Akihito Sawa

Doped indium oxides, such as indium tin oxide (ITO), have been used as transparent conducting materials and have recently attracted increasing interest as thin-film channel materials for high-performance field-effect transistors. In numerous studies on crystalline ITO, stable bixbyite-type (cubic) and metastable corundum-type (rhombohedral) phases have been investigated. Here, we demonstrate an epitaxial stabilization of the ITO polytype having Rh2O3-II-type (space group: Pbna) orthorhombic structure using an orthorhombic perovskite oxide (110) DyScO3 (DSO) substrate. Distorted-orthorhombic ITO (o-ITO) films could be epitaxially grown at substrate temperatures of approximately 350 °C. The epitaxial relationships were determined to be ITO[100]//DSO[100] and ITO[001]//DSO[001], whereas the [010] of ITO was slightly inclined relative to that of DSO because of the strain effect. The transport properties of a distorted o-ITO film were better than those of a bixbyite-type ITO film grown on yttria-stabilized zirconia substrate, indicating that o-ITO has a potential of high-performance oxide semiconductor contributing to future electronics.

Ferroelectricity in thin films of HfO₂ has been the subject of extensive studies in materials science as well as device applications. The emergence of ferroelectricity is attributable to the orthorhombic phase (Pca2₁) of HfO₂, stabilized in the films by metal-element doping, strains from substrates and electrode films, and oxygen deficiency. Recently, ferroelectricity has been reported in nanolaminates of HfO₂ with other oxides such as ZrO₂ and Al₂O₃, implying that nanolaminates are another effective way to bring about ferroelectricity in HfO₂. However, the mechanism of orthorhombic phase stabilization in nanolaminates is not fully understood. In this study, we demonstrated that ferroelectricity emerges in nanolaminates consisting of undoped HfO₂ and perovskite SrHfO₃ deposited on Sn-doped In₂O₃ bottom electrodes, when the thickness of HfO₂ layers was ≥6 nm. For nanolaminates in which the thickness of the HfO₂ layers was ≤5 nm, ferroelectricity was remarkably suppressed due to Sr-incorporation into the HfO₂ layers at the interface. In those nanolaminates, the crystal orientations of HfO₂ grains were well aligned throughout the HfO₂ layers, indicating that the HfO₂ layers grew in a pseudo-coherent manner. This study aids to understand the stabilization of the ferroelectric orthorhombic phase in nanolaminates in terms of their structural properties.

A noncontact optical imaging technique called “ferroelectrics field modulation imaging (FFMI)” is developed to visualize ferroelectric domains and domain walls. The difference image sensing technique with use of area image sensor is utilized to observe the slight modulation of transmitted optical microscope images for ferroelectric films under application of alternating electric fields. We show that the technique allows sensitive, collective and rapid visualization of ferroelectric domains in single-crystal films of hydrogen-bonded organic ferroelectrics. We also investigated further features of ferroelectric domain walls by the technique, regarding the dynamics as well as a tilt in the depth direction of the films.

Resistive switching (RS) devices have attracted increasing attention for artificial synapse applications in neural networks because of their nonvolatile and analogue resistance changes. Among the neural networks, a spiking neural network (SNN) based on spike-timing-dependent plasticity (STDP) is highly energy efficient. To implement STDP in resistive switching devices, several types of voltage spikes have been proposed to date, but there have been few reports on the relationship between the STDP characteristics and spike types. Here, we report the STDP characteristics implemented in ferroelectric tunnel junctions (FTJs) by several types of spikes. Based on simulated time evolutions of superimposed spikes and taking the nonlinear current-voltage (I-V) characteristics of FTJs into account, we propose equations for simulating the STDP curve parameters of a magnitude of the conductance change (ΔG_max) and a time window (τ_C) from the spike parameters of a peak amplitude (V_peak) and time durations (t_p and t_d) for three spike types: triangle-triangle, rectangular-triangle, and rectangular-rectangular. The power consumption experiments of the STDP revealed that the power consumption under the inactive-synapse condition (spike timing |Δt| > τ_C) was as large as 50–82% of that under the active-synapse condition (|Δt| < τ_C). This finding indicates that the power consumption under the inactive-synapse condition should be reduced to minimize the total power consumption of an SNN implemented by using FTJs as synapses.

Hf _0.5 Zr _0.5 O ₂ thin films are not always ferroelectric. This work investigates the impact of annealing temperature and time on the crystalline structures and dielectric properties of 10 nm thick Hf _0.5 Zr _0.5 O ₂ thin films. It is found that the tetragonal phase crystal is formed from the amorphous film firstly, then transforms to the orthorhombic and monoclinic phases, in accordance with the annealing temperature and time. The volume fraction of the orthorhombic phase in the film, which is known as the origin of ferroelectricity, becomes dominant in a certain range of the annealing condition. Thus, the annealing temperature and time are responsible for the anti-ferroelectric, ferroelectric, and paraelectric characteristics of Hf _0.5 Zr _0.5 O ₂ thin films. This phase transformation behavior is discussed from the viewpoint of formation energies of the respective crystal phases, which accompanies the changes of unit cell volumes. The competition of transformation rates between the tetragonal to orthorhombic and the orthorhombic to monoclinic is key for the formation of ferroelectric films.

