Systematic studies on polycrystalline Hf1–xZrxO2 films with varying Zr contents show that HfO2 films are paraelectric (monoclinic). If the Zr content is increased, films become ferroelectric (orthorhombic) and then antiferroelectric (tetragonal). HfO2 shows very good insulating properties and it is used in metal-oxide-semiconductor field-effect devices, while ZrO2 shows good piezoelectric properties, but it is antiferroelectric. In between, Hf0.5Zr0.5O2 shows good ferroelectric properties at the expense of poorer insulating and piezoelectric properties than HfO2 and ZrO2, respectively.
Here, we explore the ferroelectric, insulating, and piezoelectric properties of a series of epitaxial films of Hf1–xZrxO2 with different compositions. We show that epitaxial growth permits the stabilization of the ferroelectric phase in a whole range of Zr content (from x = 0 to x = 1). In epitaxial ZrO2 films, ferroelectricity coexists with better piezoelectric and insulating properties than Hf0.5Zr0.5O2, and in HfO2 epitaxial films, ferroelectricity coexists with better insulating properties than Hf0.5Zr0.5O2. For the case of ZrO2 films, large electroresistance is also observed. In both cases, the ferroelectric endurance is poorer than that for Hf0.5Zr0.5O2.