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ZnO multilayer films for the application of a very low resistance transparent electrode
Applied Surface Science 252 (2006) 7509–7514 www.elsevier.com/locate/apsusc
ZnO/Ag/ZnO multilayer films for the application of a very low resistance transparent electrode D.R. Sahu *, Shin-Yuan Lin, Jow-Lay Huang Department of Materials Science and Engineering, National Cheng Kung University, No. 1, Ta-Hsueh Road, Tainan 701, Taiwan Received 6 July 2005; received in revised form 2 September 2005; accepted 2 September 2005 Available online 13 October 2005
Abstract Transparent conductive ZnO/Ag/ZnO multilayer electrodes having much lower electrical resistance than the widely used transparent electrodes were prepared by simultaneous RF magnetron sputtering of ZnO and DC magnetron sputtering of Ag. An Ag film with different thickness was used as intermediate metallic layers. The optimum thickness of Ag thin films was determined to be 6 nm for high optical transmittance and good electrical conductivity. With about 20–25 nm thick ZnO films, the multilayer showed high optical transmittance in the visible range of the spectrum and had color neutrality. The electrical and optical properties of the multilayers were changed mainly by Ag film properties. A high quality transparent electrode, having sheet resistance as low as 3 ohm/sq and high transmittance of 90% at 580 nm, was obtained and could be reproduced by controlling the preparation parameter properly. The above property is suitable as transparent electrode for dye sensitized solar cells (DSSC). # 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Transparent electrode; Multilayer
1. Introduction Transparent conducting oxides (TCOs) such as impurity doped indium oxides, tin oxides, zinc oxide systems have been used in numerous optoelectronic devices such as flat panel displays [1,2] and photo * Corresponding author. Tel.: +886 6 2754410; fax: +886 6 2754410. E-mail addresses: [email protected], [email protected] (D.R. Sahu), [email protected] (J.-L. Huang).
voltaic solar cells [3,4]. However, their resistivity is rather high in some cases to adapt as a transparent electrode for improved practical application. Method of depositing thin film with reduced resistivity is being investigated in order to accommodate the increasing technological demand for large area devices with improved performance. Recently, for the improvement of the conductivity of transparent electrode, ITOmetal–ITO multilayer systems are used. A thin metal layer of about 10 nm thickness was embedded between two ITO layers [5–7]. These IMI structures
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.09.021
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have very low sheet resistance, high optical transparency in the visible range, relatively lower thickness than single-layered TCO film and better durability than single-layered metal film [8–11]. However, the major cost factors, in the production of TCO are the extremely high target cost of ITO [12]. One of the most potential candidates to substitute ITO film is being the ZnO due to its non-toxicity [13], low cost [14], material abundance [15], high stability against hydrogen plasma and heat cycling [16]. The structural characteristics, electrical and optical properties of the ZnO films have been investigated widely [17–21] while ZnO-based multilayers are still under investigation. It is well known that the optical and electrical properties of very thin metal films depend considerably on their structures. To get bulk like properties, the metal film should form a continuous structure, although they must be thin for high transmittance. Ag metal films, which have highest conductivity of all metals has been already used for ITO-based [22,5–7] multilayer for lower resistance good transparent conducting electrode. However, there is no report related to Ag and ZnO-based multilayer for the application of low resistance transparent electrode. Therefore, we used silver and developed ZnO/Ag/ZnO (ZAZ) transparent conductive film. The influence of the preparation process on the properties of the film was investigated.
The rotating speed of the substrate was 18 rpm. The thickness of the ZnO layer was varied between 20 and 100 nm and those of Ag were between 1 and 15 nm. Film thickness was measured using a surface profiler (Alpha-step 500, TENCOR) and FE-SEM (XL-40 FEG field emission scanning electron microscope). Conventional u–2u XRD studies on the films were carried out in Regaku (D/MAX 2500) diffractometer using Cu Ka radiation to investigate the crystallinity and crystal orientation of the films. Sheet resistance was measured using 4-point probe method. Optical transmittance was measured in the range of 300– 800 nm by UV–vis–IR spectrophotometer (Hewlett Packard 8452A diode array spectrophotometer).
