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Transparent conductive Sn-doped indium oxide thin films deposited by spray pyrolysis technique
Renewable Energy 29 (2004) 1671–1676 www.elsevier.com/locate/renene
Transparent conductive Sn-doped indium oxide thin films deposited by spray pyrolysis technique S.M. Rozati , T. Ganj Physics Department, The University of Guilan, Rasht 41335, Iran Received 1 August 2003; accepted 19 January 2004
Abstract Films of indium oxide doped with tin (ITO) are prepared using the low cost spray pyrolysis technique. The effect of tin doping on the physical properties of In2O3 are studied. In this study the polycrystalline ITO films with the different Sn concentration of 1 to 100-wt% SnCl2 were prepared on Corning 7059 glass substrate. These films were confirmed to show the high crystallinity by X-ray diffraction technique. Low sheet resistance (25X/_) and high visible transmission (~82%) were obtained when the films were deposited at the tin concentration of 2-wt%. # 2004 Elsevier Ltd. All rights reserved. Keywords: In2O3: Sn; Spray pyrolysis; Transparent conducting oxide
1. Introduction Thin films of SnO2:F , In2O3:F, In2O3:Sn, ZnO:Al etc. [1–4] have been widely used in opto-electronic devices [4]. In order to obtain optimal characteristics of TCO films, the parameters such as substrate temperature [5], thickness [6], dopants [7], and other deposition conditions have to be optimized. TCO films have been prepared by various deposition techniques such as vacuum evaporation [8], dip coating [7], sputtering [9], spray pyrolysis [10], sol-gel [11], etc. In this paper, results are described by preparing coatings by spray pyrolysis while varying tin concentration.
Corresponding author. Fax: +89-131-322-0066. E-mail address: [email protected] (S.M. Rozati).
0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.01.008
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The spray pyrolysis unit in this work consists of a heater capable of heating the v substrate up to 700 C and temperature controller unit to control the substrate temperature. The spray technique is one of the most commonly used techniques for preparation of transparent and conducting oxides owing to its simplicity, nonvacuum system of deposition and hence inexpensive method for large area coatings.
2. Results and discussion In order to prepare the solution for depositing undoped indium oxide films, the following procedure was adapted. The starting material was indium metal that was dissolved in HCl and the solution was heated to complete the reaction. After evaporating the excess water we obtain indium chloride (InCl3). The indium chloride thus formed was recrystallized two to three times to get pure indium chloride. This indium chloride was used in spraying solution. Chemical reactions that form the basis of the pyrolysis method are 2In þ 6HCl ! 2InCl3 þ 3H2 " 2InCl3 þ 3H2O () In2O3 þ 6HCl To optimize doping concentration, the In2O3:Sn films were deposited at a substrate v temperature of Ts ¼ 525 C and air flow rate of 6 L/min for different tin concentration (1–100wt% SnCl2). Tin is one of the most suitable dopants among the various dopants such as F, Cl, and Sb. The ionic radius of tin (0.71 G) is less than that of indium (0.81 G) [12]. Fig. 1(a–d) shows the X-ray diffraction patterns (XRD) of In2O3: Sn films deposited at different dopant levels. It is observed that these films are polycrystalline with the preferred orientation of [400]. The extent of this orientation increases with doping concentration up to a doping concentration of 30 wt% SnCl2, after which there is a decrease in the extent of preferred orientation. Similar behavior is observed for aluminum doped zinc oxide thin films [7]. No phase other than the In2O3 structure was detected in XRD patterns. However, the GD-OES (glow discharge optical emission spectroscopy) analysis (Fig. 2) on the doped films detected the Sn spectra in the medium layer. The absence of Sn in XRD results is also observed by Bender et al. [13]. The variation of the sheet resistance Rsh with doping concentration is shown in Fig. 3. It is observed that the sheet resistance decreases initially to a minimum value 25 X and then starts increasing to a maximum value of 130 X (for the same order of thickness) as the dopant concentration increases. Similar behavior is observed for ATO films deposited by spray pyrolysis [14]. The initial decrease in Rsh could be due to the tetravalent Sn atoms sit in the trivalent In sites and acting as n-type donors. The increase of Rsh for higher tin concentration may be due to interstitial Sn-atoms in the lattice that act as charged trapping centers for the electrons [14,15]. The variation of room temperature transmission in the visible region (k ¼ 550 nm) as a function of doping levels is shown in Fig. 4. It is observed that
Fig. 1. (a,b) XRD patterns of In2O3:Sn films deposited at different doping levels; (c,d) XRD patterns of In2O3:Sn films deposited at different doping levels.
