Modified Dynamic Physical Model of Valence Change Mechanism Memristors

Abstract

Valence change-type resistance switching behaviors in oxides can be understood by well-established physical models describing the field-driven oxygen vacancy distribution change. In those models, electroformed residual oxygen vacancy filaments are crucial as they work as an electric field concentrator and limit the oxygen vacancy movement along the vertical direction. Therefore, their movement outward by diffusion is negligible. However, this situation may not be applicable in the electroforming-free system, where the field-driven movement is less prominent, and the isotropic oxygen vacancy diffusion by concentration gradient is more significant, which has not been given much consideration in the conventional model. Here, we propose a modified physical model that considers the change in the oxygen vacancies’ charged state depending on their concentrations and the resulting change in diffusivity during switching to interpret the electroforming-free device behaviors. The model suggests formation of an hourglass-shaped filament constituting a lower concentration of oxygen vacancies due to the fluid oxygen diffusion in the thin oxide. Consequently, the proposed model can explain the electroforming-free device behaviors, including the retention failure mechanism, and suggest an optimized filament configuration for improved retention characteristics. The proposed model can plausibly explain both the electroformed and the electroforming-free devices. Therefore, it can be a standard model for valence change memristors.

Thermodynamic origin of nonvolatility in resistive switching

“…1a,b) based on ideal solution thermodynamics. We will later explain these differences in the context of phase separation, which can explain why some experiments show HRS retention failure 15,16,20,21 like ours, while others yield LRS retention failure 14,18,19 .…”

“…Second, while this model predicts filament dissolution, experimental measurements have shown evidence for both higher resistance from filament dissolution 14,15,18,19 (Fig. 1b) and lower resistance from filament widening 15,16,20,28 (Fig. 1c).…”

“…a) Fick's First Law, which states that mobile species diffuse from high to low concentration, suggests that the Ta2O5 will rapidly gain oxygen vacancies when in contact with a suboxide, resulting in failure shortly after fabrication. b) Most models assume that oxygen vacancies do not migrate between the two oxide layers during retention [13][14][15][16][17] . Under this modified Fickian diffusion model, the filament dissolves, resulting in increased resistance over time, a result consistent with some experiments 14,18,19 .…”

“…Fick's First Law, which assumes that defects are noninteracting and follow ideal solution thermodynamics, has been universally used to describe oxygen diffusion in VCM [14][15][16][17][18][22][23][24] . Under no applied voltages, Fig.…”

“…Because this bilayer mixing model predicts that pristine devices would become conducting and therefore fail a few hours after fabrication 25 , it is not used to model retention. Instead, it is generally assumed that oxygen vacancies do not migrate between the stoichiometric and suboxide layers during retention [13][14][15][16][17] . This modified Fickian model predicts that oxygen vacancies diffuse out of the highconcentration filament (Fig.…”