It has been shown that brain-like self-repair can arise from the interactions between neurons and astrocytes where endocannabinoids are synthesized and released from active neurons. This retrograde messenger feeds back to local synapses directly and indirectly to distant synapses via astrocytes. This direct/indirect feedback of the endocannabinoid retrograde messenger results in the modulation of the probability of release (PR) at synaptic sites. When synapses fail, there is a corresponding falloff in the firing activity of the associated neurons, and hence the strength of the direct feedback messenger diminishes. This triggers an increase in PR of healthy synapses, due to the indirect messenger from other active neurons, which is the catalyst for the repair process. In this paper, the repair process is implemented by developing a new learning rule that captures the spike-timing-dependent plasticity and Bienenstock, Cooper, and Munro learning rules. The rule is activated by the increase in PR and results in a potentiation of the weight values, which reestablishes the firing activity of neurons. In addition, this self-repairing mechanism is extended to network-level repair where astrocyte to astrocyte communications are implemented using a linear gap junction model. This facilitates the implementation of an astroglial syncytium involving multiple astrocytes, which relays the indirect feedback messenger to distant neurons: each astrocyte is bidirectionally coupled to neurons. A detailed and comprehensive set of results with analysis is presented demonstrating repair at both cellular and network levels.

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Selector device is critical in high-density cross-point resistive switching memory arrays for suppressing the sneak leakage current path. GexSe1-x based ovonic threshold switch (OTS) selectors have recently demonstrated strong performance with high on-state current, nonlinearity and endurance. Detailed study of its reliability is still lacking and the understanding on the responsible mechanisms is limited. In this work, for the first time, the endurance degradation mechanism of Ge-rich GexSe1-x OTS is identified. Accumulation of slow defects that remain delocalized at off-state and GeSe segregation/crystallization during cycling lead to the recoverable and non-recoverable leakage current, respectively. Most importantly, a refreshing program scheme is developed to recover and prevent the OTS degradation and the endurance can be therefore improved by more than five orders without adding additional material elements or process steps.

As transistor sizes are downscaled, a single trapped charge has a larger impact on smaller devices and the Random Telegraph Noise (RTN) becomes increasingly important. To optimize circuit design, one needs assessing the impact of RTN on the circuit and this can only be accomplished if there is an accurate statistical model of RTN. The dynamic Monte Carlo modelling requires the statistical distribution functions of both the amplitude and the capture/emission time (CET) of traps. Early works were focused on the amplitude distribution and the experimental data of CETs were typically too limited to establish their statistical distribution reliably. In particular, the time window used has been often small, e.g. 10 sec or less, so that there are few data on slow traps. It is not known whether the CET distribution extracted from such a limited time window can be used to predict the RTN beyond the test time window. The objectives of this work are three fold: to provide the long term RTN data and use them to test the CET distributions proposed by early works; to propose a methodology for characterizing the CET distribution for a fabrication process efficiently; and, for the first time, to verify the long term prediction capability of a CET distribution beyond the time window used for its extraction.

Comprehensive experimental and simulation evidence of the filamentary-type switching and Vth relaxation mechanism associated with defect charging/discharging in GexSe1-x ovonic threshold switching (OTS) selector is reported. For the first time, area independence of conduction current at both on/off states, Weibull distribution of time-to-switch-on/off (t-on/off), Vth relaxation and its dependence on time, bias and temperature, which is in good agreement with our first-principles simulations in density functional theory, provide strong support for filament modulation by defect delocalzation/localization that is responsible for volatile switching.Introduction: Selector device is critical to suppress the sneak path in high-density cross-point resistive switching memory arrays (Fig. 1a&b). GexSe1-x OTS selectors have achieved high on-state current, high halfbias nonlinearity and excellent endurance [1-3]. Theoretical modelling [2] also suggests that the applied electric field modulates the electronic structure of the mis-coordinated Ge-Ge bonds in the amorphous state, appearing as gap/tail states localized/delocalized in space, with signatures of simultaneous carrier hopping and filament crystallization. Despite the progress, electrical experimental evidence supported by theoretical simulation is still lacking. In this paper, based on novel characterization, supported by first-principles simulations, for the first time, we observed: (i) Area-independent conduction current at both on/off states, confirming the modulation of one dominant conduction filament. (ii) Weibull distribution of t-on/t-off, supporting a random percolation path formed by the first fire (FF) and modulated by switching; (iii) Vth relaxation and its dependence on time, bias and temperatures, in agreement with defect delocalization and localization as the dominant volatile switching process.Device and Characterization: Amorphous GexSe1-x films are prepared by room temperature physical vapor deposition (PVD). TiN/GeSe/TiN selector devices were integrated in a 300nm process flow, using a pillar (TiN) bottom electrode which defines the device size down to 50 nm (Fig. 2a). A GexSe1-x chalcogenide films control from 20 nm down to 5nm thickness was achieved and passivated with a low-temperature BEOL process scheme. Four different waveforms have been developed in this work: (1) A triangle pulse to record I-V during switching (Fig. 2b&c). (2) A constant bias square pulse to record t-on (Fig. 5a). (3) A constant bias immediately following the switching pulse to record the t-off (Fig. 6a). ( 4) A two-pulse that sandwiches a relaxation period to compare the Vth before/after the relaxation (Fig. 7a).Area-independence of conduction/leakage current at on/off states: The leakage current (Ileak) at low bias in devices of various sizes is measured before/after the first fire (FF) (Fig. 3). In a fresh device before the FF, Ileak is area-dependent, and it becomes area-independent after the FF. This indicates that a filament is formed/activated by the FF, which dominat...