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 parallel to[001]YSZ and [110]In2O3 parallel to[110]YSZ aligned within Delta omega similar to 0.31-0.41 degrees 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) Omega cm, 100 times lower than the as-deposited pristine thin films of no intentional doping. Optimal doping efficiency of eta(max)similar to 1 charge carrier per Nb-atom added, not the hoped 2, suggests an effective ionization state of Nb center dot(In), much like Nb center dot(Sn), rather than the anticipated Nb center dot center dot(In). This singly-charged state is associated with the formation of Nb2O4 molecules by drawing extra interstitial O-i atoms to gather around the substitutional Nb-In 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*=0.35m(e). A dual-band model is proposed to reconcile with the findings. (C) 2013 Elsevier B.V. All rights reserved.