Thin Solid Films 518 (2010) 4095–4099

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Nanostructured tungsten and tungsten trioxide films prepared by glancingangle deposition

Derya Deniz ⁎, David J. Frankel, Robert J. LadLaboratory for Surface Science and Technology, University of Maine, Orono, ME 04469-5708, USA

⁎ Corresponding author.E-mail address: derya.deniz@maine.edu (D. Deniz).

0040-6090/$ – see front matter © 2009 Elsevier B.V. Aldoi:10.1016/j.tsf.2009.10.153

a b s t r a c t

a r t i c l e i n f o

Article history:Received 7 July 2009Received in revised form 13 August 2009Accepted 23 October 2009Available online 31 October 2009

Keywords:Glancing angle depositionTungsten trioxideNanorodsPulsed DC sputteringScanning electron microscopyX-ray photoelectron spectroscopy

Nanostructured tungsten (W) and tungsten trioxide (WO3) films were prepared by glancing angledeposition using pulsed direct current magnetron sputtering at room temperature with continuous substraterotation. The chemical compositions of the nanostructured films were characterized by X-ray photoelectronspectroscopy, and the film structures and morphologies were investigated using X-ray diffraction and highresolution scanning electron microscopy. Both as-deposited and air annealed tungsten trioxide films exhibitnanostructured morphologies with an extremely high surface area, which may potentially increase thesensitivity of chemiresistive WO3 gas sensors. Metallic W nanorods formed by sputtering in a pure Ar plasmaat room temperature crystallized into a predominantly simple cubic β-phase with <100> texture althoughevidence was found for other random grain orientations near the film/substrate interface. Subsequentannealing at 500 °C in air transformed the nanorods into polycrystalline triclinic/monoclinic WO3 structureand the nanorod morphology was retained. Substoichiometric WO3 films grown in an Ar/O2 plasma at roomtemperature had an amorphous structure and also exhibited nanorod morphology. Post-depositionannealing at 500 °C in air induced crystallization to a polycrystalline triclinic/monoclinic WO3 phase andalso caused a morphological change from nanorods into a nanoporous network.

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1. Introduction

Tungsten trioxide (WO3) films have beenwidely used as the activeelement in conductance-type chemiresistive gas sensors [1–5] as wellas electrochromic coatings [6,7]. Since WO3 is an n-type semiconduc-tor with a band gap energy of 3.3 eV, its films are commonly grown byradio frequency (RF) magnetron sputtering. Another method, whichcan effectively prevent target poisoning, is to use a pulsed directcurrent (DC) magnetron sputtering plasma, in which charges on aninsulating target can be dissipated and the energy of reactive speciesextracted from plasma can be uniformly controlled. These plasmacharacteristics differ from those found in RF sputtering plasmas, forwhich the ion energies are distorted due to the comparable timescales of the RF cycle and the ion trajectories across the sheath regionaround the substrate [8,9].

Glancing angle deposition (GLAD), also known as oblique angledeposition, is a method to grow structures such as nanorods, nano-springs, zigzag columns, etc. by manipulating the deposition angle, α,and substrate rotation angle,φ [10,11]. It is a physical vapor depositiontechnique, in which a high melting point material flux is incident ontothe substrate from a glancing deposition angle, α, which is typicallygreater than 80° so as to exacerbate atomic shadowing effects. Low

surface mobilities compel adatoms to respond to kinetic limitationssuch as geometrical confinements and shadowing, which results in avariety of porous columnar microstructures [12–14].

