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Rapid thermal annealing of ITO films
Applied Surface Science 257 (2011) 7061–7064
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Rapid thermal annealing of ITO films Shumei Song a , Tianlin Yang a,∗ , Jingjing Liu a , Yanqing Xin a , Yanhui Li a , Shenghao Han a,b a b
School of Space Science and Physics, Shandong University at Weihai, Weihai 264209, Shandong, PR China School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, Shandong, PR China
a r t i c l e
i n f o
Article history: Received 27 December 2010 Received in revised form 1 March 2011 Accepted 1 March 2011 Available online 8 March 2011 PACS: 73.21.Ac, 73.40.Vz
a b s t r a c t Tin-doped indium oxide (ITO) films with 200 nm thickness were deposited on glass substrates by DC magnetron sputtering at room temperature. And they were annealed by rapid thermal annealing (RTA) method in vacuum ambient at different temperature for 60 s. The effect of annealing temperature on the structural, electrical and optical properties of ITO films was investigated. As the RTA temperature increases, the resistivity of ITO films decreases dramatically, and the transmittance in the visible region increases obviously. The ITO film annealed at 600 ◦ C by RTA in vacuum shows a resistivity of 1.6 × 10−4 cm and a transmittance of 92%. © 2011 Elsevier B.V. All rights reserved.
Keywords: ITO film Rapid thermal annealing Resistivity Transmittance
1. Introduction Tin-doped indium oxide (ITO) is a highly degenerate n-type semiconductor. ITO film possesses good electrical conductivity, high optical transparency in the visible region, wide optical band gap and high IR reflectivity properties, which makes it has wide applications in plat panel displays [1], solar cells [2] and organic light emitting devices [3,4] as the transparent electrodes. ITO films have been produced by a variety of deposition techniques, such as magnetron sputtering [5,6], electron beam evaporation [7], chemical vapor deposition [8], pulsed laser deposition [9] and reactive thermal evaporation [10]. Among the deposition techniques, magnetron sputtering is the most widely used method of preparing conductive and transparent thin films due to good film adhesion, large area preparation, good reproducibility and the simplicity of the growth process required. In addition, magnetron sputtering deposition can further provide high deposition rate at room temperature [11]. The ITO films deposited at room temperature are generally amorphous for thickness below 200 nm, while thicker films may be fully or partially amorphous. The conduction mechanism of ITO arises from doping of tin atom and oxygen vacancy. Oxygen vacancies dominate the conduction mechanism of ITO film by contributing two free electrons,
∗ Corresponding author. Tel.: +86 631 5688921; fax: +86 631 5688710. E-mail address: [email protected] (T. Yang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.009
while doping Sn+ ion can provide one free electron [4]. But in the amorphous state, the carrier sources are less clear, which has been verified that Sn is not efficiently activated, and carriers are contributed primarily by vacancy-like oxygen defects [12]. Post-deposition annealing is an effective and widely used way to promote crystallinity and properties of ITO films [12,13]. High temperature annealing is traditionally done in tube furnace. Conventional furnace annealing (CFA) uses resistance furnace to heat, which leads to some shortcomings such as long pre-heating and annealing time, difficulty in precisely controlling temperature and the problem of oxygen adsorption easily caused. However, rapid thermal annealing (RTA) based on tungsten halide lamp possesses some features different from CFA: high rate of heating up (50 K/s) and cooling down, very short annealing time (second for RTA vs hours for CFA) and reduction in the interference between the film and substrate, which makes RTA can reduce dramatically the oxygen adsorption on the sample surface and be propitious to industrial production. The outstanding merit of RTA is in the simplicity of the process, which makes RTA a very popular processing method in recent years [14,15]. Up to now, the investigation is abundant with reports on the effect of annealing on the optical, electrical and structural properties of ITO films. In the present work, ITO films were prepared on glass substrates by DC magnetron sputtering at room temperature, and the films were treated by RTA in vacuum. The effect of annealing temperature on the structural, optical and electrical properties of ITO films was investigated in detail.
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Fig. 1. The temperature profile during 60 s, 600 ◦ C process in the rapid thermal annealing system.
