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Enhancement in electrical and optical properties of indium tin oxide thin films grown using a pulsed laser deposition at room temperature by two-step process
Thin Solid Films 515 (2007) 3580 – 3583 www.elsevier.com/locate/tsf
Enhancement in electrical and optical properties of indium tin oxide thin films grown using a pulsed laser deposition at room temperature by two-step process Jong Hoon Kim ⁎, Byung Du Ahn, Choong Hee Lee, Kyung Ah Jeon, Hong Seong Kang, Gun Hee Kim, Sang Yeol Lee Department of Electrical and Electronic Engineering, Yonsei University, 134, Shinchon-dong, Seodaemoon-ku, Seoul, 120–749, South Korea Received 26 May 2006; received in revised form 2 November 2006; accepted 3 November 2006 Available online 15 December 2006
Abstract The optical and electrical properties of indium tin oxide (ITO) thin films deposited using a pulsed laser deposition at room temperature can be substantially enhanced by adopting a two-step process. X-ray diffraction patterns and atomic force microscopy images were used to observe the structural properties of the films. High quality ITO films grown by two-step process could be obtained with the resistivity of 3.02 × 10− 4 Ω cm, the carrier mobility of 32.07 cm2/Vs, and the transparency above 90% in visible region mainly due to the enhancement of the film crystallinity and the increase of grain size. © 2006 Elsevier B.V. All rights reserved. Keywords: Indium tin oxide; Two-step process; Electrical and optical properties
1. Introduction Indium tin oxide (ITO) is widely used as a transparent electrode in optoelectronic devices because the films have high transmittances in the visible region and low resistivity. It is well known that the low resistivity value of ITO films is due to a high carrier concentration caused by both oxygen vacancies and substitutional tin dopants. Also, the high transparency in the visible and near-IR region is caused by the wide band gap. There are several deposition techniques used to grow ITO thin films including evaporation [1,2], sputtering [3,4], chemical vapor deposition [5], and pulsed laser deposition (PLD) [6–8]. In comparison to other techniques, PLD has several advantages. The composition of the films is quite close to that of the target, and films crystallize at a low substrate temperature due to the high kinetic energies of the atoms and the ionized species in the laser-ablated plasma [7,8]. Recently, the development of a deposition process at a low temperature is very important for the application of film growth on a heat-sensitive substrate [9]. In ⁎ Corresponding author. E-mail address: [email protected] (J.H. Kim). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.006
this paper, we report the enhancement of the electrical, structural, and optical properties of ITO thin films prepared at room temperature by using the two-step process with PLD. 2. Experimental details The amorphous ITO thin films were deposited at room temperature by using PLD with a Nd:YAG laser (355 nm, 5 Hz, and full width at half maximum (FWHM) of 6 nm) with a thickness of 50 nm in the first step. The target used in this study was a sintered ITO pellet containing 5 wt.% SnO2. The ablated material was deposited on 1.25 × 1.25 cm2 glass substrates (microscope cover glass, Marienfeld). The substrates were ultrasonically cleaned in acetone and methanol, rinsed in deionized water, and subsequently dried in a flowing nitrogen gas before deposition. The basal vacuum in the chamber was as low as 1.33 × 10− 4 Pa. The target was rotated at 4 rpm to preclude pit formation and to ensure uniform ablation of the target [10]. The laser energy density on the target was 2.0 J/cm2, and the target–substrate distance was 5 cm. Oxygen gas was injected into the chamber by using a mass flow controller to create a pressure of 2 Pa. After the first deposition, ITO films
J.H. Kim et al. / Thin Solid Films 515 (2007) 3580–3583
Fig. 1. Variation of resistivity (ρ), carrier density (N ), and Hall mobility (μ) as a function of processing conditions for ITO films. All films were grown by PLD on glass with the thickness of 200 nm.
were annealed by rapid thermal annealing (RTA) in nitrogen ambient (1 atm) at 400 °C for 1 min to improve crystallinity. The two-step processed ITO film has been completed by the second deposition on the annealed crystalline ITO film with a thickness of 150 nm and with a laser repetition rate of 1 and 5 Hz respectively. Other growth conditions are the same as the first deposition. The electrical properties were measured by Hall measurements in the van der Pauw configuration at a room temperature. The magnetic field was provided by a permanent magnet of intensity B = 1 T. The structural properties of the films were characterized by using XRD (RIGAKU RINT2000) where a Ni-filtered CuKα (λ = 1.54056 Å) source was used. The surface morphology of the films was observed by AFM (Veeco Dimenson 3100). We have used AFM Contact mode with the constant force method (the force between the sample surface
Fig. 2. X-ray diffraction (θ–2θ) patterns as a function of processing conditions for ITO films deposited at room temperature. All films were grown by PLD on glass with the thickness of 200 nm.
