Electrochromic and optical properties of tungsten oxide films deposited with DC sputtering by introducing hydrogen Hsi-Chao Chen,1,2,3,* Der-Jun Jan,4 Yu-Siang Luo,1 and Kuo-Ting Huang2 1 2
Graduate School of Optoelectronics, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan
Graduate School of Science and Technology, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan 3
Department of Electronic Engineering, National Yunlin University of Science and Technology Yunlin 64002, Taiwan 4
Physics Division, the Institute of Nuclear Energy Research, Taoyuan County 32546, Taiwan *Corresponding author: [email protected] Received 27 August 2013; accepted 10 December 2013; posted 24 December 2013 (Doc. ID 196544); published 20 January 2014
Research was undertaken to investigate the electrochromic and optical properties of tungsten oxide (WO3 ) films deposited by introducing hydrogen with a direct current (DC) and pulsed DC sputtering. The results show that WO3 films have optimum electrochromic properties at a hydrogen flow of 4 and 3 sccm for DC and pulsed DC, respectively. In the Raman spectra, the peak intensity increased with the increase of hydrogen flow at both 770 cm1 and 950 cm−1 peaks, which resulted in bonds of W6 -O and W6 O, respectively. Simultaneously, the transmittance (ΔT 550 nm ) variations were 65.6% and 64.4%, and the average transmittance (ΔT 400–500 nm ) variations were 56.7% and 56.4% for DC and pulsed DC, respectively. The bleached/colored ability of the cyclic voltammograms (CVs) was DC > pulsed DC, and the resistances of AC impedance were pulsed DC > DC. © 2014 Optical Society of America OCIS codes: (310.0310) Thin films; (310.1860) Deposition and fabrication; (310.6860) Thin films, optical properties; (310.6870) Thin films, other properties. http://dx.doi.org/10.1364/AO.53.00A321
1. Introduction
Electrochromic materials and devices (ECDs), which are capable of optical property modulation under an applied voltage, generally work in the visible and nearinfrared regions [1,2]. Such ECDs include adjustablelight rear mirrors, smart energy-saving windows, sunglasses, head vision displays, and so on. The main reason for smart energy-saving windows is that electrochromic materials are able to switch between two optical states (bleaching/coloring) in response to an applied voltage. Tungsten oxide (WO3 ) thin films have drawn the attention of many scientists due to their good electrochromic activity, fast switching response, 1559-128X/14/04A321-09$15.00/0 © 2014 Optical Society of America
high coloration efficiency, and high chemical stability [3–5]. Their electrochromic properties, such as coloration efficiency (CE), cyclic durability, and kinetics of coloration, strongly depend on inherent structural, morphological, and compositional characteristics [6]. Several techniques have been applied to improve the fabrication of WO3 thin films, including chemical vapor deposition (CVD) [7], sputtering [8,9], sol–gel [10,11] and electro-deposition [12]. WO3 film is used as the main electrochromic material, which allows Li to easily pass in and out of the active layer. As such, a suitable film packing density is fabricated by the appropriate sputtering source and process. Hence, research was undertaken to investigate the electrochromic and optical properties of WO3 thin films by introducing hydrogen gas for the DC and pulsed DC plasma power. Results 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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show that WO3 thin films have good electrochromic properties with regard to the oxygen/argon ratio of 0.7, and the hydrogen flow was 4 and 3 sccm for DC and pulsed DC, respectively. The transmittance of all films was over 75% at as-deposited, and the deposition rates were all between 0.65 Ås−1 and 0.95 Ås−1 . Simultaneously, the transmittance variations (ΔT) were 65.6% and 64.4% at a wavelength of 550 nm, and the average transmittance variations were 56.7% and 56.4% from 400 to 500 nm for DC and pulsed DC, respectively. The bleached/colored ability of cyclic voltammograms (CVs) was DC > pulsed DC, and the resistances of AC impedance were pulsed DC > DC. The coloration ability and CE of WO3 thin film deposited with a plasma source of DC was better than pulsed DC with the introduction of hydrogen. 2. Experiment A.
Depositions of WO3 Thin Films
WO3 thin films were deposited at room temperature onto BK7 glass, indium tin oxide (ITO) glass, and a single crystalline p type silicon wafer using DC and pulsed DC magnetron sputtering, respectively. The system used horizontal deposition for the ion source, as shown in Fig. 1. The distance between the target and substrate was 11 cm in the chamber. The frequency of pulsed DC was 70 kHz, and a WO3 target of 99.95% purity with a diameter of 300 was used as the starting material. It is necessary to sputter the target for 10 minutes before starting the deposition to clean the oxide layer on the surface. The sputtering chamber was evacuated to 1.0 × 10−6 torr using a rotary pump and a turbo pump before introducing gases. The hydrogen flow rate was changed from 0 to 5 sccm. Simultaneously, the argon and oxygen flow rates were kept constant at 60 and 42 sccm, respectively. Table 1 is the list of deposition conditions for WO3 films. B. Measurements of Optical, Bond, and Electronic Properties
The refractive index, extinction coefficient, and film thickness of WO3 thin films were measured by a
Table 1.
Sample No. 1 2 3 4 5 6 7 8 9 10 11 12
List of Deposition Conditions for WO3 Films
Power Type
100 W DC
100 W pulsed DC
Frequency (KHz)
O2 ∕Ar Ratio
H2 (sccm)
42∕60
0 1 2 3 4 5 0 1 2 3 4 5
—
70
spectrometer (FilmTek-2000), such that film thickness accuracy could reach 1.5 Å for NIST-traceable standard oxide 1000 Å to 1 μm, and the film thickness was double-checked by an Alpha-Step Meter (Surface Profile Alpha-Step200). The Raman (RENISHAW 1000B) spectra were taken in a quasi-backscattering geometry using 100 mW at the 514.5 nm line of an Ar ion laser, focused on a square measuring 5 mm × 5 mm, as the excitation source. A threeelectrode cell was used for the measurement of static potential, and an Ag/AgCl gate was the reference electrode; the platinum sheet was a counter electrode. Lastly, AC impedance (Princeton Applied Research Versa STAT4) was used to measure the AC impedance by two non-blocking electrodes at both ends of the electrolyte and film sample. The analysis of electrical properties of the complex multi-layer film could be obtained by an impedance diagram. Hence, the AC impedance of the ionic motion could help us to understand the situation of the coloring/ bleaching for the WO3 film deposited by introducing hydrogen. C.