3. Results and discussion The crystalline structure of different multilayers was determined by XRD measurements. Fig. 1 presents the XRD patterns of as deposited ZnO and ZAZ multilayers. A strong (0 0 2) peak along with (1 0 3) was seen for ZnO film. Strong (0 0 2) preferential orientation, indicating polycrystalline nature of the film. In case of ZAZ multilayer, another (1 0 2) peak was developed but ZnO grains are mainly (0 0 2) aligned corresponding to wurtzite structure of ZnO [23]. Silver had (1 1 1) orientation. However, with increase of thickness of Ag layer, additional
2. Experimental The thin films of ZnO and ZnO/Ag/ZnO structures were sputter deposited on glass (corning 1737F) using a zinc oxide (99.9995 purity, 7.62 cm diameter, 0.64 cm thickness, target materials Inc.) and metal Ag targets (99.999% purity, 7.62 cm diameter, 0.64 cm thickness, target materials Inc.) in an inline magnetron sputter deposition system equipped with DC and RF power suppliers. The glass substrate was ultrasonically cleaned in acetone, rinsed in deionized water and subsequently dried in flowing nitrogen gas before deposition. The sputtering was performed in argon atmosphere with a target to substrate distance of 53 mm. The sputtering was carried out at a pressure 6 10 3 Torr in pure Ar with varying sputtering parameters such as argon flow rate and RF/DC power.
Fig. 1. The XRD patterns of as deposited ZnO and ZnO/Ag/ZnO multilayers.
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(2 0 0) Ag peak was developed. Addition of Ag layer enhanced the polycrystallinity of the ZnO top layer in ZAZ multilayer system. The behavior of 4–6 nm Ag film is same having higher crystallinity with increase of thickness. Fig. 2 shows the relationship between transmittance and wavelength for different thickness of Ag layer and fixed thickness of upper and lower ZnO layer. The transmittance of the single layer ZnO film (20 nm) has a peak near 450 nm and the transmittance is greater than 80% over the entire visible light wavelength range. For ZnO/Ag/ZnO film, there is a shift of the transmission peak, which is observed at 580–600 nm. The transmittance decreases in the short as well as in the long wavelength regions as the thickness of the silver layer was increased. The transmittance of the ZAZ film for different thickness of ZnO layer and a constant silver layer thickness of 6 nm was presented in Fig. 3. The peak transmittance shifted towards the long wavelength region when the thickness of the two ZnO (upper and lower) layers was increased. When the film thickness of the two ZnO film was 20 nm, a high transmittance value was obtained for the entire visible light wavelength region. With increase of ZnO thickness, surface roughness may increase resulting in more scattering of the incident light and reduction of the transmission. Fig. 4(a)–(c) shows the AFM images of multilayers with different thickness of ZnO layer. The surface roughness of multilayer film increases with increase of
thickness of ZnO layer. Film with ZnO (20 nm) shows lower roughness than that of other film, which corroborates the results of transmittance. The thickness variation of roughness was also reported by Lin et al. [24,25] for Ti- and Al-doped ZnO film, which increases with increase of thickness. Laukaitis et al. [26] reported that roughness values were very close to the morphologies of growing film, which affects the transmittance and resistivity of film. Cross-sectional SEM images of the multilayer film presented in Fig. 5 shows that Ag layer is not perfectly stable. There is diffusion of Ag inside the ZnO layer, which changes the roughness value of the film and also the morphology of films. Fig. 6 shows the sheet resistance and the percentage of maximum transmittance of a ZnO (20 nm)/Ag/ZnO (20 nm) multilayer with different average Ag layer thickness. The thickness of the silver layer was varied from 2 to 15 nm. Regardless of the thickness, the resistance of the multilayer film was always lower than that of the resistance (4.2 kV) of single layer ZnO film (film thickness: 100 nm), which lack silver layer. Further more, the multilayer film resistance decreased as the silver layer thickness increased. The enhancement of the silver layer crystallinity with thickness result in the decrease of the sheet resistance. The film resistance was 3 ohm/sq for a silver layer thickness of 6 nm. Starting with thick Ag films, it is also seen that transmission is increasing and the sheet resistance is rising slightly with decreasing Ag thickness.
