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Al-doped ZnO coatings deposited by electron beam evaporation
Applied Surface Science 253 (2007) 4886–4890 www.elsevier.com/locate/apsusc
Study on the electrical and optical properties of Ag/Al-doped ZnO coatings deposited by electron beam evaporation D.R. Sahu *, Shin-Yuan Lin, Jow-Lay Huang Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 28 August 2006; received in revised form 26 October 2006; accepted 26 October 2006 Available online 22 November 2006
Abstract A layer of silver was deposited onto the surface of glass substrates, coated with AZO (Al-doped ZnO), to form Ag/AZO film structures, using ebeam evaporation techniques. The electrical and optical properties of AZO, Ag and Ag/AZO film structures were studied. The deposition of Ag layer on the surface of AZO films resulted in lowering the effective electrical resistivity with a slight reduction of their optical transmittance. Ag (11 nm)/AZO (25 nm) film structure, with an accuracy of 0.5 nm for the thickness shows a sheet resistance as low as 5.6 0.5 V/sq and a transmittance of about 66 2%. A coating consisting of AZO (25 nm)/Ag (11 nm)/AZO (25 nm) trilayer structure, exhibits a resistance of 7.7 0.5 V/sq and a high transmittance of 85 2%. The coatings have satisfactory properties of low resistance, high transmittance and highest figure of merit for application in optoelectronics devices including flat displays, thin films transistors and solar cells as transparent conductive electrodes. # 2006 Elsevier B.V. All rights reserved. Keywords: Al-doped ZnO; Ag; TCO; Optical and electrical properties
1. Introduction Nowadays Al-doped ZnO thin films are investigated as transparent conductive electrodes for use in optoelectronics devices including flat displays, thin films transistors, solar cells because of their unique optical and electrical properties [1–3]. For the use as transparent conductive electrodes, a film has to have low resistivity, high absorption in the ultra violent light region and high optical transmission in the visible region. Different technologies such as electron beam evaporation, chemical vapor deposition, laser evaporation, DC and RF magnetron sputtering [4–8] have been reported to produce thin films of AZO with adequate performance for applications. In order to optimize the optical and electrical characteristics, these techniques usually are applied in combination with temperature annealing (during or after deposition process) [9–11]. Annealing procedures increase the optical transmittance and reduce the defects of the crystalline structures (vacancies and interstitial impurities). Then, free carrier density can be reduced and hence the reduction of film electronic conductivity can occur. In order to * Corresponding author. Tel.: +886 6 2754410; fax: +886 6 2763586. E-mail address: [email protected] (D.R. Sahu). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.10.061
increase the electronic conductivity and also to retain the high transmittance, thin silver layer can be deposited on AZO. In this study we select the silver (Ag) film coated AZO because the Ag starts with a low resistivity of 2 10 6 V cm. The sheet resistance of an Ag layer proportionally decreases with the Ag thickness. However, thicker Ag films deteriorate optical properties in the visible wavelength region such as transmittance, reflectance and reflective color. To find a way out of these difficulties, optimization of the electrical resistivity of Ag thin film along with optical properties is essential for a practical use. Recently, it is also known that electrical resistivity of Ag thin film changes with its under layer materials; the resistivity of Ag becomes low when Ag is deposited on ZnO undercoats [12]. We investigated the effect of AZO undercoats on the electrical resistivity of Ag thin films. The study of multilayer system Ag/ AZO produced by e-beam allowed us to determine the optical characteristics of the AZO films and to produce a transparent and conductive electrodes on highly transparent glass substrates. 2. Experimental Thin films of Ag/AZO structures were deposited on glass substrates (corning eagle 2000 glass) in an e-beam evaporation
D.R. Sahu et al. / Applied Surface Science 253 (2007) 4886–4890
system. The multilayer films were successively formed on glass substrates without vacuum break using an Al-doped zinc oxide sintered target [ZnO (OSAKA, 99% purity) doped with 2 wt% Al2O3 (Alcoa, 99.7% purity), pressed and sintered at 1400 8C for 2 h] and metal Ag chips (99.999% purity). The e-beam chamber was pumped down to 9 10 7 Torr prior to deposition. The substrate temperature was measured using a thermocouple gauge. The variation of substrate temperature during deposition was maintained within 5 8C. Substrate temperature was controlled in the range 50–250 8C. Deposition of AZO and Ag films were performed at a pressure of 2 10 5 Torr in the evaporation chamber. AZO films were deposited at 4 kV and 20–30 mA at a substrate temperature of 200 8C. Ag films were deposited at 8 kV and 20 mA at a substrate temperature of 30 8C. Specimens with different thickness fabricated at different conditions were prepared and characterized. The thickness of the film was measured using a surface profiler (Alpha-step 500, TENCOR, USA) and on line thickness measurement system which was further confirmed by crosssectional SEM observation within an accuracy of 0.5 nm. Conventional u–2u XRD studies on the films were carried out in Regaku (D/MAX 2500, Japan) diffractometer using Cu Ka radiation to investigate the crystalinity and crystal orientation of the films. Surface morphology was observed by field emission scanning electron microscope (FESEM, XL-40, The Netherlands). Sheet resistance was measured using four-point probe method. Optical transmittance was measured in the range 300–800 nm by UV–Vis–IR spectrophotometer (Hewlett Packard 8452A, Palo Alto CA, USA). 