2. Experimental Details ITO films were prepared on glass substrates by DC magnetron sputtering using an ITO target (purity: 99.99%, diameter: 7.62 cm, thickness: 0.5 cm, In2 O3 : SnO2 = 90: 10 wt.%) at room temperature. Prior to deposition, the substrates were ultrasonically cleaned and degreased in acetone. The sputtering chambers were initially pumped down to 1.5 × 10−4 Pa, and then pure argon gas was introduced through mass flow controllers to achieve the required pressure. During the sputtering, the deposition pressure was maintained at 0.6 Pa, and the DC power was kept at 70 W. The distance between substrate and target was 4.5 cm. The film thickness was 200 nm on average for all the samples. After the deposition, the ITO films were annealed in vacuum (0.1 Pa) from 300 to 600 ◦ C for 60 s using the RTA system. An example of wafer temperature profile in annealing process was shown in Fig. 1. The thickness of films was measured using a surface profiler (Ambios Technology Company, USA). The structural property was analyzed by the X-ray diffraction (XRD) using a Rigaku D/MAX 200 V/PC diffractometer with a Cu-K␣1 radiation source. Surface roughness and surface images were taken by atomic force microscope (AFM, Digital Instruments Inc., Nanoscope Multimode and NanoScope Dimension 3100, USA). The sheet resistance (RS ) was determined by four-point probe measurements. The optical transmittance measurements were performed with a Cary100 UV–vis spectrophotometer (Varian Company) in the range of 300–800 nm.
Fig. 2. XRD patterns of ITO films before and after RTA at different temperature.
conversion from an amorphous structure to a polycrystalline with cubic bixbyite In2 O3 structure. The ITO film favors (2 1 1), (4 0 0), (4 4 0) and (6 2 2) orientations after annealed at 300 ◦ C. When the RTA temperature increases to 400 ◦ C and higher temperature, the (2 2 2) and (4 3 1) peaks are detected, and the intensity of the (2 1 1), (4 0 0), (4 4 0) and (6 2 2) peaks increases obviously. None of the spectra indicate any characteristic peaks of Sn, SnO, and SnO2 , which means that the Sn atoms are probably doped substitutionally into the In2 O3 lattice [19,20]. The FWHM and crystallite size for (4 4 0) orientations of the ITO films annealed at different RTA temperature are shown in Fig. 3. It can be seen that the FWHM decreases monotonously and the crystallite size increases continuously with the increase of RTA temperature, which implies that the crystallinity of ITO films is improved obviously. From these results, it can be concluded that RTA is beneficial to improve the crystallinity of the ITO films. Fig. 4 exhibits the AFM images (1 m × 1 m) of ITO films before and after RTA at different temperature. As shown in Fig. 4, all the films show smooth surface morphology. The root mean square roughness (Rrms ) of the ITO film after RTA is larger than that of the as-deposited film, which is probably due to the larger crystal-
3. Results and discussion Fig. 2 shows the XRD spectra of ITO films annealed by RTA method at different temperature. A broad peak due to the amorphous nature of the glass substrate can be found between 15◦ and 35◦ . It can be seen that the XRD patterns of as-deposited ITO film does not show any peak, suggesting an amorphous film. Song et al. [16] and Minami et al. [17] both have reported that the crystallization temperature for ITO film is 150 ◦ C. The average energy of the adatoms is considered to be determined by the kinetic energy of the sputtered atoms just before arriving at the substrate and substrate temperature [18]. In our case, the as-deposited film is prepared at room temperature, which limits the adatom mobility, and the film is amorphous. After RTA in vacuum, the films show obvious
Fig. 3. Crystallite size and FWHM for (4 4 0) orientation of the ITO film as a function of RTA temperature.
