Electrical Property of Transparent TiO 2 :Nb / ZnO:Ga Layered Film
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Electrical Property of Transparent TiO2:Nb / ZnO:Ga Layered Film Seiji Ichiyanagi1, Yasuji Yamada1, Shuhei Funaki1 1 Department of Physics and Materials Science, Shimane University, 1060 Nishikawatsu, Matsue 690-8504, Japan.
ABSTRACT Nb-doped TiO2 (TNO) films, Ga-doped ZnO (GZO) films and TNO/GZO layered films were fabricated on glass substrates and electrical properties of TNO/GZO layered films were investigated in terms of interaction between TNO and GZO layers. By a thermal annealing in vacuum, the observed resistivity of the TNO/GZO layered films was lower than that of the single layered films fabricated and annealed at the same conditions. The resistivity reduction observed in the layered structure is not explained by the parallel connection of the TNO and GZO layers, indicating that there exists an interaction between these two layers. The TNO/GZO films with low resistivity have still been transparent. INTRODUCTION Doped zinc oxides such as Ga doped ZnO (GZO) is one of the most promising materials that substitutes indium-tin oxide (ITO) among some potential candidates of transparent conducting oxides (TCO) because of its low electrical resistivity and high transparency. Many researches for practical applications have been conducted and reported that areal inhomogeneity in electrical property occurs when the magnetron sputtering method is used for the production [15]. Inhomogeneity in property is significantly removed by heat treatment. Proper heat treatment improves the conductivity of doped ZnO [6,7], however, high temperature annealing in the ambient atmosphere as used for the production of dye-sensitized solar cell causes fatal degradation of its electrical and optical properties. Improvements of chemical stability with areal homogeneity increase potential application of doped ZnO. Nb-doped titanium oxide (TNO) in anatase form also exhibits high conductivity and transparency [8]. Low resistivity of TNO is achieved by room temperature deposition and thermal annealing at temperatures over 350°C with the phase transformation from amorphous to anatase. The lowest value of resistivity reported reached below 1x10-4 Ωcm on a single crystalline substrate. However, in the case on a glass substrate, which is used for typical applications, the value is typically more than 1x10-3 Ωcm, higher than doped ZnO films. Anatase has been well known and utilized as a photocatalysis material before the discovery of excellent electrical property of TNO, indicating chemical and thermal stability. Therefore, ZnO films covered by anatase layer can provide conductive, transparent and chemically durable TCO films. In the course of the research of layered films with TNO and GZO layers, we have found that the TNO/GZO composite film anomalously reduces its resistivity below that of each layer of TNO and GZO. Relevant behavior was reported on an Al-doped ZnO film capped by Si layer
for solar cell application [9]. In this paper, we report thorough investigation of the resistivity of TNO/GZO films in comparison with TNO and GZO films. EX
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