- Email: [email protected]
Dip-coating of ITO films
N
J O U R N A L OF
t,l,ll
ELSEVIER
SO]
Journal of Non-Crystalline Solids 218 (1997) 129-134
Dip-coating of ITO films Yasutaka Takahashi *, Shinya Okada, Radhouane Bel Hadj Tahar, Ken Nakano, Takayuki Ban, Yutaka Ohya Department of Chemist~', Faculty' of Engineering, Gifu Uni~,ersitv, 1-1, Yanagido, Gifu 501 - 11, Japan
Abstract Uniform, transparent indium tin oxide (ITO) films were prepared by dip-coating process using an organic sol composed of indium acetate-diethanolamine-tin octylate-n-propanol mixture and the relationship between their electrical properties, film morphology and dip-coating conditions have been investigated. The optimum Sn-doping concentration was about 4 mol% relative to In ion. The conductivity of as-prepared ITO films increased with an increase in firing temperature. Multiple coating of the layers as thin as a few tens of nanometers accelerated the growth of the ITO crystals and increased the conductivity of formed films. Thus, ITO films with a resistivity of 4 X 10 4 1~ cm could be obtained by dip-coating and by subsequent post annealing in nitrogen. Through this study we conclude that the conductivity of dip-coated films was mainly controlled by the crystallite size and, hence, by carrier mobility. © 1997 Elsevier Science B.V.
1. Introduction
Indium tin oxide (ITO) is one of the indispensable materials for current optoelectric technologies and, therefore, the development of an economical ITO film production process other than the sputtering method which is currently used assumes significance. Dip or spin coating may be one of the best available production processes. So far several sol-gel or metallo-organic processes for ITO coatings have been reported in which alkoxides [1-4], acetylacetone-In(NO3) 3 mixture [5-8], acetate [3], 2-ethylhexanoate [9,10] and aqueous hydroxide sols [11-14] were used as the starting materials. Air-baked films had the smallest resistivity of ~ 2 X 10 3 1) cm [3,8] and a transparency greater than 90% for visible light. The resistivity can be further decreased by post
* Corresponding author. Tel.: + 81-58 293 2585; fax: + 81-58 230 1893; e-mail: [email protected].
annealing in vacuum or inert (or reducing) atmospheres [3,5,6,12]. It has been reported by Arfsten et al. [1] that ITO films with electrical and optical properties comparable to those of sputtered ones could be directly dip-coated on glass plates by suitable processing. In the dip-coating, however, sometimes multiple coatings, which are troublesome for practical applications, are required to obtain satisfactory film thickness, especially when a suitable sol with high concentration can not be obtained. Considering in practice that ITO films with the resistivity as small as 2 × 10 4 Ft cm are available, minimum film thickness of about 200 nm is required to attain minimum sheet resistance of I0 f ~ / [ ] for application to LC display, for example. Therefore, if a system is developed to produce films as thick as 200 nm with greater electrical conductivity by single dip-coating, it could be applied for the industrial production of ITO films. So far, our group has been studying the
0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0022-3093(97)001 99-3
130
Y. Takahashi et aL / Journal of Non-Crystalline Solids 218 (1997) 129 134
dip-coating process of ITO films using an organic sol composed of indium acetate-diethanolamine-npropanol (or 2-propanol) mixture [3] and an aqueous sol derived from indium nitrate [13,14]. In the case of the organic system, stable sols with the concentrations as large as 0.8 M can be obtained and hence, the preparation of films with 200 nm thickness by a single dip-coating process is possible. Therefore, the dip-coatings of pure In203 and then Sn-doped (ITO) films were investigated focusing on whether or not excellent ITO films can be deposited by single coating from the sol. The relationship among the dipcoating conditions (the sol composition and concentration, substrate withdrawal speed, the number of coating application, the kind of substrate and firing temperature), film morphologies and electrical properties was examined. In the present paper the results obtained through this study are described.
