Single Atomic Layer Ferroelectric on Silicon

Enhanced polarization and endurance properties of ZrO2-based ferroelectric capacitor using HfO2interfacial layer

Recently discovered ferroelectricity in fluorite-structure ZrO2thin film has attracted increasing and intense interest due to its lower crystallization temperature and higher content in nature in comparison to hafnium oxide. Here, the effect of HfO2interfacial layer on the ferroelectric properties of ZrO2thin films is investigated systematically by designing four types of interfacial structures. It is revealed that the ferroelectric orthorhombic phase, remanent polarization, and endurance can be improved in ZrO2thin film by inserting both a top- and bottom-HfO2interfacial layer. A maximal ferroelectric remanent polarization (2Pr) of 53.4μC cm-2and an optimal endurance performance of 3 × 107field cycles under frequency of 100 kHz are achieved in Pt/HfO2/ZrO2/HfO2/Pt capacitors, with ferroelectric stacks being crystallized at 450 °C via post-deposition annealing method. X-ray photoelectron spectroscopy analysis confirms that the HfO2bottom-layer plays a very important role in the formation of a higher ratio o-phase, thus enhancing the ferroelectricity. These results suggest that designing appropriate interfaces would help achieve excellent ferroelectric properties in ZrO2films.

Hafnium-doped zirconia ferroelectric thin films with excellent endurance at high polarization

Using thermal atomic layer deposition and subsequent rapid thermal annealing without the need for metal clamping atop, a remanent polarization (P_r) of 25.5 μC cm^-2 was achieved in a 10 nm-thick ZrO₂ film deposited on a W bottom electrode. Hafnium doping was further explored to improve the ferroelectric properties in P_r as well as the endurance of zirconia-based thin films. A significantly enhanced P_r reaching 41 μC cm^-2 was obtained for 10 nm-thick hafnium-doped ZrO₂ with an optimal Zr : Hf ratio of 3 : 1. Importantly, owing to the greatly reduced leakage, the optimal hafnium-doped ZrO₂ thin films exhibited superior retention and outstanding endurance performances at relatively high polarizations, free of serious degradation for up to 2.3 × 10⁹-6.8 × 10⁹ field cycles at an initial P_r of 27 μC cm^-2 and were even capable of over 10⁷ cycles at a maximum P_r of 41 μC cm^-2. The superb ferroelectricities were demonstrated on big-sized capacitors as well as sub-micrometer ones, isolated or in array. This would empower zirconia-based ferroelectric thin films as a competitive front-runner for practical applications in ferroelectric-related nanoelectronics.

Emergent ferroelectricity in subnanometer binary oxide films on silicon

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 (ZrO2) thin films on silicon. The emergent ferroelectricity and hysteretic polarization switching in ultrathin ZrO2, 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.

Ferroelectricity in thin films driven by charges accumulated at interfaces

A simple view of ferroelectricity is proposed for a thin film with uniform polarization oriented perpendicular to its surface, starting from the assumption that this situation is always accompanied by charge accumulation in the outer metal electrodes, in the contamination layers or near the surface, in the ferroelectric film itself. Starting with the formula derived for an "elemental" dipole moment in the film, simple statistical mechanics allows one to derive hysteresis cycles, and their dependence on temperature starting with only two parameters: the dielectric constant of the material and the maximum value of the dipole moment of a unit cell. Values obtained for Curie temperatures and coercive fields agree well with experiments. "Exact" energy dependencies on the asymmetry parameter are derived, and their connection with the Landau-Ginsburg-Devonshire is proven. By considering also the dipolar interaction in a continuous model, in addition to the ordering energy in the presence of surface charge accumulation, one may estimate the distribution of the polarization inside the film and the validity of the hypothesis of uniform polarization.

The coexistence of ferroelectricity and topological phase transition in monolayer α-In2Se3 under strain engineering

The coexistence of ferroelectricity and topological phase transition in monolayer α-In₂Se₃ through strain engineering are investigated by first-principles calculation. The results show that with the spontaneous polarization increasing, the transition barrier decreases, approximately linearly related to the applied strain, and the effect of biaxial compressive strain within the in-plane is two orders of magnitude greater than that of tensile strain along the out-of-plane. The results also show that a Dirac cone with a linear dispersion relationship occurs at the high symmetry Γ point within the Brillouin region whatever strain pattern is applied. By analyzing the orbital characters of the electronic states near the Fermi level we find that the electronic structure presents obvious topological phase transition, indicating that monolayer α-In₂Se₃ is not only an excellent 2D ferroelectric material but also a topological material.

Causes of ferroelectricity in HfO2-based thin films: an ab initio perspective

The combined effects of doping and biaxial strain explain the transformation of HfO₂ grains into the ferroelectric phase observed during thermal annealing.