Thickness dependence of the crystallization and phase transition in ZrO2 thin films

Fluorite-structure binary oxides (e.g., HfO2 and ZrO2) have attracted increasing interest for a broad range of applications including thermal barrier coatings, high-k dielectrics, and novel ferroelectrics. A crystalline structure plays a crucial role in determining physical and chemical properties. Structure evolution of ZrO2 thin films, particularly down to the nanometer scale, has not been thoroughly studied. In this work, we carried out systematic annealing analysis on the ZrO2 thin films. Through in-situ high-temperature X-ray diffraction (XRD) characterizations, a thickness dependence of crystallization and phase transition is observed. Irrespective of the thickness (10-300 nm), the as-prepared amorphous ZrO2 thin films are preferentially crystallized into a tetragonal (t) structure (high-temperature phase), which can be preserved down to room temperature (RT) upon annealing at the corresponding crystallization temperature (TC). When annealing at temperatures higher than TC, the transition from t to monoclinic (m; RT phase) will occur, and the quantity of the transition strongly depends on the film thickness. Our work expands the basic understanding of the phase transition in the ZrO2 thin films, and offers a path to the selective control over the phase structure for novel functionalities.