Stability of nano-thick transparent conducting oxide films for use in a moist environment
Introduction
Most transparent electrode applications in flat panel displays (FPDs) in practical use feature Sn-doped In2O3 (ITO) and ZnO–In2O3 multicomponent oxide thin films. There are, however, problems associated with their use. In particular, the indium in the ITO used in liquid crystal displays (LCDs) constitutes the largest use of this material. However, a stable supply of indium, the principal material of these In2O3-based oxides, may be difficult to sustain for the recently expanding market for FPDs and solar cells because of its cost and scarcity. Recently, polycrystalline impurity-doped ZnO materials such as Al- or Ga-doped ZnO (AZO or GZO) have attracted much attention as promising substitutes for In2O3-based oxides such as ITO [1], [2], [3], [4], [5], [6], [7]. However, the use of these materials may be a problem since depositions of transparent electrodes on color filters in LCD applications require the preparation of thin-film transparent electrodes with a thickness below approximately 100 nm at a low temperature below 200 °C; it has been reported that ZnO-based oxide thin films deposited on low temperature substrates are more unstable in oxidizing environments than ITO thin films [8], [9], [10], [11]. Therefore, in order to be a practical alternative to ITO transparent electrodes, the resistivity of AZO and GZO thin films prepared at the required low temperatures must be stable in activated oxidizing environments such as an atmosphere with high relative humidity. Although ITO thin-film transparent electrodes prepared with a thickness below 50 nm are in practical use in many applications, there are few reports regarding the stability of transparent conducting In2O3- or ZnO-based oxide thin films in various environments.
In this paper, we describe the stability of the electrical properties of nano-thick transparent conducting ZnO-based and In2O3-based oxide thin films in a high-relative humidity environment: ITO, AZO and GZO thin films prepared with a thickness in the range from approximately 20 to 100 nm on glass substrates at a temperature below 200 °C by a pulsed laser deposition.
Section snippets
Experimental
Long-term stability tests, up to 1000 h, were carried out in a high-humidity environment: air at a relative humidity of 90% and a temperature of 60 °C. As-deposited transparent conducting oxide thin films used in the test were prepared with a thickness in the range from approximately 20 to 100 nm on glass (OA-10, Nippon Electric Glass Co. Ltd.) substrates at a temperature in the range from approximately 65 to 200 °C. The substrates temperature was monitored near rotating substrate. The AZO and
Thickness dependence of resistivity stability
It has been reported that the resistivity of transparent conducting impurity-doped ZnO thin films prepared by magnetron sputtering depositions (MSD) increased as the film thickness was decreased below approximately 300 nm [2], [13], [14]. In particular, this thickness dependence of resistivity was considerably accentuated with depositions on low temperature substrates; the extent of this dependence was strongly dependent on the film deposition method used [2], [15]. In addition, it was found
Conclusion
The stability of as-deposited transparent conducting oxide thin films was evaluated by testing in air at a relative humidity of 90% and a temperature of 60 °C. It was found that the resistivity of AZO and GZO thin films tested increased markedly with test time, whereas that of ITO remained relatively stable. Although the stability (resistivity increase) in AZO and GZO thin films was found to be considerably affected by film thickness, it was relatively independent of the deposition temperature
Acknowledgments
The authors wish to acknowledge Mr. S. Tsukada for their technical assistance in the experiments. This work was partially supported by a private university High-Tech Research Center project matching fund subsidy from MEXT (Ministry of Education, Culture, Sports, Science and Technology), 2005–2007.
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