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Liquid phase deposition film of tin oxide
Journal of Non-Crystalline Solids 210 Ž1997. 48–54
Liquid phase deposition film of tin oxide Koji Tsukuma b
a,)
, Tomoyuki Akiyama a , Hiroaki Imai
b
a Tsukuba Research Laboratory, Tosoh Corporation, 43, Miyukigaoka, Tsukuba-shi, Ibaragi-Ken 305, Japan Department of Applied Chemistry, Faculty of Science and Technology, Keio UniÕersity, 3-14-1 Hiyoshi, Yokohama 223, Japan
Received 15 February 1994; revised 11 March 1996
Abstract The thin film of tin oxide was formed in the solution containing 0.005–0.3 molrl SnF2 . The procedure of film formation was very simple; the solution, in which a substrate is immersed, is maintained above 408C for tens of hours. In this method, the hydrolysis product of SnF2 deposited as the film on a substrate. As-deposition film included 6–16 mol% fluorine. The chemical component was deduced as SnO 2y05 x Fx , where 0.17 - =- 0.5. The film was modified to pure SnO 2 by heating above 3008C. The electrical conductivity was improved to 1.4 = 10y2 V cm by heating at 5008C. The model of liquid phase deposition was proposed to extend another oxide film.
1. Introduction The thin film of tin oxide has been utilized for wide application such as transparent conductive film and infrared cutting filter. The various methods, spraying thermal decomposition w1x, sputtering w2x, chemical vapor deposition w3x and sol–gel w4–6x, have been developed to produce the tin oxide film. The liquid phase deposition method Žso-called LPD method. is a new technique to produce the thin film. Nagayama et al. w7x succeeded in forming the silica film by LPD method. In this method, silica is deposited on a substrate by the addition of H 3 BO 3 into the H 2 SiF6 solution saturated with SiO 2 . The LPD method has been extended to another oxide film in their pioneering work. For example, TiO 2 and SnO 2 films were obtained by a similar method using the solution systems of H 2TiF6 q H 3 BO 3 w8x and H 2 SnF6 q H 3 BO 3 w9x respectively. In the case of )
Corresponding author. Fax: q81-298 501 044.
SnO 2 film, the film formation has been obtained by maintaining the H 2 SnF6 solution supersaturated with SnO 2 at 358C. The supersaturation was attained by addition of a small amount of H 3 BO 3 or SnCl 4 into H 2 SnF6 solution prepared by dissolving stannous oxide into HF solution. These previous studies seem to be done in the general manner, that is, the deposition components are the reaction product between MF62y ion and additives or the precipitation product from the supersaturating H 2 MF6 solution with metal oxide. It is expected that these reactants or precipitates should include considerable amount of fluorine, and may be written as MO 2y 0.5 x ŽOH, F. x . We considered that this intermediate product with fluorine should play an essential role on the deposition phenomena. In this study, the simple reaction, hydrolysis of metal fluoride, was applied to lead to the intermediate product with fluorine. The purpose of this study is to provide the simple technique producing the tin oxide film by the deposition in a liquid phase.
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 8 3 - 2
K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
49
2. Experimental The flow diagram of film formation is depicted in Fig. 1. The reagent SnF2 was used as a starting material. The starting solution was prepared by dissolving SnF2 in pure water at room temperature. The concentration of Sn was varied between 0.005 and 0.5 molrl. The range of 0.005–0.2 molrl was preferably adopted to form the film. The substrates were immersed in the solution. Three kinds of substrates, silica glass, soda–lime glass and silicon wafer, were used. The film formation was carried out by maintaining a plastic vessel containing solution and substrate at constant temperature between 40 and 808C. The vessel was wrapped in polyethylene film. Above 1008C, the teflon-coated autoclave was utilized as a vessel. The solution became a little cloudy after several hours above 408C, which is due to precipitation of hydrolysis product of SnF2 . After keeping for tens of hours, the substrate was removed, and washed with water, and then dried at 608C. The heat treatment of film was performed at each temperature, 300, 500, and 8008C, in air for 1 h. The gravimetric analysis was conducted to deduce the fraction of precipitates occurred by hydrolysis reaction of SnF2 . The film was observed to estimate the thickness by SEM. The composition of film was analyzed by EPMA ŽJCMA-733 JEOL.. The crystalline phase was evaluated by X-ray diffraction method. The electrical conductivity of film was measured by means of the four-probe method.
