FTO films deposited in transition and oxide modes by magnetron sputtering using tin metal target Bo-Huei Liao,1 Shih-Hao Chan,2 Cheng-Chung Lee,2 Chien-Cheng Kuo,2 Sheng-Hui Chen,2 and Donyau Chiang1,* 1

Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 30076, Taiwan 2

Department of Optics and Photonics, National Central University, Chung-Li 320, Taiwan *Corresponding author: [email protected] Received 4 September 2013; revised 4 November 2013; accepted 4 November 2013; posted 5 November 2013 (Doc. ID 196882); published 9 December 2013

Fluorine-doped tin oxide (FTO) films were prepared by pulsed DC magnetron sputtering with a metal Sn target. Two different modes were applied to deposit the FTO films, and their respective optical and electrical properties were evaluated. In the transition mode, the minimum resistivity of the FTO film was 1.63 × 10−3 Ω cm with average transmittance of 80.0% in the visible region. Furthermore, FTO films deposited in the oxide mode and mixed simultaneously with H2 could achieve even lower resistivity to 8.42 × 10−4 Ω cm and higher average transmittance up to 81.1% in the visible region. © 2013 Optical Society of America OCIS codes: (310.7005) Transparent conductive coatings; (310.1860) Deposition and fabrication. http://dx.doi.org/10.1364/AO.53.00A148

1. Introduction

Doped SnO2 films are a highly transparent, widely applicable material used for IR reflecting heat mirrors, protective layers of photovoltaic cells, gas sensors, touch screens, etc. [1–3]. The mostly recommended doping elements are antimony (Sb) and fluorine (F) [4–6]. Compared to Sb-doped SnO2 , the solubility of fluorine-doped tin oxide (FTO) is preferred because its excess amount is volatile, while Sb at high doping levels tends to form segregated clusters in the lattice boundary and decrease the carrier mobility. Figure 1 shows a schematic representation of the FTO rutile structure, and the corresponding sites of the oxygen vacancies, Sn, O, F, and H atoms. When SnO2 is completely stoichiometric, it is an insulator and all oxygen sites are occupied. However, the realistic material is never stoichiometric and is invariably anion-deficient. The oxygen vacancies are always formed in the 1559-128X/14/04A148-06$15.00/0 © 2014 Optical Society of America A148

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crystal, such as situation (1) illustrated in Fig. 1. As F− anions enter into the rutile structure, they either occupy the oxygen vacancies as situation (2) or replace the O− anions as situation (3) in Fig. 1. Situations (1) [7] and (3) [8,9] usually generate more carrier concentrations, and situation (2) compared to situation (1) can improve the mobility of the FTO films [8,9]. As the hydrogen gas is introduced into the chamber during the sputtering, it reacts with O2 to generate oxygen vacancies, and also incorporates in high concentrations as a shallow donor. The H ions are reported to be stable in the FTO lattices and can modify the electrical characteristics [10–14] of FTO films. We reported that F-doped SnO2 films were prepared by a simple and cost-effective method in the former research [4]. The film formation process is shown in Fig. 2. The working gas is Ar, and reactive gases are the mixture of O2 and CF4 . SnO2 films are first synthesized from Ar, O2 , and Sn target. After that, CF4 decomposes with the aid of plasma and reacts with oxide films to become fluorine-doped oxide films. In the sputtering procedure, O2 is an

Fig. 3. Sputtering modes used for FTO preparation. Fig. 1. Schematic representation of the FTO rutile structure and the corresponding sites for the Sn, O, F, and H atoms.

