Electronic, and Electrical Properties of CdO Thin Films.

Transparent conducting metal oxides 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. Recently, CdO has received considerable attention because of its high intrinsic dopability, which coupled with excellent Hall mobilities gives a high electrical conductivity. CdO adopts face-centered cubic rocksalt structure. The conduction band is of dominant Cd 5s atomic character and has a minimum at the Γ point. A fundamental indirect gap of 0.89−0.99 eV at room temperature has been suggested. The direct allowed optical onset is found at about 2.18 eV. Both direct and indirect absorption edges show a very strong blue shift with degenerate n-type doping due to the occupation of states at the bottom of the conduction band which blocks off the lowest-energy transitions. This ensures that, despite the relatively low values of the indirect and direct electronic gaps for undoped CdO, highly doped material remains essentially transparent throughout the visible region. CdO has a much lower fundamental gap than alternative In2O3 and SnO2, where the direct forbidden gaps are, respectively, 2.9 and 3.6 eV.The properties of CdO are strongly influenced by native donor defects arising from nonstoichiometry, which is in turn strongly dependent on the synthetic procedure adopted. In fact, native donors can take CdO through a transition from a semiconductor to a degenerate metallic material. In the present study we have explored the structure, electronic, and electrical properties of CdO thin films prepared by a simple metallo-organic 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 × 1020 cm−3 provided by native donors. This system seems also promising for water treatments.