Katsumasa Kamiya

ON-OFF switching mechanism of resistive–random–access–memories based on the formation and disruption of oxygen vacancy conducting channels

We study the ON-OFF switching mechanism of oxide-based resistive–random–access–memories using theoretical calculations. Electron deficient vacancies (VO) up to 1+ charge states would stabilize a cohesive filament, while further electron removal will stabilize the disrupted VO configurations with 2+ charges. The VO cohesion-isolation transition upon carrier injection and removal is shown to be a strong driving force in the ON-OFF switching process. We also propose that bipolar or unipolar behavior is determined by how the carriers are injected into VO. The control of the carrier injection by the electrode material selection is essential for desired bipolar switching.

Nanoscale (∼10nm) 3D vertical ReRAM and NbO 2 threshold selector with TiN electrode

The scaling and 3-D integration issues of NbO₂ with threshold switching characteristics were investigated for ReRAM selector device. To avoid the process problems of Pt electrode, we tested ReRAM and selector devices with conventional electrodes (TiN and W). By adopting 10nm-thick TiN bottom electrode with low thermal conductivity, we could significantly reduce the threshold current for insulator-metal transition (I-M-T) due to the heat confinement effect. We have evaluated for the first time both 1S1R (NbO₂/TaO_x) and hybrid (NbO₂/Nb₂O₅) devices. We have confirmed the feasibility of high density vertical memory device by adopting NbO₂ I-M-T selector device.

Vacancy Cohesion-Isolation Phase Transition Upon Charge Injection and Removal in Binary Oxide-Based RRAM Filamentary-Type Switching

We report on the electronic roles in filamentary-type switching of binary oxide-based resistive random access memories using ab initio calculations. We show that charge injection and removal determine the thermodynamic stability of the vacancy filament and the diffusion in the memory devices; electron injection induces the vacancy cohesion that stabilizes the filament, whereas removal of these electrons favors the vacancy isolation that destabilizes the filament; electron removal makes the energy barrier of the vacancy diffusion processes small enough to be overcome by joule heating. The vacancy cohesion-isolation processes are induced by charge injection and removal that leads to occupation of the bonding-like electron states, which can be controlled by shifting the system Fermi level via an applied voltage during memory operation. The vacancy cohesion-isolation phase transition upon charge injection and removal is thus one of the main factors that govern resistive switching. Based on the physics, we propose three-layer stack structures for further improvement of the memory characteristics.

Charge-dependent oxygen vacancy diffusion in Al2O3-based resistive-random-access-memories

We theoretically study an oxygen vacancy (VO) diffusion in Al2O3-based resistive-random-access-memories (ReRAMs). We find that the activation energy of VO diffusion in Al2O3 strongly depends on the charge state of VO. In ReRAM, the charge state of VO can be easily changed by applying voltage and the lowest activation energy is observed at q = 2+. The operation voltage on Al2O3-based ReRAM is close to the activation energy at q = 2+, indicating that VO diffuses with doubly positive state. Moreover, the activation energy at q = 0 is close to that observed in bulk Al2O3, which explains the discrepancy between previous experimental and theoretical studies.

Physics in designing desirable ReRAM stack structure — Atomistic recipes based on oxygen chemical potential control and charge injection/removal

We clarify the importance of three-layers ReRAM stack-structures and provide guidelines for further optimization by both charge injection/removal and oxygen chemical potential. We determine atomistic structures corresponding to the ON-OFF switching process of ReRAMs using ab initio calculations. The cohesion-isolation of oxygen vacancies is found to be a strong driving force in the ON-OFF switching observed in oxide-based ReRAMs, and this phase transition can be controlled by injecting/removing charges while altering the oxygen chemical potential. Based on this concept, we propose universal guidelines for designing desirable ReRAM stack structures by introducing an oxygen vacancy barrier layer.

Structural design of AlN/GaN superlattices for deep-ultraviolet light-emitting diodes with high emission efficiency

We demonstrate on the basis of first-principles calculations that a AlN/GaN superlattice with one or two GaN monolayers is efficient for near-band-edge C-plane emission of deep-ultraviolet light-emitting diodes. We find that such superlattices lead to a significant increase of the C-plane components of the optical matrix element up to 57% relative to GaN bulk. At the same time, the energy gap of these superlattices is in the deep-UV region, where the shortest emission wavelength is 224 nm. This is remarkably shorter than that in Al-rich AlGaN alloys.

Intrinsic origin of negative fixed charge in wet oxidation for silicon carbide

We demonstrate on the basis of first-principles calculations that the formation of carbonate-like moiety in SiO2 could be the intrinsic origin of negative fixed charge in SiC thermal oxidation. We find that two possible origins for the negative fixed charges are O-lone-pair state and a negatively charged CO3 ion in SiO2. Such CO3 ion is able to be formed as a result of the existence of residual C atoms in SiO2, which are expected to be emitted from the interface between SiC and SiO2, and the incorporation of H atoms during wet oxidation.

Charge-injection phase change memory with high-quality GeTe/Sb 2 Te 3 superlattice featuring 70-μA RESET, 10-ns SET and 100M endurance cycles operations

We developed a high quality charge-injection GeTe/Sb2Te3 superlattice phase change memory. A RESET-current of 70 μA and a SET-speed of 10 ns, the fastest ever reported, were obtained. Transmission electron microscopy analysis showed that the superlattice structure was maintained after 1 million (M) endurance. Even 100 M cycle endurance was possible, and these results conclusively proved non-melting resistive switching.

Nylon-Oligomer Hydrolase Promoting Cleavage Reactions in Unnatural Amide Compounds.

The active site of 6-aminohexanoate-dimer hydrolase, a nylon-6 byproduct-degrading enzyme with a β-lactamase fold, possesses a Ser112/Lys115/Tyr215 catalytic triad similar to the one of penicillin-recognizing family of serine-reactive hydrolases but includes a unique Tyr170 residue. By using a reactive quantum mechanics/molecular mechanics (QM/MM) approach, we work out its catalytic mechanism and related functional/structural specificities. At variance with other peptidases, we show that the involvement of Tyr170 in the enzyme-substrate interactions is responsible for a structural variation in the substrate-binding state. The acylation via a tetrahedral intermediate is the rate-limiting step, with a free-energy barrier of ∼21 kcal/mol, driven by the catalytic triad Ser112, Lys115, and Tyr215, acting as a nucleophile, general base, and general acid, respectively. The functional interaction of Tyr170 with this triad leads to an efficient disruption of the tetrahedral intermediate, promoting a conformational change of the substrate favorable for proton donation from the general acid.

Physical Guiding Principles for High Quality Resistive Random Access Memory Stack with Al2O3 Insertion Layer

We theoretically clarified the atomistic role of the Al2O3 oxygen vacancy (VO) barrier layer in advanced ReRAM stacks. We found that VO filament formation in Al2O3 can be controlled by applying voltage when the Al2O3 layer is in contact with VO source layer such as Hf, although VO formation in Al2O3 is difficult in usual situation. Moreover, we proposed a physical guiding principle toward designing high quality ReRAM stacks with Al2O3 VO barrier layers.