the optical and the electrochromic properties of sputtered TiO2 thin film

Journal of the Korean Physical Society, Vol 46, No 6, June 2005, pp 1383∼1391 Effect of Substrate Temperature on the Optical and the Electrochromic Properties of Sputtered TiO2 Thin Films Kil Dong Lee∗ Department of Physics, Kyonggi University, Suwon 442-760 (Received 15 February 2005, in final form 18 April 2005) Titanium oxide films (TiO2 ) were deposited on ITO-coated glass substrates at different substrate temperatures by using an RF reactive magnetron sputtering in an Ar/O2 atmosphere, and their electrochromic properties and stability by repeated coloring and bleaching cycles were investigated for counterelectrode applications The electrochemical properties of the TiO2 films as counterelectrodes showed weak dependences on the substrate temperature The optical band gap of the film increased from 3.30 eV to 3.40 eV when the substrate temperature was increased from room temperature to 500 ◦ C The cyclic durability of the TiO2 films deposited at a substrate temperature of 200 ◦ C was found to be the most stable and was suitable for counterelectrode applications when the films were subjected to 1000 cycles between –2 V and +2 V in a 1-M solution of LiClO4 PACS numbers: 68 Keywords: Titanium oxide films, RF reactive magnetron sputtering, Stability I INTRODUCTION The preparation and the characterization of thin films of TiO2 have been receiving the greatest attention during the past two decades because of their high refractive index and their dielectric constant In addition, this material is widely used as an electrochromic (EC) layer for smart-window applications [1–5] The characteristics of a smart-window, which allows dynamic control of the solar energy passing through the window, have been investigated widely in order to increase the efficiency of energy in a building [6–12] A smart window is fabricated by using multilayer thin films, and a major element is the EC thin film This EC thin film is characterized by a reversible and persistent change of the optical properties under the action of a voltage pulse Since Deb [13] found the EC effect in tungsten-oxide thin films, EC films have been prepared using sputtering, electronbeam, spray pyrolysis, thermal evaporation, and sol-gel methods, and their physical and electrochemical properties have been studied [14–22] With all these deposition methods, TiO2 films can be made with largely varying structural, optical, and EC properties Typical EC materials are transition metal oxide films Oxides of tungsten, molybdenum, vanadium, and titanium belong to the group of cathodic EC materials, which are colored by reduction reactions On the other hand, oxides of nickel and iridium are known to be anodic EC materials, which are colored by oxidation reactions ∗ E-mail: [1, 5] In these EC materials, the preparation and the characteristics of the TiO2 films, as an optical material, have been actively investigated because of their mechanical and chemical durability, high refractive index, and high transparency [23, 24] However, the durability of the TiO2 films have not been extensively investigated for smart-window applications Even though EC films have the advantage [1] of a long open-circuit memory and a small consumption of electric power, they are not yet available for windows in buildings because of the short device lifetime The development of electrolytes as ion sources, structural improvements of the thin films, process development of thin films for EC devices, etc have been actively studied to increase the durability and performance of the EC films [1,6,10,18,20] However, studies of the durability by repeated coloring and bleaching (C/B) cycles and performance evaluation of the EC TiO2 thin films for counterelectrode applications as a function of the preparation conditions have not been performed in detail On the other hand, sputtering method for thin films deposition are widely used for industrial products because high-quality films without contamination but with high density, high adhesion, high hardness, etc., can be obtained at a low substrate temperature with good thickness uniformity over a large area In this study, we report the preparation of the TiO2 films by using an RF (radio frequency) reactive magnetron sputtering for industrial application and evaluate the influence of the substrate temperature on the electrochromic properties, the optical and the