S. Song et al. / Applied Surface Science 257 (2011) 7061–7064
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Fig. 4. The AFM images of ITO films before and after RTA at different temperature (a) as-deposited film (b) after RTA at 400 ◦ C (c) after RTA at 600 ◦ C.
lite size after RTA [21]. With the increase of RTA temperature, the Rrms nearly maintains constant. The dependence of the electrical properties (resistivity, Hall mobility and carrier concentration) of the ITO films before and after RTA at different temperature is depicted in Fig. 5. The resistivity of the ITO film after RTA at 300 ◦ C has a remarkable decrease comparing with that of the as-deposited film. As the RTA temperature increases from 300 to 400 ◦ C, the resistivity varies little with the simultaneous invariability of Hall mobility and carrier concentration. As the RTA temperature increases further to 600 ◦ C, there is another large reduction in resistivity due to both the increase of Hall mobility and carrier concentration. The lowest resistivity of 1.6 × 10−4 cm is obtained at 600 ◦ C RTA temperature, which has a remarkable 67% decrease comparing with that of the as-deposited film. From the XRD results discussed above, the crystallite size of ITO films after RTA is larger than that of the as-deposited film. Larger crystallite size results in lower density of grain boundaries,
which behave as traps for free carriers and barriers for carrier transport [22]. Therefore, increasing the crystallite size can decrease the boundary scattering and increase the carrier lifetime, which consequently leads to an increase of conductivity due to the increase of carrier concentration and Hall mobility [23]. With the improvement of crystallinity, the concentration of electrically active donor sites is improved [24], which can also increase the carrier concentration. The optical transmittance spectra of ITO films before and after RTA at different temperature are presented in Fig. 6. The average transmittance in the visible region (from 400 to 700 nm) of ITO film before RTA is 83.9%, while after RTA at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C, the transmittance is 84.1%, 90.3%, 92.3% and 92%, respectively. The transmittance is obviously increased when the RTA temperature increases from 300 to 600 ◦ C. As discussed above, the crystallization of the ITO films strongly depends on the RTA
Fig. 5. Dependence of the electrical properties (resistivity, Hall mobility and carrier concentration) of ITO films before and after RTA at different temperature.
Fig. 6. Transmittance spectra for the ITO films before and after RTA at different temperature.
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as-deposited ITO film is amorphous structure. With the increase of RTA temperature, the films show a polycrystalline cubic bixbyite In2 O3 polycrystalline structure. As the RTA temperature increases, the resistivity of ITO films decreases dramatically. The lowest resistivity of 1.6 × 10−4 cm is obtained at the RTA temperature of 600 ◦ C, which is due to the higher product of carrier concentration and Hall mobility. The transmittance in the visible region of ITO films increases with the increase of RTA temperature. High transmittance (above 90%) is exhibited by the films annealed at 400 ◦ C and above. These results indicate the promise of the present approach for achieving high quality ITO films by RTA treatment, which will widen the application of the ITO films. Acknowledgements This work was supported by the Natural Science Foundation of China (grant no.10974118 and grant no. 10778701) and Natural Science Foundation of Shandong Province (grant no.Y2008A37 and grant no. ZR2009ZM020). Fig. 7. The dependence of the absorption coefficient on the photon energy for the ITO films before and after RTA.
