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

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.