structural properties, and the cyclic stability gdlee@kyonggi.ac.kr; Fax: +82-31-253-1165 -1383- -1384- Journal of the Korean Physical Society, Vol 46, No 6, June 2005 II EXPERIMENTS The TiO2 thin films were prepared with an RF magnetron deposition sputter system The deposition sputter system was a stainless-steel vacuum chamber of about 60 dm3 The background pressure in the sputtering vacuum chamber was 1.4 ∼ 1.8 × 10−3 Pa and was maintained by using a turbomolecular pump backed up by a mechanical pump High-purity argon (99.99 %) and oxygen (99.99 %) were used as the sputtering and the reactive gases, respectively Fig shows a schematic diagram of the RF magnetron apparatus for the deposition of TiO2 films The substrate used for deposition was glass coated with ITO (indium tin oxide; In2 O3 : Sn) The sheet resistance and the thickness of the ITO layer were 10 Ω/cm2 and 2000 ˚ A, respectively The substrate was mounted in a holder The substrate was rotated (10 rpm) during sputtering in order to increase the film uniformity A titanium metal (Ti) disk, (Pure Tech Inc) 0.076 m (3 inch) in diameter, with 99.95 % purity was used as the target The thickness of the Ti target was 0.0032 m (0.125 inch) The RF power used was 300 W at 13.56 MHz The flow rates of argon and oxygen were controlled with a MFC (mass flow control), and the pressures were measured with Pirani and Penning pressure gauges The distance between the target and the substrate was about 0.1 m Before deposition, the chamber was pumped down to 1.4 ∼ 1.8 × 10−3 Pa, and the target was pre-sputtered in a pure argon atmosphere for 600 s in order to remove the surface oxide layer of the target Afterward, oxygen was introduced into the chamber and the target was sputtered in an argon and oxygen mixture The deposition parameters for sample preparation are shown in Table The oxygen flow rate was varied from to 25 sccm (standard cubic cm/min), and the argon flow rate was kept constant at 300 sccm for deposition with increasing substrate temperature This led to a change in the Ar/O2 flow ratio from 60 to 12, which covered the range of the metallic-mode sputtering mode and target poisoning the sputtering mode The oxygen concentration in Fig Schematic diagram of an RF reactive magnetron sputtering apparatus for the deposition of TiO2 films Table Deposition conditions of the RF reactive magnetron sputtering system Substrate temperature Base pressure of system Total sputtering pressure (Ar+O2 ) Sputtering power Target-substrate distance Sputtering time Variation in oxygen Target material room temperature to 500 ◦ C 1.4 ∼ 1.8 × 10−3 Pa 4.67 ∼ 4.80 × 10−1 Pa 300 W 0.1 m 4800 s ∼ 25 sccm metallic titanium the deposited films was, thus, modulated The thickness in this experiment varied between 310 nm and 1000 nm when the oxygen flow rate was changed from to 25 sccm for deposition The deposition rate is a very important parameter for practical thin-film production The thickness of the TiO2 films deposited on ITOcoated glass substrates was measured by using a α-step profilometer, and the deposition rate was calculated from the film thickness obtained for a given deposition time The values depended on the total pressure and the oxygen concentration of the sputtering atmosphere In the case of the deposition at oxygen flow rates of 15 ∼ 20 sccm, the typical deposition rate was 0.0083 nm /s Nearly no thickness variation with increasing substrate temperature was observed The substrate was heated by using a hot stainless-steel plate, the temperature of which was kept constant within about ◦ C as measured by a thermocouple Since the substrate and the plane stainless-steel plate were in close contact in vacuum, we assumed that the substrate temperature was equal to the actually measured stainless-steel plate’s temperature For the film deposited at RT (room temperature) without extra heating, the substrate temperature increased during continued sputtering owing to energetic particle bombardment, intrinsic to the sputtering process, until about 90 ◦ C, which was reached after 4800 s For this case, it is very difficult to determine the actual substrate temperature during thin film deposition, but because of the low thermal conductivity of the glass and the relatively high heat capacity of the metal plate, we assumed the substrate temperature to be about 90 ◦ C throughout the deposition We prepared a set of sample under the above sputter conditions for substrate