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Fig. 3. Effect of processing conditions on optical transmittance spectra for ITO films with an inset of optical band gap energy. All films were grown by PLD on glass with the thickness of 200 nm.
and the AFM tip (Veeco DNP-20) was kept constant by a feedback system while the surface beneath the tip was scanned). The optical transmission measurements were performed using a UV-near IR grating spectrometer (Cary 5G; 175–3300 nm). 3. Results and discussion To observe the effect of the two-step process, samples were systematically prepared in 4 different conditions. Sample A is as grown ITO film deposited at room temperature. Sample B is the film deposited at room temperature and annealed by RTA in nitrogen ambient (1 atm) at 400 °C, for 1 min. Sample C and D are two-step processed ITO films with the laser repetition rate of 1 and 5 Hz in the second deposition, respectively. All films were grown on glass substrates with a same thickness of 200 nm. Fig. 1 illustrates the variation of resistivity, carrier density, and Hall mobility as a function of processing conditions. It has been observed that the electrical properties are strongly influenced by the processing condition. Relative high resistivity of 1.23 × 10− 3 Ω cm was obtained from as grown sample (Sample A) and it decreased to a lower value of 9.19× 10− 4 and 3.02 × 10− 4 Ω cm from Sample B and C, respectively. This decrease in resistivity may be due to the reduction of severe oxygen related deficiencies which cause the lattice structural disorders and consequently reduce the mobility of carriers, by RTA treatment [11].
Fig. 4. Schematic diagram of the two-step process.
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The resistivity of Sample B is higher than that of Sample C. This may have been caused by the fact that the RTA time of 1 min is enough to crystallize the buffer layers with the thickness of 50 nm, but not enough for entire 200-nm-thick films. In the case of Sample D, high resistivity of 1.30 × 10− 3 Ω cm was observed. It is suggested that the laser repetition rate of 5 Hz is too fast to keep the ordinary arrangement of the film, because of overstacking during the second deposition. This variation in resistivity may be due to different grain sizes [12]. AFM measurement indicates that the grain size of the films changes by varying the processing conditions [13]. The AFM image of Sample A shows a very smooth surface with a root-mean-square (rms) roughness of 0.52 nm. On Sample C, the rms roughness increases to 2.48 nm because of the enhancement of crystallinity. A larger grain size results in lower density of grain boundaries, which behave as traps for free carriers and barriers for carrier transport in the film. Thus, the decrease in resistivity is accompanied by the increase in both the Hall mobility and the carrier density of the films, which result from the increased grain size. Moreover, the two-step process enhances crystallinity of the films (see Fig. 2). In ITO films, Sn atoms in crystal lattices (so called ‘activated Sn atoms’) generate carrier electrons. On the contrary, since Sn atoms in the amorphous ITO grains were not activated, they do not make carriers but neutral scattering centers, such as SnOx clusters [14]. Fig. 2 represented the XRD patterns (Θ–2Θ) of ITO films as a function of processing condition. XRD patterns of as grown ITO film (Sample A) did not show any peak, suggesting an amorphous layer. However, the structure of the ITO film became a crystalline one after RTA or the two-step process. On Sample B and C, the intensities of the major (222) peak of the ITO films increased significantly and its value of FWHM was 0.32° and 0.22°, respectively. It is well agreed with the result of electrical properties as mentioned in Fig. 1. However, the peak intensity was reduced and (222) peak splitted into two peaks in the case of Sample D. Izumi group reported a similar peak split observed in sputtered ITO films and proposed the residual stress model [15]. In our case, the development of the intrinsic