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Optical and electrical properties of indium tin oxide (ITO) nanostructured thin films deposited on polycarbonate substrates “thickness effect”
Optik 125 (2014) 1478–1481
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Optical and electrical properties of indium tin oxide (ITO) nanostructured thin films deposited on polycarbonate substrates “thickness effect” Akbar Eshaghi a,∗ , Alireza Graeli b a b
Faculty of Materials Science and Engineering, Maleke Ashtar University of Technology, Shahinshahr, Esfahan, Iran Iranian Academic Center for Education, Culture & Research, Sharif Branch, Iran
a r t i c l e
i n f o
Article history: Received 17 April 2013 Accepted 3 September 2013
Keywords: ITO Thin film Optical properties Electrical properties
a b s t r a c t In this research, indium tin oxide (ITO) thin films of various thickness (200, 250, 300, 350, 400 nm) were deposited on polycarbonate polymer substrates using a magnetron sputtering technique. The structure, morphology, surface composition, optical and electrical properties of the thin films were investigated by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), X-ray Photoelectron Spectroscopy (XPS), UV-VIS-NIR spectrophotometer and four point probe method, respectively. The results indicated that grain size increased as the thickness increased. The transmittance and sheet resistance of the ITO thin films showed that ITO thin films with 200 nm thickness had the highest transmission whereas ITO thin film with 400 nm had the best conductivity. Crown Copyright © 2013 Published by Elsevier GmbH. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) thin films have important uses in a variety of applications such as transparent electrodes for optoelectronic devices, panel displays, solar cells, touch panels, IR reflectors, etc. [1–3]. Among TCO thin films, indium tin oxide (ITO) thin film has been almost exclusively used in optoelectronic devices due to its higher transmittance in the visible region and lower electrical properties [4,5]. ITO thin films onto glass substrates are widely used as transparent and conductive electrodes. Many techniques have been used to deposit ITO films onto glass substrates such as sputtering, evaporation, sol–gel, etc. [6]. Generally, ITO thin films deposited by the techniques indicated above must later be exposed to a relatively high temperature or post annealing process to acquire satisfactory reasonably electrical and optical properties [6]. Recently, transparent polymer materials, including polycarbonate-PC, due to their unique properties such as low weight, optical transparency, electrical, and mechanical properties have become very attractive as a replacement for inorganic glass substrates in optoelectronic devices [7,8]. Therefore, ITO thin films deposited on polymer substrates at room temperature must be analyzed because of their weak thermal resistance [9–11]. Thus, one of the significant technical issues in the manufacturing processes of flexible optoelectronic devices is to develop a low temperature
∗ Corresponding author. Fax: +98 3125228530. E-mail address: [email protected] (A. Eshaghi).
deposition technique. Magnetron sputtering has been considered as a low temperature deposition technique and has been used to deposit ITO thin films. It was found that the properties of ITO thin films are strongly dependent upon the preparation conditions such as the sputtering power, chamber pressure, sputtering time and thin film thickness. Among the literatures, few studies report the influence of thickness on the characteristics of ITO thin films deposited on the PC substrates. Then, the main purpose is to understand the relationships between the characteristics of ITO thin films and their thickness. In this study, transparent conducting ITO thin films were deposited on polymer substrates by magnetron sputtering. The dependence of the thin film thickness on the structural, electrical, and optical properties of ITO thin films were investigated. 