temperatures between RT and 500 ◦ C C/B of the films were carried out by using an EC cell apparatus to examine the EC reaction and the cyclic durability Fig shows the apparatus of the EC cell to monitor the ion insertion (extraction) reaction in the TiO2 film The TiO2 film was used as the working electrode in the EC cell A platinum wire was used for the counterelectrode The Li+ ion source for C/B the films was a solution of 1-M LiClO4 propylene carbonate The col- Effect of Substrate Temperature on the Optical· · · – Kil Dong Lee -1385- Cu Kα X-ray (λ = 1.54 ˚ A) and a Ni filter X-ray diffraction studies revealed that the TiO2 films were mainly crystalline at increased substrate temperatures The surface morphology of the films was observed by means of scanning electron microscopy (SEM: 15 kV, 100 kX) In order to prevent charge build-up, we sputtered a thin 10nm-thick gold film on the sample surface before making the SEM measurements III RESULTS AND DISCUSSION Fig Schematic diagram of the EC cell apparatus prepared for EC property measurements oration area of the TiO2 film was about 0.02 × 0.015 m2 Each potential was measured against the saturated calomel electrode (SCE) For C/B, a voltage was applied between the TiO2 film and the Pt electrode under the potentiostat condition (PARC, Model 273) C/B were achieved by switching the potential between + 2.0 V and – 2.0 V at a scan rate of 40 mV/s All experiments were performed in a glove box filled with nitrogen gas During the measurements, the electrolyte was bubbled with dry nitrogen gas in order to remove dissolved oxygen and to suppress water increase in the electrolyte The spectral transmittances of the TiO2 films after C/B cycling were measured in the visible region by using a spectrophotometer (Kontron Inst, Uvikon 941 plus) After the desired number of cycles had been completed, the sample was withdrawn from the electrolyte, rinsed in distilled water without affecting coloration, blow-dried with filtered air, placed in the sample compartment of the spectrophotometer, and subjected to optical measurements When these were completed, the sample could be put back into the electrolyte and run through more C/B cycles In order to characterize the electrochromic behavior of the deposited films, we performed cyclic voltammetry experiment using a potentiostat All the cyclic voltammograms (CV) were taken at room temperature in a quiescent solution In the cyclic voltammetry experiment, the potentiostat applied a potential ramp to the working electrode to gradually change the potential; then, the scan was reversed, returning to the initial potential During the potential sweep, the potentiostat measured the current resulting from the applied potential These values were then used to plot the cyclic voltammetry graph of current versus applied potential The variation of the current flowing through film, which is related to the insertion (extraction) of ions into (out of) the film, can be measured by using a cyclic voltammetry The transmittance of the sample was calculated by integrating with respect to the solar air mass [25] and photopic spectra [26] The crystal structure of the films was investigated by using X-ray diffraction (Philips model PW 3710) with Fig shows the variation in spectral transmittance of TiO2 film deposited on ITO-coated glass at RT The film thickness measurement was performed at different locations [up (1), middle (2), and down (3)] on sample in order to examine its uniformity As the figure shows, the transmittance did not change with sample location, and at a photopic wavelength of 550-nm, the transmittance of the film was about 82 % The 550-nm wavelength indicates the peak of the photopic (human-eye response spectrum) spectrum From these data, the film was found to be very uniform in thickness In addition, the samples with substrate temperatures of 200 ◦ C, 300 ◦ C, and 500 ◦ C were observed to be very uniform The undulation in the spectra is due to optical interference caused by thin films of titanium oxide and indium tin oxide on glass, which had thicknesses comparable to the wavelength of visible light Fig the shows spectral transmittance of TiO2 films prepared at substrate temperatures of RT, 200 ◦ C, 300 ◦ C, and 500 ◦ C As the figure shows, the transmittance of the films deposited at a substrate temperature of 500 ◦ C was lower than those of the other films at wavelengths of 400 nm, 450 nm, 550 nm, and 750 nm The