WOx cluster formation in radio frequency assisted pulsed laser deposition

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Applied Surface Science 254 (2007) 1347–1351 www.elsevier.com/locate/apsusc

WOx cluster formation in radio frequency assisted pulsed laser deposition M. Filipescu a,*, P.M. Ossi b, M. Dinescu a b

a National Institute for Laser, Plasma and Radiation Physics, P.O. Box MG 16, RO-77125 Magurele, Bucharest, Romania Dipartimento di Ingegneria Nucleare & Centre for NanoEngineered Materials and Surfaces – NEMAS, Politecnico di Milano, Italy

Received 26 July 2007; received in revised form 20 September 2007; accepted 20 September 2007 Available online 3 October 2007

Abstract The influence of oxygen gas pressure and radio-frequency power on the characteristics of the WOx films produced by laser ablation of a W target at room temperature in oxygen reactive atmosphere were investigated. Changing buffer gas pressure in the hundreds of Pa range affects the bond coordination, roughness and morphology of the deposited films, as investigated by micro-Raman spectroscopy, atomic force microscopy and scanning electron microscopy. The combination of radio-frequency discharge and buffer gas pressure on film nanostructure, as reflected by bond coordination, surface morphology and roughness is discussed. # 2007 Elsevier B.V. All rights reserved. Keywords: Pulsed laser deposition; Radio frequency; Clusters

1. Introduction Thin films of the wide-gap semiconductor oxide WO3 find application as active layers in gas sensors for the detection of H2S and H2, small concentrations of NO and Cl2 [1]. There is also an increasing interest in WO3 for catalytic applications including selective oxidation of organic compounds [2], hydrodesulphurisation of fuels [3] and methanol oxidation for fuel cells [4]. For both fields of application it is desirable to deposit films with high specific effective surface, without loosing thickness uniformity, reasonable adhesion to the substrate and controlled crystallinity. In particular, cluster-assembled films, with a range of degrees of open surface, could be an excellent solution. Starting from a metallic W target pulsed laser deposition (PLD) in vacuum allows obtaining thin metallic films that are oxidised after exposure to ambient atmosphere. The nanostructure and oxidation path of the films are strongly affected when PLD is performed in a buffer gas at high pressure, up to 1 kPa [5]. The clusters, formed mainly by collisional mechanisms [6] in the expanding plasma plume, land on the substrate where they mutually aggregate. Yet, the reduced average kinetic energy of the deposited particles can be

* Corresponding author. Tel.: +40 214574470; fax: +40 214574467. E-mail address: [email protected] (M. Filipescu). 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.09.090

detrimental to the intrinsic film strength and to film-substrate adhesion. Such limitations can be overcome by the use of a hybrid deposition technique, combining the advantages of conventional PLD in a buffer gas (O2) with the enhancement of reactivity at the substrate induced by an excited and ionised beam of atoms and molecules produced by a radio-frequency discharge through oxygen [7]. The morphological and structural changes induced in WOx films by changing laser fluence, substrate temperature and RF power were recently investigated [7]. This work is a continuation of the above-mentioned research; this time all process parameters are deliberately fixed, except two, namely the oxygen pressure and RF power. The former affects cluster formation in the flying plume, whilst the combination of both determines the relative weight of high-energy species to film growth, stoichiometry and substrate adhesion. Our goal is to investigate the combined, as well as the separate effects of such parameters on WOx thin films. 2. Experimental conditions The deposition set-up consists of a Nd:YAG laser with four harmonics, a reactive chamber, a pump system that can go down to 104 Pa, a rotation system of the target, a radio-frequency generator (13.56 MHz, CESAR 1310, RF maximum power 1000 W). Micro-Raman spectra were collected from all films to

