Doping limitation due to self-compensation by native defects in In-doped rocksalt Cd_x Zn_1-x O

Cadmium oxide (CdO)-ZnO alloys (Cd_x Zn_1-xO) exhibit a transformation from the wurtzite to the rocksalt (RS) phase at a CdO composition of ∼70% with a drastic change in the band gap and electrical properties. RS-Cd_x Zn_1-xO alloys (x > 0.7) are particularly interesting for transparent conductor applications due to their wide band gap and high electron mobility. In this work, we synthesized RS-Cd_x Zn_1-xO alloys doped with different concentrations of In dopants and evaluated their electrical and optical properties. Experimental results are analyzed in terms of the amphoteric native defect model and compared directly to defect formation energies obtained by hybrid density functional theory (DFT) calculations. A saturation in electron concentration of ∼7 × 10²⁰ cm^-3 accompanied by a rapid drop in electron mobility is observed for the RS-Cd_x Zn_1-xO films with 0.7 ≤ x < 1 when the In dopant concentration [In] is larger than 3%. Hybrid DFT calculations confirm that the formation energy of metal vacancy acceptor defects is significantly lower in RS-Cd_x Zn_1-xO than in CdO, and hence limits the free carrier concentration. Mobility calculations reveal that due to the strong compensation by native defects, RS-Cd_x Zn_1-xO alloys exhibit a compensation ratio of >0.7 for films with x < 0.8. As a consequence of the compensation by native defects, in heavily doped RS-Cd_x Zn_1-xO carrier-induced band filling effect is limited. Furthermore, the much lower mobility of the RS-Cd_x Zn_1-xO alloys also results in a higher resistivity and reduced transmittance in the near infra-red region (λ > 1100 nm), making the material not suitable as transparent conductors for full spectrum photovoltaics.