decreased transmittance is attributed to insufficient oxygen incorporation in the film during deposition due to a diminished oxygen-sticking coefficient [23] In other words, the films deposited at higher temperatures are more absorbing A similar observation was made by Rao and Mohan Fig Variation in spectral transmittance at different locations in a TiO2 film deposited on ITO-coated glass at substrate temperature of 200 ◦ C -1386- Journal of the Korean Physical Society, Vol 46, No 6, June 2005 Fig Specral transmittance of TiO2 films deposited on ITO-coated glass at different substrate temperatures Fig Absorption coefficient of TiO2 films deposited on ITO-coated glass at different temperatures and by Rae et al [27,28] for electron-beam-evaporated TiO2 films The transparency of the films exhibits a sharp decrease in the ultraviolet (UV) region, as viewed from the transmittance spectra This decrease is caused by the fundamental absorption of light Generally in the visible region, the absorption coefficient, α, is influenced by the scattering of light due to the surface roughness, and it can be obtained from the approximate relation [24,29] T = (1 − R)2 exp(−α(λ)d) , − R2 exp(−2α(λ)d) (1) where T is the transmittance, R is the reflectance, d is the thickness of the film, and λ is the wavelength However, at shorter wavelengths close to the optical band gap, the scattering losses are dominated by the fundamental absorption and the following relation is often used [30]: α=d −1 ln(1/T ), (2) Above the threshold of fundamental absorption, the dependence of α on incident light energy is α = α0 (E − Eg )m , (3) where E = hν is the photon energy, Eg is the optical band gap, and α0 is a constant which does not depend on E The value of m may be taken as m = 2, a characteristic value for the indirect allowed transition, which dominates over the optical absorption, according to the theoretical and the experimental results in Refs 24 and 31 For photon energies hν > Eg , the material can absorb these photons For hν < Eg , α = and the photons cannot be absorbed anymore Fig shows the dependences α1/2 = f (E) for TiO2 films deposited on ITO-coated glass From the linear part of these dependences, one can obtain the extrapolated optical band gap, Eg , for α = for each curve The results show that the optical band gap increases as the substrate temperature increases The optical band Fig Spectral transmittance of the as-prepared TiO2 films deposited on ITO-coated glass at different oxygen flow rates gaps calculated from the extrapolate method were 3.30 eV, 3.35 eV, 3.35 eV, and 3.40 eV, respectively, for films deposited at substrate temperatures of RT, 200 ◦ C 300 ◦ C, and 500 ◦ C This changes in the optical band gap is another indication of structural modifications, but these calculated values of the optical band gap vary little in comparison with the reported results [23] The difference is attributed to microstructural variations in the film during deposition due to the different preparation conditions of the samples Fig shows the spectral transmittance of the TiO2 films prepared at different oxygen flow rates Films deposited at oxygen flow rates of 10 ∼ 20 sccm are transparent, with transmittances exceeding 80 % in the visible region However, the transmittance of the film deposited at an oxygen flow rate of sccm was very low in the visible region because the coloration of the surface had been changed to a deep blue color These results indicate that the chemical composition and the thickness of film were changed due to the oxygen content in the sputtering chamber In the case when oxygen contents of ∼ 25 sccm were introduced into the sputtering chamber, the total working pressure was changed up to 4.80 × Effect of Substrate Temperature on the Optical· · · – Kil Dong Lee Fig Variation of absorption coefficient of as-prepared TiO2 films deposited on ITO-coated glass with the oxygen flow rates -1387- Fig X-ray diffraction patterns of TiO2 films deposited on ITO-coated glass at different substrate temperatures: (a) RT, (b) 100 ◦ C, (c) 150 ◦ C, (d) 200 ◦ C, (e) 250 ◦ C, (f) 300 ◦ C, (g) 350 ◦ C, and (h) 500 ◦ C Fig Deposition rate for TiO2 films as a function of the oxygen flow rate Fig 10 Variation of deposition rate for TiO2 films with the substrate temperatures 10−1 Pa In addition, the low transmittance at shorter wavelength (λ < 300 nm) is due to ITO Fig shows the spectral variation of the absorption