Structural evolution and defect control of yttrium-doped ZrO ₂ films grown by a sol-gel method

Yttrium-doped ZrO ₂ 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 ZrO ₂ , even at low ambient process temperatures. We were therefore able to successfully synthesize yttrium-doped zirconium oxide (Y-ZrO ₂ ) 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 ZrO ₂ thin films as O ₂ ^-1 states. The conduction band offset (CBO) is also increased via yttrium doping: from 1.69 eV for ZrO ₂ to 1.99 eV for Y-ZrO ₂ in the as-grown films, and from 1.27 eV for ZrO ₂ to 1.35 eV for Y-ZrO ₂ 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 ZrO ₂ films, while the annealed films have oxygen vacancies. The reported data show that yttrium doping of ZrO ₂ 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-ZrO ₂ did not generate defect states within the silicon band gap, whereas in the ZrO ₂ 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-ZrO ₂ , as compared with undoped ZrO ₂ . Following the application of electrical stress, the reduction in interface states was found to be greater in the Y-ZrO ₂ film, which is consistent with the DFT calculation.