Search for improved transparent conducting oxides: A fundamental investigation of CdO, Cd2SnO4, and Zn2SnO4

The bulk of developmental work on transparent conducting oxides (TCOs) has been somewhat empirical. This statement applies both to more familiar materials such as indium tin oxide (ITO) and to less-well-known materials that have emerged in recent years. In this article, we place a greater emphasis on more fundamental research. Our eventual goal is to gain a thorough understanding of these materials, their potential for further improvement, whether or not they suggest new and potentially superior materials, and the way their properties are influenced by structural and other issues. We also hope to provide guidelines to other researchers working in this area. We have investigated films of cadmium oxide (CdO), cadmium stannate (Cd2SnO4 or CTO), and zinc stannate [Zn2SnO4 (ZTO)]. The CdO was prepared by chemical-vapor deposition, whereas the stannates were prepared by rf sputtering. In both cases, Corning 7059 glass substrates were used. However, some depositions were also made onto tin oxide, which had a profound effect on the nucleation of CdO, in particular. It is well known that a high free-carrier mobility is essential for a TCO with near-ideal electro-optical properties. Increasing the free-carrier concentration also increases the free-carrier absorbance but a higher mobility reduces it. We have achieved free-electron mobilities in CdO (Eg∼2.4 eV) of greater than 200 cm2 V−1 s−1, of almost 80 cm2 V−1 s−1 in CTO (Eg∼3.1 eV), but of only 10–15 cm2 V−1 s−1 in ZTO (Eg∼3.6 eV). We have characterized these materials, and will show key data, using techniques as diverse as the Nernst–Ettingshausen effect; Mössbauer, Raman, optical, and near-infrared spectroscopies; atomic-force and high-resolution electron microscopy; and x-ray diffraction. These measurements have enabled us to determine the effective mass of the free carriers and their relaxation time, the probable distributions of cations between octahedral and tetrahedral sites, the role of the deposition parameters on the carrier concentrations, and the nature of the dominant scattering mechanisms. We also consider issues relating to toxicity of cadmium and to reserves of indium. Both are of great significance to prospective large-volume manufacturers of TCO films and must be taken into account by researchers.