temperature. The increase in optical transmittance after RTA can be attributed to the improvement of crystallinity [25]. From the XRD data shown in Figs. 2 and 3, the crystallinity of ITO films is improved and the crystallite size is increased after RTA, which can decrease the scattering of incoming light and increase the transmittance. The optical absorption coefficient ˛ is determined by the band edge transmittance and film thickness by using the following relation: T = A1 exp(˛d)
(1)
where T and d are the transmittance and thickness of the film, respectively, and A1 is the constant. The absorption coefficient data are used to determine the energy band gap Eg . Since ITO belongs to a direct band gap semiconductor, the band gap can be determined by the following formula: 2
(hv˛) = A(hv − Eg )
(2)
where A is the constant, and h is the photon energy. The plot of (˛h)2 as a function of the photon energy h is shown in Fig. 7. The optical energy band gap Eg could be obtained through extrapolating the linear part of the curve to ˛h = 0. The optical energy band gap from Fig. 7 for the as-deposited ITO film is 3.635 eV, and the band gap for the film after RTA at 600 ◦ C is 4.045 eV. Difference of carrier concentration between the as-deposited and RTA films causes the band gap shift. The band gap widening with the increase of carrier concentration can be explained by Burstein–Moss effect [26]. 4. Conclusions Tin doped indium oxide thin films have been prepared on glass substrates by DC magnetron sputtering at room temperature. The
References [1] U. Betza, M.K. Olsson, J. Marthy, F. Atamny, Surf. Coat. Technol. 200 (2006) 5751–5759. [2] V.A. Dao, H. Choi, J. Heo, H. Park, K. Yoon, Y. Lee, Y. Kim, N. Lakshminarayan, J. Yi, Curr. Appl. Phys. 10 (2010) S506–S509. [3] L.Y. Yang, X.Z. Chen, H. Xu, D.Q. Ye, H. Tian, S.G. Yin, Appl. Surf. Sci. 254 (2008) 5055–5060. [4] H. Han, D. Adams, J.W. Mayer, T.L. Alford, J. Appl. Phys. 98 (2005) 1–8, 083705. [5] K.J. Kumar, N.R.C. Raju, A. Subrahmanyam, Appl. Surf. Sci. 257 (2011) 3075–3080. [6] C.H. Yang, S.C. Lee, T.C. Lin, S.C. Chen, Thin Solid Films 516 (2008) 1984–1991. [7] N. Wan, T. Wang, H.C. Sun, G. Chen, L. Geng, X.H. Gan, S.H. Guo, J. Xu, L. Xu, K.J. Chen, J. Non-Cryst. Solids 356 (2010) 911–916. [8] T. Kondo, Y. Sawada, H. Funakubo, K. Akiyama, T. Kiguchi, M.H. Wang, T. Uchida, Electrochem. Solid-State Lett. 12 (2009) D42–D44. [9] K.A. Sierros, D.R. Cairns, J.S. Abell, S.N. Kukureka, Thin Solid Films 518 (2010) 2623–2627. [10] 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) 755–757. [11] H.L. Hartnagel, A.L. Dawar, A.K. Jain, C. Jagadsik, Semiconducting Transparent Thin Films, Institute of Physics, Philadelphia, 1995, p. 91. [12] H. Morikawa, M. Fujita, Thin Solid Films 359 (2000) 61–67. [13] Y.L. Hu, X.G. Diao, C. Wang, W.C. Hao, T.M. Wang, Vacuum 75 (2004) 183–188. [14] R. Singh, M. Fakhruddin, K.F. Poole, Appl. Surf. Sci. 168 (2000) 198–203. [15] T.Y. Ma, D.K. Shim, Thin Solid Films 410 (2002) 8–13. [16] P.K. Song, Y. Shigesato, I. Yasui, C.W. Ow-Yang, D.C. Paine, Jpn. J. Appl. Phys. 37 (1998) 1870–1896. [17] T. Minami, H. Sonohara, T. Kakumu, S. Takata, Thin Solid Films 270 (1995) 37–42. [18] R. Balasundaraprabhu, E.V. Monakhov, N. Muthukumarasamy, O. Nilsen, B.G. Svensson, Mater. Chem. Phys. 114 (2009) 425–429. [19] H.R. Fallah, M. Ghasemi, A. Hassanzadeh, H. Steki, Mater. Res. Bull. 42 (2007) 487–496. [20] J. George, C.S. Menon, Surf. Coat. Technol. 132 (2000) 45–48. [21] D.F. Lii, J.L. Huang, I.J. Jen, S.S. Lin, P. Sajgalik, Surf. Coat. Technol. 192 (2005) 106–111. [22] C.Y. Hsu, T.F. Ko, Y.M. Huang, J. Eur. Ceram. Soc. 28 (2008) 3065–3070. [23] X.T. Tao, J. Ma, D.H. Zhang, T.L. Yang, H.L. Ma, Y.G. Yang, C.F. Cheng, J. Huang, Appl. Surf. Sci. 183 (2001) 137–142. [24] Y.S. Jung, J.Y. Seo, D.W. Lee, D.Y. Jeon, Thin Solid Films 445 (2003) 63–71. [25] Y. Djaoued, V.H. Phong, S. Badilescu, P.V. Ashrit, F.E. Girouard, V.-V. Truong, Thin Solid Films 293 (1997) 108–112. [26] H. Han, J.W. Mayer, T.L. Alford, J. Appl. Phys. 99 (2006) 1–7, 123711.