2. Experimental Nanostructure ITO thin films of various thicknesses (200, 250, 300, 350 and 400 nm) were prepared on polycarbonate (PC) substrates by means of a DC magnetron sputtering method (MSS160 model, High vacuum Technology Center, ACECR-Sharif University Branch, IRAN). The deposition rate and the thickness of the growing films were measured by the use of a quartz-crystal sensor, which was placed near the substrate. The ITO thin film preparation conditions in the sputtering method are indicated in Table 1. It is necessary to mention, that before coating, the PC substrates were ultrasonically cleaned in a 1% neutral detergent solution and then plasma etched according to the specifications in Table 2. The
0030-4026/$ – see front matter. Crown Copyright © 2013 Published by Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.09.011
A. Eshaghi, A. Graeli / Optik 125 (2014) 1478–1481
1479
Table 1 ITO thin film preparation condition. Target
Substrate
Power (W/cm2 )
Sputtering gas
Base pressure
Work pressure
In2 O3 /SnO2 (90/10 wt.%)
PC
2.75
Ar
2 × 10−5 mbar
1.5 × 10−2 mbar
Table 2 Plasma etching procedure condition. Power (W/cm2 )
Sputtering gas
Tem (◦ C)
Etching time (min)
Base pressure (mbar)
Work pressure
2
Ar/O2 (90/10)
40
10
1.5 × 10−5
2 × 10−2
C 1s peak at a binding energy of 285 eV is observed on the film surface. The XPS spectra for In 3d and Sn 3d are shown in the Fig. 3. The binding energy of In 3d5/2 at 445.1 eV measured from the ITO film shown in Fig. 3 can be attributed to the In3+ bonding state from In2 O3 [15]. The binding energy of Sn 3d5/2 is at 487.1 eV and corresponds to the Sn4+ bonding state from SnO2 [14]. Fig. 4 shows the transmittance spectra of the ITO thin films. It is clear that the transmittance of ITO thin film decreases when the thickness increases. This phenomenon can be related to the effect of grain size. By increasing the thin film thickness, the grain size increases (see Table 3), which causes light scattering [15]. In addition, with increasing thickness, the average transmittance decreases slightly which may be caused by free carrier Fig. 1. XRD pattern of ITO thin film (400 nm).
plasma treatment was used to improve the adhesion of the ITO thin film onto the PC substrates. The structure and morphology of the thin films were determined using a Bruker X-ray diffractometer (D8ADVANCE, Germany, Ni˚ and Field emission scanning filter, Cu K␣ radiation = 1.5406 A) electron microscopy (FE-SEM, Hitachi S4160, Cold Field Emission, voltage 20KV). The surface chemical composition of the ITO thin film was analyzed by the X-ray photoelectron spectroscopy (XPS) using an Al K␣ source (1486.6 eV). The X-ray source was operated at 15 kV with a current of 10 mA. The XPS spectra were calibrated with respect to a carbon-1s peak at 285 eV. The transmittance spectra and sheet resistance of the ITO thin films were obtained using UVVIS-NIR spectrophotometer (Shimadzu UV-3100) and a four point probe method, respectively. 3. Results and discussion The XRD pattern of the ITO thin film (400 nm) is shown in Fig. 1. Apart from a large peak at 2 = 18, related to the PC, only crystalline In2 O3 has been identified [12,13]. This means that SnO2 completely has dissolved in the In2 O3 . Fig. 2 shows FE-SEM images of the ITO thin films. It may be seen that the grain size of the ITO thin films increases alogwith an increase in the thickness. The average grain sizes of the ITO thin films are indicated in Table 3. The XPS scan spectra of the ITO thin film (200 nm thickness) are shown in Fig. 3. Photoelectron peaks for In, Sn, O and C were recorded for the ITO film in the binding energy range of 0 to 1000 eV. The binding energy of the O 1s photoelectron peak is at 534 eV. A Table 3 Crystal size, sheet resistance and resistivity of ITO thin films. Thickness (nm)
Crystal size (nm)
Rs (/Sq)
200 250 300 350 400
23 32 37 49 63
87 45 22 15 10
× 10−4 ( cm) 17.4 11.25 6.6 5.25 4
Fig. 2. FE-SEM images of ITO thin films: (a) 200; (b) 250; (c) 300; (d) 350 and (e) 400 nm.
