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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.
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...
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