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Comparative study of ITO and FTO thin films grown by spray pyrolysis
Materials Research Bulletin 44 (2009) 1458–1461
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Comparative study of ITO and FTO thin films grown by spray pyrolysis M. Ait Aouaj a, R. Diaz b, A. Belayachi a, F. Rueda b, M. Abd-Lefdil a,* a b
Laboratory of Materials Physics, University of Mohammed V-Agdal, Rabat, Morocco Departamento de Fı´sica Aplicada C-XII, Universidad Auto´noma de Madrid, Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 November 2008 Received in revised form 22 January 2009 Accepted 26 February 2009 Available online 13 March 2009
Tin doped indium oxide (ITO) and fluorine doped tin oxide (FTO) thin films have been prepared by one step spray pyrolysis. Both film types grown at 400 8C present a single phase, ITO has cubic structure and preferred orientation (4 0 0) while FTO exhibits a tetragonal structure. Scanning electron micrographs showed homogeneous surfaces with average grain size around 257 and 190 nm for ITO and FTO respectively. The optical properties have been studied in several ITO and FTO samples by transmittance and reflectance measurements. The transmittance in the visible zone is higher in ITO than in FTO layers with a comparable thickness, while the reflectance in the infrared zone is higher in FTO in comparison with ITO. The best electrical resistivity values, deduced from optical measurements, were 8 104 and 6 104 V cm for ITO (6% of Sn) and FTO (2.5% of F) respectively. The figure of merit reached a maximum value of 2.15 103 V1 for ITO higher than 0.55 103 V1 for FTO. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films C. X-ray diffraction D. Optical properties D. Electrical properties
1. Introduction Transparent conducting oxides (TCO) like tin doped indium oxide, In2O3: Sn (ITO), fluorine or antimony doped tin oxide; SnO2: F (FTO) and SnO2: Sb (ATO) are particularly attractive. They find numerous applications because of their high optical transparency in the visible region, good electrical conductivity and the high infrared reflectivity. One advantage of using binary TCO materials is that the control of chemical in film deposition is relatively easier than in ternary compounds (Zn2In2O5, GaInO3, . . .) and multicomponent oxides (ZnO–SnO2, ZnO–In2O3, . . .). There are many applications of using TCO films like in solar cells devices [1] liquid crystal displays [2], wave guide electron devices [3] and light emitted diode [4,5]. The ITO electroluminescent properties have been used also in biology to determine the concentration of glucose in a solution [6]. A large variety of techniques have been developed for TCOs thin films deposition. Chemical vapour deposition [7], reactive evaporation [8], dc and rf sputtering [9–12], sol–gel [13] and pulsed laser ablation [14] are some preparation processes currently used to prepare ITO and FTO. The properties of obtained films are strongly depending on the preparation method and the control of the process parameters. The undoped stoichiometric SnO2 and In2O3 films have very high electrical resistivity because of their low intrinsic carrier density and mobility. Therefore the challenge is to
* Corresponding author. Tel.: +212 61372116; fax: +212 37778973. E-mail address: [email protected] (M. Abd-Lefdil). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.02.019
prepare non-stoichiometric doped thin films. Dopants as antimony, Sb, indium, In, and fluorine, F, are frequently used. The spray pyrolysis is a simple technique for growing thin films, low-cost and which can be used in large-scale production as it does not require the use of vacuum. It has been successfully used to synthesize many transparent conducting oxide films [15–17]. In the present work, we report structural, optical and electrical results obtained on ITO and FTO prepared by the chemical spray pyrolysis technique. All the films grown at 400 8C have properties which are useful in most applications. On the other hand, the films do not require a post annealing to improve good TCO characteristics, as needed in alternative processes, like reactive evaporation, sputtering or pulsed laser deposition. 2. Experimental Thin layers of indium tin oxide (ITO) and fluorine doped tin oxide (FTO) have been prepared on glass substrates by using spray pyrolysis. In both cases the sprayed alcoholic solution had ethanol/ distilled water ratio of 0.25, with HCl addition to adjust the pH to 0.5. The solution contains anhydrous indium chloride InCl3 (2.5 102 M) and SnCl45H2O (2 103 to 0.75 M) as dopant for ITO. For FTO, the concentration of SnCl45H2O was 2 102 M while the fluorine dopant was NH4F with concentrations in the 5 103 to 1.5 102 M range. In both compounds the preparation conditions were: the distance between the spray nozzle and the substrates 40 cm, the carrier gas flux 2 L min1, the spray rate 2.5 mL min1. The glass substrates temperature was maintained at 400 8C during the whole spraying process.
