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ITO multilayers
Optics Communications 282 (2009) 2362–2366
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Dependence of intermediated noble metals on the optical and electrical properties of ITO/metal/ITO multilayers J.Y. Lee, J.W. Yang, J.H. Chae, J.H. Park, J.I. Choi, H.J. Park, Daeil Kim * School of Materials Science and Engineering, University of Ulsan, San 29, Mugeo Dong, Nam Gu, Ulsan 680-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 June 2008 Received in revised form 17 December 2008 Accepted 18 December 2008
Keywords: Optical properties Thin films Magnetron sputtering X-ray diffraction
a b s t r a c t Sn doped In2O3 (ITO) single layer and a sandwich structure of ITO/metal/ITO (IMI) multilayer films were deposited on a polycarbonate substrate using radio-frequency and direct-current magnetron sputtering process without substrate heating. The intermediated metal films in the IMI structure were Au and Cu films and the thickness of each layer in the IMI films was kept constant at 50 nm/10 nm/40 nm. In this study, the ITO/Au/ITO films show the lowest resistivity of 5.6 105 X cm. However the films show the lower optical transmission of 71% at 550 nm than that (81%) of as deposited ITO films. The ITO/Cu/ITO films show an optical transmittance of 54% and electrical resistivity of 1.5 104 X cm. Only the ITO/Au/ITO films showed the diffraction peaks in the XRD pattern. The figure of merit indicated that the ITO/Au/ITO films performed better in a transparent conducting electrode than in ITO single layer films and ITO/Cu/ITO films. Ó 2009 Published by Elsevier B.V.
1. Introduction A Sn-doped In2O3 (ITO) thin film is a highly degenerated, widegap semiconductor with a low electrical resistivity and high optical transmission across the visible spectrum. Because of their unique optoelectrical properties, ITO films have many applications, such as solar cells [1], gas sensor [2], and flat panel displays [3] and conventional ITO films can be reproducibly prepared by various methods including reactive magnetron sputtering [4], electron beam evaporation [5], ion beam assist deposition [6], and pulsed laser deposition (PLD) [7]. Among the methods mentioned above, reactive magnetron sputtering is the most frequently used deposition methods for ITO in optoelectronic device manufacturing. It is well known that in conventional sputter deposition processes temperatures as high as 300 °C are required for deposition and/or post-annealing to obtain ITO films with a reasonably low resistivity (2 104 X cm) and high optical transmittance (90% in the visible region) [8]. However, for certain ITO applications such as flexible optoelectronic devices, high substrate temperatures or high post-deposition annealing temperatures are undesirable due to the low thermal resistance of polymer substrates. One way to improve optoelectrical properties of ITO films without intentional substrate heating is to use a sandwich structure of ITO/metal/ITO (IMI) films [9], which have lower resistivity than ITO single layer films of the same thickness. In recent years, several
* Corresponding author. Fax: +82 52 259 1688. E-mail address: [email protected] (D. Kim). 0030-4018/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.optcom.2008.12.044
advantageous IMI structures, in which Ag alloys were used as an intermediated layer to improve the properties of ITO films, have been reported [9,10]. In this study to find new intermediated metals for the IMI films which have high quality than that of the ITO single layer films, pure Au and Cu thin films were intermediated between ITO films and then the changes in structural and optoelectrical properties were compared with ITO single layer films. 2. Experimental details ITO single layer films, ITO/Au/ITO, and ITO/Cu/ITO multilayer films were deposited on polycarbonate (PC) substrate. The schematic diagram of the three targets magnetron sputtering system used in this study is shown in Fig. 1. The RF (13.56 MHz) and DC powers were applied to the ITO (High Purity Chemicals Japan, In2O3–SnO2 (90:10 wt %), purity; 99.99%) and Au (Target materials Inc., purity; 99.95%) and Cu (Target materials Inc., purity; 99.99%) targets, respectively. The targets were 3 in. in diameter by 0.25 in. thick and the substrate was 100 lm thick PC. Prior to deposition, the chamber was evacuated to 1.3 104 Pa. Sputtering was performed at 1 102 Pa in a pure Ar/O2 gas mixture and the distance between the target and rotation substrate holder was constant at 10 cm. The substrate rotation speed was set to 10 rpm for all depositions. Substrate temperature was detected by a K-type thermocouple directly in contact with the substrate surface. Although the PC substrates were not heated intentionally during deposition, the temperature was increased to 70 °C. By controlling the deposition time, ITO/metal/ITO films, in which the 10 nm thick Au and Cu films were sandwiched by the
J.Y. Lee et al. / Optics Communications 282 (2009) 2362–2366
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Fig. 1. Schematic diagram of a magnetron sputtering system.
