Effects of strain on the electronic, optical, and ferroelectric transition properties of HfO₂:ab initiosimulation...

The amazing ferroelectricity observed in Hafnium oxide (HfO2) thin films is due to the existence of non-centrosymmetric orthorhombicPca2₁phase. In this study, the structural, electronic, optical, and polarization switching properties of the orthorhombicPca2₁phase with various strains are investigated by employing density functional theory (DFT) calculations based on the DFT. Out-of-plane and in-plane strains are taken into consideration. The results indicate that the band-gap values of orthorhombic HfO₂increase at first and then decrease from compressive strain to tensile strain due to the variation of the valence band maximum and the conduction band minimum. Moreover, orthorhombic HfO₂transforms into a direct bandgap material when compressive strain along thezdirection is larger than or equal to 5%. A red-shift occurs under both tensile and compressive strains in the absorption edge compared with instinct HfO₂in the absorption spectra. The absorption intensity of the strained HfO₂is enhanced with the energy below 5.5 eV. The energy barriers of path A and path B of the polarization switching mediated by non-polar tetragonal phase are 116.8 and 91.2 meV per formula unit (meV/f.u.), respectively. In polarization switching without mediated by non-polar phase, the energy barriers decrease with the increasing of out-of-plane compressive strain along thexdirection and tensile strain along theydirection compared with strain free structures.