Theoretical Understanding of the In-plane Tensile Strain Effects on Enhancing the Ferroelectric Performance of Hf0.5Zr0.5O2 and ZrO2 Thin Films

Abstract

The in-plane tensile strain was reported to enhance the ferroelectricity of Hf1-xZrxO2 thin films by promoting the formation of the polar orthorhombic (PO-) phase. However, its origin remains yet to be identified unambiguously, although the stain-related thermodynamic stability variation was reported. This work explores the kinetic effects that have been overlooked to provide a precise answer to the problem, supplementing the thermodynamic calculations. The in-plane strain-dependent phase fractions were identified by calculating the relative influences of thermodynamic factor (Boltzmann distribution of free energies of polymorphs) and kinetic factor (transition rate between polymorphs using the Johnson-Mehl-Avrami equation). The monoclinic (M-) phase constitutes the ground state under almost all conditions. However, its formation is kinetically suppressed by the high activation barrier for the transition from the tetragonal (T-) phase. In contrast, the PO-phase formation is dominated by thermodynamic effects and is promoted under in-plane tensile strain due to the energetic stabilization of the PO-phase, while the T- to PO-phase transition is kinetically probable due to a low activation barrier. The in-plane tensile strain also lowers the activation barrier of TM, hence, the optimal tensile strain for the PO-phase formation varies by thermal conditions. The remanent polarization was calculated using spontaneous polarization and the PO-phase fraction. The in-plane tensile strain of 2~2.5% and moderate annealing at approximately 700 K are optimum for increasing ferroelectricity by 34% in Hf0.5Zr0.5O2 and 106% in ZrO2 along <111> orientation.