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Optical and structural properties of TiO2 films deposited from Ti3O5 by electron beam
Surface & Coatings Technology 201 (2007) 5367 – 5370 www.elsevier.com/locate/surfcoat
Optical and structural properties of TiO2 films deposited from Ti3O5 by electron beam F.X. Wang a,b,⁎, C.K. Hwangbo b , B.Y. Jung b , J.H. Lee b , B.H. Park b , N.Y. Kim b a
Department of Physics and Mathematics, Shandong Institute of Architecture and Engineering, Jinan 250101, PR China b Department of Physics, Inha University, Inchon 402-751, South Korea Available online 16 November 2006
Abstract TiO2 thin f ilms were deposited by e-beam and the refractive index and extinction coefficient of the films were measured. When the oxygen pressure increases from 1.3 × 10− 5 to 2.2 × 10− 4 Torr, while the substrate temperature (300 ± 10 °C) and the depositing rate (0.1 nm/s) were kept constant, the refractive index of the films decreases from 2.41 to 2.12 (at λ = 550 nm). The structures of the films were investigated by X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM). XRD measurements revealed that the films deposited in a higher substrate temperature (300 ± 10 °C) were partially crystalline. The AFM investigation confirmed that lower oxygen pressure, higher deposition rate and higher substrate temperature produce a higher surface roughness of TiO2 f ilm. When the deposition rate (0.1 nm/s) and the substrate temperature (300 ± 10 °C) were kept constant, and the oxygen pressure was varied from 1.3 × 10− 5 to 2.2 × 10− 4 Torr, the roughness (Root-Mean-Square) of as-deposited films was changed from 3.53 to 1.91 nm. © 2006 Published by Elsevier B.V. PACS: 68.55.-a; 78.66.Nk; 78.20.Ci Keywords: TiO2 thin film; E-beam deposition; Optical properties; Film structure; Atomic Force Microscopy
1. Introduction Titanium dioxide (TiO2) thin films possess many excellent optical and electrical properties, so they are applied in many fields. For example, they are widely used in optical coating due to their high refractive index, excellent transmittance and good durability. Because of their unique electro-optical properties, they exhibit a large potential to be used in the fields such as solar energy conversing [1,2], lithium insertion batteries [3], gas sensor [4], photocatalytic detoxification of polluted water [5], air deodorisation [6], and so on. So it is reasonable that many researchers have focused their work on the fabrication of TiO2 thin films. In order to get high quality of TiO2 thin films, a lot of film preparation techniques were employed, such as electron beam (EB) [7–11], ion-assisted deposition (IAD) [12,13], magnetron ⁎ Corresponding author. Department of Physics and Mathematics, Shandong Institute of Architecture and Engineering, Jinan 250101, PR China. Tel.: +86 531 86361596. E-mail address: [email protected] (F.X. Wang). 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.07.037
sputtering technique [14], pulsed laser deposition (PLD) [15], self-assembly process [16], laser chemical vapor depositing (LCVD) [17], etc. Among them, EB is one of the most traditional methods and is widely used in scientific researches as well as in practical production. A lot of research works have been devoted to investigate the influence of varied depositing conditions on the properties of the films. For example, Pulker et al. prepared TiO2 films by reactive evaporation and observed the influence of starting material composition, substrate temperature, oxygen pressure and depositing rate on the optical properties of the deposited films [18]. Bennett et al. reported the comparison of the properties of TiO2 films prepared by various techniques [19]. Chen et al. investigated the substrate-dependent optical absorption characteristics of TiO2 films [20]. Lehmann and Frick summarized their research results and declared the optimized deposition parameters of EB evaporated TiO2 films [8]. In most of the reports, the optical properties of the TiO2 films are limited in visible light range, though the optical properties of the thin films in near infrared range are also important in practice, so it is significant to investigate the optical properties in larger irradiant range of the
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at 150 ± 10 °C and 300 ± 10 °C by auto-controller system. The thickness of deposited films was controlled at about 550 nm and all the samples were in situ annealed for 10 min after the depositing process was ended. 3. Results and discussions
Fig. 1. Refractive index and extinction coefficient versus wavelength for TiO2 thin films deposited in different oxygen pressures. Substrate temperature: 300 ± 10 °C; depositing rate: 0.1 nm/s.
