Structural evolution and defect control of yttrium-doped ZrO₂ films grown by a sol–gel method

Highlights: • Yttrium-doped ZrO₂ thin-films can be synthesized via the sol–gel method at low temperatures. • The interfacial states can be effectively reduced through yttrium doping into ZrO₂ using the sol–gel process. • Yttrium doping of ZrO₂ changes the level of interfacial states originating from oxygen vacancies out of the silicon band gap. - Abstract: Yttrium-doped ZrO₂ thin-films were prepared on Si substrates via sol–gel synthesis at a low temperature of 700 °C. During sol–gel synthesis, yttrium can easily take the place of the zirconium in ZrO₂, even at low ambient process temperatures. We were therefore able to successfully synthesize yttrium-doped zirconium oxide (Y-ZrO₂) with a clean interface without the generation of zirconium silicate, which is formed at high temperatures (∼1000 °C). Doped yttrium can eliminate the interstitial oxygen contained in ZrO₂ thin films as O₂⁻¹ states. The conduction band offset (CBO) is also increased via yttrium doping: from 1.69 eV for ZrO₂ to 1.99 eV for Y-ZrO₂ in the as-grown films, and from 1.27 eV for ZrO₂ to 1.35 eV for Y-ZrO₂ in the annealed films. The difference observed in the CBO of the as-grown films may be caused by interstitial oxygen, which is formed in the ZrO₂ films, while the annealed films have oxygen vacancies. The reported data show that yttrium doping of ZrO₂ induces the formation of a yttrium–oxygen vacancy pair, which can reduce the formation energy of oxygen vacancies. However, using the density-of-states analysis from the VASP code density functional theory (DFT) calculations, we confirm that the oxygen vacancy in the Y-ZrO₂ did not generate defect states within the silicon band gap, whereas in the ZrO₂ it did generate defect states within the silicon band gap. Using the conductance method, reductions in the interfacial trap charge densities of approximately 20% were observed near the mid-gap in Y-ZrO₂, as compared with undoped ZrO₂. Following the application of electrical stress, the reduction in interface states was found to be greater in the Y-ZrO₂ film, which is consistent with the DFT calculation