50 nm (bottom ITO layer) and 40 nm (upper ITO layer) thick ITO films, were obtained by depositing each film layer continuously without exposure of the films to the atmosphere [11]. Table 1 shows the deposition conditions of ITO and IMI films. As a reference, 100 nm thick ITO films were prepared in the same manner. After deposition, the thickness of films was measured from the several well defined edges with a surface profilometer.
Table 1 Deposition conditions of ITO and intermediated Au and Cu thin films.
Base pressure (Pa) Deposition pressure (Pa) Power density (W/cm2) Deposition rate (nm/min) Gas flow rate (Ar/O2 sccm)
ITO
Au
Cu
1.3 104 2 101 RF, 2.5 14 5/0.03
1.3 104 2 101 DC, 2.3 20 5 (Ar only)
1.3 104 2 101 DC, 2.5 17 5 (Ar only)
Fig. 3. SEM micrographs of (a) ITO, (b) ITO/Au/ITO, and (c) ITO/Cu/ITO films.
Fig. 2. XRD pattern of as deposited ITO and ITO/metal/ITO films.
It maps surface topography by dragging a sharp probe across a film surface. The structural characterization was done using X-ray diffraction (XRD) measurements with Cu Ka radiation (RAD-3C, Rigaku). The surface morphology was analyzed using a scanning electron microscope (SEM, JSM-820, JEOL) and surface average roughness measurements were performed on 2 2 lm2 areas with tapping mode atomic force microscope (AFM, Dimension 5000, Veeco) under ambient conditions. The average surface roughness of the bare PC substrates was 1.3 nm. Since the film thickness was kept constant at 100 nm, the variation in surface morphology due to thickness was ignored. After deposition, ITO/Au/ITO films were annealed in a vacuum of 1 102 Pa for 20 min at 150 °C to consider the influence of annealing temperature on the optoelectrical properties of IMI films. The electrical resistivity was obtained using the Van der Pauw method (HMS-3000, Ecopia) and optical transmittance was measured in the range of 300–800 nm by the UV– vis. spectrophotometer at the Korea Basic Science Institute (KBSI).
J.Y. Lee et al. / Optics Communications 282 (2009) 2362–2366
The data presented in this work contain the transmission of the multilayers, including the PC substrate. In order to consider the sheet resistance (Rs) stability in a high humidity environment (air at a relative humidity of 90% for 600 h and a temperature of 60 °C), the normalized Rs of the films which have the highest figure of merit (uTC) in this study were measured with four point probe method. The grain size (D) of In2O3 films was calculated using Scherrer’s formula [12]
D ¼ 0:9k=b cos h where k = 1.54 Å and b is sq. root (B2 b2), where B is the observed full width half maximum (FWHM) and b is the resolution of the spectrometer (here 0.01).
100
80
Optical transmittacne (%)
2364
60
40 PC substrae ITO single layer films ITO / Au / ITO films ITO / Cu / ITO films
20
0 400
500
600
700
800
Wavelength (nm) Fig. 5. Optical transmittance of as deposited ITO and ITO/metal/ITO films.