3. Results and discussion Fig. 1 shows the XRD spectra of AZO (25 nm) and Ag/AZO (25 nm) films for different thickness of Ag layer. The spectrum of the as deposited AZO film exhibit the existence of (0 0 2), (1 0 3), (1 0 2) ZnO crystal planes. There is no Al diffraction peak appeared in the pattern. However, in the spectrum of Ag/ AZO film, silver has (1 1 1) orientation and there is a small shift of ZnO peaks towards lower diffraction angle. Shift of (0 0 2) peaks towards the lower diffraction angle with the addition of Ag layer indicates that there is small distortion of crystallites
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Fig. 1. XRD patterns of AZO and Ag/AZO thin films for fixed thickness of AZO layer (25 nm).
[13,14]. With the increase of thickness of Ag layer, more intense (2 0 0), (2 2 0), (3 1 1) Ag peaks are also developed. Addition of Ag layer enhanced the polycrystalinity of AZO layer. The oriented Ag films with their (1 1 1) planes parallel to the substrate can be deposited on preferentially grown AZO undercoats. The SEM cross-sectional view of the Ag (11 nm)/ AZO and Ag (14 nm)/AZO thin films shown in Fig. 2 indicates that the layer structures are not parallel to the surface of substrate. There is interdiffsusion of Ag and AZO which may be due to higher reactivity of Ag [15]. Interdiffusion of particles changes the morphology films which can affect the photoelectric properties of the films. So systematic search was carried out to determine the effect of each layer on the optical and electrical properties of the films. The most crucial factor that affects the performance of the coating is the homogeneity of the metal layer as the thickness of Ag layer is not allowed beyond a certain threshold for high transmittance in the visible region. Fig. 3 shows the sheet resistance of Ag/AZO films with the thickness of silver layer. There is a report that silver film does not show significant conductivity until its thickness exceeds few nm, e.g. >30 nm, when it becomes continuous and its resistivity decreases with thickness [16–18]. However, this thickness of silver film depends on the deposition process and
Fig. 2. SEM cross-section view of: (a) Ag (11 nm)/AZO/glass and (b) Ag (14 nm)/AZO/glass film.
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Fig. 3. Variation of sheet resistance of different thin film with Ag layer thickness deposited on glass and with different thickness of AZO layer.
Fig. 5. Transmittance spectra of AZO (20 nm) and Ag/AZO (20 nm) thin film for different thickness of Ag layer.
the substrate on which it is deposited [19–21]. So different thicknesses of Ag films deposited on glass and three different thicknesses of AZO are presented in Fig. 3. It is observed that with the increase of thickness of silver layer, sheet resistance of the film decreases. At about 11 nm of Ag thickness, sheet resistance is almost constant or differs marginally for all films deposited on AZO. Thickness of AZO layer has no much influence on the sheet resistance beyond this thickness of Ag layer. The lowest sheet resistance of about 5.6 V/sq is observed for 25 nm thickness of AZO layer and 11 nm thickness of Ag layer. So 11 nm thickness of Ag layer is supposed to be continuous on AZO surface. The transmittance spectra observed for different thickness of silver layer on glass, 20 nm AZO, 25 nm AZO, 30 nm AZO along with the AZO layer are placed in Figs. 4–7. It is observed from the figures that single AZO layer shows more transmittance that that of the other film in which Ag layer is
deposited on AZO. There is an increase of transmittance with increase of thickness of silver from 3 to 11 nm. However, with further increase of thickness of Ag layer, decrease of transmittance is observed. Maximum transmittance peak was observed at about 450 nm for three Ag/AZO film but minimum transmittance peak for the only Ag film on glass. The transmittance decreases in the short as well as long wavelength regions as the thickness of the silver layer was increased in case of Ag/AZO films. With the increase of the thickness of AZO layer, there is a shift of transmittance curve to wards short wavelength region and it becomes more flat in the visible region for 25 nm AZO layers. The percentage of maximum transmission for all three types of films along with the silver film on glass in the visible spectrum (400–700 nm) is presented in Fig. 8. We have considered the maximum of transmittance in
Fig. 4. Transmittance spectra of Ag/glass thin film for different thickness of Ag layer.