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A X’Pert Pro diffractometer was used to determine the X-ray diffraction (XRD) patterns with Cu Ka radiation. The surface morphology was observed using a Scanning Electron Microscope (SEM) SEI Quanta 200 and a comparative study of the amount of dopant in the samples was analyzed with Energy Dispersive X-ray Analysis (EDAX). In order to find out the size of grains in the surface, the SEM image was studied with an image processing programme called ‘‘Analysis’’. Further the crystalline size along a line normal to the preferentially oriented crystal plane is calculated using the Scherrer’s formula [18]: D¼
0:9l B cos u
where u is the Bragg’s diffraction angle, B is the broadening of diffraction line at half its maximum intensity and l the wavelength of X-rays. Optical transmittance and reflectance were obtained by means of a Cary 17D double beam spectrophotometer in the 400– 2200 nm range, as for lower than 400 nm a strong absorption appears due to the glass substrate. A model assuming multiple reflections in compact films was used in order to determine the absorption coefficient a. The free carrier absorption, in the infrared region, was used to obtain the electrical parameters by the linear fit of (1/a) as a function of 1/l2 [19,20]. We also determine the electrical resistivity from Hall effect measurements. The film thickness d was measured with a stylus type apparatus (Talysted, Taylor & Hobson, UK). 3. Results and discussion Fig. 1 shows a XRD spectrum for ITO (6% of Sn) and FTO (2.5% of F) thin films deposited at 400 8C. The XRD pattern of ITO (6%) was indexed on the basis of cubic structure with (4 0 0) preferred orientation. For ITO prepared by pulsed laser deposition [14] and sputtering [21] techniques, the preferred orientation has been assigned to (2 2 2). The lattice parameter a is 10.130 A˚ higher than 10.118 A˚ of the stoichiometric In2O3 [22]. This result confirms that ITO is essentially formed by substitutional Sn replacing In3+ atoms from the In2O3 cubic structure. The spectrum was attributed to ITO with In1.94Sn0.06O3 composition which was determined by EDAX measurements. For FTO film with 2.5% of fluorine content, the spectrum shows a preferred orientation (1 1 0) and can be attributed to a tetragonal structure with the following lattice parameters: a = 4.687 A˚ and c = 3.160 A˚. The SnO2 lattice parameters are a = 4.7552 A˚ and c = 3.1992 A˚ [23], higher than those observed in our samples indicating the substitutional O2 by F. The grain size values estimated from XRD patterns by Scherrer’s
Fig. 1. XRD patterns for thin films. (a) ITO (% Sn = 6) and (b) FTO (% F = 2.5).
Fig. 2. Scanning electron micrographs for thin films of ITO and FTO. (a) ITO (% Sn = 6) and (b) FTO (% F = 2.5).
law, which gives the coherence length perpendicularly to the substrate, were approximately around 35 nm. The surface topography of ITO (6%) and FTO (2.5%) thin films are shown in Fig. 2a and b respectively. One can observe that the films present a homogeneous surface similar in both compounds. The average grain size visualized by SEM, which corresponds to the grain size parallel to the substrate, was 257 and 190 nm for ITO and FTO surfaces respectively, higher than the estimation presented above. Fig. 3a and b shows typical curves of the transmittance as function of wavelength for ITO and FTO sprayed films with different dopant content respectively. For ITO, the spectra show that the transmittance is around 85% in the visible wavelength zone for the sample with higher dopant content and the low thickness (ITO (6%)). In Fig. 3b, we present the transmittance of FTO for two different percent of fluorine dopant. The FTO (2.5%) presents a transmittance around 70% in the visible zone. The comparison between ITO (1.5%) and FTO (1%) transmittances which have the same thickness shows that the transmittance in the visible zone is higher in ITO than in FTO and the spectra are very different beyond 1200 nm in the near infrared region. Fig. 4 presents the reflectance for ITO and FTO samples. Analogous to transmittance, we note some differences between ITO and FTO films. FTO shows high reflectance in near infrared zone, which is absolutely necessary in solar cell devices, low optical reflectance in the visible region but high in the infrared region. These optical results are in good agreement with literature
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Fig. 3. Transmission spectra for ITO and FTO films. (a) ITO (% Sn = 1.5), ITO (% Sn = 4), ITO (% Sn = 6); and (b) FTO (% F = 1), FTO (% F = 2.5), ITO (% Sn = 4).
