Comparison of Indium-Tin-Oxide Transparent Conducting Films Fabricated by Spray CVD Using Different Tin Chlorides

Transaction of the Materials Research Society of Japan 34[2] 225-228 (2009)

Comparison of Indium-Tin-Oxide Transparent Conducting Films Fabricated By Spray CVD Using Different Tin Chlorides

Yutaka Sawada1, Takeshi Kondo1, Takeshi Aoyama1and Riko Ozao2 1Department of Industrial Chemistry, Graduate School of Engineering, Tokyo Polytechnic University, 1583 Iiyama, Atsugi, Kanagawa 2433-0297, Japan Fax: 81-046-942-9532, e-mail: [email protected] 2Department of Informatics and Media Technology, SONY Institute of Higher Education, 428 Nurumizu, Atsugi, Kanagawa 243-8501, Japan, e-mail: [email protected]

The authors reported elsewhere ITO transparent conducting films prepared by spraying ethanol solution of indium (III) chloride and tin (II) chloride using an inexpensive perfume atomizer onto a glass substrate at 350oC; Y. Sawada et al., Thin Solid Films 409, 46 (2002) and Y. Sawada, Mater. Sci. Forum 437/438, 23 (2003). In the present study, tin-doping was attempted under identical experimental conditions except for using tetravalent tin (IV) chloride which can be dissolved in ethanol more easily than divalent tin (II) chloride. The minimum resistivities (1.4×10-4 and 7.0×10-5 ohm cm), respectively, for the as-deposited and after annealing at 600oC in reducing atmosphere were approximately equal to our previous one (1.7×10-4 and 7.9×10-5 ohm cm). The lowest resistivity after the annealing was compatible with that (7.7×10-5 ohm cm) of the single-crystal ITO film deposited by pulsed-laser-deposition process onto an yttrium-stabilized zirconium oxide single crystal (Ohta et al., Appl. Phys. Lett. 76 (19) 2740 (2000)). Carrier concentration and mobility, optical transmittance and reflectance were also similar to those deposited using tin (II) chloride reported previously. These results suggested the formation of tin (II) chloride during the deposition process using tin (IV) chloride. Key words: ITO films, transparent conducting films, chemical vapor deposition, indium chloride, tin chloride

1. INTRODUCTION hotplate. The lowest resistivities of the as-deposited Transparent conducting films are inevitable for all flat films (1.9×10-4 ohm cm [9] and 1.7×10-4 ohm cm [13]) panel displays and solar cells. TCFs are also used for were approximately compatible with the films deposited shielding of infrared radiation and electromagnetic by PVD process. Kawashima et al. [10] seemed to trace waves of windows for houses, buildings and cars etc. our method and confirmed the slightly lower value Transparent heaters are important for refrigerating show (1.2×10-4 ohm cm). Fukano and Motohiro [11] reported cases and airplane windshields. Tin-doped indium oxide that the compatible resistivities ((3-4)×10-4 ohm cm) (indium-tin-oxide) is a typical transparent conducting were reproduced relatively easily. Annealing at 600oC in material and often deposited by sputtering process onto reducing atmosphere (nitrogen plus 0.1% [9] or 0.2% unheated plastic substrate (polyimide color filters) or [13] hydrogen) lowered the resistivity (9.5×10-5 or flexible plastic substrates. Spray CVD (mist CVD or 7.9×10-5 ohm cm, respectively); the latter value agreed spray pyrolysis) of ITO films is instrumentally cheap with that (7.7×10-5 ohm cm) for a single-crystal ITO and needs substrate heating so that it should be focused film deposited by PLD (pulsed laser deposition) onto an to the deposition onto glass or silicon substrates. Table I yttrium-stabilized zirconium oxide single crystal show reports [1-13] on spray CVD of ITO films using reported by Ohta et al.[18] The carrier concentration indium chloride and tin chlorides which are stable and (1.7×1021 cm-3) and the mobility (40 cm 2/V s) also 21 -3 2 relatively cheap raw materials. Tin (IV) chloride SnCl4 agreed with those (1.8×10 cm and 42 cm /V s, was used as the tin-doping source. Amongst them, the respectively) reported by Ohta et al.[18] resistivity by Benamar et al. (4.5×10-5 ohm cm) [8] is In the present study, tin-doping was attempted using apparently the lowest and compatible with the lowest tetravalent tin (IV) chloride which can be dissolved values obtained by PVD process such as Nath et al.[14] more easily than tin (II) chloride. Other deposition (7×10-5 ohm cm), Ray et al.[15] (6.8×10-5 ohm cm), parameters unchanged from our previous works to Rauf [16,17] (4.4×10-5 ohm cm) and Ohta et al.[18] examine whether the high-quality ITO films are (7.7×10-5 ohm cm). However, the authors doubt the attributed mainly to deposition system or the tin the lowest value by Benamar et al.[8] since higher value valence (or chlorine composition) of tin chloride. (4.5×10-4 ohm cm) was obtained when calculating from their sheet resistivity (5 ohm/sq.) and their thickness 2. EXPERIMENTAL (900 nm). Indium (III) chloride, InCl3·2.7H2O (purity 99.99), Our previous works [9, 13] should be the first supplied from Kojundo Chemical Lab., Inc., and tin (IV) exception to use tin (II) chloride SnCl2 as the tin-doping chloride, SnCl4·5H2O (purity >98.0%), supplied from source compound and only ethanol as the solvent. A Wako Pure Chemical Inc., were quickly weighed and cheap perfume atomizer was used to spray the solution dissolved in ethanol (purity, 99.5%; Wako Pure onto a glass substrate heated at 350oC on a laboratory Chemical Inc.) before being stirred for 12 hours or more.