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Fig. 5. Absorption coefficient spectra of ITO thin films.
and conduction bands. The absorption coefficient (˛) as a function of photon energy can be expressed as [18]: 2
(˛h) = A(h − Eg )
Fig. 3. XPS spectra of ITO thin film, wide and narrow scan spectra.
absorption that increases carrier concentration in thick film and leads to the absorption of more light. [16]. The absorption coefficient (˛) of the ITO thin films can be calculated from the following formula [17]: ˛=
1 ln t
1 T
(1)
where t is the thickness, T is the transmittance. The absorption coefficients of the ITO thin films are shown in Fig. 5. The UV absorption edge is found to shift to higher wavelengths. In addition, the band gap energy (Eg ) of the ITO thin films can be estimated by presuming a direct transition between the valence
Fig. 4. Transmittance spectra of the ITO thin film.
(2)
where h is the photon energy, A is a constant, Eg is the band gap energy. The (˛h)2 versus h curve is shown in Fig. 6. The band gap (Eg ) value of the ITO thin films can be obtained by extrapolating the linear part of the plot relating to (˛h)2 and h to ˛h = 0 as shown in the Fig. 6. The band gap of the ITO thin films decreased with an increase in thickness. The band gap values are indicated in Table 3. By increasing the thickness, the absorption coefficient increased and the band gap decreased. This could be another reason for decrees in the transmittance when thickness is increased. The relation between film thickness and band gap values can be explained by the Bursteine–Moss shift. As mentioned when discussing electrical properties, the carrier concentration of the thinnest film is the smallest one, thus the occupied states in the valence band are less leading to a small band gap value [16]. The sheet resistance (Rs ) of the ITO thin films was measured by the four point probe method and the results are indicated in Table 3. Table 3 shows that sheet resistance decreased when thickness was increased. In addition, the resistivity () of the ITO thin films was measured according to following formula [19] and shown in Table 3. = Rs × t
(3)
According to Table 3, it can be seen that there is an inverse relationship between the sheet resistance and grain size of the thin films. The sheet resistance of a thin film is expressed as: Rs =
1 t
Fig. 6. (˛h)2 versus energy plots for ITO thin films with different thicknesses.
(4)
A. Eshaghi, A. Graeli / Optik 125 (2014) 1478–1481
where is the conductivity of the film. The conductivity is given by: = nq
(5)
where n is the free electron concentration, q is the electron charge and is the electron mobility. The electron mobility depends on several scattering mechanisms including lattice scattering, neutral impurity scattering and grain boundary scattering [15]. The grain boundary scattering is represented as [20]: g =
ıq exp 2 mn ∗ kT
−∅ b
(6)
kT
Here ı is the grain size, mn * is the electron effective mass, ∅b is the grain boundary potential barrier, k is the Boltzmann constant and T is the absolute temperature. If the grain boundary scattering is considered to be the major scattering mechanism, then the sheet resistance Rs can be indicated as: Rs =
1 1 = nqg t Mı
(7)
where M=
nq2 t (2 mn ∗ kT )
1/2
exp
−∅ b
kT
Thus, an inverse relationship between the sheet resistance and the grain size of the thin film can be anticipated. It is evident from Table 3 that the sheet resistances are more or less inversely related to the grain sizes. Therefore, the predominate mechanism of different conductivity is grain boundary scattering. Grain boundary can scatter the charge carriers. Thus, by increasing the grain size and decreasing grain boundary, conductivity increased, which caused decreased resistivity [20]. 4. Conclusion In this research, ITO thin films with various thicknesses were deposited on PC substrates using a DC magnetron sputtering technique. The grain sizes of the thin films were found to increase with film thickness. The sheet resistance measurement shows that thicker films have better electrical properties. The visible transmittance shows that transmittance decreases with film thickness. It was confirmed that the electrical and optical properties of ITO thin films on PC substrates strongly depended on film thickness. References [1] C.J.M. Emmott, A. Urbina, J. Nelson, Environmental and economic assessment of ITO-free electrodes for organic solar cells, Sol. Energy Mater. Sol. Cells 97 (2012) 14–21.
