Native Defects in CdO and CdCuO Alloys
Indium tin oxide (ITO) is the most extensively researched and most commonly used transparent conductive oxide. Despite ITO's popularity its long term use by industry presents several significant challenges. Indium is an expensive and a non-abundant metal. The demand for optoelectronic devices is growing far faster than the supply of indium. In the future, this will result in even higher prices for an already expensive material. In addition to cost, ITO has a critical performance flaw. All ITO films and current substitutes for ITO, such as doped zinc oxide and doped tin oxide, achieve low resistivies because they have very high carrier concentrations, not because they have very high mobilities. Significant free carrier absorption exists in the low energy portion of the visible and near infrared solar spectrum. For many devices, this results in a loss of transparency that can negatively affect device performance.
Cadmium oxide (CdO) does not suffer from either the cost or the performance limitation of ITO. CdO displays exceptional mobilities that are approximately five times greater than most ITO. This means free carrier absorption is significantly reduced. Despite CdO's promise, it has not displaced ITO in the market. The main reason is that this material is not well understood. Before a new transparent conductive oxide is used in a device, a detailed understanding of the optimal deposition parameters, defect behavior, dopant behavior and processing behavior all have to be established. The goal of this work is to provide that understanding. It is shown that the amphoteric defect model can be used to provide a unified explanation for several unresolved questions in CdO, namely, why the carrier concentration in CdO is typically between 2*1019 to 2*1020 cm-3, why no p-type films have been fabricated and why there is such a large spread in the reported optical absorption edge, ranging from 2.2 to 2.6 eV. In addition, the optimal deposition and annealing conditions have been determined which yield the lowest carrier concentration and highest mobility undoped CdO ever published. This is an important step for commercialization. Finally, the influence of high doses of irradiation on CdO and CdCuO is considered. Here, it is shown that irradiation provides a method to increase the optical absorption edge and reduce the resistivity by increasing the number of free carriers. However the irradiation dose must be carefully chosen because doses that are too small provide little improvement whiles doses that are too large lead to poor optical performance.