3. Results To confirm the influence of the intermediated noble Au and Cu layer in ITO crystallization, XRD analysis was performed. Fig. 2 shows the XRD pattern of the as deposited ITO and ITO/metal/ ITO films. Only ITO/Au/ITO films were oriented along In2O3 (4 0 0), while ITO films was amorphous phase. To this end, the Au intermediate layer of the IMI structure effectively promoted thin film crystallization of the ITO films. In order to know the crystallinity of ITO/Au/ITO films, grain size of the In2O3 (4 0 0) from XRD pattern was evaluated was about 12 nm. Fig. 3 shows the SEM micrographs of the films. The as deposited ITO and ITO/Cu/ITO films are amorphous in the XRD pattern. In the SEM images, there is no significant difference between the ITO and ITO/Cu/ITO films. But ITO/Au/ITO films showed some granular growth on the film surface. This is consisted results with XRD measurements. Fig. 4 shows AFM images of the as deposited ITO single layer films and ITO/Au/ITO multilayer films which show some spherical grain growth on the surface. The as deposited ITO film showed an average surface roughness of 2.5 nm, while the ITO/Cu/ITO film was 0.98 nm and ITO/Au/ITO films was 0.51 nm. Ghosh [13] reported that ITO films deposited on quartz substrate are smoother than the films deposited on amorphous glass substrate and Purica [14] and Diplas et al. [15] also observed that ITO films deposited on glass substrate showed increased roughness compared to the film deposited on crystallized Si (1 0 0) substrate. From the previous reports [13–15], it may concluded that the crystallized Au interlayer in ITO/Au/ITO multilayer films results in the
Table 2 Comparison of carrier density (1019/cm3), mobility (cm2/Vs), resistivity (104 X cm), optical transmittance contained PC substrate (T, (%)) at 550 nm, and figure of merit (uTC, 103 X1) of ITO and IMI films.
Fig. 4. AFM images of ITO/metal/ITO multilayered films (X-axis: 0.5 lm/Div., Yaxis: 10.0 nm/Div.). (a) ITO 50 nm/Au 10 nm/ITO 40 nm, (b) ITO 50 nm/Cu 10 nm/ ITO 40 nm.
Carrier density Mobility Resistivity T (%) uTC
ITO
ITO/Au/ITO
ITO/Cu/ITO
2.3 85 31.2 81 0.38
220 47 0.56 72 6.6
54 39 1.51 54 0.14
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J.Y. Lee et al. / Optics Communications 282 (2009) 2362–2366
ity than that of the IMI films, suggesting that the two interfaces between the ITO and intermediated metal films may act as a defect to carrier movement. Although ITO films had a higher mobility than IMI films, they had a lower resistivity due to the low carrier density. ITO films had a resistivity of 4 103 X cm, and ITO/Cu/ITO films had a resistivity as low as 1.5 104 X cm. The lowest resistivity of 5.6 105 X cm was obtained from the ITO/Au/ITO films. Fig. 6 shows the normalized Rs of ITO/Au/ITO films which have the higher uTC than the other films as a function of exposure time in the humidity test. Since the normalized Rs measured in the same humidity condition was kept constantly, it may conclude that ITO/ Au/ITO films are relatively stable with moisture condition within measured exposure time of 600 h. The figure of merit (uTC) is an important index for evaluating the performance of transparent conducting oxide (TCO) films [16]. The uTC is defined by
Normalized sheet resistance (R/R I)
3.0
2.5
2.0
1.5
1.0
/TC ¼ T 10 =RS
0.5
0.0 0
100
200
300
400
500
600
700
Exposure time (Hour) Fig. 6. Normalized sheet resistance as a function of exposure time at humidity condition.
more flat surface morphology than that of the as deposited ITO single layer films. Fig. 5 shows the optical transmittance of a wavelength range of 300–800 nm. The bare PC substrates and ITO films deposited on the PC substrate have 90% and 81% optical transmittance at 550 nm, respectively, which is consistent with previously reported conventional magnetron sputtering results without substrate heating [9]. Multilayered ITO/Au/ITO and ITO/Cu/ITO films showed an optical transmission of 71% and 54% at 550 nm, respectively. Table 2 shows the electrical properties of the ITO and IMI multilayer films. In the case of a conventional ITO film, the carrier concentration was mainly determined by oxygen vacancies or the concentration of substituted Sn4+ on In3+ sites [4]. However, in this study, all process parameters, such as the O2/Ar flow rate, the Sn concentration in the sputtering target, and the substrate temperature, which all may affect carrier concentration, were kept constant. Thus, the high carrier concentration in the IMI films compared with that in the ITO films was attributed to the high carrier concentration in the metal layer on the IMI films. Although intermediated Au and Cu films have the same thickness of 10 nm in IMI films, ITO/Au/ITO films have a higher carrier density than that of the ITO/Cu/ITO films. It is well known that a larger grain size (small grain boundary density) increases the carrier density because grain boundaries behave as traps for free carriers in the film. Thus, the lowest resistivity obtained in the ITO/Au/ITO film was accompanied by an increase in the carrier density of the ITO/Au/ ITO films, which resulted from the crystallization of the films as shown in XRD spectra. However, the ITO films had a higher mobil-
where T is the optical transmittance (at 550 nm in this study), and Rs is a sheet resistance. The uTC for IMI multilayer and ITO films are compared in Table 2. The uTC reached a maximum of 5.8 103 X1 for ITO/Au/ITO films, which was higher than the uTC of the ITO films. The ITO/Cu/ITO films had the lowest uTC of 0.14 103 X1. A higher uTC resulted in better quality TCO films [17] suggesting that the ITO/Au/ITO films had better optoelectrical properties than those of the ITO/Cu/ITO films and ITO films. Table 3 shows the comparison of figure of merit and optoelectrical properties of Ag and Au intermediated IMI films which have similar thickness. Although ITO/ Au/ITO films show the lower optical transmittance of 72% in a wavelength of 550 nm than that of the ITO/Ag/ITO films [18], they have the high figure of merit due to the low sheet resistance of the films. After post deposition annealing, the uTC of ITO/Au/ITO films increased to 12.7 103 X1 because of improved transmittance as high as 75% which is comparable with ITO/Ag/ITO films [18]. 4. Conclusion ITO single layer and ITO/metal/ITO (IMI) multilayer films, which had Au and Cu intermediated films were prepared on a PC substrate using an RF and DC magnetron sputtering process, respectively. In IMI films, only ITO/Au/ITO films were aligned (4 0 0) which corresponding to the indium oxide bixbyite structure. To this end, the Au interlayer of the IMI structure effectively promoted thin film crystallization of the ITO films. In electrical resistivity measurements, the ITO films had a resistivity of 4 103 X cm and ITO/Cu/ITO films had a resistivity as low as 1.5 104 X cm. The lowest resistivity of 5.6 105 X cm was obtained from the ITO/Au/ITO films. Also ITO/Au/ITO films show the stable normalized Rs with moisture condition (90%, 60 °C) within measured exposure time of 600 h. In this study, as deposited ITO/Au/ITO films had a figure of merit (uTC) of 6.6 103 X1 and after post deposition annealing, the uTC of the films increased as high as 12.7 103 X1.
Table 3 The comparison of figure of merit (uTC) and optoelectrical property of the IMI films. Optical transmittance at 550 nm (T, %) contained PC substrate. Both ITO/Cu/ITO and ITO/Au/ ITO films have same composition of 50/10/40 nm. Post deposition vacuum annealed ITO/Au/ITO films also compared with Ag and Au intermediated IMI films, respectively. Structure(Thickness)
T (%)
Resistance (X/h)
uTC
Reference
ITO/Ag/ITO (50/10/50 nm) ITO/Cu/ITO ITO/Au/ITO ITO/Au/ITO (Annealed at 150 °C)
79
23
4.3
[18]
54 72 75
15.1 5.6 4.4
0.14 6.6 12.7
This study This study This study
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Acknowledgement This work was financed from the budget sources for science research by the Korea Sanhak Foundation in the years 2007–2008. References [1] [2] [3] [4] [5] [6]
D. Rached, R. Mostefaoui, Thin Solid Films 516 (2008) 5087. V.S. Vaishnav, P.D. Patel, N.G. Patel, Thin Solid Films 487 (2005) 277. U. Betz, M. Olsson, J. Martly, M. Escola, Surf. Coat. Technol. 200 (2006) 5751. W. Wu, B. Chiou, Thin Solid Films 298 (1997) 221. D. Raoufi, A. Kiasatpour, H. Fallah, A. Rozatian, Appl. Surf. Sci. 253 (2007) 9085. L. Meng, J. Gao, R. Silva, S. Song, Thin Solid Films 516 (2008) 5454.