TiO2 films deposited in different ambient. On the other hand, according to our knowledge, there is no published work related to investigate the influence of depositing conditions on the surface structure of TiO2 thin films. Ti3O5 is one of the Ti oxides and preferable to be used as a starting material in depositing TiO2 film [7]. In the present work, TiO2 films were deposited from Ti3O5 materials by EB in different ambient. The optical properties of as-grown TiO2 films were investigated in visible and near infrared range (300–2000 nm). The structure of the films was checked by Xray Diffraction (XRD) and Atomic Force Microscopy (AFM) techniques.
To investigate the optical characteristics of the TiO2 films, the transmittances of the films were measured by using spectrophotometer (Cary500, Varian). By using an envelope method, the refractive index and extinction coefficient of the films at wavelengths of quarter wave and half wave thickness could be extracted from these transmittance spectra. Fig. 1 shows the dispersion curves of the refractive index and extinction coefficient in the wavelength range of 400 to 1800 nm. It can be seen from the figure that a higher oxygen pressure produces lower refractive index TiO2 films. It must be pointed out that to get higher refractive index TiO2 films, lower oxygen pressure could be used in the depositing process, but the oxygen pressure could not be too low, otherwise the TiO2 film would be deficient in oxygen. According to our experimental results, the lowest oxygen pressure in the depositing process is about 1.0 × 10− 5 Torr, and the depositing rate cannot be higher than 0.1 nm/s in such an oxygen pressure. Fig. 1 shows that the differences of extinction coefficient values among as-deposited TiO2 films are small. The fact that the higher oxygen pressure produces lower refractive index TiO2 films has been explained by Cevro and Carter [9]. They pointed out that as the oxygen pressure increases, the collision of the evaporated species with oxygen molecules would increase, and more collision would let the mobility of the species on the substrate during condensation of the film would be reduced, so a less densely packed film with lower refractive index would be resulted. This explanation
2. Experimental details TiO2 films were deposited by typical EB evaporation. The base pressure of the chamber was below 2.4 × 10− 6 Torr by a rotary pump and cryo-pump. The starting material was Ti3O5 (99.9%, CERAC) and the evaporating voltage was 9.01 kV. Well-polished B270 glasses were employed as the substrates. The oxygen pressure was maintained by controlling the flow rate of O2 through the gas valve. The depositing rate and film thickness were controlled by quartz crystal monitor. The chamber was heated by halogen lamps and the temperature of the substrate was checked by a thermocouple meter which was placed directly on the back of the substrate. The distance from the substrate to the EB source is about 55 cm. In the depositing process, oxygen pressure, depositing rate and substrate temperature are main parameters which should exert important influence on the properties of deposited films. In the present work, the oxygen pressures were varied from 1.3 × 10− 5 to 2.2 × 10− 4 Torr and the deposition rates were varied from 0.1 to 0.6 nm/s. The substrate temperature was kept
Fig. 2. Refractive index and extinction coefficient versus wavelength for TiO2 thin films deposited in different depositing rates. Substrate temperature: 300 ± 10 °C; oxygen pressure: 1.0 × 10− 4 Torr.
F.X. Wang et al. / Surface & Coatings Technology 201 (2007) 5367–5370
Fig. 3. Refractive index and extinction coefficient versus wavelength for TiO2 thin films deposited in different substrate temperatures. Depositing rate: 0.1 nm/s; oxygen pressure: 1.6 × 10− 4 Torr.
