Dielectric Characterization of Thin Films Consisting of Tin Doped Indium Oxide Nanoparticles
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ABSTRACT In recent years there has been a growing interest in cheap and easy production techniques for transparent conducting thin films. One way of making these uses a nanoparticle dispersion. We prepared thin films of tin doped indium oxide by spin-coating of a solution of nanoparticles. The sintering behavior of these ceramic particles was studied by dielectric spectroscopy and by grazing incidence X-ray diffraction. The grain growth was found to start at 1000°C and to be prominent at 1250'C. However, the electrical conductivity reached a maximum below these temperatures.
INTRODUCTION There are a number of technological applications in which transparent conducting thin films of materials such as tin doped indium oxide (ITO), aluminium doped zinc oxide, or fluorine doped tin oxide are used, e.g. in light emitting diodes [1], heat mirrors [2], and other kinds of energy efficient window applications [3, 4]. Sometimes there is a need for a patterned structure, e.g. for solar cells and liquid crystal displays. Today's standard production methods sputtering, chemical vapour deposition, or spray pyrolysis - followed by lithography and etching have drawbacks in long production times, high costs, and hazardous by-products. Much would be gained if instead the thin film could be printed in a one-step procedure [5-7]. A dispersion of nanoparticles can be used as an "ink" in such a process. In the present work we
study the conduction mechanisms and sintering properties of spin coated thin films consisting of nanoparticles of ITO. We believe that these layers have the same properties as printed layers. This paper presents data on grain growth as a function of sintering temperature, Ts, from grazing incidence X-ray diffraction (XRD). We also report impedance spectroscopy results taken under the application of an AC voltage of variable frequency to the film. The latter technique provides useful information on charge transfer and polarisation phenomena [8].
EXPERIMENTAL Our samples were prepared from an ITO nanopowder doped with tin, with the Sn/(In+Sn) ratio being 0.05. The powder was dispersed in water and spin-coated onto glass, A120 3 , and fused silica substrates. These samples were then sintered in air at different temperatures, followed by reduction in N 2+H2 gas by annealing at TA = 480' C (Table I). The temperature in the oven was increased by 8 to 10°/min until the final temperature was attained. The sintering or annealing temperature was then kept constant for 30 minutes. The samples were cooled down with the furnace shut off. Film thicknesses between 41 and 87 nm were calculated from X-ray fluorescence measurements. These data correspond to the mass thickness and do not account for the porosity. Visual inspection revealed that the adhesion of the film to the substrate was good 491 Mat. Res. Soc. Symp. Proc. Vol. 581 © 2000 Materials Research Society
Table I: Heat treatment and resistivity values. Ts (TA) denotes sintering (annealing) temperature. Post heating for reduction in N 2 + 7 vol% H 2 TA Resistivity (°C)
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