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study the local bonding coordination. The 514 nm line of an Ar+ laser was focused on the sample by a 50 Leica Germany optical objective (NA = 0.75 corresponding to 0.5 mm nominal spot diameter for this excitation wavelength). The laser power was fixed at 0.5 mW to avoid local annealing and photo-induced structural modifications of the films. The backscattered light was collected by the same objective and analyzed by a Renishaw inVia Raman Microscope equipped with a holographic Notch filter (cut-off at 100 cm1), a 1800 lines mm1 diffraction grating and a thermoelectrically cooled RenCam CCD detector. The local morphology of all surfaces samples and the formation of WOx nano-clusters were studied by an atomic force microscope (Nomad type) in the intermittent contact mode [7], analysing (10  10), or (2  2) mm2 areas. AFM data were complemented by scanning electron microscopy (SEM) observations, using a Zeiss Supra 40 field ion microscope. Starting from a metallic target of tungsten kept at different reactive oxygen pressure values, WOx cluster-assembled films were deposited on (1 0 0)-silicon substrates, either with the assistance or of the RF discharge, to find out the most suitable conditions to obtain nanostructured films based on the assembling of WOx clusters. Si was chosen because it is commonly used as a reference realistic substrate (much more than glass). Substrate choice is critical when the development of film morphology is to be studied, because its chemical nature and surface structuring strongly affect cluster dynamics and aggregation, resulting in a film with specific nanostructure and adhesion properties. Unfortunately in the case of WO3, glass substrates would be preferable to perform Raman measurements [7]. The substrate was set parallel to the target at a distance of 4 cm and was kept at room temperature during all depositions. Using a wavelength of 266 nm, 10,000 laser pulses were shot and the laser fluence was set after a parametric study to 4 J cm2 [7]. The deposition parameters that were changed are summarized in Table 1 together with the performed analyses. 3. Results and discussions At room temperature and atmospheric pressure the perovskite-like structure of single crystal WO3 is based on Table 1 Summary of deposition conditions and analyses performed on each sample Sample

poxygen (Pa)

PRF (W)

Analyses

211 212 213 214 215 216 217 218 219 220 221

100 100 100 300 300 500 500 700 700 900 900

0 50 100 0 100 0 100 0 100 0 100

R; R; R; R; R; R; R; R; R; R; R;

A A A A; S A; S A A; S A A A A

R: Raman spectroscopy; A: atomic force microscopy; S: scanning electron microscopy.

Fig. 1. Collection of visible Raman spectra from WO3 films deposited at different oxygen partial pressures and RF power values.

corner sharing of WO6 regular octahedra; polycrystalline WO3 consists of a mixture of the monoclinic g-phase P21 =nðC52h Þ and ¯ 1 Þ with very similar Raman features. the triclinic d-phase P1ðC i In the spectra, besides the low frequency lattice modes, the peaks between 200 and 400 cm1 are associated to O–W–O bending modes and the peaks between 600 and 900 cm1 are related to W–O stretching modes [8]. The Raman spectra of amorphous tungsten oxides (a-WOx) and nanostructured tungsten oxides (ns-WOx) are characterised by two large bands extending over the same frequency ranges where the above discussed peaks are found in crystalline phases [9]. In Fig. 1 are shown representative spectra of films grown in different conditions (see Table 1), recorded between 100 and 1200 cm1, where the meaningful material features lie. We study both the separate and the combined influence of oxygen partial pressure and RF power on bond coordination in the films and its evolution. The difficulties in the analysis of the spectra (compare the spectra in [7]) associated to the choice of (1 0 0) Si wafers as the substrates are due to the transparency of WOx to the used radiation, its comparatively low Raman scattering cross section and the intensity of Si Raman peaks. Moving from the bottom to the top, the spectra refer to films deposited at increasing oxygen partial pressure in the deposition chamber and between films prepared at the same oxygen pressure (218, 219) at increasing RF power. The main features are broad bands around 300, 630, 820 cm1. Slightly defined and sometimes not observed is a shoulder around 250 cm1. The feature at about 820 cm1, associated to W–O stretching mode, is present in the Raman spectra of all crystalline phases of WO3 (lying between 805 and 820 cm1) and is often taken as a fingerprint of compound presence. Given the broadness of the above bands it is likely that the radiation is coherently scattered by small-sized WOx domains, either noncrystalline, or nano-crystalline. The intense peaks around 530 and 970 cm1 are due to the Si substrate and confirm that all films, whose thickness is comparable, are non-metallic and transparent at the wavelength adopted. The high wavenumber Si peak obscures any contribution around 950 cm1, due to W O stretching vibration, usually associated to the development of a nanostructure in the film [10]. Also, at least a contribution to the band at 300 cm1 is due to a second order signal from Si.