coefficient for TiO2 films prepared at different oxygen flow rates The values of the absorption coefficients were calculated on the basis of Fig In the 190 – 400-nm wavelength range, the absorption coefficient of the TiO2 film deposited at an oxygen flow rate of sccm was found to be much higher than those of the samples deposited at oxygen flow rates of 15 ∼ 25 sccm In the case of the films deposited at oxygen flow rates of 15 ∼ 25 sccm, the spectral absorption coefficient did not change considerably From these results, the fundamental absorption edges due to ITO (band gap: 3.5 eV) and to the TiO2 film (band gap: 3.2 eV) were found to occur in the UV region Thus, the samples deposited at oxygen flow rates of 15 ∼ 25 sccm showed that no differences in the absorption coefficient However, the high absorption coefficient in the TiO2 film deposited at oxygen flow rate of sccm might be due to insufficient oxygen incorporation in the film during deposition When reactive sputtering is used for the preparation of compound films, it is very important to know the target surface conditions during sputtering For this purpose, it is very useful to moni- tor the deposition rate’s dependence on the oxygen flow rate Fig shows the deposition rate of the film as a function of the oxygen flow rate In these measurements the argon flow rate was kept at 300 sccm, and the total pressure was about 4.67 ∼ 4.80 × 10−1 Pa The deposition rate was determined by using a the film thickness obtained with a profilometer divided by the deposition time As the fig shows, the abrupt decrease in the deposition rate at oxygen flow rates of – 15 sccm is the result of target oxidation and the resulting low sputtering yield of the oxide However, the deposition rate did not depend much on the oxygen content in the sputtering chamber at oxygen flow rates above 15 sccm The target was found to be near completely oxidized in this oxygen flow range In the case of the TiO2 film deposited at an oxygen flow rate of sccm, the film thickness was about 1000 nm, as measured by using a profilometer Also, the optical transmittance was reduced for higher oxygen flow rates No obvious steep absorption band was seen in the transparent region examined, suggesting that the deposited films did not include any impurity ion or defect -1388- Journal of the Korean Physical Society, Vol 46, No 6, June 2005 Fig 11 Spectral transmittance of electrochromic TiO2 films deposited on ITO-coated glass at different substrate temperatures: (a) RT, (b) 200 ◦ C, (c) 300 ◦ C, and (d) 500 ◦ C centers Fig shows the diffraction patterns of films deposited at different substrate temperatures As the figure shows, A, B, C, D, E, F, G, and H are samples prepared at substrate temperatures of RT, 100 ◦ C, 150 ◦ C, 200 ◦ C, 250 ◦ C, 300 ◦ C, 350 ◦ C and 500 ◦ C, respectively The diffraction patterns show no distinguishable features In films A-G, a strong amorphous background is seen The brookite, anatase, and rutile phases were not observed in any of the films investigated Pawlewicz and Busch [32] observed mixed phases of anatase and rutile for substrate temperatures in the range 200 – 500 ◦ C A similar amorphous-to-crystalline transition around 350 – 400 ◦ C was observed by Bange et al [33] for evaporated films and by Willams and Hess [34] for RF sputtered films In all cases, the mixed phase is present up to around 600 ◦ C; at higher temperature, only the rutile structure prevailed These different results in comparison with other investigator’s results [33,34] might be due to differences in sample preparation conditions Fig 10 shows the variation in the deposition rate with substrate temperature Obviously, the deposition rate was not influenced by the substrate temperature until substrate was heated to 350 ◦ C Most probably, this result suggests that the deposition rate depends only on the number of sputtered Ti atoms that subsequently reach the substrate The effects in this experiment, such as the local pressure reduction in the sputtering plasma, the variation in the sticking coefficients of Ti or oxygen, and the re-evaporation from the substrate, are neglect In addition, in the case of the sample with a substrate temperature of 500 ◦ C, the larger value of the deposition rate in the sputtering chamber compared to that obtained at lower substrate temperatures might be due to a differences among the sputtered atoms, the substrate temperatures, and contents of oxygen Fig 