**8. Effect of grain surface area and grain boundary**

Interfacial type resistive switching (RS) in memristive device is dominated by the surface morphology and properties. Under the application of an external voltage bias, the density distribution of defects/vacancies can vary along the film thickness, which affects the bandgap of material and produces unpredictable behavior in resistive switching response. These uncertain changes at the interface, barrier bandgap leads to a change in the sample's resistance in an interfacial RS which is extremely sensitive to interfacial properties.

Generally, interface is defined between two different materials systems but sometimes interface can also be defined at the regions which have the same composition and crystal structure but different crystal orientations (even inside the same solid such as grain boundaries (GBs)-including tilt and twist boundaries-twin boundaries and stacking faults) [34]. Moreover, an interface is affected by the several external environment conditions such as air, vacuum, moisture and some other material properties such as the crystallinity of solids at the interface. However, the defect chemistry is comparatively different in the proximity of a charged interface (GB) from the bulk situation (single crystal). Basically, charged interface induces the redistribution of the mobile charge carriers in the space-charge layer region while in bulk, the electroneutrality has been played an important role (at equilibrium) between differently charged point defects. In this section, the effect of grain boundaries (GBs) and grain surfaces (GSs) on RS have discussed.

Oxygen vacancies and defects are considered responsible for resistive switching phenomena in oxides materials. For nanoscaled materials, GBs conductivity is directly proportional to grain size, and it may modulate according to the direction of current flow (perpendicular and parallel direction of GBs) [63]. However, the position of the GBs is dependent on the shape and size of the grain J. Maier [63] has reported that in case of yttria-doped zirconia if the grain size decreases at particular dimension (~ 50 nm in diameter) then most of the current passes perpendicular to the GB axis and the conductivity parallel to GB becomes negligible. Further, the formation of GBs when two adjacent and equally oriented grains are rotated to each other and twisted GB occurs when the rotation axis is perpendicular to the boundary. On the other hand, if the rotation axis is lied in the boundary plane, a tilted boundary is resulted [34]. Moreover, the degree of rotation also affects the coherence of the final grain boundary. GB with minimum rotation angle can be treated as an group of edge dislocations and aggregation of screw dislocations [34]. The concentration of oxygen vacancies (Ov) is comparatively higher at GBs which leads to leakage current flow through the GBs predominantly [64]. Any variation in Ov will be closely related to grain boundary and grain surface area. In some polycrystalline oxide thin film, structural defects, grain boundaries and local nonstoichiometric regions are responsible for high leakage current. Further, high electrical stress due to applied electrical potential, induces traps/cracks along the GBs. Induced traps/ cracks also increases the leakage current and size of the conduction region at the GBs as compared to the grain regions [65]. In recent years, the ab initio calculations and conductive atomic force microscopy (CAFM) have demonstrated to study the charge transport through grain boundaries in polycrystalline HfO2 [66].

The space-charge conduction model for acceptor-doped zirconia suggests that the lower ionic conductivity in zirconia occurs due to the depletion of oxygen vacancies and excess the positive charge laying in the GB core [67, 68]. However, dislocations appeared in YSZ single crystal due to plastic deformation does not improve the material's electrical transport significantly [69]. Further, the resistive switching in WO3 thin film is dominated by the grain surface region, not by the GB [70].

## *Effect of Surface Variations on Resistive Switching DOI: http://dx.doi.org/10.5772/intechopen.97562*

In case of polycrystalline oxide films, GBs contain a high density of defects, which will accumulate the more traps inside the grain regions. A traps present inside the grain may cause a percolation path under a high electric field [65]. In filamentary type conduction, oxygen vacancies/ions would form a filaments throughout the oxide layer (highly conductive path) via GB [71]. Another interpretation of conductive filament's formation is connected with the motion of the O2− ions which usually appears near the crystal defects such as oxygen vacancies and GBs [56, 72, 73]. A filament forming behavior in switching oxide film can be controlled by controlling the grain size underneath the top electrode and smaller grain size indicate the large number of GBs. These types of correlation can be identified by varying the electrode size and the number of GB underneath. However, these correlation becomes are not impactful if electrode size less than the individual grain size [74]. In addition, Das *et al*. have discussed the effect of GBs, GSs, and surface morphology such as (hillocks, lattice mismatch) on the statistical variation of RS parameters (forming voltage, set-reset voltage) in yttriabased resistive switching device (**Figure 2**) [64]. Successive RS operations depend on the inhomogeneous changes in defect structure, and as a result, the switching parameters also vary persistently. During RS process, there are several type of sources in different oxides which provoke variability in device parameters. However, the formation and recombination of oxygen vacancies is highly stochastic in nature and play dominant role in deciding degree of variability. After analyzing the experimental data, Monte Carlo simulation has established a potential stochastic model that relates subsequent RS behavior to the initial states of contact in resistive memory cells [75].

#### **Figure 2.**

*Yttria layer (~80 nm) is deposited at different substrate temperatures of 300 (Y3), 400 (Y4), and 500°C (Y5) by dual ion beam sputtering system. Scanning electron microscope (FESEM) images of (a) Y3 (b) Y4 (c) Y5. (d) Mean (M) and standard deviation (*σ*) of the set and reset voltages. (e) Mean (M) and standard deviation (*σ*) of the grain surface area (figure d and e. reprinted with permission: Ref. [64]).*