seems to be reasonable, which our experimental results supported well. When the deposition rate was used as a variable parameter, while other parameters (TSUB = 300 ± 10 °C, PO2 = 1.0×10− 4Torr) were kept constant, the changes of refractive index and extinction coefficient of the films are shown in Fig. 2. The figure shows that the higher depositing rate produces higher refractive index film. This result is in agreement with that of Pulker et al. [18] and Cevro and Carter [9]. It seems reasonable that a higher deposition rate produces a higher refractive index film, because to get a higher deposition rate, stronger e-beam current is needed, so the evaporated species can get more kinetic energy and higher packing density film can be condensed. The results shown in Fig. 2 suggest that to get higher refractive index TiO2 films a higher deposition rate is desired in the depositing process, but the deposition rate is limited by other parameters. In fact there are several points that must be taken into consideration in the actual depositing process of TiO2 thin film. First, a higher deposition rate needs a higher oxygen pressure, otherwise, the deposited film could show oxygen deficiency. Second, the higher deposition rate allows the surface roughness of the deposited films to increase, which will be proved by AFM investigation later. Third, the thickness of the deposited film is more difficult to be controlled accurately when the deposition rate is high. Synthesizing the affections of several depositing parameters, we suggest that the deposition rate below 0.3 nm/s in TiO2 film depositing process by e-beam is reasonable. Besides oxygen pressure and depositing rate, the substrate temperature is another important parameter in the depositing process. Fig. 3 shows the dependence of the refractive index and extinction coefficient of the films on substrate temperature. The figure indicates that a higher substrate temperature allows the deposited film to get a higher refractive index. Note that it is
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difficult to keep exactly the substrate temperature constant because the emitted heat from the crucible allows the temperature to rise even though the auto-controlling system of the temperature is employed in the operated chamber. In the present case, the substrate temperature is kept at 150 ± 10 °C and 300 ± 10 °C, and the XRD result indicates that the samples prepared at high temperature (300 ± 10 °C) are partially crystalline. The surface morphology of the deposited TiO2 films was investigated by AFM. When the deposition rate (0.1 nm/s) and substrate temperature (300 ± 10 °C) were kept constant, and the varied oxygen pressures were 1.3 × 10− 5, 3.2 × 10− 5, 1.0 × 10− 4, 1.6 × 10− 4 and 2.2 × 10− 4 Torr, respectively, the Root-MeanSquare (RMS) of as-deposited films is 3.53, 2.96, 2.84, 2.31 and 1.91 nm, respectively. When the oxygen pressure (1.0 × 10− 4 Torr) and substrate temperature (300 ± 10 °C) were kept constant and the deposition rate was 0.1, 0.3 and 0.6 nm/s, respectively, the RMS of as-deposited films is 2.84, 3.76 and 4.95 nm respectively. When the oxygen pressure (1.6 × 10− 4 Torr) and the deposition rate (0.1 nm/s) were kept constant and the substrate temperature was varied to be about 300 and 150 °C, the RMS of as-deposited films is 2.31 and 1.84 nm, respectively. From these experimental results, we confirmed that lower oxygen pressure, higher deposition rate and higher substrate temperature produce a higher surface roughness TiO2 film. 4. Summary TiO2 films have been deposited from Ti3O5 material by EB in different ambient. The films deposited at high temperature are partially crystalline. The optical properties and structure of the films were influenced strongly by the depositing conditions. Lower oxygen pressure, higher deposition rate and higher substrate temperature produced higher refractive index TiO2 films. At the same time, the varied deposition conditions that can increase the refractive index also allow the surface roughness of the film to increase. When we deposit the TiO2 film with a certain demand on limited surface roughness of the film, the influence of depositing conditions must be considered seriously before starting to deposit. Acknowledgements This work was supported by BK21 project, Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
B. O'Regan, M. Grätzel, Nature 353 (1991) 737. A. Hugot-le Goff, P. Falaras, J. Electrochem. Soc. 142 (1995) L38. L. Kavan, K. Kratochvilova, M. Grätzel, J. Electroanal. Chem. 394 (1995) 93. H.M. Lin, C.H. Keng, C.Y. Tung, Nanostruct. Mater. 9 (1997) 747. Y.-M. Gao, H.-S. Shen, K. Dwight, A. Wold, Mater. Res. Bull. 27 (1992) 1023. I. Sopyan, S. Murasawa, K. Hashimot, A. Fujishima, Chem. Lett. (1994) 723. K. Balasubramanian, X.F. Han, K.H. Guenther, Appl. Opt. 32 (28) (1993) 5594. Hans W. Lehmann, K. Frick, Appl. Opt. 27 (23) (1988) 4920. M. Cevro, G. Carter, J. Phys. D, Appl. Phys. 28 (1995) 1962. Y. Leprince-Wang, K. Yu-Zhang, V. Nguyen Van, D. Souche, J. Rivory, Thin Solid Films 307 (1997) 38. [11] H.K. Pulker, Thin Solid Films 34 (1976) 343.
