Phase formation and stability of reactive sputtered zirconium dioxide thin films

As materials properties are greatly influenced by their phase constitution, therefore it's of high importance to understand and address the mechanisms driving their phase formation and stability. In this respect, zirconium oxide (ZrO2) has been the focus of a special attention for the last couple of decades regarding the stabilization of its cubic (c) phase at room temperature. In the present study, the role of the film chemistry i.e. of oxygen vacancies and of energy deposited during the film growth is investigated. To this purpose, 100 nm thick films of zirconium oxide are grown in the poisoned mode as well as in the transition zone with the help of voltage feedback control unit (Speedflo mini from Gencoa UK). During the film growth, to have a fast response from the feedback unit and thus a tight control over the film chemistry (i.e. O/Zr ratio), oxygen is injected just at the target surface. By systematically varying the working parameters, it is observed that for films grown at 200 mA, 10 mTorr in the poisoned mode, the XRD diffractograms only exhibits reflections from the low-temperature stable monoclinic (m) phase. To the contrary, while working inside the transition zone i.e. by growing sub-stoichiometric zirconium oxide thin films as demonstrated by careful elemental characterization, the film phase is dramatically modified and only the c reflections are observed. Theoretical calculations at the Density Functional Theory level are in remarkable agreement with the experimental data, hence highlighting that the incorporation of oxygen vacancies is the sole responsible mechanism for the stabilization of the c-phase. It is also observed that any deviation from the optimized working conditions i.e. change in discharge current or pressure leads to the change in film phase constitution. Thermal annealing analysis performed in air and N2 shows the oxygen vacancy stabilized zirconia films are stable up-to 750 C. Above 750 C, the mechanical stress, generated in the film due to the mismatch of the thermal expansion coefficients of both the zirconia film and the substrate, apparently surpasses a critical value and leads to the appearance of m-phase. In conclusion, c-phase of zirconia can be stabilized at room temperature (up to 750°C) by solely incorporating oxygen vacancies in the zirconia lattice. However, increasing the energy flux during film growth or the mechanical stress may induce the transformation of the oxygen vacancy stabilized cubic phase of zirconia into the m-phase.