[7] C. Viespe, I. Nicolae, R. Medianu, Thin Solid Films 515 (2007) 8771. [8] S. Boycheva, A. Krasilnikova, A. Piegari, Thin Solid Films 515 (2007) 8474. [9] M. Bender, W. Seelig, C. Daube, H. Frankenberger, B. Ocker, J. Stollenwerk, Thin Solid Films 326 (1998) 67. [10] S. Chen, C. Koo, Mater. Lett. 61 (2007) 4097. [11] D.R. Sahu, J. Huang, Appl. Surf. Sci. 253 (2006) 915. [12] B.D. Cullity, Elements of X-ray Diffraction, second ed., Addison-Wesley Publishing Company, 1977. p. 102. [13] S. Ghosh, H. Kim, K. Hong, C. Lee, Mater. Sci. Eng. B 95 (2002) 171. [14] M. Purica, F. Iacomi, C. Baban, P. Prepelita, N. Apetroaei, D. Mardare, D. Luca, Thin Solid Films 515 (2007) 8674. [15] S. Diplas, A. Ulyashin, K. Maknys, Thin Solid Films 515 (2007) 8539. [16] G. Haacke, J. Appl. Phys. 47 (1976) 4086. [17] D. Kim, Opt. Mater. 24 (2003) 471. [18] E. Bertran, C. Corbella, M. Vives, A. Pinyol, C. Person, I. Porqueras, Solid State Ionics 165 (2003) 139.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Dependence of intermediated noble metals on the optical and electrical properties of ITO/metal/ITO multilayers J.Y. Lee, J.W. Yang, J.H. Chae, J.H. Park, J.I. Choi, H.J. Park, Daeil Kim * School of Materials Science and Engineering, University of Ulsan, San 29, Mugeo Dong, Nam Gu, Ulsan 680-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 26 June 2008 Received in revised form 17 December 2008 Accepted 18 December 2008
Keywords: Optical properties Thin films Magnetron sputtering X-ray diffraction
a b s t r a c t Sn doped In2O3 (ITO) single layer and a sandwich structure of ITO/metal/ITO (IMI) multilayer films were deposited on a polycarbonate substrate using radio-frequency and direct-current magnetron sputtering process without substrate heating. The intermediated metal films in the IMI structure were Au and Cu films and the thickness of each layer in the IMI films was kept constant at 50 nm/10 nm/40 nm. In this study, the ITO/Au/ITO films show the lowest resistivity of 5.6 105 X cm. However the films show the lower optical transmission of 71% at 550 nm than that (81%) of as deposited ITO films. The ITO/Cu/ITO films show an optical transmittance of 54% and electrical resistivity of 1.5 104 X cm. Only the ITO/Au/ITO films showed the diffraction peaks in the XRD pattern. The figure of merit indicated that the ITO/Au/ITO films performed better in a transparent conducting electrode than in ITO single layer films and ITO/Cu/ITO films. Ó 2009 Published by Elsevier B.V.
1. Introduction A Sn-doped In2O3 (ITO) thin film is a highly degenerated, widegap semiconductor with a low electrical resistivity and high optical transmission across the visible spectrum. Because of their unique optoelectrical properties, ITO films have many applications, such as solar cells [1], gas sensor [2], and flat panel displays [3] and conventional ITO films can be reproducibly prepared by various methods including reactive magnetron sputtering [4], electron beam evaporation [5], ion beam assist deposition [6], and pulsed laser deposition (PLD) [7]. Among the methods mentioned above, reactive magnetron sputtering is the most frequently used deposition methods for ITO in optoelectronic device manufacturing. It is well known that in conventional sputter deposition processes temperatures as high as 300 °C are required for deposition and/or post-annealing to obtain ITO films with a reasonably low resistivity (2 104 X cm) and high optical transmittance (90% in the visible region) [8]. However, for certain ITO applications such as flexible optoelectronic devices, high substrate temperatures or high post-deposition annealing temperatures are undesirable due to the low thermal resistance of polymer substrates. One way to improve optoelectrical properties of ITO films without intentional substrate heating is to use a sandwich structure of ITO/metal/ITO (IMI) films [9], which have lower resistivity than ITO single layer films of the same thickness. In recent years, several
* Corresponding author. Fax: +82 52 259 1688. E-mail address: [email protected] (D. Kim). 0030-4018/$ - see front matter Ó 2009 Published by Elsevier B.V. doi:10.1016/j.optcom.2008.12.044