Fig. 1. Flow diagram of film formation procedure.
The preparation conditions such as SnF2 concentration and holding temperature were examined in detail. The result is shown in Fig. 3. The SnF2 concentration and temperature are major factors determining whether the film is formed or not. The film could be deposited on a substrate in concentration varying between 0.005 and 0.3 molrl, but not observed in 0.5 molrl. The deposition rate increased with increasing SnF2 concentration, with increased acceleration at a concentration of more than 0.03 molrl, as shown in Fig. 4. The growth rate reached 50 nmrh under conditions of 0.2 molrl and 808C. The temperature also influenced the deposition rate. The deposition rate was enhanced with increasing temperature, for example, 2 nmrh at 408C, 20 nmrh at 608C and 30 nmrh at 808C in 0.03 molrl SnF2 solution. As shown in Fig. 3, when the keeping temperature of the solution was higher than 608C, film damage was often observed. The early damaging phenomena was the partial corrosion of film, with later the complete disappearance of film.
3. Experimental results 3.1. Film formation We found that the thin film can be deposited on a substrate in a dilute SnF2 solution. SEM photograph of as-deposition film is shown in Fig. 2. This film was obtained in the solution with 0.03 molrl SnF2 , after 15 h at 608C. X-ray diffraction pattern of the film fired at 3008C corresponded to the crystalline SnO 2 with the rutile-type structure. These pre-studies disclosed that there is a simple method with which the thin film of tin oxide can be produced by maintaining a substrate in a dilute tin fluoride solution.
Fig. 2. Scanning electron micrograph of the film deposited in SnF2 solution.
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K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
Fig. 3. Preparation conditions of film formation. The region surrounded by a square shows that the film could be formed. The boundary, – P – and indicate that the film on soda– lime glass Žor silicon. and that on silica glass were damaged above the lines, respectively.
The fraction of precipitates which occurred in the solution during film formation is shown in Fig. 5. The membrane Žpore size 0.05 mm. was used to discrete precipitates. It is obvious that these precipitates are formed by hydrolysis reaction of SnF2 . The fraction of precipitates increased as the solution became dilute. The fraction of precipitates was low under overall conditions, less than 15% in the solution with 0.01–0.17 molrl. Even after 48 h holding of 0.17 molrl solution at 808C, the fraction showed
Fig. 4. Relation between film thickness and SnF2 concentration. Preparation condition: `: 608C, 24 h. ^: 808C, 24 h.
Fig. 5. Fraction of precipitated particles formed by hydrolysis. Preparation condition: `: 408C, 109 h. ^: 808C, 15 h.
only a small increment of 3%. This result suggests that very fine colloids of hydrolysis product remained in the solution, except for precipitates. The significance of this hydrolysis behavior will be discussed in the Section 4.
Fig. 6. X-ray diffraction patterns of the precipitated particles dried at room temperature. ŽA. dependence on SnF2 concentration, ŽB. dependence on holding temperature.
K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
51
the common principle of crystal growth, that is, in a dilute solution, the generating number of nuclei is so small that each nucleus can keep a gradual growth, which gives high degree of crystallinity. 3.2. Characterization of film
Fig. 7. Influence of SnF2 concentration on F content in the film.
X-ray diffraction patterns of precipitated particles dried at room temperature are shown in Fig. 6. The result indicates that the hydrolysis products are composed of the crystalline tin oxide with rutile-type structure. The degree of crystallinity increased with decreasing Sn concentration. This is interpreted by
The result of EPMA analysis for as-deposition film is shown in Table 1. Sn, 0 and F were detected for all samples. It is noticed that as-deposition film contains as much as 6–16 mol% fluorine. As shown in Fig. 7, the fluorine content was closely related to SnF2 concentration in the solution. The fluorine in film increased from 6 to 16 mol% when SnF2 in the solution increased from 0.005 to 0.3 molrl. If the structure was described by Eq. Ž1., 6 mol%F and 16 mol%F should be deduced to be n s 4 and n s 1, respectively:
Ž1.
It is noted that fluorine in the film never exceeds 16 mol%. This suggests that the possible structure unit is that F bonding with Sn exists alternately. As-deposition film was heated at temperatures between 300–8008C for 1 h in air. The compositional change in the film is shown in Table 1. Fluorine was removed by heating above 3008C. The pure SnO 2 film is constructed by heating. X-ray diffraction patterns of the fired film are shown in
Fig. 8. X-ray diffraction patterns of the fired film.