2. Experimental procedure Organic sols of ITO were prepared according to the method described previously [3]. In this study, however, n-propanol and tin octylate were used instead of 2-propanol and tin isopropoxide as the solvent and the precursor for tin-doping, respectively, because n-propanol yielded more stable sols than 2-propanoi and tin octylate had no harmful impurities such as sodium ions. Diethanolamine (DEA) was found to be the best additive for sol stabilization. Monoethanolamine (MEA) was examined in the place of DEA as another possible additive, expecting to afford better films, because it has less carbon content and, hence, is more volatile than DEA. But unfortunately MEA did not yield homogeneous sols. Indium concentrations in the ITO sols (with Sn-dopant 0 to 10 mol%) used in this study were in a range of 0.3 to 0.7 M. Using these sols, uniform and transparent ITO films with the total thickness 200 to 300 nm were dip-coated on glass plates (Coming #7059) with different withdrawal speeds (2 to 18 c m / m i n ) and different number of times of the coating cycle (coating-drying-firing). Firing was done at temperatures between 400 and 700°C in air for 30 min for each coating cycle. The film thickness per coating cycle was found to increase proportional with the square root of substrate
withdrawal speed as expected in the dip-coating process. Formed films were characterized with visible spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The estimation of film thickness and refractive index was made using the interference bands observed in the visible spectra. Sometimes the thicknesses were directly confirmed by SEM images of a cross-section. The errors of the film thickness value and hence refractive index are in 5%. The mean crystallite size (t) of the films was evaluated from line broadening of (222) XRD peak of ITO using Scherrer's equation: t = 0 . 9 A / ( B ~ - B 2 ) ' / 2 cos 0, where A is the wavelength of the X-ray ( C u - K a : 0.154 nm), 0 the diffraction angle, BM (in radian) the half width measured for the (222) peak and Bs (in radian) the half width of the standard at the angle 0. Silicon powder which was prepared by crashing a single crystal and annealing was used as the standard sample. The error of the estimated values is within 3% for small size around 10 nm and within 10% for large size around 40 nm. The resistivity data of all films were collected using a conventional instrument with four terminal probe. At least the resistivity data obtained by this method have an accuracy higher than 95%. For several typical samples including a commercially available sputter-coated film which was used as a reference, the conductivity, Hall mobility and carrier concentration were measured at room temperature with the van der Pauw method in a magnetic field of 0.5 to 1 T. In the latter measurement I n - S n solder was used as the contact between the film and Au wire leads. Here, it is interesting to note that this alloy solder could tightly adhere to the dip-coated films but not to the sputtered sample, indicative of some difference between them in respect to the film morphology or porosity. Therefore, in the latter case gold paste was applied to assist for soldering of the alloy. The data obtained by the van der Pauw method were reproducible, so that the errors of the resistivity and hence carrier concentration would be defined by those of the film thickness evaluation which are in the order of 5%. It was found that the resistivities of the as-prepared ITO films decreased during storage in air (about one order of the magnitude in one week) [3].
E Takahashi et al./Journal of Non-Crystalline Solids 218 (1997) 129-134
The resistivities observed about one week after the time of deposition are cited in this paper. The effect of post annealing of the films at 650°C in nitrogen atmosphere or in vacuo on the variation in the conductivity was examined for several samples.
131 10-1
~g
0.7M, Sn:4mol%
16 O
~t=
•
•
14 10-2 E"
w~
~ 12
O
:J.
~ lO
0
0
3. Results
0 I
In203 films (n = about 1.95) obtained from organic sols on glass plates (Coming Co. #7059) are less porous than those from aqueous sols and have greater electrical conductivities (for as prepared films, 13 to 90 S cm - ] ) than those from the aqueous sols [11-14]. Therefore, in this study, we exclusively examined the properties of ITO films prepared from organic sols. The effects of Sn-doping concentration on the properties of the three-time coated films prepared at 650°C from 0.5 M sol are tabulated in Table 1. This table shows that (1) the films are composed of fine crystallites of 12 to 24 nm in size, (2) the crystallite sizes and the refractive indices of the films decreased with the doping concentration, (3) the optimum doping concentration is about 4 mol% for the smallest resistivity, 1.7 × 10 - 3 ~-~ cm. All films are uniform to the eye and have a transmittance larger than 90% for visible light. The effects of firing temperature on the resistivities and the crystallite size of 4 mol% doped ITO films prepared from 0.7 M sol by two cycle coatings are traced in Fig. 1. Mean crystallite size evaluated from the half width of ITO (222) XRD peak increased from 10 to 15 nm with an increase in firing temperature from 400 to 700°C and simultaneously,
400
I
I
Sn conc. (mol%)
Resistivity (11 cm)
Refractive index (wavelength) (nm)
Crystallite size (nm)
0 2 4 7 10
5.7 × 10 -3 1.4× 10 -3 1.7 × 10 -3 2.0×10 3 2.7×10 3
1.84-1.95 (528-530) 1.81 (561) 1.78 (584) 1.74(620) 1.75(595)
24 19 17 14 12
700
Fig. 1. The effects of firing temperature on the resistivity and crystallite size of ITO films prepared from 0.7 M sol by two-time coating with the substrate withdrawal speed of 6 cm/min.
the resistivities decreased almost by one order of magnitude, suggesting that the resistivity mainly depends on the crystallite size, namely the mobility of the carrier but not on the cartier concentration, as will be shown later (Table 2). It is interesting to note that the crystallite size of the multi-layered films with total thickness of 200 to 300 nm was dependent on the number of the coating cycle as illustrated in Fig. 2. The mean crystallite sizes (40 to 50 nm) of multi-coated films of thin layers (about 20 nm) are four to five times larger than those (10 nm) of singly coated ones. This result may be ascribed to vaporization of the organic components from thin films during the heat treatment and, more importantly, to homoepitaxial effect on the crystallization of the oxide on the crystallized films with the same composition. The morphologies
50 Sn:4mol%
40 Table 1 The effect of tin-doping on the resistivity, refractive index (with the wavelength for the evaluation) and crystallite size of ITO films prepared at 650°C by three-time coating using 0.5 M sol
10-3
I
500 600 Firing temperature (°C)
o
6509C
30
r~
~, 20 I0 0
I
I
I
100
200
300
Thickness/coating (nm) Fig. 2. Relationship between the crystallite size and the thickness of layer per coating cycle in the multi-layered films with total thickness of about 200 nm.
