Simulation of TiN/HfO2/Pt memristor I–V curve for different conductive filament thickness

The operation of the TiN/HfO2/Pt bipolar memristor has been simulated by the finite elements method using the Maxwell steady state equations as a mathematical basis. The simulation provided knowledge of the effect of conductive filament thickness on the shape of the I–V curve. The conductive filament has been considered as the highly conductive Hf ion enriched HfOx phase (x < 2) whose structure is similar to a Magneli phase. In this work a mechanism has been developed describing the formation, growth and dissolution of the HfOx phase in bipolar mode of memristor operation which provides for oxygen vacancy flux control. The conductive filament has a cylindrical shape with the radius varying within 5–10 nm. An increase in the thickness of the conductive filament leads to an increase in the area of the hysteresis loop of the I–V curve due to an increase in the energy output during memristor operation. A model has been developed which allows quantitative calculations and hence can be used for the design of bipolar memristors and assessment of memristor heat loss during operation.

The Relation between Drift, Entropy Distribution and Kirkendall Plane Position during Diffusion

The Kirkendall effect appears due to the unbalanced diffusion fluxes causing the vacancy flux. There are several numerical methods that allow to predict the position of Kirkendall plane after the diffusion couple annealing. In this work for the first time the entropy density distribution is used to estimate the trajectory of the Kirkendall plane. The entropy density distribution is calculated with use of the bi-velocity method, which combines: (1) the volume continuity, (2) the conservation of mass, (3) momentum and (4) entropy-density. The method is applied to simulate the diffusion in Ni-Pd diffusion couple.

The Nature of the Vacancy-Wind Effect Occurring in Diffusion via Six-Jump-Cycles in B2 Intermetallics

First discovered by the late Dr John Manning, the vacancy-wind effect is a subtle phenomenon that occurs when two or more atomic species compete for vacancies in a net vacancy flux. The vacancy-wind effect is incorporated in (for example) the vacancy-wind or Manning factor that appears in the Darken-Manning Equation relating the interdiffusivity, the tracer diffusivities and the thermodynamic factor. The mechanism of the vacancy-wind phenomenon has long been very poorly understood. Recently, a moving reference frame Monte Carlo method was used to illustrate graphically how the vacancy-wind effect operates in both ionic conductivity in an ionic solid with a dilute solute and chemical interdiffusion in concentrated alloys and ionic compounds. That strategy is extended in this paper to show graphically how the vacancy-wind effect operates in interdiffusion in a stoichiometric intermetallic taking the B2 structure. A simple 4-frequency vacancy diffusion model is used. In previous work, it was shown that depending on composition and temperature, this model can exhibit the six-jump-cycle mechanism. It is shown that in the limit of perfect order that there is no vacancy-wind effect associated with this mechanism when both types of cycle operate equally (zero net vacancy flux). The non-unity value of the vacancy-wind factor found for this mechanism under zero vacancy flux conditions is purely a consequence of a particular geometric mix of tracer and collective atom displacements. The concept that a non-zero off-diagonal phenomenological coefficient provides the vacancy-wind effect is verified.

Simulation of Intrinsic Diffusion in Multicomponent Multiphase Systems

ABSTRACTA simulation model for intrinsic diffusion of multicomponent multiphase systems is presented. The model is not restricted onto a certain number of components or phases. For simplicity, Manning's random alloy model with vanishing vacancy wind effect is used. Then the cross terms of the diffusion flux can be neglected. The simulation routine uses equations for the fluxes, the equation of continuity and an equation for the change of the thickness of volume elements due to the vacancy flux. With this model diffusions paths, concentration profiles, fluxes of the components as well as marker positions can be calculated. The shift of interfaces and the growth of new phases can also be determined. The simulation results were compared with experimental data of the Cu-Fe-Ni system. Diffusion was studied in single-phase areas and across interfaces.

The Behavior of Oxidation Stacking Faults During O2/NF3 Oxidation of Silicon

ABSTRACTThe behavior of oxidation—induced stacking faults(OSFs) during the O2 /NF3 oxidation of silicon has been investigated in the temperature range of 850–1100°C with varying concentration of NF 3 (0.011—0.044 vol%). In this study, we report the rapid and non-linear shrinkage of pregrown OSFs with time by a new oxidation process including small(ppm) additions of fluorine compound to the oxidant. It is also found that the shrinkage rate of OSF decreases as the oxidation time is increased. It is proposed that the fast OSF shrinkage is due to excessive vacancy flux as a result of the reaction of fluorine at the Si/SiO2 interface during the initial transient state, and subsequently the shrinkage rate is reduced as the steady—state condition of vacancy—interstitial recombination is approached.

Cavity nucleation on special boundaries

All present theories of cavity formation assume cavities are nucleated during creep on geometrical irregularities at grain boundaries and triple junctions. The resulting (required) tensile stress concentration enhances diffusional creep in the vicinity of the boundary producing a high vacancy flux which ends up nucleating a cavity. No mention is done of the role played by the structure of the boundary. In this paper we present experimental evidence indicating that cavity nucleation is dependent on the structure of the boundary and that cavities in pure metals are nucleated at special boundaries.According to Bollman, a lattice dislocation impinging on a boundary can dissociate into grain boundary dislocations with non crystalline Burgers vectors (GBDs). Also, Pond has shown that further dissociation into partials can occur in certain boundaries. When the degree of good fit decreases, the Burgers vector of any possible GBDs decrease and dislocations tend to be largely spread out becoming “dissolved” into the boundary. Boundaries capable of sustaining dislocations with relatively large Burgers vectors (long range strain fields) are called special boundaries.

STEM microanalysis of tin segregation in germanium

STEM investigations using very fine incident electron beam diameters and x-ray analysis facilities provide a powerful tool for determining the segregation levels of solute elements at, or near,grain boundaries. As yet however relatively few studies have made use of this potential. It is particularily interesting to apply the technique to the study of Sn segregation to germanium grain boundaries, because the boundaries can be driven to migrate by being heated in the presence of the Sn. It is proposed that this grain boundary migration is caused by a process called Diffusion Induced Boundary Migration, (DIGM). The suggested mechanism for DIGM is that a grain boundary Kirkendall effect occurs due to the uneven fluxes of solute and solvent atoms down the interface. The resulting net vacancy flux stimulates the movement of grain boundary dislocations, which in turn cause boundary migration. A significant feature of this model is that solute enriched grains will be left behind each migrating boundary.