on ITO [24] and also with Rottkay and Rubin results on galss/SnO2/ SiO2/SnO2: F multicomponents [25]. The absorption coefficient a increase up to 104 for energies higher than 2.3 eV. In the infrared region, the electrical parameters were computed by the linear fit of (1/a) as function of 1/l2 (Fig. 5). Table 1 summarizes several films parameters, i.e. the thickness, the percent of dopant and the electrical parameters determined from optical measurements. For ITO, the lowest electrical resistivity value is 8 104 V cm obtained for 6% of Sn. An increase of the Sn concentration above 6%, in the solution, leads to a drastic increase of the electrical resistivity since no more Sn-atoms can be embedded in the In2O3 lattice. This result is in agreement with Frank et al. work [26] where the maximum solubility of Sn in In2O3 was limited to about 5% of Sn. For FTO, the minimum of electrical resistivity measured is 6 104 V cm, obtained for 2.5% of F. Hall effect measurements have been performed on ITO and FTO samples. The values of carrier density obtained are 36.7 1019 and 18.4 1019 cm3 for ITO (6% of Sn) and FTO (2.5% of F) respectively, lowers than these determined from optical measurements. The
Fig. 4. Reflectance spectra for ITO and FTO films. (a) ITO (% Sn = 1.5), ITO (% Sn = 4), ITO (% Sn = 6); and (b) FTO (% F = 1), FTO (% F = 2.5), ITO (% Sn = 4).
difference can be attributed to substantial disordered states with dopant atoms not activated between crystalline grains, when taking the electrical measurements, which lead to an increase of the electrical resistivity. We obtain r = 0.13 102 V cm and Table 2 Electrical resistivity of FTO and ITO films as reported by other authors and in the present study.
r 104 (V cm1) FTO
43 5.7 15 4 5.1
ITO
13 27.1 3.3 10 100
Preparation technique
Reference
Spray 2.5%(F) – Spray 7.5% (F) Spray Spray
This work [29] [16] [30] [31]
Spray 6% (Sn) Spray 5% (Sn) PLD 5% (Sn) Thermal evaporation Sputtering
This work [17] [14] [32] [12]
Table 1 Some typical parameters of Sprayed ITO and FTO films. Sample
% of dopant
d (nm)
r 102 (V cm)
m (cm2 V1 s1)
n 1019 cm3
R (V)
FTC 104 V1
ITO ITO ITO FTO FTO
1.5 (Sn) 4 (Sn) 6 (Sn) 1 (F) 2.5 (F)
200 200 160 200 180
1.03 0.35 0.08 0.18 0.06
13.3 14.3 12.4 26.5 33.5
4.6 12.5 63.0 13.1 31.1
515 175 50.0 90.0 33.0
0.63 1.2 21.5 0.11 5.5
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4. Conclusion Spray pyrolysis at 400 8C was shown to be a simple and inexpensive method for producing indium tin oxide and fluorine doped tin oxide with high optical transparency in the visible region and high near infrared reflectivity. The best electrical resistivity values, deduced from optical measurements, were 8 104 to 6 104 V cm for ITO (6% of Sn) and FTO (2.5% of F) respectively. ITO and FTO prepared by this alternative technique can be used in many devices such as a window in solar cells, in optoelectronic devices and as a work electrode in electrodeposition process. Acknowledgment Fig. 5. Linear fit of 1/a as a function of 1/l2 in the infrared region for ITO (% Sn = 4) and FTO (% F = 2.5).
This work has been done in the framework of the MoroccoHispano University Collaboration by the Spanish Agency of International Cooperation Project No. A14436/07.
r = 0.43 102 V cm as electrical resistivity for ITO (6% of Sn) and FTO (2.5% of F) respectively. From Table 2, the electrical resistivity values of our samples are in the same order of magnitude as in the literature. They correspond to degenerate semiconductors with high free-electron concentration, more than 1019 cm3. In ITO with cubic structure, it is due to contribution of substitutional Sn and to oxygen vacancies (native donors). Similarly, F substitutes O2 in the tin oxide lattice for FTO films. For this substitution, the electrical perturbation is largely confined to the filled valence band and the scattering of electrons is reduced leading to a decrease of resistivity. The values of mobility are small by less than one order of magnitude of the best published values [14,27] because of the ionized scattering centers. The degeneracy of our samples is confirmed by the evaluation of the Fermi energy level using the relation [28]: ! 2 h 3n 2=3 EF ¼ 8m p where m* is the value of effective mass, 0.19me (me is the rest mass of electron) and n is the carrier concentration. The calculated EF values are 1.43 eV for ITO and 0.89 eV for FTO respectively, which are higher than the energy corresponding to the room temperature. In order to summarize the properties of our TCO’s, the figure of merit was calculated using Haacke’s [29] equation: F TC ¼
T 10 R
where T is the value of optical transmission at l = 550 nm and R is the sheet resistance given in Table 1. One can see that FTC values are 0.55 103 V1 and 2.15 103 V1 for FTO and ITO respectively, in a good agreement with recent works [11,17]. A possible explanation of these good values of FTC may be the effect of ethanol, which possibly reduces the oxide and provokes a higher oxygen vacancy concentration not compensated by the oxygen of air at relatively low temperatures. These FTC results indicate the promise of these films to potential applications in transparent conducting electrode like solar cells and as a work electrode in electrodeposition process [33].