132
E Takahashi et aL / Journal of Non-Crystalline Solids 218 (1997) 129-134
Table 2 Relationship between coating conditions, film thickness, crystallite size, resistivity, carrier concentration and mobility of ITO films Run
1 2 3 4 5 6 7 8
9 10 11 a 12 a 13 b
Sol conc. (M)
Sn conc. (mol%)
Coating rate (cm/min)
Application number
Deposition. temp. (°C)
Film thickness (nm)
Crystallite size (nm)
Resistivity (1) cm)
Carrier conc. (102°/cm 3)
Mobility (cm2/Vs)
0.5 0.5 0.5 0.7 0.3 0.7 0.7 0.7 0.7 0.7 0.7 0.4 -
0 4 10 4 4 4 4 4 4 4 4 6 ?
6 6 6 18 2 6 6 6 6 6 6 6 -
3 3 3 1 12 2 2 2 2 2 2 5 -
650 650 650 650 650 400 500 600 650 700 650 600 -
210 240 240 300 210 310 280 240 280 230 280 300 200
24 17 12 13 36 10 11 14 15 15 14 18 60
5.7 X 1.7 X 2.1 X 6.4 X 8.1 X 3.7 X 6.6X 2.2 x 1.8 x 1.3 X 5.8 X 3.3 x 1.6×
0.30 2.9 3.0 1.7 3.8 1.3 2.4 3.1 2.9 3.5 5.6 10.0 11.0
35 13 9.8 5.7 20 1.3 4.0 9.3 12 14 19 21 33
10 3 10 -3 10 3 10 3 10 -4 10 2 10 3 10 3 10 -3 10 -3 10 -4 10 -4 10 4
a Runs 11 and 12 denote the data of post-annealed samples. Run 11 is the data observed after storage for a long time in air after the treatment. Just after annealing, this sample had shown the resistivity of 4.0 X 10 -4 1) cm. Run 12 was cited from Ref. [3]. b The sample is a commercially available ITO film sputter-coated on 7059 glass. Film thickness and other data were evaluated by us.
of the multi-layered films also varied with the thick-
film obtained by
ness of each layer. Fig. 3a and b show typical SEM
w i t h a c o n c e n t r a t i o n as l o w a s 0.1 M w i t h 6 c m / m i n
photographs
of two
extreme
samples
prepared
at
15 c o a t i n g s u s i n g t h e d i l u t e d s o l
withdrawal speed. Both films have almost same film
6 0 0 ° C . O n e is s i n g l y d i p - c o a t e d I T O f i l m s f r o m t h e
thickness,
concentrated
seen that the multi-layered film has a well developed
withdrawal
organic sol (0.65 M) with the substrate s p e e d o f 18 c m / m i n
a n d t h e o t h e r is a
~ 2 0 0 n m . F r o m F i g . 3 it c a n b e c l e a r l y
c o l u m n a r s t r u c t u r e w h i l e s i n g l e - l a y e r e d f i l m is c o m -
Fig. 3. SEM photographs of the cross-sections of two extreme samples prepared at 600°C; (a) singly dip-coated ITO films from concentrated organic sol (0.65 M) with the substrate withdrawal speed of 18 c m / m i n and (b) the film obtained by 15-time coating from a diluted sol of 0.1 M concentration with 6 c m / m i n as the withdrawal speed.
Y. Takahashi et a l . / Journal of Non-Crystalline Solids 218 (1997) 129-134
Sn:4mol% 650°(2
5 X 10 -3
O0 ._~
• lO-3
10
! • • |8 210
• I
30
40
Crystallite size (nm) Fig. 4. Relationship between the resistivity and the crystallite size in the films prepared by varied conditions.
posed of particles with semi-spherical form and with sizes 15 to 20 nm. Ignoring the effect of coating conditions and firing temperatures, the relationship between the crystallite sizes and resistivities of many ITO films prepared in this study are plotted in Fig. 4. As Fig. 4 suggests, the resistivities decrease with the increase in the crystallite size. In order to obtain quantitative information about the effect of film morphology and dip-coating conditions on the resistivity, carrier concentration and mobility were measured for several typical samples and the observed data are summarized in Table 2. Table 2 indicates that the conductivities are mainly controlled by the mobility except for the effect of Sn-doping (run 1, 2 and 3) which caused an increase in carrier concentration as expected. As described above, the increase in the firing temperature causes an increase in crystallite size and simultaneously an increase in mobility (runs 6 to 10). Similarly, comparison between the data of runs 2, 4 and 5 shows that the largest crystallite size of 12-layered film (run 5) had the largest mobility of 2 0 c m Z / V s. In order to increase carrier concentration or the conductivity, some post-treatment such as annealing in vacuo, or in nitrogen atmosphere is required. With such treatment, a film with conductivities as large as 3000 S cm-1 were obtained at 650°C. The data for the two post annealed samples are shown on runs 11 and 12. The former had a resistivity of 4.0 X 10 - 4 cm just after the annealing, but the value increased to 5.8 × 10 4 f~ cm several months later from the time of the annealing (run 11).