The objective of this study was to use GLAD methodology tofabricate nanostructured W and WO3 films and employ post-deposition annealing treatments as a means of obtaining WO3 gassensing films with high surface to volume ratios. Since the sensitivityof chemiresistive gas sensors is highly dependent on the surface tovolume ratio of the films [15], the resulting nanostructured morphol-ogies may be beneficial for improved sensitivity and also producelarge surface areas for placing selective catalytic dopants. Previousstudies have shown that WO3 films deposited by physical vapordeposition techniques at room temperature have amorphous struc-ture [16,17]. In order to grow polycrystalline or epitaxial WO3

configurations, one has to work at elevated temperatures, whichallows sufficient adatom mobilities for thermodynamic driving forcesto dominate. For instance, Moulzolf et al. [18] report deposition ofamorphous phase, heteroepitaxial tetragonal phase, and heteroepi-taxial monoclinic phase WO3 films on sapphire substrates at roomtemperature, 450 °C, and 650 °C, respectively, using RF magnetronsputtering. There have been a few attempts to deposit nanostructuredWO3 films by the GLAD technique using DC magnetron sputtering aswell as thermal evaporation [19–21]. However, in these studiesthere is not a detailed discussion or evidence of crystallinity andcolumnar nature of the nanostructures. In this study, it is demon-strated that high surface to volume ratio WO3 nanostructured films

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can be produced via direct pulsed DC sputter deposition or via post-deposition annealing of films prepared using GLAD.

2. Experimental details

Films were fabricated in a high vacuum chamber operating with abase pressure of <1×10−5Pa using the GLAD experimental config-uration shown in Fig. 1. A home-made GLAD manipulator equippedwith a DC motor maintained the azimuthal sample rotation at a speedof 5 rpm during each deposition. The substrate tilt angle wasmanuallyset to 80° before each run. All films were deposited at roomtemperature onto 2.5 cm×2.5 cm fused silica substrates having anrms roughness <1 nm. A 7.5 cm diameter W target with a purity of99.95% was located 11 cm from centerline of the substrate and waspresputtered in Ar for 10 min prior to film deposition. There was nocollimation employed in the experiments. The deposition rate andfilm thickness were measured in-situ by means of a quartz crystaloscillator calibrated against surface profilometry, X-ray reflectivity,and scanning electron microscopy (SEM) measurements. Metallictungsten films with a thickness of ∼50 nm were grown in a pure Arplasma using a 5 sccm flow rate at 0.26 Pa, a DC power of 100 W, and a410 V discharge voltage. WO3 films with a thickness of ∼100 nmweregrown in Ar and O2 gas mixtures at a total pressure of 0.26 Pa usingflow rates of 4 and 6 sccm, respectively. In this case, the DC power was100 W with a 685 V discharge voltage. The deposition rates of W andWO3 films were 2.3 and 3.0 nm/min, respectively. Films werecharacterized immediately after deposition and also following post-deposition annealing treatments at 500 °C in air for 5 h.

Film stoichiometry was determined using X-ray photoelectronspectroscopy (XPS) analysis of the O 1 s andW 4f peaks acquired withAl Kα X-rays and a SPECS hemispherical analyzer operating at 20 eVpass energy. All XPS peaks were aligned to the adventitious C 1 s withbinding energy of 285.0 eV due to steady state sample charging.Empirical XPS sensitivity factors were obtained by measuring spectrafrom a stoichiometric WO3 film grown by conventional RF magnetronsputtering at 500 °C that was further annealed and oxidized at 600 °C,0.13 Pa O2 for 4 h. The accuracy of XPS composition measurements

Fig. 1. Sketch of the GLAD configuration.

based on these factors was estimated to be±2 at.%. This filmwas usedas a reference by comparing its O 1 s andW 4f XPS peaks to those fromthe GLAD samples. Film structure was characterized by means of θ–2θscans or grazing incidence angle X-ray diffraction (XRD) and pole figuremeasurementswith achromatic Cu Kα radiation (λavg=1.5406 Å)usingline and point foci on a Panalytical X'pert MRD Pro Diffractometer.Sample morphology was investigated using a Zeiss Nvision 40 SEM onrandom areas of the films. Film resistivities were measured with astandard four point probe technique. As-grownW films had a resistivityof 50 μΩ-cm while all the WO3 films were insulating (>2 MΩ-cm).