active oxidizing gas. It reacts with the decomposed carbon atoms from CF4 to become CO gas and reduce the contamination. Besides, oxygen atoms can also create more fluorine atoms. However, in the former research the smallest resistivity of FTO films is 1.63 × 10−3 Ω cm and the average transmittance measured from wavelengths of 400–700 nm is just 80%. We hope to achieve FTO films with resistivity less than 1 × 10−3 Ω cm and average transmittance more than 80% within the visible spectrum. To further improve the optical and electrical properties of FTO films for the transparent conducting application, we propose two different methods, and the sputtering reaction modes are illustrated schematically in Fig. 3. When the Sn target reacts with no oxygen or a small amount of oxygen during the sputtering, the properties of the deposited film appeared as metallic properties, rather than dielectric properties. The method to prepare the FTO films under the no-oxygen condition was named the metallic mode. The cathode voltage (absolute value) of the metallic mode is higher. In our first method, the SnO2 films with oxygen vacancies are deposited in transition mode (1), and then CF4 gas is injected to form F-doped SnO2 films. The cathode voltage (absolute value) between the target and substrate decreases when increasing the oxygen gas in the transition mode. When the oxygen concentration

Fig. 2. Schematic diagram of the new deposition method.

continues increasing, the reaction enters the oxide mode and the cathode voltage (absolute value) remains constant and at the lowest value compared to the previous two modes. In our second method, SnO2 films are first prepared in the oxide mode and the voltage is at the smallest value, and then CF4 gas and H2 gas are introduced into the chamber to optimize the electrical characteristics. The reduction effect of hydrogen can create more oxygen vacancies, and the cathode voltage (absolute value) between target and substrate would increase. 2. Experiment A. Film Preparation

Figure 4 shows a schematic representation of a pulsed DC magnetron sputtering system. A tin target was mounted on one cathode. The Sn target was 15.24 cm (6 in.) in diameter and set approximately 9 cm below the substrate, and all of the films were deposited at power of 100 W. There was a pulse generator with a frequency of 20 kHz, and it was located between the DC power supply and the sputtering cathode. The pulse generator can help to decrease the arcing and maintain the stable plasma. FTO films were coated on soda lime substrates 2.54 cm (1 in.) in diameter and 0.5 mm thick by a magnetron sputtering at 300°C. After injecting Ar gas (52 sccm) into the chamber, the mixture gases of CF4, O2 , and H2 at different ratios were added to form the undoped and doped SnO2 films. In the

Fig. 4. Schematic diagram of the sputtering system. 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

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transition mode, the FTO films were prepared with fixed O2 flow rate of 19.5 sccm but with different ratios of CF4 to O2 gases, which were 0 (0∕19.5), 0.153 (3∕19.5), 0.179 (3.5∕19.5), and 0.231 (4.5∕19.5), respectively. In the oxide mode, the FTO films were prepared with fixed O2 flow rate of 23.5 sccm and fixed CF4 flow rate of 6.5 sccm, but different H2 flow rates. The total pressure was maintained between 1 × 10−3 and 3 × 10−3 torr. Before conducting the experiments, the substrates were cleaned in a UV photo cleaner for 10 min prior to the deposition process. The UV light cleaner has two main working wavelengths, 185 and 254 nm, and the organic dusts are dissociated into CO, CO2 , and H2 O gases during the cleaned exposure. The deposition chamber was pumped down to a base pressure of less than 6 × 10−6 torr by a cryopump. B.

Film Characterization

The transmittance of thin films on soda lime substrates was measured with a Hitachi U4100 spectrometer. From the spectral analysis, the refractive index, extinction coefficient, and physical thickness were determined by the envelope method [15]. The resistivity, carrier mobility, and donor concentration of films were estimated by the Accent HL5500PC hall measurement system. The crystallization phase of films was analyzed using x-ray diffraction (XRD) with Cu Kα radiation (λ  1.54056 Å). 3. Results and Discussion A. Sputtered Film Characteristics Prepared in Transition Mode

Figure 5 shows the transmittance spectra of FTO films deposited with fixed Ar (52 sccm) and O2 (19.5 sccm) flow rates but with different ratios of CF4 to O2 mixed gases, which were 0 (0∕19.5), 0.153 (3∕19.5), 0.179 (3.5∕19.5), and 0.231 (4.5∕19.5), respectively. The oxide films were not completely oxidized in the transition mode at no CF4 injection (0 ratio of CF4 to O2 ) and possessed lower transmittance in the visible range. The average transmittance in the visible range increased to 81.0% after the CF4 ∕O2 ratio increased to 0.153 and the phenomenon was consistent with the result of the extinction coefficient described in Fig. 6. The extinction coefficient of

Fig. 5. Transmittance spectrum and average transmittance of FTO films deposited with different ratios of CF4 to O2 gases. A150

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Fig. 6. Extinction coefficient of undoped and doped SnO2 films deposited with various CF4 to O2 gases.