Electrochromatism of WO3 Thin Films
Electrochromatism of the WO3 thin films resulted from the injection and extraction of an electron and a proton by metal ions [13], as shown in Fig. 2, which is given by WO3 xe−1 xM ↔Mx WO3 ;
Fig. 1. Schematic drawing of magnetron sputtering with DC and pulsed DC plasma sources. A322
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(1)
where M was standard for the protons, such as H, Li , Na , or K . 1 M LiClO4 was used as the electrolyte in the propylene carbonate, and the solution was prepared from dried products and stored over molecular sieves. The potentials, which were used for coloring and bleaching, were −1.5 and 1.5 V, respectively. The transmittance spectra of the semi-finished cells, colored and bleached, were measured by an ultraviolet-visible spectrometer (Solid Space-3700). The variation in optical density (ΔOD) of the WO3 thin films at a dominative wavelength of 550 nm and 400–500 nm for the colored and bleached state, was calculated by the following relationship:
Fig. 2. Schematic drawing of an electrochromic device.
T bleaching ΔOD log Acoloring − Ableaching ; T coloring
Fig. 3. Sputtering rate of WO3 films deposited by different hydrogen flows and plasma sources.
(2)
where T bleaching and T coloring are the transmittance of the WO3 thin films in the bleached and colored states, respectively. CE (η) is introduced to describe the efficiency of the change of these charges in inducing absorption in an electrochromic device. The CE is defined as the variation in optical density (ΔOD) per unit of inserted charges, and is calculated by the following equation: CE η
ΔOD ; Q
(3)
where ΔOD is the optical density and Q is the density of charges (Q q∕A). Then, the charge is equal to the current multiplied by time. Hence, the condition of the experiment was that the current (I) was kept at 1 mA, the time (t) needed was 60 s, and the active area of WO3 thin film was 3.0 cm × 3.0 cm. By calculation, therefore, the density of charge (Q) was 0.02 C∕cm2. To evaluate the electrochromic properties of WO3 film deposition using different plasma sources, CVs were conducted in a 1 M LiClO4 electrolyte solution between −1.5 V and 1.5 V (versus Ag/AgCl) with a scan rate of 40 mV s−1 sweeping back and forth.
Hence, plasma density and deposition rates were slightly reduced by increasing the hydrogen flow, unlike the rapid reduction caused by increasing oxygen [14]. The sputtering rate has the maximum variation with regard to the DC plasma source, where the hydrogen flow changes from 0 to 5 sccm; therefore, the pulsed DC plasma had the lowest sputtering rate [15]. B. Optical Properties
The optical properties of the refractive index (n) are exhibited in Fig. 4 with different hydrogen flows and plasma sources at a wavelength of 550 nm. The DC power proved simple with regard to tapping out ions from the target, but it was also easy to lose the oxygen ion during deposition. The deposition rate of DC was faster than the pulsed DC power described in Section 3.A. The refractive index of the WO3 thin film was between 2.1 and 2.2 with the optimum parameters of the coloring range. However, when the O2 ∕Ar ratio was 0.7, the refractive index of the WO3 thin film was between 2.10 and 2.24, with the increase in the flow rate of hydrogen in this experiment.
3. Result and Discussion A.
Deposition Rate
Figure 3 shows the sputtering rates for different hydrogen flows in the deposited film thicknesses, being controlled at 300 nm, for DC and pulsed DC plasma sources. The deposition rates decreased a little with the increase of hydrogen flow for all films. This phenomenon resulted from the ionization energy of oxygen (48.76 eV) being higher than that of argon (15.76 eV) and hydrogen (13.6 eV), and the dissociation rate of oxygen being lower than that of argon and hydrogen. Although the O2 and Ar ratios are constant at 42 and 60 sccm, respectively, the increase in hydrogen affects dissociation of the target.
Fig. 4. Refractive index (n) of WO3 thin films deposited by different hydrogen flows and plasma sources at λ 550 nm. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 5. Raman spectra of WO3 films deposited by DC magnetron sputtering with different hydrogen flows of (a) 0 sccm, (b) 1 sccm, (c) 2 sccm, (d) 3 sccm, (e) 4 sccm, and (f) 5 sccm.
WO3 thin film has an optimal refractive index at around 2.2 in the literature [16], and this was attested to in our results. The packing density of WO3 thin film increases with the increase of the refractive index using the optical analysis of the Lorentz– Lorenz Equation [17,18]. This phenomenon suggests that the WO3 thin film is far from a metallic state and approaches an oxidation state to reduce the extinction coefficients. C.
Raman Spectra
Raman spectra with varied hydrogen flows from 0 sccm to 5 sccm and plasma power are exhibited in Figs. 5 and 6. These WO3 thin films were amorphous and were checked by x-ray diffraction (XRD). They did not show obvious peaks. The Raman spectra results showed that the three low ionic bonding of W4, W5 -O, and W5 O exhibited no emergence in DC plasma sources. The two ionic bondings of W4 and W5 O appeared, but these were smaller than the
Fig. 7. Raman peak area ratios of WO3 films deposited by DC and pulsed DC magnetron sputtering with various hydrogen flows.
two ionic bonds of W6 -O and W6 O in pulsed DC plasma sources in high hydrogen flow. The hexvalence peaks of W6 O and O-W6 -O were enhanced by increasing the hydrogen flow for both plasma sources. The DC and pulsed DC magnetron sputtering had the same power, so the DC magnetron had a higher voltage than that of the pulsed DC magnetron. The deposition rate increased due to the high voltage to cover a little impurity. Therefore, the film was looser, and the WO3 film deposited by DC was easier to color than that deposited by pulsed DC [19]. Additionally, the optimum coloration of WO3 film deposited with different hydrogen flows could be controlled by the area ratio of 807 and 719 cm−1 peaks in the O-W6 -O bonding of Raman spectra. From Table 2, the maximum peak area ratio was 4.81 and 2.66 at the hydrogen flows of 4 and 3 sccm for DC and pulsed DC, respectively. Figure 7 clearly shows the peak ratio of the WO3 film deposited with different hydrogen flows and power sources. The phenomena also meet by the next transmittance spectra of colored WO3 films. D.
