Stability of the orthorhombic phase in (111)-oriented YO_1.5-substituted HfO₂ films

Abstract

Ferrolectric orthorhombic HfO₂-based films are potential candidates for future ferroelectirc applications. The stability of the orthorhombic phase of epitaxial YO_1.5-substituted HfO₂ films on (111) yttria-stabilized zirconia substrates was investigated for various compositions, thicknesses and underlying layers. In the case of 14 nm-thick films, only 7 mol % film consisted of a single orthrohombic phase. With decreasing amount of YO_1.5 or increasing thickness, the paraelectric monoclinic phase formed in the films. On the contrary, the highly symmetric phase was grown by the increasing amount of YO_1.5. The effect of the underlying layer was also investigated. Differences in the degree of relaxation of the buffer layer give different amounts of the monoclinic phase. These results imply the importance of the compostions as well as of the strain state to (111)-oriented epitaxial Y-substituted HfO₂ films.