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

Transition metal doped ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ 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 ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ doped with several transition metal (TM) species (${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{TM}$) 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 ${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{TM}$ thin films lies in the range of $50\text{--}75\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{\ensuremath{-}}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{\ensuremath{-}}1}$, much higher than the typical mobility of $30\text{--}40\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\text{\ensuremath{-}}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\text{\ensuremath{-}}1}$ 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 ${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{TM}$ at the bottom of the conduction band ${m}_{o}^{*}$ ($0.11\text{--}0.14{m}_{o}$) is much smaller than the reported ${m}_{o}^{*}\ensuremath{\sim}0.18\ensuremath{-}0.30{m}_{o}$ 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 $1\text{--}2\ifmmode\times\else\texttimes\fi{}{10}^{\text{\ensuremath{-}}4}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$, we find that ${\mathrm{In}}_{2}{\mathrm{O}}_{3}:\mathrm{TM}$ films have \ensuremath{\sim}4--10 times lower absorption coefficient at $\ensuremath{\lambda}=1300\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ due to free carrier absorption and have their plasma reflection edge extended to \ensuremath{\sim}1.7 \ensuremath{\mu}m compared to \ensuremath{\sim}1.2--1.4 \ensuremath{\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. These materials will be potentially important as transparent conductors for optoelectronic devices utilizing NIR photons.