Tin dioxide thin films prepared by chemical vapor deposition from tin(II) acetylacetonate

Solar Energy Materials and Solar Cells 28 (1992) 209-215 North-Holland

Solar Energy Materials and Solar Cells

Tin dioxide thin films prepared by chemical vapor deposition from tin(II) acetylacetonate Toshiro M a r u y a m a and Yoshiaki Ikuta Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606, Japan Received 20 February 1992; in revised form 8 July 1992 Tin dioxide thin films were prepared by a low-temperature atmospheric-pressure chemical vapor deposition method in air. The raw material was tin(II) acetylacetonate. At a reaction temperature above 230°C, polycrystalline thin films were obtained with a high deposition rate. When the thickness of film is larger than 1000 nm, the texture coefficients show a dominant [002] orientation, which is independent of both substrate temperature and vapor pressure of source material.

1. Introduction

Tin dioxide (SnO 2) is an n-type semiconductor which has many applications. In particular, SnO 2 thin films doped with antimony or fluorine are widely used in practice as transparent conductive films. The advantages of SnO 2 films are high chemical and mechanical stabilities even at high temperatures. These advantages find some applications in modern optoelectronic devices, such as solar cells. High quality SnO 2 film has been prepared by spray pyrolysis and chemical vapor deposition (CVD) methods [1-3]. The CVD method uses tin(IV) compounds, such as, organotin [1,2] and SnCI 4 [3] as a tin precursor. Recently, we [4] proposed to use tin(II) complexes in the preparation of transparent conductive SnO2 films. We obtained SnO 2 films with high electric conductivity by using CVD of tin(II) acetate at relatively low deposition temperature in air. In this paper, tin(II) acetylacetonate is proposed as a source material for obtaining SnO 2 films. Tin(II) acetylacetonate is nontoxic and easy to handle in liquid form at room temperature. It has a high vapor pressure and a low thermal decomposition temperature. The preparation conditions and structure of SnO2 film will be discussed by comparing those of SnO 2 films prepared from tin(II) acetate. Particular emphasis is placed on the crystalline orientation of the film. The texture coefficient of the films defined by Barret and Massalski [5] will be used to describe the preferred orientation. Correspondence to: T. Maruyama, Department of Chemical Engineering, Faculty of Engineering, Kyoto University, Yoshida-Honmachi, Sakyo-ku, Kyoto 606, Japan. 0927-0248/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved

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2. Experimental

Tin(II) acetylacetonate (Sn(CsH702)2, Matsumoto Chemical Industry Co., Ltd.) was used as the source material for preparing SnO 2. Sn(CsH702) 2 was heated at temperatures of 60-150°C. The generated gases were entrained by carrier gases, which were mainly nitrogen. The flow rate of the carrier gas was 0.3 t/min. Borosilicate glass plates and quartz glass plates were used as the substrates, The substrate was placed on a temperature-controlled electric heater. The substrate temperature ranged from 100°C to 600°C. The depositions were carried out mainly in air. (Preparations of SnO film in an inert atmosphere were made by using a closed tube reactor.) The composition of the film was measured by X-ray photoelectron spectroscopy. The crystallinity of the film was analyzed by the X-ray diffraction method with Cu K s radiation. The morphology of the film was measured by scanning electron microscopy. The electric resistivity of the film was measured by the van der Pauw method. The carrier concentration and mobility were measured by using the Hall effect.

3. Results and discussion

In an inert (nitrogen) atmosphere, no film could be obtained. When n 2 0 vapor was mixed with the source gas, however, SnO films were obtained through hydrolysis reaction of Sn(CsH702) 2, i.e. Sn(CsH702) z + H 2 0 ~ SnO + 2 C5H80 z. The byproduct acetylacetone C5H802 is not toxic. Fig. 1 shows a typical example of the X-ray diffraction pattern of the SnO film on a borosilicate glass substrate at a substrate temperature of 300°C. The pattern indicates that the SnO film is composed of crystallites of high crystallinity and with strong (00l) (l = 1, 2, 4) plane texturing. Thus, the thin SnO film was oriented with its (001) direction perpendicular to the plane of the glass substrate. 5

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T. Maruyama, Y. Ikuta / Tin dioxide thin films

