Design of n-Type Transparent Conducting Oxides: The Case of Transition Metal Doping in In₂O₃

Design of novel n-type transparent conducting oxides beyond Sn-doped In₂O₃ has stimulated extensive interest in the past decade. One of the approaches can be using transition metals (TMs) as dopants. In this article, using In₂O₃ as an example, it is shown that TM doping in oxides can be classified into three categories (type-I, II, or III) based on their TM d-orbital energy levels reference to the bottom of the conduction band of In₂O₃. It is found that although Mo is proposed to replace Sn as a promising TM donor to achieve higher carrier density in In₂O₃, it actually exhibits unusual dual-doping behaviors, i.e., it can act as either a deep donor when it occupies the In 8b-sites (type-I) or shallow single donor when it occupies the In 24d-sites (type-II). The calculated ionization of Mo in In₂O₃ increases as the growth temperature increases, in good agreement with experimental observations but contradict to previous theoretical studies. It is also identified that Zr, Hf, and Ta (type-III) are better potential donors than Mo and Sn in In₂O₃ for achieving higher carrier mobility and density. The analysis and approach can also be used to improve the doping performance in other oxides.