Sources of Conductivity and Doping Limits in CdO from Hybrid Density Functional Theory

CdO has been studied for decades as a prototypical wide band gap transparent conducting oxide with excellent n-type ability. Despite this, uncertainty remains over the source of conductivity in CdO and over the lack of p-type CdO, despite its valence band maximum (VBM) being high with respect to other wide band gap oxides. In this article, we use screened hybrid DFT to study intrinsic defects and hydrogen impurities in CdO and identify for the first time the source of charge carriers in this system. We explain why the oxygen vacancy in CdO acts as a shallow donor and does not display negative-U behavior similar to all other wide band gap n-type oxides. We also demonstrate that p-type CdO is not achievable, as n-type defects dominate under all growth conditions. Lastly, we estimate theoretical doping limits and explain why CdO can be made transparent by a large Moss-Burstein shift caused by suitable n-type doping.