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Hafnium oxide is found to be a favorable material for ferroelectric nonvolatile memory devices. Its compatibility with complementary metal–oxide–semiconductor processes, the relatively low crystallization temperature when zirconium‐doped, and the thickness scaling are among the advantageous properties of hafnium oxide. Different requirements must be fulfilled for different applications of hafnium oxide. Herein, high‐temperature annealing and operation conditions are analyzed in order to investigate nonvolatile memories for automotive applications. A strong imprint behavior (shift in coercive voltages) is observed after annealing hafnium–zirconium–oxide thin films at temperatures varied between 100 and 200 °C. The imprint behavior is a significant challenge in many applications. Therefore, to reduce/recover the undesirable imprint behavior caused by high‐temperature treatment, two different ways are successfully examined and delineated here: endurance cycling and applying high electric fields. [ABSTRACT FROM AUTHOR]

Due to its ferroelectricity, hafnium oxide has attracted a lot of attention for ferroelectric memory devices. Amongst different dopant elements, zirconium is found to be beneficial due to the relatively low crystallization temperature of hafnium‐zirconium‐oxide (HZO), thus it is back‐end‐of‐line (BEoL) compatible. The thickness of HZO has a significant impact on ferroelectric device reliability. High operation temperatures and high endurance are important criteria depending on the application. Herein, various HZO thicknesses (7, 8, and 10 nm) in MFM (metal‐ferroelectric‐metal) capacitors are investigated at varying operation temperatures (25 to 175 °C) at varying electric fields (±3 to ±5.4 MV cm−1) with respect to polarization, leakage current, endurance, and retention. 7 nm HZO showed promising results with an endurance of 107 cycles, with a low leakage current density, and almost no retention loss after 10 years. Extrapolated results at operation conditions (±2 MV cm−1 and 10 MHz) showed an endurance of 1010 cycles. [ABSTRACT FROM AUTHOR]

Pure samples of colorless, air‐stable Ba(BO2OH) crystals were obtained from Ba(NO3)2 and H3BO3 under the ultra‐alkaline conditions of a KOH hydroflux at about 250 °C. The product formation depends on the water‐base molar ratio and the molar ratio of the starting materials. B(OH)3 acts as a proton donor (Brønsted acid) rather than a hydroxide acceptor (Lewis acid). Ba(BO2OH) crystallizes in the non‐centrosymmetric orthorhombic space group P212121. Hydrogen bonds connect the almost planar (BO2OH)2− anions, which are isostructural to HCO3−, into a syndiotactic chain. IR and Raman spectroscopy confirm the presence of hydroxide groups, which are involved in weak hydrogen bonds. Upon heating in air to about 450 °C, Ba(BO2OH) dehydrates to Ba2B2O5. Moreover, the non‐centrosymmetric structure of Ba(BO2OH) crystals was verified with power‐dependent confocal Second Harmonic Generation (SHG) microscopy indicating large conversion efficiencies in ambient atmosphere. [ABSTRACT FROM AUTHOR]

The properties of hybrid ferroelectric (FE) and antiFE (AFE) films integrated in a single capacitor stack is reported. The stack lamination (4 × 5 nm) or (2 × 10 nm) using an Alumina (Al2O3) interlayer, material type (Si‐doped HfO2 (HSO) and Zr doped HfO2 (HZO)), precursor condition (TEMA‐Hf and Hf/ZrCl4), or dopant concentration (Si and Zr) are investigated for laminate stack properties. Optimized FE properties (higher 2Pr and a lower fraction of the monoclinic phase) are observed at (2 × 10 nm) laminates compared to a single 20 nm film thickness. The hybrid laminate stack as FE‐FE, AFE‐FE, DE‐FE, or DE‐AFE using (2 × 10 nm) based laminates are explored in terms of the output FE hysteresis (2Pr, 2Ec) and structural properties by X‐ray diffraction. The hybrid AFE‐FE stack shows the potential of tailoring the output FE hysteresis 2Ec by varying the fraction of the AFE phase. The hybrid AFE‐FE stack is studied for the HSO and HZO materials at optimal FE content for the first laminate layer while varying the Si or Zr content to stabilize different degrees of an AFE phase in the second laminate layer. The superposition of the hybrid AFE‐FE hysteresis shows a systematic 2Ec control. A model is developed to describe the tailored output FE hysteresis via the tuning of a hybrid AFE‐FE stack. The role of stack lamination at hybrid‐stabilized phases inside a single stack is explored with the aim for a controlled and optimized FE hysteresis shape toward a low power (2Ec) operation. [ABSTRACT FROM AUTHOR]