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[12] D. Bhattacharyya, N.K. Sahoo, S. Thakur, N.C. Das, Thin Solid Films 360 (2000) 96. [13] G. Atanassov, J. Turlo, J.K. Fu, Y.S. Dai, Thin Solid Films 342 (1999) 83. [14] S. Bhagwat, R.P. Howson, Surf. Coat. Technol. 111 (1999) 162. [15] C. Garapon, C. Champeaux, J. Mugnier, G. Panczer, P. Marchet, A. Catherinot, B. Jacquier, Appl. Surf. Sci. 96–98 (1996) 836. [16] D. Huang, Z.D. Xiao, J.H. Gu, N.P. Huang, C.W. Yuan, Thin Solid Films 305 (1997) 110.
[17] A. Watanabe, Y. Imai, Thin Solid Films 348 (1999) 63. [18] H.K. Pulker, G. Paesold, E. Ritter, Appl. Opt. 15 (12) (1976) 2986. [19] J.M. Bennett, E. Pelletier, G. Albrand, J.P. Borgogno, B. Lszarides, C.K. Carniglia, R.A. Schmell, T.H. Allen, T.T. Hart, K.H. Guenther, A. Saxer, Appl. Opt. 28 (15) (1989) 3303. [20] J.S. Chen, S. Chao, J.S. Kao, G.R. Lai, W.H. Wang, Appl. Opt. 36 (19) (1997) 4403.
Optical and structural properties of TiO2 films deposited from Ti3O5 by electron beam F.X. Wang a,b,⁎, C.K. Hwangbo b , B.Y. Jung b , J.H. Lee b , B.H. Park b , N.Y. Kim b a
Department of Physics and Mathematics, Shandong Institute of Architecture and Engineering, Jinan 250101, PR China b Department of Physics, Inha University, Inchon 402-751, South Korea Available online 16 November 2006
Abstract TiO2 thin f ilms were deposited by e-beam and the refractive index and extinction coefficient of the films were measured. When the oxygen pressure increases from 1.3 × 10− 5 to 2.2 × 10− 4 Torr, while the substrate temperature (300 ± 10 °C) and the depositing rate (0.1 nm/s) were kept constant, the refractive index of the films decreases from 2.41 to 2.12 (at λ = 550 nm). The structures of the films were investigated by X-ray Diffraction (XRD) and Atomic Force Microscopy (AFM). XRD measurements revealed that the films deposited in a higher substrate temperature (300 ± 10 °C) were partially crystalline. The AFM investigation confirmed that lower oxygen pressure, higher deposition rate and higher substrate temperature produce a higher surface roughness of TiO2 f ilm. When the deposition rate (0.1 nm/s) and the substrate temperature (300 ± 10 °C) were kept constant, and the oxygen pressure was varied from 1.3 × 10− 5 to 2.2 × 10− 4 Torr, the roughness (Root-Mean-Square) of as-deposited films was changed from 3.53 to 1.91 nm. © 2006 Published by Elsevier B.V. PACS: 68.55.-a; 78.66.Nk; 78.20.Ci Keywords: TiO2 thin film; E-beam deposition; Optical properties; Film structure; Atomic Force Microscopy
1. Introduction Titanium dioxide (TiO2) thin films possess many excellent optical and electrical properties, so they are applied in many fields. For example, they are widely used in optical coating due to their high refractive index, excellent transmittance and good durability. Because of their unique electro-optical properties, they exhibit a large potential to be used in the fields such as solar energy conversing [1,2], lithium insertion batteries [3], gas sensor [4], photocatalytic detoxification of polluted water [5], air deodorisation [6], and so on. So it is reasonable that many researchers have focused their work on the fabrication of TiO2 thin films. In order to get high quality of TiO2 thin films, a lot of film preparation techniques were employed, such as electron beam (EB) [7–11], ion-assisted deposition (IAD) [12,13], magnetron ⁎ Corresponding author. Department of Physics and Mathematics, Shandong Institute of Architecture and Engineering, Jinan 250101, PR China. Tel.: +86 531 86361596. E-mail address: [email protected] (F.X. Wang). 0257-8972/$ - see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.surfcoat.2006.07.037
