Thickness dependence of the crystallization and phase transition in ZrO 2 thin films

Abstract

Fluorite-structure binary oxides (e.g., HfO₂ and ZrO₂) have attracted increasing interest for a broad range of applications including thermal barrier coatings, high-<i>k</i> 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 <i>in-situ</i> 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 (<i>T</i>_C). When annealing at temperatures higher than <i>T</i>_C, 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.