We have fabricated compact optical modulators consisting of a Si waveguide with a VO₂ cladding layer. These devices showed a sharp decrease in transmittance at around 60 °C, which is attributable to the metal–insulator transition of the VO₂ cladding layer. By systematically varying the length of the device, we evaluated the transmission losses per unit length of the device to be 1.27 dB/µm, when the VO₂ cladding layer was in the insulating (ON) state and 4.55 dB/µm when it was in the metallic (OFF) state. Furthermore, we found that the device showed an additional loss in the OFF state, which is attributable to a structural effect. As a result, an 8-µm-long device showed a large extinction ratio of more than 33 dB.

Many hydrogen-bonded organic ferroelectrics exhibit low-field switching of large spontaneous polarizations. Although the switchable electric dipoles of π-conjugated organic molecules account for the large spontaneous polarizations, their relevant optoelectronic processes have not been used to probe the ferroelectricity. We show that the variation in electro-optic response enables visualization of the ferroelectric domains and domain walls in single-crystal films of a hydrogen-bonded molecular cocrystal. Highly sensitive and rapid visualization is realized by difference optical image sensing between the forward and reverse field applications. We call this technique “ferroelectrics field modulation imaging (FFMI).” The unique optical-probe nature reveals the existence of two types of domain walls showing different three-dimensional orientations within the films; one is roughly perpendicular to the film plane, whereas the other is considerably tilted from the normal to the plane. We explain that both of the domain walls are stabilized to generate substantial neutrality by being directed parallel to the direction of polarization. This study opens a route for exploring the three-dimensional topological nature of domain walls in ferroelectric materials.

A charge excitation in a two-dimensional Mott insulator is strongly coupled with the surrounding spins, which is observed as magnetic-polaron formations of doped carriers and a magnon sideband in the Mott-gap transition spectrum. However, the dynamics related to the spin sector are difficult to measure. Here, we show that pump-probe reflection spectroscopy with seven-femtosecond laser pulses can detect the optically induced spin dynamics in Nd₂CuO₄, a typical cuprate Mott insulator. The bleaching signal at the Mott-gap transition is enhanced at ~18 fs. This time constant is attributable to the spin-relaxation time during magnetic-polaron formation, which is characterized by the exchange interaction. More importantly, ultrafast coherent oscillations appear in the time evolution of the reflectivity changes, and their frequencies (1400–2700 cm⁻¹) are equal to the probe energy measured from the Mott-gap transition peak. These oscillations can be interpreted as the interference between charge excitations with two magnons originating from charge–spin coupling.

The electronic phase change from an antiferromagnetic insulator to an antiferromagnetic/ferromagnetic-like metal has been investigated for the electron-doped EuTiO3, i.e., the single crystals of Eu1−xLaxTiO3 (0 ≤ x ≤ 0.07) prepared by the floating zone method. The EuTiO3 (x = 0) crystal is an insulator, and Eu 4f electrons exhibit an antiferromagnetic spin ordering at TN ∼ 5.5 K. Upon the substitution of Eu2+ with La3+, a metallic state appears for x ≥ 0.01; this state is similar to the isostructural Sr1−xLaxTiO3. As x increases from 0.01 to 0.07, TN decreases from 5.5 to 3 K. For x = 0.07, the transition to the antiferromagnetic state seems to be followed by the appearance of a ferromagnetic-like state at a higher temperature (∼4.5 K). In the temperature profiles of resistivity for the crystals with 0.01 ≤ x ≤ 0.07, an anomaly is observed at the temperature for the spin ordering of Eu 4f electrons, which reflects the exchange coupling between itinerant Ti 3d and Eu 4f electrons. Magnetotransport properti...

We report on the growth of polycrystalline yttrium-5%-doped HfO₂ (YHO) films with sharp and uniform interfaces over a large area on indium-tin oxide (ITO) layers and their ferroelectric properties. On substrates of thermally oxidized silicon, YHO/ITO bilayers were deposited in amorphous form by means of pulsed-laser deposition and were subsequently crystalized by two-step post-deposition annealing (PDA) at low and high temperatures. The crystallized YHO/ITO heterostructures had ultraflat surfaces: atomic-force-microscopy images exhibited a root-mean-square roughness of 0.15 nm over an area of 100 μm². The roughness was much smaller than that of single-component polycrystalline films of ITO. X-ray diffraction and transmission electron microscopy (TEM) suggested the formation of YHO in higher symmetry phases including ferroelectric orthorhombic (o) YHO. TEM also confirmed crystallization of YHO layers in a pseudo-coherent manner from the YHO/ITO interfaces and resultant sharp and uniform YHO/ITO interfaces. These results are attributable to good lattice matching between the o-YHO layer and the ITO layer. It is suggested that this lattice matching was realized by the reduction of ITO during the crystallization in the two-step PDA process. The reduction might occur in the vicinity of the YHO/ITO interface via H₂O incorporated in the amorphous bilayers. Pt/YHO/ITO junctions fabricated by using the two-step PDA method showed a polarization–electric field (P–E) hysteresis loop with remanent polarizations (P_r) of no less than 13 μC/cm² at YHO thicknesses of 12 and 6 nm. These results indicate that the two-step PDA method can be used to fabricate high-quality YHO/ITO heterostructures in terms of surface and interface morphology, as well as ferroelectric properties.