Effects of free carriers on the optical properties of high mobility transition metal doped In2O3 transparent conductors

Transition metal doped In2O3 with high mobility can be used as a transparent conductor with enhanced transparency spectral window. In this work, we carried out a comprehensive study on the electrical and optical properties of In2O3 doped with several transition metal (TM) species (In2O3 : TM) including W, Zr, Mo, and Ti. Detailed optical properties obtained by spectroscopic ellipsometry (SE) are correlated with electrical properties obtained by Hall effect measurements. We find that the mobility of In2O3 : TM thin films lies in the range of 50-75 cm(2) V-1 s(-1), much higher than the typical mobility of 30-40 cm(2) V-1 s(-1) for conventional ITO. The complex dielectric functions of the thin films reveal remarkable carrier density dependent changes in the optical properties. SE analyses show that the electron effective mass of In2O3 : TMat the bottom of the conduction band m(o)(*) (0.11-0.14m(o)) is much smaller than the reported m(o)(*) similar to 0.18-0.30m(o) for ITO, which directly results in their higher mobility. This low m(o)(*) is consistent with recent theoretical studies which proposed that 4d donor states of the TMs are resonance in the CB. For films with comparably low resistivity of 1-2 x 10(-4) Omega cm, we find that In2O3 : TM films have similar to 4-10 times lower absorption coefficient at lambda = 1300 nm due to free carrier absorption and have their plasma reflection edge extended to similar to 1.7 mu m compared to similar to 1.2-1.4 mu m for ITO. Hence, using TM doping we have achieved transparent conductors with conductivity comparable to ITO but with transmission extended to>1600 nm. Thesematerials will be potentially important as transparent conductors for optoelectronic devices utilizing NIR photons.