Transmittance Spectra of Colored WO3 Film
Figures 8 and 9 show the visible transmission spectra of colored WO3 films with various hydrogen flows Table 2.
Peak Area
Fig. 6. Raman spectra of WO3 films deposited by pulsed DC magnetron sputtering with different hydrogen flows of (a) 0 sccm, (b) 1 sccm, (c) 2 sccm, (d) 3 sccm, (e) 4 sccm, and (f) 5 sccm. A324
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1 DC 2 3 4 5 6 7 Pulsed DC 8 9 10 11 12
List of Raman Peak Area Ratios for WO3 Films
A1 of 807 cm−1 A2 of 719 cm−1 Ratio of H2 (O-W6 -O) (O-W6 -O) A1 ∕A2 (sccm) 3304.72 2950.90 2697.54 5658.03 5011.85 30928.44 3310.94 8335.67 2985.90 9683.05 16062.62 14440.73
1767.41 1128.69 828.57 1280.21 1041.70 7618.52 1777.36 3362.02 1133.69 3635.64 7277.14 7697.98
1.86 2.61 3.25 4.41 4.81 4.05 1.86 2.47 2.63 2.66 2.20 1.87
0 1 2 3 4 5 0 1 2 3 4 5
Fig. 8. Visible transmission spectra of colored WO3 thin films with various hydrogen flows by DC magnetron sputtering.
for each different power source. Transmittance decreases with the increase in hydrogen flow; therefore, the minimum transmittance of the color process was at hydrogen flows of 4 and 3 sccm for DC and pulsed DC plasma sources, respectively. Simultaneously, Figs. 10 and 11 show the transmittance spectra of colored WO3 at wavelengths of 550 and 400–500 nm with different hydrogen flows and plasma sources. The transmittances were all up 75% at a wavelength of 550 nm with different hydrogen flows and plasma sources. This phenomenon suggests that these films exhibit good performance with regard to good deposition conditions, without any low bonding of W4 and W5 to form the color-center generation. The results are supported by results presented in Section 3.C. The minimum transmittances of the color process also met at 4 and 3 sccm for DC and pulsed DC plasma sources, respectively, as seen in Figs. 10 and 11. E.
Transmittance Spectra of Coloring and Bleaching
Figures 12 and 13 show the transmittance spectra of coloring and bleaching with varied hydrogen flows for different plasma sources. In Fig. 12, the WO3 thin
Fig. 9. Visible transmission spectra of colored WO3 thin films with various hydrogen flows by pulsed DC magnetron sputtering.
Fig. 10. Transmittance spectra of colored WO3 at λ 550 nm with different hydrogen and plasma sources.
film began the coloring and bleaching at a hydrogen flow of 0–5 sccm, and the optimal variation was at the flow of 4 sccm for DC power. Furthermore, in Fig. 13 for DC pulse power, the WO3 thin film began the coloring and bleaching at a hydrogen flow of 0–5 sccm, but the optimal variation changed at the flow of 3 sccm. F. Optical Density, Coloration Efficiency, and Cyclic Voltammetry
Figure 14 shows the optical density of WO3 thin films deposited by different hydrogen flows at a wavelength of 550 nm with different plasma sources. Figure 15 shows the optical density of WO3 thin films deposited by different hydrogen flows at an average wavelength from 400 to 500 nm with different plasma sources. The optical density could reach ΔT 550 nm 65.6% and ΔT 400–500 nm 56.7% at the hydrogen flow of 4 sccm for DC magnetron sputtering. The optical density could reach ΔT 550 nm 64.4% and ΔT 400–500 nm 56.4% at the hydrogen flow
Fig. 11. Transmittance spectra of colored WO3 at 400–500 nm with different hydrogen and plasma sources. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 12. Transmittance spectra of WO3 thin films deposited by DC power with different hydrogen flows of (a) 0 sccm, (b) 1 sccm, (c) 2 sccm, (d) 3 sccm, (e) 4 sccm, and (f) 5 sccm.
of 3 sccm for pulsed DC magnetron sputtering. Figure 16 shows CE increasing with the increase of hydrogen flow at a wavelength of 550 nm. Figure 17
shows CE increasing with the increase in hydrogen flow at an average wavelength of 400–500 nm. The amount of CE reached 25.91 cm2 ∕C (550 nm) and
Fig. 13. Transmittance spectra of WO3 thin films deposited by pulsed DC power with different hydrogen flows of (a) 0 sccm, (b) 1 sccm, (c) 2 sccm, (d) 3 sccm, (e) 4 sccm, and (f) 5 sccm. A326
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Fig. 14. Optical density of WO3 thin films deposited by different hydrogen flows at λ 550 nm with different plasma sources.
25.28 cm2 ∕C (400–500 nm) at a hydrogen flow of 4 sccm for DC magnetron sputtering. For pulsed DC magnetron sputtering, the CE reached 22.61 cm∕C (550 nm) and 21.80 cm2 ∕C (400–500 nm) at a hydrogen flow of 3 sccm, respectively. CVs were investigated for WO3 thin film depositions with different plasma sources and these are shown in Fig. 18. All curves display typical behavior, as sharp peaks do not appear for either the bleached or colored processes. It can be seen that, with increasing voltage, the current increased steadily from a negative value to a maximum, and then decreased to close to zero, which corresponded to bleaching of the film. When the voltage decreased, the current went on decreasing and attained a negative value, which corresponded to coloring of the film [20,21]. The area of the CVs’ curvatures by the integration of the charge density (q), was 80.34 and 70.20 mC for DC and pulsed DC plasma sources, respectively. The area size of the CVs were DC > pulsed DC. Hence, film deposited by DC has the maximum CVs area resulting from the
Fig. 15. Optical density of WO3 thin films deposited by different hydrogen flows at λ 400–500 nm with different plasma sources.