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[2] S.H. Mohamed, F.M. El-Hossary, G.A. Gamal, M.M. Kahlid, Properties of indium tin oxide thin films deposited on polymer substrates, Acta Phys. Pol. A 115 (3) (2009) 704–708. [3] J.W. Park, G. Kim, S.H. Lee, E.H. Kim, G.H. Lee, The effect of film microstructures on cracking of transparent conductive oxide (TCO) coatings on polymer substrates, Surf. Coat. Technol. 205 (2010) 915–921. [4] M.J. Keum, J.G. Han, Preparation of ITO thin film by using DC magnetron sputtering, J. Korean Phys. Soc. 53 (3) (2008) 1580–1583. [5] J.H. Jung, S.J. Lee, Preparation of In-based transparent conducting oxides, J. Korean Phys. Soc. 53 (1) (2008) 322–326. [6] L.J. Meng, E. Liang, J. Gao, V. Teixeira, M.P. dos Santos, Study of indium tin oxide thin films deposited on acrylics substrates by ion beam assisted deposition technique, J. Nanosci. Nanotechnol. 9 (2009) 4151–4155. [7] Y.H. Ko, M.S. Kim, J.S. Yu, Controllable electrochemical synthesis of ZnO nanorod arrays on flexible ITO/PET substrate and their structural and optical properties, Appl. Surf. Sci. 259 (15) (2012) 99–104. [8] T. Hanada, T. Negishi, I. Shiroishi, T. Shiro, Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays, Thin Solid Films 518 (2010) 3089–3092. [9] J. Lee, H. Jung, D. Lim, K. Yang, W. Song, J. Yi, Effects of bias voltage on the properties of ITO films prepared on polymer substrates, Thin Solid Films 480–481 (2005) 157–161. [10] K.H. Ri, Y.B. Wang, W.L. Zhou, J.X. Gao, X.J. Wang, J. Yu, The effect of SiO2 buffer layer on the electrical and structural properties of Al-doped ZnO films deposited on soda lime glasses, Appl. Surf. Sci. 257 (2011) 5471–5475. [11] C.N. de Carvalho, G. Lavareda, E. Fortunato, A. Amaral, Properties of ITO films deposited by r.f.-PERTE on unheated polymer substratesdependence on oxygen partial pressure, Thin Solid Films 427 (2003) 215–218. [12] B.S. Chiou, S.T. Hsieh, R.f. magnetron-sputtered in ion-etched acrylic substrate, Thin Solid Films 229 (1993) 146–155. [13] C. Guillen, J. Herrero, Comparison study of ITO thin films deposited by sputtering at room temperature onto polymer and glass substrates, Thin Solid Films 480–481 (2005) 129–132. [14] M.J. Alam, D.C. Cameron, Optical and electrical properties of transparent conductive ITO thin films deposited by sol–gel process, Thin Solid Films 377–378 (2000) 455–459. [15] A. Eshaghi, M. Pakshir, R. Mozaffarinia, Preparation and photo-induced superhydrophilicity of composite TiO2 –SiO2 –In2 O3 thin film, Appl. Surf. Sci. 256 (2010) 7062–7066. [16] F. Wang, M.Z. Wu, Y.Y. Wang, Y.M. Yu, X.M. Wu, L.J. Zhuge, Influence of thickness and annealing temperature on the electrical, optical and structural properties of AZO thin films, Vacuum 89 (2013) 127–131. [17] T. Asanuma, T. Matsutani, C. Liu, T. Mihara, M. Kiuchi, Structural and optical properties of titanium dioxide films deposited by reactive magnetron sputtering in pure oxygen plasma, J. Appl. Phys. 95 (2004) 6011–6016. [18] M.S. Huh, B.S. Yang, J. Song, J. Heo, S.J. Won, J.K. Jeong, C.S. Hwang, H.J. Kim, Improving the morphological and optical properties of sputtered indium tin oxide thin films by adopting ultralow-pressure sputtering, J. Electrochem. Soc. 156 (1) (2009) J6–J11. [19] S. Fernandez, A. Martinez-Steele, J.J. Gandia, F.B. Naranjo, Radio frequency sputter deposition of high-quality conductive and transparent ZnO: Al films on polymer substrates for thin film solar cells applications, Thin Solid Films 517 (2009) 3152–3156. [20] A.K. Kulkarni, H. Kirk, T.S. Schulz, M. Lim, Khan, Dependence of the sheet resistance of indium-tin-oxide thin films on grain size and grain orientation determined from X-ray diffraction techniques, Thin Solid Films 345 (1999) 273–277.