FTO films prepared by the CF4 ∕O2 ratio of 0.153 at the short wavelength range was around three times smaller than that prepared without CF4 gas. The explanation was that the F− anions replaced the oxygen vacancies and the chemical stoichiometry became better, so the extinction coefficient decreased and the average transmittance increased. However, the average transmittance in the visible range decreased when further increasing the CF4 ∕O2 ratio. The decrease in transmittance was attributable to the contamination produced by the carbon atoms generated from CF4 . In the NIR ranges, the transmittance decreased as the CF4 ∕O2 ratio increased. The F− anions originating from the CF4 gas could replace the oxygen anions and generate more free electrons so that the transmittance in the NIR ranges decreased. Figure 7 shows the electrical properties of the FTO samples. The resistivity decreased with an increase of the CF4 ∕O2 ratio and slightly increased at a ratio of 0.231. The minimum resistivity obtained was 1.63 × 10−3 Ω cm, as the CF4 ∕O2 ratio was 0.179. The initial increase of the Hall mobility (μ) in the FTO films resulted from the oxygen sites replaced by F− anions in the SnO2 lattice, which improved the lattice structure [6]. However, at high ratios of CF4 ∕O2, the mobility decreased again. The decrease in mobility was attributed to the formation of Sn–F complexes in the grain boundaries [8]. The carrier concentration increased monotonically as the CF4 ∕O2 ratio increased within the studied ranges. The increase in free carrier concentration

Fig. 7. Electrical properties of FTO films deposited with different ratios of CF4 to O2 gases.

was due to the substitution of O2− anions by F− anions [6]. B.

Sputtered Film Characteristics Prepared in Oxide Mode

Figure 8 shows the transmittance spectra of SnO2 films deposited at 300°C with three different gas flow rate conditions. They are condition (1) with the sufficient supply of O2 gas at a flow rate of 23.5 sccm, condition (2) with introducing an O2 gas flow rate of 23.5 sccm and a CF4 gas flow rate of 6.5 sccm, and condition (3) with O2 gas flow rate of 23.5 sccm, CF4 gas flow rate of 6.5 sccm, and various H2 gases. In condition (1), SnO2 films were deposited in the oxide mode, in which the tin was fully oxidized and the average transmittance from 400 to 700 nm could reach 84.8%. After the oxide film was formed, CF4 gas was introduced and the F− anions replaced the O2− anions and generated more free electrons. The replacement caused the slightly decreasing transmittance in the NIR range. Finally, both CF4 and H2 mixture gases were introduced as condition (3). The average transmittance from 400 to 700 nm decreased from 83.6% to 79.9% as the H2 gas flow rate increased from 18 to 26 sccm. This resulted from the increase of oxygen vacancies concentration as H2 gas was introduced into the deposition process. In addition, the transmittance in the NIR region also decreased when increasing the H2 gas. The H atoms could form stable interstitial H in the lattice, which led to an increase in the carrier concentration [10– 14]. The increasing carrier concentration would raise the plasma resonance frequency and obviously decrease the transmittance in the NIR region. The dispersion of refractive index of undoped and doped SnO2 films deposited with CF4 (6.5 sccm) or CF4 (6.5 sccm) and H2 (22 sccm) gases at the 100 W sputtering power is shown in Fig. 9. The decrease of refractive index as films were deposited with CF4 and H2 gases was due to the generation of fluoride whose refractive index was lower than that of SnO2. Figure 10 shows the extinction coefficient of undoped and doped SnO2 films deposited with CF4 (6.5 sccm) and with the mixed gases of CF4 (6.5 sccm) and H2 (22 sccm). The extinction coefficient increased when introducing the H2 gas. H2 gas introduction increased more oxygen vacancies and led to the higher extinction coefficient in the vis-

Fig. 8. Transmittance spectra of SnO2 and FTO films deposited with CF4 and the mixed gases of CF4 and H2 with different ratios.