Fig. 16. CE at different hydrogen flows and plasma sources at λ 550 nm.
Fig. 17. CE at different hydrogen flows and plasma sources at λ 400–500 nm.
Fig. 18. Variation of CVs in WO3 films deposited with different plasma sources. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 19. AC impedance of the WO3 films (a) without hydrogen, and (b) with 4 sccm hydrogen for DC power.
Fig. 20. AC impedance of the WO3 films (a) without hydrogen, and (b) with 3 sccm hydrogen for pulsed DC power.
high package density [22]. We hypothesize that the film structure of WO3 film deposited by DC provides a channel for Li ions to bleach and color. G.
AC Impedance
Figure 19 shows the variation of AC impedance of the WO3 films without and with 4 sccm hydrogen for DC power. The resistance could be reduced from 170 to 66 KΩ by introducing hydrogen gas. Simultaneously, Fig. 20 exhibits a variation of AC impedance for the WO3 films without and with 3 sccm hydrogen for pulsed DC power. The resistance could be reduced from 193 to 72 KΩ by introducing hydrogen gas. Hence, the WO3 film deposited with pulsed DC power had a higher resistance of 193 KΩ than that of 170 KΩ with DC power without the introduction of hydrogen gas. In the same situation, a resistance of 72 KΩ for pulsed DC power, was higher than 66 KΩ of DC power with the introduction of hydrogen gas. These phenomena show that WO3 film deposited with DC sputtering have better ionic motion than that those deposited with pulsed DC, with or without the introduction of hydrogen gas. This situation is verified by OD, CE, and CVs in Section 3.F. 4. Conclusions
The best deposition rates were 0.83 Ås−1 and 0.66 Ås−1 for the DC and pulsed DC magnetron sputtering by introducing hydrogen gas, respectively. Hence, the optimal deposition rate was between A328
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0.65 and 0.95 for WO3 film. WO3 film only had a high bonding of W6 O and not a low bonding of W5 O and W4 O, as shown by analysis of the Raman spectra. The refractive index could reach above 2.0 and the extinction coefficient was reduced below 10−3 at suitable O2 ∕Ar ratios for DC and pulsed DC plasma sources. Moreover, WO3 thin films had the best coloring and bleaching at hydrogen flows of 4 and 3 sccm for DC and pulsed DC sputtering, respectively. Simultaneously, the transmittance variations were 57% and 53% for DC and pulsed DC sputtering at a wavelength of 550 nm, respectively. Hence, the CE could reach CE 25.91 cm2 ∕C (550 nm) and CE 25.28 cm2 ∕C (400–500 nm) at a hydrogen flow of 4 sccm for DC power sources; the CE could reach CE 22.61 cm2 ∕C (550 nm) and CE 21.80 cm2 ∕C (400–500 nm) at a hydrogen flow of 3 sccm for pulsed DC power sources. DC power has a better optical density of 57% and CE of 25.91 cm2 ∕C (550 nm) and 25.28 cm2 ∕C (400–500 nm) over those of the pulsed DC power. The bleached/colored ability of CVs were DC > pulsed DC, and the resistances of AC impedance were pulsed DC > DC. Hence, DC is a good choice for depositing WO3 thin films by introducing hydrogen gas. The authors thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under contract No. NSC 1022220-E-224-002- and NSC 102—2622—E—224— 003—CC3.
References 1. S. K. Deb, “A novel electrophotographic system,” Appl. Opt. 3, 192–195 (1969). 2. C. G. Granqvist, Handbook of Inorganic Electrochromic Materials (Elsevier, 1995). 3. F. Hamelmann, K. Gesheva, T. Ivanov, A. M. Szekeres, M. A. Brashev, and U. Heinzmann, “Optical and electrochromic characterization of multilayered mixed metal oxide thin films,” J. Optoelectron. Adv. Mater. 7, 393–396 (2005). 4. J. M. Berak and M. J. Sienko, “Effect of oxygen-deficiency on electrical transport properties of tungsten trioxide crystals,” J. Solid State Chem. 2, 109–133 (1970). 5. S. J. Babinec, “A quartz crystal microbalance analysis of ion insertion into WO3,” Sol. Energy Mater. Sol. Cells 25, 269–271 (1992). 6. P. V. Ashrit, K. Benaissa, G. Bader, F. E. Girouard, and V. V. Truong, “Lithiation studies on some transition-metal oxides for an all-solid thin-film electrochromic system,” Solid State Ionics 59, 47–57 (1993). 7. H. C. Chen, D. J. Jan, and C. H. Chen, “Investigation of optical and electrochromic properties of tungsten oxide deposited with horizontal DC and DC pulse magnetron sputtering,” Jpn. J. Appl. Phys. 51, 045503 (2012). 8. K. D. Lee, “Preparation and electrochromic properties of WO3 coating deposited by the sol-gel method,” Sol. Energy Mater. Sol. Cells 57, 21–24 (1999). 9. B. Munro, S. Kraner, P. Zapp, and H. Krug, “Characterization of electrochromic WO3-layers prepared by sol-gel nanotechnology,” J. Sol-Gel Sci. Technol. 13, 673–678 (1998). 10. L. Jackson, “Auto industry embraces optical technologies,” Opt. Photon. News 11, 30–35 (2000). 11. B. W. Faughnan, R. S. Crandall, and P. M. Heyman, “Electrochromism in tungsten (VI) oxide amorphous films,” RCA Rev. 36, 177–197 (1975). 12. S. H. Lee, M. Hyeonsik, and K. D. Satyen, “Raman spectroscopic studies of electrochromic a-WO3,” Electrochemistry 44, 3111–3115 (1999).