Contents lists available at ScienceDirect
Optik journal homepage: www.elsevier.de/ijleo
Optical and electrical properties of indium tin oxide (ITO) nanostructured thin films deposited on polycarbonate substrates “thickness effect” Akbar Eshaghi a,∗ , Alireza Graeli b a b
Faculty of Materials Science and Engineering, Maleke Ashtar University of Technology, Shahinshahr, Esfahan, Iran Iranian Academic Center for Education, Culture & Research, Sharif Branch, Iran
a r t i c l e
i n f o
Article history: Received 17 April 2013 Accepted 3 September 2013
Keywords: ITO Thin film Optical properties Electrical properties
a b s t r a c t In this research, indium tin oxide (ITO) thin films of various thickness (200, 250, 300, 350, 400 nm) were deposited on polycarbonate polymer substrates using a magnetron sputtering technique. The structure, morphology, surface composition, optical and electrical properties of the thin films were investigated by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), X-ray Photoelectron Spectroscopy (XPS), UV-VIS-NIR spectrophotometer and four point probe method, respectively. The results indicated that grain size increased as the thickness increased. The transmittance and sheet resistance of the ITO thin films showed that ITO thin films with 200 nm thickness had the highest transmission whereas ITO thin film with 400 nm had the best conductivity. Crown Copyright © 2013 Published by Elsevier GmbH. All rights reserved.
1. Introduction Transparent conductive oxide (TCO) thin films have important uses in a variety of applications such as transparent electrodes for optoelectronic devices, panel displays, solar cells, touch panels, IR reflectors, etc. [1–3]. Among TCO thin films, indium tin oxide (ITO) thin film has been almost exclusively used in optoelectronic devices due to its higher transmittance in the visible region and lower electrical properties [4,5]. ITO thin films onto glass substrates are widely used as transparent and conductive electrodes. Many techniques have been used to deposit ITO films onto glass substrates such as sputtering, evaporation, sol–gel, etc. [6]. Generally, ITO thin films deposited by the techniques indicated above must later be exposed to a relatively high temperature or post annealing process to acquire satisfactory reasonably electrical and optical properties [6]. Recently, transparent polymer materials, including polycarbonate-PC, due to their unique properties such as low weight, optical transparency, electrical, and mechanical properties have become very attractive as a replacement for inorganic glass substrates in optoelectronic devices [7,8]. Therefore, ITO thin films deposited on polymer substrates at room temperature must be analyzed because of their weak thermal resistance [9–11]. Thus, one of the significant technical issues in the manufacturing processes of flexible optoelectronic devices is to develop a low temperature
∗ Corresponding author. Fax: +98 3125228530. E-mail address: [email protected] (A. Eshaghi).
