Conduction Channel Formation and Dissolution Due to Oxygen Thermophoresis/Diffusion in Hafnium Oxide Memristors

Abstract

Transition metal oxide memristors, or resistive random-access memory (RRAM) switches, are under intense development for storage-class memory because of their favorable operating power, endurance, speed, and density. Their commercial deployment critically depends on predictive compact models based on understanding nanoscale physicochemical forces, which remains elusive and controversial owing to the difficulties in directly observing atomic motions during resistive switching, Here, using scanning transmission synchrotron x-ray spectromicroscopy to study in-situ switching of hafnium oxide memristors, we directly observed the formation of a localized oxygen-deficiency-derived conductive channel surrounded by a low-conductivity ring of excess oxygen. Subsequent thermal annealing homogenized the segregated oxygen, resetting the cells towards their as-grown resistance state. We show that the formation and dissolution of the conduction channel are successfully modeled by radial thermophoresis and Fick diffusion of oxygen atoms driven by Joule heating. This confirmation and quantification of two opposing nanoscale radial forces that affect bipolar memristor switching are important components for any future physics-based compact model for the electronic switching of these devices.Keywords: Memristors, thermophoresis, operating mechanism, oxygen migration, filament.The recent surge in technological and commercial interest in transition-metal-oxide memristors, especially those utilizing hafnium oxide as the switching material, is accompanied by urgent efforts to formulate a compact predictive model of their behavior in large-scale integrated circuits. 1-9 Several efforts in this direction include first-principles and analytical modeling, 8,10,11 materials characterization, 2,12,13 and circuit characterization and modeling. 14,15 The resultant models are incomplete and controversial owing to a lack of understanding of the nanoscale physico-chemical forces that determine atomic motions during switching, particularly with regard to the presence and sign of temperaturegradient-driven thermophoresis of oxygen atoms, and quantification of the concentration-gradient-driven Fick diffusion. 7,8,11,16,17 Direct in-situ and in-operando studies of localized atomic motion during memristor switching can resolve these issues and improve our modeling, but such observations face steep experimental challenges due to the extremely high resolution and sensitivity required to detect atomic motions inside a functioning cell. 2,4,18,19 In order to non-destructively study the chemical and position changes associated with oxygen atoms during memristor operation, we utilized a synchrotron-based scanning transmission x-ray microscopy (STXM) system tuned to the O K-edge with a spatial resolution of <31 nm and a spectral resolution of ~70 meV. 20 We analyzed a prototype device that had only one oxide layer to permit an unambiguous analysis of the results. To enable x-ray transmission experiments, operational memristor cells for this study wer...

Current Localization and Redistribution as the Basis of Discontinuous Current Controlled Negative Differential Resistance in NbOx

“…No significant change in device resistance was observed after the initial snapback response and identical characteristics were measured during a second sweep, consistent 5 | P a g e with the current bifurcating into domains of high and low current-density, as illustrated in Figure 2h. However, repeated cycling did eventually cause a reduction in resistance and a concomitant reduction in both threshold-voltage and threshold-current, consistent with the creation of a permanent filament 28 . These results are consistent with the predictions of the core-shell model in that the lowconductivity film (x=2.6) (i.e.…”

Low‐Conductance and Multilevel CMOS‐Integrated Nanoscale Oxide Memristors

“…[40][41][42] In our case, we think the SR is a combination of thermal and electric field-driven process. [43] Future studies of cross-sectional transmission electron microscopy (TEM) and nanoscale chemical analysis, as well as dynamical process during HR, will help to fully understand the material change and thermal dynamics that are involved in the process. However, HR is an extreme process that likely involves a much higher temperature, which is crucial for rupturing the filament completely; yet the device does not suffer a damage that would prevent it from repeatedly switching on and off (in the case of a successful HR).…”

“…[43][44][45] Second, wider electrode bars have more thermal conductance and thermal capacitance that aid the heat escaping from the filament. There are two reasons that the HR power is memristor sizedependent.…”

Memristors with organic‐inorganic halide perovskites

“…In correspondence of RESET process, V I migration can be activated by two processes: the Fick diffusion process and V I drift process. The driving force of Fick diffusion is the V I concentration gradient . The bottom electrode acts as V I reservoir, the gradient direction is from the bottom electrode to the top electrode.…”

“…The driving force of Fick diffusion is the V I concentration gradient. 105,106 The bottom electrode acts as V I reservoir, the gradient direction is from the bottom electrode to the top electrode. Another important V I migration factor in the RESET process is the electric-field.…”