Effects of strain on the electronic, optical, and ferroelectric transition properties of HfO2: ab initio simulation study

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.