advantageous IMI structures, in which Ag alloys were used as an intermediated layer to improve the properties of ITO films, have been reported [9,10]. In this study to find new intermediated metals for the IMI films which have high quality than that of the ITO single layer films, pure Au and Cu thin films were intermediated between ITO films and then the changes in structural and optoelectrical properties were compared with ITO single layer films. 2. Experimental details ITO single layer films, ITO/Au/ITO, and ITO/Cu/ITO multilayer films were deposited on polycarbonate (PC) substrate. The schematic diagram of the three targets magnetron sputtering system used in this study is shown in Fig. 1. The RF (13.56 MHz) and DC powers were applied to the ITO (High Purity Chemicals Japan, In2O3–SnO2 (90:10 wt %), purity; 99.99%) and Au (Target materials Inc., purity; 99.95%) and Cu (Target materials Inc., purity; 99.99%) targets, respectively. The targets were 3 in. in diameter by 0.25 in. thick and the substrate was 100 lm thick PC. Prior to deposition, the chamber was evacuated to 1.3 104 Pa. Sputtering was performed at 1 102 Pa in a pure Ar/O2 gas mixture and the distance between the target and rotation substrate holder was constant at 10 cm. The substrate rotation speed was set to 10 rpm for all depositions. Substrate temperature was detected by a K-type thermocouple directly in contact with the substrate surface. Although the PC substrates were not heated intentionally during deposition, the temperature was increased to 70 °C. By controlling the deposition time, ITO/metal/ITO films, in which the 10 nm thick Au and Cu films were sandwiched by the
J.Y. Lee et al. / Optics Communications 282 (2009) 2362–2366
2363
Fig. 1. Schematic diagram of a magnetron sputtering system.
50 nm (bottom ITO layer) and 40 nm (upper ITO layer) thick ITO films, were obtained by depositing each film layer continuously without exposure of the films to the atmosphere [11]. Table 1 shows the deposition conditions of ITO and IMI films. As a reference, 100 nm thick ITO films were prepared in the same manner. After deposition, the thickness of films was measured from the several well defined edges with a surface profilometer.
Table 1 Deposition conditions of ITO and intermediated Au and Cu thin films.
Base pressure (Pa) Deposition pressure (Pa) Power density (W/cm2) Deposition rate (nm/min) Gas flow rate (Ar/O2 sccm)
ITO
Au
Cu
1.3 104 2 101 RF, 2.5 14 5/0.03
1.3 104 2 101 DC, 2.3 20 5 (Ar only)
1.3 104 2 101 DC, 2.5 17 5 (Ar only)
Fig. 3. SEM micrographs of (a) ITO, (b) ITO/Au/ITO, and (c) ITO/Cu/ITO films.
Fig. 2. XRD pattern of as deposited ITO and ITO/metal/ITO films.
It maps surface topography by dragging a sharp probe across a film surface. The structural characterization was done using X-ray diffraction (XRD) measurements with Cu Ka radiation (RAD-3C, Rigaku). The surface morphology was analyzed using a scanning electron microscope (SEM, JSM-820, JEOL) and surface average roughness measurements were performed on 2 2 lm2 areas with tapping mode atomic force microscope (AFM, Dimension 5000, Veeco) under ambient conditions. The average surface roughness of the bare PC substrates was 1.3 nm. Since the film thickness was kept constant at 100 nm, the variation in surface morphology due to thickness was ignored. After deposition, ITO/Au/ITO films were annealed in a vacuum of 1 102 Pa for 20 min at 150 °C to consider the influence of annealing temperature on the optoelectrical properties of IMI films. The electrical resistivity was obtained using the Van der Pauw method (HMS-3000, Ecopia) and optical transmittance was measured in the range of 300–800 nm by the UV– vis. spectrophotometer at the Korea Basic Science Institute (KBSI).
J.Y. Lee et al. / Optics Communications 282 (2009) 2362–2366
The data presented in this work contain the transmission of the multilayers, including the PC substrate. In order to consider the sheet resistance (Rs) stability in a high humidity environment (air at a relative humidity of 90% for 600 h and a temperature of 60 °C), the normalized Rs of the films which have the highest figure of merit (uTC) in this study were measured with four point probe method. The grain size (D) of In2O3 films was calculated using Scherrer’s formula [12]
D ¼ 0:9k=b cos h where k = 1.54 Å and b is sq. root (B2 b2), where B is the observed full width half maximum (FWHM) and b is the resolution of the spectrometer (here 0.01).