References [1] N. Di Lalla, A. Lamagna, 26th Photovoltaic specialists conference California, 1997, p. 459. [2] H. Ryu, J. Kang, Y. Han, D. Kim, J.J. Park, W.K. Park, M.S. Yang, J. Electron. Mater. 23 (2003) 919. [3] R. Lessman, I.A. Hummelgen, Mater. Res. 7 (2004) 447. [4] H. Chen, C. Qiu, M. Wong, H.S. Kwak, IEEE Electron Device Lett. 24 (2003) 315. [5] A.G. Macedo, E.A. de Vascooncelos, R. Valaski, F. Muchenski, E.F. da Silva Jr., A.F. da Silva, L.S. Roman, Thin Solid Films 517 (2008) 870. [6] T. Miyasaka, Y. Jinbo, K. Sakai, Y. Yoshimi, Food Biproducts Process. 76 (1998) 102. [7] J.R. Brown, P.W. Haycock, L.M. Smith, A.C. Jones, E.W. Williams, Sensors Actuators B 63 (2000) 109. [8] K. Omura, P. Veluchamy, M. Tsuji, T. Nishio, M. Murozono, J. Electrochem. Soc. 146 (1999) 2113. [9] C.W. OW-Yang, D. Spinner, Y. Shigesato, D.C. Paine, J. Appl. Phys. 83 (1998) 145. [10] Y. Farber, F.K. Arefi, J. Amoureux, Thin Solid films 241 (1994) 282. [11] H.J. Park, J.H. Park, J.I. Choi, J.Y. Lee, J.H. Chae, D. Kim, Vacuum 83 (2009) 448. [12] S. Boycheva, A.K. Sytchkova, M.L. Grilli, A. Piegari, Thin Solid Films 515 (2007) 8469. [13] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 82 (1997) 865. [14] J.H. Kim, K.A. Jeon, G.H. Kim, S.Y. Lee, Appl. Surf. Sci. 252 (2006) 4834. [15] J. Wienke, A.S. Booij, Thin Solid films 516 (2008) 4508. [16] K.K. Purushothaman, M. Dhanashankar, G. Muralidharan, Curr. Appl. Phys. 9 (2009) 67. [17] A.V. Moholkar, S.M. Pawar, K.Y. Rajpure, V. Ganesan, C.H. Bhosale, J. Alloys Compd. 464 (2008) 387. [18] P. Scherrer, Gottinger Nachrichten 2 (1918) 98. [19] I.A. Rauf, Mater. Lett. 23 (1995) 73. [20] T. Watanabe, T. Sameshima, K. Nakahata, T. Kamiya, I. Shimizu, Thin Solid Films 383 (2001) 248. [21] C. Guillen, J. Herrero, Mater. Chem. Phys. 112 (2008) 641. [22] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 83 (1998) 2631. [23] International Center for Diffraction Data V2.1, 2000. [24] H. Bisht, H.T. Eun, A. Merhrtens, M.A. Aegerter, Thin Solid Films 351 (1999) 109. [25] K.V. Rottkay, M. Rubin, Mater. Res. Soc. Symp. Proc. 426 (1996) 449. [26] G. Frank, H. Kostlin, A. Rabenau, Phys. Status Solidi A 52 (1979) 231. [27] T. Fukano, T. Motohiro, Sol. Energy Mater. Sol. Cells 82 (2004) 576. [28] B. Thangaraju, Thin Solid Films 402 (2002) 71. [29] G. Haacke, J. Appl. Phys. 47 (1976) 4086. [30] A.L. Martinez, D.R. Acosta, G. Cedilli, Thin Solid Films 490 (2) (2005) 118. [31] A.V. Moholkar, S.M. Pawar, K.Y. Rajpure, S.N. Almari, P.S. Patil, C.H. Bhosale, Sol. Energy Mater. Sol. Cells 92 (2008) 1439. [32] G.S. Belo, B.J.P. da Silva, E.A. de Vasconcelos, W.M. de Azevedo, E.F. da Silva Jr., Appl. Surf. Sci. 255 (2008) 755. [33] M. Ait Aouaj, M. Abd-Lefdil, F. Cherkaoui El Moursli, F. Hajji, Eur. Phys. J. Appl. Phys. 40 (2007) 55.
