Most research to develop highly transparent and conductive thin films has focused onn-type semiconductors consisting of metal oxides. Historically, transparent conducting oxide (TCO) thin films composed of binary compounds such as SnO2and In2O3were developed by means of chemical- and physical-deposition methods. Impurity-doped SnO2(Sb- or F-doped SnO2, e.g., SnO2:Sb or SnO2: F) and In2O3: Sn (indium tin oxide, ITO) films are in practical use. In addition to binary compounds, ternary compounds such as Cd2SnO4, CdSnO3, and CdIn2O4were developed prior to 1980, but their TCO films have not yet been used widely.