225 226 Comparison of Indium-Tin-Oxide Transparent Conducting Films Fabricated By Spray CVD Using Different Tin Chlorides

Table I: ITO films deposited by spray CVD using indium chloride and tin chlorides. as-deposited Resistivity solutes solvents year authors or annealed /ohm cm InCl3 SnCl4 SnCl2 H2O MeOH EtOH HCl BuAc 1958 Ishikawa 5.5x10-4 X X X 1966 Groth 2.3x10-4 X X X 1978 Nagatomo et al. 2.0x10-4 X X X X 1981 Manifecier et al. 5.0x10-4 X X X X X -4 as-deposited or 1981 Pommier et al. 2.0x10 X X X X no description 1987 Kaneyasu et al. 1.2x10-4 X X X X about annealing 1999 Bishet et al. 3.0x10-4 X X X X 1999 Benamar et al. 4.5x10-5 X X X X 2002 Sawada et al. 1.9x10-4 X X X 2003 Kawashima et al. 1.2x10-4 X X X 2004 Fukano et al. (3-4)x10-4 X X X 1982 Frank and Kostlin 1.3x10-4 X X X annealed 2002 Sawada et al. 9.5x10-5 X X X 2003 Sawada et al. 7.7x10-5 X X X

30 The thickness and the composition (Sn/(In+Sn) ratio) of the oxide films were determined by an X-ray SnCl4 fluorescence analysis (model JSX-3200, JEOL; SnCl2 fundamental parameter method). The thickness was 20 checked with a surface profiler (model Dektak3ST, Veeco). The crystalline state of the films was evaluated by an X-ray diffraction analysis (model RINT-2500V, Rigaku) with a Cu target (40 kV, 300 mA) and a 10 graphite monochrometer. The optical transmittance and reflectance was measured with a conventional spectrometer (model Ubest V-570DS, JASCO); an aluminium plate was used as the reference (100% Film composition / at.%Sn / composition Film reflectance). The resistivity of the films was measured 0 10 20 30 by the four-point-probe method (probe distance, 0.65 mm; probe current, 1 mA) with a multi-meter (model Solution composition / at.%Sn 34401A, Hewlett-Packard). The Hall coefficient was Fig. 1: The dependence of film composition on the measured by the van der Pauw method (model MI-675, solution composition. Sanwa Radio Measurement Works Co., Ltd.; probe current, 1 mA).

The concentration of the total metal ions in the solution 400 was fixed at approx. 0.1 mol/l. Solutions with 0-30 at.% SnCl4 Sn as indicated by Sn/(In+Sn) were prepared. Corning SnCl2 #7059 glass substrates (25×75×0.7 mm3) were cleaned 300 ultrasonically for 10 min in deionized water with a detergent (Semicoclean 56, Furuuchi Chemical Co., Ltd.) and rinsed several times in deionized water. The 200 glass substrates were soaked in boiling acetone for 10 min and then quickly pulled out into the air. The substrate was heated on a laboratory hot plate (Corning o

