On the Bonding and Electrochemical Performance of Sputter Deposited WO3 Thin Films

Tungsten oxide (WO3) thin films have attained a distinct and special place in the field of electrochromism. Therefore, any sort of study towards this material will invariably has lot of importance and technological significance. The films of WO3 were deposited by reactive magnetron sputtering at various partial pressures and annealed at 475°C. There is a tremendous change in the bonding characteristics, which eventually shows significant change in the optical and electrochemical behavior. The bandgap of WO3 films is found to be increasing with different oxygen partial pressure. A systematic study of cyclic voltammetry has been done to analyze the electrochemical behavior of WO3 films. Oxidation and reduction peak currents have shown an increasing trend with the oxygen partial pressure. Raman spectroscopy has revealed the improvement in the atomic ordering in WO3 films.


Introduction
Tungsten oxide (WO 3 ) thin films acquired great technological significance due to its best electrochromic performance [1]. WO 3 is n-type semiconductor, which has colour memory and shows high coloration efficiency. In the oxidized state it is in W6+ state appearing colour less, and converts to deep blue colour in reduced state i.e W5+ state. Briefly, when metal ions M + (M = H, Li, Na, or K) intercalate into the WO 3 lattice structure the colour of the film changes to dark blue. While de-intercalating of these metal ions from the WO 3 lattice, the film turns out to transparent or colour less. The general equation which explains this is as follows.
Thus, the optical property of WO 3 films transforms from transparent to an absorbing nature due to the filling of t2 band of perovskite structure by the excess electrons [2].
There are different models to explain the coloured and bleached behaviour of these films. One such model says, a polaron [3]for which electrons got trapped at tungsten sites when injected into WO 3 lattice to form W5 + centres. A small poloran would be formed from the perturbation of surrounding lattice. It is also evident that there will be an optical transfer of the polaron from a W5+ site to a close  [4,5]. The conduction there is explained by a free electron model of Drude type. Furthermore, it is not obvious whether a unified description can be applied to the large variety of materials produced by methods as different as thermal evaporation, radiofrequency reactive sputtering, anodic oxidation, etc.
Different methods are used to prepare WO 3 films such as thermal evaporation [6], anodic oxidation [7], spray pyrolysis [8], sol-gel [9], and molecular beam deposition [6] and sputtering [10]. Among these, sputtering has the advantage getting highly dense films, high directional deposition and smooth films also large area deposition [11]. 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 WO 3 were deposited by RF magnetron sputtering of metallic tungsten target under various oxygen partial pressures. The influence of oxygen partial pressure on the structural, surface morphology, optical properties and electrochemical properties was systematically studied and the results were reported.

Experimental details
The WO 3 thin films were deposited on Corning glass and FTO coated glass substrates. Corning glass and FTO coated glass substrates cleaned ultrasonically using soap solution followed by DI water and rinse and finally dried using nitrogen gas. Before deposition again cleaned with acetone and placed in vacuum chamber. Vacuum sputtering chamber with an ultimate low pressure of 1.0× 10 -6 mbar is achieved using 1000LPS Turbo molecular pump backed by a1000 LPM rotary pump. Tungstentrioxide thin films are prepared using high pure tungsten target of 3 inch diameter and 3mm thick was sputtered in Oxygen (O)-Argon (Ar) ambient atmosphere. Substrates were kept at a distance of9 cm from the target during the deposition. Before every deposition, pre-sputtering was performed in Argon environment for 15 min to remove the adsorbed contaminants and the native oxide from the surface of the tungsten target. The films were prepared on the substrates held at room temperature at different oxygen partial pressures in the ranges 2×10 −4 to 1×10 −3 mbar. The RF power fed to the sputter target was 40W using RF power source. Deposition conditions maintained during the preparation ofWO 3 films are given in Table 1.  Figure 1 shows the Raman spectrum of films deposited at various partial pressures.

Figure 1. Raman spectra of WO 3 films deposited at various oxygen partial pressures.
From the figure it is evident that with the oxygen partial pressure there is an improvement in the sharpness of the characteristic peaks, which infers increase in the atomic ordering in the WO 3 lattice. Figure 2 shows the optical transmittance of the films deposited at various partial pressures. From the figure it is evident that, the transparency has been increasing with the partial pressure.  Optical absorption coefficient (α) of prepared thin films are found using below equation αhυ = (hυ − Eg) n n=1/2 for direct allowed transitions and 2 for indirect transitions, 'α' is the absorption coefficient, 'E g ' is optical bandgap and 'hυ' is the incident photon energy. From the Tauc-plots with n=1/2, we found the variation in the optical bandgap of WO 3 films by taking a straight line fit of (αhυ) 2 on to the energy axis. Figure 3 shows the WO3 bandgap variation with different oxygen partial pressures.         Figure 9. CV plots at different scan rates of WO 3 films deposited at pO 2 =1x10 -3 mbar.

Optical properties:
The peak current values during oxidation and reduction of these WO 3 films during CV measurements is shown in Figure 10.