Structural, Optical, and Luminescence Properties of Reactive Magnetron Sputtered Tungsten Oxide Thin Films

Abstract

Tungsten oxide (WO₃) thin films were deposited on to unheated Corning glass and silicon substrates by RF magnetron sputtering of metallic tungsten target at various oxygen partial pressures in the range 4×10⁻²–1×10⁻¹ Pa. The influence of oxygen partial pressure on the structure and surface morphology and the optical and photoluminescence properties of the films were investigated. X-ray diffraction studies revealed that the deposited films were amorphous in nature. Fourier transform infrared transmission spectra confirmed that the presence of stretching vibration of W-O-W and deformation of W-O bonds related to the WO₃. The optical transmittance of the films at wavelengths >500 nm increased from 62% to 85% with the increase of oxygen partial pressure. The optical band gap of the films increased from 3.00 to 3.14 eV and the refractive index of the films decreased from 2.26 to 2.08 with the increase of oxygen partial pressure from 4×10⁻² to 1×10⁻¹ Pa, respectively. The photoluminescence studies indicated that the intense blue emission which was assigned to band-to-band transition was observed at oxygen partial pressure of 6×10⁻² Pa.

1. Introduction

Transition metal oxide thin films have attractive technological importance for electrochromic devices. Among the transition metal oxides, tungsten oxide (WO₃) is an n-type semiconductor, exhibits electrochromic properties in the visible and infrared regions, is relatively low cost, and has high color efficiency [1]. It possesses excellent coloration efficiency and electrochemical stability after insertion with protons or other small monovalent cations such as Li⁺ and Ni⁺ [2]. Hussain et al. [3] reported on pulsed laser deposited WO₃ films, formed at substrate temperature 573 K, that the crystallinity of the triclinic WO₃ decreased with the increase of oxygen partial pressure from 10 to 26 Pa, and at higher oxygen partial pressure of 40 Pa the films were almost in amorphous structure. Kaushal and Kaur [4] achieved nanocrystalline WO₃ films by pulsed DC magnetron sputtering at substrate temperature of 873 K with the optical band gap of 3.39 eV and the crystallite size, and optical band gap of the films increased with the increase of oxygen partial pressure. Bittencourt et al. [5, 6] reported on the WO₃ thin films formed by RF magnetron sputtering at different argon: oxygen ratios on the structural and composition of the films and achieved the stoichiometric films at 1 : 1 of argon and oxygen ratio. Subramanyam and Karuppasamy [7] formed the WO₃ films at room temperature by pulsed DC magnetron sputtering in active arc suspension mode at different oxygen partial pressures in the range 1.5–5.2 Pa. The films formed at high oxygen partial pressure of 5.2 Pa exhibited the refractive index of 2.1 and optical band gap of 3.12 eV and the refractive index decreased with the increase of oxygen partial pressure.

Different deposition methods have been used to prepare WO₃ films such as the chemical and physical deposition processes like electrospinning [8], spray pyrolysis [9], electrodeposition [10], thermal evaporation [11], plasma assisted evaporation [12], and DC and RF magnetron sputtering techniques [6, 7, 13]. Among these, magnetron sputtering has the advantage to deposit uniform films on large area substrates. The metal oxide films can be deposited from metallic target in the presence of reactive gas of oxygen in the process of reactive sputtering. In this reactive magnetron sputtering technique, magnetron cathode has the advantage in the growth of uniform films at low applied voltages compared to diode sputtering. The physical properties of the magnetron sputtered films mainly depend on the deposition parameters such as oxygen partial pressure, substrate temperature, and sputter power. In the present investigation, thin films of WO₃ were formed by RF magnetron sputtering of metallic tungsten target under various oxygen partial pressures. The influence of oxygen partial pressure on the compositional, structure, and surface morphology and optical and photoluminescence properties was systematically studied and the results were reported.

2. Experimentation

The WO₃ thin films were deposited on Corning glass and Si substrates were maintained at room temperature by RF magnetron sputtering of (99.99% pure) tungsten target of (50 mm diameter) under different oxygen partial pressures. The vacuum pumping system was employed for sputter deposition using a combination of diffusion and rotary pumps to achieve an ultimate pressure of 4×10⁻⁴ Pa. Pure oxygen and argon were used as reactive and sputter gases, respectively. Required quantities of oxygen and argon gases were admitted into the sputter chamber through fine controlled needle valves and their flow rates were monitored individually employing Aalborg mass flow controllers. Digital Pirani and Penning gauges were used to measure the pressure in the sputter chamber. The films were prepared on the substrates held at room temperature at different oxygen partial pressures in the ranges 4×10⁻²–1×10⁻¹ Pa. The RF power fed to the sputter target was 150 W using Advance Energy RF power source and the deposition time was 120 min. Deposition conditions maintained during the preparation of WO₃ films are given in Table 1.

