it can be deduced that the films annealed at 600 and00 ◦C could be employed in EC devices, as they would show betterlectro and gasochromic properties than the other samples.

.2. LIL processed WO3 thin films

WO3 thin films have been characterized by FESEM after the

anostructuring process. Fig. 6b shows that during the LIL processhe top layer is removed and that the ablation of the material isroduced in the same form as the simulated pattern (Fig. 6a). Thisample, annealed at 600 ◦C, shows a hexagonal symmetry with a

ulation of the process: 3-beam configuration, angle of incidence of 45◦ and energynfiguration, angle of incidence of 35◦ and energy of 35 mJ and (d) processed sample

period of 340 nm and the diameter of the nanostructured holesis 150 nm. The obtained structure shown in Fig. 6d for a sampleannealed at 900 ◦C has a square symmetry and a higher period,440 nm, and the hole diameter is 160 nm.

The ablation process does not affect the crystalline structureof the samples as observed in the diffraction pattern of Fig. 7. Thetetragonal phase of samples annealed at 600 ◦C is maintained beforeand after being processed with LIL.

As for the optical properties, Eg calculated for direct and indi-rect allowed transitions of the samples does not vary after theprocessing. As can be seen in Fig. 8, the reflectance of the sam-ples does not change significantly, except for the reflectance peaks

Fig. 7. XRD patterns of samples annealed at 600 ◦C: as grown and processed by LIL.

234 I. Castro-Hurtado et al. / Applied Surfa

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ig. 8. Reflectance characteristics of the samples before (a) and after (b) LIL nanos-ructuring.

emperature, which suggests a slight difference in the periodicalattern for the different temperatures.

This section shows that WO3 nanostructured films can bebtained with LIL providing better morphological properties foroth gas sensing and optical applications. Thus, replacing the con-inuous film with the samples processed with LIL in gas sensingpplications will not only provide higher diffusion of the gas butlso a greater volume to surface ratio. Hence, the sensitivity isxpected to improve as reported in [38], where a comparison ofhe response of SnO2 wires obtained by ebeam and continuousnO2 film is carried out. Moreover, in [14] it is shown that theore volume is the dominating factor for the response of WO3 thinlms to NO2. On the other hand, LIL nanostructured films could bemployed in photonic crystals. As it is explained in Ref. [1], tunableC photonic crystals can be obtained by intercalating WO3 withi+ ions. In the example a hexagonal arrangement of microporess fabricated by using polystyrene templates. In this work a similarrrangement at a nanometre scale has been obtained together with

square geometry.

. Conclusions

In the present work the effect of the annealing temperature onhe crystalline structure of WO3 sputtered thin films has been stud-ed. The XRD diagrams show a phase transition from tetragonal to

onoclinic when the samples are annealed at temperatures higherhan 800 ◦C. Furthermore, the change of morphology of the thinlms due to temperature could be highlighted. Very smooth lay-rs have been obtained with annealing temperatures of 600 and00 ◦C, whereas the samples annealed at 800 ◦C show a granular

◦

tructure. Moreover, in the case of 900 C, the layers become nonniform and present zones with different thicknesses. The opti-al energy band gap of the sputtered WO3 thin films decreasesith annealing temperature (3.65–3.5 eV and 3.5–3.05 eV for direct

[

ce Science 276 (2013) 229– 235

and indirect transitions respectively) influenced by the change incrystalline structure and morphology of the films.

On the other hand, nanostructured WO3 thin films have beenobtained by the ablation of the thin films with LIL. This techniquehas improved the morphological properties of all type of films,regardless of the annealing temperature. Periodic nanostructureshave been obtained provided with a granular surface and holes,fundamental for gas diffusion. Hence, outstanding nanostructuredfilms have been developed which could be used as sensing layersfor the detection of hazardous gases such as NO2 and H2S.

Acknowledgments

Funding for this work was provided by the Ministry of Sci-ence and Education of Spain within the framework of the NAMIRISproject no. TEC2010-21375-C05-01 “Modular system based onadvanced micro- and nanotechnologies for safety and air-qualityapplications”.

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