Extended Data Fig. 3: Ferroic phase insights: proximity to temperature-dependent phase transition.
From: Ultrathin ferroic HfO2–ZrO2 superlattice gate stack for advanced transistors
a, Schematic of temperature-dependent AFE–FE phase evolution in fluorite-structure oxides. At lower temperatures, the higher symmetry tetragonal phase is expected to transition to the lower symmetry orthorhombic phase. b, Schematic crystal field splitting diagram for fluorite-structure polymorphs; the symmetry-induced e-splitting (rhombic distortion, ∆R), besides the typical t2-e splitting (tetrahedral distortion, ∆T) present in all fluorite-structure phases, provides a spectroscopic signature for the polar O phase (Methods). c, Temperature-dependent XAS at the oxygen K edge for a 2-nm HZH bare film demonstrating clearer spectroscopic signatures of the FE O phase emerge slightly below room temperature.d, Simulated oxygen K-edge XAS spectra (Materials Project) for the respective O and T phases. XAS provides spectroscopic signatures to distinguish between the O and T phases (difficult to resolve from GI-XRD). e, Prototypical C–V behaviour for mixed AFE–FE (shoulder-like features in addition to the characteristic butterfly-like shape) and FE films (just butterfly-like) in MIM capacitor structures. f, Temperature-dependent C–V for thicker HZH multilayers of the same periodicity (in MIM capacitor structure) demonstrating an evolution from mixed-ferroic to FE-like hysteresis upon cooling slightly below room temperature. Thinner HZH multilayers films suffer from leakage limitations, preventing such hysteretic C–V measurements. The thicker HZH multilayers of the same periodicity—annealed at the same low-temperature condition to maintain the multilayer structure—demonstrate a similar mixed ferroic to FE phase transition slightly below room temperature as the thinner 2-nm multilayer (c).