Table 1 Composition of the deposition film No.
1 2 3 4 5 6 7 8
SnF2 sol. Žmolrl.
Synthesis temp. Ž8C.
time Žh.
0.01 0.03
60 60
24 24
0.10
60
24
0.20 0.30
60 60
24 24
Film treatment
As-dep As-dep 3008C 5008C As-dep 5008C As-dep As-dep
Composition F-content Žmol%.
SnO 2y x r2 Fx 2 y xr2
x
FrO molar ratio Ž%.
6.4 8.9 0.4 0.0 15.5 0.0 14.8 15.2
1.90 1.86 2.00 2.00 1.75 2.00 1.76 1.75
0.20 0.27 0.01 0.00 0.50 0.00 0.48 0.50
0.11 0.20 0.01 0.00 0.29 0.00 0.27 0.29
K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
52
Table 2 Electrical conductivity of the film fired at 5008C No.
1 2 3 4 5
SnF2 sol. Žmolrl.
0.005 0.030 0.030 0.100 0.200
Synthesis temp. Ž8C.
time Žh.
80 60 60 60 60
15 15 24 24 24
Film treatment
Conductivity Ž=10y2 V cm.
5008C 5008C 5008C 5008C 5008C
18 1.5 1.4 2.6 2.6
Fig. 8. The diffraction peaks corresponding to rutiletype structure became sharp with elevated temperature. This is due to an increase in crystallinity. The electrical conductivity of the film fired at 5008C is shown in Table 2. The values of specific resistance were in a range of 1.4–2.6 = 10y2 V cm. The dependence on preparation conditions could not be clearly recognized. The conductivity of as-deposition film was too high to measure the values.
4. Discussion The tentative scheme of reaction route for film formation is illustrated in Fig. 9. The hydrolysis reaction of SnF2 is well-known as chemical property of its aqueous solution. SnF2 is easily soluble in water, the solubility is 42 gr100 g at room temperature, but hydrolyzed gradually by heating. This hydrolysis reaction possesses the property that highly corrosive hydrofluoric acid is by-produced with for-
mation of stannic acid. As shown in Fig. 9, the hydrolysis product must be the stannous oxide with fluorine, SnO 2y x r2yy r2 ŽOH. x Fy . This material will form the precipitates by polymerization. However, as shown in Fig. 5, the probability reaching to the precipitates is relatively low. The majority of stannous oxide remains in an earlier stage of polymerization, that is, oligomer or monomer. It is probable that the corrosive action of hydrofluoric acid is helpful in maintaining such a colloidal solution. The hydrofluoric acid introduces the complex reactions such as decomposition, reaction and ion exchange reaction, written by Eqs. Ž2. and Ž3., respectively: 2SnO 2 q 2HF | 2SnO1.5 F q H 2 O SnO Ž OH . q HF | SnO1.5 F q H 2 O
Ž 2. Ž 3.
The decomposition reaction Eq. Ž2. plays a role in retarding polymerization. Thus, fine units such as oligomer or monomer can remain in a solution. Such a solution state is supposed to be necessary for the film deposition. It is thought that the film damaging phenomena, shown in Fig. 3, is due to the corrosive force of hydrofluoric acid. At higher temperatures, hydrofluoric acid behaves by dissolving not only colloidal particles of stannous oxide, but also the freshly deposited film. The damaging phenomenon was dependent on the kind of substrate material; with sodalime glass or silicon it became difficult to obtain the film at higher temperature compared to silica glass. It is expected that hydrofluoric acid makes a corrosive attack on a substrate before the film deposition.
Fig. 9. Reaction route for deposition film.