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In air, transparent SnO 2 films were obtained at substrate temperatures above 100°C. Thus, similar to the CVD from Sn(II) acetate [4], oxygen gas makes a role in oxidation of Sn(II) into Sn(IV). The lower limit of the substrate temperature is lower than that for Sn(II) acetate (200°C) [4]. Fig. 2 shows the Arrhenius plot of deposition rates of SnO2, which were obtained at source temperatures of 60°C and 120°C and at a carrier gas flow rate of 0.3 //min. The values of the deposition rate are of the same order of magnitude as those reported previously using Sn(II) acetate. Fig. 3 shows the deposition rates as a function of source temperature. An exponential increase of the deposition 100

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T. Maruyama, Y. lkuta / Tin dioxide thin films

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Table 1 Resistivity, mobility and carrier concentration of SnO 2 film Substrate temperature (°C)

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Mobility ( c m 2 / V s)

Carrier concentration (cm - 3)

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4.28 X 1019 5.42 × 1019 6 . 3 3 x 1019 1.35 x 1019

rates with increasing source temperature suggests that the deposition rates is proportional to the vapor pressure of S n ( C s H 7 0 2 ) 2. The X-ray diffraction measurements showed that the film obtained at a substrate temperature above 230°C was polycrystalline. Table 1 shows the resistivity, mobility and carrier concentration for some polycrystalline SnO 2 films. These

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Fig. 5. Texture coefficients as a function of film thickness for (a) substrate temperature 300°C and source temperature 60°C, (b) substrate temperature 300°C and source temperature 120°C and (c) substrate temperature 500"C and source temperature 60°C.

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T. Maruyama, Y Ikuta / Tin dioxide thin films

Fig. 6. Cleaved profile of a scanning electron micrograph of SnO 2 film.

values are comparable to those for films prepared by other CVD methods [1,3]. Figs. 4a and 4b show typical examples of the X-ray diffraction pattern of the film on a borosilicate glass substrate at a substrate temperature of 300°C. The patterns indicate that the SnO 2 films are composed of crystallites with tetragonal rutile structure. Fig. 4a reveals a non-oriented polycrystaUine nature for the 305 nm thick film, while Fig. 4b shows prominent (002) peak for the 1719 nm thick film. Figs. 5 a - 5 c show the texture coefficients [5], texture coefficient ( h k l ) =

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as a function of film thickness. At thicknesses less than about 200 nm, the film is non-oriented on the amorphous substrate. With increasing film thickness from 200 to 1000 nm, the (002) plane texturing increases and the (103) plane texturing keeps a constant value, while another plane texturing decreases. At thicknesses larger than 1000 nm, the texture coefficients show a dominating [002] orientation of the films, which is independent of both substrate temperature and vapor pressure of the source material. From these results, the preferred orientation is inferred to be controlled by the same mechanism, that is, it is not due to initial nucleation on a bare substrate but due to oriented overgrowth as a result of preferred nucleation on the growing surface. Fig. 6 shows the cleaved profile of a scanning electron micrograph of the SnO 2 film. Evidently, there appears an oriented columnar grain structure perpendicular to the glass substrate.

T. Maruyama, Y. Ikuta / Tin dioxide thin films

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4. Conclusions Tin dioxide thin films were prepared by a low-temperature atmospheric-pressure chemical vapor deposition method in air. The raw material was tin(II) acetylacetonate. At a substrate temperature above 230°C, polycrystalline thin films were obtained with a high deposition rate. When the thickness of film is larger than 1000 nm, the texture coefficients show a dominant (002) orientation, which is independent of both substrate temperature and vapor pressure of source material.

Acknowledgements This work was supported by the Iketani Science and Technology Foundation, Yazaki Science and Technology Foundation, Nippon Sheet Glass Foundation, General Sekiyu Research & Development Encouragement & Assistance Foundation and a Grant-in-Aid for Scientific Research on Priority Area from the Ministry of Education, Science and Culture. The authors would like to thank K. Nakanishi of Nippon Sheet Glass Co., Ltd. for the Hall coefficient measurements.

References [1] [2] [3] [4] [5]

R. Mutoh and S. Furuuchi, Oyo Buturi 41 (1972) 41. T.P. Chow, M. Ghezzo and B.J. Baliga, J. Electrochem. Soc. 129 (1982) 1040. A.K. Saxena, R. Thangaraj, S.P. Singh and O.P. Agnihotri, Thin Solid Films 131 (1985) 121. T. Maruyama and K. Tabata, J. Appl. Phys. 68 (1990) 4282. C. Barret and T.B. Massalski, Structure of Metals (Pergamon Press, Oxford, 1980) p. 204.