Emergent ferroelectricity in subnanometer binary oxide films on silicon

Abstract

The critical size limit of voltage-switchable electric dipoles has extensive implications for energy-efficient electronics, underlying the importance of ferroelectric order stabilized at reduced dimensionality. We report on the thickness-dependent antiferroelectric-to-ferroelectric phase transition in zirconium dioxide (ZrO 2 ) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO 2 , conventionally a paraelectric material, notably persists down to a film thickness of 5 angstroms, the fluorite-structure unit-cell size. This approach to exploit three-dimensional centrosymmetric materials deposited down to the two-dimensional thickness limit, particularly within this model fluorite-structure system that possesses unconventional ferroelectric size effects, offers substantial promise for electronics, demonstrated by proof-of-principle atomic-scale nonvolatile ferroelectric memory on silicon. Additionally, it is also indicative of hidden electronic phenomena that are achievable across a wide class of simple binary materials.

Neuromorphic devices based on fluorite‐structured ferroelectrics

“…In 2020, it was suggested that the robust ferroelectricity of (Hf,Zr)O 2 can be retained even when the film thickness decreases to 1 or 1.5 nm, and it was further suggested that unit‐cell‐scale information storage would theoretically be possible because of the flat phonon band of ferroelectric HfO 2 31,32 . Moreover, it has been experimentally demonstrated that ferroelectricity in fluorite‐structured ferroelectric could be retained below 5 Å or even at a single atomic layer 33,34 . Recently, freestanding fluorite‐structured ferroelectrics for flexible devices have also been reported with robust ferroelectricity 35,36 .…”

“…31,32 Moreover, it has been experimentally demonstrated that ferroelectricity in fluoritestructured ferroelectric could be retained below 5 Å or even at a single atomic layer. 33,34 Recently, freestanding fluorite-structured ferroelectrics for flexible devices have also been reported with robust ferroelectricity. 35,36 These achievements demonstrated the intrinsic advantage of fluorite-structured ferroelectrics for nano-electronic devices, which are the basic building blocks for neuromorphic computing.…”

Freestanding complex-oxide membranes

“…Expanding this strategy to FE oxides would provide opportunities, for example, to add nonvolatile functionality or develop neuromorphic devices [190]. In this regard, binary oxides with fluorite structure are promising candidates for such applications: polycrystalline films of Hf 0.8 Zr 0.2 O 2 and ZrO 2 grown by atomic-layer deposition on silicon display FE behavior down to just a single unit cell [191,192]; however, interface optimization, as well as integration of single crystal films may motivate researchers to expand the epitaxial lift-off techniques to fluorite based compounds.…”

Ferroelectricity in HfO2 from a Coordination Number Perspective

“…Ferroelectrics based on hafnia, such as doped HfO 2 − and Hf 0.5 Zr 0.5 O 2 , − are quite unusual compared with conventional perovskite ferroelectrics. − First of all, though these oxides consist of mixed ionic-covalent bonding, it is unclear whether the covalence should play any role in the ferroelectricity. On the other hand, in perovskite oxide ferroelectrics, the existence of orbital hybridization and covalent bonding is important for the ferroelectric distortion .…”