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Exceptionally electron‐rich, nearly trigonal‐planar tricyanidometalate anions [Fe(CN)3]7− and [Ru(CN)3]7− were stabilized in LiSr3[Fe(CN)3] and AE3.5[M(CN)3] (AE=Sr, Ba; M=Fe, Ru). They are the first examples of group 8 elements with the oxidation state of −IV. Microcrystalline powders were obtained by a solid‐state route, single crystals from alkali metal flux. While LiSr3[Fe(CN)3] crystallizes in P63/m, the polar space group P63 with three‐fold cell volume for AE3.5[M(CN)3] is confirmed by second harmonic generation. X‐ray diffraction, IR and Raman spectroscopy reveal longer C−N distances (124–128 pm) and much lower stretching frequencies (1484–1634 cm−1) than in classical cyanidometalates. Weak C−N bonds in combination with strong M−C π‐bonding is a scheme also known for carbonylmetalates. Instead of the formal notation [Fe−IV(CN−)3]7−, quantum chemical calculations reveal non‐innocent intermediate‐valent CN1.67− ligands and a closed‐shell d10 configuration for Fe, that is, Fe2−. [ABSTRACT FROM AUTHOR]

Ferroelectric hafnium oxide (HfO2) is considered a very prospective material for applications in integrated devices due to its considerably large spontaneous polarization and superior thickness scaling. In fact, the evolution of the ferroelectric hysteresis upon field cycling plays an important role in most applications; especially the so‐called wake‐up effect that describes the increase of remanent polarization for initial field cycling, needs a profound understanding in HfO2. Herein, the discovery of electric field–induced crystallization in hafnium oxide is reported. In addition, differences in the wake‐up behavior are addressed that finally can be categorized into five different cases, all being relevant when describing the evolution of ferroelectricity. Moreover, analysis of the temperature dependence and transmission Kikuchi diffraction provides insight into the underlying physical mechanisms of different wake‐up behavior scenarios, and proves ferroelastic switching as the origin for the observed antiferroelectric‐like behavior. This knowledge provides clear procedures of 1) how to experimentally quantify and 2) how to prepare and manufacture hafnium oxide phases for the five different wake‐up types. [ABSTRACT FROM AUTHOR]

The hafnium oxide (HfO2) material system offers a unique combination of outstanding physical properties, that enable a manifold of novel integrated ferroelectric, piezoelectric, and pyroelectric applications. Long‐term stability is an essential concern for nonvolatile memory devices, sensors, and nanoelectromechanical systems. Herein, the aging effects of the pyroelectric response in polycrystalline Si‐doped HfO2 thin films in the field‐free case are reported. It is observed that aging effects are accelerated by high temperatures, lower film thicknesses, and higher dopant concentration. The decay of the pyroelectric coefficients and the dielectric permittivity exhibits a logarithmic time dependence. The full pyroelectric response is restored by repeated electric field cycling (i.e., deaging). After the aging process, a significant internal bias field is observed. It is concluded that the migration of positively charged oxygen vacancies in the films is responsible for this aging process. [ABSTRACT FROM AUTHOR]

Optical second harmonic generation (SHG) at surfaces and interfaces is an exciting new technique. In order to achieve high surface specificity and sensitivity of the SHG technique, smaller particles of SHG materials are highly desired. Borate halides are regarded as a great source of SHG materials because of their outstanding nonlinear optical properties. Phase‐pure nanostructured samples of three borate halides of the type A2B5O9X were synthesized at low temperature in task‐specific ionic liquids (IL). Optimized reactions of acetates A(OAc)2 (A = Sr, Ba) and boric acid B(OH)3 with the mixed IL [P66614]X (X = Cl, Br) and LiNTf2 led to nano‐rods of Sr2B5O9Cl, nano‐needles of Sr2B5O9Br, and nano‐sheets of Ba2B5O9Cl. The reaction route presented here might trigger the synthesis of other borate compounds. Efficient second‐order nonlinear conversion by the nanostructured borate halides was confirmed using confocal SHG microscopy. [ABSTRACT FROM AUTHOR]

The in‐plane piezoelectric response of 20 nm thick Si‐doped HfO2 is examined by exploiting thermal expansion of the substrate upon rapid temperature cycling. The sample is heated locally by a deposited metal film, and the subsequently registered pyroelectric current is found to be frequency dependent in the observed range of 5 Hz to 35 kHz. While the intrinsic response remains constant, the secondary contribution can be switched off in the high‐frequency limit due to substrate clamping. As this secondary response is generated by thermal expansion and the piezoelectric effect, this allows for extraction of the corresponding in‐plane response. By comparing pyroelectric measurements in low‐ and high‐frequency limits, a piezoelectric coefficient d31 of −11.5 pm V −1 is obtained, which is more than five times larger than that of AlN. The magnitude of piezoelectric response increases upon electric field cycling, which is associated with a transition from antiferroelectric‐like behavior to a purely ferroelectric polarization hysteresis. The hafnium oxide material system is proposed as a promising candidate for future CMOS compatible piezoelectric micro‐ and nano‐electromechanical systems (MEMS and NEMS). [ABSTRACT FROM AUTHOR]

