In article number 2100009, the ab initio calculations by Cheol Seong Hwang, Jung-Hae Choi, and co-workers show that wurtzite-type AlN can have a suitable range of ferroelectric switching barrier (Ea) and spontaneous polarization (Ps) by forming superlattice structures with ScN. The Ea is dominated by the local structure instability, while the Ps is significantly affected by the structural parameters and the Born effective charge.

In article number 2100028, Jiyoung Kim, Si Joon Kim, and co-workers comprehensively review the various factors such as doping effects, deposition method, annealing method and conditions, and substrate material for obtaining fluorite-structure ferroelectrics with low-thermal-budget processes (400 °C or less). These low-thermal-budget processes facilitate not only the integration of ferroelectric circuits in the back-end-of-line, but also the applications to flexible and wearable electronics.

With the help of transmission Kikuchi diffraction the major physical mechanism of antiferroelectric-like behavior as well as of the wake-up effect is determined by Maximilian Lederer, Konrad Seidel, and co-workers in article number 2100086. Furthermore, guidelines on how to influence this behavior by crystallographic texture and adjacent layers, as well as process conditions are identified. Finally, electric-field-induced crystallization is discovered as a new effect in hafnium oxide thin films.

Ferroelectricity facilitated by field-induced phase transition is demonstrated by Takao Shimizu, Hiroshi Funakubo, and co-workers in article number 2000589. Their X-ray diffraction and scanning transparent electron microscopy studies show the as-deposited paraelectric tetragonal phase transforms to the ferroelectric orthorhombic phase in the Y-doped Hf_0.5Zr_0.5O₂ film by applying a strong electric field.

HfO₂-based ferroelectrics applications in nanoelectronics are discussed, especially in topics not well developed up to now, such as microwaves, energy harvesting, and neuromorphic devices working as artificial neurons and synapses.

Epitaxial control of hafnia thin films can be effective while mediating the phase competition of monoclinic, orthorhombic, and rhombohedral phases. Therefore, the effects of growth temperature, epitaxial strain from substrate and bottom electrode, oxygen content, and film thickness variations are reviewed. Subsequent influences on ferroelectricity such as wake-up effect, imprint, and retention are also discussed.

The various factors such as doping effects, deposition method, annealing method and conditions, and substrate material for obtaining fluorite-structure ferroelectrics with low-thermal-budget (400 °C or less) processes are reviewed. These low-thermal-budget processes facilitate not only the integration of ferroelectric circuits in the back-end-of-line but also applications for flexible and wearable electronics.

An analog resistive memory device is fabricated with a back-end-of-line compatible process. Two TiN electrodes are separated by a ferroelectric, 4.5 nm-thick HfZrO₄ (HZO) layer, and a semiconducting TiO_x interlayer. Synaptic functionality is obtained, in the pristine state as well as after wake-up, and originates from the ferroelectric properties of the HZO layer.

The influence of the oxygen concentration in the IrO_x electrodes during crystallization anneal on the ferroelectric properties is characterized by electrical, chemical, and structural methods. A significant change in the ferroelectric properties after annealing in forming gas, O₂, and N₂ atmospheres is found as a result of the interaction of the top electrode with the dielectric layer.

Rapid ferroelectric switching, within 200 ns and no obvious loss of polarization for over 8000 cycles are observed in 45 nm-thick (Al,Sc)N thin films deposited by reactive sputtering. This newly discovered nitride ferroelectric material offers large and stable remanent polarizations of 115 μC cm⁻² and compatibility with complementary metal–oxide–semiconductor (CMOS) transistor processing.

The phase transition in ferroelectric Y-doped Hf_0.5Zr_0.5O₂ epitaxial thin film induced by an electric field is demonstrated, which triggers the emergence of ferroelectric characteristics from the nonpolar tetragonal phase to the polar orthorhombic phase. This ferroelectricity induced by the electric-field-induced phase transition is quite an important mechanism by which ferroelectricity is exhibited in flurorite-oxides ferroelectrics.

During annealing, thin-film HfO₂ in capacitors initially crystallizes into a ferroelectric phase but then transforms to a monoclinic structure within seconds. The kinetic barrier to this conversion is above 1 eV f.u.⁻¹ Fast temperature ramp rates (which minimize the time above the crystallization temperature) can boost remanent polarization more than twofold.

The ferroelectric properties (switching barrier [E_a] and spontaneous polarization [P_s]) of (AlN)_n/(ScN)_m superlattice structures are investigated and compared with those of (Al,Sc)N solid solutions, based on density functional theory calculations. The E_a and P_s values decrease as the ScN fraction increases, which are attributed to the structural parameters and local structural instability.

