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The role of defects on the electrochromic response time of sputter-deposited Ni oxide films
Solid State Ionics 156 (2003) 433 – 437 www.elsevier.com/locate/ssi
The role of defects on the electrochromic response time of sputter-deposited Ni oxide films Kwang-Soon Ahn, Yoon-Chae Nah, Yung-Eun Sung* Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 500-712, South Korea Received 1 April 2002; accepted 15 July 2002
Abstract The role of defects on the electrochromic response time of sputter-deposited Ni oxide films grown by RF magnetron sputtering system was examined. In order to create an excess interstitial oxygens and voids, the sputter-deposited Ni oxides were grown at varying Ar/O2 ambient ratios and RF power, respectively. The electrochromic response time was evaluated by an in situ transmittance measurement during a pulse potential cycling test. Although all of the sputter-deposited Ni oxide films had a similar, amorphous crystallographic structure, the excess interstitial oxygens and the voids affect the electrochromic response time, leading to a disturbance in proton intercalation/deintercalation during the coloring/bleaching processes. Moreover, the voids in the film played a more critical role on the electrochromic response time than an excess of interstitial oxygen. D 2003 Elsevier Science B.V. All rights reserved. PACS: 78.40.Ha; 81.40.Tv; 81.15.Cd Keywords: Electrochromics; Ni oxide; Transmittance; Response time; Sputtering
1. Introduction Electrochromism (EC) is defined as reversible changes in optical properties under an applied electric field [1,2]. Due to their low power consumption ( < 2 V), high coloration efficiency, and good memory effect under open circuit potential, EC devices have many potential applications in EC windows such as smart windows, mirrors, and eyewear as well as in EC displays including mobile phones, smart cards, and
*
Corresponding author. Tel.: +82-62-970-2302; fax: +82-62970-2304. E-mail address: [email protected] (Y.-E. Sung).
price labels [1,2]. However, the response time of EC devices may limit their application to only EC windows. Studies in this laboratory have previously shown that a Ni metal nanolayer rather than indium tin oxide (ITO), currently the most widely used material for the transparent conducting layer, could be utilized to prepare an EC device with a faster response time [3,4]. The ion diffusivity and electron conductivity of the coloration materials are also important factors in the response time of an EC device. WO3 and Ni oxides are currently in widespread use as cathodic and anodic coloration electrochromic materials, respectively [1,2]. WO3 has been extensively studied [1,2,5 –8] and has been shown to have a faster response time than Ni oxide [3,4].
0167-2738/02/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 6 9 6 - 3
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K.-S. Ahn et al. / Solid State Ionics 156 (2003) 433–437
Therefore, the response time of Ni oxide may limit the total response time of a complementary EC device. The electrochromic response time of Ni oxide may be strongly affected by defects in the film. In this paper, we report on an investigation of the role of defects, such as excess interstitial atoms and voids, on the electrochromic response time of sputter-deposited Ni oxide films. In order to create excess interstitial atoms and voids in the sputter-deposited Ni oxide film, the sputtering processes were performed at varying Ar/O2 ambient ratios and RF power, respectively. We also report on which type of the defect in the Ni oxide is the more dominant factor in limiting electrochromic response speed.
counter and reference electrodes, respectively. Sputter-deposited Ni oxide films and aqueous 1 M NaOH were used as the working electrode and the electrolyte, respectively. Continuous potential cycling (or linear-sweep potential cycling) was first carried out for up to 50 cycles in the range of 0.9 to 0.62 V at a scan rate of 10 mV/s, as described elsewhere [9]. This process was required to achieve a stable performance of the electrochromic reaction, because the sputter-deposited Ni oxides were transformed into Ni(OH)2, the active electrochromic phase, during potential cycling, and then reversibly reacted by the following electrochromic reaction [1,9,10,11,12]:
2. Experimental
NiðOHÞ2 X NiOOH þHþ þ e colored bleached oxidized reduced
Ni oxide films were grown using an RF magnetron sputtering system. ITO (Samsung Corning)-coated transparent glass was used as the substrate. NiO was used as the target material and the distance between the target and the substrate was 8 cm. The base pressure was below 5 10 6 Torr and the working pressure was 5 10 3 Torr. Prior to the actual sputtering, a presputtering process was performed for 30 min in order to eliminate contamination from the target. Sputtering was then conducted at different Ar/O2 ratios at 60 W and different RF power at an Ar/ O2 ratio of 1:1, respectively, at room temperature (RT). The thicknesses of all the films examined here ˚ thick and were controlled so as to be about 2000 A were measured by cross-sectional scanning electron microscopy (SEM). The O/Ni ratio and film density of the sputter-deposited Ni oxides were measured by Rutherford backscattering spectroscopy (RBS). A dose of 10 AC of He2 + ions was used to obtain the RBS spectra. The ions had an incident energy of 2.24 MeV and a scattering angle of 165j. Hypra, Charls Evans and Associates’ simulation software program was used for the analysis of RBS data. Crystallographic structure was examined by X-ray diffraction (XRD) measurements, using a Rigaku diffractometer operated with a Cu Ka radiation source at 40 kV and 40 mA. All electrochemical potential cycling tests were performed using an Autolab PGSTAT30 Potentiostat/Galvanostat. Pt and Ag wires were used as the
Pulse potential cycling tests were then performed between 0.9 and 0.6 V with a duration time of 30 s and provided kinetic information such as the electrochromic response time during the coloring/bleaching processes, respectively. The transmittance (633 nm, 10 mW He/Ne laser) was simultaneously measured in situ during the pulse potential cycling tests. The transmittance of the ITO/glass with the electrolyte and the cell window was assumed to be 100%.