Fig. 9. Refractive index of undoped and doped SnO2 films deposited with CF4 (6.5 sccm) and the mixed gases of CF4 (6.5 sccm) and H2 (22 sccm).

ible range. The extinction coefficient of the doped SnO2 films deposited with the mixed gases of CF4 (6.5 sccm) and H2 (22 sccm) in NIR was larger than that in the visible range. As the CF4 and H2 gases were introduced to the deposition, the F− anions would replace the O2− anions [6] and the deionized H atoms could form stable interstitial H as the donors in the lattice. Both of them could increase carrier concentration and raise the plasma resonance frequency so the extinction coefficient increased in the NIR region. The resistivity ρ, carrier concentration N, Hall mobility μ, and average transmittance of the FTO samples depicted in Fig. 8 are summarized in Table 1. As expected, SnO2 films prepared in the oxide mode possessed the highest resistivity and transmittance because of the intrinsic oxide characteristics. After introducing CF4 (6.5 sccm), the films transferred to F-doped SnO2 films and the resistivity was 4000 times smaller than that of oxide mode SnO2 films. Finally, when both CF4 and H2 gases were introduced as in condition (3), the effect of hydrogen introduction would reduce the oxide mode FTO film. The average transmittance in the visible region decreased from 84.3% to 81.1% after introducing 22 sccm H2 gas, and the phenomena indicated that there were more oxygen vacancies in the FTO films. Besides, according to the first principal calculations, the hydrogen in the FTO films could form stable interstitial H cations as the donors in the lattice

Fig. 10. Extinction coefficient of undoped and doped SnO2 films deposited with CF4 (6.5 sccm) and the mixed gases of CF4 (6.5 sccm) and H2 (22 sccm). 1 February 2014 / Vol. 53, No. 4 / APPLIED OPTICS

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Table 1.

Electrical and Optical Properties of the Undoped and Doped SnO2 Films

Samples SnO2 SnO2 SnO2 SnO2 SnO2

(oxide mode) (with 6.5 sccm CF4 ) (with 6.5 sccm CF4 and 18 sccm H2 ) (with 6.5 sccm CF4 and 22 sccm H2 ) (with 6.5 sccm CF4 and 26 sccm H2 )

Mobility (cm2 V−1 s−1 )

Carrier Concentration (cm−3 )

Resistivity (Ω cm)

Average Transmittance (400–700 nm) (%)

/ 11.2 19.8 11 9.07

/ 4.64 × 1019 2.43 × 1020 6.75 × 1020 4.35 × 1020

4.8 × 10 1.20 × 10−2 1.30 × 10−3 8.42 × 10−4 1.58 × 10−3

84.8% 84.3% 83.6% 81.1% 79.9%

Fig. 11. XRD patterns for oxide mode SnO2 , FTO with 6.5 sccm CF4 , and FTO:H with 6.5 sccm CF4 and 22 sccm H2 .

[10–14]. Both of the oxygen vacancies and the stable interstitial H cations could increase the carrier concentration. Hence, the highest carrier concentration was achieved when 22 sccm H2 gas was introduced. Nevertheless, the mobility started to decrease as H2 gas was introduced larger than 18 sccm. The excess stable interstitial H cations would become the scattering center and result in poor mobility and high resistivity. The lowest resistivity of 8.42 × 10−4 Ω cm could be reached when introducing 6.5 sccm CF4 and 22 sccm H2 mixed gas. Figure 11 shows the XRD patterns for oxide mode SnO2 films, FTO films deposited with 6.5 sccm CF4 , and FTO:H films deposited with 6.5 sccm CF4 and 22 sccm H2 at 300°C substrate temperature. All the films revealed polycrystallinity and contained the SnO2 tetragonal structure. They exhibited the presence of peaks, such as (1 1 0), (1 0 1), (2 0 0), and (2 1 1), and the preferred orientation with a (1 0 1) plane. The FTO films deposited with 6.5 sccm CF4 show the (1 0 1) preferred orientation with the lowest intensity peaks compared to those of the oxide mode SnO2 films and FTO:H films. The decrease in intensity was attributed to the formation of Sn–F complexes in the grain boundaries. After introducing H2 (22 sccm) gas, the FTO:H film had the highest intensity at a (1 0 1) peak. It showed that the hydrogen doping made better crystallinity and explained why FTO:H films had high carrier concentration but still possessed the same mobility as FTO films deposited with 6.5 sccm CF4 . 4. Summary