Fig. 2. Dependence of transmittance of ZnO/Ag/ZnO multilayer film on the thickness of silver layer.
Fig. 3. Dependence of transmittance of ZnO/Ag/ZnO film on the ZnO layer thickness.
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Fig. 4. The AFM images of ZnO/Ag/ZnO film with different thickness of ZnO layer: (a) 20 nm, (b) 50 nm and (c) 100 nm.
This behavior changes drastically at a thickness of approximately 6 nm. Transmittance of the layer stack also decreases with further reduction of the Ag layer thickness. This is due to the absorption in the aggregated Ag film. In the same range of Ag thickness, a sharp rise in the sheet resistance was also observed. As reported [27], a continuous layer of silver has low absorption and very good electrical conductivity. A thin layer of silver becomes transparent in the visible spectrum range. Below a critical layer thickness, the materials properties differ considerably from those of bulk material. Both the electrical
resistivity and the absorption of light rapidly increase with a further decrease in the layer thickness beyond this point. This behavior is attributed to a transition from a continuous film to the formation of distinct islands of silver atoms (aggregated state) [28]. The critical thickness for this transition depends upon the substrate and the deposition conditions, and is mostly between 5 and 15 nm average thickness. In order to check the influence of the ZnO layer to the total sheet resistance of the multilayer structure, different sample series with an average Ag layer thickness between 2 and 20 nm were prepared.
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Fig. 5. The cross-sectional SEM image of ZnO (50 nm)/Ag (6 nm)/ ZnO (50 nm) film. Fig. 7. Comparison between different series of ZnO/Ag/ZnO film with different Ag thickness. The embedding ZnO layers are of 20, 30 and 50 nm.
Fig. 6. Dependence of sheet resistance and maximum transmittance on the Ag thickness for a ZnO(20 nm)/Ag/ZnO(20 nm) film.
Samples differ in ZnO thickness was chosen to be 20, 30 and 50 nm for each ZnO layer. For each Ag thickness, the ratios between the sheet resistances are examined. The results are given in Fig. 7 It is observed that for very thin films there is a remarkable difference in the sheet resistance between the stacks. For optimum thickness, there is almost no difference in the sheet resistance between the multilayers. Fahland et al. [27] also observed the same type of behavior for silver-based ITO multilayers. From these results, it is cleared that the quality of ZnO regarding its electrical properties is only of minor importance for the whole structure. This is the key of substituting the single ZnO film with three-layer stack. The quality is determined by the properties of the Ag film instead of the ZnO
properties. This means that the basic requirement of high substrate temperature can be avoided for low resistivity electrodes. Of course, Ag can also show some problem. It concerns corrosion or a restriction in the maximum transmittance, even for very thin films. Nevertheless, the demonstrated solution can be a new method to synthesize low resistivity electrodes. However, properties of ZnO are also very important for the performance of the multilayer electrodes. The exact shape, percentage of transmission and low resistance can be determined by the thickness of the ZnO embedding the Ag layer by controlling the exact process parameter.
4. Conclusions Different multilayer structure of ZnO/Ag/ZnO has been examined and developed as transparent conductive film with low resistance. The multilayer stack can be optimized to have sheet resistance of 3 ohm/sq at a total transmittance over 90% at 580 nm. This makes it possible to synthesize low resistivity electrode at room temperature without using high substrate temperature or post annealing process. This condition may also be favorable for deposition on polymer materials, which can be better used as transparent electrode for solar cell and other display application.
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Acknowledgements This research has been carried out under the financial support by the National Science Council of Taiwan under Contract no. NSC-93-2811-M-006-016. Author D.R. Sahu is thankful to NSC for providing him a postdoctoral position to carry out this work.
