Hiroyuki Akinaga

Resistive Random Access Memory (ReRAM) Based on Metal Oxides

In this paper, we review the recent progress in the resistive random access memory (ReRAM) technology, one of the most promising emerging nonvolatile memories, in which both electronic and electrochemical effects play important roles in the nonvolatile functionalities. First, we provide a brief historical overview of the research in this field. We also provide a technological overview and the epoch-making achievements, followed by an account of the current understanding of both bipolar and unipolar ReRAM operations. Finally, we summarize the challenges facing the ReRAM technology as it moves toward the beyond-2X-nm generation of nonvolatile memories and the so-called beyond complementary metal-oxide-semiconductor (CMOS) device.

Nonpolar resistance switching of metal/binary-transition-metal oxides/metal sandwiches : Homogeneous/inhomogeneous transition of current distribution

Exotic features of a metal/oxide/metal (MOM) sandwich, which will be the basis for a drastically innovative nonvolatile memory device, is brought to light from a physical point of view. Here the insulator is one of the ubiquitous and classic binary-transition-metal oxides (TMO), such as Fe2O3, NiO, and CoO. The sandwich exhibits a resistance that reversibly switches between two states: one is a highly resistive off-state and the other is a conductive on-state. Several distinct features were universally observed in these binary TMO sandwiches: namely, nonpolar switching, non-volatile threshold switching, and current--voltage duality. From the systematic sample-size dependence of the resistance in on- and off-states, we conclude that the resistance switching is due to the homogeneous/inhomogeneous transition of the current distribution at the interface.

High Speed Unipolar Switching Resistance RAM (RRAM) Technology

We have successfully achieved high speed (~50 ns) unipolar operation in RRAM devices comprised of titanium oxynitride (TiON) combined with a control resistor connected in series. For unipolar switching, programming and erasing pulses can be the same width, typically, a few tens of nano-seconds. This enables high speed and high density cross-point RRAM memory arrays. In addition, we demonstrate how switching characteristics can be controlled by a series resistor

Spin-polarized light-emitting diode using metal/insulator/semiconductor structures

We have succeeded in growing ferromagnetic metals (Co, Fe, and NiFe)/Al2O3/AlGaAs heterostructures with homogeneous flat interfaces. The electroluminescence from a light-emitting diode with a metal/insulator/semiconductor (MIS) structure depends on the magnetization direction of the ferromagnetic electrode. This fact shows that a spin injection from the ferromagnetic metal to the semiconductor is achieved. The spin-injection efficiency is estimated to be the order of 1% at room temperature.

Epitaxial growth of zinc-blende CrAs/GaAs multilayer

Zinc-blende CrAs/GaAs multilayers were grown by molecular beam epitaxy. It was certified that each CrAs layer maintains an epitaxial relationship with the zinc-blende GaAs structure judging from the reflection high-energy electron diffraction observation. The film contains thicker zinc- blende CrAs layers in total than the CrAs thin film directly grown on the GaAs substrate which has the critical thickness of 3 nm. It was clarified that the optimum thicknesses of CrAs and GaAs to keep a good epitaxial relationship are 2 ML and 2 ML, respectively. The electronic structure of the multilayer is thought to be close to that of the (Ga, Cr)As thin film which has 50% of Cr content judging from x-ray absorption spectroscopy measurements.

Room-temperature thousandfold magnetoresistance change in MnSb granular films: Magnetoresistive switch effect

A huge positive magnetoresistance effect has been discovered in MnSb granular films. Granular film consisting of nanoscale MnSb dots that are grown on a sulfur-passivated GaAs (001) substrate by molecular-beam epitaxy, then covered with an Sb thin layer, exhibits magnetic-field-sensitive current–voltage characteristics. When a constant voltage, above the threshold value, is applied to the film, more than 1000% change in the current, which we term magnetoresistive switch, is driven by the magnetoresistance effect under a relatively low magnetic field (less than 0.5 T) at room temperature.

X-Ray Diffraction Study on the Crystal Structure of Nd1+xBa2-xCu3O7-δ

Crystal structures in the Nd1+xBa2-xCu3O7-δ system have been investigated by X-ray powder diffraction combined with Rietveld analysis. The compound NdBa2Cu3O7-δ is isomorphic with orthorhombic YBa2Cu3O7-δ. With the increase of x the orthorhombic distortion decreases, and the compound becomes tetragonal at x=0.2. The tetragonal structure is isomorphic with the tetragonal form of YBa2Cu3O7-δ. Tc of the compounds with x=0 and 0.05 is above 90 K, which decreases monotonically with x and the compound becomes semiconducting at x=0.4.

A magnetic force microscope using CoFe-coated carbon nanotube probes

Ferromagnetic-film-coated carbon nanotube (CNT) probes were employed in a magnetic force microscope (MFM) observation. We succeeded in making a uniform ferromagnetic film on the CNT probes by improving the coating process and selecting materials. The performance of the CoFe-coated CNT probe was evaluated in ultra-high-density perpendicular magnetic storage media with densities from 600 to 1100?k flux changes per inch (FCI). The magnetic domain structure of the magnetic storage media was clearly observed up to 1100?kFCI. The ultimate lateral resolution of the newly developed MFM probes is down to about 10?nm, which exceeds the bit length of a magnetic recording with a density of Tbit?inch?2.

Hole Concentration Compensation Effect and Superconducting Properties of Nd1+xBa2-xCu3O7-δ

The superconducting transition temperature Tc and normal-state resistivity have been investigated as a function of x for the Nd1+xBa2-xCu3O7-? system, whose crystal structures have been determined at room temperature by X-ray powder diffraction combined with the Rietveld method. Tc of the samples with x=0 and 0.05 is above 90 K, which decreases monotonically with x except for around x=0.25?0.30 (Tc?35 K) and the sample becomes semiconducting at x=0.40. The orthorhombic distortion of the compounds decreases with x and the compound becomes tetragonal at x=0.20. The variation of Tc with x is insensitive to the orthorhombic-tetragonal phase transition. A correlation of Tc with hole concentration in copper-oxide-type superconductors is discussed based on a possible hole concentration compensation effect of excess Nd.

Fabrication and magneto-optical properties of epitaxial ferromagnetic Mn1 − xSb thin films grown on GaAs and sapphire

Abstract We succeeded in growing epitaxial ferromagnetic Mn 1 − xSb thin films by molecular beam epitaxy (MBE). The crystal structures were of NiAs-type and their orientations changed depending on the substrate, (10.1) for (001) GaAs and (00.1) for (00.1) sapphire. The sharp X-ray diffraction patterns showed that these epitaxially grown films were of the highest crystal quality reported so far. Under the present growth conditions, the compositions x of the films were automatically settled to be about 0.2 ( Mn 0.8 Sb ) which was determined by electron probe microanalysis and Auger analysis. The Curie temperature, T C , was about 620 K, which was evaluated by the temperature dependence of the magnetization of the film grown on the GaAs substrate. The polar magneto-optical Kerr rotation spectrum of the film on the GaAs substrate was almost the same as that reported previously in the bulk samples. However, the spectrum of the (10.1) film on the GaAs substrate revealed a clearly different structure compared with that of the (00.1) film on the sapphire substrate.