Seong-Geon Park

Impact of Oxygen Vacancy Ordering on the Formation of a Conductive Filament in

The electronic properties of rutile TiO2 with an ordered arrangement of oxygen vacancies show a transition from a resistive to conductive oxide as a function of vacancy ordering. Vacancy ordering along two different directions [110] and [001], studied by the density functional theory, predicts that the geometries in which the vacancy-to-vacancy interaction is the strongest, within the nearest neighbor coordination, are thermodynamically favorable and of technological importance. The oxygen vacancies induce several occupied defect states of Ti 3d character, and according to our model, the vacancies are the mediators of electron conduction, while the conductive filament is formed by Ti ions. We propose that the formation of these types of conductive filament is intrinsically connected to the observed defect-assisted tunneling processes and oxide breakdown issues.

\hbox{TiO}_{2}

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.

for Resistive Switching Memory

Abstract First principles density functional theory calculations were employed to study the band structure of reduced rutile TiO2 and the implications of oxygen vacancy charge states on the switching mechanisms observed in resistance change random access memory devices. The formation of conductive paths in TiO2 composed of oxygen vacancy filaments were investigated using the LDA + U d + U p method to calculate the band-structure, electron localization functions, partial charge densities of defect states and the Bader charge decompositions. The thermodynamic stability of charged oxygen vacancies on resistive switching are discussed based on the defect formation energies and a resistive switching mechanism is proposed based on models of filament formation and rupture.

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

The electronic properties of defective rutile TiO2 with oxygen vacancy were studied by ab initio methods to understand resistance switching mechanism for non-volatile memory applications. Density functional theory calculations employing the Hubbard-U on-site correction to the local density approximation (LDA) were performed to determine the structural and electronic modifications introduced by oxygen vacancy formation in bulk TiO2. It has been found that introducing on-site Coulomb interaction between 2p electrons (Up) in addition to on-site Coulomb interaction between 3d electrons (Ud), a corrected description of the band gap state induced by the oxygen vacancy is obtained. The calculated band gap energy within the LDA+Ud+p method is in excellent agreement with the experimental value of 3.0 eV. We find that the oxygen vacancy produces a defect state within the band gap. This state is occupied by two electrons that are highly localized on Ti 3d orbitals of the nearest Ti atoms to the vacancy. The defect state is not a shallow donor, but a deep level at ~0.7 eV below the conduction band edge, indicating that the electron doping at room temperature is not favorable. The atomic relaxation effects are mainly on the Ti and O atoms in the immediate vicinity of the vacancy, however the induced displacements are rather small. The electrons localize on Ti after vacancy creation, and therefore a spatial electronic charge redistribution is observed. This change in electronic density around the vacancy increases the possibility of electron hopping in multi-vacancy systems that align to form a conductive path and ultimately lead to the low resistance state.

First principles modeling of charged oxygen vacancy filaments in reduced TiO 2 –implications to the operation of non-volatile memory devices

We have developed a novel method and model to describe the switching and degradation phenomena of uni-polar type ReRAM using conventional electric field dependent dielectric breakdown model. By this method, we can clearly describe the relationship between “forming”, “set”, “reset”, and degradation process of TiO2 based ReRAM. In this paper, we will demonstrate our method and discuss the switching and degradation model of uni-polar type ReRAM regarding its future potential for commercialization.

First-principles study of resistance switching in rutile TiO 2 with oxygen vacancy

We performed first-principle simulation for the study of oxygen vacancy defect in rutile TiO 2 based on density functional theory. The effects of a vacancy on the electronic structure of rutile TiO 2 were studied. Here we have employed neutral and charged oxygen vacancy in the supercell to address the resistance switching mechanism. Neutral vacancy induces the band gap states at deep level, ∼0.7 eV below the conduction band minimum, which is occupied by highly localized electrons. The calculation results of positively charged oxygen vacancy show that larger atomic relaxation surrounding oxygen vacancy results in the stretching of Ti-O bond around vacancy, thus band gap states are formed near the conduction band minimum.

Electric field dependent switching and degradation of Resistance Random Access Memory

We study the ON-OFF switching mechanism of ReRAM via oxygen vacancy (VO) based conducting channels using first-principles calculations. We find cohesion-dispersion transition of VO upon carrier injection and removal is a strong driving force in the ON-OFF switching in binary-oxide-based ReRAMs. The physical origins of the transition is the formation of bonding-like hybridized orbitals of VO defect levels, whose occupation can be controlled by changing system Fermi level by applying a voltage.

Lattice and electronic effects in rutile TiO 2 containing charged oxygen defects from ab initio calculations

Computational Science became crucial techniques for designing future micro electronics engineering. In this paper, we describe the examples which aim the designing of future micro electronics devices such as charge trap memories and resistive random access memories based on the computational science.