Tingkun Gu

First-principles simulations on bulk Ta2O5 and Cu/Ta2O5/Pt heterojunction: Electronic structures and transport properties

Electronic structures and transport properties of bulk Ta2O5 and Cu/Ta2O5/Pt heterojunction have been studied from first principles. Of the two room-temperature phases of bulk Ta2O5, β-, and δ-Ta2O5, our calculated results showed that the β phase has much narrower band gap than the δ-Ta2O5. For Cu/δ-Ta2O5/Pt heterojunction, the p-type Schottky barriers between the Cu (Pt) and Ta2O5 were estimated as 0.9–1.2 eV. Both the standard density-functional calculation and the nonequilibrium Green’s function showed that no conducting channels were formed from Cu to Pt through δ-Ta2O5.

Conductive path formation in the Ta₂O₅ atomic switch: first-principles analyses.

The conductive path formed by the interstitial Cu or oxygen vacancies in the Ta(2)O(5) atomic switch were investigated in detail by first-principles methods. The calculated results indicated that the defect state induced by the interstitial Cu is located just at the Fermi level of the Cu and Pt electrodes in the Cu/Ta(2)O(5)/Pt heterostructure and that a conduction channel is formed in the Ta(2)O(5) film via the interstitial Cu. On the other hand, oxygen vacancies in Ta(2)O(5) do not form such a conduction channel because of the lower energy positions of their defect states. The above results suggest that the conductive path could be formed by interstitial Cu in the Ta(2)O(5) atomic switch, whereas the oxygen vacancies do not contribute to the formation of the conductive path.

Excess-silver-induced bridge formation in a silver sulfide atomic switch

Structural properties and electron transport of a Ag2S atomic switch composed of Ag–Ag2S–Ag heterostructure are investigated by nonequilibrium Green’s function calculations considering the effect of excess Ag in the Ag2S layer. In addition to confirming experimentally the formation of the Ag bridge inside Ag2S, the bridge is found to consist of units having a structure similar to that of the Ag (111) face in the bulk Ag. The analyses of Mulliken population, transmission spectra, and current-voltage characteristics reveal that the bridge has a conductive and metallic nature.

Conduction paths in Cu/amorphous-Ta2O5/Pt atomic switch: First-principles studies

We have examined the structure of Cu filaments in Cu/amorphous-Ta2O5 (a-Ta2O5)/Pt atomic switch from first principles. We have found that the Cu single atomic chains are unstable during the molecular dynamics (MD) simulation and thus cannot work as conduction paths. On the other hand, Cu nanowires with various diameters are stable and can form conductive paths. In this case, the Cu-Cu bonding mainly contributes to the conductive, delocalized defect state. These make a sharp contrast with the case of single Cu chains in crystalline Ta2O5, which can be conductive paths through the alternant Cu-Ta bonding structure. A series of MD simulations suggest that even Cu nanowires with a diameter of 0.24 nm can work as conduction paths. The calculations of the transport properties of Cu/a-Ta2O5/Pt heterostructures with Cu nanowires between two electrodes further confirm the conductive nature of the Cu nanowires in the a-Ta2O5.

Migration of Ag in low-temperature Ag2S from first principles

Using the density-functional theory combined with the nudged elastic band method, we have calculated migration pathways and estimated the activation energy barriers for the diffusion of Ag ions in low-temperature Ag2S. The activation energy barriers for four essential migrations for an Ag ion, namely, from a tetrahedral (T) site to an adjacent T vacancy (VT), from an octahedral (O) site to an adjacent O vacancy (VO), from T to VO, and from O to VT, are estimated as 0.461, 0.668, 0.212, and 0.318 eV, respectively, which are comparable to experimental values. This means that diffusions of Ag ions between nonequivalent sites are preferable to those between equivalent sites, and that direct T-VT and O-VO diffusions are less likely to occur than indirect T-VO-T and O-VT-O diffusions. These diffusion behaviors between nonequivalent sites have also been supported by ab initio molecular dynamics simulations, in which the diffusion pathways are directly observed.