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Fig. 2. GD-OES analysis on the optimum In2O3:Sn films.
the visible transmission decreases as the doping level is increased from 5 to 100 wt% SnCl2. Similar results are also observed by [16]. It is believed that the decrease in the transmittance at doping level greater than 5 wt% Sn is due to increased light scattering.
Fig. 3. Variation of sheet resistance with doping concentration.
S.M. Rozati, T. Ganj / Renewable Energy 29 (2004) 1671–1676
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Fig. 4. Variation of transmission in the visible region as a function of doping level.
The data obtained from spectral transmittance were used to calculate the film thickness of ITO films. The thickness of the film can be calculated by using an interference pattern observed in the region (0:4 lm < k < 0:9 lm) following the formula given by Manifacier et al. [17,18]. Mk 1 k 2 t¼ 2 nf ðk 1 Þ k 2 n f ðk 2 Þk 1 where M is the number of oscillations between the two extrema occurring at k1 and k2 .The value of M is equal to 1/2 for very thin film where there is only one minimum and one maximum. nf(k1) and nf(k2) are the corresponding refractive indices at k1 and k2, respectively. The thickness of the films was found to lie in the range of 4000–4500G. In conclusion, indium tin oxide thin films were deposited by spray pyrolysis technique. The physical properties are well governed by the amount of tin chloride in the solution. It was observed that 2 wt% of SnCl2 is the optimum doping amount to achieve the best electrical as well as optical properties of the ITO films.
Acknowledgements This work was supported by the Department of Research at the University of Guilan. The authors are grateful to Professor Zanjanchi, Head of the Higher Education Department for using X-ray system.
References [1] Hanthi S, Subrananian, Ramasamy P. Cryst Res Technol 1999;34;46. [2] Kim H, Pique A, Murata JS, Kafafi ZH, Gilmore CM, Christy DB. Thin Solid Films 2000;377:798.
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[3] Haitjema H, Elich JJPh, Hoogendoorn CJ. Solar Energy Mater 1989;18:283. [4] Hartnagel H, Dawar AL, Jain AK, Jagadish C. Semiconducting transparent thin films. Institute of Physics Publishing Bristol and Philadelphia; 1995. [5] Rozati SM, Ganj T. To be published. [6] Agashe C, Marateh BR. J Phys D: Appl Phys 1993;26:2049. [7] Seki S, Sawada Y, Nishida T. Thin Solid Films 2001;388:22. [8] Rozati SM, Mirzapour S, Takwale MG, Marathe BR, Bhide VG. Mat Chem Phys 1993;19:341. [9] Zahirul AHM, Saha PK, Hata T, Sasaki K. Thin Solid Films 1999;52:133. [10] Ramaiah KS, Raja VS, Bhathagar AK, Tomlinso RD, Pilkington RD, Hill AB, Chang SJ, Suy K, Juang FS. Semicon Sci Technol 2000;15:676. [11] Atashbar MZ, Gong B, Sun HT, Wlodrski, Lamb R. Thin Solid Films 1999;354:222. [12] Kittle C. Introduction to solid state physics. Wiley; 1986. [13] Bender M, Trube J, Stollenwerk J. Thin Solid Films 1999;354:100. [14] Bisht H, Eun HT, Mehrtens A, Aegerter MA. Thin Solid Films 1999;351:109. [15] Hamberg I, Granqvist CG. J Appl Phys 1986;60:R123. [16] Juarez AS, Silver AT, Ortiz A. Solar Energy Mater Solar Cells 1998;52:301. [17] Manifacier JC, Fillard JP, Bind JM. Thin Solid Films 1981;77:67. [18] Manifacierm JC, De Murcia M, Fillard JP. Thin Solid Films 1977;41:127.
