Structural evolution of epitaxial CeO2-HfO2 thin films using atomic-scale observation: Formation of ferroelectric phase and domain structure

Abstract

We investigated the structural evolution of 0.1CeO2–0.9HfO2 epitaxial films upon thermal annealing using X-ray diffraction and scanning transmission electron microscopy. The films were deposited via a sputtering method at room temperature onto (001)yttria-stabilized zirconia substrates and subsequently annealed. The crystalline phase of the deposited films changed from amorphous to highly symmetric phase (tetragonal and/or cubic phases) and then to orthorhombic with increasing annealing temperature. Isothermal annealing at 900 °C resulted in a series of transitions from the highly symmetric phase to a mixture of highly symmetric and orthorhombic phases, and finally to the orthorhombic phase as a function of time. Atomic-scale observations revealed the formation of nanosized orthorhombic domains within the dominant tetragonal phase; their number density and size increased with the annealing time. However, in-situ transmission electron microscopy of the crystallized film with the dominant orthorhombic phase revealed that the orthorhombic phase transformed back to the tetragonal phase above 540 °C upon reheating. Therefore, the tetragonal phase is a prerequisite for forming the orthorhombic phase, and the tetragonal-to-orthorhombic phase transition is reversible. Thus, this study offers a strategy to control the formation of tetragonal and orthorhombic phases of Ce-substituted HfO2 ferroelectric films by optimizing the annealing conditions.