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

作者：Wang, Ying; Chow, Yuen Yan; Kwok, Cheuk Kai Gary; Leung, Yau Fung; Yu, Kin Man^*

来源：Journal of Alloys and Compounds, 2023, 935: 168116.

DOI：10.1016/j.jallcom.2022.168116

• Transparent conducting oxides
• Transition metal dopant
• Cadmium oxide
• Native defects
• Rapid thermal annealing

Cadmium oxide (CdO) is known to have the highest electron mobility among common transparent con-ductive 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 mu increases with rapid thermal annealing (RTA) temperature, N decreases when the temperature is > 400 degrees 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 degrees C further reveals that H interstitial donors incorporated during room temperature film growth are unstable at temperature < 300 degrees C. Overall, both CMO and CWO can achieve a low resistivity of similar to 1.5 x 10-4 Omega-cm with mu > 100 cm2/V-s and N similar to 3.5 x 1020 and 4 x 1020 cm-3, respectively, as well as an overall transmittance > 80% up to lambda > 1600 nm with x less than or similar to 0.01 after RTA at 600 degrees C in N2. Finally, changes in optical absorption edges Eopt 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 mu at high x.