stress in the film could be caused by the overstacking during the second deposition with a repetition rate of 5 Hz. The effect of processing condition on the optical properties of ITO films is shown in Fig. 3. It is known that the increase in optical transmittance can be attributed to the improvement of crystallinity and stoichiometry [16]. High average transmittance values in the visible region were exhibited from Sample B and C (89.1% and 91.7%, respectively). This high transmittance may be due to the enhancement of the crystallinity and stoichiometry of the ITO films and it is well agreed with the electrical and structural properties illustrated in Figs. 1 and 2, respectively. On Sample A, the transmittance is relatively low because the crystallization energy is not sufficient to form a stoichiometric film. The inset of Fig. 3 shows the variation of the optical band gap energy as a function of processing condition. The optical band gap could be obtained from the intercept of α2 (α: absorption coefficient) versus hν for direct allowed transitions. The value of the optical band gap is observed to vary from 3.76 (Sample A) to
4.13 eV (Sample C). The change of optical band gap can be explained by Burstein–Moss shift [17]. This increase in the band gap is due to the increase in free electron concentration in the films evidenced in Fig. 1. It is the result of a large increase in the free carrier concentration and the corresponding upward shift of the Fermi level to the top of the band edge. Generally, it is well known that the broadening in absorption edge is mainly due to the increase of disorders in semiconductor films, which lead to the appearance of localized electron and/or hole states [18]. Sample A shows broader absorption edge compared to Sample C, and this result also supports the electrical and structural properties of these films mentioned in Figs. 1 and 2. Fig. 4 shows the schematic diagram of the two-step process. The amorphous ITO thin films were grown at room temperature in the first deposition and annealed by RTA to improve crystallinity. A new ITO film was grown epitaxially on the annealed crystalline ITO film which had a role as a seed layer. By varying the repetition rate, the amount of material deposited per pulse was same for each deposition condition, but the time between pulses was varied [19]. In the case of Sample D, there is not enough time for the material to accommodate in the structure before the next pulse arrives. It seems that this is the origin of the intrinsic stress which causes the degradation of electrical and optical properties of Sample D. As mentioned in Fig. 1, there are two main reasons of carrier generation in ITO films; native oxygen vacancies and substitutional tin dopants. The oxygen vacancies create free electrons in the films because one oxygen vacancy creates two extra electrons in the films. And Sn atoms which substituted In sites in crystal lattices generate carrier electrons. These two factors are in trade-off. Both relatively low resistivity and high electron mobility were consistent with the enhancement of optical transmittance in our two-step processed crystalline ITO films. 4. Conclusions In summary, we have suggested a process to enhance the properties of ITO thin films. By using the two-step process, films have larger grain size and better crystallinity which is the origin of the enhancement of the electrical and optical properties. High quality ITO films with a resistivity of 3.02 × 10− 4 Ω cm, a carrier mobility of 32.07 cm2/Vs, and a transparency above 90% in visible region was able to be obtained. These results indicate the promise of the present approach for making good quality ITO films at low temperature with a potential application in transparent electrode. Acknowledgement This work was supported by KOSEF through National Core Research Center for Nanomedical Technology (R15–2004– 024–0000–0). References [1] T. Ishida, H. Kobayshi, Y. Nakato, J. Appl. Phys. 73 (1993) 4344. [2] D. Paine, T. Whitson, D. Janiac, R. Beresford, C. Yang, B. Lewis, J. Appl. Phys. 85 (1999) 8445.
