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 In2O3 or SnO2 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 xTM exceeded ∼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 xTM. 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 xTM 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 xTM might be due to compensation by oxygen interstitials Oi acceptors, enhanced electron correlation effect, and/or TM aggregation. Moreover, we find that CdO:TM films with xTM>∼2% demonstrated enhanced environmental stability in ambient air (with ∼45% relative humidity), possibly attributed to a lower concentration of VO-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.