Indium tin oxide thin films prepared by thermal decomposition of ethylene glycol solution
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Michio Inagaki Department of Applied Chemistry, Faculty of Engineering, Hokkaido University, Kita-Ku, Sapporo, 060 Japan (Received 15 February 1992; accepted 20 April 1992)
Indium tin oxide (ITO) films were prepared on a glass substrate by thermal decomposition of ethylene glycol (EG) solutions which were prepared by dissolving indium and tin chlorides into ethylene glycol and then heating the solution at 200 °C. Homogeneous ITO film with a thickness of 1 fim without any cracks was obtained from the EG solution with a viscosity of about 50 cSt by spin coating, with the rotation of the substrate at 2000 rpm. The crystallization of the film thus obtained occurred above 400 °C. A well-crystallized ITO film was obtained by heat-treating at 600 °C. The resistivity of the ITO film obtained on a glass substrate showed a minimum value of 1.1 X 10" 4 ohm cm at about 7 at. % Sn.
I. INTRODUCTION
II. EXPERIMENTAL
Transparent conductive oxide films have many applications; for example, the materials with high transmittance in visible light region but high reflectance in the infrared portion are important for radiation insulation and solar energy collection. Recently, the indium-tin oxide (ITO) film has attracted much attention because of its high transparency and low electrical resistivity, and so several preparation procedures have been proposed using various techniques, such as vacuum vapor deposition, r.f. sputtering, d.c. sputtering, r.f. plating, etc. 1 5 These physical vapor deposition (PVD) methods require complex equipment and high vacuum conditions. In contrast, the thermal decomposition of metal alkoxides and metal complex salts can give ITO films with comparable transmittance by using relatively simple equipment under atmospheric pressure.6'7 This method has the possibility for easily forming patterned transparent electrodes by applying printing techniques. Electrical resistivity of ITO films is strongly dependent on preparation conditions such as heat-treatment, atmosphere, and raw materials. Some investigators have reported that their films must be heat-treated in a reducing atmosphere to achieve low resistivity, whereas others indicated that no heattreatment was necessary.
A. Preparation of ITO film The procedure employed in the present work for preparing ITO thin films is shown as a block diagram in Fig. 1. The raw materials, indium and tin chlorides of reagent grade, were dissolved separately into ethylene glycol (EG) which had been purified by distillation. The concentration of each chloride ranged from 0.1 to 0.5 mol/1. These two EG solutions were mixed, making the tin content in the resulting solution a constant, InCl, + Ethylene glycol
SnCl
+ Ethylene glycol
I
Mixing under stirring at 200 °C Concentration by heating at 200 °C
I Spin coating ( 1000-3000 rpm )
I 200 °C, 1 hour, in air
300-600 °C, 1 hour, in air
In the present work, an ethylene glycol solution of indium and tin chlorides was used as a starting material for preparing ITO film. The crystallization of ITO film at temperatures ranging f
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