3. Results and discussion

Thermodynamic equilibrium is achieved for most binary systemsat temperatures typically above half their melting points. However,for the most part, thin film growth processes are carried out at lowertemperatures where thermodynamic equilibrium is difficult to attain.Therefore, the vast majority of thin film growth processes are at non-equilibrium. Thin films are generally deposited by condensation atvarious working pressures, which makes it even more difficult toapply equilibrium phase diagrams. Nevertheless, arriving adatoms forlow melting point materials have high enough surface mobilities tonucleate thermodynamically and grow in stable phases in short timeperiods even at low temperatures. This is not generally true formaterials with high melting points, such as tungsten, due to theirmuch lower adatom mobilities. These limited mobilities were foundto be sufficiently small at room temperature to form bothW andWO3

nanorod morphologies using our GLAD geometry.

3.1. As-deposited and post-deposition oxidized metallic tungsten nanorods

As-grown metallic tungsten films sputtered in a pure Ar plasmaexhibited a simple cubic β-phase structure, which is thermodynam-ically unstable at room temperature. Fig. 2 a) shows an XRD patternfrom a tungsten film acquired in a θ–2θ scan. The spectrum containsdominant (200) and (210) peaks as well as additional very weak (hkl)peaks, all from the β-phase W; no evidence for α-phase W was foundas has been reported previously [22,23]. A (200) pole figure (notshown) confirms the <100> β-phase nanorod fiber texture with a fullwidth at half maximum (FWHM) of 34° indicating a large mosaicspread on the substrate surface. The (210) pole figure showed verylow intensity at φ=0° and a ring at φ=26° indicating no azimuthalin-plane crystallographic alignment. The angle between (200) and(210) planes of 26.56° coupled with the large mosaic spread accountsfor the large (210) peak in the θ–2θ scan. The weak (hkl) peaks aredue to a small number of random polycrystalline grains as alsoconfirmed by the grazing incidence angle XRD data.

Fig. 2 b) shows a grazing incidence angle XRD pattern from the as-deposited W film. Since the film has textured <100> rods, theintensity of the (200) peak would be expected to be very small ormissing in grazing incidence scans. However, a strong (200) peak aswell as large (210) and (211) peaks are evident. We interpret this asbeing due to the ∼34° mosaic spread combined with accidentalsatisfaction of the (200), (210), and (211) diffraction conditions in thegrazing geometry from the <100> textured nanorods. The fact thatseveral additional weak (hkl) peaks are visible in both the θ–2θ andgrazing incidence scans indicates that the textured W nanorodscoexist with a set of very small completely random grains at thesubstrate–film interface. Previous work [22,23] has shown that duringoblique angle deposition of metallic tungsten, α- and β-phase islandscoexist at the early stages of the film growth, but the β-phase has ahigher vertical growth rate than the α-phase due to lower adatommobility on these islands. The β-phase islands get taller as the filmthickens, thereby shading the shorter α-phase islands, and thusnanorods are formed as a result of the shadowing effects. We show,however, that there exists a variety of completely random β-phase

Fig. 2. a) θ–2θ scan and b) grazing incidence angle XRD pattern from as-depositedtungsten film.

Fig. 3. a) θ–2θ scan and b) grazing incidence angle XRD pattern from post-deposition airannealed tungsten film.

Fig. 4. High resolution XPS W 4f peaks from as-deposited and post-deposition airannealed tungsten film.

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grains at the early stages of film growth and the <100> orientedβ-phase grains become dominant as the film grows.