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Comparative study of ITO and FTO thin films grown by spray pyrolysis M. Ait Aouaj a, R. Diaz b, A. Belayachi a, F. Rueda b, M. Abd-Lefdil a,* a b
Laboratory of Materials Physics, University of Mohammed V-Agdal, Rabat, Morocco Departamento de Fı´sica Aplicada C-XII, Universidad Auto´noma de Madrid, Madrid, Spain
A R T I C L E I N F O
A B S T R A C T
Article history: Received 6 November 2008 Received in revised form 22 January 2009 Accepted 26 February 2009 Available online 13 March 2009
Tin doped indium oxide (ITO) and fluorine doped tin oxide (FTO) thin films have been prepared by one step spray pyrolysis. Both film types grown at 400 8C present a single phase, ITO has cubic structure and preferred orientation (4 0 0) while FTO exhibits a tetragonal structure. Scanning electron micrographs showed homogeneous surfaces with average grain size around 257 and 190 nm for ITO and FTO respectively. The optical properties have been studied in several ITO and FTO samples by transmittance and reflectance measurements. The transmittance in the visible zone is higher in ITO than in FTO layers with a comparable thickness, while the reflectance in the infrared zone is higher in FTO in comparison with ITO. The best electrical resistivity values, deduced from optical measurements, were 8 104 and 6 104 V cm for ITO (6% of Sn) and FTO (2.5% of F) respectively. The figure of merit reached a maximum value of 2.15 103 V1 for ITO higher than 0.55 103 V1 for FTO. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: A. Thin films C. X-ray diffraction D. Optical properties D. Electrical properties
1. Introduction Transparent conducting oxides (TCO) like tin doped indium oxide, In2O3: Sn (ITO), fluorine or antimony doped tin oxide; SnO2: F (FTO) and SnO2: Sb (ATO) are particularly attractive. They find numerous applications because of their high optical transparency in the visible region, good electrical conductivity and the high infrared reflectivity. One advantage of using binary TCO materials is that the control of chemical in film deposition is relatively easier than in ternary compounds (Zn2In2O5, GaInO3, . . .) and multicomponent oxides (ZnO–SnO2, ZnO–In2O3, . . .). There are many applications of using TCO films like in solar cells devices [1] liquid crystal displays [2], wave guide electron devices [3] and light emitted diode [4,5]. The ITO electroluminescent properties have been used also in biology to determine the concentration of glucose in a solution [6]. A large variety of techniques have been developed for TCOs thin films deposition. Chemical vapour deposition [7], reactive evaporation [8], dc and rf sputtering [9–12], sol–gel [13] and pulsed laser ablation [14] are some preparation processes currently used to prepare ITO and FTO. The properties of obtained films are strongly depending on the preparation method and the control of the process parameters. The undoped stoichiometric SnO2 and In2O3 films have very high electrical resistivity because of their low intrinsic carrier density and mobility. Therefore the challenge is to
* Corresponding author. Tel.: +212 61372116; fax: +212 37778973. E-mail address: [email protected] (M. Abd-Lefdil). 0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2009.02.019
prepare non-stoichiometric doped thin films. Dopants as antimony, Sb, indium, In, and fluorine, F, are frequently used. The spray pyrolysis is a simple technique for growing thin films, low-cost and which can be used in large-scale production as it does not require the use of vacuum. It has been successfully used to synthesize many transparent conducting oxide films [15–17]. In the present work, we report structural, optical and electrical results obtained on ITO and FTO prepared by the chemical spray pyrolysis technique. All the films grown at 400 8C have properties which are useful in most applications. On the other hand, the films do not require a post annealing to improve good TCO characteristics, as needed in alternative processes, like reactive evaporation, sputtering or pulsed laser deposition. 2. Experimental Thin layers of indium tin oxide (ITO) and fluorine doped tin oxide (FTO) have been prepared on glass substrates by using spray pyrolysis. In both cases the sprayed alcoholic solution had ethanol/ distilled water ratio of 0.25, with HCl addition to adjust the pH to 0.5. The solution contains anhydrous indium chloride InCl3 (2.5 102 M) and SnCl45H2O (2 103 to 0.75 M) as dopant for ITO. For FTO, the concentration of SnCl45H2O was 2 102 M while the fluorine dopant was NH4F with concentrations in the 5 103 to 1.5 102 M range. In both compounds the preparation conditions were: the distance between the spray nozzle and the substrates 40 cm, the carrier gas flux 2 L min1, the spray rate 2.5 mL min1. The glass substrates temperature was maintained at 400 8C during the whole spraying process.