Table 1 

Deposition conditions for preparation of WO₃ films.

The thickness of the films deposited on Corning glass substrates was measured by using 𝛼-step profilometer. Seifert X-ray diffraction (model 3003TT) with Cu Kα radiation source (𝜆=0.15406 nm) was used to determine the crystallographic structure of the films. The chemical composition of the films formed on glass substrates was analyzed with energy dispersive X-ray analysis (Philips XL 308) attaching to scanning electron microscope. The chemical bonding configuration of the films formed on silicon substrates was obtained from the Fourier transform infrared spectrophotometer (Nicolet model 5700 FTIR). The surface morphology of the films was carried out by using atomic force microscope (Digital Instrument 3100 series). The optical transmittance and reflectance of the films formed on glass substrates were recorded by using UV-Vis-NIR (Perkin Elmer model Lambda 950) double beam spectrophotometer in the wavelength range 300–1000 nm. Photoluminescence spectra of the films were obtained using a Hitachi U-4700 spectrophotometer under excitation wavelength of 375 nm.

3. Results and Discussions

In order to study the influence of oxygen partial pressure on the physical properties, tungsten oxide films were deposited under various oxygen partial pressures in the range 4×10⁻²–1×10⁻¹ Pa and maintained other deposition parameters as constant. The measured thickness of the films was in the range 313–398 nm. The variation of deposition rate with oxygen partial pressure of the WO₃ films is shown in Figure 1. It is seen that the deposition rate was decreased with the increase of oxygen partial pressure. The deposition rate of the films formed at low oxygen partial pressure of 4×10⁻² Pa was 3.44 nm/min and it decreased to 2.62 nm/min with the increase of oxygen partial pressure to 1×10⁻¹ Pa. The high deposition rate at low oxygen partial pressure was due to the high sputtering yield of metallic tungsten and availability of insufficient oxygen for reaction with tungsten. Sun et al. [14] also noted the deposition rate of 4 nm/min in pulsed DC magnetron sputtered WO₃ films formed with sputter power of 100 W. The decrease of deposition rate at higher oxygen partial pressures was due to availability of sufficient oxygen to react with the tungsten ions and also it leads to formation of oxide layer on the surface of the target which generally reduces the deposition rate [15].

Figure 1 

Variation of deposition rate of WO3 films with oxygen partial pressure.

3.1. Chemical Composition

The chemical composition of the films deposited on glass substrate was analyzed with energy dispersive X-ray analysis. The atomic ratio of oxygen to tungsten was at low value of 2.66 at low oxygen partial pressure of 4×10⁻² Pa due to the growth of tungsten along with tungsten oxide in the films. A representative EDAX spectrum of the films formed at oxygen partial pressure of 6×10⁻² Pa is shown in Figure 2. For the films formed at oxygen partial pressure of 6×10⁻² Pa, the atomic ratio of oxygen to tungsten was 2.98 and was almost constant at higher oxygen partial pressures. It indicated that the films formed at oxygen partial pressure of 6×10⁻² Pa were of nearly stoichiometric WO₃.

Figure 2 

EDAX spectrum of WO3 films formed at oxygen partial pressure of 6×10−2 Pa.

3.2. Structural Properties

Figure 3 shows the X-ray diffraction profiles of the films formed at different oxygen partial pressures. The broad diffraction pattern of the films formed at different oxygen partial pressures indicated that the films were amorphous in nature. Mohamed et al. [16] reported that amorphous WO₃ films formed on unheated glass substrates at oxygen partial pressure >5.7×10⁻³ Pa by DC magnetron sputtering.

Figure 3 

XRD profiles of WO3 films formed at different oxygen partial pressures.

Fourier transform infrared spectra of WO₃ films formed at different oxygen partial pressures in the wavenumber range 400–1500 cm⁻¹ are shown in Figure 4. The films formed at low oxygen partial pressure of 4×10⁻² Pa showed a broad absorption band in between 600 and 1000 cm⁻¹. The broadness of the absorption band suggested that the deposited films were amorphous in nature. When the oxygen partial pressure increased to 6×10⁻² Pa, the broad absorption band centered at 780 cm⁻¹ and a shoulder at about 590 cm⁻¹ was observed. The absorption band observed at 780 cm⁻¹ attributed to the O-W-O stretching mode of tungsten oxide [17]. The band seen at 590 cm⁻¹ related to the deformation mode of W-O in WO₃ [18]. In addition, an absorption band seen at 1150 cm⁻¹ was due to the presence of Si–O band of the silicon oxide present on the silicon substrate and the peaks observed at 1338 and 1430 cm⁻¹ were attributed to the weakly bounded water due to exposure of the films to the ambient air. The presence of such broad bands was also noticed by the Rougier et al. [19] in pulsed laser deposited WO₃ films formed at different oxygen partial pressures.