11 shows the spectral transmittance of the EC films prepared at substrate temperatures of RT, 200 ◦ C, 300 ◦ C, and 500 ◦ C As the fig shows, the transmittance of colored and bleached films did not significantly change compared with those of the asprepared film These results indicate that the TiO2 films deposited at various substrate temperatures can be used as counterelectrodes in electrochromic devices (glass/ITO/WO3 /LiAlF4 /TiO2 /glass) such as smartwindows Counterelectrodes with cathode coloring characteristics should have a small decrease in transparency on the insertion of ions Therefore from the transmittance data, the TiO2 films can have the properties of passive counterelectrodes (ion storage) on the insertion/extraction of ions in the Fig 11 The TiO2 films are well known to have potential advantages as counterelectrodes due to their microstructural features being favorable for ion transport [35] Fig 12 shows variation of charge capacity (electric charge density) as a function of the cyclic number for samples prepared at substrate temperatures of RT [sample (a)], 200 ◦ C [sample (b)], 300 ◦ C [sample (c)], and 500 ◦ C [sample (d)] in order to evaluate during 1000 cycles the electrochromic performance for counterelectrode application The charge capacity is the effective intercalated charge that is reversibly transferred upon cycling [36] Maximization of this charge gives the maximum transmittance difference Further, in EC materials, the charge capacity is related to the coloration process, i.e., cathodic or anodic charge capacity The usual units are expressed per area, mC/cm2 The charge capacity was obtained from CV and were recorded during cathodic Effect of Substrate Temperature on the Optical· · · – Kil Dong Lee -1389- Fig 12 Dependence of the charge density of electrochromic TiO2 films on the C/B cycles for substrate temperatures of (a) RT, (b) 200 ◦ C, (c) 300 ◦ C, and (d) 500 ◦ C Fig 13 Dependence of the cyclic voltammograms of electrochromic TiO2 films on the C/B cycles for substrate temperatures of (a) RT, (b) 200 ◦ C, (c) 300 ◦ C, and (d) 500 ◦ C bleaching in order to avoid any possible contribution due to oxygen evolution The transferred charge was calculated by integrating the measured intensity from the CV plot As the figure shows, the TiO2 films deposited with substrate temperatures of 200 ◦ C gave maximum values of both the inserted coloring charge density and the extracted bleaching charge density after 1000 cycles This means the cyclic durability of the TiO2 films pre- pared at substrate temperatures of 200 ◦ C for counterelectrode materials is stronger than those of the other samples Such a dependence of the transferred charge on the substrate temperature may be due to the difference in both the boundaries and the surfaces of the TiO2 microcrystallites Therefore, the large amount of transferred charge in sample (b) after 1000 cycles can be attributed to numerous grain boundaries Grain bound- -1390- aries are well known to be good diffusion channels for ions to be injected into or extracted from films In other words, this result indicates that the space between TiO2 crystallites become larger, which makes it easier for ions to be injected into or extracted from the films under cycling In addition, the total charge contents in films with increasing the number of cycles were nearly zero This suggests that the electrochromic reaction is a reversible process Fig 13 shows the CV curves of the TiO2 films prepared under different substrate temperatures for evaluation of the electrochemical properties The charge capacity of the TiO2 films changes rapidly in the first stage of cyclic voltammetry, so for stability, we measure its value after 10 cycles As the figure shows, the C/B process for the samples with substrate temperatures of RT [sample (a)], 200 ◦ C [sample (b)], 300 ◦ C [sample (c)], and 500 ◦ C [sample (d)] is nearly reversible The redox reaction is slightly shifted toward a lower negative potential after 1000 cycles Coloration (upon oxidation) and bleaching (upon reduction) are associated with redox peaks The anodic and the cathodic peaks of the samples is decreased in size after 1000 cycles These results indicate that there was little ion transport into this film, leading to a weak blue coloration Thus, the TiO2 film’s microstructure was clearly modified during the 1000 cycles On the other hand, in the case of sample (b), the redox reaction was stronger