The characteristics of SnO2 :F thin films deposited by a pulsed DC magnetron sputtering method using a A152

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metal Sn target were reported. The reaction mechanism of this simple and cost-effective deposition method was described. The optical and electrical properties of the films deposited at substrate temperature of 300°C were evaluated by introducing different ratios of CF4 to O2 gas. Besides, the FTO films were prepared in transition or oxide modes. In the transition mode, the minimum resistivity of the FTO film was 1.63 × 10−3 Ω cm with the average transmittance of 80.0% in visible region. Furthermore, FTO films deposited in the oxide mode and interacting with H2 gas reached the lowest resistivity of 8.42 × 10−4 Ω cm and higher average transmittance of 81.1%. The process to prepare FTO films in the oxide mode with hydrogen introduction is a workable process suitable for the application of manufacture in the real-world optical industry. The authors thank the National Science Council of Taiwan for financial support under Contract No. NSC 101-2622-E-492-008-CC3 and also thank the Gredmann Taiwan Ltd. for the kind sponsorship. References 1. C. G. Granqvist, “Transparent conductors as solar energy materials: a panoramic review,” Sol. Energy Mater. Sol. Cells 91, 1529–1598 (2007). 2. T. Kawashima, H. Matsui, and N. Tanabe, “New transparent conductive films: FTO coated ITO,” Thin Solid Films 445, 241–244 (2003). 3. R. Valaskia, C. D. Canestraroa, L. Micaronib, R. M. Q. Mellob, L. S. Romana, and N. Kaiser, “Organic photovoltaic devices based on polythiophene films electrodeposited on FTO substrates,” Sol. Energy Mater. Sol. Cells 91, 684–688 (2007). 4. B.-H. Liao, C.-C. Kuo, P.-J. Chen, and C.-C. Lee, “Fluorinedoped tin oxide films grown by pulsed direct current magnetron sputtering with an Sn target,” Appl. Opt. 50, C106–C110 (2011). 5. H. Kim and A. Piqué, “Transparent conducting Sb-doped SnO2 thin films grown by pulsed-laser deposition,” Appl. Phys. Lett. 84, 218–221 (2004). 6. H. Kim, R. C. Y. Auyeung, and A. Piqué, “Transparent conducting F-doped SnO2 thin films grown by pulsed laser deposition,” Thin Solid Films 516, 5052–5056 (2008). 7. E. Kuantama, D.-W. Han, Y.-M. Sung, J.-E. Song, and C.-H. Han, “Structure and thermal properties of transparent conductive nanoporous F:SnO2 films,” Thin Solid Films 517, 4211–4214 (2009). 8. P. M. Gorley, V. V. Khomyak, S. V. Bilichuk, I. G. Orletsky, P. P. Horley, and V. O. Grechko, “SnO2 films: formation, electrical and optical properties,” Mater. Sci. Eng. B 118, 160–163 (2005). 9. K. Omura, P. Veluchamy, M. Tsuji, T. Nishio, and M. Murozono, “A pyrosol technique to deposit highly transparent,

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FTO films deposited in transition and oxide modes by magnetron sputtering using tin metal target.

Fluorine-doped tin oxide (FTO) films were prepared by pulsed DC magnetron sputtering with a metal Sn target. Two different modes were applied to depos...
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