100
80
Optical transmittacne (%)
2364
60
40 PC substrae ITO single layer films ITO / Au / ITO films ITO / Cu / ITO films
20
0 400
500
600
700
800
Wavelength (nm) Fig. 5. Optical transmittance of as deposited ITO and ITO/metal/ITO films.
3. Results To confirm the influence of the intermediated noble Au and Cu layer in ITO crystallization, XRD analysis was performed. Fig. 2 shows the XRD pattern of the as deposited ITO and ITO/metal/ ITO films. Only ITO/Au/ITO films were oriented along In2O3 (4 0 0), while ITO films was amorphous phase. To this end, the Au intermediate layer of the IMI structure effectively promoted thin film crystallization of the ITO films. In order to know the crystallinity of ITO/Au/ITO films, grain size of the In2O3 (4 0 0) from XRD pattern was evaluated was about 12 nm. Fig. 3 shows the SEM micrographs of the films. The as deposited ITO and ITO/Cu/ITO films are amorphous in the XRD pattern. In the SEM images, there is no significant difference between the ITO and ITO/Cu/ITO films. But ITO/Au/ITO films showed some granular growth on the film surface. This is consisted results with XRD measurements. Fig. 4 shows AFM images of the as deposited ITO single layer films and ITO/Au/ITO multilayer films which show some spherical grain growth on the surface. The as deposited ITO film showed an average surface roughness of 2.5 nm, while the ITO/Cu/ITO film was 0.98 nm and ITO/Au/ITO films was 0.51 nm. Ghosh [13] reported that ITO films deposited on quartz substrate are smoother than the films deposited on amorphous glass substrate and Purica [14] and Diplas et al. [15] also observed that ITO films deposited on glass substrate showed increased roughness compared to the film deposited on crystallized Si (1 0 0) substrate. From the previous reports [13–15], it may concluded that the crystallized Au interlayer in ITO/Au/ITO multilayer films results in the
Table 2 Comparison of carrier density (1019/cm3), mobility (cm2/Vs), resistivity (104 X cm), optical transmittance contained PC substrate (T, (%)) at 550 nm, and figure of merit (uTC, 103 X1) of ITO and IMI films.
Fig. 4. AFM images of ITO/metal/ITO multilayered films (X-axis: 0.5 lm/Div., Yaxis: 10.0 nm/Div.). (a) ITO 50 nm/Au 10 nm/ITO 40 nm, (b) ITO 50 nm/Cu 10 nm/ ITO 40 nm.
Carrier density Mobility Resistivity T (%) uTC
ITO
ITO/Au/ITO
ITO/Cu/ITO
2.3 85 31.2 81 0.38
220 47 0.56 72 6.6
54 39 1.51 54 0.14
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ity than that of the IMI films, suggesting that the two interfaces between the ITO and intermediated metal films may act as a defect to carrier movement. Although ITO films had a higher mobility than IMI films, they had a lower resistivity due to the low carrier density. ITO films had a resistivity of 4 103 X cm, and ITO/Cu/ITO films had a resistivity as low as 1.5 104 X cm. The lowest resistivity of 5.6 105 X cm was obtained from the ITO/Au/ITO films. Fig. 6 shows the normalized Rs of ITO/Au/ITO films which have the higher uTC than the other films as a function of exposure time in the humidity test. Since the normalized Rs measured in the same humidity condition was kept constantly, it may conclude that ITO/ Au/ITO films are relatively stable with moisture condition within measured exposure time of 600 h. The figure of merit (uTC) is an important index for evaluating the performance of transparent conducting oxide (TCO) films [16]. The uTC is defined by
Normalized sheet resistance (R/R I)
3.0
2.5
2.0
1.5
1.0
/TC ¼ T 10 =RS
0.5
0.0 0
100
200
300
400
500
600
700
Exposure time (Hour) Fig. 6. Normalized sheet resistance as a function of exposure time at humidity condition.