3. Results and discussion Fig. 1 shows the growth rates of sputter-deposited Ni oxide films grown as a function of (a) Ar/O2 ratio and (b) RF power. The growth rate was independent of the Ar/O2 ratio but increased with increasing the RF power due to the higher sputtering yield achieved. X-ray diffraction (XRD) showed that all the films examined here had an amorphous crystallographic structure (not shown here). Fig. 2 shows (a) a typical RBS spectrum of 60 W Ni oxide (a simple expression for denoting the sputter-deposited Ni oxide grown at 60 W), and (b, c) O/Ni ratios and film densities for the Ni oxides grown as function of Ar/O2 ratio and RF power, respectively. The actual film densities were calculated from the area densities, obtained from the RBS analysis, with an actual film thickness, measured by
K.-S. Ahn et al. / Solid State Ionics 156 (2003) 433–437
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thus, nearly independent of RF power. On the contrary, the film density decreased significantly at an RF power above 100 W, due to the rapid growth rate. This indicates that the 140-W Ni oxide film includes many voids. Defects such as excess interstitial oxygens and voids, as mentioned above, may affect the electrochromic response time during the coloring/
Fig. 1. Growth rates of sputter-deposited Ni oxide films grown as a function of (a) Ar/O2 ratio and (b) RF power, respectively.
cross-sectional SEM images, according to the following equation [13]: Area density ðatoms=cm2 Þ ¼ Dactual Tactual where Dactual and Tactual indicate the actual film density and actual film thickness, respectively. Fig. 2(b) shows that the O/Ni ratios used in the production of sputter-deposited Ni oxide films increase for Ar/O2 ratios above 1:1, indicating that excess oxygens occupy the interstitial sites in the Ni oxide film. The film densities were found to be independent of the Ar/O2 ratio, due to their similar growth rate, as shown in Fig. 1(a). However, the dependency of RF power on the O/Ni ratio and film density was quite different from that found for different Ar/O2 ratios. Fig. 2(c) shows that the O/Ni ratios of sputterdeposited Ni oxide films are nearly constant and,
Fig. 2. (a) Typical RBS data for 60-W Ni oxide (open circles: experimental, solid line: simulated). (b, c) O/Ni ratio and film densities for sputter-deposited Ni oxide films grown as a function of Ar/O2 ratio and RF power, respectively.
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Ni oxide has good cycling performance. In the same manner, all the Ni oxide films examined here also showed stable cycling performance, good coloration efficiency (about 36 cm2 C 1), and good memory effect. However, the kinetics such as the electrochromic response time of the Ni oxide films was considerably different. The electrochromic response times during the coloring/bleaching processes were quantitatively calculated from normalized transmittance data of the first transmittance curves of the pulse potential cycling data, as described previously [3,9]. For example, Fig. 3(b) shows the normalized transmittance during the coloring/bleaching processes of the 100 W Ni oxide. The electrochromic response times were calculated on the level of a 90% trans-
Fig. 3. (a) Voltage profile and in situ transmittance curve for the pulse potential cycling test of a 100 W Ni oxide film. (b) Normalized transmittance during the first pulse potential cycling of the oxide film.
bleaching processes. The electrochromic response time was evaluated from data on the pulse potential cycling tests, as described in the Experimental section. Fig. 3(a) shows typical applied voltage and in situ transmittance curves with time during the pulse potential cycling test. These data show that 100 W
Fig. 4. Electrochromic response times as a function of (a) Ar/O2 ratio and (b) RF power, respectively, as calculated from normalized transmittance data.