Fig. 3a) and b ) show θ–2θ scan and grazing incidence angle XRDpatterns, respectively, from the W film annealed in air for 5 h at500 °C. This annealing treatment causes the tungsten nanorods tocrystallize into a polycrystalline triclinic/monoclinic WO3 structure,which is confirmed by the data provided in the Joint Committee ofPowder Diffraction Standards (JCPDS) cards No. 00-032-1395 and 01-083-0950 [24,25]. Both the peak positions and the intensity ratios arein agreementwith those in the JCPDS files. We note, however, that ourdata indistinguishably match to both monoclinic and triclinicstructures. This is because these WO3 structures are very similar toeach other and differ only by small distortions of the WO6 octahedralbuilding blocks within the crystal lattice, especially when thestructure contains oxygen vacancies [26]. Overlaps occur among thepeaks that cannot be distinguishedwithin the grain size broadening ofthe spectra. For example, the (200), (020) and (002) peaks fall withina single peak envelope and cannot be deconvoluted. The color of thetungsten film changed from dark brown to transparent following theannealing, which is expected for a stoichiometric WO3 film [27].

Fig. 4 shows high resolution XPSW 4f peaks from as-deposited andannealed tungsten films. Two pairs of spin–orbit split W 4f doubletsindicate the presence of some surface oxidation on the metallictungsten nanorods. FWHM of O 1 s and W 4f 7/2 peaks from the as-deposited film are determined to be 2.05 and 2.0 eV, respectively. Thesurface composition of the as-depositedW film was 25 at.%W, 36 at.%C, and 39 at.% O. Following post-deposition annealing at 500 °C in air,a stoichiometric WO3 film was formed as evidenced by narrow andsymmetric O 1 s and W 4f 7/2 peaks with FWHM of 1.91 and 1.8 eVand also the single spin–orbit split W 4f doublet. The film compositiondetermined by XPS in this case was 29 at.% W, 6 at.% C, and 65 at.% O.

Carbon peaks in the XPS spectra were present due to surface con-tamination from air exposure.

Fig. 5 a) shows a high resolution SEM image of an as-depositedtungsten film showing nanorod structure and highly porous mor-phology. One can see that these nanorods are standing straight up onthe substrate surface; that is, rods are uniformly distributedthroughout the substrate surface as displayed by uniform spacingsamong them. The competition between different rod shapes can bereadily deduced from these images, e. g, taller rods are surrounded bymuch shorter and smaller rods. Although we are not capable ofidentifying the crystalline orientation of each individual nanorod, ourresults suggest that short rods are polycrystalline and tall rods are<100> textured β-phaseW. The post-deposition annealing treatment

Fig. 5. Plan view SEM images from a) an as-deposited tungsten film and b) a tungstenfilm after 500 °C annealing in air for 5 h.

Fig. 6. a) θ–2θ scan from post-deposition annealed WO3 film and b) grazing incidenceangle XRD patterns from as-deposited and annealed WO3 films.

Fig. 7. High resolution XPS W 4f peaks from as-deposited and post-deposition airannealed WO3 film.

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did not change the nanomorphology of the oxidized tungsten film asis evident from Fig. 5 b).

3.2. As-deposited and post-deposition annealed tungsten trioxide nanorods

As-grown WO3 films sputtered in an Ar/O2 plasma at room tem-perature exhibited an amorphous structure as determined by XRD.However, the post-deposition annealing treatment transformed thefilms into a polycrystalline triclinic/monoclinicWO3 phase as shown bythe θ–2θ scan (Fig. 6 a). The grazing incidence angle XRD patterns fromas-deposited and post-deposition annealed WO3 films (Fig. 6 b) showwell-defined triclinic/monoclinicWO3peaks that againmatch the JCPDScards No. 00-032-1395 and 01-083-0950 [24,25]. The fact that thegrazing incidence angle and θ–2θ scans look identical indicates that thefilms have random polycrystalline structure.

