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Strain effect on the stability in ferroelectric HfO2 simulated by first-principles calculations

The strain effect on the phase stability of bulk HfO2 is theoretically investigated with the density functional calculations. The origin of the ferroelectric (FE) characteristic in HfO2 is considered to be the formation of a non-centrosymmetric polar orthorhombic (PO) phase. The centrosymmetric nonpolar orthorhombic (O) phase is an antiferroelectric (AFE) structure, which has a smaller energy than PO-phase at unstrained condition. Applied biaxial strain can manipulate the relative stability among phases and also the ferroelectricity. Compressive strain is favored to transform HfO2 from monoclinic to PO-phase. The PO-phase HfO2 possesses the ferroelectricity with the remnant polarization of 66 µC/cm2 and the energy barrier of 70 meV/atom for the polarization switching from up to down. Tensile strain is found out to raise the relative energy of AFE state with respect to FE state. Furthermore, the Al doping in HfO2 could lower the energy barrier for polarization switching and also enhance the population of PO-phase.