Fig. 6. Transmittance spectra of AZO (25 nm) and Ag/AZO (25 nm) thin film for different thickness of Ag layer.
D.R. Sahu et al. / Applied Surface Science 253 (2007) 4886–4890
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Fig. 7. Transmittance spectra of AZO (30 nm) and Ag/AZO (30 nm) thin film for different thickness of Ag layer.
Fig. 9. Transmittance spectra of AZO/Ag/AZO multilayer structure deposited by e-beam evaporation. Spectra correspond to different thickness of AZO and different thickness of Ag.
the visible spectrum because this value is independent of the interference phenomena associated to multiple reflections of the film and is indicative of their optical observation. From Fig. 8, it is observed that average maximum percentage of transmittance for the films is observed at 11 nm thickness of Ag layer. With increase or decrease of thickness of Ag layer beyond 11 nm, there is a decrease of percentage of transmission. Film with 25 nm AZO show maximum percentage of transmittance which is about 66%. As reported [22], a continuous layer of silver has low absorption and very good electrical conductivity. The absorption of light rapidly increases with further decrease of the layer thickness. This behavior is attributed to a transition from a continuous film to the formation of distinct islands of silver atoms (aggregated state) [19]. With the increase of the thickness of Ag layer beyond 11 nm, it behaves like a reflecting mirror, so there is
more reflection of light and less transmittance. So transmittance decreases and reflection increases with further increase of silver thickness. The multilayer structures AZO/Ag/AZO/glass work in a different manner than the samples based on Ag/AZO/glass. In the multilayer structure, the Ag layer mainly acts as a conductive layer, but its low transmittance is a serious drawback limiting the performance of the complete Ag/AZO structure. The AZO layer in the structure has a double role, its relatively high conductivity assures a good electrical connection between the Ag layer with any point of the structure surface and on the other hand, this transparent layer acts as an antireflection coating reducing the high reflectivity of the Ag layer. Figs. 9 and 10 are diverse comparative plots of the transmittance of several structures. Fig. 9 is a comparative plot of structures AZO/Ag/AZO/glass showing the important
Fig. 8. Variation of optimal maximum transmittance in the visible spectrum for the thin films deposited on glass and different thickness of AZO for different thickness of Ag layer.
Fig. 10. Comparison of transmittance spectra of different AZO, Ag, Ag/AZO and AZO/Ag/AZO thin film on glass.
