Theoretical understanding of the in-plane tensile strain effects on enhancing the ferroelectric performance of HfZrO and ZrO thin films.

In-plane tensile strain was reported to enhance the ferroelectricity of HfZrO thin films by promoting the formation of a polar orthorhombic (PO-) phase. However, its origin remains yet to be identified unambiguously, although a strain-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 the thermodynamic factor (Boltzmann distribution of free energies of polymorphs) and the 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, 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 PO-phase formation varies depending on the 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 were optimum for increasing ferroelectricity by 34% in HfZrO and 106% in ZrO along the 〈111〉 orientation.