Transparent conductive Sn-doped indium oxide thin films deposited by spray pyrolysis technique S.M. Rozati , T. Ganj Physics Department, The University of Guilan, Rasht 41335, Iran Received 1 August 2003; accepted 19 January 2004
Abstract Films of indium oxide doped with tin (ITO) are prepared using the low cost spray pyrolysis technique. The effect of tin doping on the physical properties of In2O3 are studied. In this study the polycrystalline ITO films with the different Sn concentration of 1 to 100-wt% SnCl2 were prepared on Corning 7059 glass substrate. These films were confirmed to show the high crystallinity by X-ray diffraction technique. Low sheet resistance (25X/_) and high visible transmission (~82%) were obtained when the films were deposited at the tin concentration of 2-wt%. # 2004 Elsevier Ltd. All rights reserved. Keywords: In2O3: Sn; Spray pyrolysis; Transparent conducting oxide
1. Introduction Thin films of SnO2:F , In2O3:F, In2O3:Sn, ZnO:Al etc. [1–4] have been widely used in opto-electronic devices [4]. In order to obtain optimal characteristics of TCO films, the parameters such as substrate temperature [5], thickness [6], dopants [7], and other deposition conditions have to be optimized. TCO films have been prepared by various deposition techniques such as vacuum evaporation [8], dip coating [7], sputtering [9], spray pyrolysis [10], sol-gel [11], etc. In this paper, results are described by preparing coatings by spray pyrolysis while varying tin concentration.
Corresponding author. Fax: +89-131-322-0066. E-mail address: [email protected] (S.M. Rozati).
0960-1481/$ - see front matter # 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2004.01.008
1672
S.M. Rozati, T. Ganj / Renewable Energy 29 (2004) 1671–1676
The spray pyrolysis unit in this work consists of a heater capable of heating the v substrate up to 700 C and temperature controller unit to control the substrate temperature. The spray technique is one of the most commonly used techniques for preparation of transparent and conducting oxides owing to its simplicity, nonvacuum system of deposition and hence inexpensive method for large area coatings.
2. Results and discussion In order to prepare the solution for depositing undoped indium oxide films, the following procedure was adapted. The starting material was indium metal that was dissolved in HCl and the solution was heated to complete the reaction. After evaporating the excess water we obtain indium chloride (InCl3). The indium chloride thus formed was recrystallized two to three times to get pure indium chloride. This indium chloride was used in spraying solution. Chemical reactions that form the basis of the pyrolysis method are 2In þ 6HCl ! 2InCl3 þ 3H2 " 2InCl3 þ 3H2O () In2O3 þ 6HCl To optimize doping concentration, the In2O3:Sn films were deposited at a substrate v temperature of Ts ¼ 525 C and air flow rate of 6 L/min for different tin concentration (1–100wt% SnCl2). Tin is one of the most suitable dopants among the various dopants such as F, Cl, and Sb. The ionic radius of tin (0.71 G) is less than that of indium (0.81 G) [12]. Fig. 1(a–d) shows the X-ray diffraction patterns (XRD) of In2O3: Sn films deposited at different dopant levels. It is observed that these films are polycrystalline with the preferred orientation of [400]. The extent of this orientation increases with doping concentration up to a doping concentration of 30 wt% SnCl2, after which there is a decrease in the extent of preferred orientation. Similar behavior is observed for aluminum doped zinc oxide thin films [7]. No phase other than the In2O3 structure was detected in XRD patterns. However, the GD-OES (glow discharge optical emission spectroscopy) analysis (Fig. 2) on the doped films detected the Sn spectra in the medium layer. The absence of Sn in XRD results is also observed by Bender et al. [13]. The variation of the sheet resistance Rsh with doping concentration is shown in Fig. 3. It is observed that the sheet resistance decreases initially to a minimum value 25 X and then starts increasing to a maximum value of 130 X (for the same order of thickness) as the dopant concentration increases. Similar behavior is observed for ATO films deposited by spray pyrolysis [14]. The initial decrease in Rsh could be due to the tetravalent Sn atoms sit in the trivalent In sites and acting as n-type donors. The increase of Rsh for higher tin concentration may be due to interstitial Sn-atoms in the lattice that act as charged trapping centers for the electrons [14,15]. The variation of room temperature transmission in the visible region (k ¼ 550 nm) as a function of doping levels is shown in Fig. 4. It is observed that
Fig. 1. (a,b) XRD patterns of In2O3:Sn films deposited at different doping levels; (c,d) XRD patterns of In2O3:Sn films deposited at different doping levels.
