New n -Type Transparent Conducting Oxides
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Transparent Conducting Oxides Tadatsugu Minami
Introduction Most research to develop highly transparent and conductive thin films has focused on n-type semiconductors consisting of metal oxides. Historically, transparent conducting oxide (TCO) thin films composed of binary compounds such as SnO2 and In2O3 were developed by means of chemical- and physical-deposition methods.1–3 Impurity-doped SnO2 (Sb- or F-doped SnO2 , e.g., SnO2Sb or SnO2F) and In2O3Sn (indium tin oxide, ITO) films are in practical use. In addition to binary compounds, ternary compounds such as Cd2SnO4, CdSnO3, and CdIn2O4 were developed prior to 1980,1,2 but their TCO films have not yet been used widely. For the purpose of obtaining lower resistivities than that of TCO films such as ITO and impurity-doped SnO2, other TCO materials have been developed as alternatives. In the 1980s, impurity-doped ZnO, an inexpensive and abundant binary compound material, was developed.3 Recently, ZnOAl and ZnOGa thin films have attracted much attention as the transparent electrode for thin-film solar cells; a resistivity of the order of 1 104 cm was obtained in these films prepared by various deposition methods. To obtain TCO films suitable for specialized applications, new TCO materials have been actively studied in recent years. In the 1990s, new TCO materials consisting of multicomponent oxides have been developed—for example, combinations of binary compounds such as ZnO, CdO, In2O3, and SnO2. In these materials systems, new TCO materials consisting of ternary compounds such as Zn2SnO4,4 MgIn2O4,5 CdSb2O6Y,6 ZnSnO3,7 GaInO3,8 Zn2In2O5,9 and In4Sn3O12,10 as well as multicomponent oxides composed of combinations of these ternary compounds, were developed.11 The use of multicomponent oxide materials makes possible the design of TCO films suitable for specialized applications be38
cause their electrical, optical, chemical, and physical properties can be controlled by altering their chemical compositions. This article introduces newly developed n-type TCO materials consisting of binary and ternary compounds and multicomponent oxides. Although TCO films are usually evaluated by their resistivity and transmittance in the visible range, the following discussion focuses on resistivity, since the transmittance of TCO films can be controlled by altering film thickness.
Binary Compound TCO Materials One advantage of using binary compounds as TCO materials is that their chemical composition in film depositions is relatively easier to control than that of ternary compounds and multicomponent oxides. Until now, impurity-doped SnO2, In2O3, ZnO, and CdO films have been developed as TCO materials consisting of binary compounds. It is well known that the transparent conducting thin films of these metal oxides can also be prepared without intentional impurity doping.1–3 These films are n-type degenerated semiconductors, with free-electron concentrations of the order of 1020 cm3 provided by native donors such as oxygen vacancies and/or interstitial metal atoms. Howev
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