Takashi Nakamura

Bio: Takashi Nakamura is an academic researcher from Rohm. The author has contributed to research in topics: Ferroelectricity & Dielectric. The author has an hindex of 25, co-authored 99 publications receiving 2383 citations. Previous affiliations of Takashi Nakamura include Japan Advanced Institute of Science and Technology.

Preparation of Pb(Zr,Ti)O3 thin films on electrodes including IrO2

Abstract: The development of ferroelectric memory devices requires an improvement of the fatigue properties of ferroelectric thin films. Pb(Zr,Ti)O3(PZT) thin films obtained by the sol‐gel method on Pt/Ti electrodes have reduced residual polarization by continuous polarization reverses about of 108 cycles. The electric characteristics such as fatigue properties have mostly depended on electrode materials. We propose Ir, IrO2, and these layer films as electrode materials and evaluate electric characteristics of PZT thin film capacitors. PZT thin films using Ir/IrO2 and Pt/IrO2 electrodes show no fatigue up to 1012 cycles of ±5 V switching pulses. Moreover, good properties of PZT capacitors, not only on SiO2 but also on polycrystalline silicon, were obtained by using IrO2.

Preparation of Pb(Zr,Ti)O3 Thin Films on Ir and IrO2 Electrodes

Abstract: Pb(Zrx Ti1-x )O3 (PZT) thin films were prepared on Ir and IrO2 electrodes. Ir has very similar properties to Pt, and IrO2 is a conductive oxide. Perovskite single-phase PZT thin films were obtained on their electrodes. PZT thin films were grown by the conventional sol-gel method with rapid thermal annealing (RTA) at 700° C. When Pt thin films were deposited directly on poly-Si, PtSi layers were formed, and PZT thin films on the Pt had very poor crystallinity. When an IrO2 layer was formed between PZT and poly-Si, a high-quality PZT thin film was obtained. Moreover, when electrodes including the IrO2 layer were used, fatigue properties of PZT thin films were drastically improved.

High performance SiC trench devices with ultra-low ron

Abstract: We have developed SiC trench structure Schottoky diodes and SiC double-trench MOSFETs. We succeeded in improving device performance by the reduction of the electric field through the introduction of the aforementioned trench structures. The threshold voltage of the trench structure Schottky diode is 0.48V smaller than the planar. Also, the lowest on-resistance in SiC MOSFETs was achieved.

Generation of very fast states by nitridation of the SiO[2]/SiC interface

Abstract: Fast states at SiO2/SiC interfaces annealed in NO at 1150–1350 °C have been investigated. The response frequency of the interface states was measured by the conductance method with a maximum frequency of 100 MHz. The interface state density was evaluated based on the difference between quasi-static and theoretical capacitances (C−ψS method). Very fast states, which are not observed in as-oxidized samples, were generated by NO annealing, while states existing at an as-oxidized interface decreased by approximately 90%. The response frequency of the very fast states was higher than 1 MHz and increased when the energy level approaches the conduction band edge. For example, the response frequency (time) was 100 MHz (5 ns) at EC−ET = 0.4 eV and room temperature. The SiO2/SiC interface annealed in NO at 1250 °C showed the lowest interface state density, and NO annealing at a temperature higher than 1250 °C is not effective because of the increase in the very fast states.

Accurate evaluation of interface state density in SiC metal-oxide-semiconductor structures using surface potential based on depletion capacitance

Abstract: We propose a method to accurately determine the surface potential (ψS) based on depletion capacitance, and the interface state density (DIT) was evaluated based on the difference between quasi-static and theoretical capacitances in SiC metal-oxide-semiconductor capacitors (C−ψS method). We determined that this method gives accurate values for ψS and DIT. From the frequency dependence of the capacitance measured at up to 100 MHz, a significant fast-interface-state response exists at 1 MHz, which results in the overestimation of ψS if it is determined based on the flatband capacitance at 1 MHz. The overestimation of ψS directly affects the accuracy of the energy level. DIT at a specific energy level is underestimated by the overestimation of ψS. Furthermore, the fast interface states that respond at 1 MHz cannot be detected by the conventional high(1 MHz)-low method. The C−ψS method can accurately determine the interface state density including the fast states without high-frequency measurements.