K.-S. Ahn et al. / Solid State Ionics 156 (2003) 433–437
mittance change; I and II in Fig. 3(b) correspond to the electrochromic response times during the bleaching and coloring processes, respectively. Fig. 4 shows the electrochromic response time as a function of (a) Ar/O2 ratios and (b) RF power, respectively, as calculated from the normalized transmittance data. The response time during the coloring process was, in any case, higher than that during the bleaching process. This can be caused by differences in conductivity between the low conductive Ni(OH)2 and the highly conductive NiOOH [9,14]. Fig. 4(a) shows that the electrochromic response time increases for Ar/O2 ratios above 1:1, indicating that the excess interstitial oxygens disturb the proton intercalation/ deintercalation during the coloring/bleaching processes. Fig. 4(b) shows that the electrochromic response time increases for RF power levels above 100 W. This indicates that the voids in the film, caused by a low film density as shown in Fig. 2(c), also interfere with proton intercalation/deintercalation during the coloring/bleaching processes, in a manner similar to that of excess interstitial oxygens. Moreover, Fig. 4 also shows that the electrochromic response time is more strongly dependent on the RF power than the Ar/O2 ratio. Therefore, the voids in the film represent decisive determinants of the electrochromic response time and are more important than the presence of an excess of interstitial oxygens.
4. Conclusions Sputter-deposited Ni oxide films were grown as a function of the Ar/O2 ratio and RF power at RT in order to create an excess of interstitial oxygens and voids in the films. An RBS analysis confirmed the existence of excess interstitial oxygens and voids for the films grown at high Ar/O2 ratio and RF power, respectively. The electrochromic response times during the coloring/bleaching processes were calculated from in situ transmittance data during a pulse potential cycling test. Despite the similar, amorphous crystallographic structure of all the films, defects such as excess interstitial oxygens and voids in the
437
film resulted in a disruption of proton intercalation/ deintercalation during the coloring/bleaching processes, thus leading to an increase in the electrochromic response time. It is noteworthy that voids in the film played a more critical role for the electrochromic response time than an excess of interstitial oxygens.
Acknowledgements This work was supported by the Korea Energy Management Corporation and the Brain Korea 21 Project from the Ministry of Education in Korea.
References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [2] P.J. Gellings, H.J.M. Bouwmeester, The CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, 1997, Chap. 16. [3] D.J. Jeong, W.S. Kim, Y.-E. Sung, Jpn. J. Appl. Phys. 40 (2001) L708. [4] D.J. Jeong, W.S. Kim, Y.K. Choi, Y.-E. Sung, J. Electroanal. Chem. 511 (2001) 79. [5] S.H. Lee, H.M. Cheong, C.E. Tracy, A. Mascarenhas, J.R. Pitts, G. Jorgensen, S.K. Deb, Appl. Phys. Lett. 76 (2000) 3908. [6] H. Kaneko, K. Miyake, J. Appl. Phys. 66 (1989) 845. [7] L.C. Chen, K.C. Ho, Electrochim. Acta 46 (2001) 2151. [8] C.G. Granqvist, Appl. Phys., A 57 (1993) 3. [9] K.S. Ahn, Y.C. Nah, J.H. Yum, Y.-E. Sung, Jpn. J. Appl. Phys. 41 (2002) L212. [10] S.R. Jiang, B.X. Feng, P.X. Yan, X.M. Cai, S.Y. Lu, Appl. Surf. Sci. 174 (2001) 125. [11] R.S. Conell, D.A. Corrigan, B.R. Powell, Sol. Energy Mater. 25 (1992) 301. [12] Y. Wu, G. Wu, X. Ni, X. Wu, Fourth International Conference on Thin Films Physics and Applications, Proceedings of SPIE, vol. 4086, 2000, p. 418. [13] C.R. Brundle, C.A. Evans Jr., S. Wilson, Encyclopedia of Materials Characterization, Butterworth-Heinemann, Stoneham, 1992, pp. 476 – 487. [14] A. Gorenstein, F. Dechker, W. Estrada, C. Esteves, A. Andersson, S. Passerini, S. Pantaloni, B. Scrosati, J. Electroanal. Chem. 277 (1990) 277.