J.H. Kim et al. / Thin Solid Films 515 (2007) 3580–3583 [3] P. Thilakan, C. Minarini, S. Loreti, E. Terzini, Thin Solid Films 388 (2001) 34. [4] E. Nishimura, M. Ando, K. Onisawa, M. Takabatake, T. Minemura, Jpn. J. Appl. Phys. 35 (1996) 2788. [5] T. Maruyama, K. Fukui, Thin Solid Films 203 (1991) 297. [6] H. Fang, T. Miller, B.R. Rogers, R.H. Magruder III, R.A. Weller, J. Appl. Phys. 97 (2005) 083502. [7] H. Kim, J.S. Horwitz, G. Kushto, A. Pique, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, J. Appl. Phys. 88 (2000) 6021. [8] M.A. Morales-Paliza, R.F. Haglund Jr., L.C. Feldman, Appl. Phys. Lett. 80 (2002) 3757. [9] P.F. Carcia, R.S. McLean, M.H. Reily, Z.G. Li, L.J. Pillone, R.F. Messier, Appl. Phys. Lett. 81 (2002) 1800. [10] Y.S. Jeong, S.Y. Lee, H.K. Jang, I.S. Yang, S.H. Moon, B.D. Oh, Appl. Surf. Sci. 109 (1997) 424. [11] H. Kim, C.M. Gilmore, A. Pique, J.S. Horwitz, H. Mattousi, H. Murata, D.B. Chrisey, J. Appl. Phys. 86 (1999) 6451.
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[12] K. Daoudi, C.S. Sandu, V.S. Teodorescu, C. Ghica, B. Canut, M.G. Blanchin, J.A. Roger, M. Oueslati, B. Bessais, Cryst. Eng. 5 (2002) 187. [13] C. Liu, T. Matsutani, T. Asanuma, K. Murai, M. Kiuchi, E. Alves, M. Reis, J. Appl. Phys. 93 (2003) 2262. [14] H. Izumi, F.O. Adurodija, T. Kaneyoshi, T. Ishihara, H. Yoshioka, M. Motoyama, J. Appl. Phys. 91 (2002) 1213. [15] F.O. Adurodija, H. Izumi, T. Ishihara, H. Yoshioka, M. Motoyama, J. Mater. Sci., Mater. Electron. 12 (2001) 57. [16] Y. Djaoued, V.H. Phong, S. Badilescu, P.V. Ashrit, F.E. Girouard, V.-V. Truong, Thin Solid Films 293 (1997) 108. [17] B.E. Sernelius, K.F. Berggen, Z.C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244. [18] C.F. Klingshirn, Semiconductor Optics, Springer-Verlag, Berlin, 1995, p. 171. [19] J.L. Arias, M.B. Mayor, J. Pou, B. Leon, M. Perez-Amor, Vacuum 67 (2002) 653.
Enhancement in electrical and optical properties of indium tin oxide thin films grown using a pulsed laser deposition at room temperature by two-step process Jong Hoon Kim ⁎, Byung Du Ahn, Choong Hee Lee, Kyung Ah Jeon, Hong Seong Kang, Gun Hee Kim, Sang Yeol Lee Department of Electrical and Electronic Engineering, Yonsei University, 134, Shinchon-dong, Seodaemoon-ku, Seoul, 120–749, South Korea Received 26 May 2006; received in revised form 2 November 2006; accepted 3 November 2006 Available online 15 December 2006
Abstract The optical and electrical properties of indium tin oxide (ITO) thin films deposited using a pulsed laser deposition at room temperature can be substantially enhanced by adopting a two-step process. X-ray diffraction patterns and atomic force microscopy images were used to observe the structural properties of the films. High quality ITO films grown by two-step process could be obtained with the resistivity of 3.02 × 10− 4 Ω cm, the carrier mobility of 32.07 cm2/Vs, and the transparency above 90% in visible region mainly due to the enhancement of the film crystallinity and the increase of grain size. © 2006 Elsevier B.V. All rights reserved. Keywords: Indium tin oxide; Two-step process; Electrical and optical properties
1. Introduction Indium tin oxide (ITO) is widely used as a transparent electrode in optoelectronic devices because the films have high transmittances in the visible region and low resistivity. It is well known that the low resistivity value of ITO films is due to a high carrier concentration caused by both oxygen vacancies and substitutional tin dopants. Also, the high transparency in the visible and near-IR region is caused by the wide band gap. There are several deposition techniques used to grow ITO thin films including evaporation [1,2], sputtering [3,4], chemical vapor deposition [5], and pulsed laser deposition (PLD) [6–8]. In comparison to other techniques, PLD has several advantages. The composition of the films is quite close to that of the target, and films crystallize at a low substrate temperature due to the high kinetic energies of the atoms and the ionized species in the laser-ablated plasma [7,8]. Recently, the development of a deposition process at a low temperature is very important for the application of film growth on a heat-sensitive substrate [9]. In ⁎ Corresponding author. E-mail address: [email protected] (J.H. Kim). 0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2006.11.006