PC-400) at 300-350 C. The substrate temperature was nm / thickness Film 100 monitored using a thermocouple that was placed on the glass substrate. The solution was sprayed in air with an atomizer used for cosmetic purposes (a spray bottle, 50 ml, PV14033-00 Shoubi-Do, Co., Ltd.). The spraying in 0 10 20 30 the present work was executed vertically (downwardly) Film composition / at.%Sn by which the compositional controllability and the film uniformity were improved [13] while the spraying at an Fig. 2: The dependence of the film thickness on the film oblique direction scattered the data as reported in our composition. first paper [9]. The distance between the spray nozzle and the substrate was 15 cm; the value was experimentally optimized [13]. The spraying in air was 3. RESULTS AND DISCUSSION repeated 200 times and consumed approximately 40 ml The dependence of film composition on the solution of solution. Some films were annealed at 600oC for 2 h composition is shown in Fig. 1. In this figure, the results in a reducing atmosphere (N2-0.2%H2; gas flow rate, using tin (II) chloride as described in our previous paper 300 ml/min). [13] are plotted for reference; the film composition Y. Sawada et al. Transaction of the Materials Research Society of Japan 34[2] 225-228 (2009) 227

(Sn/(In+Sn) as indicated by at.%) agreed approximately with the solution composition. The dependence of the as-daposited(SnCl 4) film thickness on the film composition is shown in Fig. annealed(SnCl4)

2. Film thickness was scattered and independent of the -3 as-deposited(SnCl2) film composition as well as in case of our previous annealed(SnCl2) cm 2 work. 21

annealed 4000

In O powder 3000 2 3 1 (for reference) 2000

ITO film 1000 Sn 4 at.% as-deposited Carrier concentration / 10 concentration Carrier Intensity / cps 0 0 0 10 20 30 (400) (222) Film composition / at.%Sn 30 32 34 36 2θ/° Fig. 5: The dependence of carrier concentration on the film composition. Fig. 3: A typical diffraction spectrum of the ITO film deposited using tin (IV) chloride. 100

All X-ray diffraction peaks agreed with those of cubic as-daposited(SnCl4)

In2O3 suggesting the formation of ITO (solid solution) as -1 80 annealed(SnCl4) well as in case of the previous work. A typical s as-deposited(SnCl ) -1 2 diffraction spectrum of the film is indicated in Fig. 3. In V annealed(SnCl2) this figure, the spectrum of the randomly-oriented In O 2 2 3 60 powders is indicated for reference. The peak intensity ratios show that the crystal orientation of the present film is approximately random although the 400 peak is slightly strong. The preferred orientation was less 40 remarkable compared with the previous films. The lower peak angle compared with that of undoped In2O3 is interpreted as the lattice expansion caused by tin doping. 20

The narrow peak width for the films indicates the cm / mobility Carrier evolution of large crystallites. 0 10 20 30 10-2 as-daposited(SnCl ) 4 Film composition / at.%Sn annealed(SnCl4)

as-deposited(SnCl2) Fig. 6: The dependence of carrier mobility on the film annealed(SnCl2) composition.

10-3 Dependence of resistivity on the film composition is indicated in Fig. 4. The results in our previous work using SnCl2 are plotted in this figure for reference. Resistivities for the most as-deposited ITO films were in as-deposited the order of 10-4 ohm cm; the lowest resisitivity in the cm / ohm Resistivity present case was 1.4×10-4 ohm cm (4.0 at.% Sn; thickness, 144 nm) which was slightly lower than the 10-4 annealed previous films (1.7×10-4 ohm cm; 8.3 at.% Sn; thickness, 201 nm) deposited using SnCl2. These values were compatible with those for the films deposited by PVD 0 10 20 30 processes such as sputtering. In this figure, the Film composition / at.%Sn resisitivities after annealing in a N2-0.2%H2 atmosphere are also plotted. The lowest resistivity in the present -5 Fig. 4: Dependence of resistivity on the film work using SnCl4 was 7.0×10 ohm cm (4.0 at.% Sn; composition. thickness, 210 nm). These values are slightly lower than 228 Comparison of Indium-Tin-Oxide Transparent Conducting Films Fabricated By Spray CVD Using Different Tin Chlorides