The spherical recess channel transistor structure is applied to metal–ferroelectric–metal–insulator–semiconductor ferroelectric field-effect transistors (FeFETs), and through numerical simulation based on various materials and device design parameters, the direction of high-performance FeFETs is presented.

The behavior of hydrogen-incorporated hafnium zirconium oxide (HZO) and the effect of post O₂ plasma treatment are presented. HZO thin film fabricated by H₂O₂ oxidants shows a leaky electrical property resulting from hydrogen-related trap sites. O₂ plasma treatment is proposed to reduce the leakage current by reducing the degree of hydrogen incorporation and oxygen vacancies inside the film.

The ferroelectric hafnium oxide material system offers a unique combination of physical properties, enabling novel sensor applications and nanoelectromechanical systems, for which long-term stability is an essential concern. In this work, aging effects in polycrystalline, Si-doped HfO₂ thin films are assessed, which are found to be accelerated by temperature, low film thickness, and high dopant content.

The choice of gas atmosphere during rapid thermal annealing (RTA) impacts the chemistry of top electrode and interface of ferroelectric IrO₂/Hf_0.5Zr_0.5O₂/IrO₂ capacitors. Strongly modified chemical states are observed for RTA in forming gas, where a reduction to metallic Ir with incorporated and possibly mobile oxygen is found—which may affect the metal–insulator–metal (MIM)'s performance instabilities.

Enhanced endurance (10¹⁰ cycles) of Hf-rich hafnium zirconate using 1.6% La dopant is reported. Faster wake-up behavior is promoted by stacking the ferroelectric layer with ZrO₂. A correlation of ferroelectric characteristics (remanent polarization and endurance) with the initial content of the phase extracted by Rietveld refinement of the X-ray diffraction spectra brings new insights into modalities to control the ferroelectric behavior of 10 nm hafnium zirconate films grown by atomic layer deposition, with potential application in ferroelectric random-access memory.

Ferroelectric aluminum scandium nitride film bulk acoustic resonators (FBARs), with a quality factor of 572, electromechanical coupling coefficient of 11.4%, and operating at 3.17 GHz, are demonstrated. The operations of such ferroelectric FBARs are studied at the two polarization states (i.e., nitrogen polar and metal-polar). Such devices are used in tunable filters in the super-high-frequency range.

In situ monitoring with glow discharge optical emission spectroscopy during growth of Al_0.6Sc_0.4N films reveals a connection between target mode and film properties. Poisoned target mode favors synthesis of ferroelectric (0002) textured wurtzite films, whereas metallic target mode favors (111) rocksalt growth and leads to clear microstructure heterogeneities, degradation of wurtzite phase purity, and nonferroelectric films.

A novel combinatorial approach to investigate the ferroelectricity of Hf_xZr_1−xO₂ thin films via a saturated/non-saturated atomic layer deposition (ALD) technique is proposed. Detailed observation of continuous variation of the ferroelectric–antiferroelectric transition is successfully applied. It is possible to achieve a higher efficiency method for studying ferroelectricity change in terms of the doping and composition of HfO₂-based ferroelectric thin films through this combinatorial approach.

From first-principles investigation, two inequivalent, nonpolar, orthorhombic phases of the same space group Pbca (no. 61) are found to exist. Their energy is below the ferroelectric Pca21 (no. 29) and they may reduce the remanent polarization of HZO thin films.

Pulsed laser irradiation of a solid substrate results in extremely fast heating and cooling rates of a subsurface region, which gives rise to quenching of metastable phases during crystallization. The possibility of forming a ferroelectric phase in Hf_0.5Zr_0.5O₂ (HZO) thin films by their crystallization with laser pulses of millisecond and nanosecond duration is demonstrated.

Antiferroelectricity in hafnium oxide is caused by ferroelastic switching due to the in-plane stress σ of the thin film. In addition, electric field–induced crystallization into the ferroelectric phase and the crystallographic texture affect the ferroelectric response over field cycling. Multiple process parameters, such as annealing temperature, influence this behavior.

A reactive magnetron sputtering process, based on segmented aluminum–scandium targets, is developed for growth of ferroelectric scandium–aluminum–nitride (Sc_xAl_1−xN) films. Sc_xAl_1−xN films as thin as 25 nm with full width at half maximum better than 2.8° are achieved. The process provides large tuning of scandium content over and residual stress, enabling coercive field scaling for integrated device applications.