K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
The previous studies w7–10x, showed that the deposition film of metal oxides such as SiO 2 , TiO 2 and SnO 2 can be obtained in a liquid phase, which is called the LPD method. LPD studies indicated that the film can be deposited by using the method of deriving the hydrolysis oxides from MF62y ion ŽM s Si, Ti, Sn. in H 2 MF6 solution. Two techniques were used to get hydrolysis oxide in H 2 MF6 solution. One was utilization of the reaction between H 2 MF6 and additives ŽH 3 BO 3 , AlCl 3 etc.., for example, H 2 SiF6 q 2H 3 BO 3 ™ SiO 2 q 2BF3 H 2 O q 2H 2 O. The other was the deposition of hydrolysis oxide from H 2 MF6 solution supersaturated with oxides. This case was limited in only the H 2 SiF2 system, in which the supersaturated state was brought from a difference in saturated solubility of SiO 2 by temperature change w10x. The present study showed that the deposition film can be obtained from a more simple reaction, hydrolysis of metal fluoride solution. According to our recent study w11x, this simple reaction could be applied to obtain more different kinds of oxide film such as TiO 2 , Sb 2 O5 , In 2 O 3 and SnO 2 doped with Sb 2 O5 . The present study is different in the method of deriving the hydrolysis material from the LPD method. However, it appears that the materials participating in film deposition are very similar, probably the fine colloids Žsuch as oligomer or monomer. of MO 2y x r2 ŽOH. x with fluorine. The tentative model of film deposition is illustrated in Fig. 10. The model is inferred from analogy to the adsorption mechanism of surfactant on a solid surface w12x. The oligomer, whose surface is covered with fluorine, prefers to attach onto a solid surface because the fluorine gives hydrophobic affinity to the oligomer. These oligomers polymerize on a substrate with each other by means of ion exchange reaction between F and OH and dehydrationrcondensation reaction.
Fig. 10. Tentative model of deposition on a substrate. ŽA. Adsorption due to chemical affinity, ŽB. F–OH ion exchange, ŽC. dehydration, ŽD. deposition of colloidal particle.
53
It is attractive to discuss the comparisons between the present study and the previous studies on the electroless deposition technique w12–14x. The electroless deposition technique was applied to produce many kinds of oxides film, In 2 O 3 , ZnO, SnO 2 , Cd 2 SnO4 etc. The basic principle of this technique is based on the controlled homogeneous precipitation of metal hydroxides facilitated by the slow reaction of anions and cations using freezing agents such as NH 4 F, NH 3 , etc. For example, stannic acid film was deposited on a substrate by adding NaOH into the mixed solution of tin chloride, freezing agent ŽNH 4 F. and catalyst ŽAgNO 3 .. It is apparent that the basic reaction in this technique is neutralization, on the other hand that in ours is hydrolysis. However, the common concept exists in both techniques; only the very fine colloids, which are in a stable dispersion state in a solution, are allowed to join to the film deposition. The electrical property of tin oxide film prepared by sol–gel method has been reported on the previous studies w4,5x. In comparison with the conductivity of the sol–gel film, the deposition film, which underwent the same heat treatment Žfired at 5008C., showed a slightly higher value. This suggests that maybe more densified and well crystallized film can be formed by the deposition method.
5. Conclusion It was found that a thin film of tin oxide can be formed in a simple solution system containing only SnF2 Ž0.005–0.3 molrl.. The thin film was formed by the deposition of hydrolysisrpolymerization products of SnF2 . The film growth was strongly dependent on the preparation conditions, SnF2 concentration in a solution and keeping temperature. The increase in SnF2 concentration and temperature enhanced the growth rate but the trend toward the film damage was intensified owing to an increase in corrosive power of solution. As-deposition film had a characteristic feature, that was that a large amount of fluorine, 6–16 mol%, was included in the film. The fluorine content was related to the preparation conditions and especially the SnF2 concentration. The maximum limited value of 16 mol% suggested that structurally an Sn–F bond exists alternately in
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K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
Sn–O network. The film fired at 5008C showed the electrical conductivity of 1–3 = 10y2 V cm.
References w1x E. Shanthi and V. Dutta, J. Appl. Phys. 51 Ž1980. 6243. w2x H.W. Lehmann and R. Widmer, Thin Solid Films 27 Ž1975. 359. w3x J. Kane and W. Kern, J. Electrochem. Soc. 123 Ž1976. 270. w4x T. Tsuchiya and J. Koizumi, J. Ceram. Soc. Jpn. 98 Ž1990. 1011. w5x A. Tsunashima, H. Honda, K. Kodaira, S. Shimada and T. Matsushita, J. Mater. Sci. 21 Ž1986. 2731. w6x G.J.R. Gonzalez–Oliver and I. Kato, J. Non-Cryst. Solids 82 Ž1986. 400. w7x H. Nagayama, H. Honda and H. Kawahara, J. Electrochem. Soc. 135 Ž1988. 2013.
