RESUMEN
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.
RESUMEN
The thin-film growth conditions in a plasma-enhanced atomic layer deposition for the (3.0-4.5) nm thick HfO2 film were optimized to use the film as the resistive switching element in a neuromorphic circuit. The film was intended to be used as a feasible synapse with analog-type conductance-tuning capability. The 4.5 nm thick HfO2 films on both conventional TiN and a new RuO2 bottom electrode required the electroforming process for them to operate as a feasible resistive switching memory, which was the primary source of the undesirable characteristics as the synapse. Therefore, electroforming-free performance was necessary, which could be accomplished by thinning the HfO2 film down to 3.0 nm. However, the device with only the RuO2 bottom electrode offered the desired functionality without involving too high leakage or shorting problems, which are due to the recovery of the stoichiometric composition of the HfO2 near the RuO2 layer. In conjunction with the Ta top electrode, which provided the necessary oxygen vacancies to the HfO2 layer, and the high functionality of the RuO2 as the scavenger of excessive incorporated oxygen vacancies, which appeared to be inevitable during the repeated switching operation, the Ta/3.0 nm HfO2/RuO2 provided a highly useful synaptic device component in the neuromorphic hardware system.
RESUMEN
Chalcogenide materials have been regarded as strong candidates for both resistor and selector elements in passive crossbar arrays owing to their dual capabilities of undergoing threshold and resistance switching. This work describes the bipolar resistive switching (BRS) of amorphous GeSe thin films, which used to show Ovonic threshold switching (OTS) behavior. The behavior of this new functionality of the material follows filament-based resistance switching when Ti and TiN are adopted as the top and bottom electrodes, respectively. The detailed analysis revealed that the high chemical affinity of Ti to Se produces a Se-deficient GexSe1-x matrix and the interfacial Ti-Se layer. Electroforming-free BRS behavior with reliable retention and cycling endurance was achieved. The performance improvement was attributed to the Ti-Se interfacial layer, which stabilizes the composition of GeSe during the electrical switching cycles by preventing further massive Se migration to the top electrode. The conduction mechanism analysis denotes that the resistance switching originates from the formation and rupture of the high-conductance semiconducting Ge-rich GexSe1-x filament. The high-resistance state follows the modified Poole-Frenkel conduction.
RESUMEN
The high nonuniformity and low endurance of the resistive switching random access memory (RRAM) are the two major remaining hurdles at the device level for mass production. Incremental step pulse programming (ISPP) can be a viable solution to the former problem, but the latter problem requires material level innovation. In valence change RRAM, electrodes have usually been regarded as inert (e.g., Pt or TiN) or oxygen vacancy (VO) sources (e.g., Ta), but different electrode materials can serve as a sink of VO. In this work, an RRAM using a 1.5 nm-thick Ta2O5 switching layer is presented, where one of the electrodes was VO-supplying Ta and the other was either inert TiN or VO-sinking RuO2. Whereas TiN could not remove the excessive VO in the memory cell, RuO2 absorbed the unnecessary VO. By carefully tuning (balancing) the capabilities of VO-supplying Ta and VO-sinking RuO2 electrodes, an almost invariant ISPP voltage and a greatly enhanced endurance performance can be achieved.