13. M. Stowell, J. Muller, M. Ruske, M. Lutz, and T. Linz, “RFsuperimposed DC and pulsed DC sputtering for deposition of transparent conductive oxides,” Thin Solid Films 515, 7654–7657 (2007). 14. K. S. Rao, B. R. Kanth, G. S. Devi, and P. K. Mukhopadhyay, “Structural and optical properties of nanocrystalline WO3 thin films,” J. Mater. Sci. 22, 1466–1472 (2011). 15. C. P. Smyth, Dielectric Behavior and Structure, 1st ed. (McGraw-Hill, 1995). 16. C. Brigouleix, P. Topart, E. Bruneton, F. Sabary, G. Nouhaut, and G. Campet, “Roll-to-roll pulsed dc magnetron sputtering deposition of WO3 for electrochromic windows,” Electrochemistry 46, 1931–1936 (2001). 17. H. Inaba, M. Iwaku, K. Nakase, H. Yasukawa, I. Seo , and N. Oyama, “Electrochromic display device of tungsten trioxide and Prussian Blue films using polymer gel electrolyte of methacrylate,” Electrochim. Acta 40, 227–232 (1995). 18. J. Zhang, X. L. Wang, X. H. Xia, C. D. Gu, and J. P. Tu, “Electrochromic behavior of WO3 nanotree films prepared by hydrothermal oxidation,” Sol. Energy Mater. Sol. Cells 95, 2107–2112 (2011). 19. M. G. Hutchins, N. A. Kamel, and A. Hady, “Effect of oxygen content on the electrochromic properties of sputtered tungsten oxide films with Li+ insertion,” Vacuum 51, 433–439 (1998). 20. S. Ezhivalavan and T. Y. Tseng, “Electrical properties of Ta2O5 thin films deposited on Cu,” Thin Solid Films 360, 268–273 (2000). 21. H. C. Chen, K. S. Lee, and C. C. Lee, “Annealing dependence of residual stress and optical properties of TiO2 thin film deposited by different deposition methods,” Appl. Opt. 47, C284–C287 (2008). 22. H. C. Chen, D. J. Jan, C. H. Chen, and K. T. Huang, “Bond and electrochromic properties of WO3 films deposited with horizontal DC, pulsed DC, and RF sputtering,” Electrochim. Acta 93, 307–313 (2013).
1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Graduate School of Optoelectronics, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan
Graduate School of Science and Technology, National Yunlin University of Science and Technology, Yunlin 64002, Taiwan 3
Department of Electronic Engineering, National Yunlin University of Science and Technology Yunlin 64002, Taiwan 4
Physics Division, the Institute of Nuclear Energy Research, Taoyuan County 32546, Taiwan *Corresponding author: [email protected] Received 27 August 2013; accepted 10 December 2013; posted 24 December 2013 (Doc. ID 196544); published 20 January 2014
Research was undertaken to investigate the electrochromic and optical properties of tungsten oxide (WO3 ) films deposited by introducing hydrogen with a direct current (DC) and pulsed DC sputtering. The results show that WO3 films have optimum electrochromic properties at a hydrogen flow of 4 and 3 sccm for DC and pulsed DC, respectively. In the Raman spectra, the peak intensity increased with the increase of hydrogen flow at both 770 cm1 and 950 cm−1 peaks, which resulted in bonds of W6 -O and W6 O, respectively. Simultaneously, the transmittance (ΔT 550 nm ) variations were 65.6% and 64.4%, and the average transmittance (ΔT 400–500 nm ) variations were 56.7% and 56.4% for DC and pulsed DC, respectively. The bleached/colored ability of the cyclic voltammograms (CVs) was DC > pulsed DC, and the resistances of AC impedance were pulsed DC > DC. © 2014 Optical Society of America OCIS codes: (310.0310) Thin films; (310.1860) Deposition and fabrication; (310.6860) Thin films, optical properties; (310.6870) Thin films, other properties. http://dx.doi.org/10.1364/AO.53.00A321
1. Introduction
Electrochromic materials and devices (ECDs), which are capable of optical property modulation under an applied voltage, generally work in the visible and nearinfrared regions [1,2]. Such ECDs include adjustablelight rear mirrors, smart energy-saving windows, sunglasses, head vision displays, and so on. The main reason for smart energy-saving windows is that electrochromic materials are able to switch between two optical states (bleaching/coloring) in response to an applied voltage. Tungsten oxide (WO3 ) thin films have drawn the attention of many scientists due to their good electrochromic activity, fast switching response, 1559-128X/14/04A321-09$15.00/0 © 2014 Optical Society of America
high coloration efficiency, and high chemical stability [3–5]. Their electrochromic properties, such as coloration efficiency (CE), cyclic durability, and kinetics of coloration, strongly depend on inherent structural, morphological, and compositional characteristics [6]. Several techniques have been applied to improve the fabrication of WO3 thin films, including chemical vapor deposition (CVD) [7], sputtering [8,9], sol–gel [10,11] and electro-deposition [12]. WO3 film is used as the main electrochromic material, which allows Li to easily pass in and out of the active layer. As such, a suitable film packing density is fabricated by the appropriate sputtering source and process. Hence, research was undertaken to investigate the electrochromic and optical properties of WO3 thin films by introducing hydrogen gas for the DC and pulsed DC plasma power. Results 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
A321
show that WO3 thin films have good electrochromic properties with regard to the oxygen/argon ratio of 0.7, and the hydrogen flow was 4 and 3 sccm for DC and pulsed DC, respectively. The transmittance of all films was over 75% at as-deposited, and the deposition rates were all between 0.65 Ås−1 and 0.95 Ås−1 . Simultaneously, the transmittance variations (ΔT) were 65.6% and 64.4% at a wavelength of 550 nm, and the average transmittance variations were 56.7% and 56.4% from 400 to 500 nm for DC and pulsed DC, respectively. The bleached/colored ability of cyclic voltammograms (CVs) was DC > pulsed DC, and the resistances of AC impedance were pulsed DC > DC. The coloration ability and CE of WO3 thin film deposited with a plasma source of DC was better than pulsed DC with the introduction of hydrogen. 2. Experiment A.