w8x H. Ino, M. Hishinuma, H. Nagayama and H. Kawahara, Japanese Patent H1-93443 Ž1989.; H. Nagata, Japanese Patent, H426516 Ž1992.; H. Nagata and M. Hishinima, Japanese Patent, H3-285821, H3-285822 Ž1991.; H. Kawahara and H. Honda, Japanese Patent, S59-141441 Ž1984.. w9x H. Ino, M. Hishinuma, H. Nagayama and H. Kawahara, Japanese Patent S63-134667 Ž1988.. w10x Y. Sakai, T. Goda, A. Hishinuma and H. Kawahara, in: Proc. of the Int. Ceramics Conf., Perth, Western Australia, AUSTCERAM 90, 26–31 Aug. 1990, p. 474. w11x K. Tsukuma and T. Akiyama, J. Am. Ceram. Soc., to be published. w12x D. Raviendra and J.K. Sharma, J. Appl. Phys. 58 Ž2. 15 Ž1985. 838. w13x W. Mindt, J. Electron. Soc. 117 Ž1970. 615. w14x Y.L. Chen, S. Chen, C. Frank and J. Israelachivili, J. Solid Interf. Sci. 153 Ž1992. 244.
Liquid phase deposition film of tin oxide Koji Tsukuma b
a,)
, Tomoyuki Akiyama a , Hiroaki Imai
b
a Tsukuba Research Laboratory, Tosoh Corporation, 43, Miyukigaoka, Tsukuba-shi, Ibaragi-Ken 305, Japan Department of Applied Chemistry, Faculty of Science and Technology, Keio UniÕersity, 3-14-1 Hiyoshi, Yokohama 223, Japan
Received 15 February 1994; revised 11 March 1996
Abstract The thin film of tin oxide was formed in the solution containing 0.005–0.3 molrl SnF2 . The procedure of film formation was very simple; the solution, in which a substrate is immersed, is maintained above 408C for tens of hours. In this method, the hydrolysis product of SnF2 deposited as the film on a substrate. As-deposition film included 6–16 mol% fluorine. The chemical component was deduced as SnO 2y05 x Fx , where 0.17 - =- 0.5. The film was modified to pure SnO 2 by heating above 3008C. The electrical conductivity was improved to 1.4 = 10y2 V cm by heating at 5008C. The model of liquid phase deposition was proposed to extend another oxide film.
1. Introduction The thin film of tin oxide has been utilized for wide application such as transparent conductive film and infrared cutting filter. The various methods, spraying thermal decomposition w1x, sputtering w2x, chemical vapor deposition w3x and sol–gel w4–6x, have been developed to produce the tin oxide film. The liquid phase deposition method Žso-called LPD method. is a new technique to produce the thin film. Nagayama et al. w7x succeeded in forming the silica film by LPD method. In this method, silica is deposited on a substrate by the addition of H 3 BO 3 into the H 2 SiF6 solution saturated with SiO 2 . The LPD method has been extended to another oxide film in their pioneering work. For example, TiO 2 and SnO 2 films were obtained by a similar method using the solution systems of H 2TiF6 q H 3 BO 3 w8x and H 2 SnF6 q H 3 BO 3 w9x respectively. In the case of )
Corresponding author. Fax: q81-298 501 044.
SnO 2 film, the film formation has been obtained by maintaining the H 2 SnF6 solution supersaturated with SnO 2 at 358C. The supersaturation was attained by addition of a small amount of H 3 BO 3 or SnCl 4 into H 2 SnF6 solution prepared by dissolving stannous oxide into HF solution. These previous studies seem to be done in the general manner, that is, the deposition components are the reaction product between MF62y ion and additives or the precipitation product from the supersaturating H 2 MF6 solution with metal oxide. It is expected that these reactants or precipitates should include considerable amount of fluorine, and may be written as MO 2y 0.5 x ŽOH, F. x . We considered that this intermediate product with fluorine should play an essential role on the deposition phenomena. In this study, the simple reaction, hydrolysis of metal fluoride, was applied to lead to the intermediate product with fluorine. The purpose of this study is to provide the simple technique producing the tin oxide film by the deposition in a liquid phase.