this paper, we report the enhancement of the electrical, structural, and optical properties of ITO thin films prepared at room temperature by using the two-step process with PLD. 2. Experimental details The amorphous ITO thin films were deposited at room temperature by using PLD with a Nd:YAG laser (355 nm, 5 Hz, and full width at half maximum (FWHM) of 6 nm) with a thickness of 50 nm in the first step. The target used in this study was a sintered ITO pellet containing 5 wt.% SnO2. The ablated material was deposited on 1.25 × 1.25 cm2 glass substrates (microscope cover glass, Marienfeld). The substrates were ultrasonically cleaned in acetone and methanol, rinsed in deionized water, and subsequently dried in a flowing nitrogen gas before deposition. The basal vacuum in the chamber was as low as 1.33 × 10− 4 Pa. The target was rotated at 4 rpm to preclude pit formation and to ensure uniform ablation of the target [10]. The laser energy density on the target was 2.0 J/cm2, and the target–substrate distance was 5 cm. Oxygen gas was injected into the chamber by using a mass flow controller to create a pressure of 2 Pa. After the first deposition, ITO films
J.H. Kim et al. / Thin Solid Films 515 (2007) 3580–3583
Fig. 1. Variation of resistivity (ρ), carrier density (N ), and Hall mobility (μ) as a function of processing conditions for ITO films. All films were grown by PLD on glass with the thickness of 200 nm.
were annealed by rapid thermal annealing (RTA) in nitrogen ambient (1 atm) at 400 °C for 1 min to improve crystallinity. The two-step processed ITO film has been completed by the second deposition on the annealed crystalline ITO film with a thickness of 150 nm and with a laser repetition rate of 1 and 5 Hz respectively. Other growth conditions are the same as the first deposition. The electrical properties were measured by Hall measurements in the van der Pauw configuration at a room temperature. The magnetic field was provided by a permanent magnet of intensity B = 1 T. The structural properties of the films were characterized by using XRD (RIGAKU RINT2000) where a Ni-filtered CuKα (λ = 1.54056 Å) source was used. The surface morphology of the films was observed by AFM (Veeco Dimenson 3100). We have used AFM Contact mode with the constant force method (the force between the sample surface
Fig. 2. X-ray diffraction (θ–2θ) patterns as a function of processing conditions for ITO films deposited at room temperature. All films were grown by PLD on glass with the thickness of 200 nm.
3581
Fig. 3. Effect of processing conditions on optical transmittance spectra for ITO films with an inset of optical band gap energy. All films were grown by PLD on glass with the thickness of 200 nm.
and the AFM tip (Veeco DNP-20) was kept constant by a feedback system while the surface beneath the tip was scanned). The optical transmission measurements were performed using a UV-near IR grating spectrometer (Cary 5G; 175–3300 nm). 3. Results and discussion To observe the effect of the two-step process, samples were systematically prepared in 4 different conditions. Sample A is as grown ITO film deposited at room temperature. Sample B is the film deposited at room temperature and annealed by RTA in nitrogen ambient (1 atm) at 400 °C, for 1 min. Sample C and D are two-step processed ITO films with the laser repetition rate of 1 and 5 Hz in the second deposition, respectively. All films were grown on glass substrates with a same thickness of 200 nm. Fig. 1 illustrates the variation of resistivity, carrier density, and Hall mobility as a function of processing conditions. It has been observed that the electrical properties are strongly influenced by the processing condition. Relative high resistivity of 1.23 × 10− 3 Ω cm was obtained from as grown sample (Sample A) and it decreased to a lower value of 9.19× 10− 4 and 3.02 × 10− 4 Ω cm from Sample B and C, respectively. This decrease in resistivity may be due to the reduction of severe oxygen related deficiencies which cause the lattice structural disorders and consequently reduce the mobility of carriers, by RTA treatment [11].
Fig. 4. Schematic diagram of the two-step process.
