Impact of the Mechanical Stress on Switching Characteristics of Electrochemical Resistive Memory

Abstract

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...

(Invited) Stress-Induced Asymmetric Switching and Filament Instability in Electrochemical Memories

“…The same picture is supported by direct in-situ observations of ECM operation [14,15]. While these models are accurate enough to serve for device switching under different working conditions, such as different compliance current and pulse width, there is still a need for a deeper understanding of switching and reliability mechanisms, such as resistance instabilities and fluctuations in the LRS [12,13,[16][17][18]. For this purpose, a careful experimental study of switching characteristics as a function of the compliance current and under pulsed operation is needed.…”

Data retention statistics and modelling in HfO<inf>2</inf> resistive switching memories

“…12 [7]. Due to mass transfer during the set process, a stress field develops close to the CF in LRS at relatively large R, corresponding to partially-formed CFs.…”

“…The stress gradient provides a driving force for defect migration back to the TE, causing R to increase, reaching an equilibrium point (Fig. 12b) [7]. HRS instead displays an opposite stress field, due to the opposite flow of defects during reset (Fig.…”

“…However, many challenges should still be addressed to reach the stage of industrial production. As an example, device reliability is still unsatisfactory due to noise and random resistance changes [3]- [7]. In particular, set state was shown to be unstable with respect to thermallyactivated diffusion [8].…”

“…In particular, set state was shown to be unstable with respect to thermallyactivated diffusion [8]. Also, the mechanical stress around the conductive filament (CF) was shown to play a role in accelerating set failure in conductive bridge resistive memory [7]. On the other hand, the reset state was shown to display resistance fluctuations [6] and drift due to thermally-activated diffusion [9].…”