Effects of transition metal dopants (Mo and W) on electrical and optical properties of CdO thin films

Cadmium oxide (CdO) is known to have the highest electron mobility among common transparent conductive oxides (TCOs). Here, we carried out a systematic investigation on the properties of CdO films doped with transition metal donors, namely Mo (CMO) and W (CWO) which have been reported to enhance the mobility in In₂O₃. We find that the electron concentration N in as-grown CMO and CWO increases with dopant concentration to > 10²¹ cm⁻³. While the mobility μ increases with rapid thermal annealing (RTA) temperature, N decreases when the temperature is > 400 °C, suggesting that Mo and W are unstable donors. The stronger affinity of the Mo and W to diffuse out in an O₂ environment results in an even stronger decrease in N when annealed in O₂. Isothermal RTA at 300 °C further reveals that H interstitial donors incorporated during room temperature film growth are unstable at temperature< 300 °C. Overall, both CMO and CWO can achieve a low resistivity of ∼1.5 × 10⁻⁴ Ω-cm with μ > 100 cm²/V-s and N∼3.5 × 10²⁰ and 4 × 10²⁰ cm^–3, respectively, as well as an overall transmittance> 80% up to λ > 1600 nm with x ≲ 0.01 after RTA at 600 °C in N₂. Finally, changes in optical absorption edges E_opt of CMO and CWO are consistent with the modification of the conduction band (CB) due to anticrossing interactions of the dopant d–states with the CdO CB, which also flattens the CB dispersion which dramatically lowers the μ at high x.