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J.H. Kim et al. / Thin Solid Films 515 (2007) 3580–3583
The resistivity of Sample B is higher than that of Sample C. This may have been caused by the fact that the RTA time of 1 min is enough to crystallize the buffer layers with the thickness of 50 nm, but not enough for entire 200-nm-thick films. In the case of Sample D, high resistivity of 1.30 × 10− 3 Ω cm was observed. It is suggested that the laser repetition rate of 5 Hz is too fast to keep the ordinary arrangement of the film, because of overstacking during the second deposition. This variation in resistivity may be due to different grain sizes [12]. AFM measurement indicates that the grain size of the films changes by varying the processing conditions [13]. The AFM image of Sample A shows a very smooth surface with a root-mean-square (rms) roughness of 0.52 nm. On Sample C, the rms roughness increases to 2.48 nm because of the enhancement of crystallinity. A larger grain size results in lower density of grain boundaries, which behave as traps for free carriers and barriers for carrier transport in the film. Thus, the decrease in resistivity is accompanied by the increase in both the Hall mobility and the carrier density of the films, which result from the increased grain size. Moreover, the two-step process enhances crystallinity of the films (see Fig. 2). In ITO films, Sn atoms in crystal lattices (so called ‘activated Sn atoms’) generate carrier electrons. On the contrary, since Sn atoms in the amorphous ITO grains were not activated, they do not make carriers but neutral scattering centers, such as SnOx clusters [14]. Fig. 2 represented the XRD patterns (Θ–2Θ) of ITO films as a function of processing condition. XRD patterns of as grown ITO film (Sample A) did not show any peak, suggesting an amorphous layer. However, the structure of the ITO film became a crystalline one after RTA or the two-step process. On Sample B and C, the intensities of the major (222) peak of the ITO films increased significantly and its value of FWHM was 0.32° and 0.22°, respectively. It is well agreed with the result of electrical properties as mentioned in Fig. 1. However, the peak intensity was reduced and (222) peak splitted into two peaks in the case of Sample D. Izumi group reported a similar peak split observed in sputtered ITO films and proposed the residual stress model [15]. In our case, the development of the intrinsic stress in the film could be caused by the overstacking during the second deposition with a repetition rate of 5 Hz. The effect of processing condition on the optical properties of ITO films is shown in Fig. 3. It is known that the increase in optical transmittance can be attributed to the improvement of crystallinity and stoichiometry [16]. High average transmittance values in the visible region were exhibited from Sample B and C (89.1% and 91.7%, respectively). This high transmittance may be due to the enhancement of the crystallinity and stoichiometry of the ITO films and it is well agreed with the electrical and structural properties illustrated in Figs. 1 and 2, respectively. On Sample A, the transmittance is relatively low because the crystallization energy is not sufficient to form a stoichiometric film. The inset of Fig. 3 shows the variation of the optical band gap energy as a function of processing condition. The optical band gap could be obtained from the intercept of α2 (α: absorption coefficient) versus hν for direct allowed transitions. The value of the optical band gap is observed to vary from 3.76 (Sample A) to
4.13 eV (Sample C). The change of optical band gap can be explained by Burstein–Moss shift [17]. This increase in the band gap is due to the increase in free electron concentration in the films evidenced in Fig. 1. It is the result of a large increase in the free carrier concentration and the corresponding upward shift of the Fermi level to the top of the band edge. Generally, it is well known that the broadening in absorption edge is mainly due to the increase of disorders in semiconductor films, which lead to the appearance of localized electron and/or hole states [18]. Sample A shows broader absorption edge compared to Sample C, and this result also supports the electrical and structural properties of these films mentioned in Figs. 1 and 2. Fig. 4 shows the schematic diagram of the two-step process. The amorphous ITO thin films were grown at room temperature in the first deposition and annealed by RTA to improve crystallinity. A new ITO film was grown epitaxially on the annealed crystalline ITO film which had a role as a seed layer. By varying the repetition rate, the amount of material deposited per pulse was same for each deposition condition, but the time between pulses was varied [19]. In the case of Sample D, there is not enough time for the material to accommodate in the structure before the next pulse arrives. It seems that this is the origin of the intrinsic stress which causes the degradation of electrical and optical properties of Sample D. As mentioned in Fig. 1, there are two main reasons of carrier generation in ITO films; native oxygen vacancies and substitutional tin dopants. The oxygen vacancies create free electrons in the films because one oxygen vacancy creates two extra electrons in the films. And Sn atoms which substituted In sites in crystal lattices generate carrier electrons. These two factors are in trade-off. Both relatively low resistivity and high electron mobility were consistent with the enhancement of optical transmittance in our two-step processed crystalline ITO films. 4. Conclusions In summary, we have suggested a process to enhance the properties of ITO thin films. By using the two-step process, films have larger grain size and better crystallinity which is the origin of the enhancement of the electrical and optical properties. High quality ITO films with a resistivity of 3.02 × 10− 4 Ω cm, a carrier mobility of 32.07 cm2/Vs, and a transparency above 90% in visible region was able to be obtained. These results indicate the promise of the present approach for making good quality ITO films at low temperature with a potential application in transparent electrode. Acknowledgement This work was supported by KOSEF through National Core Research Center for Nanomedical Technology (R15–2004– 024–0000–0). References [1] T. Ishida, H. Kobayshi, Y. Nakato, J. Appl. Phys. 73 (1993) 4344. [2] D. Paine, T. Whitson, D. Janiac, R. Beresford, C. Yang, B. Lewis, J. Appl. Phys. 85 (1999) 8445.