XPS analysis of the as-depositedWO3 film yields O 1 s andW4f 7/2peaks with FWHMof 1.99 and 1.75 eV, whereas the O 1 s andW 4f 7/2peaks from the annealed WO3 film have FWHM of 1.59 and 1.40 eV,respectively. This result suggests that the as-deposited film wassubstoichiometric and became close to stoichiometric WO3 afterannealing. The narrow and symmetric O 1 s and W 4f 7/2 peaks areconsistent with the stoichiometric referenceWO3 as seen in Fig. 7. Theas-depositedWO3 film has a 26 at.%W, 16 at.% C, and 58 at.% O surfacecomposition, while the annealed WO3 film has a compositionalmakeup of 29 at.% W, 7 at.% C, and 64 at.% O.

Fig. 8 a) and b) show cross-sectional and plan viewSEM images fromthe as-grownWO3 film. The columnar nature of the nanorods is readilyapparent and the plan view image displays a ‘cauliflower’morphology.The columnar formation is a result of the competitive growth process.Atomic shadowingwill stochastically lead to apreferential growthof thetallest features on the surface. As with the amorphous films, theroughness fluctuations on the substrate surface make some regionsreceive more material flux than the others due to shadowing. Constant

planar density is sustained by an increase in the column diameter as theshorter features become diminished during film growth [28]. Further-more, according to the standard sputtering zone model [29], WO3

should fall within zone 1, where kinetic constraints dominate thestructure formation resulting in a top surface morphology with‘cauliflower’ shape. Post-deposition annealing of amorphous WO3

films yielded crystallization to the monoclinic/triclinic WO3 phaseaccompanied by a morphological deviation into an extremely nanopor-ous WO3 network as shown in Fig. 8 c). As previously reported, such astructure with high surface area and porosity may permit fast diffusionof ions, which causes an increase in electrochromic efficiency of WO3

[30]. Faster kinetics in these structures may allow rapid electrochromic

Fig. 8. a) Cross-sectional and b) plan view SEM images from as-deposited WO3 filmc) plan view SEM image from WO3 film after annealing at 500 °C in air for 5 h.

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switching effects under infrared (IR) and near-infrared (NIR) radiationexposures. It should be noted that this film is about twice as thick as theWO3 film fabricated by annealing a 50 nm metallic tungsten film.Consequently, onemight have to grow filmsmuch thinner than 100 nmin order to maintain a nanorod morphology through the annealingprocess.

According to bulk thermodynamics, the stable equilibrium phasesof WO3 depend on temperature: tetragonal above 720 °C, orthorhom-bic between 320 °C and 720 °C, monoclinic between 17 °C and 320 °C,and triclinic below 17 °C [25]. One might expect WO3 to crystallize inthe orthorhombic phase when annealed in air at 500 °C. However,especially in the case of films with extremely high surface to volume

ratio in nanorod morphology, the surface and interface energiesapparently dominate the free energies of the system and a stable filmstructure that differs considerably from the bulk thermodynamicpredictions is formed.

4. Conclusions

Pulsed DC magnetron sputtering of a tungsten target using a GLADgeometry at room temperature can be used to fabricate high surface tovolume ratio tungsten and tungsten trioxide films with nanorodmorphologies. It has been demonstrated that metallic tungsten filmsdeposited in a pure Ar plasma crystallize with a dominant <100>textured simple cubic β-phase structure and nanorod morphology.Deposition using an Ar/O2 plasma yields an amorphous tungstentrioxide film also with a nanorod morphology. Post-depositionannealing treatments at 500 °C in air cause both types of as-depositedfilms to crystallize into the triclinic/monoclinic WO3 structure. Theannealing treatment does not affect the nanomorphology of theoxidized tungsten nanorods but it does change theWO3 nanorods intoa highly nanoporous network. The sensitivity of chemiresistive metaloxide gas sensors can be markedly increased by fabricating nanos-tructured films with very high surface to volume ratio. Therefore, wesuggest that one of the possible applications of these nanoengineeredWO3 film structures is for gas sensing applications.

Acknowledgements

This work was supported by the W.M. Keck Foundation. Theauthors are grateful to Scott Moulzolf from the University of Maine foraiding with the SEM measurements.

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