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differences in transmission due likely to a non-optimized tuning of the thicknesses of AZO layers. From this figures, no important differences can be discerned between 12 and 11 nm of the Ag layer for fixed thickness of AZO layer. Fig. 10 compares the transmittances of a very thin Ag layer (11 nm), an AZO (25 nm), Ag (11 nm)/AZO (25 nm), and AZO/Ag/AZO (61 nm) structures having 25 nm AZO layer and 11 nm Ag layer. This plot evidences no presences of interferences in the transmittance of the structure AZO/Ag/AZO because of the low thickness of AZO layers and an increase of the transmittance in the visible range of the AZO/Ag/AZO structure with respect to transmittance of Ag single layer and Ag/AZO film, despite both samples have an 11 nm Ag layer. Moreover, the transmittance of AZO/Ag/AZO structure is little bit similar to the averaged transmittance of AZO layer (showing 25 nm). Looking at the electrical behavior of the samples, this multilayer structures AZO (25 nm)/Ag (11 nm)/ AZO (25 nm) show sheet resistance of 7.7 V/sq which is little higher than of Ag/AZO layer. This film also shows high transmittance of about 85%. Film of this structure show highest figure of merit (2.55 10 2 V 1) which is a measure for the performance of the transparent conductive oxide (TCO) evaluated using the formula F TC = T10/Rs [23], where T is the transmittance of film and Rs the sheet resistance of the film. So TCO of this multilayer structure can be produced economically with a considerable saving of expensive materials. This technology is compatible with a continuous production line using a complete low temperature processing. 4. Conclusions Multilayer having high transmittance more than 85% and low sheet resistance of about 7.7 V/sq were deposited by ebeam evaporation method. The technology of using 200 8C of substrate temperature for AZO and room temperature for Ag has provided the Ag/AZO film of 36 nm and AZO/Ag/AZO film of 61 nm with the highest TCO figure of merit. These multilayer structures can be produced economically than the
costly used materials for the use of TCO. These Ag based multilayer structure is better choice as TCO for practical application. Acknowledgement Financial support provided by National Science Council of Taiwan under Contract No. NSC-95-2120-M-006-001 is highly acknowledged. References [1] D. Song, A.G. Aberle, J. Xia, Appl. Surf. Sci. 195 (2002) 291. [2] T. Schuler, M.A. Aegerter, Thin Solid Films 351 (1999) 125. [3] K. Matsubara, P. Fons, K. Iwata, A. Yamada, K. Sakurai, H. Tampo, S. Niki, Thin Solid Films 431/432 (2003) 369. [4] M. Jin, J. Feng, Z. De-Heng, M. Hon-Hlei, L. Shu-Yang, Thin Solid Films 357 (1999) 98. [5] V. Musat, B. Teixeira, E. Fortunato, R.C.C. Monteiro, P. Vilarinho, Surf. Coat. Technol. 180/181 (2004) 659. [6] H.M. Suarez, A. Maldonado, M.D.L. Olvera, A. Reyes, R.C. Perez, G.T. Delgado, R. Asomoza, Appl. Surf. Sci 193 (2002) 52. [7] J. Mass, P. Bhattacharya, R.S. Katiyar, Mater. Sci. Eng. B 103 (2003) 9. [8] S.S. Lin, J.L. Huang, P. Sajgalik, Surf. Coat. Technol. 190 (2005) 39. [9] P. Nunes, D. Costa, E. Fortunato, R. Martins, Vacuum 64 (2002) 293. [10] G.J. Fang, D. Li, B.L. Yao, Thin Solid Films 418 (2002) 156. [11] S.S. Lin, J.L. Huang, P. Sajgalik, Surf. Coat. Technol. 185 (2004) 254. [12] Y. Tsuda, H. Omoto, K. Tanaka, H. Ohsaki, Thin Solid Films 502 (2006) 223. [13] T.L. Tansley, D.F. Neely, Thin Solid Films 121 (1984) 95. [14] Putnis, Introduction to Mineral Sciences, vol. 100, 1992. [15] K. Chiba, Thin Solid Films 112 (1984) 359. [16] M. Arbab, MRS Bull. 22 (1997) 27. [17] M. Ohring, The Materials Science of Thin Films, Academic Press, San Diego, 1992. [18] J. Szczrbowski, A. Dietrich, K. Hartig, Sol. Energy Mater. 16 (1987) 103. [19] C. Charton, M. Fahland, in: Proceedings of the Conferences on Plasma and Surface Engineering (PSE), 2000. [20] D.R. Sahu, S.Y. Lin, J.L. Huang, Appl. Surf. Sci. 252 (2006) 7509. [21] K.H. Choi, J.Y. Kim, Y.S. Lee, H.J. Kim, Thin Solid Films 341 (1999) 152. [22] M. Fahland, P. Karlsson, C. Charton, Thin Solid Films 392 (2001) 334. [23] G. Haacke, J. Appl. Phys. 44 (1976) 1981.