the previous one (7.9×10-5 ohm cm; 7.7 at.% Sn; sufficiently high and agreed with that (81%) in the thickness, 274 nm) using SnCl2 [13] and the one previous work [13] using SnCl2. Annealing lowered the (7.7×10-5 ohm cm, single crystal ITO film) deposited on transmittance and increased the reflectance in the a YSZ single crystal by pulsed laser deposition by Ohta infrared region. Reflectance initiated at relatively short et al. [18] wavelength (approximately 1000 nm); this supported The dependence of carrier concentration on the film high carrier concentration. composition is shown in Fig. 5. The values were Thus, electrical and optical properties did not differ approximately doubled by annealing. The highest one in whether the films were deposited using SnCl2 or SnCl4. 21 -3 the present work deposited using SnCl4 (2.1×10 cm ; This can be understood if we assume tentatively that 4.0 at.% Sn; thickness, 210 nm) was slightly higher than SnCl4 decomposes to SnCl2 (SnCl4(gas) → SnCl2(gas) + that of the previous work deposited using SnCl2 Cl2(gas)) and that the reaction of SnCl2 is the predominant (1.7×1021 cm-3; 7.7 at.% Sn; thickness, 249 nm) [13] and factor to determine the tin doping reaction during the Ohta et al [18]. The dependence of carrier mobility on formation process of indium oxide films. the film composition is shown in Fig. 6. The mobility decreased drastically by the tin doping and saturated. 4. CONCLUSIONS The mobility doesn’t seem to be influenced by the Ethanol solutions of indium (III) chloride and tin (IV) annealing and the species of the doping chloride (SnCl2 chloride were sprayed onto a Corning #7059 glass 2 or SnCl4). The mobility was 40 cm /V s for the film with substrate. The electrical and optical properties agreed the lowest resistivity. with those in our previous work [9, 13] fabricated by the same deposition parameters except for the tin source compound (SnCl2). The film properties were 100 approximately independent of the valence of tin chloride. glass substrate The lowest resistivities for the as-deposited and 80 annealed films were 1.4×10-4 and 7.0×10-5 ohm cm, respectively. 60 as-deposited REFERENCES [1] T. Ishikawa, Yogyo-kyokai-shi, 66(7), c250-59 40 visible range (1958) (in Japanese). [2] Groth, phys. sat. sol., 14, 69-75 (1966). Transmittance / % 20 [3] T. Nagatomo and O. Oomoto, Oyo-butsuri, 47 (7), annealed 618-23 (1978) (in Japanese). [4] J. C. Manifacier, J. P. Rillard and J. M. Bind, 0 Thin 500 1000 1500 2000 2500 Solid Films, 77, 67-81 (1981). [5] R. Pommier, C. Grill and J. Marucchi, Thin Solid Wavelength / nm Films, 77, 91-97 (1981). [6] K. Kaneyasu, K. Adachi, T. Hirayama and H. Sakata, Fig. 7: Optical transmission of ITO films. DENKI KAGAKU,55 (3), 245-50 (1987) (in Japanese). [7] H. Bisht, H. -T. Eun, A. Mehrtens and M. A. Aegerter, Thin Solid Films, 351, 109-14 (1999). 100 [8] E. Benamar, M. Rami, C. Messaoudi, D. Sayah and annealed A. Ennaoui, Solar Energy Materials and Solar Cells, 56, 80 125-39 (1999). [9] Y. Sawada, C. Kobayashi, S. Seki and H. Funakubo, 409 (2002) 46-50. Visible range Thin Solid Films 60 [10] T. Kawashima, H. Matsui and N. Tanabe, Thin Solid Films, 445, 241-244 (2003). 40 [11] T. Fukano and T. Motohiro, Solar Energy Materials

Reflectance / % Reflectance as-deposited & Solar Cells 82 (2004) 567-575. 20 [12] G. Frank and H. Kostlin, Appl. Phys., A27, 69-75 (1982). 0 [13] Y. Sawada, Mater. Sc i. Forum 437/438, 23-26 500 1000 1500 2000 2500 (2003). [14] P. Nath, R. F. Bunshah, B. M. Basol and O.M. Wavelength / nm Staffsud, Thin Solid Films, 72,463-68 (1980). [15] S. Ray, R. Banerjee, N. Basu, A. K. Batabyal, and Fig. 8: Optical reflectance of ITO films. A. K. Barua, J. Appl. Phys., 54(6),3497-3501(1983). [16] I. A. Rauf, J. Mater. Sci. Lett., 12,1902-05(1993). [17] I. A. Rauf, J. Appl. Phys., 79(8), 4057-65(1996). Optical transmittance and reflectance for a typical [18] H. Ohta, M. Orita, M. Hirano, H. Tanji, H. ITO film are shown in Figs. 7 and 8, respectively. The Kawazoe, H. Hosono, Appl. Ph ys. Lett . 76 (19) (2000) average transmission (80%) in the visible range 2740-2742. (380-780 nm) in the present work using SnCl are 4 (Received December 23, 2008; Accepted February 3, 2009)