sputtering technique [14], pulsed laser deposition (PLD) [15], self-assembly process [16], laser chemical vapor depositing (LCVD) [17], etc. Among them, EB is one of the most traditional methods and is widely used in scientific researches as well as in practical production. A lot of research works have been devoted to investigate the influence of varied depositing conditions on the properties of the films. For example, Pulker et al. prepared TiO2 films by reactive evaporation and observed the influence of starting material composition, substrate temperature, oxygen pressure and depositing rate on the optical properties of the deposited films [18]. Bennett et al. reported the comparison of the properties of TiO2 films prepared by various techniques [19]. Chen et al. investigated the substrate-dependent optical absorption characteristics of TiO2 films [20]. Lehmann and Frick summarized their research results and declared the optimized deposition parameters of EB evaporated TiO2 films [8]. In most of the reports, the optical properties of the TiO2 films are limited in visible light range, though the optical properties of the thin films in near infrared range are also important in practice, so it is significant to investigate the optical properties in larger irradiant range of the
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F.X. Wang et al. / Surface & Coatings Technology 201 (2007) 5367–5370
at 150 ± 10 °C and 300 ± 10 °C by auto-controller system. The thickness of deposited films was controlled at about 550 nm and all the samples were in situ annealed for 10 min after the depositing process was ended. 3. Results and discussions
Fig. 1. Refractive index and extinction coefficient versus wavelength for TiO2 thin films deposited in different oxygen pressures. Substrate temperature: 300 ± 10 °C; depositing rate: 0.1 nm/s.
TiO2 films deposited in different ambient. On the other hand, according to our knowledge, there is no published work related to investigate the influence of depositing conditions on the surface structure of TiO2 thin films. Ti3O5 is one of the Ti oxides and preferable to be used as a starting material in depositing TiO2 film [7]. In the present work, TiO2 films were deposited from Ti3O5 materials by EB in different ambient. The optical properties of as-grown TiO2 films were investigated in visible and near infrared range (300–2000 nm). The structure of the films was checked by Xray Diffraction (XRD) and Atomic Force Microscopy (AFM) techniques.
To investigate the optical characteristics of the TiO2 films, the transmittances of the films were measured by using spectrophotometer (Cary500, Varian). By using an envelope method, the refractive index and extinction coefficient of the films at wavelengths of quarter wave and half wave thickness could be extracted from these transmittance spectra. Fig. 1 shows the dispersion curves of the refractive index and extinction coefficient in the wavelength range of 400 to 1800 nm. It can be seen from the figure that a higher oxygen pressure produces lower refractive index TiO2 films. It must be pointed out that to get higher refractive index TiO2 films, lower oxygen pressure could be used in the depositing process, but the oxygen pressure could not be too low, otherwise the TiO2 film would be deficient in oxygen. According to our experimental results, the lowest oxygen pressure in the depositing process is about 1.0 × 10− 5 Torr, and the depositing rate cannot be higher than 0.1 nm/s in such an oxygen pressure. Fig. 1 shows that the differences of extinction coefficient values among as-deposited TiO2 films are small. The fact that the higher oxygen pressure produces lower refractive index TiO2 films has been explained by Cevro and Carter [9]. They pointed out that as the oxygen pressure increases, the collision of the evaporated species with oxygen molecules would increase, and more collision would let the mobility of the species on the substrate during condensation of the film would be reduced, so a less densely packed film with lower refractive index would be resulted. This explanation
