Structural and optical characterization of WO3 deposited on glass and ITO

doi:10.1016/S0042-207X(01)00300-1

Vacuum

Volume 64, Issues 3–4, January 2002, Pages 287-291

https://doi.org/10.1016/S0042-207X(01)00300-1 Get rights and content

Abstract

Electrochromic materials exhibit variable and reversible optical properties under the action of voltage pulses. The interest in these materials has increased in the last few years due to their potential application in a wide variety of optical modulation devices. Tungsten oxide (WO₃) is typical in such devices. In this work, we present a study of the structural and optical properties of tungsten oxide films deposited on glass and indium tin oxide. The films were produced by reactive dc magnetron sputtering at different temperatures (room temperature and 200°C) and bias voltage (−60 to +60 V). The sputtering atmosphere was composed of an Ar+O₂ mixture so that sample could be deposited with different oxygen partial pressure (0.2⩽ p(O₂)⩽0.8). Spectral transmission in the visible and near infrared was measured using a double-beam spectrophotometer. X-ray diffraction was used in order to characterize the film structure. The surface microtopography was analyzed by atomic force microscopy (AFM). XRD results show that for low p(O₂) the films present an amorphous WO₃ phase, while for high oxygen partial pressure they present a mixture of a more crystalline WO₃ phase and a W₂₀O₅₈ phase. The structure change is for p(O₂)=0.5–0.6. A corresponding minimum was observed in the transmission and in the optical band gap, at these oxygen partial pressures. These results show that the optical properties of the films are dependent on the presence of oxygen deficient regions on the films. This was also observed in the samples deposited with an applied bias voltage. Microstructure (AFM) and X-ray measurements in these samples show that electron bombardment (positive bias) favors crystallinity and ion bombardment (negative bias) favors amorphization of the tungsten oxide phases present on the films.

Introduction

Electrochromic thin films are of considerable technological interest due to their potential application on “smart-windows” [1], [2], [3], automobile mirrors [4], low-refractive materials in filters [5] and non-emissive displays. Electrochromism can be described by a reversible change in transmittance and/or reflectance, caused by an applied external voltage [6], [7]. To observe this change, it is necessary to incorporate the electrochromic layer in an electrochromic device [8]. The device consists of an electrochromic and ion storage layer separated by an ion-conducting layer [8]. Two external transparent conducting layers are used to apply the voltage. Tungsten oxide is one of the most studied electrochromic material [9], [10] and it can be prepared by RF sputtering [11], CVD [12], sol–gel [10], thermal evaporation [13] and reactive direct-current (DC) magnetron sputtering [14]. Depending on the deposition conditions and techniques films may present considerably different structural, optical and electrical behaviour, and consequently different electrochromic behaviour [3], [4]. With DC sputtering from a compressed target, thin film properties can be improved by controlling the reactive gas atmosphere. In this work, we present a study of the structural and optical properties of WO₃ films deposited by reactive magnetron sputtering on glass and indium tin oxide (ITO) coated glass. The deposition was performed at different oxygen partial pressures, both at room temperature and 200°C. The effect of applying a bias voltage to the ITO layer during WO₃ deposition was also studied.

Section snippets

Experimental

Tungsten oxide films were prepared by DC reactive magnetron sputtering on microscope slide glass and ITO coated glass (Delta Technologies, Ltd), both at room temperature and 200°C. The target was composed of a compressed tungsten powder with 99.95% purity. Sputtering was done with an atmosphere of Ar+O₂ gas mixture that was kept at a 1.3×10⁻² mbar total work pressure in all cases. However, the partial pressure of oxygen (p(O₂)=P(O₂)/[P(O₂)+P(Ar)]) was varied between 0.2 and 0.8 in our study. The

Results and discussion

Figs. 1a and b show typical X-ray diffraction spectra measured on the tungsten oxide films. It is observed that films deposited with low oxygen partial pressure (p(O₂)⩽0.5), at room temperature and 200°C, present an amorphous WO₃ phase with peaks near 25.96 (0 0 1), 35.02 (0 2 1) and 53.80 (0 0 2). However, as p(O₂) increases the structure of the films start to change so that for p(O₂)⩾0.5 the films present a mixture of a more crystalline WO₃ phase and a W₂₀O₅₈. To determine the oxygen concentration

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WO₃ is grown by hot-filament metal oxide deposition (HFMOD) technique under atmospheric pressure and an oxygen atmosphere. By X-ray diffraction obtains that WO₃ presents mainly monoclinic crystalline phase. The chemical stoichiometry is obtained by X-ray Photoelectron Spectroscopy (XPS). The IR spectrum of the as-grown WO₃ presents broad peaks in the range of 1100 to 3600 cm⁻¹. A broad band in the 2200 to 3600 cm⁻¹ region and the peaks sited at 1645 and 1432 cm⁻¹ are well resolved, which are originated from moisture and are assigned to ν(OH) and δ(OH) modes of adsorbed water and the corresponding tungsten oxide vibrations are in infrared region from 400 to 1453 cm⁻¹ and around 3492 cm⁻¹, which correspond to tungsten–oxygen (W–O) stretching, bending and lattice modes. The Raman spectrum shows intense peaks at 801, 710, 262 and 61 cm⁻¹ that are typical Raman peaks of crystalline WO₃ (m-phase) that correspond to stretching vibrations of the bridging oxygen, which are assigned to W–O stretching (ν) and W–O bending (δ) modes, respectively. By transmittance measurements obtains that the WO₃ band gap can be varied from 2.92 to 3.13 eV in the investigated annealing temperature range.
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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.

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Structural and optical characterization of WO3 deposited on glass and ITO

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Thin Solid Films

Thin Solid Films

Thin Solid Films

Thin Solid Films

Thin Solid Films

Solid State Ion

Thin Solid Films

Thin Solid Films

Structural and optical characterization of WO₃ deposited on glass and ITO