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With reference to a spectrum from a bare (1 0 0) Si wafer, the spectra of films prepared at low O2 pressure (211–213) show only weak signatures from WOx, even in the presence of the highest RF power (sample 213). Only beyond a threshold O2 pressure (300 Pa) a trend towards better-defined features is found looking at the 300, 630 and 820 cm1 band (sample 215) and a weak shoulder at about 250 cm1 appears. Higher values

of the O2 pressure (300–900 Pa) in the presence of RF power (100 W) results in an increased definition of the spectra, as may be seen comparing samples 218 to 219; the same trend is observed in films 214–215 and 216–217. At variance with the Si spectrum, all the above spectra show an up-bending trend in the region between 500 and 900 cm1. This is characteristic of WOx spectra. In the presence of the same RF power (100 W), a higher O2 pressure does not lead to any meaningful spectral modification (compare spectra 215, 219, 221), but a better

Fig. 2. 3D AFM images of samples deposited using the same PLD conditions, but different RF power (a) 0 W, (b) 50 W and (c) 100 W.

Fig. 3. 3D AFM images for samples deposited using the same PLD conditions, but at different oxygen pressure (a) 500 Pa, (b) 700 Pa and (c) 900 Pa.

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definition of the band at 820 cm1 at the highest O2 pressure (221). From Raman spectroscopy the combined effect of O2 pressure and RF power on film formation emerges. If buffer gas pressure is too low, the number density of excited/ionised O atoms and molecules produced by collisions in the plasma is insufficient to significantly affect film growth. Only for gas pressure beyond a lower threshold the oxygen plasma effectively contributes to film formation, as shown by the

Fig. 4. 3D AFM images for sample deposited using the same RF–PLD conditions, but at different oxygen pressure (a) 500 Pa, (b) 700 Pa and (c) 900 Pa.

strong difference between samples 211 and 215. Notwithstanding the high RF power, at high O2 pressure (above 500 Pa, in our experiments) bond coordination of the growing films is not strongly affected (films 215, 219, 221). The influence of the RF discharge (at different powers) on the surface morphology of the deposited films was investigated by AFM. Indeed, keeping constant the oxygen pressure at 100 Pa and increasing the RF power from 0 to 100 W, the droplets become progressively smaller and even disappear at 100 W; correspondingly, the roughness (RMS) of samples 211, 212, 213 decreases below 1 nm, as shown in Fig. 2. It is likely that this is due to low-intensity bombardment of the growing films. When PLD is performed in O2 buffer gas at pressure progressively higher from 500 to 900 Pa (samples 216, 218, 220) the roughness increases, too (Fig. 3), being associated to cluster formation and growth in the plume. Such clusters land on the substrate with monotonically decreasing kinetic energy at higher O2 pressure thereby at fixed substrate temperature their possibility to migrate and reciprocally aggregate giving bigger agglomerates is progressively reduced [11]. From Fig. 4, at 100 W RF power, the film roughness strongly increases with increasing high O2 pressure (samples 217, 219, 221). Such an increase is consistent with the formation of big clusters, compacted more and more effectively in the highenergy conditions provided at film surface by the RF discharge. The detailed local information on WOx film morphology was complemented by SEM observations. Microstructure differences are evident among three groups of samples, in agreement with AFM results. The surfaces of the films deposited at lowest oxygen pressure (211–213; not shown) are flat and uniform, without any structuring; a few spherical droplets (average size around 100 nm) are observed. Films 214, 216, 218 and 220, deposited by PLD at increasing O2 pressure, show smooth surfaces and a random distribution of particles with irregular shape, whose size ranges between 15 and 30– 40 nm, as shown in Fig. 5. Somewhere such particles, whose composition is the same as that of the films, are agglomerated to

Fig. 5. Representative surface microstructure of a film (sample 214) deposited without RF power.

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4. Conclusions The combined effect of buffer gas (oxygen) pressure and RF power discharge on bond coordination, surface morphology and roughness of WOx films prepared by PLD has been investigated, identifying ranges of parameter values that correspond to film nanostructuring and to the achievement of a high open surface area. Acknowledgement The authors are grateful to Mr. A. Mantegazza, Politecnico di Milano, for his support with Raman and SEM measurements. References Fig. 6. Representative surface microstructure of a film (sample 215) deposited with the assistance of RF power (100 W).

give bigger, irregularly shaped flakes, ranging from 150 to 600 nm in size. The peculiar microstructure of samples 215, 217, 219 and 221 that were synthesised in the presence of a high RF power (100 W) consists of a dense nanostructure made of irregularly shaped agglomerates (Fig. 6) whose typical size lowers with increasing O2 pressure, from about 80 nm (sample 215) to 40 nm (217), to 20 nm (219 and 221). The above observations, both AFM and SEM, indicate that RF power is effective to induce film nano-structuring, resulting in an open microstructure much prone to oxidisation. Within the range of explored deposition conditions, sample 219 appears the best compromise in terms of bond coordination, surface morphology and roughness.

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