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.

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In this article we examine the structural, electrical, and optical properties of several ternary alloy thin films. The alloys are zinc oxide and cadmium oxide, each of which was reacted with both indium oxide and tin oxide to form sputtering targets. The films were deposited by rf sputtering. X-ray diffraction spectroscopy showed that cadmium stannate, cadmium indate, and zinc stannate films were all polycrystalline spinel phase, but only when deposited at room temperature in pure oxygen and then annealed in argon/cadmium sulfide. The fourth alloy, zinc indate, exhibited a hexagonal phase when prepared under identical conditions. Cadmium stannate has one of the lowest resistivities of any transparent conducting oxide (TCO), has low absorbance in the visible spectrum, and is an excellent compromise between electrical and optical requirements. For this material, we show that a single phase is essential for the highest electrical conductivity and lowest optical absorbance. Zinc stannate has a resistivity more than one order of magnitude higher than cadmium stannate but, because of a larger band gap, has an even lower absorbance. We conclude that there is no “best” TCO, and the decision regarding which to use depends on the specific application and the weighting given to electrical versus optical properties. The zinc-bearing alloys are nontoxic, which may also be an attractive feature for many applications.

Abstract:The Boltzmann transport equation can be solved to give analytical solutions to the resistivity, Hall, Seebeck, and Nernst coefficients. These solutions may be solved simultaneously to give the density-of-states (DOS) effective mass ( m d * ), the Fermi energy relative to either the conduction or valence band, and a scattering parameter that is related to a relaxation time and the Fermi energy. The Nernst coefficient is essential for determining the scattering parameter and, thereby, the effective scattering mechanism(s). We constructed equipment to measure these four transport coefficients simultaneously over a temperature range of 30-350 K for thin, semiconducting films deposited on insulating substrates. We measured these coefficients for rf magnetron-sputtered zinc oxide, both doped and 2

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