Influence of nitrogen doping on the sputter-deposited WO3 films

Thin Solid Films 518 (2009) 1430–1433

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Influence of nitrogen doping on the sputter-deposited WO3 films Amit Kumar Chawla a, Sonal Singhal a,b, Hari Om Gupta b, Ramesh Chandra a,⁎ a b

Nanoscience Laboratory, Institute Instrumentation Center, Indian Institute of Technology Roorkee, Roorkee, 247667, India Department of Electrical Engineering, Indian Institute of Technology Roorkee, Roorkee, 247667, India

a r t i c l e

i n f o

Available online 21 September 2009 Keywords: Reactive sputtering XRD FE-SEM

a b s t r a c t Tungsten oxide films were produced using reactive rf magnetron sputtering. In this work, nitrogen doping was used to modify structural and, optical properties of the material in the presence of two inert gases (argon and helium). Substituting helium gas with argon results in a decrease in the particle sizes and thus affects the band gap values. Bandgap values were obtained over the range of 2.43 to 3.01 eV via incorporating oxygen–nitrogen–argon/helium mixtures in the gas ambient. It was also observed that the atomic mass of the sputtering gas plays a major role for changing the primary crystallite size as well as the surface morphology and texture. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Thin films of transition metal oxides have attracted a great deal of attention because of their important applications such as electrochromic devices and solid state sensors. Recently the electrochromic devices made by using the electrochromic oxides like tungsten trioxide (WO3) have been extensively studied to regulate the radiated energy through glass with modifying their optical properties for the application in ‘smart windows’ [1–4]. Bandgap values from 2.6 to 3.4 eV, reported for polycrystalline and amorphous WO3 thin films [5] only allow for absorption in the near ultraviolet and blue region of the solar spectrum. Bandgap modifications would be necessary to extend the light harvesting capability to a wider portion of the solar spectrum. For metal oxide semiconductors, doping of the material can result in band edge modifications. Ideally there is a reduction of band gap via raising of the valance band and separately an increase of the conduction band to reduce the potential shifts. Impurity doping of photoactive metal oxides for the purpose of improving photoresponse has been proposed in the past [6]. Recent detailed studies were conducted on substitutional doping of TiO2 [7–9]. Comparing the effect of several anion dopants through density of states calculations, Asahi et al. concluded that nitrogen was the most effective species because its p states mix with O 2p states, resulting in bandgap narrowing [7,8]. The introduction of nitrogen into the oxide lattice or oxygen into the nitride lattice resulted in modifications of the film properties. Mohamed et al. have reported in their study that for the formation of tungsten oxy nitrides, a partial pressure of nitrogen must be near 80% of the total gas pressure [10,11]. Here in this study we show the effect of nitrogen gas on tungsten oxide films at lower partial pressure of nitrogen gas. Little work has been published to date on the doping of WO3 with anion species ⁎ Corresponding author. Tel.: +91 1332 285743; fax: +91 1332 286303. E-mail address: ramesfi[email protected] (R. Chandra). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.09.060

specifically for the purpose of bandgap modification. Arguments qualitatively similar to those for TiO2 can be applied to nitrogen doping of WO3. These considerations prompted the effort, reported in this paper, to modify the bandgap of reactively sputtered WO3 thin films using nitrogen (N2) gas in the sputtering ambient as a doping agent in argon and helium atmosphere. Similar studies for other sputterdeposited nanocrystalline films are quite rare. We also show how the nature of the sputtering gas (argon and helium) along with nitrogen doping affects the surface morphology, texture and the optical properties of the nanocrystalline tungsten oxide films. 2. Experimental details Tungsten oxide nanocrystalline thin films were deposited by rf magnetron sputtering in a custom built 12″-diameter chamber (Excel Instruments). The base vacuum of the sputtering chamber was 6.6 × 10− 4 Pa. Thereafter, fixed high purity oxygen (20%) was bled into the chamber along with varying nitrogen pressure. Sputtering was carried out in the presence of two different inert gas (He and Ar) atmospheres. The ratio of the gas mixtures was controlled and measured using mass flow controller and Capacitance manometers (MKS) respectively. The gas pressure was kept at 1.99 Pa for all depositions. Sputtering was done for a set period of time at a fixed power of 100 W on the corning glass, kept at 400 ºC. The substrate to target (50.1 mm diameter tungsten target of 99.97% purity) distance was fixed at 50 mm. All the parameters, except nitrogen partial pressure were kept constant during sputtering. The WO3 films were characterized by XRD for the structural properties by using an X-ray diffractometer (Bruker D8 Advance). The coherently diffracting domain size (dXRD) was calculated from the integral width of the diffraction lines using the well known Scherrer equation [12] after background subtraction and correction for instrumental broadening. The surface morphology and the microstructure of

