Structural, Electronic, and Electrical Properties of an Undoped n-Type CdO Thin Film with High Electron Concentration

Transparent conducting metal oxides (TCOs) combine the properties of optical transparency in the visible region with a high electrical conductivity. They are a critical component as the window electrode in liquid crystal and electroluminescent display devices, as well as in many designs of solar cells now under development. Sn-doped In2O3 is currently the most important TCO, but it suffers from some drawbacks. These include the high cost of indium, weak optical absorption in the blue-green region, as well as chemical instability that leads to corrosion phenomena in organic light-emitting devices. Indium tin oxide (ITO) films are also brittle and of relatively low durability. A number of other systems are therefore of interest as potential alternatives to Sn-doped In2O3. Recently, CdO has received considerable attention because of its high intrinsic dopability, which coupled with excellent Hall mobilities gives a high electrical conductivity. In the present study we have explored the structure, electronic, and electrical properties of CdO thin films prepared by a simple metalloorganic chemical vapor deposition route. The techniques employed include X-ray diffraction, atomic force microscopy, photoelectron spectroscopy, optical absorption spectroscopy, and luminescence measurements. Electrical properties were characterized by four-point probe and Hall effect measurements in the temperature range between 80 and 460 K. The electrical measurements demonstrated that the CdO thin films are degenerate semiconductors, with free-electron concentrations of around 7 x 10(20) cm(-3) provided by native donors.