2. Experimental details TiO2 films were deposited by typical EB evaporation. The base pressure of the chamber was below 2.4 × 10− 6 Torr by a rotary pump and cryo-pump. The starting material was Ti3O5 (99.9%, CERAC) and the evaporating voltage was 9.01 kV. Well-polished B270 glasses were employed as the substrates. The oxygen pressure was maintained by controlling the flow rate of O2 through the gas valve. The depositing rate and film thickness were controlled by quartz crystal monitor. The chamber was heated by halogen lamps and the temperature of the substrate was checked by a thermocouple meter which was placed directly on the back of the substrate. The distance from the substrate to the EB source is about 55 cm. In the depositing process, oxygen pressure, depositing rate and substrate temperature are main parameters which should exert important influence on the properties of deposited films. In the present work, the oxygen pressures were varied from 1.3 × 10− 5 to 2.2 × 10− 4 Torr and the deposition rates were varied from 0.1 to 0.6 nm/s. The substrate temperature was kept
Fig. 2. Refractive index and extinction coefficient versus wavelength for TiO2 thin films deposited in different depositing rates. Substrate temperature: 300 ± 10 °C; oxygen pressure: 1.0 × 10− 4 Torr.
F.X. Wang et al. / Surface & Coatings Technology 201 (2007) 5367–5370
Fig. 3. Refractive index and extinction coefficient versus wavelength for TiO2 thin films deposited in different substrate temperatures. Depositing rate: 0.1 nm/s; oxygen pressure: 1.6 × 10− 4 Torr.
seems to be reasonable, which our experimental results supported well. When the deposition rate was used as a variable parameter, while other parameters (TSUB = 300 ± 10 °C, PO2 = 1.0×10− 4Torr) were kept constant, the changes of refractive index and extinction coefficient of the films are shown in Fig. 2. The figure shows that the higher depositing rate produces higher refractive index film. This result is in agreement with that of Pulker et al. [18] and Cevro and Carter [9]. It seems reasonable that a higher deposition rate produces a higher refractive index film, because to get a higher deposition rate, stronger e-beam current is needed, so the evaporated species can get more kinetic energy and higher packing density film can be condensed. The results shown in Fig. 2 suggest that to get higher refractive index TiO2 films a higher deposition rate is desired in the depositing process, but the deposition rate is limited by other parameters. In fact there are several points that must be taken into consideration in the actual depositing process of TiO2 thin film. First, a higher deposition rate needs a higher oxygen pressure, otherwise, the deposited film could show oxygen deficiency. Second, the higher deposition rate allows the surface roughness of the deposited films to increase, which will be proved by AFM investigation later. Third, the thickness of the deposited film is more difficult to be controlled accurately when the deposition rate is high. Synthesizing the affections of several depositing parameters, we suggest that the deposition rate below 0.3 nm/s in TiO2 film depositing process by e-beam is reasonable. Besides oxygen pressure and depositing rate, the substrate temperature is another important parameter in the depositing process. Fig. 3 shows the dependence of the refractive index and extinction coefficient of the films on substrate temperature. The figure indicates that a higher substrate temperature allows the deposited film to get a higher refractive index. Note that it is
5369