Study on the electrical and optical properties of Ag/Al-doped ZnO coatings deposited by electron beam evaporation D.R. Sahu *, Shin-Yuan Lin, Jow-Lay Huang Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Received 28 August 2006; received in revised form 26 October 2006; accepted 26 October 2006 Available online 22 November 2006
Abstract A layer of silver was deposited onto the surface of glass substrates, coated with AZO (Al-doped ZnO), to form Ag/AZO film structures, using ebeam evaporation techniques. The electrical and optical properties of AZO, Ag and Ag/AZO film structures were studied. The deposition of Ag layer on the surface of AZO films resulted in lowering the effective electrical resistivity with a slight reduction of their optical transmittance. Ag (11 nm)/AZO (25 nm) film structure, with an accuracy of 0.5 nm for the thickness shows a sheet resistance as low as 5.6 0.5 V/sq and a transmittance of about 66 2%. A coating consisting of AZO (25 nm)/Ag (11 nm)/AZO (25 nm) trilayer structure, exhibits a resistance of 7.7 0.5 V/sq and a high transmittance of 85 2%. The coatings have satisfactory properties of low resistance, high transmittance and highest figure of merit for application in optoelectronics devices including flat displays, thin films transistors and solar cells as transparent conductive electrodes. # 2006 Elsevier B.V. All rights reserved. Keywords: Al-doped ZnO; Ag; TCO; Optical and electrical properties
1. Introduction Nowadays Al-doped ZnO thin films are investigated as transparent conductive electrodes for use in optoelectronics devices including flat displays, thin films transistors, solar cells because of their unique optical and electrical properties [1–3]. For the use as transparent conductive electrodes, a film has to have low resistivity, high absorption in the ultra violent light region and high optical transmission in the visible region. Different technologies such as electron beam evaporation, chemical vapor deposition, laser evaporation, DC and RF magnetron sputtering [4–8] have been reported to produce thin films of AZO with adequate performance for applications. In order to optimize the optical and electrical characteristics, these techniques usually are applied in combination with temperature annealing (during or after deposition process) [9–11]. Annealing procedures increase the optical transmittance and reduce the defects of the crystalline structures (vacancies and interstitial impurities). Then, free carrier density can be reduced and hence the reduction of film electronic conductivity can occur. In order to * Corresponding author. Tel.: +886 6 2754410; fax: +886 6 2763586. E-mail address: [email protected] (D.R. Sahu). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.10.061
increase the electronic conductivity and also to retain the high transmittance, thin silver layer can be deposited on AZO. In this study we select the silver (Ag) film coated AZO because the Ag starts with a low resistivity of 2 10 6 V cm. The sheet resistance of an Ag layer proportionally decreases with the Ag thickness. However, thicker Ag films deteriorate optical properties in the visible wavelength region such as transmittance, reflectance and reflective color. To find a way out of these difficulties, optimization of the electrical resistivity of Ag thin film along with optical properties is essential for a practical use. Recently, it is also known that electrical resistivity of Ag thin film changes with its under layer materials; the resistivity of Ag becomes low when Ag is deposited on ZnO undercoats [12]. We investigated the effect of AZO undercoats on the electrical resistivity of Ag thin films. The study of multilayer system Ag/ AZO produced by e-beam allowed us to determine the optical characteristics of the AZO films and to produce a transparent and conductive electrodes on highly transparent glass substrates. 2. Experimental Thin films of Ag/AZO structures were deposited on glass substrates (corning eagle 2000 glass) in an e-beam evaporation
D.R. Sahu et al. / Applied Surface Science 253 (2007) 4886–4890
system. The multilayer films were successively formed on glass substrates without vacuum break using an Al-doped zinc oxide sintered target [ZnO (OSAKA, 99% purity) doped with 2 wt% Al2O3 (Alcoa, 99.7% purity), pressed and sintered at 1400 8C for 2 h] and metal Ag chips (99.999% purity). The e-beam chamber was pumped down to 9 10 7 Torr prior to deposition. The substrate temperature was measured using a thermocouple gauge. The variation of substrate temperature during deposition was maintained within 5 8C. Substrate temperature was controlled in the range 50–250 8C. Deposition of AZO and Ag films were performed at a pressure of 2 10 5 Torr in the evaporation chamber. AZO films were deposited at 4 kV and 20–30 mA at a substrate temperature of 200 8C. Ag films were deposited at 8 kV and 20 mA at a substrate temperature of 30 8C. Specimens with different thickness fabricated at different conditions were prepared and characterized. The thickness of the film was measured using a surface profiler (Alpha-step 500, TENCOR, USA) and on line thickness measurement system which was further confirmed by crosssectional SEM observation within an accuracy of 0.5 nm. Conventional u–2u XRD studies on the films were carried out in Regaku (D/MAX 2500, Japan) diffractometer using Cu Ka radiation to investigate the crystalinity and crystal orientation of the films. Surface morphology was observed by field emission scanning electron microscope (FESEM, XL-40, The Netherlands). Sheet resistance was measured using four-point probe method. Optical transmittance was measured in the range 300–800 nm by UV–Vis–IR spectrophotometer (Hewlett Packard 8452A, Palo Alto CA, USA). 3. Results and discussion Fig. 1 shows the XRD spectra of AZO (25 nm) and Ag/AZO (25 nm) films for different thickness of Ag layer. The spectrum of the as deposited AZO film exhibit the existence of (0 0 2), (1 0 3), (1 0 2) ZnO crystal planes. There is no Al diffraction peak appeared in the pattern. However, in the spectrum of Ag/ AZO film, silver has (1 1 1) orientation and there is a small shift of ZnO peaks towards lower diffraction angle. Shift of (0 0 2) peaks towards the lower diffraction angle with the addition of Ag layer indicates that there is small distortion of crystallites
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Fig. 1. XRD patterns of AZO and Ag/AZO thin films for fixed thickness of AZO layer (25 nm).