0022-3093r97r$17.00 Copyright q 1997 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 3 0 9 3 Ž 9 6 . 0 0 5 8 3 - 2
K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
49
2. Experimental The flow diagram of film formation is depicted in Fig. 1. The reagent SnF2 was used as a starting material. The starting solution was prepared by dissolving SnF2 in pure water at room temperature. The concentration of Sn was varied between 0.005 and 0.5 molrl. The range of 0.005–0.2 molrl was preferably adopted to form the film. The substrates were immersed in the solution. Three kinds of substrates, silica glass, soda–lime glass and silicon wafer, were used. The film formation was carried out by maintaining a plastic vessel containing solution and substrate at constant temperature between 40 and 808C. The vessel was wrapped in polyethylene film. Above 1008C, the teflon-coated autoclave was utilized as a vessel. The solution became a little cloudy after several hours above 408C, which is due to precipitation of hydrolysis product of SnF2 . After keeping for tens of hours, the substrate was removed, and washed with water, and then dried at 608C. The heat treatment of film was performed at each temperature, 300, 500, and 8008C, in air for 1 h. The gravimetric analysis was conducted to deduce the fraction of precipitates occurred by hydrolysis reaction of SnF2 . The film was observed to estimate the thickness by SEM. The composition of film was analyzed by EPMA ŽJCMA-733 JEOL.. The crystalline phase was evaluated by X-ray diffraction method. The electrical conductivity of film was measured by means of the four-probe method.
Fig. 1. Flow diagram of film formation procedure.
The preparation conditions such as SnF2 concentration and holding temperature were examined in detail. The result is shown in Fig. 3. The SnF2 concentration and temperature are major factors determining whether the film is formed or not. The film could be deposited on a substrate in concentration varying between 0.005 and 0.3 molrl, but not observed in 0.5 molrl. The deposition rate increased with increasing SnF2 concentration, with increased acceleration at a concentration of more than 0.03 molrl, as shown in Fig. 4. The growth rate reached 50 nmrh under conditions of 0.2 molrl and 808C. The temperature also influenced the deposition rate. The deposition rate was enhanced with increasing temperature, for example, 2 nmrh at 408C, 20 nmrh at 608C and 30 nmrh at 808C in 0.03 molrl SnF2 solution. As shown in Fig. 3, when the keeping temperature of the solution was higher than 608C, film damage was often observed. The early damaging phenomena was the partial corrosion of film, with later the complete disappearance of film.
3. Experimental results 3.1. Film formation We found that the thin film can be deposited on a substrate in a dilute SnF2 solution. SEM photograph of as-deposition film is shown in Fig. 2. This film was obtained in the solution with 0.03 molrl SnF2 , after 15 h at 608C. X-ray diffraction pattern of the film fired at 3008C corresponded to the crystalline SnO 2 with the rutile-type structure. These pre-studies disclosed that there is a simple method with which the thin film of tin oxide can be produced by maintaining a substrate in a dilute tin fluoride solution.
Fig. 2. Scanning electron micrograph of the film deposited in SnF2 solution.
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K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
Fig. 3. Preparation conditions of film formation. The region surrounded by a square shows that the film could be formed. The boundary, – P – and indicate that the film on soda– lime glass Žor silicon. and that on silica glass were damaged above the lines, respectively.
The fraction of precipitates which occurred in the solution during film formation is shown in Fig. 5. The membrane Žpore size 0.05 mm. was used to discrete precipitates. It is obvious that these precipitates are formed by hydrolysis reaction of SnF2 . The fraction of precipitates increased as the solution became dilute. The fraction of precipitates was low under overall conditions, less than 15% in the solution with 0.01–0.17 molrl. Even after 48 h holding of 0.17 molrl solution at 808C, the fraction showed
Fig. 4. Relation between film thickness and SnF2 concentration. Preparation condition: `: 608C, 24 h. ^: 808C, 24 h.
Fig. 5. Fraction of precipitated particles formed by hydrolysis. Preparation condition: `: 408C, 109 h. ^: 808C, 15 h.
only a small increment of 3%. This result suggests that very fine colloids of hydrolysis product remained in the solution, except for precipitates. The significance of this hydrolysis behavior will be discussed in the Section 4.
Fig. 6. X-ray diffraction patterns of the precipitated particles dried at room temperature. ŽA. dependence on SnF2 concentration, ŽB. dependence on holding temperature.
K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
51
the common principle of crystal growth, that is, in a dilute solution, the generating number of nuclei is so small that each nucleus can keep a gradual growth, which gives high degree of crystallinity. 3.2. Characterization of film
Fig. 7. Influence of SnF2 concentration on F content in the film.
