Effect of surface/interface energy and stress on the ferroelectric properties

The ferroelectricity in fluorite-structure type oxide thin films can be attributed to the stabilization of a noncentrosymmetric orthorhombic phase (space group: Pca2₁), which is a nonequilibrium phase of bulk HfO₂ or ZrO₂. Various mechanisms, including surface or interface/grain boundary energy and film stress, have been suggested to stabilize the metastable ferroelectric phase, and kinetic mechanisms have been shown to inhibit the formation of the stable monoclinic phase. Film thickness has been demonstrated to be a critical factor that strongly influences the structure and ferroelectric response of films. Although the film thickness is a key factor for the formation of the ferroelectric phase in fluorite-structure type thin films, details of it have not been clearly elucidated yet. In this chapter, the existing literature on the thickness-dependent ferroelectricity in fluorite-type oxides is comprehensively reviewed, and the previous studies to mitigate the thickness effects are discussed.