Transparent Conductive In-doped Cd 3 TeO 6 Thin Films with Perovskite Structure Deposited by Radio Frequency Magnetron S
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Kiyotaka Wasa Faculty of Science, Yokohama City University, Nara 631-0045, Japan (Received 10 April 2005; accepted 15 June 2005)
Transparent conductive In-doped Cd3TeO6 thin films were deposited on silica glass substrate by radio frequency magnetron sputtering using targets composed of CdO, TeO2, and In2O3 powders, and their electrical and optical properties were examined. The electrical resistivity of 3.2 × 10−3 ⍀cm and an average transmittance above 80% in the visible region (400–800 nm) were obtained for the films deposited at the substrate temperature above 300 °C. The maximum optical band gap was 3.92 eV for the film deposited at 700 °C, demonstrating a large optical band gap comparable to indium tin oxide.
Transparent conducting oxides (TCO), such as indium tin oxide (ITO) and tin oxide (SnO2) etc., are substances having a unique combination of high optical transparency in the visible region and excellent electrical transport properties.1–5 Several years ago, we focused on an ordered perovskite oxide, Cd3TeO6,6,7 and found it possesses some remarkable features similar to the above oxides: (i) a low resistivity (∼10−1 ⍀cm), (ii) a wide band gap (∼2.8 eV), and (iii) all the composed positive ions have a same closed-shell configuration (4d105s0). It is very clear that Cd3TeO6 could be a transparent conductor. We recently succeeded in making transparent conductive Cd3TeO6 thin films by radio frequency (rf) magnetron sputtering. The electrical resistivity of 1.9 × 10−2 ⍀cm and an averaged transmittance above 85% in the visible region (400–800 nm) were obtained after annealing the film at 500 °C. Their carrier density and Hall mobility were 8.7 × 1019 cm−3 and 6.8 cm2 V−1 s−1, respectively. Among very few study reports on the perovskite TCO film,8,9 Cd3TeO6 films showed the best performance as perovskite TCO, which could be made by the simplest method. In addition, for the electronic band structure of this compound, the bottom of conduction band is formed from a single 5s character of Cd, which has a very high dispersion with the 0.2 me small electroneffective mass, comparable to that of ITO.10 Therefore, it
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0308 2256
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J. Mater. Res., Vol. 20, No. 9, Sep 2005 Downloaded: 06 Oct 2015
is expected to exhibit a pronounced shift of the absorption edge with increasing electron carrier density by carrier doping. Moreover, according to our experience, the electrical conductivity of the poly- and single-crystal Cd3TeO6 can be increased over 2 or 3 digits by the atomic substitution of In3+ cations for Cd2+ ions.11,12 In other words, it is possible that the performance of Cd3TeO6 thin film will be improved through electron doping. In this work, we tried to make an In-doped Cd3TeO6 thin film, investigate the effects of element substitution on electrical and optical properties in the film, and verify the relationship between characteristics and the manufacturing process. Films were fabricated
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