X-ray diffraction patterns of precipitated particles dried at room temperature are shown in Fig. 6. The result indicates that the hydrolysis products are composed of the crystalline tin oxide with rutile-type structure. The degree of crystallinity increased with decreasing Sn concentration. This is interpreted by
The result of EPMA analysis for as-deposition film is shown in Table 1. Sn, 0 and F were detected for all samples. It is noticed that as-deposition film contains as much as 6–16 mol% fluorine. As shown in Fig. 7, the fluorine content was closely related to SnF2 concentration in the solution. The fluorine in film increased from 6 to 16 mol% when SnF2 in the solution increased from 0.005 to 0.3 molrl. If the structure was described by Eq. Ž1., 6 mol%F and 16 mol%F should be deduced to be n s 4 and n s 1, respectively:
Ž1.
It is noted that fluorine in the film never exceeds 16 mol%. This suggests that the possible structure unit is that F bonding with Sn exists alternately. As-deposition film was heated at temperatures between 300–8008C for 1 h in air. The compositional change in the film is shown in Table 1. Fluorine was removed by heating above 3008C. The pure SnO 2 film is constructed by heating. X-ray diffraction patterns of the fired film are shown in
Fig. 8. X-ray diffraction patterns of the fired film.
Table 1 Composition of the deposition film No.
1 2 3 4 5 6 7 8
SnF2 sol. Žmolrl.
Synthesis temp. Ž8C.
time Žh.
0.01 0.03
60 60
24 24
0.10
60
24
0.20 0.30
60 60
24 24
Film treatment
As-dep As-dep 3008C 5008C As-dep 5008C As-dep As-dep
Composition F-content Žmol%.
SnO 2y x r2 Fx 2 y xr2
x
FrO molar ratio Ž%.
6.4 8.9 0.4 0.0 15.5 0.0 14.8 15.2
1.90 1.86 2.00 2.00 1.75 2.00 1.76 1.75
0.20 0.27 0.01 0.00 0.50 0.00 0.48 0.50
0.11 0.20 0.01 0.00 0.29 0.00 0.27 0.29
K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
52
Table 2 Electrical conductivity of the film fired at 5008C No.
1 2 3 4 5
SnF2 sol. Žmolrl.
0.005 0.030 0.030 0.100 0.200
Synthesis temp. Ž8C.
time Žh.
80 60 60 60 60
15 15 24 24 24
Film treatment
Conductivity Ž=10y2 V cm.
5008C 5008C 5008C 5008C 5008C
18 1.5 1.4 2.6 2.6
Fig. 8. The diffraction peaks corresponding to rutiletype structure became sharp with elevated temperature. This is due to an increase in crystallinity. The electrical conductivity of the film fired at 5008C is shown in Table 2. The values of specific resistance were in a range of 1.4–2.6 = 10y2 V cm. The dependence on preparation conditions could not be clearly recognized. The conductivity of as-deposition film was too high to measure the values.
4. Discussion The tentative scheme of reaction route for film formation is illustrated in Fig. 9. The hydrolysis reaction of SnF2 is well-known as chemical property of its aqueous solution. SnF2 is easily soluble in water, the solubility is 42 gr100 g at room temperature, but hydrolyzed gradually by heating. This hydrolysis reaction possesses the property that highly corrosive hydrofluoric acid is by-produced with for-
mation of stannic acid. As shown in Fig. 9, the hydrolysis product must be the stannous oxide with fluorine, SnO 2y x r2yy r2 ŽOH. x Fy . This material will form the precipitates by polymerization. However, as shown in Fig. 5, the probability reaching to the precipitates is relatively low. The majority of stannous oxide remains in an earlier stage of polymerization, that is, oligomer or monomer. It is probable that the corrosive action of hydrofluoric acid is helpful in maintaining such a colloidal solution. The hydrofluoric acid introduces the complex reactions such as decomposition, reaction and ion exchange reaction, written by Eqs. Ž2. and Ž3., respectively: 2SnO 2 q 2HF | 2SnO1.5 F q H 2 O SnO Ž OH . q HF | SnO1.5 F q H 2 O
Ž 2. Ž 3.
The decomposition reaction Eq. Ž2. plays a role in retarding polymerization. Thus, fine units such as oligomer or monomer can remain in a solution. Such a solution state is supposed to be necessary for the film deposition. It is thought that the film damaging phenomena, shown in Fig. 3, is due to the corrosive force of hydrofluoric acid. At higher temperatures, hydrofluoric acid behaves by dissolving not only colloidal particles of stannous oxide, but also the freshly deposited film. The damaging phenomenon was dependent on the kind of substrate material; with sodalime glass or silicon it became difficult to obtain the film at higher temperature compared to silica glass. It is expected that hydrofluoric acid makes a corrosive attack on a substrate before the film deposition.
