Fumiyoshi Takano

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.

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.

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.

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.

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.

Enhancement in ordering of Fe50Pt50 film caused by Cr and Cu additives

A 100-nm-thick Fe50Pt50 film with high coercivity was fabricated by magnetron sputtering. On introducing a Cr top/bottom layer, the ordering temperature (Tor) of the film decreased considerably from 500 to 380 °C. On introducing both Cr top/bottom layer and a 0.2-nm-thick Cu interlayer, the Tor decreased even further to 340 °C. During the annealing process, the number of Cr atoms diffused into the Fe50Pt50 layer increased. This increased the Fe diffusivity, lowering the Tor in the process. The diffusion of the additive and the effects of annealing were verified by selected-area electron diffraction observations and by investigating the temperature dependence of the magnetization. The superlattice structure of the ordered Fe50Pt50 film containing Cr and Cu additives was confirmed by the transmission electronic microscopy. Because of the Cr and Cu additives, the average grain size in the ordered Fe50Pt50 film increased slightly; however, the coercivity of the ordered Fe50Pt50 film is improved considerably.

Anomalous temperature-dependent exchange bias in Fe films deposited on Si substrates with the native oxide layer

The temperature dependence of the exchange bias has been investigated in Fe films deposited on the Si substrates with the native oxidize layer. A crossover from negative to positive exchange bias has been observed with increasing temperature at 30 K, accompanied with the maximum of the coercivity. This effect could be understood as a result of the exchange coupling between the ferromagnetic Fe film and the spin-glasslike Fe oxides particles formed spontaneously at the interface, where the direction of their end spins controls the sign of exchange bias.

Reactive Ion Etching Process of Transition-Metal Oxide for Resistance Random Access Memory Device

The reactive ion etching (RIE) of the binary transition-metal oxides (TMOs) NiO, CuO and CoO, which are expected to be key materials of resistance random access memory (RRAMTM), was investigated. We found that inductively coupled plasma using CHF3-based discharge, which is highly compatible with conventional semiconductor RIE, is effective for the TMOs studied here. Furthermore, device fabrication using Pt/CoO/Pt trilayers is carried out, and a large change in resistance, which is an essential functionality of RRAM, was successfully observed. This should be definite evidence of a successful RIE realized in the present device fabrication.

Characterization of mn-doped 3C-SiC prepared by ion implantation

The characterization of Mn-doped 3C-SiC prepared by ion implantation is reported. Implantation of Mn ions at a dose of 1×1016cm−2 into a 3C-SiC homoepitaxial wafer was carried out. High temperature annealing following the implantation process was found to enhance the ferromagnetic ordering. Transmission magnetic circular dichroism and magnetization investigations demonstrated a ferromagnetic behavior below 245K. The lattice relaxation induced by the postannealing is considered a possible mechanism of this outcome.

Characterization of room temperature ferromagnetic Mn–Si compound synthesized on SiC substrate

Manganese silicide (Mn–Si) related compound with a Curie temperature (TC) of 300K was synthesized on a 4H-SiC homoepitaxial wafer. Thin Mn layer was deposited on the SiC wafer, and then annealing was performed to diffuse the Mn atoms into the SiC epitaxial layer. Transmission magnetic circular dichroism spectra reflected that of the SiC wafer, although the structural investigations suggested that the dominant phase of the synthesized layer was paramagnetic Mn5Si2. At this stage, two possible explanations are considered the origin of the observed ferromagnetism. One is that a small amount of carbon (C) incorporated into the Mn5Si2 host induced the ferromagnetic order in Mn5Si2. On the other hand, the partial formation of a ferromagnetic SiC-related component at the vicinity of the lower side of the interface between SiC and Mn5Si2 layers is considered the other possible explanation.