Ferroelectricity emerging in strained (111)-textured ZrO₂ thin films

(Anti-)ferroelectricity in complementary metal-oxide-semiconductor (CMOS)-compatible binary oxides have attracted considerable research interest recently. Here, we show that by using substrate-induced strain, the orthorhombic phase and the desired ferroelectricity could be achieved in ZrO₂ thin films. Our theoretical analyses suggest that the strain imposed on the ZrO₂ (111) film by the TiN/MgO (001) substrate would energetically favor the tetragonal (t) and orthorhombic (o) phases over the monoclinic (m) phase of ZrO₂, and the compressive strain along certain 〈11-2〉 directions may further stabilize the o-phase. Experimentally ZrO₂ thin films are sputtered onto the MgO (001) substrates buffered by epitaxial TiN layers. ZrO₂ thin films exhibit t- and o-phases, which are highly (111)-textured and strained, as evidenced by X-ray diffraction and transmission electron microscopy. Both polarization-electric field (P-E) loops and corresponding current responses to voltage stimulations measured with appropriate applied fields reveal the ferroelectric sub-loop behavior of the ZrO₂ films at certain thicknesses, confirming that the ferroelectric o-phase has been developed in the strained (111)-textured ZrO₂ films. However, further increasing the applied field leads to the disappearance of ferroelectric hysteresis, the possible reasons of which are discussed.