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Rapid thermal annealing of ITO films Shumei Song a , Tianlin Yang a,∗ , Jingjing Liu a , Yanqing Xin a , Yanhui Li a , Shenghao Han a,b a b
School of Space Science and Physics, Shandong University at Weihai, Weihai 264209, Shandong, PR China School of Physics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, Shandong, PR China
a r t i c l e
i n f o
Article history: Received 27 December 2010 Received in revised form 1 March 2011 Accepted 1 March 2011 Available online 8 March 2011 PACS: 73.21.Ac, 73.40.Vz
a b s t r a c t Tin-doped indium oxide (ITO) films with 200 nm thickness were deposited on glass substrates by DC magnetron sputtering at room temperature. And they were annealed by rapid thermal annealing (RTA) method in vacuum ambient at different temperature for 60 s. The effect of annealing temperature on the structural, electrical and optical properties of ITO films was investigated. As the RTA temperature increases, the resistivity of ITO films decreases dramatically, and the transmittance in the visible region increases obviously. The ITO film annealed at 600 ◦ C by RTA in vacuum shows a resistivity of 1.6 × 10−4 cm and a transmittance of 92%. © 2011 Elsevier B.V. All rights reserved.
Keywords: ITO film Rapid thermal annealing Resistivity Transmittance
1. Introduction Tin-doped indium oxide (ITO) is a highly degenerate n-type semiconductor. ITO film possesses good electrical conductivity, high optical transparency in the visible region, wide optical band gap and high IR reflectivity properties, which makes it has wide applications in plat panel displays [1], solar cells [2] and organic light emitting devices [3,4] as the transparent electrodes. ITO films have been produced by a variety of deposition techniques, such as magnetron sputtering [5,6], electron beam evaporation [7], chemical vapor deposition [8], pulsed laser deposition [9] and reactive thermal evaporation [10]. Among the deposition techniques, magnetron sputtering is the most widely used method of preparing conductive and transparent thin films due to good film adhesion, large area preparation, good reproducibility and the simplicity of the growth process required. In addition, magnetron sputtering deposition can further provide high deposition rate at room temperature [11]. The ITO films deposited at room temperature are generally amorphous for thickness below 200 nm, while thicker films may be fully or partially amorphous. The conduction mechanism of ITO arises from doping of tin atom and oxygen vacancy. Oxygen vacancies dominate the conduction mechanism of ITO film by contributing two free electrons,
∗ Corresponding author. Tel.: +86 631 5688921; fax: +86 631 5688710. E-mail address: [email protected] (T. Yang). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.03.009
while doping Sn+ ion can provide one free electron [4]. But in the amorphous state, the carrier sources are less clear, which has been verified that Sn is not efficiently activated, and carriers are contributed primarily by vacancy-like oxygen defects [12]. Post-deposition annealing is an effective and widely used way to promote crystallinity and properties of ITO films [12,13]. High temperature annealing is traditionally done in tube furnace. Conventional furnace annealing (CFA) uses resistance furnace to heat, which leads to some shortcomings such as long pre-heating and annealing time, difficulty in precisely controlling temperature and the problem of oxygen adsorption easily caused. However, rapid thermal annealing (RTA) based on tungsten halide lamp possesses some features different from CFA: high rate of heating up (50 K/s) and cooling down, very short annealing time (second for RTA vs hours for CFA) and reduction in the interference between the film and substrate, which makes RTA can reduce dramatically the oxygen adsorption on the sample surface and be propitious to industrial production. The outstanding merit of RTA is in the simplicity of the process, which makes RTA a very popular processing method in recent years [14,15]. Up to now, the investigation is abundant with reports on the effect of annealing on the optical, electrical and structural properties of ITO films. In the present work, ITO films were prepared on glass substrates by DC magnetron sputtering at room temperature, and the films were treated by RTA in vacuum. The effect of annealing temperature on the structural, optical and electrical properties of ITO films was investigated in detail.
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Fig. 1. The temperature profile during 60 s, 600 ◦ C process in the rapid thermal annealing system.