deposition technique. Magnetron sputtering has been considered as a low temperature deposition technique and has been used to deposit ITO thin films. It was found that the properties of ITO thin films are strongly dependent upon the preparation conditions such as the sputtering power, chamber pressure, sputtering time and thin film thickness. Among the literatures, few studies report the influence of thickness on the characteristics of ITO thin films deposited on the PC substrates. Then, the main purpose is to understand the relationships between the characteristics of ITO thin films and their thickness. In this study, transparent conducting ITO thin films were deposited on polymer substrates by magnetron sputtering. The dependence of the thin film thickness on the structural, electrical, and optical properties of ITO thin films were investigated. 2. Experimental Nanostructure ITO thin films of various thicknesses (200, 250, 300, 350 and 400 nm) were prepared on polycarbonate (PC) substrates by means of a DC magnetron sputtering method (MSS160 model, High vacuum Technology Center, ACECR-Sharif University Branch, IRAN). The deposition rate and the thickness of the growing films were measured by the use of a quartz-crystal sensor, which was placed near the substrate. The ITO thin film preparation conditions in the sputtering method are indicated in Table 1. It is necessary to mention, that before coating, the PC substrates were ultrasonically cleaned in a 1% neutral detergent solution and then plasma etched according to the specifications in Table 2. The
0030-4026/$ – see front matter. Crown Copyright © 2013 Published by Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.ijleo.2013.09.011
A. Eshaghi, A. Graeli / Optik 125 (2014) 1478–1481
1479
Table 1 ITO thin film preparation condition. Target
Substrate
Power (W/cm2 )
Sputtering gas
Base pressure
Work pressure
In2 O3 /SnO2 (90/10 wt.%)
PC
2.75
Ar
2 × 10−5 mbar
1.5 × 10−2 mbar
Table 2 Plasma etching procedure condition. Power (W/cm2 )
Sputtering gas
Tem (◦ C)
Etching time (min)
Base pressure (mbar)
Work pressure
2
Ar/O2 (90/10)
40
10
1.5 × 10−5
2 × 10−2
C 1s peak at a binding energy of 285 eV is observed on the film surface. The XPS spectra for In 3d and Sn 3d are shown in the Fig. 3. The binding energy of In 3d5/2 at 445.1 eV measured from the ITO film shown in Fig. 3 can be attributed to the In3+ bonding state from In2 O3 [15]. The binding energy of Sn 3d5/2 is at 487.1 eV and corresponds to the Sn4+ bonding state from SnO2 [14]. Fig. 4 shows the transmittance spectra of the ITO thin films. It is clear that the transmittance of ITO thin film decreases when the thickness increases. This phenomenon can be related to the effect of grain size. By increasing the thin film thickness, the grain size increases (see Table 3), which causes light scattering [15]. In addition, with increasing thickness, the average transmittance decreases slightly which may be caused by free carrier Fig. 1. XRD pattern of ITO thin film (400 nm).
plasma treatment was used to improve the adhesion of the ITO thin film onto the PC substrates. The structure and morphology of the thin films were determined using a Bruker X-ray diffractometer (D8ADVANCE, Germany, Ni˚ and Field emission scanning filter, Cu K␣ radiation = 1.5406 A) electron microscopy (FE-SEM, Hitachi S4160, Cold Field Emission, voltage 20KV). The surface chemical composition of the ITO thin film was analyzed by the X-ray photoelectron spectroscopy (XPS) using an Al K␣ source (1486.6 eV). The X-ray source was operated at 15 kV with a current of 10 mA. The XPS spectra were calibrated with respect to a carbon-1s peak at 285 eV. The transmittance spectra and sheet resistance of the ITO thin films were obtained using UVVIS-NIR spectrophotometer (Shimadzu UV-3100) and a four point probe method, respectively. 3. Results and discussion The XRD pattern of the ITO thin film (400 nm) is shown in Fig. 1. Apart from a large peak at 2 = 18, related to the PC, only crystalline In2 O3 has been identified [12,13]. This means that SnO2 completely has dissolved in the In2 O3 . Fig. 2 shows FE-SEM images of the ITO thin films. It may be seen that the grain size of the ITO thin films increases alogwith an increase in the thickness. The average grain sizes of the ITO thin films are indicated in Table 3. The XPS scan spectra of the ITO thin film (200 nm thickness) are shown in Fig. 3. Photoelectron peaks for In, Sn, O and C were recorded for the ITO film in the binding energy range of 0 to 1000 eV. The binding energy of the O 1s photoelectron peak is at 534 eV. A Table 3 Crystal size, sheet resistance and resistivity of ITO thin films. Thickness (nm)
Crystal size (nm)
Rs (/Sq)
200 250 300 350 400
23 32 37 49 63
87 45 22 15 10
× 10−4 ( cm) 17.4 11.25 6.6 5.25 4
Fig. 2. FE-SEM images of ITO thin films: (a) 200; (b) 250; (c) 300; (d) 350 and (e) 400 nm.