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Fig. 2. GD-OES analysis on the optimum In2O3:Sn films.
the visible transmission decreases as the doping level is increased from 5 to 100 wt% SnCl2. Similar results are also observed by [16]. It is believed that the decrease in the transmittance at doping level greater than 5 wt% Sn is due to increased light scattering.
Fig. 3. Variation of sheet resistance with doping concentration.
S.M. Rozati, T. Ganj / Renewable Energy 29 (2004) 1671–1676
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Fig. 4. Variation of transmission in the visible region as a function of doping level.
The data obtained from spectral transmittance were used to calculate the film thickness of ITO films. The thickness of the film can be calculated by using an interference pattern observed in the region (0:4 lm < k < 0:9 lm) following the formula given by Manifacier et al. [17,18]. Mk 1 k 2 t¼ 2 nf ðk 1 Þ k 2 n f ðk 2 Þk 1 where M is the number of oscillations between the two extrema occurring at k1 and k2 .The value of M is equal to 1/2 for very thin film where there is only one minimum and one maximum. nf(k1) and nf(k2) are the corresponding refractive indices at k1 and k2, respectively. The thickness of the films was found to lie in the range of 4000–4500G. In conclusion, indium tin oxide thin films were deposited by spray pyrolysis technique. The physical properties are well governed by the amount of tin chloride in the solution. It was observed that 2 wt% of SnCl2 is the optimum doping amount to achieve the best electrical as well as optical properties of the ITO films.
Acknowledgements This work was supported by the Department of Research at the University of Guilan. The authors are grateful to Professor Zanjanchi, Head of the Higher Education Department for using X-ray system.
References [1] Hanthi S, Subrananian, Ramasamy P. Cryst Res Technol 1999;34;46. [2] Kim H, Pique A, Murata JS, Kafafi ZH, Gilmore CM, Christy DB. Thin Solid Films 2000;377:798.
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S.M. Rozati, T. Ganj / Renewable Energy 29 (2004) 1671–1676
[3] Haitjema H, Elich JJPh, Hoogendoorn CJ. Solar Energy Mater 1989;18:283. [4] Hartnagel H, Dawar AL, Jain AK, Jagadish C. Semiconducting transparent thin films. Institute of Physics Publishing Bristol and Philadelphia; 1995. [5] Rozati SM, Ganj T. To be published. [6] Agashe C, Marateh BR. J Phys D: Appl Phys 1993;26:2049. [7] Seki S, Sawada Y, Nishida T. Thin Solid Films 2001;388:22. [8] Rozati SM, Mirzapour S, Takwale MG, Marathe BR, Bhide VG. Mat Chem Phys 1993;19:341. [9] Zahirul AHM, Saha PK, Hata T, Sasaki K. Thin Solid Films 1999;52:133. [10] Ramaiah KS, Raja VS, Bhathagar AK, Tomlinso RD, Pilkington RD, Hill AB, Chang SJ, Suy K, Juang FS. Semicon Sci Technol 2000;15:676. [11] Atashbar MZ, Gong B, Sun HT, Wlodrski, Lamb R. Thin Solid Films 1999;354:222. [12] Kittle C. Introduction to solid state physics. Wiley; 1986. [13] Bender M, Trube J, Stollenwerk J. Thin Solid Films 1999;354:100. [14] Bisht H, Eun HT, Mehrtens A, Aegerter MA. Thin Solid Films 1999;351:109. [15] Hamberg I, Granqvist CG. J Appl Phys 1986;60:R123. [16] Juarez AS, Silver AT, Ortiz A. Solar Energy Mater Solar Cells 1998;52:301. [17] Manifacier JC, Fillard JP, Bind JM. Thin Solid Films 1981;77:67. [18] Manifacierm JC, De Murcia M, Fillard JP. Thin Solid Films 1977;41:127.