Abstract: Here the concept of a 2D resistor network (2D RN) is applied in order to model the electrical conductivity along sheet‐like domain walls (DWs) in single crystalline lithium niobate (sc‐LNO). The only input to the RN modeling approach is the DW inclination angle distribution, as measured previously with respect to the polar c‐axis. The simulations then show that a 2D network of Ohmic resistors not only adequately accounts for the different boundary conditions envisaged in experiments, but equally well provides a direct link between the local domain wall conductivity (DWC) and the DW inclination angle α. Moreover, the RN simulations can be directly compared to local‐scale transport measurements, as obtained by scanning probe techniques. The conceptual simplicity and the low computational effort make the present RN modeling approach a useful tool for both the advanced interpretation and evaluation of potential DWC ferroelectrics. [ABSTRACT FROM AUTHOR]

We present the first in situ 3D observation of domain wall dynamics close to a ferroelectric-paraelectric phase transition, which we obtain via second-harmonic generation microscopy (SHGM). Showing a pure second-order phase transition at its Curie temperature of 49 °C, triglycine sulfate (TGS) is a model ferroelectric material to study such phase transitions. After annealing, in TGS various qualitatively different domain patterns, e.g., stripes or small lenticular domains, have been reported so far. By applying SHGM, we can show that these findings do not necessarily contradict each other. In fact, qualitatively different domain patterns may coexist in the same TGS sample at the same time. [ABSTRACT FROM AUTHOR]

We apply here spectral-domain optical coherence tomography (SD-OCT) for the precise detection and temporal tracking of ferroelectric domain walls (DWs) in magnesium-doped periodically poled lithium niobate (Mg:PPLN). We reproducibly map static DWs at an axial (depth) resolution down to ∼ 0.6 μm, being located up to 0.5 mm well inside the single crystalline Mg:PPLN sample. We show that a full 3-dimensional (3D) reconstruction of the DW geometry is possible from the collected data, when applying a special algorithm that accounts for the nonlinear optical dispersion of the material. Our OCT investigation provides valuable reference information on the DWs' polarization charge distribution, which is known to be the key to the electrical conductivity of ferroelectric DWs in such systems. Hence, we carefully analyze the SD-OCT signal dependence both when varying the direction of incident polarization, and when applying electrical fields along the polar axis. Surprisingly, the large backreflection intensities recorded under extraordinary polarization are not affected by any electrical field, at least for field strengths below the switching threshold, while no significant signals above noise floor are detected under ordinary polarization. Finally, we employed the high-speed SD-OCT setup for the real-time DW tracking upon ferroelectric domain switching under high external fields. [ABSTRACT FROM AUTHOR]

Lithium niobate (LNO) and lithium tantalate (LTO) see widespread use in fundamental research and commercial technologies reaching from electronics over classical optics to integrated quantum communication. The mixed crystal system lithium niobate tantalate (LNT) allows for the dedicate engineering of material properties by combining the advantages of the two parental materials LNO and LTO. Vibrational spectroscopies such as Raman spectroscopy or (Fourier transform) infrared (IR) spectroscopy are vital techniques to provide detailed insight into the material properties, which is central to the analysis and optimization of devices. This work presents a joint experimental–theoretical approach allowing to unambiguously assign the spectral features in the LNT material family through both Raman and IR spectroscopy, as well as providing an in‐depth explanation for the observed scattering efficiencies based on first‐principles calculations. The phononic contribution to the static dielectric tensor is calculated from the experimental and theoretical data using the generalized Lyddane–Sachs–Teller relation and compared with the results of the first‐principles calculations. [ABSTRACT FROM AUTHOR]

Poling and structuring of ferroelectric domains form the basis for developing prospective applications using materials from the lithium‐niobate (LN) family. Applications range from second harmonic generation to electro‐optic modulators or surface acoustic wave devices. In the work presented here, hysteresis measurements are used as a standard method to quantitatively determine the poling properties of these ferroelectrics, including their spontaneous polarization Ps as well as their forward and reverse coercive fields Ec,+ and Ec,−. Systematic measurements that depend on parameters such as the ramp rate R of the applied poling voltage and the waiting time twait between domain inversions are investigated and compared between the congruent variants of LN, lithium tantalate (LT), their magnesium‐doped analogues, and stoichiometric LN. For bulk magnesium‐doped LN, for example, it is found that the resulting coercive field strongly depends on the speed of the voltage ramp, with Ec$E_{\text{c}}$ values ranging from 11 to 21 kV mm−1. These investigations are used as fundamental input for poling ferroelectric lithium niobate‐tantalate solids (LNT), a system that offers a high potential for tuning the material parameters beyond what is possible for LN or LT. [ABSTRACT FROM AUTHOR]