Depositions of WO3 Thin Films
WO3 thin films were deposited at room temperature onto BK7 glass, indium tin oxide (ITO) glass, and a single crystalline p type silicon wafer using DC and pulsed DC magnetron sputtering, respectively. The system used horizontal deposition for the ion source, as shown in Fig. 1. The distance between the target and substrate was 11 cm in the chamber. The frequency of pulsed DC was 70 kHz, and a WO3 target of 99.95% purity with a diameter of 300 was used as the starting material. It is necessary to sputter the target for 10 minutes before starting the deposition to clean the oxide layer on the surface. The sputtering chamber was evacuated to 1.0 × 10−6 torr using a rotary pump and a turbo pump before introducing gases. The hydrogen flow rate was changed from 0 to 5 sccm. Simultaneously, the argon and oxygen flow rates were kept constant at 60 and 42 sccm, respectively. Table 1 is the list of deposition conditions for WO3 films. B. Measurements of Optical, Bond, and Electronic Properties
The refractive index, extinction coefficient, and film thickness of WO3 thin films were measured by a
Table 1.
Sample No. 1 2 3 4 5 6 7 8 9 10 11 12
List of Deposition Conditions for WO3 Films
Power Type
100 W DC
100 W pulsed DC
Frequency (KHz)
O2 ∕Ar Ratio
H2 (sccm)
42∕60
0 1 2 3 4 5 0 1 2 3 4 5
—
70
spectrometer (FilmTek-2000), such that film thickness accuracy could reach 1.5 Å for NIST-traceable standard oxide 1000 Å to 1 μm, and the film thickness was double-checked by an Alpha-Step Meter (Surface Profile Alpha-Step200). The Raman (RENISHAW 1000B) spectra were taken in a quasi-backscattering geometry using 100 mW at the 514.5 nm line of an Ar ion laser, focused on a square measuring 5 mm × 5 mm, as the excitation source. A threeelectrode cell was used for the measurement of static potential, and an Ag/AgCl gate was the reference electrode; the platinum sheet was a counter electrode. Lastly, AC impedance (Princeton Applied Research Versa STAT4) was used to measure the AC impedance by two non-blocking electrodes at both ends of the electrolyte and film sample. The analysis of electrical properties of the complex multi-layer film could be obtained by an impedance diagram. Hence, the AC impedance of the ionic motion could help us to understand the situation of the coloring/ bleaching for the WO3 film deposited by introducing hydrogen. C.
Electrochromatism of WO3 Thin Films
Electrochromatism of the WO3 thin films resulted from the injection and extraction of an electron and a proton by metal ions [13], as shown in Fig. 2, which is given by WO3 xe−1 xM ↔Mx WO3 ;
Fig. 1. Schematic drawing of magnetron sputtering with DC and pulsed DC plasma sources. A322
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(1)
where M was standard for the protons, such as H, Li , Na , or K . 1 M LiClO4 was used as the electrolyte in the propylene carbonate, and the solution was prepared from dried products and stored over molecular sieves. The potentials, which were used for coloring and bleaching, were −1.5 and 1.5 V, respectively. The transmittance spectra of the semi-finished cells, colored and bleached, were measured by an ultraviolet-visible spectrometer (Solid Space-3700). The variation in optical density (ΔOD) of the WO3 thin films at a dominative wavelength of 550 nm and 400–500 nm for the colored and bleached state, was calculated by the following relationship:
Fig. 2. Schematic drawing of an electrochromic device.
T bleaching ΔOD log Acoloring − Ableaching ; T coloring
Fig. 3. Sputtering rate of WO3 films deposited by different hydrogen flows and plasma sources.
(2)
where T bleaching and T coloring are the transmittance of the WO3 thin films in the bleached and colored states, respectively. CE (η) is introduced to describe the efficiency of the change of these charges in inducing absorption in an electrochromic device. The CE is defined as the variation in optical density (ΔOD) per unit of inserted charges, and is calculated by the following equation: CE η
ΔOD ; Q
(3)
where ΔOD is the optical density and Q is the density of charges (Q q∕A). Then, the charge is equal to the current multiplied by time. Hence, the condition of the experiment was that the current (I) was kept at 1 mA, the time (t) needed was 60 s, and the active area of WO3 thin film was 3.0 cm × 3.0 cm. By calculation, therefore, the density of charge (Q) was 0.02 C∕cm2. To evaluate the electrochromic properties of WO3 film deposition using different plasma sources, CVs were conducted in a 1 M LiClO4 electrolyte solution between −1.5 V and 1.5 V (versus Ag/AgCl) with a scan rate of 40 mV s−1 sweeping back and forth.
Hence, plasma density and deposition rates were slightly reduced by increasing the hydrogen flow, unlike the rapid reduction caused by increasing oxygen [14]. The sputtering rate has the maximum variation with regard to the DC plasma source, where the hydrogen flow changes from 0 to 5 sccm; therefore, the pulsed DC plasma had the lowest sputtering rate [15]. B. Optical Properties
The optical properties of the refractive index (n) are exhibited in Fig. 4 with different hydrogen flows and plasma sources at a wavelength of 550 nm. The DC power proved simple with regard to tapping out ions from the target, but it was also easy to lose the oxygen ion during deposition. The deposition rate of DC was faster than the pulsed DC power described in Section 3.A. The refractive index of the WO3 thin film was between 2.1 and 2.2 with the optimum parameters of the coloring range. However, when the O2 ∕Ar ratio was 0.7, the refractive index of the WO3 thin film was between 2.10 and 2.24, with the increase in the flow rate of hydrogen in this experiment.
