Stability of ferroelectric and antiferroelectric hafnium–zirconium oxide thin films

Abstract

Hafnium–zirconium oxide (HZO) thin films are of interest due to their ability to form ferroelectric (FE) and antiferroelectric (AFE) oxide phases. Density functional theory is employed to elucidate the stabilization mechanisms of both FE HZO thin films and AFE ZrO2 films. The FE orthorhombic phase is primarily stabilized by in-plane tensile strain, which spontaneously occurs during the synthesis process, and this is more effective for HZO than HfO2. Layer-by-layer stack models and core-matrix three-dimensional models of the polymorphs reveal that the electrostatic component of interfacial free energy can play a critical role in the formation of the AFE tetragonal phase in ZrO2 and the “wake-up” effect for FE HZO.

Hafnium–zirconium oxide interface models with a semiconductor and metal for ferroelectric devices

“…The atoms in the HZO layer had to be frozen to prevent dipoles from rotating to the side. 13,14 In all the models in this study, bridging O atoms at the interfaces were allowed to relax and other atoms were fixed at their bulk equilibrium positions. The validity of relaxing just one layer of interfacial O atoms was checked by allowing additional layers of interfacial atoms to relax for a subset of the calculations.…”

Local Epitaxial Templating Effects in Ferroelectric and Antiferroelectric ZrO2

“…The AFE to FE phase transition is thermodynamically exothermic in the bulk state, that is, Δ U < 0, but the change in the interfacial interactions, Δγ = (FE) – γ­(AFE), depends on various factors such as electrical characteristics of the phase and bond strains at the interface. The interfacial interactions have crucial effects on the phase stability as the grain size decreases . In the MIS structure, each of the grains has interfaces to either the top or bottom electrodes (TiN or SiO 2 ) or neighboring grains, so there are three different interfacial free energies: grain boundary, SiO 2 , and TiN.…”

High‐Throughput Electronic Structures and Ferroelectric Interfaces of HfO2 by GGA+U(d,p) Calculations

“…However, it was recently noted that the DFT energy for the electrode–dielectric system actually follows total energies once the strain and lattice‐matching conditions are included for the FE oxide and TiN electrode, by the attraction of Ti sites to O sites in HfO 2 . [ 34 ]…”

“…However, it was recently noted that the DFT energy for the electrode-dielectric system actually follows total energies once the strain and latticematching conditions are included for the FE oxide and TiN electrode, by the attraction of Ti sites to O sites in HfO 2 . [34] We built the ferroelectric-phase HfO 2 interface with compound metal TiN as the contact metal, as TiN is used in HfO 2 ferroelectric field-effect transistors. [36] To minimize the mismatch, the (3 Â 5 p t) TiN( 111) is stacked on top of (2 Â 3) HfO 2 (001) plane, with the lattice constants of 10.54 Å and 15.12 Å in the xand y-direction, respectively.…”

“…This U(d,p) approach also allows us to study the effect of interfaces, grain and domain boundaries, and phase energies on FE and antiferroelectricity (AFE) in HfO 2 and its alloys, [34] without restriction due to the artificial lowering of bandgap in the GGA approach. The kinetics of phase transitions during annealing of HfO 2 are often assumed to follow Ostwald's rule of phases, [35] that the system steps sequentially from the least stable tetragonal phase to the most stable monoclinic phase via the orthorhombic FE phase.…”