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

摘要

Stabilization of high-temperature phases such as tetragonal (t-) 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 t-phase as the annealing temperature increased. Despite high-temperature annealing, the t-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 t(111)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_x) between the ZrO2 and Si.