Figure 4 

FTIR spectra of WO3 films formed at different oxygen partial pressures.

3.3. Surface Morphology

Figure 5 shows three- and two-dimensional atomic force microscope images of WO₃ films deposited at different oxygen partial pressures. The surface of the films deposited at lower oxygen partial pressure of 4×10⁻² Pa (Figure 5(a)) was smooth with small size grains and surface roughness of 2.26 nm. The films formed at oxygen partial pressure of 6×10⁻² Pa (Figure 5(b)) showed the uniformly distributed grains on the surface. In a further increase of oxygen partial pressure to 1×10⁻¹ Pa, the films exhibited pyramidal shape grains as shown in Figure 5(c). The average grain size of the films increased from 93 to 252 nm and the surface roughness of the films decreased from 2.26 nm to 1.57 nm with the increase of oxygen partial pressure from 4×10⁻² to 1×10⁻¹ Pa, respectively. The morphological evolution of growing surface with oxygen partial pressure was greatly affected due to variation in the energy of impinging particles and the rate of impingement and the mobility of the arrived particles onto the surface of the substrates.

(a)

(b)

(c)

Figure 5 

AFM images of WO3 films formed at oxygen partial pressures: (a) 4×10−2 Pa, (b) 6×10−2 Pa, and (c) 1×10−1 Pa.

3.4. Optical Properties

The optical properties of the films were found to be strongly influenced by the oxygen partial pressure. The optical transmittance spectra of the WO₃ films formed at different oxygen partial pressures are shown in Figure 6. The optical transmittance of the films at wavelengths ≥500 nm increased from 60% to 82% with the increase of oxygen partial pressure from 4×10⁻² to 1×10⁻¹ Pa, respectively. At lower oxygen partial pressure, high energy sputtered species were traveled through plasma and large number of tungsten ions reached at the substrate due to heavier mass of the tungsten compared to oxygen atoms. Due to this reason, the films formed at low oxygen partial pressure exhibited lower optical transmittance of 60%. With the increase of oxygen partial pressure of 6×10⁻² Pa in the sputtering chamber, both the oxygen and tungsten will effectively react on the substrate surface and form nearly stoichiometric films, hence the optical transmittance increased to 70%. At higher oxygen partial pressure of 1×10⁻¹ Pa, the excess of oxygen atoms was reached the substrate, hence the films showed high transmittance of 82% [20]. Kaushal and Kaur [4] reported such an increase in the optical transmittance of WO₃ films with the increase of oxygen partial pressure formed by pulsed laser deposition. Lethy et al. [21] also reported on the pulsed laser deposition films that the optical transmittance increased from 40% to 80% in WO₃ films with the increase of oxygen partial pressures from 0.006 to 0.12 mbar which was correlated with the charge transport of the sputter species present in the sputter chamber. The sharp absorption edge was observed in the wavelength region 450–500 nm and the absorption edge shifted towards lower wavelengths side with the increase of oxygen partial pressure. The optical reflectance of the films formed at different oxygen partial pressures is shown in Figure 7. The optical absorption coefficient (𝛼) of the films was calculated from the optical transmittance (𝑇) and reflectance (𝑅) data using the following relation: 1𝛼=−𝑡𝑇ln(1−𝑅)², (1) where 𝑡 is the film thickness. The optical absorption in the films was fitted to the following relation: (𝛼ℎ𝜈)²=𝐴ℎ𝜈−𝐸𝑔, (2) assuming that the direct transitions take place from the top of the valance band and bottom of the conduction band. Figure 8 shows the plots of (𝛼ℎ𝜈)² versus photon energy (ℎ𝜈) of the films formed at different oxygen partial pressures. The extrapolation of linear portion of the plots to 𝛼=0 leads to the optical band gap. The optical band gap of the WO₃ films increased from 3.00 to 3.14 eV with the increase of oxygen partial pressure from 4×10⁻² to 1×10⁻¹ Pa, respectively. The increase of optical band gap with the increase of oxygen partial pressure was due to reduction in the oxygen vacancies. The achieved optical band gap values were in good agreement with the DC magnetron sputtering tungsten oxide films, where the optical band gap increased from 2.97 to 3.14 eV with the increase of oxygen partial pressure from 1.5 to 5.2 Pa [7]. Chawla et al. [22] reported that the optical band gap of the RF sputtered films increased from 2.76 to 3.07 eV with increase of oxygen content from 10% to 40% in sputter chamber by RF magnetron sputtering. Mohamed et al. [16] reported that the optical band gap of the films increased from 2.90 to 3.18 eV with the increase of oxygen partial from 0.20 to 0.52 Pa. The increased of optical band gap with oxygen partial pressure can be explained due to the decrease in the defect centers by improving the stoichiometry of the films. The difference in the optical band gap values was due to process parameters maintained for the growth of the films which were different for different deposition methods.