133
For comparison with a sputtered film, the data (run 12) observed by us for a commercial sample are added to Table 2: crystallite size = ~ 60 nm, resistivity = 1.6 X 1 0 - 4 ~'~ cm, carrier concentration = 1.1 X 1 0 - 2 1 / c m 3 and mobility = 34 c m Z / V s. The mobility of the sputtered film is almost comparable to that of non-doped In203 (run 11 of Table 2) and somewhat larger than that of the dip-coated ITO films as shown in Table 2. Expecting some effect of pre-coating of the glass plate with a crystallized film which would assist the crystallization and growth of ITO by heteroepitaxial effect and would act as an effective barrier for the diffusion of some elements from the substrate [15], glass plates were coated in advance with TiO 2 or ZrO 2 before ITO coating application. But we could not find an effect of the pre-coating as expected, especially in the case of multiple coated films. In this study temperature profile during heating and cooling processes had no important effect on the film properties.
4. D i s c u s s i o n
The results obtained through this study indicate that the conductivity of dip-coated films are mainly controlled by the crystallite size and, hence, by carrier mobility, and that multiple coating of the layers as thin as a few tens nanometer can assist the growth of the ITO crystals and increased the conductivity of formed films. Thus, ITO films with a resistivity of 4 X 10 4 l) cm could be obtained by dip-coating and by subsequent post annealing in nitrogen. The acceleration of crystal growth by the multiple coatings would be ascribed to the effect of an epitaxial crystal growth which has been observed for dip-coating process [ 15-17]. The columnar morphology of multiple coating films also well supports this idea. Pre-coating of glass plates by a crystalline TiO 2 or ZrO 2 film had no effects on the crystal growth of multiple coated films. This result may be partly due to the homoepitaxial effect of the first ITO layer. The multiple coating process, however, may be too elaborate to be applied in an industrial scale.
134
Y. Takahashi et al. / Journal of Non-Co'stalline Solids 218 (1997) 129-134
Therefore, in order to m a k e the dip-coating process m o r e useful, s o m e breakthrough, for e x a m p l e , the d e v e l o p m e n t o f a sol system to e f f e c t i v e l y increase the rate o f crystal growth and to lead to full densification is required. A n o t h e r important concern is an increase in the resistivity during storage. This m a y be attributed to the porous nature of the dip-coated I T O films, as supposed f r o m a l o w e r refractive index (about 1.9) o f dip-coated I T O films. The d e v e l o p m e n t o f a sol system to produce w e l l - d e n s i f i e d I T O films w o u l d also be a better solution to this concern.
5. Conclusions W e c o n c l u d e that I T O films with a resistivity c o m p a r a b l e to that o f sputtered films can be prepared by multiple dip-coating process. Piling-up o f I T O layers using diluted sols or slow substrate withdrawal speed is an important processes for i m p r o v ing electrical properties. B e c a u s e dip-coating is a very simple and e c o n o m i c a l process and the films prepared by this process h a v e e x c e l l e n t electrical and optical properties, they are e x p e c t e d to h a v e their o w n usage, for example, as a key material o f touch panel.
Acknowledgements This w o r k was supported by G r a n t - i n - A i d for Scientific R e s e a r c h ( S u b j e c t No. 08455301), the
Ministry o f Education, Science, Sports and Culture, Japan.
References [1] N.J. Arfsten, R. Kaufmann, H. Dislich, in: Ultrastructure Processing of Ceramics, Glasses and Composites, ed. L.L. Hench and D.R. Ulrich (Wiley Interscience, New York, 1984) p. 189. [2] D.M. Mattox, Thin Solid Films 204 (1991) 25. [3] Y. Takahashi, H. Hayashi, Y. Ohya, Mater. Res. Soc. Symp. Proc. 271 (1992) 401. [4] M.J. van Bommel, T.N.M. Bernards, W. Talen, Mater. Res. Soc. Symp. Proc. 346 (1994) 469. [5] S. Ogihara, K. Kinugawa, Yogyo-Kyokai-Shi 90 (1982) 157. [6] T. Maruyama, A. Kojima, Jpn. J. Appl. Phys. 27 (1988) L 1829. [7] D. Gallagher, F. Scanlan, R. Houriet, H.J. Mathieu, T.A. Ring, J. Mater. Res. 8 (1993) 3135. [8] K. Nishio, T. Sei, T. Tsuchiya, J. Mater. Sci. 31 (1996) 1761. [9] T. Furusaki, K. Kodaira, M. Yamamoto, S. Shimada, T. Matsushita, Mater. Res. Bull. 21 (1986) 803. [10] J.J. Xu, A.S. Shaikh, R.W. Vest, Thin Solid Films 161 (1988) 273. [11] T. Furussaki, J. Takahashi, K. Kodaira, J. Ceram. Soc. Jpn. 102 (1994) 200. [12] T. Furusaki, K. Kodaira, High Performance Ceramic Films and Coatings, 1991, pp. 241-247. [13] Y. Takahashi, R. Bel Hadj Tahar, K. Shimaoka, T. Ban, Y. Ohya, in: Proc. Symp. of MRS Japan, May 1996, Trans. Mater. Res. Soc. Jpn. 20 (1996) 538. [14] Y. Takahashi, R. Bel Hadj Tahar, K. Shimaoka, T. Ban, Y. Ohya, J. Am. Ceram. Soc., submitted. [15] Y. Takahashi, Y. Wada, J. Electrochem Soc. 137 (1990) 267. [16] Y. Takahashi, K. Yamaguchi, J. Mater. Sci. 25 (1990) 3950. [17] Y. Takahashi, Y. Ohchi, T. Sasaki, High Performance Ceramic Films and Coatings, 1991, pp. 127-136.