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A X’Pert Pro diffractometer was used to determine the X-ray diffraction (XRD) patterns with Cu Ka radiation. The surface morphology was observed using a Scanning Electron Microscope (SEM) SEI Quanta 200 and a comparative study of the amount of dopant in the samples was analyzed with Energy Dispersive X-ray Analysis (EDAX). In order to find out the size of grains in the surface, the SEM image was studied with an image processing programme called ‘‘Analysis’’. Further the crystalline size along a line normal to the preferentially oriented crystal plane is calculated using the Scherrer’s formula [18]: D¼
0:9l B cos u
where u is the Bragg’s diffraction angle, B is the broadening of diffraction line at half its maximum intensity and l the wavelength of X-rays. Optical transmittance and reflectance were obtained by means of a Cary 17D double beam spectrophotometer in the 400– 2200 nm range, as for lower than 400 nm a strong absorption appears due to the glass substrate. A model assuming multiple reflections in compact films was used in order to determine the absorption coefficient a. The free carrier absorption, in the infrared region, was used to obtain the electrical parameters by the linear fit of (1/a) as a function of 1/l2 [19,20]. We also determine the electrical resistivity from Hall effect measurements. The film thickness d was measured with a stylus type apparatus (Talysted, Taylor & Hobson, UK). 3. Results and discussion Fig. 1 shows a XRD spectrum for ITO (6% of Sn) and FTO (2.5% of F) thin films deposited at 400 8C. The XRD pattern of ITO (6%) was indexed on the basis of cubic structure with (4 0 0) preferred orientation. For ITO prepared by pulsed laser deposition [14] and sputtering [21] techniques, the preferred orientation has been assigned to (2 2 2). The lattice parameter a is 10.130 A˚ higher than 10.118 A˚ of the stoichiometric In2O3 [22]. This result confirms that ITO is essentially formed by substitutional Sn replacing In3+ atoms from the In2O3 cubic structure. The spectrum was attributed to ITO with In1.94Sn0.06O3 composition which was determined by EDAX measurements. For FTO film with 2.5% of fluorine content, the spectrum shows a preferred orientation (1 1 0) and can be attributed to a tetragonal structure with the following lattice parameters: a = 4.687 A˚ and c = 3.160 A˚. The SnO2 lattice parameters are a = 4.7552 A˚ and c = 3.1992 A˚ [23], higher than those observed in our samples indicating the substitutional O2 by F. The grain size values estimated from XRD patterns by Scherrer’s
Fig. 1. XRD patterns for thin films. (a) ITO (% Sn = 6) and (b) FTO (% F = 2.5).
Fig. 2. Scanning electron micrographs for thin films of ITO and FTO. (a) ITO (% Sn = 6) and (b) FTO (% F = 2.5).
law, which gives the coherence length perpendicularly to the substrate, were approximately around 35 nm. The surface topography of ITO (6%) and FTO (2.5%) thin films are shown in Fig. 2a and b respectively. One can observe that the films present a homogeneous surface similar in both compounds. The average grain size visualized by SEM, which corresponds to the grain size parallel to the substrate, was 257 and 190 nm for ITO and FTO surfaces respectively, higher than the estimation presented above. Fig. 3a and b shows typical curves of the transmittance as function of wavelength for ITO and FTO sprayed films with different dopant content respectively. For ITO, the spectra show that the transmittance is around 85% in the visible wavelength zone for the sample with higher dopant content and the low thickness (ITO (6%)). In Fig. 3b, we present the transmittance of FTO for two different percent of fluorine dopant. The FTO (2.5%) presents a transmittance around 70% in the visible zone. The comparison between ITO (1.5%) and FTO (1%) transmittances which have the same thickness shows that the transmittance in the visible zone is higher in ITO than in FTO and the spectra are very different beyond 1200 nm in the near infrared region. Fig. 4 presents the reflectance for ITO and FTO samples. Analogous to transmittance, we note some differences between ITO and FTO films. FTO shows high reflectance in near infrared zone, which is absolutely necessary in solar cell devices, low optical reflectance in the visible region but high in the infrared region. These optical results are in good agreement with literature
M.A. Aouaj et al. / Materials Research Bulletin 44 (2009) 1458–1461
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Fig. 3. Transmission spectra for ITO and FTO films. (a) ITO (% Sn = 1.5), ITO (% Sn = 4), ITO (% Sn = 6); and (b) FTO (% F = 1), FTO (% F = 2.5), ITO (% Sn = 4).