2. Experimental Details ITO films were prepared on glass substrates by DC magnetron sputtering using an ITO target (purity: 99.99%, diameter: 7.62 cm, thickness: 0.5 cm, In2 O3 : SnO2 = 90: 10 wt.%) at room temperature. Prior to deposition, the substrates were ultrasonically cleaned and degreased in acetone. The sputtering chambers were initially pumped down to 1.5 × 10−4 Pa, and then pure argon gas was introduced through mass flow controllers to achieve the required pressure. During the sputtering, the deposition pressure was maintained at 0.6 Pa, and the DC power was kept at 70 W. The distance between substrate and target was 4.5 cm. The film thickness was 200 nm on average for all the samples. After the deposition, the ITO films were annealed in vacuum (0.1 Pa) from 300 to 600 ◦ C for 60 s using the RTA system. An example of wafer temperature profile in annealing process was shown in Fig. 1. The thickness of films was measured using a surface profiler (Ambios Technology Company, USA). The structural property was analyzed by the X-ray diffraction (XRD) using a Rigaku D/MAX 200 V/PC diffractometer with a Cu-K␣1 radiation source. Surface roughness and surface images were taken by atomic force microscope (AFM, Digital Instruments Inc., Nanoscope Multimode and NanoScope Dimension 3100, USA). The sheet resistance (RS ) was determined by four-point probe measurements. The optical transmittance measurements were performed with a Cary100 UV–vis spectrophotometer (Varian Company) in the range of 300–800 nm.
Fig. 2. XRD patterns of ITO films before and after RTA at different temperature.
conversion from an amorphous structure to a polycrystalline with cubic bixbyite In2 O3 structure. The ITO film favors (2 1 1), (4 0 0), (4 4 0) and (6 2 2) orientations after annealed at 300 ◦ C. When the RTA temperature increases to 400 ◦ C and higher temperature, the (2 2 2) and (4 3 1) peaks are detected, and the intensity of the (2 1 1), (4 0 0), (4 4 0) and (6 2 2) peaks increases obviously. None of the spectra indicate any characteristic peaks of Sn, SnO, and SnO2 , which means that the Sn atoms are probably doped substitutionally into the In2 O3 lattice [19,20]. The FWHM and crystallite size for (4 4 0) orientations of the ITO films annealed at different RTA temperature are shown in Fig. 3. It can be seen that the FWHM decreases monotonously and the crystallite size increases continuously with the increase of RTA temperature, which implies that the crystallinity of ITO films is improved obviously. From these results, it can be concluded that RTA is beneficial to improve the crystallinity of the ITO films. Fig. 4 exhibits the AFM images (1 m × 1 m) of ITO films before and after RTA at different temperature. As shown in Fig. 4, all the films show smooth surface morphology. The root mean square roughness (Rrms ) of the ITO film after RTA is larger than that of the as-deposited film, which is probably due to the larger crystal-
3. Results and discussion Fig. 2 shows the XRD spectra of ITO films annealed by RTA method at different temperature. A broad peak due to the amorphous nature of the glass substrate can be found between 15◦ and 35◦ . It can be seen that the XRD patterns of as-deposited ITO film does not show any peak, suggesting an amorphous film. Song et al. [16] and Minami et al. [17] both have reported that the crystallization temperature for ITO film is 150 ◦ C. The average energy of the adatoms is considered to be determined by the kinetic energy of the sputtered atoms just before arriving at the substrate and substrate temperature [18]. In our case, the as-deposited film is prepared at room temperature, which limits the adatom mobility, and the film is amorphous. After RTA in vacuum, the films show obvious
Fig. 3. Crystallite size and FWHM for (4 4 0) orientation of the ITO film as a function of RTA temperature.