References [1] M. Fahland, P. Karlsson, C. Charton, Thin Solid Films 392 (2001) 334. [2] Y.H. Tak, K.B. Kim, H.G. Park, K.H. Lee, J.R. Lee, Thin Solid Films 411 (2002) 12. [3] H.S. Ullal, K. Zwaibel, B. Von Roedern, in: Proceedings of the 29th IEEE Photovolatic Specialists Conference, 2002, p. 472. [4] J. Pia, M. Tamasi, R. Rizzoli, M. Losurdo, E. Centurioni, C. Summonte, F. Rubinelli, Thin Solid Films 425 (2003) 185. [5] J. Stollenwerk, B. Ockler, K.H. Kretschmer, Digest of SIDDMTC (1995) 111. [6] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Thin Solid Films 326 (1998) 72. [7] K.H. Choi, J.Y. Kim, Y.S. Lee, H.J. Kim, Thin Solid Films 341 (1999) 152. [8] A. Kloppel, W. Kriegseis, B.K. Meyer, A. Scharmann, C. Daube, J. Stollenwerk, J. Trube, Thin Solid Films 365 (2000) 139. [9] A. Kloppel, M. Meyer, J. Trube, Thin Solid Films 392 (2001) 311. [10] M. Sawada, M. Higuchi, S. Kondo, H. Saka, Jpn. J. Appl. Phys. 40 (2001) 3332.
[11] Y.S. Jung, Y.W. Choi, H.C. Lee, D.W. Lee, Thin Solid Films 440 (2003) 278. [12] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Thin Solid Films 326 (1998) 67. [13] K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1. [14] S.S. Lin, J.L. Huang, D.F. Lii, Mat. Chem. Phys. 90 (2005) 22. [15] D. Song, A.G. Aberle, J. Xia, Appl. Surf. Sci. 195 (2002) 291. [16] J. Hu, R.G. Gordon, J. Appl. Phys. 71 (1992) 880. [17] I. Sayago, M. Aleixandre, L. Ares, M.J. Fernandez, J.P. Santos, J. Gutierrez, M.C. Horrillo, Appl. Surf. Sci. 245 (2005) 173. [18] A.B. Kashyout, M. Soliman, M.E. Gamal, M. Fathy, Mat. Chem. Phys. 90 (2005) 230. [19] M. Berber, V. Bulto, R. Klib, H. Hahn, Scripta Mater. 53 (2005) 457. [20] Y.W. Sun, J. Gospodyn, P. Kursa, J. Sit, R.G. DeCorby, Y.Y. Tsui, Appl. Surf. Sci. 248 (2005) 392. [21] N. Scarisoreanu, D.G. Matei, G. Dinescu, G. Epurescu, C. Ghica, L.C. Nistor, M. Dinescu, Appl. Surf. Sci. 247 (2005) 518. [22] M. Sawada, M. Higuchi, S. Kondo, H. Saka, Jap. J. Appl. Phys. 40 (2001) 3332. [23] Powder diffraction file, Data card 5-644, 3c PDS International Center for Diffraction Data, Swartmore, PA. [24] S.S. Lin, J.L. Huang, D.F. Lii, Surf. Coat. Technol. 190 (2005) 372. [25] S.S. Lin, J.L. Huang, Ceram. Inter. 30 (2004) 497. [26] G. Laukaitis, S. Lindroos, S. Tamulevieius, M. Leskela, Appl. Surf. Sci. 185 (2001) 134. [27] M. Fahland, P. Karlsson, C. Charton, Thin Solid Films 392 (2001) 334. [28] C. Charton, M. Fahland, in: Proceedings of the Conference on Plasma and Surface Engineering (PSE), 2000.
ZnO/Ag/ZnO multilayer films for the application of a very low resistance transparent electrode D.R. Sahu *, Shin-Yuan Lin, Jow-Lay Huang Department of Materials Science and Engineering, National Cheng Kung University, No. 1, Ta-Hsueh Road, Tainan 701, Taiwan Received 6 July 2005; received in revised form 2 September 2005; accepted 2 September 2005 Available online 13 October 2005
Abstract Transparent conductive ZnO/Ag/ZnO multilayer electrodes having much lower electrical resistance than the widely used transparent electrodes were prepared by simultaneous RF magnetron sputtering of ZnO and DC magnetron sputtering of Ag. An Ag film with different thickness was used as intermediate metallic layers. The optimum thickness of Ag thin films was determined to be 6 nm for high optical transmittance and good electrical conductivity. With about 20–25 nm thick ZnO films, the multilayer showed high optical transmittance in the visible range of the spectrum and had color neutrality. The electrical and optical properties of the multilayers were changed mainly by Ag film properties. A high quality transparent electrode, having sheet resistance as low as 3 ohm/sq and high transmittance of 90% at 580 nm, was obtained and could be reproduced by controlling the preparation parameter properly. The above property is suitable as transparent electrode for dye sensitized solar cells (DSSC). # 2005 Elsevier B.V. All rights reserved. Keywords: ZnO; Transparent electrode; Multilayer
1. Introduction Transparent conducting oxides (TCOs) such as impurity doped indium oxides, tin oxides, zinc oxide systems have been used in numerous optoelectronic devices such as flat panel displays [1,2] and photo * Corresponding author. Tel.: +886 6 2754410; fax: +886 6 2754410. E-mail addresses: [email protected], [email protected] (D.R. Sahu), [email protected] (J.-L. Huang).