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the films were studied using a field emission scanning electron microscope (FEI Quanta 200 F). Compositional analysis was carried by an energy dispersive system attached with FE-SEM. Optical transmittance and absorption were measured in the 200–800 nm wavelength range using UV–Vis–NIR spectrophotometer (Cary 5000 Varian). Samples A, B, C and D were deposited in Ar–O2–N2 mixture with nitrogen partial pressure of 2, 3, 5, and 10% respectively while samples E, F, G and H were deposited in He–O2–N2 mixture with similar nitrogen partial pressures. 3. Results and discussions Effect of nitrogen doping on grain structure in the WO3 film in two different inert gas atmospheres is illustrated by the X-ray diffractograms shown in Fig. 1(a) and (b). XRD pattern (Fig. 1(a)) of the nanocrystalline thin films consists of samples deposited in Ar atmosphere with different nitrogen concentration. The XRD patterns for samples A–D exhibit a broad diffuse background, with a single peak at 2θ ≈ 23°, indicating the presence of a WO3 crystalline phase. These patterns are consistent with results reported in our previous study regarding reactively sputtered WO3 thin films deposited with O2 partial pressure greater than 10% resulting in a crystalline phase [13]. There is no pronounced effect with increasing nitrogen partial pressure in samples A–D. Fig. 1(b) shows the XRD patterns of the nanocrystalline thin films of tungsten oxide deposited in He atmosphere with varying nitrogen concentration. Broadening in diffraction peak at 2θ ≈ 23° corresponds to the decrease in particle size. It is evident from Fig. 1(b) as the nitrogen concentration is

Fig. 1. XRD patterns of the WO3 films with varying nitrogen concentration (a) in Ar atmosphere (b) in He atmosphere.

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Table 1 Summary of deposition conditions for sputtered tungsten oxide films. Sample N2 gas N2 obtained by EDS dXRD Band gap Refractive (sccm) (wt.%) (nm) (eV) index

Thickness (nm)

A B C D E F G H

1108 956 935 945 295 264 256 272

2% 3% 5% 10% 2% 3% 5% 10%

0.74 0.84 1.23 1.54 2.05 2.34 2.67 2.96

20.3 21.7 17.7 14.8 15.4 14.7 – –

2.70 2.65 2.55 2.43 3.01 2.94 2.82 2.68

2.37 2.45 2.31 2.42 2.47 2.38 2.41 2.52

increased intensity of the (002) peak starts to diminish. Randomness due to nitrogen incorporation may explain this inhibition of crystallite growth [14,15]. It can also be seen that there is an emanation of two more orientations in the samples (E–H) deposited in the presence of helium. A potential reason for this is the process of penning ionization that occurs in plasma [13,16,17]. According to which the possibility of the oxygen molecule getting ionized is greater in the presence of helium than argon. Therefore a few extra orientations of WO3 appear when sputtering was done in helium atmosphere. In order to calculate the particle size, d of the samples Scherrer analysis is employed [12]. The calculated particle sizes are shown in Table 1. It was observed that the films deposited in He atmosphere

Fig. 2. (a) Optical transmission curve of WO3 films deposited at 400 ºC with different nitrogen doping in Ar atmosphere (b) in He atmosphere.

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Fig. 3. Bandgap values in electron volt for samples deposited in argon and helium atmosphere with varying nitrogen gas.

have smaller particle size as compared to those deposited in Ar atmosphere. This can be explained on the basis of mean free path. The mean free path of the atoms in a gas is given by [18]: λ=