Figure 6 

Wavelength dependence of optical transmittance of WO3 films.

Figure 7 

Wavelength dependence of reflectance spectra of WO3 films.

Figure 8 

Plots of (𝛼ℎ𝜈)2 versus photon energy (ℎ𝜈) of WO3 films.

The refractive index (𝑛) of the films was determined from the optical transmittance interference data employing Swanepoel’s envelope method [23] using the following relation: 𝑛(𝜆)=𝑁+𝑁²-𝑛²₀𝑛²₁^1/21/2,𝑁=2𝑛₀𝑛₁𝑇_max−𝑇_min𝑇_max𝑇_min+𝑛²₀+𝑛²₁2, (3) where 𝑛_𝑜 and 𝑛₁ are the refractive indices of air and substrate and 𝑇_max and 𝑇_min are the optical transmittance maximum and minimum, respectively.

Figure 9 shows the wavelength dependence of refractive index of WO₃ films formed at different oxygen partial pressures. For all the films, the refractive index was decreased with the increase of wavelength. At a wavelength of 550 nm, the refractive index of the WO₃ films decreased from 2.26 to 2.08 with the increase of oxygen partial pressure from 4×10⁻² to 1×10⁻¹ Pa. The decrease in the refractive index at higher oxygen partial pressures was due to the decrease in the packing density of the films. In the literature, Mohamed et al. [16] reported that refractive indices at wavelength of 550 nm of the films decreased from 2.16 to 2.08 with the increase of oxygen partial pressure from 2×10⁻² to 5.2×10⁻² Pa in DC magnetron sputtered WO₃ films. Baloukas et al. [24] also observed the decrease in the refractive index from 2.21 to 1.97 with the increase of oxygen partial pressure from 1×10⁻¹ to 8×10⁻¹ Pa in the films formed by RF magnetron sputtering technique.

Figure 9 

Variation of refractive index with wavelength of WO3 films.

3.5. Photoluminescence

Figure 10 shows the photoluminescence spectra of WO₃ films formed at oxygen partial pressures of 4×10⁻² and 6×10⁻² Pa. The films deposited at low oxygen partial pressure of 4×10⁻² Pa exhibited two emission bands centered at about 410 and 440 nm. The band seen at 410 nm was attributed to near ultraviolet emission. It was due to the presence of oxygen vacancies. Park et al. [25] reported that the occurred band was attributed to the localized state of oxygen vacancies in WO_3-x needle-like structure. A weak shoulder peak seen at 440 nm was the bluish green emission due to the presence of defects in the films. At oxygen partial pressure of 6×10⁻² Pa, the peak shifted to 465 nm attributed to blue emission due to the reduction of oxygen vacancies. Niederberger et al. [26] and Luo et al. [27] suggested that the blue emission was attributed to the band-to-band transition. The possible reason that sufficient oxygen is available in the sputter chamber leads to formation of stoichiometric WO₃ films as explained in EDAX analysis and the films have O/W ratio of 2.98 at oxygen partial pressure of 6×10⁻² Pa.

Figure 10 

Photoluminescence spectra of WO3 films formed at oxygen partial pressures: (a) 4×10−2 Pa and (b) 6×10−2 Pa.

4. Conclusions

WO₃ thin films were deposited on Corning glass and silicon substrates held at room temperature by RF magnetron sputtering of tungsten target at different oxygen partial pressures in the range 4×10⁻²–1×10⁻¹ Pa. The effect of oxygen partial pressure on the structure and surface morphology and the optical properties of the films were systematically investigated. The deposition rate of the films formed at low oxygen partial pressure of 4×10⁻² Pa was 3.30 nm/min and decreased to 2.62 nm/min with increase of oxygen partial pressure to 1×10⁻¹ Pa. EDAX analysis confirmed that the nearly stoichiometric films were obtained at an oxygen partial pressure of 6×10⁻² Pa. X-ray diffraction studies revealed that the films were amorphous in nature. A broad hump in FTIR spectrum in the wavenumber range 500–1000 cm⁻¹ was due to the stretching vibration of O-W-O observed in the films formed at higher oxygen partial pressures. The optical band gap of the films increased from 3.00 to 3.14 eV and refractive index decreased from 2.26 to 2.08 with the increase of oxygen partial pressure from 4×10⁻² to 1×10⁻¹ Pa, respectively. Blue emission was observed for the films deposited at an oxygen partial pressure of 6×10⁻² Pa. The amorphous WO₃ films formed at oxygen partial pressure of 6×10⁻² Pa with optical band gap of 3.06 eV will be quite useful for electrochromic device applications.

Acknowledgment

One of the authors, V. Madhavi, is thankful to the University Grants Commission, New Delhi, India, for providing Junior Research Fellowship through sanctioned UGC-RFSMS during the course of the work.