J O U R N A L OF
t,l,ll
ELSEVIER
SO]
Journal of Non-Crystalline Solids 218 (1997) 129-134
Dip-coating of ITO films Yasutaka Takahashi *, Shinya Okada, Radhouane Bel Hadj Tahar, Ken Nakano, Takayuki Ban, Yutaka Ohya Department of Chemist~', Faculty' of Engineering, Gifu Uni~,ersitv, 1-1, Yanagido, Gifu 501 - 11, Japan
Abstract Uniform, transparent indium tin oxide (ITO) films were prepared by dip-coating process using an organic sol composed of indium acetate-diethanolamine-tin octylate-n-propanol mixture and the relationship between their electrical properties, film morphology and dip-coating conditions have been investigated. The optimum Sn-doping concentration was about 4 mol% relative to In ion. The conductivity of as-prepared ITO films increased with an increase in firing temperature. Multiple coating of the layers as thin as a few tens of nanometers accelerated the growth of the ITO crystals and increased the conductivity of formed films. Thus, ITO films with a resistivity of 4 X 10 4 1~ cm could be obtained by dip-coating and by subsequent post annealing in nitrogen. Through this study we conclude that the conductivity of dip-coated films was mainly controlled by the crystallite size and, hence, by carrier mobility. © 1997 Elsevier Science B.V.
1. Introduction
Indium tin oxide (ITO) is one of the indispensable materials for current optoelectric technologies and, therefore, the development of an economical ITO film production process other than the sputtering method which is currently used assumes significance. Dip or spin coating may be one of the best available production processes. So far several sol-gel or metallo-organic processes for ITO coatings have been reported in which alkoxides [1-4], acetylacetone-In(NO3) 3 mixture [5-8], acetate [3], 2-ethylhexanoate [9,10] and aqueous hydroxide sols [11-14] were used as the starting materials. Air-baked films had the smallest resistivity of ~ 2 X 10 3 1) cm [3,8] and a transparency greater than 90% for visible light. The resistivity can be further decreased by post
* Corresponding author. Tel.: + 81-58 293 2585; fax: + 81-58 230 1893; e-mail: [email protected].
annealing in vacuum or inert (or reducing) atmospheres [3,5,6,12]. It has been reported by Arfsten et al. [1] that ITO films with electrical and optical properties comparable to those of sputtered ones could be directly dip-coated on glass plates by suitable processing. In the dip-coating, however, sometimes multiple coatings, which are troublesome for practical applications, are required to obtain satisfactory film thickness, especially when a suitable sol with high concentration can not be obtained. Considering in practice that ITO films with the resistivity as small as 2 × 10 4 Ft cm are available, minimum film thickness of about 200 nm is required to attain minimum sheet resistance of I0 f ~ / [ ] for application to LC display, for example. Therefore, if a system is developed to produce films as thick as 200 nm with greater electrical conductivity by single dip-coating, it could be applied for the industrial production of ITO films. So far, our group has been studying the
0022-3093/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PH S0022-3093(97)001 99-3
130
Y. Takahashi et aL / Journal of Non-Crystalline Solids 218 (1997) 129 134
dip-coating process of ITO films using an organic sol composed of indium acetate-diethanolamine-npropanol (or 2-propanol) mixture [3] and an aqueous sol derived from indium nitrate [13,14]. In the case of the organic system, stable sols with the concentrations as large as 0.8 M can be obtained and hence, the preparation of films with 200 nm thickness by a single dip-coating process is possible. Therefore, the dip-coatings of pure In203 and then Sn-doped (ITO) films were investigated focusing on whether or not excellent ITO films can be deposited by single coating from the sol. The relationship among the dipcoating conditions (the sol composition and concentration, substrate withdrawal speed, the number of coating application, the kind of substrate and firing temperature), film morphologies and electrical properties was examined. In the present paper the results obtained through this study are described.