S. Song et al. / Applied Surface Science 257 (2011) 7061–7064
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Fig. 4. The AFM images of ITO films before and after RTA at different temperature (a) as-deposited film (b) after RTA at 400 ◦ C (c) after RTA at 600 ◦ C.
lite size after RTA [21]. With the increase of RTA temperature, the Rrms nearly maintains constant. The dependence of the electrical properties (resistivity, Hall mobility and carrier concentration) of the ITO films before and after RTA at different temperature is depicted in Fig. 5. The resistivity of the ITO film after RTA at 300 ◦ C has a remarkable decrease comparing with that of the as-deposited film. As the RTA temperature increases from 300 to 400 ◦ C, the resistivity varies little with the simultaneous invariability of Hall mobility and carrier concentration. As the RTA temperature increases further to 600 ◦ C, there is another large reduction in resistivity due to both the increase of Hall mobility and carrier concentration. The lowest resistivity of 1.6 × 10−4 cm is obtained at 600 ◦ C RTA temperature, which has a remarkable 67% decrease comparing with that of the as-deposited film. From the XRD results discussed above, the crystallite size of ITO films after RTA is larger than that of the as-deposited film. Larger crystallite size results in lower density of grain boundaries,
which behave as traps for free carriers and barriers for carrier transport [22]. Therefore, increasing the crystallite size can decrease the boundary scattering and increase the carrier lifetime, which consequently leads to an increase of conductivity due to the increase of carrier concentration and Hall mobility [23]. With the improvement of crystallinity, the concentration of electrically active donor sites is improved [24], which can also increase the carrier concentration. The optical transmittance spectra of ITO films before and after RTA at different temperature are presented in Fig. 6. The average transmittance in the visible region (from 400 to 700 nm) of ITO film before RTA is 83.9%, while after RTA at 300 ◦ C, 400 ◦ C, 500 ◦ C and 600 ◦ C, the transmittance is 84.1%, 90.3%, 92.3% and 92%, respectively. The transmittance is obviously increased when the RTA temperature increases from 300 to 600 ◦ C. As discussed above, the crystallization of the ITO films strongly depends on the RTA
Fig. 5. Dependence of the electrical properties (resistivity, Hall mobility and carrier concentration) of ITO films before and after RTA at different temperature.
Fig. 6. Transmittance spectra for the ITO films before and after RTA at different temperature.
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as-deposited ITO film is amorphous structure. With the increase of RTA temperature, the films show a polycrystalline cubic bixbyite In2 O3 polycrystalline structure. As the RTA temperature increases, the resistivity of ITO films decreases dramatically. The lowest resistivity of 1.6 × 10−4 cm is obtained at the RTA temperature of 600 ◦ C, which is due to the higher product of carrier concentration and Hall mobility. The transmittance in the visible region of ITO films increases with the increase of RTA temperature. High transmittance (above 90%) is exhibited by the films annealed at 400 ◦ C and above. These results indicate the promise of the present approach for achieving high quality ITO films by RTA treatment, which will widen the application of the ITO films. Acknowledgements This work was supported by the Natural Science Foundation of China (grant no.10974118 and grant no. 10778701) and Natural Science Foundation of Shandong Province (grant no.Y2008A37 and grant no. ZR2009ZM020). Fig. 7. The dependence of the absorption coefficient on the photon energy for the ITO films before and after RTA.
temperature. The increase in optical transmittance after RTA can be attributed to the improvement of crystallinity [25]. From the XRD data shown in Figs. 2 and 3, the crystallinity of ITO films is improved and the crystallite size is increased after RTA, which can decrease the scattering of incoming light and increase the transmittance. The optical absorption coefficient ˛ is determined by the band edge transmittance and film thickness by using the following relation: T = A1 exp(˛d)
(1)
where T and d are the transmittance and thickness of the film, respectively, and A1 is the constant. The absorption coefficient data are used to determine the energy band gap Eg . Since ITO belongs to a direct band gap semiconductor, the band gap can be determined by the following formula: 2
(hv˛) = A(hv − Eg )
(2)
where A is the constant, and h is the photon energy. The plot of (˛h)2 as a function of the photon energy h is shown in Fig. 7. The optical energy band gap Eg could be obtained through extrapolating the linear part of the curve to ˛h = 0. The optical energy band gap from Fig. 7 for the as-deposited ITO film is 3.635 eV, and the band gap for the film after RTA at 600 ◦ C is 4.045 eV. Difference of carrier concentration between the as-deposited and RTA films causes the band gap shift. The band gap widening with the increase of carrier concentration can be explained by Burstein–Moss effect [26]. 4. Conclusions Tin doped indium oxide thin films have been prepared on glass substrates by DC magnetron sputtering at room temperature. The
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