−20

2:33 × 10 ðPδ2m Þ

T

where T is the temperature, P is the pressure and δm is the atomic (or molecular) diameter of the sputtering gas. Thus as the size of the

sputtering gas atoms increases the mean free path of the inert gas atoms decreases and hence the collision frequency increases. We can therefore expect that as the diameter of the gas molecule increases the sputtered tungsten atoms would also undergo multiple collisions leading to a higher probability of agglomeration and growth even before arriving at the substrate. In such a case, we would expect an increase in the particle size with increase in atomic mass of the sputtering gas [19]. EDS (Energy Dispersive Spectra) analysis shows that nitrogen concentration increases monotonously with increase in the nitrogen flow rate for all the deposited samples. The nitrogen amount in wt.% as obtained from EDS is shown in Table 1. The transmittance spectra for the samples deposited in Ar and He atmospheres with different concentrations of nitrogen are shown in Fig. 2(a) and (b) respectively. The oscillations in the spectrum with wavelength are due to interference effect. From literature it was evident that the optical transmittance of the WO3 film depends on the oxygen content of the films [13]. Since we have deposited the samples in 20% oxygen atmosphere, the average transmittance is above 80% in the visible region of the spectrum for both series of samples. Transmission data is used to obtain the refractive index of the films by using a model proposed by Manifacier et al. [20]. We have also calculated the thickness of the deposited films making use of refractive index and are given in Table 1. No significant variation in the refractive indices has been found for both the inert gases considered in this study. The absorption spectra of the nanocrystalline WO3 were also recorded as a function of the photon energy, hν, in the wavelength range 200–800 nm. The indirect optical bandgap (Eg) of nanocrystalline WO3 was determined from the absorption coefficient

Fig. 4. FE-SEM images for samples deposited in argon (a) and (b) and in helium atmosphere (c) and (d) with 2 and 10% nitrogen gas respectively.

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(α) using the Tauc relation [21]. Resulting band gap values are shown in Fig. 3. It is observed that the bandgap values of nanocrystalline films of WO3 deposited in He atmosphere are greater than that in Ar atmosphere. It can be explained with the help of XRD pattern and the FE-SEM micrographs (shown below). Both XRD and FE-SEM show that the particle size is bigger in argon than that in helium and calculated bandgap values indicate the same. It is also observed that with increase in nitrogen concentration there is a slight decrease in bandgap of the WO3 films irrespective of the inert gas used. This is due to the introduction of additional nitrogen induced states near the conduction band. This explanation is consistent with Asahi's density of state calculations for titanium dioxide, which concluded that a combination of interstitial and substitutional doping of nitrogen in TiO2 crystals introduces new states positioned energetically below the conduction band edge [7]. An analogous argument for the nitrogen doped WO3 films in this work is supported by the optical results. Specifically it was observed that the absorption edge of WO3 films in this study was affected by nitrogen doping level, indicating the introduction of new band gap states. Fig. 4(a) and (b) shows the surface morphology of the samples deposited in argon atmosphere with 2 and 10% of nitrogen respectively. Reduction in grain size is observed with increase in nitrogen concentration and is in conjunction with the particle sizes calculated from the XRD data. It is also observed that there is a uniform particle size distribution for both doping levels. Change in the surface morphology occurs with change in inert gas atmosphere as observed from Fig. 4(c) and (d) where the samples are deposited in helium with similar nitrogen concentration. Surface morphology degrades when deposited in helium environment and this can be explained on the basis of structural disorder produced due to nitrogen incorporation. But it seems a bit surprising that the incorporation of nitrogen has produced disorder in the sample structures deposited in helium atmosphere but almost negligible effect on the ones deposited in argon. The reason for this may be attributed to the differences in the atomic diameter of the reacting gases. The atomic diameter of He and Argon is 0.98 and 1.76 Å respectively while that of N2 molecule is 3.1 Å. Thus the mismatch in the atomic diameter of the He and N2 is more as compared to that of Ar and N2 and this explains why effect of nitrogen is pronounced in case of samples deposited in He. 4. Conclusions Nitrogen doping of reactively sputtered tungsten oxide demonstrates the feasibility of significant bandgap modification in the

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presence of two different inert gas environments. Change in the crystallite size by substituting helium with argon is explained on the basis of mean free path of inert gas used. Nitrogen doping for a considered pressure range was shown to have a messier surface of the samples deposited in helium atmosphere as confirmed by FE-SEM images. Since these results could have a significant impact on important applications of these nanocrystalline WO3 films, further experiments are necessary to examine in more detail the energetic positions and characteristics of the nitrogen induced states.

Acknowledgement The financial support by DST [Grant no. SR/S5/NM–32/2005], New Delhi is gratefully acknowledged.

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