difficult to keep exactly the substrate temperature constant because the emitted heat from the crucible allows the temperature to rise even though the auto-controlling system of the temperature is employed in the operated chamber. In the present case, the substrate temperature is kept at 150 ± 10 °C and 300 ± 10 °C, and the XRD result indicates that the samples prepared at high temperature (300 ± 10 °C) are partially crystalline. The surface morphology of the deposited TiO2 films was investigated by AFM. When the deposition rate (0.1 nm/s) and substrate temperature (300 ± 10 °C) were kept constant, and the varied oxygen pressures were 1.3 × 10− 5, 3.2 × 10− 5, 1.0 × 10− 4, 1.6 × 10− 4 and 2.2 × 10− 4 Torr, respectively, the Root-MeanSquare (RMS) of as-deposited films is 3.53, 2.96, 2.84, 2.31 and 1.91 nm, respectively. When the oxygen pressure (1.0 × 10− 4 Torr) and substrate temperature (300 ± 10 °C) were kept constant and the deposition rate was 0.1, 0.3 and 0.6 nm/s, respectively, the RMS of as-deposited films is 2.84, 3.76 and 4.95 nm respectively. When the oxygen pressure (1.6 × 10− 4 Torr) and the deposition rate (0.1 nm/s) were kept constant and the substrate temperature was varied to be about 300 and 150 °C, the RMS of as-deposited films is 2.31 and 1.84 nm, respectively. From these experimental results, we confirmed that lower oxygen pressure, higher deposition rate and higher substrate temperature produce a higher surface roughness TiO2 film. 4. Summary TiO2 films have been deposited from Ti3O5 material by EB in different ambient. The films deposited at high temperature are partially crystalline. The optical properties and structure of the films were influenced strongly by the depositing conditions. Lower oxygen pressure, higher deposition rate and higher substrate temperature produced higher refractive index TiO2 films. At the same time, the varied deposition conditions that can increase the refractive index also allow the surface roughness of the film to increase. When we deposit the TiO2 film with a certain demand on limited surface roughness of the film, the influence of depositing conditions must be considered seriously before starting to deposit. Acknowledgements This work was supported by BK21 project, Republic of Korea. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
B. O'Regan, M. Grätzel, Nature 353 (1991) 737. A. Hugot-le Goff, P. Falaras, J. Electrochem. Soc. 142 (1995) L38. L. Kavan, K. Kratochvilova, M. Grätzel, J. Electroanal. Chem. 394 (1995) 93. H.M. Lin, C.H. Keng, C.Y. Tung, Nanostruct. Mater. 9 (1997) 747. Y.-M. Gao, H.-S. Shen, K. Dwight, A. Wold, Mater. Res. Bull. 27 (1992) 1023. I. Sopyan, S. Murasawa, K. Hashimot, A. Fujishima, Chem. Lett. (1994) 723. K. Balasubramanian, X.F. Han, K.H. Guenther, Appl. Opt. 32 (28) (1993) 5594. Hans W. Lehmann, K. Frick, Appl. Opt. 27 (23) (1988) 4920. M. Cevro, G. Carter, J. Phys. D, Appl. Phys. 28 (1995) 1962. Y. Leprince-Wang, K. Yu-Zhang, V. Nguyen Van, D. Souche, J. Rivory, Thin Solid Films 307 (1997) 38. [11] H.K. Pulker, Thin Solid Films 34 (1976) 343.
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[12] D. Bhattacharyya, N.K. Sahoo, S. Thakur, N.C. Das, Thin Solid Films 360 (2000) 96. [13] G. Atanassov, J. Turlo, J.K. Fu, Y.S. Dai, Thin Solid Films 342 (1999) 83. [14] S. Bhagwat, R.P. Howson, Surf. Coat. Technol. 111 (1999) 162. [15] C. Garapon, C. Champeaux, J. Mugnier, G. Panczer, P. Marchet, A. Catherinot, B. Jacquier, Appl. Surf. Sci. 96–98 (1996) 836. [16] D. Huang, Z.D. Xiao, J.H. Gu, N.P. Huang, C.W. Yuan, Thin Solid Films 305 (1997) 110.
[17] A. Watanabe, Y. Imai, Thin Solid Films 348 (1999) 63. [18] H.K. Pulker, G. Paesold, E. Ritter, Appl. Opt. 15 (12) (1976) 2986. [19] J.M. Bennett, E. Pelletier, G. Albrand, J.P. Borgogno, B. Lszarides, C.K. Carniglia, R.A. Schmell, T.H. Allen, T.T. Hart, K.H. Guenther, A. Saxer, Appl. Opt. 28 (15) (1989) 3303. [20] J.S. Chen, S. Chao, J.S. Kao, G.R. Lai, W.H. Wang, Appl. Opt. 36 (19) (1997) 4403.