Factors limiting the doping efficiency of transparent conductors: A case study of Nb-doped In2O3 epitaxial thin-films

In2O3 epitaxial thin films with heavy Nb doping have been investigated in expectation of achieving higher doping efficiency per doping atom than what has been achieved by Sn-doping because Nb has one more available valence electron. The films were deposited by co-sputtering of Nb and In2O3 on (001) YSZ substrates, and are found to follow the epitaxial relations of [001]In2O3||[001]YSZ and [110]In2O3||[110]YSZ aligned within Δω∼0.31–0.41° of full rocking width at half maximum (FWHM). The doped thin films present optical transparencies of 97–99 % with electrical resistivities down to 10−4 Ω cm, 100 times lower than the as-deposited pristine thin films of no intentional doping. Optimal doping efficiency of ηmax∼1 charge carrier per Nb-atom added, not the hoped 2, suggests an effective ionization state of Nb·In, much like Nb·Sn, rather than the anticipated Nb··In. This singly-charged state is associated with the formation of Nb2O4 molecules by drawing extra interstitial Oi atoms to gather around the substitutional NbIn sites, as confirmed by XPS based on the chemical-shift of the X-ray photoelectron energy that measures the binding energy of core–shell electrons. Plasma oscillation analysis by FTIR optical spectroscopy shows an anomalously-high effective mass, about 10 times larger than the reported m⁎=0.35me. A dual-band model is proposed to reconcile with the findings.