Hiro Akinaga

Material Design of Half-Metallic Zinc-Blende CrAs and the Synthesis by Molecular-Beam Epitaxy

A new class of half-metallic ferromagnets has been found in the zinc-blende crystal structure. The previously nonexistent zinc-blende CrAs thin films have been synthesized on GaAs (001) substrates by molecular-beam epitaxy, and show a ferromagnetic behavior at room temperature. The zinc-blende CrAs has been designed by ab initio calculations based on the local spin-density approximation, and the calculation predicts the highly spin-polarized electronic band structure.

Resistance switching in the metal deficient-type oxides: NiO and CoO

The resistance switching properties in Pt∕Ni–O∕Pt and Pt∕Co–O∕Pt synthesized by the magnetron sputtering have been investigated. The oxygen partial pressure during sputtering and the post-thermal process are crucial to forming of the trilayer. By investigating x-ray photoemission spectroscopy spectra, the increase of initial resistance in Ni–O was caused by the variation of the stoichiometry, while that in the Co–O was accompanied by the phase transformation between CoO and Co3O4. The resistance switching in Pt∕Co–O∕Pt and Pt∕Ni–O∕Pt exhibits the analogous electrode area and temperature dependences. As a result of the I-V measurements at the elevated temperature, the assistance of Joule heating in the reset process is implied.

Fabrication, magnetic properties, and electronic structures of nanoscale zinc-blende MnAs dots (invited)

Ferromagnetic nanoscale zinc-blende MnAs dots were successfully fabricated on a sulfur-passivated GaAs (001) surface by molecular-beam epitaxy. Transmission electron microscopy and selected area electron diffraction showed that the crystalline structure was not the same as that of bulk MnAs with NiAs-type hexagonal crystalline structure, but of zinc-blende type. In in situ photoemission spectroscopy of the zinc-blende MnAs dots, the Fermi edge was not clearly observed and the Mn 3d partial density of states was similar to that of the diluted ferromagnetic semiconductor Ga1−xMnxAs, which also supports the fabrication of zinc-blende MnAs in the nanoscale.

Voltage polarity dependent low-power and high-speed resistance switching in CoO resistance random access memory with Ta electrode

Structural and resistance switching properties were investigated in the CoO resistance random access memory (RRAM) with the Ta electrode. The intermediate layer consisting of Co and Ta oxides was confirmed at the interface by the transmission electron microscopy and electron energy loss spectroscopy. The great affinity with oxygen in Ta together with a high resistivity of the Ta oxide improves the operational performance of RRAM. The controllability of the resistance after forming and the low-current operation property were substantially improved by using the load resistor connected in series with CoO RRAM with the Ta electrode. The reset current less than 0.2 mA and the switching speed faster than 20 ns were demonstrated.

Switchable rectifier built with Pt/TiOx/Pt trilayer

The switchable rectifier built with Pt/TiOx/Pt was proposed and the reproducible rectification switching was demonstrated. The rectification switching is considered to be originated from the electrical control of the oxygen vacancy concentration at the interfaces between Pt and TiOx and resultant variations in the Schottky barrier height. The electrode area dependence of the current conduction after the rectification switching revealed that this switching occurs at the almost entire interface region. The reproducible switching in the Pt/TiOx/Pt rectifier is demonstrated by changing the polarity of the pulse voltage, evidencing the excellent functionality of the engineered metal/oxide interface.

Electrical Spin Injection from Out-of-Plane Magnetized FePt/MgO Tunneling Junction into GaAs at Room Temperature

We have succeeded in demonstrating zero-magnetic-field spin injection in metal–insulator–semiconductor (MIS) structure at room temperature using FePt/MgO/GaAs-based light-emitting diode heterojunction with out-of-plane magnetization. The spin polarization was investigated by spin-polarized electroluminescence (EL). The lower estimate injected at remanence was 1.5% and that injected at 1 T was reached up to 11.5%. The spin injection efficiency was estimated at least 29%. The bias dependence of the EL circular polarization showed that it decreases with increasing bias voltage for both at 1 T and at remanence.

Magnetic-field-controllable avalanche breakdown and giant magnetoresistive effects in Gold/semi-insulating-GaAs Schottky diode

Gold (Au)∕semi-insulating (SI)-GaAs Schottky diode was fabricated by the standard photolithography method using wet etching. Magnetic-field-dependent avalanche breakdown phenomena were observed in the current–voltage curves measured under magnetic field. The avalanche breakdown due to impact ionization was postponed to higher electrical field under applied magnetic field. Accordingly, threshold voltages of avalanche breakdown increased with the applied magnetic field. Above 0.2T, avalanche breakdown was totally quenched. When Au‐SI‐GaAs Schottky diode was operated above the threshold voltage, giant mangetoresistive effects up to 100 000% were achieved under magnetic field of 0.8T.

Control of resistance switching voltages in rectifying Pt∕TiOx∕Pt trilayer

The Pt∕TiOx∕Pt trilayer with electrically asymmetrical interface have been synthesized by means of the reactive sputtering technique followed by the oxygen annealing. The initial current-voltage characteristics in the Pt∕TiOx∕Pt trilayer cell have rectifying behavior originated from the Schottky junction formed between TiOx and Pt top electrode layer. The series connection of Pt∕TiOx∕Pt trilayer cells brings about the control of the reset and set voltages depending on the resistance of the connected Schottky diode, which is the demonstration of the resistance switching in the resistance random access memory with the one diode and one resistance structure using Schottky barrier diode.

Rectifying characteristic of Pt/TiOx/metal/Pt controlled by electronegativity

Current voltage characteristic of the TiO x /metal interface has been studied by the systematic investigation on the top electrode (TE) material dependence of the carrier transport through the TiO x /metal interface. Rather than work function of TE ( ϕ M ) , electronegativity ( χ M ) of TE plays a dominant role on current conduction and carrier transport of Pt/ TiO x /metal (TE) devices. Pt/ TiO x /metal (TE) exhibits rectifying property, if χ M of TE is high. On the other hands, a symmetric I - V curves were observed if χ M of TE is low. Plots of Schottky barrier at TiO x /metal (TE) interface versus χ M of TE provides an index of interface behavior S ≈ 0.55 , suggesting partial Fermi-level pinning at TiO x /metal interface.

Synthesis and Characterization of Pt/Co-O/Pt Trilayer Exhibiting Large Reproducible Resistance Switching

The resistance switching in Pt/Co–O/Pt trilayers has been successfully demonstrated. The trilayers were prepared by radio-frequency magnetron sputtering. The partial pressure of oxygen during sputtering and the post thermal process for the trilayer are crucial to realize the reproducible resistance switching. By adjusting oxygen partial pressure as well as post-annealing temperature and time, large resistance switching was steadily obtainable in both the as deposited and post-annealed Pt/Co–O/Pt trilayers. The resistance switching ratio exceeds 103, being sufficiently large for the resistance random access memory (RRAM). Co–O is regarded as a very promising oxide for RRAM having compatibilities with the conventional complementary metal–oxide semiconductor process.