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and Processing of Transparent Conducting Oxides

Brian G. Lewis and David C. Paine Introduction The first report of a transparent conducting oxide (TCO) was published in 1907, when Badeker1 reported that thin films of Cd metal deposited in a glow discharge chamber could be oxidized to become transparent while remaining electrically conducting. Since then, the commercial value of these thin films has been recognized, and the list of potential TCO materials has expanded to include, for example, Al-doped ZnO, GdInOx , SnO2 , F-doped In2O3, and many others. Since the 1960s, the most widely used TCO for optoelectronic device applications has been tin-doped indium oxide (ITO). At present, and likely well into the future, this material offers the best available performance in terms of conductivity and transmissivity, combined with excellent environmental stability, reproducibility, and good surface morphology. The use of other TCOs in large quantities is application-specific. For example, tin oxide is now widely used in architectural glass applications.

Unique Properties of TCOs TCOs are an essential part of technologies that require both large-area electrical contact and optical access in the visible portion of the light spectrum. High transparency, combined with useful electrical conductivity (103 1 cm1), is achieved by selecting a wide-bandgap oxide that is rendered degenerate through the introduction of native or substitutional dopants. Most of the useful oxide-based materials are n-type conductors that ideally have a wide bandgap (3 eV), the ability to be doped to degeneracy, and a conductionband shape (dictating electron effective mass) that ensures that the plasmaabsorption edge lies in the infrared range.

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The most widely used TCO in optoelectronic devices is ITO. Other TCOs are also available and find use in specialized applications where ease of deposition, cost, or IR reflectivity is favored over optimum optical transmission and minimum sheet resistance. For example, heat-efficient windows that reflect in the infrared range are created during the manufacture of architectural glass by means of the direct deposition of SnO2 using chloride-based spray pyrolysis. In this passive application, good electrical conductivity is sacrificed for IR reflectivity, high transparency in the visible regime, and processing convenience and economics. The production of energy-efficient architectural glass occurs in quantities measured in tens of square kilometers per year. This market is continuing to grow. For optoelectronic applications, the transparent conductor must be carefully processed to maximize optical transmissivity in the visible regime, while achieving minimum electrical resistivity. Optimization of these properties will depend on the application, but in general, achieving the required performance in the as-deposited condition requires careful process control. The deposition of ITO in a manufacturing environment is typically by means of dc-magnetron sputtering. The choice of target—ceramic or metal—depends o