Fundamental Advances in Transparent Conducting Oxides

Increasingly large-volume markets for large-area, flat-panel displays and photovoltaic panels are likely to be established in the early years of the next century and transparent conducting oxides (TCOs) of improved opto-electronic properties will be required to enable some of these applications to be realized. Our work is focusing on improving both the fabrication-limited properties of the materials (extrinsic), and materials-limited properties (intrinsic). The emphasis on achieving improved electrical and optical properties hinghes on achieving higher electron mobility via intrinsic and/or extrinsic properties. To this end, we have investigated the properties of several TCOs including cadmium oxide, tin oxide, zinc oxide, cadmium stannate and zinc stannate. These may be deposited by chemical vapor deposition (CVD) or sputtering and we hope to establish the capability to fabricate compounds and alloys in the cadmium oxide, tin oxide, zinc oxide ternary phase diagram. These properties of the materials have been investigated using a wide variety of techniques including high-resolution electron microscopy, atomic force microscopy and X-ray diffraction, as well as Mössbauer, Raman and UV/visible/NIR spectroscopies. We have measured the transport properties (conductivity, Hall, Seebeck and Nernst coefficients) and have obtained the effective mass, relaxation time, Fermi energy, and scattering parameter. This information has been obtained as a function of carrier concentration, which depends on the deposition and annealing procedures. We have found that the mobilities of free-electrons in the cadmium-bearing compounds are greatly superior to those in the other materials, because they have much longer electron relaxation times. In the case of cadmium oxide, there is also great benefit from a much lower effective mass. We are gaining a clearer understanding of the fundamental microscopic attributes needed for TCOs, which will be required in more-demanding, and rapidly emerging, applications.