J.H. Kim et al. / Thin Solid Films 515 (2007) 3580–3583 [3] P. Thilakan, C. Minarini, S. Loreti, E. Terzini, Thin Solid Films 388 (2001) 34. [4] E. Nishimura, M. Ando, K. Onisawa, M. Takabatake, T. Minemura, Jpn. J. Appl. Phys. 35 (1996) 2788. [5] T. Maruyama, K. Fukui, Thin Solid Films 203 (1991) 297. [6] H. Fang, T. Miller, B.R. Rogers, R.H. Magruder III, R.A. Weller, J. Appl. Phys. 97 (2005) 083502. [7] H. Kim, J.S. Horwitz, G. Kushto, A. Pique, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, J. Appl. Phys. 88 (2000) 6021. [8] M.A. Morales-Paliza, R.F. Haglund Jr., L.C. Feldman, Appl. Phys. Lett. 80 (2002) 3757. [9] P.F. Carcia, R.S. McLean, M.H. Reily, Z.G. Li, L.J. Pillone, R.F. Messier, Appl. Phys. Lett. 81 (2002) 1800. [10] Y.S. Jeong, S.Y. Lee, H.K. Jang, I.S. Yang, S.H. Moon, B.D. Oh, Appl. Surf. Sci. 109 (1997) 424. [11] H. Kim, C.M. Gilmore, A. Pique, J.S. Horwitz, H. Mattousi, H. Murata, D.B. Chrisey, J. Appl. Phys. 86 (1999) 6451.
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[12] K. Daoudi, C.S. Sandu, V.S. Teodorescu, C. Ghica, B. Canut, M.G. Blanchin, J.A. Roger, M. Oueslati, B. Bessais, Cryst. Eng. 5 (2002) 187. [13] C. Liu, T. Matsutani, T. Asanuma, K. Murai, M. Kiuchi, E. Alves, M. Reis, J. Appl. Phys. 93 (2003) 2262. [14] H. Izumi, F.O. Adurodija, T. Kaneyoshi, T. Ishihara, H. Yoshioka, M. Motoyama, J. Appl. Phys. 91 (2002) 1213. [15] F.O. Adurodija, H. Izumi, T. Ishihara, H. Yoshioka, M. Motoyama, J. Mater. Sci., Mater. Electron. 12 (2001) 57. [16] Y. Djaoued, V.H. Phong, S. Badilescu, P.V. Ashrit, F.E. Girouard, V.-V. Truong, Thin Solid Films 293 (1997) 108. [17] B.E. Sernelius, K.F. Berggen, Z.C. Jin, I. Hamberg, C.G. Granqvist, Phys. Rev. B 37 (1988) 10244. [18] C.F. Klingshirn, Semiconductor Optics, Springer-Verlag, Berlin, 1995, p. 171. [19] J.L. Arias, M.B. Mayor, J. Pou, B. Leon, M. Perez-Amor, Vacuum 67 (2002) 653.