3. Result and Discussion A.
Deposition Rate
Figure 3 shows the sputtering rates for different hydrogen flows in the deposited film thicknesses, being controlled at 300 nm, for DC and pulsed DC plasma sources. The deposition rates decreased a little with the increase of hydrogen flow for all films. This phenomenon resulted from the ionization energy of oxygen (48.76 eV) being higher than that of argon (15.76 eV) and hydrogen (13.6 eV), and the dissociation rate of oxygen being lower than that of argon and hydrogen. Although the O2 and Ar ratios are constant at 42 and 60 sccm, respectively, the increase in hydrogen affects dissociation of the target.
Fig. 4. Refractive index (n) of WO3 thin films deposited by different hydrogen flows and plasma sources at λ 550 nm. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 5. Raman spectra of WO3 films deposited by DC magnetron sputtering with different hydrogen flows of (a) 0 sccm, (b) 1 sccm, (c) 2 sccm, (d) 3 sccm, (e) 4 sccm, and (f) 5 sccm.
WO3 thin film has an optimal refractive index at around 2.2 in the literature [16], and this was attested to in our results. The packing density of WO3 thin film increases with the increase of the refractive index using the optical analysis of the Lorentz– Lorenz Equation [17,18]. This phenomenon suggests that the WO3 thin film is far from a metallic state and approaches an oxidation state to reduce the extinction coefficients. C.
Raman Spectra
Raman spectra with varied hydrogen flows from 0 sccm to 5 sccm and plasma power are exhibited in Figs. 5 and 6. These WO3 thin films were amorphous and were checked by x-ray diffraction (XRD). They did not show obvious peaks. The Raman spectra results showed that the three low ionic bonding of W4, W5 -O, and W5 O exhibited no emergence in DC plasma sources. The two ionic bondings of W4 and W5 O appeared, but these were smaller than the
Fig. 7. Raman peak area ratios of WO3 films deposited by DC and pulsed DC magnetron sputtering with various hydrogen flows.
two ionic bonds of W6 -O and W6 O in pulsed DC plasma sources in high hydrogen flow. The hexvalence peaks of W6 O and O-W6 -O were enhanced by increasing the hydrogen flow for both plasma sources. The DC and pulsed DC magnetron sputtering had the same power, so the DC magnetron had a higher voltage than that of the pulsed DC magnetron. The deposition rate increased due to the high voltage to cover a little impurity. Therefore, the film was looser, and the WO3 film deposited by DC was easier to color than that deposited by pulsed DC [19]. Additionally, the optimum coloration of WO3 film deposited with different hydrogen flows could be controlled by the area ratio of 807 and 719 cm−1 peaks in the O-W6 -O bonding of Raman spectra. From Table 2, the maximum peak area ratio was 4.81 and 2.66 at the hydrogen flows of 4 and 3 sccm for DC and pulsed DC, respectively. Figure 7 clearly shows the peak ratio of the WO3 film deposited with different hydrogen flows and power sources. The phenomena also meet by the next transmittance spectra of colored WO3 films. D.
Transmittance Spectra of Colored WO3 Film
Figures 8 and 9 show the visible transmission spectra of colored WO3 films with various hydrogen flows Table 2.
Peak Area
Fig. 6. Raman spectra of WO3 films deposited by pulsed DC magnetron sputtering with different hydrogen flows of (a) 0 sccm, (b) 1 sccm, (c) 2 sccm, (d) 3 sccm, (e) 4 sccm, and (f) 5 sccm. A324
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1 DC 2 3 4 5 6 7 Pulsed DC 8 9 10 11 12
List of Raman Peak Area Ratios for WO3 Films
A1 of 807 cm−1 A2 of 719 cm−1 Ratio of H2 (O-W6 -O) (O-W6 -O) A1 ∕A2 (sccm) 3304.72 2950.90 2697.54 5658.03 5011.85 30928.44 3310.94 8335.67 2985.90 9683.05 16062.62 14440.73
1767.41 1128.69 828.57 1280.21 1041.70 7618.52 1777.36 3362.02 1133.69 3635.64 7277.14 7697.98
1.86 2.61 3.25 4.41 4.81 4.05 1.86 2.47 2.63 2.66 2.20 1.87
0 1 2 3 4 5 0 1 2 3 4 5
Fig. 8. Visible transmission spectra of colored WO3 thin films with various hydrogen flows by DC magnetron sputtering.
for each different power source. Transmittance decreases with the increase in hydrogen flow; therefore, the minimum transmittance of the color process was at hydrogen flows of 4 and 3 sccm for DC and pulsed DC plasma sources, respectively. Simultaneously, Figs. 10 and 11 show the transmittance spectra of colored WO3 at wavelengths of 550 and 400–500 nm with different hydrogen flows and plasma sources. The transmittances were all up 75% at a wavelength of 550 nm with different hydrogen flows and plasma sources. This phenomenon suggests that these films exhibit good performance with regard to good deposition conditions, without any low bonding of W4 and W5 to form the color-center generation. The results are supported by results presented in Section 3.C. The minimum transmittances of the color process also met at 4 and 3 sccm for DC and pulsed DC plasma sources, respectively, as seen in Figs. 10 and 11. E.
Transmittance Spectra of Coloring and Bleaching
Figures 12 and 13 show the transmittance spectra of coloring and bleaching with varied hydrogen flows for different plasma sources. In Fig. 12, the WO3 thin
Fig. 9. Visible transmission spectra of colored WO3 thin films with various hydrogen flows by pulsed DC magnetron sputtering.
