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.