The role of defects on the electrochromic response time of sputter-deposited Ni oxide films Kwang-Soon Ahn, Yoon-Chae Nah, Yung-Eun Sung* Department of Materials Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), Kwangju 500-712, South Korea Received 1 April 2002; accepted 15 July 2002
Abstract The role of defects on the electrochromic response time of sputter-deposited Ni oxide films grown by RF magnetron sputtering system was examined. In order to create an excess interstitial oxygens and voids, the sputter-deposited Ni oxides were grown at varying Ar/O2 ambient ratios and RF power, respectively. The electrochromic response time was evaluated by an in situ transmittance measurement during a pulse potential cycling test. Although all of the sputter-deposited Ni oxide films had a similar, amorphous crystallographic structure, the excess interstitial oxygens and the voids affect the electrochromic response time, leading to a disturbance in proton intercalation/deintercalation during the coloring/bleaching processes. Moreover, the voids in the film played a more critical role on the electrochromic response time than an excess of interstitial oxygen. D 2003 Elsevier Science B.V. All rights reserved. PACS: 78.40.Ha; 81.40.Tv; 81.15.Cd Keywords: Electrochromics; Ni oxide; Transmittance; Response time; Sputtering
1. Introduction Electrochromism (EC) is defined as reversible changes in optical properties under an applied electric field [1,2]. Due to their low power consumption ( < 2 V), high coloration efficiency, and good memory effect under open circuit potential, EC devices have many potential applications in EC windows such as smart windows, mirrors, and eyewear as well as in EC displays including mobile phones, smart cards, and
*
Corresponding author. Tel.: +82-62-970-2302; fax: +82-62970-2304. E-mail address: [email protected] (Y.-E. Sung).
price labels [1,2]. However, the response time of EC devices may limit their application to only EC windows. Studies in this laboratory have previously shown that a Ni metal nanolayer rather than indium tin oxide (ITO), currently the most widely used material for the transparent conducting layer, could be utilized to prepare an EC device with a faster response time [3,4]. The ion diffusivity and electron conductivity of the coloration materials are also important factors in the response time of an EC device. WO3 and Ni oxides are currently in widespread use as cathodic and anodic coloration electrochromic materials, respectively [1,2]. WO3 has been extensively studied [1,2,5 –8] and has been shown to have a faster response time than Ni oxide [3,4].
0167-2738/02/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 6 9 6 - 3
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K.-S. Ahn et al. / Solid State Ionics 156 (2003) 433–437
Therefore, the response time of Ni oxide may limit the total response time of a complementary EC device. The electrochromic response time of Ni oxide may be strongly affected by defects in the film. In this paper, we report on an investigation of the role of defects, such as excess interstitial atoms and voids, on the electrochromic response time of sputter-deposited Ni oxide films. In order to create excess interstitial atoms and voids in the sputter-deposited Ni oxide film, the sputtering processes were performed at varying Ar/O2 ambient ratios and RF power, respectively. We also report on which type of the defect in the Ni oxide is the more dominant factor in limiting electrochromic response speed.
counter and reference electrodes, respectively. Sputter-deposited Ni oxide films and aqueous 1 M NaOH were used as the working electrode and the electrolyte, respectively. Continuous potential cycling (or linear-sweep potential cycling) was first carried out for up to 50 cycles in the range of 0.9 to 0.62 V at a scan rate of 10 mV/s, as described elsewhere [9]. This process was required to achieve a stable performance of the electrochromic reaction, because the sputter-deposited Ni oxides were transformed into Ni(OH)2, the active electrochromic phase, during potential cycling, and then reversibly reacted by the following electrochromic reaction [1,9,10,11,12]:
2. Experimental
NiðOHÞ2 X NiOOH þHþ þ e colored bleached oxidized reduced
Ni oxide films were grown using an RF magnetron sputtering system. ITO (Samsung Corning)-coated transparent glass was used as the substrate. NiO was used as the target material and the distance between the target and the substrate was 8 cm. The base pressure was below 5 10 6 Torr and the working pressure was 5 10 3 Torr. Prior to the actual sputtering, a presputtering process was performed for 30 min in order to eliminate contamination from the target. Sputtering was then conducted at different Ar/O2 ratios at 60 W and different RF power at an Ar/ O2 ratio of 1:1, respectively, at room temperature (RT). The thicknesses of all the films examined here ˚ thick and were controlled so as to be about 2000 A were measured by cross-sectional scanning electron microscopy (SEM). The O/Ni ratio and film density of the sputter-deposited Ni oxides were measured by Rutherford backscattering spectroscopy (RBS). A dose of 10 AC of He2 + ions was used to obtain the RBS spectra. The ions had an incident energy of 2.24 MeV and a scattering angle of 165j. Hypra, Charls Evans and Associates’ simulation software program was used for the analysis of RBS data. Crystallographic structure was examined by X-ray diffraction (XRD) measurements, using a Rigaku diffractometer operated with a Cu Ka radiation source at 40 kV and 40 mA. All electrochemical potential cycling tests were performed using an Autolab PGSTAT30 Potentiostat/Galvanostat. Pt and Ag wires were used as the
Pulse potential cycling tests were then performed between 0.9 and 0.6 V with a duration time of 30 s and provided kinetic information such as the electrochromic response time during the coloring/bleaching processes, respectively. The transmittance (633 nm, 10 mW He/Ne laser) was simultaneously measured in situ during the pulse potential cycling tests. The transmittance of the ITO/glass with the electrolyte and the cell window was assumed to be 100%.