on ITO [24] and also with Rottkay and Rubin results on galss/SnO2/ SiO2/SnO2: F multicomponents [25]. The absorption coefficient a increase up to 104 for energies higher than 2.3 eV. In the infrared region, the electrical parameters were computed by the linear fit of (1/a) as function of 1/l2 (Fig. 5). Table 1 summarizes several films parameters, i.e. the thickness, the percent of dopant and the electrical parameters determined from optical measurements. For ITO, the lowest electrical resistivity value is 8 104 V cm obtained for 6% of Sn. An increase of the Sn concentration above 6%, in the solution, leads to a drastic increase of the electrical resistivity since no more Sn-atoms can be embedded in the In2O3 lattice. This result is in agreement with Frank et al. work [26] where the maximum solubility of Sn in In2O3 was limited to about 5% of Sn. For FTO, the minimum of electrical resistivity measured is 6 104 V cm, obtained for 2.5% of F. Hall effect measurements have been performed on ITO and FTO samples. The values of carrier density obtained are 36.7 1019 and 18.4 1019 cm3 for ITO (6% of Sn) and FTO (2.5% of F) respectively, lowers than these determined from optical measurements. The
Fig. 4. Reflectance spectra for ITO and FTO films. (a) ITO (% Sn = 1.5), ITO (% Sn = 4), ITO (% Sn = 6); and (b) FTO (% F = 1), FTO (% F = 2.5), ITO (% Sn = 4).
difference can be attributed to substantial disordered states with dopant atoms not activated between crystalline grains, when taking the electrical measurements, which lead to an increase of the electrical resistivity. We obtain r = 0.13 102 V cm and Table 2 Electrical resistivity of FTO and ITO films as reported by other authors and in the present study.
r 104 (V cm1) FTO
43 5.7 15 4 5.1
ITO
13 27.1 3.3 10 100
Preparation technique
Reference
Spray 2.5%(F) – Spray 7.5% (F) Spray Spray
This work [29] [16] [30] [31]
Spray 6% (Sn) Spray 5% (Sn) PLD 5% (Sn) Thermal evaporation Sputtering
This work [17] [14] [32] [12]
Table 1 Some typical parameters of Sprayed ITO and FTO films. Sample
% of dopant
d (nm)
r 102 (V cm)
m (cm2 V1 s1)
n 1019 cm3
R (V)
FTC 104 V1
ITO ITO ITO FTO FTO
1.5 (Sn) 4 (Sn) 6 (Sn) 1 (F) 2.5 (F)
200 200 160 200 180
1.03 0.35 0.08 0.18 0.06
13.3 14.3 12.4 26.5 33.5
4.6 12.5 63.0 13.1 31.1
515 175 50.0 90.0 33.0
0.63 1.2 21.5 0.11 5.5
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4. Conclusion Spray pyrolysis at 400 8C was shown to be a simple and inexpensive method for producing indium tin oxide and fluorine doped tin oxide with high optical transparency in the visible region and high near infrared reflectivity. The best electrical resistivity values, deduced from optical measurements, were 8 104 to 6 104 V cm for ITO (6% of Sn) and FTO (2.5% of F) respectively. ITO and FTO prepared by this alternative technique can be used in many devices such as a window in solar cells, in optoelectronic devices and as a work electrode in electrodeposition process. Acknowledgment Fig. 5. Linear fit of 1/a as a function of 1/l2 in the infrared region for ITO (% Sn = 4) and FTO (% F = 2.5).
This work has been done in the framework of the MoroccoHispano University Collaboration by the Spanish Agency of International Cooperation Project No. A14436/07.
r = 0.43 102 V cm as electrical resistivity for ITO (6% of Sn) and FTO (2.5% of F) respectively. From Table 2, the electrical resistivity values of our samples are in the same order of magnitude as in the literature. They correspond to degenerate semiconductors with high free-electron concentration, more than 1019 cm3. In ITO with cubic structure, it is due to contribution of substitutional Sn and to oxygen vacancies (native donors). Similarly, F substitutes O2 in the tin oxide lattice for FTO films. For this substitution, the electrical perturbation is largely confined to the filled valence band and the scattering of electrons is reduced leading to a decrease of resistivity. The values of mobility are small by less than one order of magnitude of the best published values [14,27] because of the ionized scattering centers. The degeneracy of our samples is confirmed by the evaluation of the Fermi energy level using the relation [28]: ! 2 h 3n 2=3 EF ¼ 8m p where m* is the value of effective mass, 0.19me (me is the rest mass of electron) and n is the carrier concentration. The calculated EF values are 1.43 eV for ITO and 0.89 eV for FTO respectively, which are higher than the energy corresponding to the room temperature. In order to summarize the properties of our TCO’s, the figure of merit was calculated using Haacke’s [29] equation: F TC ¼
T 10 R
where T is the value of optical transmission at l = 550 nm and R is the sheet resistance given in Table 1. One can see that FTC values are 0.55 103 V1 and 2.15 103 V1 for FTO and ITO respectively, in a good agreement with recent works [11,17]. A possible explanation of these good values of FTC may be the effect of ethanol, which possibly reduces the oxide and provokes a higher oxygen vacancy concentration not compensated by the oxygen of air at relatively low temperatures. These FTC results indicate the promise of these films to potential applications in transparent conducting electrode like solar cells and as a work electrode in electrodeposition process [33].