more flat surface morphology than that of the as deposited ITO single layer films. Fig. 5 shows the optical transmittance of a wavelength range of 300–800 nm. The bare PC substrates and ITO films deposited on the PC substrate have 90% and 81% optical transmittance at 550 nm, respectively, which is consistent with previously reported conventional magnetron sputtering results without substrate heating [9]. Multilayered ITO/Au/ITO and ITO/Cu/ITO films showed an optical transmission of 71% and 54% at 550 nm, respectively. Table 2 shows the electrical properties of the ITO and IMI multilayer films. In the case of a conventional ITO film, the carrier concentration was mainly determined by oxygen vacancies or the concentration of substituted Sn4+ on In3+ sites [4]. However, in this study, all process parameters, such as the O2/Ar flow rate, the Sn concentration in the sputtering target, and the substrate temperature, which all may affect carrier concentration, were kept constant. Thus, the high carrier concentration in the IMI films compared with that in the ITO films was attributed to the high carrier concentration in the metal layer on the IMI films. Although intermediated Au and Cu films have the same thickness of 10 nm in IMI films, ITO/Au/ITO films have a higher carrier density than that of the ITO/Cu/ITO films. It is well known that a larger grain size (small grain boundary density) increases the carrier density because grain boundaries behave as traps for free carriers in the film. Thus, the lowest resistivity obtained in the ITO/Au/ITO film was accompanied by an increase in the carrier density of the ITO/Au/ ITO films, which resulted from the crystallization of the films as shown in XRD spectra. However, the ITO films had a higher mobil-
where T is the optical transmittance (at 550 nm in this study), and Rs is a sheet resistance. The uTC for IMI multilayer and ITO films are compared in Table 2. The uTC reached a maximum of 5.8 103 X1 for ITO/Au/ITO films, which was higher than the uTC of the ITO films. The ITO/Cu/ITO films had the lowest uTC of 0.14 103 X1. A higher uTC resulted in better quality TCO films [17] suggesting that the ITO/Au/ITO films had better optoelectrical properties than those of the ITO/Cu/ITO films and ITO films. Table 3 shows the comparison of figure of merit and optoelectrical properties of Ag and Au intermediated IMI films which have similar thickness. Although ITO/ Au/ITO films show the lower optical transmittance of 72% in a wavelength of 550 nm than that of the ITO/Ag/ITO films [18], they have the high figure of merit due to the low sheet resistance of the films. After post deposition annealing, the uTC of ITO/Au/ITO films increased to 12.7 103 X1 because of improved transmittance as high as 75% which is comparable with ITO/Ag/ITO films [18]. 4. Conclusion ITO single layer and ITO/metal/ITO (IMI) multilayer films, which had Au and Cu intermediated films were prepared on a PC substrate using an RF and DC magnetron sputtering process, respectively. In IMI films, only ITO/Au/ITO films were aligned (4 0 0) which corresponding to the indium oxide bixbyite structure. To this end, the Au interlayer of the IMI structure effectively promoted thin film crystallization of the ITO films. In electrical resistivity measurements, the ITO films had a resistivity of 4 103 X cm and ITO/Cu/ITO films had a resistivity as low as 1.5 104 X cm. The lowest resistivity of 5.6 105 X cm was obtained from the ITO/Au/ITO films. Also ITO/Au/ITO films show the stable normalized Rs with moisture condition (90%, 60 °C) within measured exposure time of 600 h. In this study, as deposited ITO/Au/ITO films had a figure of merit (uTC) of 6.6 103 X1 and after post deposition annealing, the uTC of the films increased as high as 12.7 103 X1.
Table 3 The comparison of figure of merit (uTC) and optoelectrical property of the IMI films. Optical transmittance at 550 nm (T, %) contained PC substrate. Both ITO/Cu/ITO and ITO/Au/ ITO films have same composition of 50/10/40 nm. Post deposition vacuum annealed ITO/Au/ITO films also compared with Ag and Au intermediated IMI films, respectively. Structure(Thickness)
T (%)
Resistance (X/h)
uTC
Reference
ITO/Ag/ITO (50/10/50 nm) ITO/Cu/ITO ITO/Au/ITO ITO/Au/ITO (Annealed at 150 °C)
79
23
4.3
[18]
54 72 75
15.1 5.6 4.4
0.14 6.6 12.7
This study This study This study
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Acknowledgement This work was financed from the budget sources for science research by the Korea Sanhak Foundation in the years 2007–2008. References [1] [2] [3] [4] [5] [6]
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