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A. Eshaghi, A. Graeli / Optik 125 (2014) 1478–1481
Fig. 5. Absorption coefficient spectra of ITO thin films.
and conduction bands. The absorption coefficient (˛) as a function of photon energy can be expressed as [18]: 2
(˛h) = A(h − Eg )
Fig. 3. XPS spectra of ITO thin film, wide and narrow scan spectra.
absorption that increases carrier concentration in thick film and leads to the absorption of more light. [16]. The absorption coefficient (˛) of the ITO thin films can be calculated from the following formula [17]: ˛=
1 ln t
1 T
(1)
where t is the thickness, T is the transmittance. The absorption coefficients of the ITO thin films are shown in Fig. 5. The UV absorption edge is found to shift to higher wavelengths. In addition, the band gap energy (Eg ) of the ITO thin films can be estimated by presuming a direct transition between the valence
Fig. 4. Transmittance spectra of the ITO thin film.
(2)
where h is the photon energy, A is a constant, Eg is the band gap energy. The (˛h)2 versus h curve is shown in Fig. 6. The band gap (Eg ) value of the ITO thin films can be obtained by extrapolating the linear part of the plot relating to (˛h)2 and h to ˛h = 0 as shown in the Fig. 6. The band gap of the ITO thin films decreased with an increase in thickness. The band gap values are indicated in Table 3. By increasing the thickness, the absorption coefficient increased and the band gap decreased. This could be another reason for decrees in the transmittance when thickness is increased. The relation between film thickness and band gap values can be explained by the Bursteine–Moss shift. As mentioned when discussing electrical properties, the carrier concentration of the thinnest film is the smallest one, thus the occupied states in the valence band are less leading to a small band gap value [16]. The sheet resistance (Rs ) of the ITO thin films was measured by the four point probe method and the results are indicated in Table 3. Table 3 shows that sheet resistance decreased when thickness was increased. In addition, the resistivity () of the ITO thin films was measured according to following formula [19] and shown in Table 3. = Rs × t
(3)
According to Table 3, it can be seen that there is an inverse relationship between the sheet resistance and grain size of the thin films. The sheet resistance of a thin film is expressed as: Rs =
1 t
Fig. 6. (˛h)2 versus energy plots for ITO thin films with different thicknesses.
(4)
A. Eshaghi, A. Graeli / Optik 125 (2014) 1478–1481
where is the conductivity of the film. The conductivity is given by: = nq
(5)
where n is the free electron concentration, q is the electron charge and is the electron mobility. The electron mobility depends on several scattering mechanisms including lattice scattering, neutral impurity scattering and grain boundary scattering [15]. The grain boundary scattering is represented as [20]: g =
ıq exp 2 mn ∗ kT
−∅ b
(6)
kT
Here ı is the grain size, mn * is the electron effective mass, ∅b is the grain boundary potential barrier, k is the Boltzmann constant and T is the absolute temperature. If the grain boundary scattering is considered to be the major scattering mechanism, then the sheet resistance Rs can be indicated as: Rs =
1 1 = nqg t Mı
(7)
where M=
nq2 t (2 mn ∗ kT )
1/2
exp
−∅ b
kT
Thus, an inverse relationship between the sheet resistance and the grain size of the thin film can be anticipated. It is evident from Table 3 that the sheet resistances are more or less inversely related to the grain sizes. Therefore, the predominate mechanism of different conductivity is grain boundary scattering. Grain boundary can scatter the charge carriers. Thus, by increasing the grain size and decreasing grain boundary, conductivity increased, which caused decreased resistivity [20]. 4. Conclusion In this research, ITO thin films with various thicknesses were deposited on PC substrates using a DC magnetron sputtering technique. The grain sizes of the thin films were found to increase with film thickness. The sheet resistance measurement shows that thicker films have better electrical properties. The visible transmittance shows that transmittance decreases with film thickness. It was confirmed that the electrical and optical properties of ITO thin films on PC substrates strongly depended on film thickness. References [1] C.J.M. Emmott, A. Urbina, J. Nelson, Environmental and economic assessment of ITO-free electrodes for organic solar cells, Sol. Energy Mater. Sol. Cells 97 (2012) 14–21.
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