Resonant transition metal Ti or Ta doping in high mobility transparent conducting CdO: The effects of doping concentration

Recent studies have shown that transparent conducting oxides (TCOs) such as $\mathrm{I}{\mathrm{n}}_{2}{\mathrm{O}}_{3}$ or $\mathrm{Sn}{\mathrm{O}}_{2}$ exhibited enhanced mobility through resonant transition metal (TM) doping. However, our previous investigations of CdO, another TCO, doped with TM elements with partially filled 3d- or $4d$ shells, such as Sc, Ti, V, Mo, and W, could exhibit high electron concentration $N$ but experienced a notable reduction in electron mobility \ensuremath{\mu} when the TM concentration ${x}_{\mathrm{TM}}$ exceeded $\ensuremath{\sim}4%$. Here, we conducted computational and experimental analyses to explore the electronic structure and optoelectronic properties of CdO doped with two distinct representative TM dopants, Ti and Ta, featuring partially filled 3d- and $5d$ shells, respectively, with varying ${x}_{\mathrm{TM}}$. Density-functional theory calculations unveiled that the localized $d$-donor states of TM interact with the extended CdO conduction-band states, resulting in a lower occupied ${E}^{\ensuremath{-}}$ subband and an upper unoccupied ${E}^{+}$ subband, consistent with predictions from the band anticrossing model. The reduced \ensuremath{\mu} in the TM-doped CdO with relatively high ${x}_{\mathrm{TM}}$ can thus be attributed to both the relatively small dispersion of this ${E}^{\ensuremath{-}}$ conduction band and increased defect scattering. This is also supported by the increased electron effective mass ${m}^{*}$ and reduced mean scattering time \ensuremath{\tau} derived from spectroscopic ellipsometry analysis using the Drude model. Furthermore, the decrease in $N$ with high ${x}_{\mathrm{TM}}$ might be due to compensation by oxygen interstitials ${\mathrm{O}}_{i}$ acceptors, enhanced electron correlation effect, and/or TM aggregation. Moreover, we find that CdO:TM films with ${x}_{\mathrm{TM}}>\ensuremath{\sim}2%$ demonstrated enhanced environmental stability in ambient air (with $\ensuremath{\sim}45%$ relative humidity), possibly attributed to a lower concentration of ${V}_{\mathrm{O}}\ensuremath{-}\mathrm{related}$ defects. Comparatively, Ta doping in CdO yielded electrical properties superior than Ti, owing to the relatively weaker electron correlation in Ta $5d$ orbitals. These findings offer valuable insights into TM doping of CdO and other TCOs.