3. Results and discussion Fig. 1 shows the growth rates of sputter-deposited Ni oxide films grown as a function of (a) Ar/O2 ratio and (b) RF power. The growth rate was independent of the Ar/O2 ratio but increased with increasing the RF power due to the higher sputtering yield achieved. X-ray diffraction (XRD) showed that all the films examined here had an amorphous crystallographic structure (not shown here). Fig. 2 shows (a) a typical RBS spectrum of 60 W Ni oxide (a simple expression for denoting the sputter-deposited Ni oxide grown at 60 W), and (b, c) O/Ni ratios and film densities for the Ni oxides grown as function of Ar/O2 ratio and RF power, respectively. The actual film densities were calculated from the area densities, obtained from the RBS analysis, with an actual film thickness, measured by
K.-S. Ahn et al. / Solid State Ionics 156 (2003) 433–437
435
thus, nearly independent of RF power. On the contrary, the film density decreased significantly at an RF power above 100 W, due to the rapid growth rate. This indicates that the 140-W Ni oxide film includes many voids. Defects such as excess interstitial oxygens and voids, as mentioned above, may affect the electrochromic response time during the coloring/
Fig. 1. Growth rates of sputter-deposited Ni oxide films grown as a function of (a) Ar/O2 ratio and (b) RF power, respectively.
cross-sectional SEM images, according to the following equation [13]: Area density ðatoms=cm2 Þ ¼ Dactual Tactual where Dactual and Tactual indicate the actual film density and actual film thickness, respectively. Fig. 2(b) shows that the O/Ni ratios used in the production of sputter-deposited Ni oxide films increase for Ar/O2 ratios above 1:1, indicating that excess oxygens occupy the interstitial sites in the Ni oxide film. The film densities were found to be independent of the Ar/O2 ratio, due to their similar growth rate, as shown in Fig. 1(a). However, the dependency of RF power on the O/Ni ratio and film density was quite different from that found for different Ar/O2 ratios. Fig. 2(c) shows that the O/Ni ratios of sputterdeposited Ni oxide films are nearly constant and,
Fig. 2. (a) Typical RBS data for 60-W Ni oxide (open circles: experimental, solid line: simulated). (b, c) O/Ni ratio and film densities for sputter-deposited Ni oxide films grown as a function of Ar/O2 ratio and RF power, respectively.
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K.-S. Ahn et al. / Solid State Ionics 156 (2003) 433–437
Ni oxide has good cycling performance. In the same manner, all the Ni oxide films examined here also showed stable cycling performance, good coloration efficiency (about 36 cm2 C 1), and good memory effect. However, the kinetics such as the electrochromic response time of the Ni oxide films was considerably different. The electrochromic response times during the coloring/bleaching processes were quantitatively calculated from normalized transmittance data of the first transmittance curves of the pulse potential cycling data, as described previously [3,9]. For example, Fig. 3(b) shows the normalized transmittance during the coloring/bleaching processes of the 100 W Ni oxide. The electrochromic response times were calculated on the level of a 90% trans-
Fig. 3. (a) Voltage profile and in situ transmittance curve for the pulse potential cycling test of a 100 W Ni oxide film. (b) Normalized transmittance during the first pulse potential cycling of the oxide film.
bleaching processes. The electrochromic response time was evaluated from data on the pulse potential cycling tests, as described in the Experimental section. Fig. 3(a) shows typical applied voltage and in situ transmittance curves with time during the pulse potential cycling test. These data show that 100 W
Fig. 4. Electrochromic response times as a function of (a) Ar/O2 ratio and (b) RF power, respectively, as calculated from normalized transmittance data.