Fig. 10. Transmittance spectra of colored WO3 at λ 550 nm with different hydrogen and plasma sources.
film began the coloring and bleaching at a hydrogen flow of 0–5 sccm, and the optimal variation was at the flow of 4 sccm for DC power. Furthermore, in Fig. 13 for DC pulse power, the WO3 thin film began the coloring and bleaching at a hydrogen flow of 0–5 sccm, but the optimal variation changed at the flow of 3 sccm. F. Optical Density, Coloration Efficiency, and Cyclic Voltammetry
Figure 14 shows the optical density of WO3 thin films deposited by different hydrogen flows at a wavelength of 550 nm with different plasma sources. Figure 15 shows the optical density of WO3 thin films deposited by different hydrogen flows at an average wavelength from 400 to 500 nm with different plasma sources. The optical density could reach ΔT 550 nm 65.6% and ΔT 400–500 nm 56.7% at the hydrogen flow of 4 sccm for DC magnetron sputtering. The optical density could reach ΔT 550 nm 64.4% and ΔT 400–500 nm 56.4% at the hydrogen flow
Fig. 11. Transmittance spectra of colored WO3 at 400–500 nm with different hydrogen and plasma sources. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
λ
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Fig. 12. Transmittance spectra of WO3 thin films deposited by DC power with different hydrogen flows of (a) 0 sccm, (b) 1 sccm, (c) 2 sccm, (d) 3 sccm, (e) 4 sccm, and (f) 5 sccm.
of 3 sccm for pulsed DC magnetron sputtering. Figure 16 shows CE increasing with the increase of hydrogen flow at a wavelength of 550 nm. Figure 17
shows CE increasing with the increase in hydrogen flow at an average wavelength of 400–500 nm. The amount of CE reached 25.91 cm2 ∕C (550 nm) and
Fig. 13. Transmittance spectra of WO3 thin films deposited by pulsed DC power with different hydrogen flows of (a) 0 sccm, (b) 1 sccm, (c) 2 sccm, (d) 3 sccm, (e) 4 sccm, and (f) 5 sccm. A326
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Fig. 14. Optical density of WO3 thin films deposited by different hydrogen flows at λ 550 nm with different plasma sources.
25.28 cm2 ∕C (400–500 nm) at a hydrogen flow of 4 sccm for DC magnetron sputtering. For pulsed DC magnetron sputtering, the CE reached 22.61 cm∕C (550 nm) and 21.80 cm2 ∕C (400–500 nm) at a hydrogen flow of 3 sccm, respectively. CVs were investigated for WO3 thin film depositions with different plasma sources and these are shown in Fig. 18. All curves display typical behavior, as sharp peaks do not appear for either the bleached or colored processes. It can be seen that, with increasing voltage, the current increased steadily from a negative value to a maximum, and then decreased to close to zero, which corresponded to bleaching of the film. When the voltage decreased, the current went on decreasing and attained a negative value, which corresponded to coloring of the film [20,21]. The area of the CVs’ curvatures by the integration of the charge density (q), was 80.34 and 70.20 mC for DC and pulsed DC plasma sources, respectively. The area size of the CVs were DC > pulsed DC. Hence, film deposited by DC has the maximum CVs area resulting from the
Fig. 15. Optical density of WO3 thin films deposited by different hydrogen flows at λ 400–500 nm with different plasma sources.
Fig. 16. CE at different hydrogen flows and plasma sources at λ 550 nm.
Fig. 17. CE at different hydrogen flows and plasma sources at λ 400–500 nm.
Fig. 18. Variation of CVs in WO3 films deposited with different plasma sources. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS
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Fig. 19. AC impedance of the WO3 films (a) without hydrogen, and (b) with 4 sccm hydrogen for DC power.
Fig. 20. AC impedance of the WO3 films (a) without hydrogen, and (b) with 3 sccm hydrogen for pulsed DC power.
high package density [22]. We hypothesize that the film structure of WO3 film deposited by DC provides a channel for Li ions to bleach and color. G.
AC Impedance
Figure 19 shows the variation of AC impedance of the WO3 films without and with 4 sccm hydrogen for DC power. The resistance could be reduced from 170 to 66 KΩ by introducing hydrogen gas. Simultaneously, Fig. 20 exhibits a variation of AC impedance for the WO3 films without and with 3 sccm hydrogen for pulsed DC power. The resistance could be reduced from 193 to 72 KΩ by introducing hydrogen gas. Hence, the WO3 film deposited with pulsed DC power had a higher resistance of 193 KΩ than that of 170 KΩ with DC power without the introduction of hydrogen gas. In the same situation, a resistance of 72 KΩ for pulsed DC power, was higher than 66 KΩ of DC power with the introduction of hydrogen gas. These phenomena show that WO3 film deposited with DC sputtering have better ionic motion than that those deposited with pulsed DC, with or without the introduction of hydrogen gas. This situation is verified by OD, CE, and CVs in Section 3.F. 4. Conclusions
The best deposition rates were 0.83 Ås−1 and 0.66 Ås−1 for the DC and pulsed DC magnetron sputtering by introducing hydrogen gas, respectively. Hence, the optimal deposition rate was between A328
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0.65 and 0.95 for WO3 film. WO3 film only had a high bonding of W6 O and not a low bonding of W5 O and W4 O, as shown by analysis of the Raman spectra. The refractive index could reach above 2.0 and the extinction coefficient was reduced below 10−3 at suitable O2 ∕Ar ratios for DC and pulsed DC plasma sources. Moreover, WO3 thin films had the best coloring and bleaching at hydrogen flows of 4 and 3 sccm for DC and pulsed DC sputtering, respectively. Simultaneously, the transmittance variations were 57% and 53% for DC and pulsed DC sputtering at a wavelength of 550 nm, respectively. Hence, the CE could reach CE 25.91 cm2 ∕C (550 nm) and CE 25.28 cm2 ∕C (400–500 nm) at a hydrogen flow of 4 sccm for DC power sources; the CE could reach CE 22.61 cm2 ∕C (550 nm) and CE 21.80 cm2 ∕C (400–500 nm) at a hydrogen flow of 3 sccm for pulsed DC power sources. DC power has a better optical density of 57% and CE of 25.91 cm2 ∕C (550 nm) and 25.28 cm2 ∕C (400–500 nm) over those of the pulsed DC power. The bleached/colored ability of CVs were DC > pulsed DC, and the resistances of AC impedance were pulsed DC > DC. Hence, DC is a good choice for depositing WO3 thin films by introducing hydrogen gas. The authors thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under contract No. NSC 1022220-E-224-002- and NSC 102—2622—E—224— 003—CC3.
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