Defect-dependent environmental stability of high mobility transparent conducting In-doped CdO

Highly degenerate n-type CdO with high electron mobility is a promising transparent conducting oxide (TCO) for optoelectronic devices utilizing a spectrum in the Vis-NIR range. In particular, it has been shown that doped CdO thin films can show much superior transparency of >80% in the NIR region compared to conventional transparent conducting oxide (e.g., Sn-doped In₂O₃) thin films with a similar sheet resistance. However, CdO thin films typically experience rapid degradation in their electron mobilities when exposed to environmental conditions with H₂O moisture. Here, we studied the effects of thermal annealing on the environmental stability of In-doped CdO (CdO:In) using a combination of different analytical techniques. CdO:In thin films with different In concentration (0%-8.3%) synthesized by magnetron sputtering were subjected to different post-thermal annealing (PTA) and then aged in different environmental conditions with varying relative humidity (RH) in the range of 0%-85%. Our results reveal that the degradation of CdO:In thin films can be primarily attributed to the oxygen vacancy-related defects at the grain boundaries, which can readily react with the OH⁻ in the moisture. The moisture induced degradation can be mitigated by appropriate PTA at high temperatures (>400 °C) where grain boundary defects, primarily associated with Cd vacancies, can be passivated through hydrogen (H), thus enhancing their environmental stability. The present study provides a comprehensive understanding of the instability mechanisms and defect passivation in transparent conducting CdO:In thin films, which can also be relevant for other wide gap oxides. © 2024 Author(s). Published under an exclusive license by AIP Publishing.