voltaic solar cells [3,4]. However, their resistivity is rather high in some cases to adapt as a transparent electrode for improved practical application. Method of depositing thin film with reduced resistivity is being investigated in order to accommodate the increasing technological demand for large area devices with improved performance. Recently, for the improvement of the conductivity of transparent electrode, ITOmetal–ITO multilayer systems are used. A thin metal layer of about 10 nm thickness was embedded between two ITO layers [5–7]. These IMI structures
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.09.021
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D.R. Sahu et al. / Applied Surface Science 252 (2006) 7509–7514
have very low sheet resistance, high optical transparency in the visible range, relatively lower thickness than single-layered TCO film and better durability than single-layered metal film [8–11]. However, the major cost factors, in the production of TCO are the extremely high target cost of ITO [12]. One of the most potential candidates to substitute ITO film is being the ZnO due to its non-toxicity [13], low cost [14], material abundance [15], high stability against hydrogen plasma and heat cycling [16]. The structural characteristics, electrical and optical properties of the ZnO films have been investigated widely [17–21] while ZnO-based multilayers are still under investigation. It is well known that the optical and electrical properties of very thin metal films depend considerably on their structures. To get bulk like properties, the metal film should form a continuous structure, although they must be thin for high transmittance. Ag metal films, which have highest conductivity of all metals has been already used for ITO-based [22,5–7] multilayer for lower resistance good transparent conducting electrode. However, there is no report related to Ag and ZnO-based multilayer for the application of low resistance transparent electrode. Therefore, we used silver and developed ZnO/Ag/ZnO (ZAZ) transparent conductive film. The influence of the preparation process on the properties of the film was investigated.
The rotating speed of the substrate was 18 rpm. The thickness of the ZnO layer was varied between 20 and 100 nm and those of Ag were between 1 and 15 nm. Film thickness was measured using a surface profiler (Alpha-step 500, TENCOR) and FE-SEM (XL-40 FEG field emission scanning electron microscope). Conventional u–2u XRD studies on the films were carried out in Regaku (D/MAX 2500) diffractometer using Cu Ka radiation to investigate the crystallinity and crystal orientation of the films. Sheet resistance was measured using 4-point probe method. Optical transmittance was measured in the range of 300– 800 nm by UV–vis–IR spectrophotometer (Hewlett Packard 8452A diode array spectrophotometer).
3. Results and discussion The crystalline structure of different multilayers was determined by XRD measurements. Fig. 1 presents the XRD patterns of as deposited ZnO and ZAZ multilayers. A strong (0 0 2) peak along with (1 0 3) was seen for ZnO film. Strong (0 0 2) preferential orientation, indicating polycrystalline nature of the film. In case of ZAZ multilayer, another (1 0 2) peak was developed but ZnO grains are mainly (0 0 2) aligned corresponding to wurtzite structure of ZnO [23]. Silver had (1 1 1) orientation. However, with increase of thickness of Ag layer, additional
2. Experimental The thin films of ZnO and ZnO/Ag/ZnO structures were sputter deposited on glass (corning 1737F) using a zinc oxide (99.9995 purity, 7.62 cm diameter, 0.64 cm thickness, target materials Inc.) and metal Ag targets (99.999% purity, 7.62 cm diameter, 0.64 cm thickness, target materials Inc.) in an inline magnetron sputter deposition system equipped with DC and RF power suppliers. The glass substrate was ultrasonically cleaned in acetone, rinsed in deionized water and subsequently dried in flowing nitrogen gas before deposition. The sputtering was performed in argon atmosphere with a target to substrate distance of 53 mm. The sputtering was carried out at a pressure 6 10 3 Torr in pure Ar with varying sputtering parameters such as argon flow rate and RF/DC power.