References [1] N. Di Lalla, A. Lamagna, 26th Photovoltaic specialists conference California, 1997, p. 459. [2] H. Ryu, J. Kang, Y. Han, D. Kim, J.J. Park, W.K. Park, M.S. Yang, J. Electron. Mater. 23 (2003) 919. [3] R. Lessman, I.A. Hummelgen, Mater. Res. 7 (2004) 447. [4] H. Chen, C. Qiu, M. Wong, H.S. Kwak, IEEE Electron Device Lett. 24 (2003) 315. [5] A.G. Macedo, E.A. de Vascooncelos, R. Valaski, F. Muchenski, E.F. da Silva Jr., A.F. da Silva, L.S. Roman, Thin Solid Films 517 (2008) 870. [6] T. Miyasaka, Y. Jinbo, K. Sakai, Y. Yoshimi, Food Biproducts Process. 76 (1998) 102. [7] J.R. Brown, P.W. Haycock, L.M. Smith, A.C. Jones, E.W. Williams, Sensors Actuators B 63 (2000) 109. [8] K. Omura, P. Veluchamy, M. Tsuji, T. Nishio, M. Murozono, J. Electrochem. Soc. 146 (1999) 2113. [9] C.W. OW-Yang, D. Spinner, Y. Shigesato, D.C. Paine, J. Appl. Phys. 83 (1998) 145. [10] Y. Farber, F.K. Arefi, J. Amoureux, Thin Solid films 241 (1994) 282. [11] H.J. Park, J.H. Park, J.I. Choi, J.Y. Lee, J.H. Chae, D. Kim, Vacuum 83 (2009) 448. [12] S. Boycheva, A.K. Sytchkova, M.L. Grilli, A. Piegari, Thin Solid Films 515 (2007) 8469. [13] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 82 (1997) 865. [14] J.H. Kim, K.A. Jeon, G.H. Kim, S.Y. Lee, Appl. Surf. Sci. 252 (2006) 4834. [15] J. Wienke, A.S. Booij, Thin Solid films 516 (2008) 4508. [16] K.K. Purushothaman, M. Dhanashankar, G. Muralidharan, Curr. Appl. Phys. 9 (2009) 67. [17] A.V. Moholkar, S.M. Pawar, K.Y. Rajpure, V. Ganesan, C.H. Bhosale, J. Alloys Compd. 464 (2008) 387. [18] P. Scherrer, Gottinger Nachrichten 2 (1918) 98. [19] I.A. Rauf, Mater. Lett. 23 (1995) 73. [20] T. Watanabe, T. Sameshima, K. Nakahata, T. Kamiya, I. Shimizu, Thin Solid Films 383 (2001) 248. [21] C. Guillen, J. Herrero, Mater. Chem. Phys. 112 (2008) 641. [22] R.B.H. Tahar, T. Ban, Y. Ohya, Y. Takahashi, J. Appl. Phys. 83 (1998) 2631. [23] International Center for Diffraction Data V2.1, 2000. [24] H. Bisht, H.T. Eun, A. Merhrtens, M.A. Aegerter, Thin Solid Films 351 (1999) 109. [25] K.V. Rottkay, M. Rubin, Mater. Res. Soc. Symp. Proc. 426 (1996) 449. [26] G. Frank, H. Kostlin, A. Rabenau, Phys. Status Solidi A 52 (1979) 231. [27] T. Fukano, T. Motohiro, Sol. Energy Mater. Sol. Cells 82 (2004) 576. [28] B. Thangaraju, Thin Solid Films 402 (2002) 71. [29] G. Haacke, J. Appl. Phys. 47 (1976) 4086. [30] A.L. Martinez, D.R. Acosta, G. Cedilli, Thin Solid Films 490 (2) (2005) 118. [31] A.V. Moholkar, S.M. Pawar, K.Y. Rajpure, S.N. Almari, P.S. Patil, C.H. Bhosale, Sol. Energy Mater. Sol. Cells 92 (2008) 1439. [32] G.S. Belo, B.J.P. da Silva, E.A. de Vasconcelos, W.M. de Azevedo, E.F. da Silva Jr., Appl. Surf. Sci. 255 (2008) 755. [33] M. Ait Aouaj, M. Abd-Lefdil, F. Cherkaoui El Moursli, F. Hajji, Eur. Phys. J. Appl. Phys. 40 (2007) 55.