Fig. 9. Reaction route for deposition film.
K. Tsukuma et al.r Journal of Non-Crystalline Solids 210 (1997) 48–54
The previous studies w7–10x, showed that the deposition film of metal oxides such as SiO 2 , TiO 2 and SnO 2 can be obtained in a liquid phase, which is called the LPD method. LPD studies indicated that the film can be deposited by using the method of deriving the hydrolysis oxides from MF62y ion ŽM s Si, Ti, Sn. in H 2 MF6 solution. Two techniques were used to get hydrolysis oxide in H 2 MF6 solution. One was utilization of the reaction between H 2 MF6 and additives ŽH 3 BO 3 , AlCl 3 etc.., for example, H 2 SiF6 q 2H 3 BO 3 ™ SiO 2 q 2BF3 H 2 O q 2H 2 O. The other was the deposition of hydrolysis oxide from H 2 MF6 solution supersaturated with oxides. This case was limited in only the H 2 SiF2 system, in which the supersaturated state was brought from a difference in saturated solubility of SiO 2 by temperature change w10x. The present study showed that the deposition film can be obtained from a more simple reaction, hydrolysis of metal fluoride solution. According to our recent study w11x, this simple reaction could be applied to obtain more different kinds of oxide film such as TiO 2 , Sb 2 O5 , In 2 O 3 and SnO 2 doped with Sb 2 O5 . The present study is different in the method of deriving the hydrolysis material from the LPD method. However, it appears that the materials participating in film deposition are very similar, probably the fine colloids Žsuch as oligomer or monomer. of MO 2y x r2 ŽOH. x with fluorine. The tentative model of film deposition is illustrated in Fig. 10. The model is inferred from analogy to the adsorption mechanism of surfactant on a solid surface w12x. The oligomer, whose surface is covered with fluorine, prefers to attach onto a solid surface because the fluorine gives hydrophobic affinity to the oligomer. These oligomers polymerize on a substrate with each other by means of ion exchange reaction between F and OH and dehydrationrcondensation reaction.
Fig. 10. Tentative model of deposition on a substrate. ŽA. Adsorption due to chemical affinity, ŽB. F–OH ion exchange, ŽC. dehydration, ŽD. deposition of colloidal particle.
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It is attractive to discuss the comparisons between the present study and the previous studies on the electroless deposition technique w12–14x. The electroless deposition technique was applied to produce many kinds of oxides film, In 2 O 3 , ZnO, SnO 2 , Cd 2 SnO4 etc. The basic principle of this technique is based on the controlled homogeneous precipitation of metal hydroxides facilitated by the slow reaction of anions and cations using freezing agents such as NH 4 F, NH 3 , etc. For example, stannic acid film was deposited on a substrate by adding NaOH into the mixed solution of tin chloride, freezing agent ŽNH 4 F. and catalyst ŽAgNO 3 .. It is apparent that the basic reaction in this technique is neutralization, on the other hand that in ours is hydrolysis. However, the common concept exists in both techniques; only the very fine colloids, which are in a stable dispersion state in a solution, are allowed to join to the film deposition. The electrical property of tin oxide film prepared by sol–gel method has been reported on the previous studies w4,5x. In comparison with the conductivity of the sol–gel film, the deposition film, which underwent the same heat treatment Žfired at 5008C., showed a slightly higher value. This suggests that maybe more densified and well crystallized film can be formed by the deposition method.
5. Conclusion It was found that a thin film of tin oxide can be formed in a simple solution system containing only SnF2 Ž0.005–0.3 molrl.. The thin film was formed by the deposition of hydrolysisrpolymerization products of SnF2 . The film growth was strongly dependent on the preparation conditions, SnF2 concentration in a solution and keeping temperature. The increase in SnF2 concentration and temperature enhanced the growth rate but the trend toward the film damage was intensified owing to an increase in corrosive power of solution. As-deposition film had a characteristic feature, that was that a large amount of fluorine, 6–16 mol%, was included in the film. The fluorine content was related to the preparation conditions and especially the SnF2 concentration. The maximum limited value of 16 mol% suggested that structurally an Sn–F bond exists alternately in
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Sn–O network. The film fired at 5008C showed the electrical conductivity of 1–3 = 10y2 V cm.
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