[13,14]. With the increase of thickness of Ag layer, more intense (2 0 0), (2 2 0), (3 1 1) Ag peaks are also developed. Addition of Ag layer enhanced the polycrystalinity of AZO layer. The oriented Ag films with their (1 1 1) planes parallel to the substrate can be deposited on preferentially grown AZO undercoats. The SEM cross-sectional view of the Ag (11 nm)/ AZO and Ag (14 nm)/AZO thin films shown in Fig. 2 indicates that the layer structures are not parallel to the surface of substrate. There is interdiffsusion of Ag and AZO which may be due to higher reactivity of Ag [15]. Interdiffusion of particles changes the morphology films which can affect the photoelectric properties of the films. So systematic search was carried out to determine the effect of each layer on the optical and electrical properties of the films. The most crucial factor that affects the performance of the coating is the homogeneity of the metal layer as the thickness of Ag layer is not allowed beyond a certain threshold for high transmittance in the visible region. Fig. 3 shows the sheet resistance of Ag/AZO films with the thickness of silver layer. There is a report that silver film does not show significant conductivity until its thickness exceeds few nm, e.g. >30 nm, when it becomes continuous and its resistivity decreases with thickness [16–18]. However, this thickness of silver film depends on the deposition process and
Fig. 2. SEM cross-section view of: (a) Ag (11 nm)/AZO/glass and (b) Ag (14 nm)/AZO/glass film.
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Fig. 3. Variation of sheet resistance of different thin film with Ag layer thickness deposited on glass and with different thickness of AZO layer.
Fig. 5. Transmittance spectra of AZO (20 nm) and Ag/AZO (20 nm) thin film for different thickness of Ag layer.
the substrate on which it is deposited [19–21]. So different thicknesses of Ag films deposited on glass and three different thicknesses of AZO are presented in Fig. 3. It is observed that with the increase of thickness of silver layer, sheet resistance of the film decreases. At about 11 nm of Ag thickness, sheet resistance is almost constant or differs marginally for all films deposited on AZO. Thickness of AZO layer has no much influence on the sheet resistance beyond this thickness of Ag layer. The lowest sheet resistance of about 5.6 V/sq is observed for 25 nm thickness of AZO layer and 11 nm thickness of Ag layer. So 11 nm thickness of Ag layer is supposed to be continuous on AZO surface. The transmittance spectra observed for different thickness of silver layer on glass, 20 nm AZO, 25 nm AZO, 30 nm AZO along with the AZO layer are placed in Figs. 4–7. It is observed from the figures that single AZO layer shows more transmittance that that of the other film in which Ag layer is
deposited on AZO. There is an increase of transmittance with increase of thickness of silver from 3 to 11 nm. However, with further increase of thickness of Ag layer, decrease of transmittance is observed. Maximum transmittance peak was observed at about 450 nm for three Ag/AZO film but minimum transmittance peak for the only Ag film on glass. The transmittance decreases in the short as well as long wavelength regions as the thickness of the silver layer was increased in case of Ag/AZO films. With the increase of the thickness of AZO layer, there is a shift of transmittance curve to wards short wavelength region and it becomes more flat in the visible region for 25 nm AZO layers. The percentage of maximum transmission for all three types of films along with the silver film on glass in the visible spectrum (400–700 nm) is presented in Fig. 8. We have considered the maximum of transmittance in
Fig. 4. Transmittance spectra of Ag/glass thin film for different thickness of Ag layer.