2. Experimental procedure Organic sols of ITO were prepared according to the method described previously [3]. In this study, however, n-propanol and tin octylate were used instead of 2-propanol and tin isopropoxide as the solvent and the precursor for tin-doping, respectively, because n-propanol yielded more stable sols than 2-propanoi and tin octylate had no harmful impurities such as sodium ions. Diethanolamine (DEA) was found to be the best additive for sol stabilization. Monoethanolamine (MEA) was examined in the place of DEA as another possible additive, expecting to afford better films, because it has less carbon content and, hence, is more volatile than DEA. But unfortunately MEA did not yield homogeneous sols. Indium concentrations in the ITO sols (with Sn-dopant 0 to 10 mol%) used in this study were in a range of 0.3 to 0.7 M. Using these sols, uniform and transparent ITO films with the total thickness 200 to 300 nm were dip-coated on glass plates (Coming #7059) with different withdrawal speeds (2 to 18 c m / m i n ) and different number of times of the coating cycle (coating-drying-firing). Firing was done at temperatures between 400 and 700°C in air for 30 min for each coating cycle. The film thickness per coating cycle was found to increase proportional with the square root of substrate
withdrawal speed as expected in the dip-coating process. Formed films were characterized with visible spectroscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD). The estimation of film thickness and refractive index was made using the interference bands observed in the visible spectra. Sometimes the thicknesses were directly confirmed by SEM images of a cross-section. The errors of the film thickness value and hence refractive index are in 5%. The mean crystallite size (t) of the films was evaluated from line broadening of (222) XRD peak of ITO using Scherrer's equation: t = 0 . 9 A / ( B ~ - B 2 ) ' / 2 cos 0, where A is the wavelength of the X-ray ( C u - K a : 0.154 nm), 0 the diffraction angle, BM (in radian) the half width measured for the (222) peak and Bs (in radian) the half width of the standard at the angle 0. Silicon powder which was prepared by crashing a single crystal and annealing was used as the standard sample. The error of the estimated values is within 3% for small size around 10 nm and within 10% for large size around 40 nm. The resistivity data of all films were collected using a conventional instrument with four terminal probe. At least the resistivity data obtained by this method have an accuracy higher than 95%. For several typical samples including a commercially available sputter-coated film which was used as a reference, the conductivity, Hall mobility and carrier concentration were measured at room temperature with the van der Pauw method in a magnetic field of 0.5 to 1 T. In the latter measurement I n - S n solder was used as the contact between the film and Au wire leads. Here, it is interesting to note that this alloy solder could tightly adhere to the dip-coated films but not to the sputtered sample, indicative of some difference between them in respect to the film morphology or porosity. Therefore, in the latter case gold paste was applied to assist for soldering of the alloy. The data obtained by the van der Pauw method were reproducible, so that the errors of the resistivity and hence carrier concentration would be defined by those of the film thickness evaluation which are in the order of 5%. It was found that the resistivities of the as-prepared ITO films decreased during storage in air (about one order of the magnitude in one week) [3].
E Takahashi et al./Journal of Non-Crystalline Solids 218 (1997) 129-134
The resistivities observed about one week after the time of deposition are cited in this paper. The effect of post annealing of the films at 650°C in nitrogen atmosphere or in vacuo on the variation in the conductivity was examined for several samples.
131 10-1
~g
0.7M, Sn:4mol%
16 O
~t=
•
•
14 10-2 E"
w~
~ 12
O
:J.
~ lO
0
0
3. Results
0 I
In203 films (n = about 1.95) obtained from organic sols on glass plates (Coming Co. #7059) are less porous than those from aqueous sols and have greater electrical conductivities (for as prepared films, 13 to 90 S cm - ] ) than those from the aqueous sols [11-14]. Therefore, in this study, we exclusively examined the properties of ITO films prepared from organic sols. The effects of Sn-doping concentration on the properties of the three-time coated films prepared at 650°C from 0.5 M sol are tabulated in Table 1. This table shows that (1) the films are composed of fine crystallites of 12 to 24 nm in size, (2) the crystallite sizes and the refractive indices of the films decreased with the doping concentration, (3) the optimum doping concentration is about 4 mol% for the smallest resistivity, 1.7 × 10 - 3 ~-~ cm. All films are uniform to the eye and have a transmittance larger than 90% for visible light. The effects of firing temperature on the resistivities and the crystallite size of 4 mol% doped ITO films prepared from 0.7 M sol by two cycle coatings are traced in Fig. 1. Mean crystallite size evaluated from the half width of ITO (222) XRD peak increased from 10 to 15 nm with an increase in firing temperature from 400 to 700°C and simultaneously,
400
I
I
Sn conc. (mol%)
Resistivity (11 cm)
Refractive index (wavelength) (nm)
Crystallite size (nm)
0 2 4 7 10
5.7 × 10 -3 1.4× 10 -3 1.7 × 10 -3 2.0×10 3 2.7×10 3
1.84-1.95 (528-530) 1.81 (561) 1.78 (584) 1.74(620) 1.75(595)
24 19 17 14 12
700
Fig. 1. The effects of firing temperature on the resistivity and crystallite size of ITO films prepared from 0.7 M sol by two-time coating with the substrate withdrawal speed of 6 cm/min.
the resistivities decreased almost by one order of magnitude, suggesting that the resistivity mainly depends on the crystallite size, namely the mobility of the carrier but not on the cartier concentration, as will be shown later (Table 2). It is interesting to note that the crystallite size of the multi-layered films with total thickness of 200 to 300 nm was dependent on the number of the coating cycle as illustrated in Fig. 2. The mean crystallite sizes (40 to 50 nm) of multi-coated films of thin layers (about 20 nm) are four to five times larger than those (10 nm) of singly coated ones. This result may be ascribed to vaporization of the organic components from thin films during the heat treatment and, more importantly, to homoepitaxial effect on the crystallization of the oxide on the crystallized films with the same composition. The morphologies
50 Sn:4mol%
40 Table 1 The effect of tin-doping on the resistivity, refractive index (with the wavelength for the evaluation) and crystallite size of ITO films prepared at 650°C by three-time coating using 0.5 M sol
10-3
I
500 600 Firing temperature (°C)
o
6509C
30
r~
~, 20 I0 0
I
I
I
100
200
300
Thickness/coating (nm) Fig. 2. Relationship between the crystallite size and the thickness of layer per coating cycle in the multi-layered films with total thickness of about 200 nm.