Fig. 1. The XRD patterns of as deposited ZnO and ZnO/Ag/ZnO multilayers.
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(2 0 0) Ag peak was developed. Addition of Ag layer enhanced the polycrystallinity of the ZnO top layer in ZAZ multilayer system. The behavior of 4–6 nm Ag film is same having higher crystallinity with increase of thickness. Fig. 2 shows the relationship between transmittance and wavelength for different thickness of Ag layer and fixed thickness of upper and lower ZnO layer. The transmittance of the single layer ZnO film (20 nm) has a peak near 450 nm and the transmittance is greater than 80% over the entire visible light wavelength range. For ZnO/Ag/ZnO film, there is a shift of the transmission peak, which is observed at 580–600 nm. The transmittance decreases in the short as well as in the long wavelength regions as the thickness of the silver layer was increased. The transmittance of the ZAZ film for different thickness of ZnO layer and a constant silver layer thickness of 6 nm was presented in Fig. 3. The peak transmittance shifted towards the long wavelength region when the thickness of the two ZnO (upper and lower) layers was increased. When the film thickness of the two ZnO film was 20 nm, a high transmittance value was obtained for the entire visible light wavelength region. With increase of ZnO thickness, surface roughness may increase resulting in more scattering of the incident light and reduction of the transmission. Fig. 4(a)–(c) shows the AFM images of multilayers with different thickness of ZnO layer. The surface roughness of multilayer film increases with increase of
thickness of ZnO layer. Film with ZnO (20 nm) shows lower roughness than that of other film, which corroborates the results of transmittance. The thickness variation of roughness was also reported by Lin et al. [24,25] for Ti- and Al-doped ZnO film, which increases with increase of thickness. Laukaitis et al. [26] reported that roughness values were very close to the morphologies of growing film, which affects the transmittance and resistivity of film. Cross-sectional SEM images of the multilayer film presented in Fig. 5 shows that Ag layer is not perfectly stable. There is diffusion of Ag inside the ZnO layer, which changes the roughness value of the film and also the morphology of films. Fig. 6 shows the sheet resistance and the percentage of maximum transmittance of a ZnO (20 nm)/Ag/ZnO (20 nm) multilayer with different average Ag layer thickness. The thickness of the silver layer was varied from 2 to 15 nm. Regardless of the thickness, the resistance of the multilayer film was always lower than that of the resistance (4.2 kV) of single layer ZnO film (film thickness: 100 nm), which lack silver layer. Further more, the multilayer film resistance decreased as the silver layer thickness increased. The enhancement of the silver layer crystallinity with thickness result in the decrease of the sheet resistance. The film resistance was 3 ohm/sq for a silver layer thickness of 6 nm. Starting with thick Ag films, it is also seen that transmission is increasing and the sheet resistance is rising slightly with decreasing Ag thickness.
Fig. 2. Dependence of transmittance of ZnO/Ag/ZnO multilayer film on the thickness of silver layer.
Fig. 3. Dependence of transmittance of ZnO/Ag/ZnO film on the ZnO layer thickness.
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Fig. 4. The AFM images of ZnO/Ag/ZnO film with different thickness of ZnO layer: (a) 20 nm, (b) 50 nm and (c) 100 nm.
This behavior changes drastically at a thickness of approximately 6 nm. Transmittance of the layer stack also decreases with further reduction of the Ag layer thickness. This is due to the absorption in the aggregated Ag film. In the same range of Ag thickness, a sharp rise in the sheet resistance was also observed. As reported [27], a continuous layer of silver has low absorption and very good electrical conductivity. A thin layer of silver becomes transparent in the visible spectrum range. Below a critical layer thickness, the materials properties differ considerably from those of bulk material. Both the electrical
resistivity and the absorption of light rapidly increase with a further decrease in the layer thickness beyond this point. This behavior is attributed to a transition from a continuous film to the formation of distinct islands of silver atoms (aggregated state) [28]. The critical thickness for this transition depends upon the substrate and the deposition conditions, and is mostly between 5 and 15 nm average thickness. In order to check the influence of the ZnO layer to the total sheet resistance of the multilayer structure, different sample series with an average Ag layer thickness between 2 and 20 nm were prepared.