Fig. 6. Transmittance spectra of AZO (25 nm) and Ag/AZO (25 nm) thin film for different thickness of Ag layer.
D.R. Sahu et al. / Applied Surface Science 253 (2007) 4886–4890
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Fig. 7. Transmittance spectra of AZO (30 nm) and Ag/AZO (30 nm) thin film for different thickness of Ag layer.
Fig. 9. Transmittance spectra of AZO/Ag/AZO multilayer structure deposited by e-beam evaporation. Spectra correspond to different thickness of AZO and different thickness of Ag.
the visible spectrum because this value is independent of the interference phenomena associated to multiple reflections of the film and is indicative of their optical observation. From Fig. 8, it is observed that average maximum percentage of transmittance for the films is observed at 11 nm thickness of Ag layer. With increase or decrease of thickness of Ag layer beyond 11 nm, there is a decrease of percentage of transmission. Film with 25 nm AZO show maximum percentage of transmittance which is about 66%. As reported [22], a continuous layer of silver has low absorption and very good electrical conductivity. The absorption of light rapidly increases with further decrease of the layer thickness. This behavior is attributed to a transition from a continuous film to the formation of distinct islands of silver atoms (aggregated state) [19]. With the increase of the thickness of Ag layer beyond 11 nm, it behaves like a reflecting mirror, so there is
more reflection of light and less transmittance. So transmittance decreases and reflection increases with further increase of silver thickness. The multilayer structures AZO/Ag/AZO/glass work in a different manner than the samples based on Ag/AZO/glass. In the multilayer structure, the Ag layer mainly acts as a conductive layer, but its low transmittance is a serious drawback limiting the performance of the complete Ag/AZO structure. The AZO layer in the structure has a double role, its relatively high conductivity assures a good electrical connection between the Ag layer with any point of the structure surface and on the other hand, this transparent layer acts as an antireflection coating reducing the high reflectivity of the Ag layer. Figs. 9 and 10 are diverse comparative plots of the transmittance of several structures. Fig. 9 is a comparative plot of structures AZO/Ag/AZO/glass showing the important
Fig. 8. Variation of optimal maximum transmittance in the visible spectrum for the thin films deposited on glass and different thickness of AZO for different thickness of Ag layer.
Fig. 10. Comparison of transmittance spectra of different AZO, Ag, Ag/AZO and AZO/Ag/AZO thin film on glass.
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differences in transmission due likely to a non-optimized tuning of the thicknesses of AZO layers. From this figures, no important differences can be discerned between 12 and 11 nm of the Ag layer for fixed thickness of AZO layer. Fig. 10 compares the transmittances of a very thin Ag layer (11 nm), an AZO (25 nm), Ag (11 nm)/AZO (25 nm), and AZO/Ag/AZO (61 nm) structures having 25 nm AZO layer and 11 nm Ag layer. This plot evidences no presences of interferences in the transmittance of the structure AZO/Ag/AZO because of the low thickness of AZO layers and an increase of the transmittance in the visible range of the AZO/Ag/AZO structure with respect to transmittance of Ag single layer and Ag/AZO film, despite both samples have an 11 nm Ag layer. Moreover, the transmittance of AZO/Ag/AZO structure is little bit similar to the averaged transmittance of AZO layer (showing 25 nm). Looking at the electrical behavior of the samples, this multilayer structures AZO (25 nm)/Ag (11 nm)/ AZO (25 nm) show sheet resistance of 7.7 V/sq which is little higher than of Ag/AZO layer. This film also shows high transmittance of about 85%. Film of this structure show highest figure of merit (2.55 10 2 V 1) which is a measure for the performance of the transparent conductive oxide (TCO) evaluated using the formula F TC = T10/Rs [23], where T is the transmittance of film and Rs the sheet resistance of the film. So TCO of this multilayer structure can be produced economically with a considerable saving of expensive materials. This technology is compatible with a continuous production line using a complete low temperature processing. 4. Conclusions Multilayer having high transmittance more than 85% and low sheet resistance of about 7.7 V/sq were deposited by ebeam evaporation method. The technology of using 200 8C of substrate temperature for AZO and room temperature for Ag has provided the Ag/AZO film of 36 nm and AZO/Ag/AZO film of 61 nm with the highest TCO figure of merit. These multilayer structures can be produced economically than the
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