Phase Transition and Microstructural Changes of Sol–Gel Derived ZrO₂/Si Films by Thermal Annealing: Possible Stability of Tetragonal Phase without Transition to Monoclinic Phase

Stabilization of high-temperature phases such as tetragonal (<i>t</i>-) or cubic phases has been a pivotal issue for technological applications of polymorphic ZrO₂. In this work, we fabricated ZrO₂/Si films using a sol–gel deposition route and investigated the phase transformation, microstructural evolution, surface morphological changes, and interfacial chemical structures by thermal annealing. The ZrO₂ precursor solution was prepared using a zirconium acetylacetonate, coated, dried on Si substrates, and finally annealed at 300–950 °C in ambient air. The sol–gel-derived ZrO₂ layer crystallized into the <i>t</i>-phase as the annealing temperature increased. Despite high-temperature annealing, the <i>t</i>-phase was stabilized without a noticeable transition to the monoclinic phase, probably because of the relatively low film thickness (∼15 nm), enlarged surface/interface area due to thermal grooving, and strain effects. The probable <i>t</i>(112) orientation was developed after annealing at ≥800 °C, which could be related to minimization of the sum of the surface, interface, and strain energies. High-temperature thermal annealing resulted in the contraction of the ZrO₂ layer as a result of the pyrolysis of the remnant organics, surface roughening by thermal grooving, and thickening of the amorphous interface layer (predominantly SiO_<i>x</i>) between the ZrO₂ and Si