AVS 46th International Symposium
    Vacuum Technology Division Friday Sessions
       Session VT-FrM

Invited Paper VT-FrM9
John A. Thornton Memorial Award Lecture: The Search for Improved Transparent Conducting Oxides: An Investigation of CdO, Cd@sub 2@SnO@sub 4@, and Zn@sub 2@SnO@sub 4@

Friday, October 29, 1999, 11:00 am, Room 610

Session: Vacuum Systems, Design, and Engineering
Presenter: T.J. Coutts, National Renewable Energy Laboratory
Authors: T.J. Coutts, National Renewable Energy Laboratory
D.L. Young, National Renewable Energy Laboratory
X. Li, National Renewable Energy Laboratory
W.P. Mulligan, Sunpower Inc.
Correspondent: Click to Email
The bulk of developmental work on transparent conducting oxides (TCOs) has been largely experimental and, historically, repetitive. 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 paper, we place a greater emphasis on more fundamental research. Our eventual goal is to gain a thorough understanding of potentially suitable materials, the possibilities for significant improvement, whether new and superior materials are suggested by generalizations, and the way their properties are influenced by structural and other issues. We also hope to provide directions for those working in this area. The work concerns cadmium oxide (CdO), cadmium stannate (Cd@sub 2@SnO@sub 4@ or CTO), and zinc stannate (Zn@sub 2@SnO@sub 4@ or ZTO). The CdO was prepared by chemical vapor deposition, whereas the stannates were prepared by r.f. sputtering. In both cases, Corning 7059 glass substrates were used. However, some depositions were also made onto tin oxide, which had a marked effect on the nucleation of CdO in particular. Various substrate temperatures were used. For a TCO to have acceptable electro-optical properties, a high free-carrier mobility is essential. Increasing the free-carrier concentration increases the free-carrier absorbance, but a higher mobility (at the same concentration) reduces it. We have achieved free-electron mobilities in CdO (E@sub g@ ~2.4 eV) greater than 200 cm@super 2@ V@super -1@ s@super -1@, of almost 80 cm@super 2@ V@super -1@ s@super -1@ in Cd@sub 2@SnO@sub 4@ (E@sub g@ ~3.4 eV), but only 20-30 cm@super 2@ V@super -1@ s@super -1@ in Zn@sub 2@SnO@sub 4@ (E@sub g@ ~ 3.6 eV). The first two of these are significantly higher than other TCO materials of which we are aware. We have characterized these materials using a variety of techniques, including the Nernst-Ettingshausen effect, Mössbauer, Raman, optical, near-infrared and ellipsometric spectroscopies, atomic force and high-resolution electron microscopy, and X-ray diffraction. These measurements enable us to determine the effective mass of the free carriers and their relaxation time, the probable distributions of cations between octahedral and tetrahedral sites for the stannate compounds, the role of the deposition parameters on the carrier concentrations, and the nature of the dominant scattering mechanisms. We account for the significant differences between the carrier concentrations and mobilities of the three compounds studied. We further discuss the crystallographic phases observed, as well as the high degree of structural perfection of intragrain material in the cadmium-containing materials. Finally, issues of toxicity of cadmium and relatively limited reserves of indium are considered. Both are of great significance to prospective manufacturers of TCO films in large-volume and must be taken into account by researchers.