than it was for samples (a), (c), and (d) This result is consistent with that of the sample (b) (Fig 12) with a good ion storage material characteristic Therefore, sample (b) (Fig 13) was also confirmed to be suitable as a counterelectrode material for use in electrochromic devices because of its almost transparent property in the inserted and the extracted charge states Fig 14 shows the surface morphology of the TiO2 film prepared at substrate temperatures of RT [sample (a)], 200 ◦ C [sample (b)], and 300 ◦ C [sample (c)] As the micrographs show, the grains sizes of the TiO2 grains are found to be fine, and the grain size is found to increase with increasing substrate temperature, but films appear to be a little porous At lower substrate temperatures, the deposited atoms are expected to have restricted surface mobility Restrictive diffusion of atoms prevents crystal growth at energetically favorable sites and causes atoms to nucleate at new sites [37] This results in a structure of smaller grains and relatively weaker preferred orientation compared to the structure obtained at higher substrate temperatures At higher substrate temperatures, atoms have enough energy to diffuse to the preferred nucleation sites Easy propagation of atoms results in the development of a strong preferred orientation [37] In the case of sample (b) with a substrate temperature of 200 ◦ C, the grain size is more uniform than it is for the samples prepared at substrate temperatures of RT and 300 ◦ C Sample (c) with a substrate temperature of 300 ◦ C shows closely packed grains, but the grain size is clearly larger than it is in samples (a) and (b) This Journal of the Korean Physical Society, Vol 46, No 6, June 2005 Fig 14 SEM micrographs of TiO2 films prepared on ITOcoated glass at different substrate temperature: (a) RT, (b) 200 ◦ C, and (c) 300 ◦ C result indicates that the grain size increases whereas the grain boundary decreases at higher temperatures, which might result in a weak electrochromic reaction Therefore, variation of the grain sizes was found to be correlated with the electrochromic performance of the TiO2 film From those results, we concluded that the best film for large ion storage with counterelectrode characteristics had been obtained at a substrate temperature of 200 ◦ C The surface morphology of that sample was found to have a slightly porous structure consisting of uniform grains for weak electrochromic reactions with good counterelectrode characteristics IV CONCLUSIONS TiO2 films were deposited on ITO-coated glass at different substrate temperatures by using an RF reactive magnetron sputtering apparatus The optical band gap of the TiO2 films increased from 3.30 eV to 3.40 eV as the substrate temperature was varied from RT to 500 ◦ C This change in the optical band gap is another indication of the structural modifications As a result of examining the Ti target oxidation phenomenon in sputtering vacuum chamber, the abrupt decrease in the deposition rate at oxygen flow rates of – 15 sccm was the result of target oxidation and to the sputtering yield Effect of Substrate Temperature on the Optical· · · – Kil Dong Lee of the Ti oxide being lower than that of the Ti metal Significant changes in the X-ray diffraction patterns for the TiO2 films with increasing substrate temperatures were not observed The electrochromic behavior of the sputtered films for counterelectrode applications weakly depended on the substrate temperature during RF magnetron sputtering As a result of analyzing the CV measurement results of the films, we confirmed that the films deposited at a substrate temperatures of 200 ◦ C showed excellent counterelectrode characteristics The surface morphology of the film deposited at that temperature was found to have a slightly porous structure consisting of uniform grains for a weak electrochromic reaction REFERENCES [1] C G Granqvist, Handbook of Inorganic Electrochromic Materials (Elsevier Science, Amsterdam, 1995) [2] R Cinnsealach, G Boschloo, S N Rao and D Fitzmaurice, Sol Energy Mater & Sol Cells 57, 107 (1999) [3] K Yoshimura, T Miki and S Tanemura, J Vac Sci Technol A 15, 2673 (1997) [4] F Campus, P Bonhˆ ote, M Gr tzel, S 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C, 300 ◦ C, and 500 ◦ C As the figure shows, the transmittance of the films deposited at a substrate temperature of 500 ◦ C was lower than those of the other films at wavelengths of 400 nm, 450