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Fig. 5. The cross-sectional SEM image of ZnO (50 nm)/Ag (6 nm)/ ZnO (50 nm) film. Fig. 7. Comparison between different series of ZnO/Ag/ZnO film with different Ag thickness. The embedding ZnO layers are of 20, 30 and 50 nm.
Fig. 6. Dependence of sheet resistance and maximum transmittance on the Ag thickness for a ZnO(20 nm)/Ag/ZnO(20 nm) film.
Samples differ in ZnO thickness was chosen to be 20, 30 and 50 nm for each ZnO layer. For each Ag thickness, the ratios between the sheet resistances are examined. The results are given in Fig. 7 It is observed that for very thin films there is a remarkable difference in the sheet resistance between the stacks. For optimum thickness, there is almost no difference in the sheet resistance between the multilayers. Fahland et al. [27] also observed the same type of behavior for silver-based ITO multilayers. From these results, it is cleared that the quality of ZnO regarding its electrical properties is only of minor importance for the whole structure. This is the key of substituting the single ZnO film with three-layer stack. The quality is determined by the properties of the Ag film instead of the ZnO
properties. This means that the basic requirement of high substrate temperature can be avoided for low resistivity electrodes. Of course, Ag can also show some problem. It concerns corrosion or a restriction in the maximum transmittance, even for very thin films. Nevertheless, the demonstrated solution can be a new method to synthesize low resistivity electrodes. However, properties of ZnO are also very important for the performance of the multilayer electrodes. The exact shape, percentage of transmission and low resistance can be determined by the thickness of the ZnO embedding the Ag layer by controlling the exact process parameter.
4. Conclusions Different multilayer structure of ZnO/Ag/ZnO has been examined and developed as transparent conductive film with low resistance. The multilayer stack can be optimized to have sheet resistance of 3 ohm/sq at a total transmittance over 90% at 580 nm. This makes it possible to synthesize low resistivity electrode at room temperature without using high substrate temperature or post annealing process. This condition may also be favorable for deposition on polymer materials, which can be better used as transparent electrode for solar cell and other display application.
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Acknowledgements This research has been carried out under the financial support by the National Science Council of Taiwan under Contract no. NSC-93-2811-M-006-016. Author D.R. Sahu is thankful to NSC for providing him a postdoctoral position to carry out this work.
References [1] M. Fahland, P. Karlsson, C. Charton, Thin Solid Films 392 (2001) 334. [2] Y.H. Tak, K.B. Kim, H.G. Park, K.H. Lee, J.R. Lee, Thin Solid Films 411 (2002) 12. [3] H.S. Ullal, K. Zwaibel, B. Von Roedern, in: Proceedings of the 29th IEEE Photovolatic Specialists Conference, 2002, p. 472. [4] J. Pia, M. Tamasi, R. Rizzoli, M. Losurdo, E. Centurioni, C. Summonte, F. Rubinelli, Thin Solid Films 425 (2003) 185. [5] J. Stollenwerk, B. Ockler, K.H. Kretschmer, Digest of SIDDMTC (1995) 111. [6] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Thin Solid Films 326 (1998) 72. [7] K.H. Choi, J.Y. Kim, Y.S. Lee, H.J. Kim, Thin Solid Films 341 (1999) 152. [8] A. Kloppel, W. Kriegseis, B.K. Meyer, A. Scharmann, C. Daube, J. Stollenwerk, J. Trube, Thin Solid Films 365 (2000) 139. [9] A. Kloppel, M. Meyer, J. Trube, Thin Solid Films 392 (2001) 311. [10] M. Sawada, M. Higuchi, S. Kondo, H. Saka, Jpn. J. Appl. Phys. 40 (2001) 3332.
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