The resistive switching mechanism of 20-to 57-nm-thick TiO 2 thin films grown by atomic-layer deposition was studied by current-voltage measurements and conductive atomic force microscopy. Electric pulse-induced resistance switching was repetitively ͑Ͼ a few hundred times͒ observed with a resistance ratio ӷ10 2 . Both the low-and high-resistance states showed linear log current versus log voltage graphs with a slope of 1 in the low-voltage region where switching did not occur. The thermal stability of both conduction states was also studied. Atomic force microscopy studies under atmosphere and high-vacuum conditions showed that resistance switching is closely related to the formation and elimination of conducting spots. The conducting spots of the low-resistance state have a few tens times higher conductivity than those of the high-resistance state and their density is also a few tens times higher which results in a ϳ10 3 times larger overall conductivity. An interesting finding was that the area where the conducting spots do not exist shows a few times different resistance between the low-and high-resistance state films. It is believed that this resistance change is due to the difference in point defect density that was generated by the applied bias field. The point defects possibly align to form tiny conducting filaments in the high-resistance state and these tiny conducting filaments gather together to form stronger and more conducting filaments during the transition to the low-resistance state.

The filamentary resistance switching mechanism of a Pt∕40nm TiO2∕Pt capacitor structure in voltage sweep mode was investigated. It was unambiguously found that the conducting filaments propagate from the cathode interface and that the resistance switching is induced by the rupture and recovery of the filaments in the localized region (3–10nm thick) near the anode. The electrical conduction behavior in the high resistance state was well explained by the space charge limited current (SCLC) mechanism that occurs in the filament-free region. The various parameters extracted from the SCLC fitting supported the localized rupture and formation of filaments near the anode.

Electric-pulse-induced resistive switching of 43nm thick TiO2 thin films grown by metalorganic chemical vapor deposition was studied by current-voltage (I-V) and constant voltage-time measurements. The resistance ratio between the two stable states of the film constitutes approximately 1000. The allowed current level and voltage step width during the sweep mode I-V measurements influenced switching parameters, such as the switching voltage, time before switching, and resistance values. However, it was clearly observed that the power imparted to the film controlled mainly switching. The required power for successful switching was almost invariant irrespective of other measurement variables.

with electrode size as small as 5 nm. [ 26 ] Whereas low-power operation [ 3 ] and fast switching [ 15 ] were demonstrated in ECMs, recent experiments on ECM devices [ 25,27 ] have questioned the long-term stability of the CF, which is necessary to enable nonvolatile storage of data. To assess the switching behavior and CF stability in ECMs, the detailed evolution of electrical, chemical and mechanical forces in the CF must be understood. Figure 2 a shows the measured current-voltage ( I -V ) curve for an ECM with Ag top electrode, GeS 2 electrolyte and W bottom electrode during set and reset transition. Details about the preparation of the ECM devices can be found elsewhere. [ 15 ] The set transition from high to low resistance takes place at V set ≈ 0.3 V, while the reset transition to high resistance occurs at V reset . In the set process, the CF is fi rst formed by nucleation [ 28,29 ] followed by CF growth, [ 6,24 ] which is activated by positive ion migration as shown in Figure 2 b: metallic ions from the reactive-metal electrode (Ag) hop among localized states separated by energy barriers E A0 in amorphous GeS 2 . The electric fi eld lowers the ion-migration barrier to the value:where α = 0.3 is a barrier-lowering coeffi cient (see Figure S1 in the Supporting Information for a complete list of all model parameters used in the simulation) and V is the voltage across the electrolyte. [ 6,24,30 ] Barrier lowering enhances the hopping rate in the fi eld direction, thus increasing the growth rate of the CF diameter φ according to:where A is a pre-exponential constant proportional to cation mobility, T is the local temperature at the CF and E A was obtained from Equation 1 . In Equation 2 , E A controls ion migration in the vertical direction from top to bottom electrode, while the accumulation of defects along the CF causes growth in the radial direction, which is captured by the parameter φ (e.g., see Figure 1 c). Note that the parameter φ represents an effective CF diameter, to properly describe non-cylindrical (e.g., conical) CF shapes and the case of multiple fi laments contributing to the resistive switching. [ 12,26 ] For instance, in the case of multiple fi laments the effective diameter in Equation 2 obeys φ 2 = Σ φ i 2 , where φ i represents the diameter of the individual i -th fi lament and φ properly describes the resistance R ∝ φ −2 of the set state.To control the size of the growing CF during the set transition, the current was kept equal to a compliance value I C = 1 mA in Figure 2 a, thus resulting in a resistance of about 0.3 kΩ in the set state. The current-controlled CF growth was Resistive switching in oxides and other insulating materials provides a promising approach to nanoscale memory devices, where the stored logic state can be changed by activating/deactivating a conductive fi lament (CF). [ 1,2 ] Among resistive switching devices, the electrochemical memory (ECM) attracts strong interest because of outstanding properties such as extremely small programming current, [ 3 ] fast switching...

Phase change random access memory appears to be the strongest candidate for next-generation high density nonvolatile memory. The fabrication of ultrahigh density phase change memory (≫1 Gb) depends heavily on the thin film growth technique for the phase changing chalcogenide material, most typically containing Ge, Sb and Te (Ge–Sb–Te). Atomic layer deposition (ALD) at low temperatures is the most preferred growth method for depositing such complex materials over surfaces possessing extreme topology. In this study, [(CH3)3Si]2Te and stable alkoxy-Ge (Ge(OCH3)4) and alkoxy-Sb (Sb(OC2H5)3) metal–organic precursors were used to deposit various layers with compositions lying on the GeTe2–Sb2Te3 tie lines at a substrate temperature as low as 70 °C using a thermal ALD process. The adsorption of Ge precursor was proven to be a physisorption type while other precursors showed a chemisorption behavior. However, the adsorption of Ge precursor was still self-regulated, and the facile ALD of the pseudobinary solid solutions with composition (GeTe2)(1‑x)(Sb2Te3) x were achieved. This chemistry-specific ALD process was quite robust against process variations, allowing highly conformal, smooth, and reproducible film growth over a contact hole structure with an extreme geometry. The detailed ALD behavior of binary compounds and incorporation behaviors of the binary compounds in pseudobinary solid solutions were studied in detail. This new composition material showed reliable phase change and accompanying resistance switching behavior, which were slightly better than the standard Ge2Sb2Te5 material in the nanoscale. The local chemical environment was similar to that of conventional Ge2Sb2Te5 materials.