Effective mass of high-mobility In2O3 -based transparent conductive oxides fabricated by solid-phase crystallization

Polycrystalline ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$ films, which are solid-phase crystallized (spc) from amorphous films doped with hydrogen (H) and transition metals (TMs), exhibit remarkably high mobilities $(100--160\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$; moreover, their specific resistance is equivalent to $2--3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{\ensuremath{\Omega}}\phantom{\rule{0.16em}{0ex}}\mathrm{cm}$ or less, even when the carrier density is reduced to one-fifth $(1--4\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3})$ that of conventional transparent conductive oxide (TCO) films. The high mobility and low carrier density significantly reduce free-carrier absorption, and the transparent region of the TCO films extends across the visible and near-infrared regions. In this study, we experimentally demonstrate that spc-${\mathrm{In}}_{2}{\mathrm{O}}_{3}$:H and ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$:TM,H films with an impurity concentration of less than a few at. % exhibit superior mobility compared to that of optimized ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$:Sn,H films $(\ensuremath{\sim}70\phantom{\rule{0.16em}{0ex}}{\mathrm{cm}}^{2}\phantom{\rule{0.16em}{0ex}}{\mathrm{V}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1})$ with similar Sn and close-carrier concentrations. Photoelectron spectroscopy measurements revealed that the electronic state of the spc films, including the valence of TM impurities, changed with varying In/O ratios of the amorphous films. Furthermore, the In, O, TM, and H compositions in the amorphous films were found to be critical for effective activation of ${\mathrm{TM}}_{\mathrm{In}}^{+}$, ${\mathrm{H}}_{\mathrm{O}}^{+}$, and ${{\mathrm{H}}_{\mathrm{i}}}^{+}$ donors after crystallization. To determine the origin of the high electron mobility, the effective mass and relaxation time of electrons in these films were evaluated by spectroscopic ellipsometry. The results showed that the high mobility could be primarily attributed to the long relaxation time instead of the small effective mass. Additionally, the dispersion of conduction bands near the Fermi energy was found to be almost independent of the type of metallic impurity (Sn, H, Ce, Zr, and W) for the investigated impurity concentrations. Moreover, the increase in relaxation time by H and TM doping was examined.