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 μ when the TM concentration $x_{\mathrm{TM}}$ exceeded $\sim 4\%$. 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 $E^{-}$ subband and an upper unoccupied $E^{+}$ subband, consistent with predictions from the band anticrossing model. The reduced μ in the TM-doped CdO with relatively high $x_{\mathrm{TM}}$ can thus be attributed to both the relatively small dispersion of this $E^{-}$ conduction band and increased defect scattering. This is also supported by the increased electron effective mass $m^{*}$ and reduced mean scattering time τ 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 $x_{\mathrm{TM}}>\sim 2\%$ demonstrated enhanced environmental stability in ambient air (with $\sim 45\%$ relative humidity), possibly attributed to a lower concentration of $V_{\mathrm{O}}-\mathrm{related}$ 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.

Figure 1

The calculated electronic band structure of TM-doped CdO (a)–(f) with different $x_{\mathrm{TM}}$, i.e., $x_{\mathrm{Ti}}=3\%$ (a), $x_{\mathrm{Ti}}=6\%$ (b), $x_{\mathrm{Ti}}=12\%$ (c), $x_{\mathrm{Ta}}=3\%$ (d), $x_{\mathrm{Ta}}=6\%$ (e), $\mathrm{and}{x}_{\mathrm{Ta}}=12\%$ (f), and In-doped CdO with different $x_{\mathrm{In}}$, i.e., $x_{\mathrm{In}}=3\%$ (g), $x_{\mathrm{In}}=6\%$ (h), and $x_{\mathrm{In}}=12\%$ (i). The components of the host bands and of the introduced TM $d$ bands or In $s$ band are indicated by the blue and red balls, respectively, while their radii represent the band weight for each species. As an example, the inset in (a) shows the CB splitting owing to the interaction between the host CB and the impurity $d$ bands, leading to an unoccupied upper $E^{+}$ subband and an occupied lower $E^{-}$ subband.

Figure 2

GIXRD patterns of Ti- (a) and Ta (b)-doped CdO films with different doping concentrations $x_{\mathrm{TM}}$; and grain sizes of TM-doped CdO films as a function of $x_{\mathrm{TM}}$. The corresponding environmental stability for these TM-doped CdO films is labeled as stable in the yellow region or unstable in the green region (c).

Figure 3

Electrical properties [i.e., electron concentration $N$ in (a), electron mobility μ in (b), and resistivity ρ in (c)] of TM (i.e., Ti and Ta)-doped CdO thin films as a function of dopant concentration $x$. The electrical properties of In-doped CdO films are also shown for comparison. The dashed lines are to guide the eyes.

Figure 4

Absorption coefficient α of CdO films (including the nominally undoped, TM-doped, and In-doped) with different dopant concentration and $N$ (a); optical band gap $E_G^{\mathrm{opt}}$ of CdO films as a function of $N$, with the dotted line the corresponding theoretical values (b); and transmission spectra of four representative CdO films and an ITO film with similar sheet resistance $R_{\mathrm{S}}$ of $\sim 10\mathrm{\Omega }/\square$ on glass (c).

Figure 5

Electron effective mass $m^{*}$ for CdO with different dopants as a function of $N$. The dopant concentration for each sample is also indicated. The blue dashed line represents the nonlinear fitting for the In-doped CdO by taking into account the nonparabolicity of the CB of CdO according to Eq. (4).

Figure 6

Schematic band-structure diagrams for the doped CdO (i.e., CdO:Ti, CdO:Ta, and CdO:In) with varying doping concentration $x$. The black dashed line represents the CB of undoped CdO, while the black dotted line indicates the corresponding Fermi level $E_{\mathrm{F}}$. The Ti $3d$ and Ta $5d$ impurity bands are depicted by green and purple wavy lines, respectively. The mobility μ is associated with the electron effective mass $m^{*}$ and the mean scattering time τ, expressed as $\mu \propto \tau /m^{*}$.

Figure 7

XPS core-level spectra of CdO:Ti [i.e., Cd $3d$ (a); Ti $2p$ (b); and O $1s$ (c)] and CdO:Ta [i.e., Cd $3d$ (d), Ta $4f$ (e), and O $1s$ (f)]. Peak area ratios of ${\mathrm{O}}_{\mathrm{II}}/{\mathrm{O}}_{\mathrm{I}}$ and ${\mathrm{O}}_{\mathrm{III}}/{\mathrm{O}}_{\mathrm{I}}$ obtained from XPS analysis for the TM-doped CdO with different $x_{\mathrm{TM}}$ (g). The corresponding environmental stability for these TM-doped CdO films is labeled as stable in the yellow region or unstable in the green region.