K.-S. Ahn et al. / Solid State Ionics 156 (2003) 433–437
mittance change; I and II in Fig. 3(b) correspond to the electrochromic response times during the bleaching and coloring processes, respectively. Fig. 4 shows the electrochromic response time as a function of (a) Ar/O2 ratios and (b) RF power, respectively, as calculated from the normalized transmittance data. The response time during the coloring process was, in any case, higher than that during the bleaching process. This can be caused by differences in conductivity between the low conductive Ni(OH)2 and the highly conductive NiOOH [9,14]. Fig. 4(a) shows that the electrochromic response time increases for Ar/O2 ratios above 1:1, indicating that the excess interstitial oxygens disturb the proton intercalation/ deintercalation during the coloring/bleaching processes. Fig. 4(b) shows that the electrochromic response time increases for RF power levels above 100 W. This indicates that the voids in the film, caused by a low film density as shown in Fig. 2(c), also interfere with proton intercalation/deintercalation during the coloring/bleaching processes, in a manner similar to that of excess interstitial oxygens. Moreover, Fig. 4 also shows that the electrochromic response time is more strongly dependent on the RF power than the Ar/O2 ratio. Therefore, the voids in the film represent decisive determinants of the electrochromic response time and are more important than the presence of an excess of interstitial oxygens.
4. Conclusions Sputter-deposited Ni oxide films were grown as a function of the Ar/O2 ratio and RF power at RT in order to create an excess of interstitial oxygens and voids in the films. An RBS analysis confirmed the existence of excess interstitial oxygens and voids for the films grown at high Ar/O2 ratio and RF power, respectively. The electrochromic response times during the coloring/bleaching processes were calculated from in situ transmittance data during a pulse potential cycling test. Despite the similar, amorphous crystallographic structure of all the films, defects such as excess interstitial oxygens and voids in the
437
film resulted in a disruption of proton intercalation/ deintercalation during the coloring/bleaching processes, thus leading to an increase in the electrochromic response time. It is noteworthy that voids in the film played a more critical role for the electrochromic response time than an excess of interstitial oxygens.
Acknowledgements This work was supported by the Korea Energy Management Corporation and the Brain Korea 21 Project from the Ministry of Education in Korea.
References [1] C.G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995. [2] P.J. Gellings, H.J.M. Bouwmeester, The CRC Handbook of Solid State Electrochemistry, CRC Press, Boca Raton, 1997, Chap. 16. [3] D.J. Jeong, W.S. Kim, Y.-E. Sung, Jpn. J. Appl. Phys. 40 (2001) L708. [4] D.J. Jeong, W.S. Kim, Y.K. Choi, Y.-E. Sung, J. Electroanal. Chem. 511 (2001) 79. [5] S.H. Lee, H.M. Cheong, C.E. Tracy, A. Mascarenhas, J.R. Pitts, G. Jorgensen, S.K. Deb, Appl. Phys. Lett. 76 (2000) 3908. [6] H. Kaneko, K. Miyake, J. Appl. Phys. 66 (1989) 845. [7] L.C. Chen, K.C. Ho, Electrochim. Acta 46 (2001) 2151. [8] C.G. Granqvist, Appl. Phys., A 57 (1993) 3. [9] K.S. Ahn, Y.C. Nah, J.H. Yum, Y.-E. Sung, Jpn. J. Appl. Phys. 41 (2002) L212. [10] S.R. Jiang, B.X. Feng, P.X. Yan, X.M. Cai, S.Y. Lu, Appl. Surf. Sci. 174 (2001) 125. [11] R.S. Conell, D.A. Corrigan, B.R. Powell, Sol. Energy Mater. 25 (1992) 301. [12] Y. Wu, G. Wu, X. Ni, X. Wu, Fourth International Conference on Thin Films Physics and Applications, Proceedings of SPIE, vol. 4086, 2000, p. 418. [13] C.R. Brundle, C.A. Evans Jr., S. Wilson, Encyclopedia of Materials Characterization, Butterworth-Heinemann, Stoneham, 1992, pp. 476 – 487. [14] A. Gorenstein, F. Dechker, W. Estrada, C. Esteves, A. Andersson, S. Passerini, S. Pantaloni, B. Scrosati, J. Electroanal. Chem. 277 (1990) 277.