Yasushi Hotta

Determination of the optimal cation composition of ferroelectric (ZnxCd1−x)S thin films for applications to silicon-based nonvolatile memories

Thin films of ferroelectric binary mixed II–VI compounds such as (ZnxCd1−x)S, as well as (ZnyCd1−y)Te and (ZnzCd1−z)Se (0⩽x,y,z⩽1), were examined from the standpoint of the application to Si-based nonvolatile memories. Electronic-band discontinuities at the ferroelectric–Si interface decreased significantly with increase in the atomic number of the constituent chalcogenide atoms, which favored (ZnxCd1−x)S as the most potential gate ferroelectrics among the three compounds. Polarization-field (P–E) characteristics of the (ZnxCd1−x)S films were found to largely depend on the cation composition. No hysteretic behaviors in the P–E curves were observed for high-Zn concentrations above x=0.5, while the P–E curves traced hysteretic loops due to the ferroelectricity for x<0.5. The remnant polarization was greatly dependent on the Zn concentration, and yielded a maximum of 0.03 μC/cm2 for x=0.3. On the other hand, the coercive field was not composition dependent, and was approximately 12 kV/cm.

Noise-driven signal transmission using nonlinearity of VO2 thin films

We demonstrated signal transmission using the nonlinearity of VO2 in the manner of stochastic resonance (SR). A correlated insulator state of VO2 changes nonlinearly to a metallic state when an applied bias voltage increases beyond a threshold. The transition of the states between insulator and metal is adaptable to the SR theory. In this study, the response to a weak pulse signal was optimized by a particular level of noise via SR. Numerical SR simulations suggests the existence of multiple threshold channels in the insulating state, spontaneously enhancing the reliability of signal transmission.

Optimization of electronic-band alignments at ferroelectric (ZnxCd1−x)S/Si(100) interfaces

We have obtained ferroelectric characteristics from nonoxide (ZnxCd1−x)S (x=0.1–0.3) thin films. On the basis of x-ray photoelectron and visible–ultraviolet light absorption spectroscopy measurements, the conduction-band discontinuity at the (ZnxCd1−x)S/Si(100) interfaces is found to vary between 0.4 and 1.3 eV with a change in composition x between 0.1 and 0.9. The leakage current density, which strongly depends on the conduction-band discontinuity, is reduced to less than 10−6 A/cm2 at a gate voltage of 4 V.

Low leakage current characteristics of YMnO3 on Si(111) using an ultrathin buffer layer of silicon oxynitride

We have grown YMnO3 (YMO) thin films on Si(111) using silicon oxynitride (SiON) as a buffer layer. Thickness of SiON buffer layer was well controlled within 2 nm. High resistance of ultrathin SiON layer (dSiON∼0.7 nm) to Si oxidation was confirmed by x-ray photoelectron spectroscopy (XPS). Using the ultrathin SiON layer, we obtained c-axis oriented ferroelectric phase of YMO. Although capacitance–voltage curves of Al/YMO/SiON/Si(111) showed hystereses attributed to ferroelectricity of the YMO films, the memory window was not sufficient (0.2 V), seemingly due to poor crystallinity of the YMO films. On the other hand, leakage current characteristic was good enough for application. The typical value of leakage current density was 10−8 A/cm2 at a drive voltage of ±5 V. In this article, the details of the characterization elucidated by using x-ray diffraction, atomic force microscopy, and XPS will be shown as well.

Effects of an ultrathin silicon oxynitride buffer layer on electrical properties of ferroelectric Bi4Ti3O12 thin films on p-Si(100) surfaces

Electrical properties of ferroelectric Bi4Ti3O12 (BiT) films on Si(100) using a 1 nm thick silicon oxynitride (SiON) buffer were investigated. The capacitance–voltage (C–V) characteristics of Au/BiT/SiON/Si(100) exhibited hysteresis loops with a memory window of 2 V due to the ferroelectricity, and did not show large carrier injections. The effects of the SiON buffer were demonstrated in current–voltage characteristics. In the reverse bias region, a leakage current density of the specimen without the SiON buffer was much larger than that of the specimen with the buffer. Apart from these electrical measurements, anomalous features appeared in C–V characteristics of the illuminated specimen, which were likely to be due to the ac response of the optically generated electrons in some trap states at the interface.