132
E Takahashi et aL / Journal of Non-Crystalline Solids 218 (1997) 129-134
Table 2 Relationship between coating conditions, film thickness, crystallite size, resistivity, carrier concentration and mobility of ITO films Run
1 2 3 4 5 6 7 8
9 10 11 a 12 a 13 b
Sol conc. (M)
Sn conc. (mol%)
Coating rate (cm/min)
Application number
Deposition. temp. (°C)
Film thickness (nm)
Crystallite size (nm)
Resistivity (1) cm)
Carrier conc. (102°/cm 3)
Mobility (cm2/Vs)
0.5 0.5 0.5 0.7 0.3 0.7 0.7 0.7 0.7 0.7 0.7 0.4 -
0 4 10 4 4 4 4 4 4 4 4 6 ?
6 6 6 18 2 6 6 6 6 6 6 6 -
3 3 3 1 12 2 2 2 2 2 2 5 -
650 650 650 650 650 400 500 600 650 700 650 600 -
210 240 240 300 210 310 280 240 280 230 280 300 200
24 17 12 13 36 10 11 14 15 15 14 18 60
5.7 X 1.7 X 2.1 X 6.4 X 8.1 X 3.7 X 6.6X 2.2 x 1.8 x 1.3 X 5.8 X 3.3 x 1.6×
0.30 2.9 3.0 1.7 3.8 1.3 2.4 3.1 2.9 3.5 5.6 10.0 11.0
35 13 9.8 5.7 20 1.3 4.0 9.3 12 14 19 21 33
10 3 10 -3 10 3 10 3 10 -4 10 2 10 3 10 3 10 -3 10 -3 10 -4 10 -4 10 4
a Runs 11 and 12 denote the data of post-annealed samples. Run 11 is the data observed after storage for a long time in air after the treatment. Just after annealing, this sample had shown the resistivity of 4.0 X 10 -4 1) cm. Run 12 was cited from Ref. [3]. b The sample is a commercially available ITO film sputter-coated on 7059 glass. Film thickness and other data were evaluated by us.
of the multi-layered films also varied with the thick-
film obtained by
ness of each layer. Fig. 3a and b show typical SEM
w i t h a c o n c e n t r a t i o n as l o w a s 0.1 M w i t h 6 c m / m i n
photographs
of two
extreme
samples
prepared
at
15 c o a t i n g s u s i n g t h e d i l u t e d s o l
withdrawal speed. Both films have almost same film
6 0 0 ° C . O n e is s i n g l y d i p - c o a t e d I T O f i l m s f r o m t h e
thickness,
concentrated
seen that the multi-layered film has a well developed
withdrawal
organic sol (0.65 M) with the substrate s p e e d o f 18 c m / m i n
a n d t h e o t h e r is a
~ 2 0 0 n m . F r o m F i g . 3 it c a n b e c l e a r l y
c o l u m n a r s t r u c t u r e w h i l e s i n g l e - l a y e r e d f i l m is c o m -
Fig. 3. SEM photographs of the cross-sections of two extreme samples prepared at 600°C; (a) singly dip-coated ITO films from concentrated organic sol (0.65 M) with the substrate withdrawal speed of 18 c m / m i n and (b) the film obtained by 15-time coating from a diluted sol of 0.1 M concentration with 6 c m / m i n as the withdrawal speed.
Y. Takahashi et a l . / Journal of Non-Crystalline Solids 218 (1997) 129-134
Sn:4mol% 650°(2
5 X 10 -3
O0 ._~
• lO-3
10
! • • |8 210
• I
30
40
Crystallite size (nm) Fig. 4. Relationship between the resistivity and the crystallite size in the films prepared by varied conditions.
posed of particles with semi-spherical form and with sizes 15 to 20 nm. Ignoring the effect of coating conditions and firing temperatures, the relationship between the crystallite sizes and resistivities of many ITO films prepared in this study are plotted in Fig. 4. As Fig. 4 suggests, the resistivities decrease with the increase in the crystallite size. In order to obtain quantitative information about the effect of film morphology and dip-coating conditions on the resistivity, carrier concentration and mobility were measured for several typical samples and the observed data are summarized in Table 2. Table 2 indicates that the conductivities are mainly controlled by the mobility except for the effect of Sn-doping (run 1, 2 and 3) which caused an increase in carrier concentration as expected. As described above, the increase in the firing temperature causes an increase in crystallite size and simultaneously an increase in mobility (runs 6 to 10). Similarly, comparison between the data of runs 2, 4 and 5 shows that the largest crystallite size of 12-layered film (run 5) had the largest mobility of 2 0 c m Z / V s. In order to increase carrier concentration or the conductivity, some post-treatment such as annealing in vacuo, or in nitrogen atmosphere is required. With such treatment, a film with conductivities as large as 3000 S cm-1 were obtained at 650°C. The data for the two post annealed samples are shown on runs 11 and 12. The former had a resistivity of 4.0 X 10 - 4 cm just after the annealing, but the value increased to 5.8 × 10 4 f~ cm several months later from the time of the annealing (run 11).
