Moon Young Yang

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.

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.

Interstitial oxygen induced Fermi level pinning in the Al2O3-based high-k MISFET with heavy-doped n-type poly-Si gates

We study the origin of substantial threshold voltage (Vth) shifts observed in Al2O3-based MISFETs with n+poly-Si gate, by focusing on the effect of an interstitial oxygen (Oi) formation in Al2O3. We observed that the Oi level in Al2O3 is 1 eV above the valence band top of Al2O3 by first-principles calculation. Therefore, Oi formation and subsequent electron transfer from Fermi level to the Oi level allows the system to overcome the energy loss by the Oi formation, which depends on the position of Fermi level. In case of n+poly-Si gate, this electron transfer across the interface occurs and results in substantial Vth shifts. The proposed mechanism reproduces experimental result and provides a good understanding of nano-interfacial interactions.

Energetics and electron states of Au/Ag incorporated into crystalline/amorphous silicon

We report behaviors of gold (Au) and silver (Ag) incorporated into crystalline silicon (c-Si) and amorphous silicon (a-Si) based on first-principles calculations. Our results show that both Au and Ag exhibit different behaviors in between c-Si and a-Si. The Au/Ag atom locates at the tetrahedral interstitial site in c-Si and it takes a positively charged state as an energetically stable state for the wide range of band gap of c-Si. On the other hand, the Au/Ag atom tends to take a neutral charge state in a-Si. We reveal that both d- and s-orbitals of Au/Ag are deeply concerned in those different behaviors. This study indicates that Au/Ag changes the way to interact with Si depending on the geometric structure.

First principles guiding principles for the switching process in oxide ReRAM

First principles calculations became crucial techniques for designing future electronics engineering. In this paper, we describe the typical examples which aim the designing of future electron devices such as modern resistive random access memories (ReRAM) by using the knowledge obtained by the first principles calculations.

Substrate-mediated proton relay mechanism for the religation reaction in topoisomerase II.

The DNA religation reaction of yeast type II topoisomerase (topo II) was investigated to elucidate its metal-dependent general acid/base catalysis. Quantum mechanical/molecular mechanical calculations were performed for the topo II religation reaction, and the proton transfer pathway was examined. We found a substrate-mediated proton transfer of the topo II religation reaction, which involves the 3′ OH nucleophile, the reactive phosphate, water, Arg781, and Tyr782. Metal A stabilizes the transition states, which is consistent with a two-metal mechanism in topo II. This pathway may be required for the cleavage/religation reaction of topo IA and II and will provide a general explanation for the catalytic mechanism in the topo IA and II.