Artificial Control of Order Degree State of B-Site Ions in Ba(Zr,Ti)O3 by a Superlattice Technique

Artificial control for arranging the B-site ion has been demonstrated by superlattices to elucidate relaxor behavior. The relaxor behavior of ferroelectric material depends on order state of the constructing ion in the crystal. To clarify the mechanism of this relaxor behavior, Ba(Zr,Ti)O3 is chosen as the target material because its B-site ions are not naturally ordered. The superlattice samples were prepared by using a pulsed laser deposition (PLD) technique. Observed X-ray diffraction (XRD) patterns of the superlattices and their theoretical calculation revealed that the degree of order state was well controlled by our technique. The relaxor behavior occurs in the order degree (OD) ratio below -25% indicating that the OD is a key function for determining the origin of the relaxor behavior.

Noise-driven attractor switching device.

Problems with artificial neural networks originate from their deterministic nature and inevitable prior learnings, resulting in inadequate adaptability against unpredictable, abrupt environmental change. Here we show that a stochastically excitable threshold unit can be utilized by these systems to partially overcome the environmental change. Using an excitable threshold system, attractors were created that represent quasiequilibrium states into which a system settles until disrupted by environmental change. Furthermore, noise-driven attractor stabilization and switching were embodied by inhibitory connections. Noise works as a power source to stabilize and switch attractors, and endows the system with hysteresis behavior that resembles that of stereopsis and binocular rivalry in the human visual cortex. A canonical model of the ring network with inhibitory connections composed of class 1 neurons also shows properties that are similar to the simple threshold system.

Room-temperature-photoinduced magnetism and spin-electronic functions of spinel ferrite with a spin-cluster structure

Room-temperature-photoinduced magnetization (PIM) was observed in spinel ferrite Al0.2Ru0.8Fe2O4 thin films with a spin-cluster-glass structure. Additionally, the films exhibited significant properties as spintronic materials, showing a low saturation magnetization under 0.6μB/unit cell and good conductivity with a high spin polarized electron level of over 75%. A combination of high-temperature PIM and the electronic properties associated with spintronics would generate an area of research and development that utilize the degrees of freedom offered by optical systems in the field of spintronics.

Cooperative Dynamics of an Artificial Stochastic Resonant System

We have investigated cooperative dynamics of an artificial stochastic resonant system, which is a recurrent ring connection of neuron-like signal transducers (NSTs) based on stochastic resonance (SR), using electronic circuit experiments. The ring showed quasi-periodic, tunable oscillation driven by only noise. An oscillation coherently amplified by noise demonstrated that SR may lead to unusual oscillation features. Furthermore, we found that the ring showed synchronized oscillation in a chain network composed of multiple rings. Our results suggest that basic functions (oscillation and synchronization) that may be used in the central pattern generator of biological system are induced by collective integration of the NST element.

Interface control of Bi4Ti3O12 film growth on Si(100) by use of an ultrathin silicon oxynitride buffer layer

Electrical properties of Bi4Ti3O12 (BiT) films on Si(100) were improved due to insertion of silicon oxynitride (SiON) buffer layers with thicknesses of 1–2 nm. Capacitance–voltage measurements indicated that the improvement was largely attributable to better Si interface properties rather than to the difference of the BiT film quality. By means of x-ray photoelectron spectroscopy and high-resolution transmission microscopy, the Si interfaces of the specimens with and without the SiON buffer layers were investigated. Consequently, we found that a postannealing treatment at 680 °C inevitably resulted in nonuniform growth of Si oxide layers at the Si interface of the specimen without the SiON buffer layer, and that the layer thickness mounted to approximately 10 nm. In contrast, 1–2-nm-thick SiON buffer layers terminated the growth of the additional oxide layer of less than about 3 nm, and the resulting Si oxide layers were quite uniform.