133
For comparison with a sputtered film, the data (run 12) observed by us for a commercial sample are added to Table 2: crystallite size = ~ 60 nm, resistivity = 1.6 X 1 0 - 4 ~'~ cm, carrier concentration = 1.1 X 1 0 - 2 1 / c m 3 and mobility = 34 c m Z / V s. The mobility of the sputtered film is almost comparable to that of non-doped In203 (run 11 of Table 2) and somewhat larger than that of the dip-coated ITO films as shown in Table 2. Expecting some effect of pre-coating of the glass plate with a crystallized film which would assist the crystallization and growth of ITO by heteroepitaxial effect and would act as an effective barrier for the diffusion of some elements from the substrate [15], glass plates were coated in advance with TiO 2 or ZrO 2 before ITO coating application. But we could not find an effect of the pre-coating as expected, especially in the case of multiple coated films. In this study temperature profile during heating and cooling processes had no important effect on the film properties.
4. D i s c u s s i o n
The results obtained through this study indicate that the conductivity of dip-coated films are mainly controlled by the crystallite size and, hence, by carrier mobility, and that multiple coating of the layers as thin as a few tens nanometer can assist the growth of the ITO crystals and increased the conductivity of formed films. Thus, ITO films with a resistivity of 4 X 10 4 l) cm could be obtained by dip-coating and by subsequent post annealing in nitrogen. The acceleration of crystal growth by the multiple coatings would be ascribed to the effect of an epitaxial crystal growth which has been observed for dip-coating process [ 15-17]. The columnar morphology of multiple coating films also well supports this idea. Pre-coating of glass plates by a crystalline TiO 2 or ZrO 2 film had no effects on the crystal growth of multiple coated films. This result may be partly due to the homoepitaxial effect of the first ITO layer. The multiple coating process, however, may be too elaborate to be applied in an industrial scale.
134
Y. Takahashi et al. / Journal of Non-Co'stalline Solids 218 (1997) 129-134
Therefore, in order to m a k e the dip-coating process m o r e useful, s o m e breakthrough, for e x a m p l e , the d e v e l o p m e n t o f a sol system to e f f e c t i v e l y increase the rate o f crystal growth and to lead to full densification is required. A n o t h e r important concern is an increase in the resistivity during storage. This m a y be attributed to the porous nature of the dip-coated I T O films, as supposed f r o m a l o w e r refractive index (about 1.9) o f dip-coated I T O films. The d e v e l o p m e n t o f a sol system to produce w e l l - d e n s i f i e d I T O films w o u l d also be a better solution to this concern.
5. Conclusions W e c o n c l u d e that I T O films with a resistivity c o m p a r a b l e to that o f sputtered films can be prepared by multiple dip-coating process. Piling-up o f I T O layers using diluted sols or slow substrate withdrawal speed is an important processes for i m p r o v ing electrical properties. B e c a u s e dip-coating is a very simple and e c o n o m i c a l process and the films prepared by this process h a v e e x c e l l e n t electrical and optical properties, they are e x p e c t e d to h a v e their o w n usage, for example, as a key material o f touch panel.
Acknowledgements This w o r k was supported by G r a n t - i n - A i d for Scientific R e s e a r c h ( S u b j e c t No. 08455301), the
Ministry o f Education, Science, Sports and Culture, Japan.
References [1] N.J. Arfsten, R. Kaufmann, H. Dislich, in: Ultrastructure Processing of Ceramics, Glasses and Composites, ed. L.L. Hench and D.R. Ulrich (Wiley Interscience, New York, 1984) p. 189. [2] D.M. Mattox, Thin Solid Films 204 (1991) 25. [3] Y. Takahashi, H. Hayashi, Y. Ohya, Mater. Res. Soc. Symp. Proc. 271 (1992) 401. [4] M.J. van Bommel, T.N.M. Bernards, W. Talen, Mater. Res. Soc. Symp. Proc. 346 (1994) 469. [5] S. Ogihara, K. Kinugawa, Yogyo-Kyokai-Shi 90 (1982) 157. [6] T. Maruyama, A. Kojima, Jpn. J. Appl. Phys. 27 (1988) L 1829. [7] D. Gallagher, F. Scanlan, R. Houriet, H.J. Mathieu, T.A. Ring, J. Mater. Res. 8 (1993) 3135. [8] K. Nishio, T. Sei, T. Tsuchiya, J. Mater. Sci. 31 (1996) 1761. [9] T. Furusaki, K. Kodaira, M. Yamamoto, S. Shimada, T. Matsushita, Mater. Res. Bull. 21 (1986) 803. [10] J.J. Xu, A.S. Shaikh, R.W. Vest, Thin Solid Films 161 (1988) 273. [11] T. Furussaki, J. Takahashi, K. Kodaira, J. Ceram. Soc. Jpn. 102 (1994) 200. [12] T. Furusaki, K. Kodaira, High Performance Ceramic Films and Coatings, 1991, pp. 241-247. [13] Y. Takahashi, R. Bel Hadj Tahar, K. Shimaoka, T. Ban, Y. Ohya, in: Proc. Symp. of MRS Japan, May 1996, Trans. Mater. Res. Soc. Jpn. 20 (1996) 538. [14] Y. Takahashi, R. Bel Hadj Tahar, K. Shimaoka, T. Ban, Y. Ohya, J. Am. Ceram. Soc., submitted. [15] Y. Takahashi, Y. Wada, J. Electrochem Soc. 137 (1990) 267. [16] Y. Takahashi, K. Yamaguchi, J. Mater. Sci. 25 (1990) 3950. [17] Y. Takahashi, Y. Ohchi, T. Sasaki, High Performance Ceramic Films and Coatings, 1991, pp. 127-136.