Lanthanum-substituted bismuth titanate for use in non-volatile memories

Abstract: Non-volatile memory devices are so named because they retain information when power is interrupted; thus they are important computer components. In this context, there has been considerable recent interest1,2 in developing non-volatile memories that use ferroelectric thin films—‘ferroelectric random access memories’, or FRAMs—in which information is stored in the polarization state of the ferroelectric material. To realize a practical FRAM, the thin films should satisfy the following criteria: compatibility with existing dynamic random access memory technologies, large remnant polarization (Pr) and reliable polarization-cycling characteristics. Early work focused on lead zirconate titanate (PZT) but, when films of this material were grown on metal electrodes, they generally suffered from a reduction of Pr (‘fatigue’) with polarity switching. Strontium bismuth tantalate (SBT) and related oxides have been proposed to overcome the fatigue problem3, but such materials have other shortcomings, such as a high deposition temperature. Here we show that lanthanum-substituted bismuth titanate thin films provide a promising alternative for FRAM applications. The films are fatigue-free on metal electrodes, they can be deposited at temperatures of ∼650 °C and their values of Pr are larger than those of the SBT films.

‘Memristive’ switches enable ‘stateful’ logic operations via material implication

Abstract: The authors of the International Technology Roadmap for Semiconductors-the industry consensus set of goals established for advancing silicon integrated circuit technology-have challenged the computing research community to find new physical state variables (other than charge or voltage), new devices, and new architectures that offer memory and logic functions beyond those available with standard transistors. Recently, ultra-dense resistive memory arrays built from various two-terminal semiconductor or insulator thin film devices have been demonstrated. Among these, bipolar voltage-actuated switches have been identified as physical realizations of 'memristors' or memristive devices, combining the electrical properties of a memory element and a resistor. Such devices were first hypothesized by Chua in 1971 (ref. 15), and are characterized by one or more state variables that define the resistance of the switch depending upon its voltage history. Here we show that this family of nonlinear dynamical memory devices can also be used for logic operations: we demonstrate that they can execute material implication (IMP), which is a fundamental Boolean logic operation on two variables p and q such that pIMPq is equivalent to (NOTp)ORq. Incorporated within an appropriate circuit, memristive switches can thus perform 'stateful' logic operations for which the same devices serve simultaneously as gates (logic) and latches (memory) that use resistance instead of voltage or charge as the physical state variable.

Ferroelectric thin films: Review of materials, properties, and applications

Abstract: An overview of the state of art in ferroelectric thin films is presented. First, we review applications: microsystems' applications, applications in high frequency electronics, and memories based on ferroelectric materials. The second section deals with materials, structure (domains, in particular), and size effects. Properties of thin films that are important for applications are then addressed: polarization reversal and properties related to the reliability of ferroelectric memories, piezoelectric nonlinearity of ferroelectric films which is relevant to microsystems' applications, and permittivity and loss in ferroelectric films-important in all applications and essential in high frequency devices. In the context of properties we also discuss nanoscale probing of ferroelectrics. Finally, we comment on two important emerging topics: multiferroic materials and ferroelectric one-dimensional nanostructures. (c) 2006 American Institute of Physics.

Thermodynamic stability of binary oxides in contact With silicon

Abstract: Using tabulated thermodynamic data, a comprehensive investigation of the thermo-dynamic stability of binary oxides in contact with silicon at 1000 K was conducted. Reactions between silicon and each binary oxide at 1000 K, including those involving ternary phases, were considered. Sufficient data exist to conclude that all binary oxides except the following are thermodynamically unstable in contact with silicon at 1000 K: Li2O, most of the alkaline earth oxides (BeO, MgO, CaO, and SrO), the column IIIB oxides (Sc2O3, Y2O3, and Re2O3, where Re is a rare earth), ThO2, UO2, ZrO2, HfO2, and Al2O3. Of these remaining oxides, sufficient data exist to conclude that BeO, MgO, and ZrO2 are thermodynamically stable in contact with silicon at 1000 K. Our results are consistent with reported investigations of silicon/binary oxide interfaces and identify candidate materials for future investigations.

Memory devices and applications for in-memory computing

Abstract: Traditional von Neumann computing systems involve separate processing and memory units. However, data movement is costly in terms of time and energy and this problem is aggravated by the recent explosive growth in highly data-centric applications related to artificial intelligence. This calls for a radical departure from the traditional systems and one such non-von Neumann computational approach is in-memory computing. Hereby certain computational tasks are performed in place in the memory itself by exploiting the physical attributes of the memory devices. Both charge-based and resistance-based memory devices are being explored for in-memory computing. In this Review, we provide a broad overview of the key computational primitives enabled by these memory devices as well as their applications spanning scientific computing, signal processing, optimization, machine learning, deep learning and stochastic computing. This Review provides an overview of memory devices and the key computational primitives for in-memory computing, and examines the possibilities of applying this computing approach to a wide range of applications.