Does a Morphotropic Phase Boundary Exist in Zr_xHf_1-xO₂-Based Thin Films?

Zr_xHf_1-xO₂-based materials have attracted considerable interest due to their excellent scalability and compatibility with complementary metal-oxide-semiconductor technology. Following the discovery of ferroelectricity in these materials, it has opened new avenues not only redefining ferroelectric memory but also as a promising high-k dielectric for advanced non-volatile and volatile memory applications by utilizing specific crystalline phases to tune the electrical parameters. Various fundamental factors influencing the formation of ferroelectricity in Zr_xHf_1-xO₂ have been identified and distinguished from those in the classical perovskite ferroelectrics. Notably, a sharp increase in the dielectric constant near 0 V observed in Zr-rich Zr_xHf_1-xO₂ films achieved by tuning the fabrication parameters has been attributed to the presence of a morphotropic phase boundary. This study investigates 6 nm Zr-rich Zr_xHf_1-xO₂ thin film metal-insulator-metal capacitors using a combination of experimental methods and machine learning-based molecular dynamics simulations. The study provides insight into the physical mechanisms that enhance the dielectric constant near 0 V and attributes it to the orthorhombic phase rather than a morphotropic phase boundary. The work discusses the limitations in the practical application of the high dielectric constant observed near 0 V. Additionally, it highlights similarities and differences between Zr_xHf_1-xO₂ and the well-known morphotropic phase boundary in PbZr_xTi_1-xO₃.