Z. Wei

Demonstration of high-density ReRAM ensuring 10-year retention at 85°C based on a newly developed reliability model

A new oxygen diffusion reliability model for a high-density bipolar ReRAM is developed based on hopping conduction in filaments, which allows statistical predication of activation energy. The filament in the active cells is confirmed by EBAC and TEM directly for the first time. With optimized filament size, a 256-kbit ReRAM with long-term retention exceeding 10 years at 85°C is successfully demonstrated.

Electroforming and resistance-switching mechanism in a magnetite thin film

The electroforming and the resistance-switching behaviors in magnetite, Fe3O4, by the application of an appropriate electric field are demonstrated on a lateral device with multiple electrodes. By means of this device, both the location and the nature of the change in Fe3O4 are specified from the electrical measurements and Raman spectroscopy. The switching phenomenon is caused in maghemite, γ-Fe2O3, which is formed by oxidation of Fe3O4, near an interface of an anode. The authors argue that the switching motion is originated in a redox reaction between the Fe3O4 and γ-Fe2O3.

Fast switching and long retention Fe-O ReRAM and its switching mechanism

A novel iron oxide (Fe-O) ReRAM is proposed and its high-speed resistance-switching of 10 ns is demonstrated. The switching mechanism is confirmed as a redox reaction between Fe₃O₄ and y-Fe₂O₃. Based on this model, we have achieved long-retention characteristics by introducing Zn atoms to suppress the reduction process.

Conductive filament scaling of TaO x bipolar ReRAM for long retention with low current operation

We demonstrate for the first time that the density of oxygen vacancy in a conductive filament plays a key role in ensuring data retention. We achieve very good retention results up to 100 hours at 150°C even under the low current operation due to the scaling of conductive filament size while retaining sufficiently high density of oxygen vacancy.

Retention Model for High-Density ReRAM

A retention model for both the high resistance state and low resistance state of the bipolar ReRAM is developed. Degradation of resistance is caused by the oxygen vacancy profile in filament changing due to oxygen diffusion.

Consideration of conductive filament for realization of low-current and highly-reliable TaO x ReRAM

Characteristics and their origin of a conductive filament in TaOx ReRAM are investigated. The results of systematic experimentation demonstrate that the formation of a small conductive filament with high density of oxygen vacancies, achieved by controlling the oxygen content of the resistance-switching material and forming/set current, is the key to achieving low-current switching combined with long retention.

A ReRAM-based physically unclonable function with bit error rate < 0.5% after 10 years at 125°C for 40nm embedded application

This paper presents a secure application-a physically unclonable function (PUF)-that uses the physical property of resistive random access memory (ReRAM). The proposed PUF-generating method and reproducing algorithm achieves highly reliable with bit error rate (BER) <; 0.5% and reproduction exceeding 1010 times at -40 to 125°C after 10 years at 125°C and high uniqueness as evidenced by passing NIST tests. Evaluations on 40nm ReRAM test chips have demonstrated the feasibility of a scaled-down ReRAM cell enhanced with PUF.

Reduction of cycle-to-cycle variability in ReRAM by filamentary refresh

In this paper, we clarify a filamentary “refresh” mechanism of a resistive random access memory (ReRAM) cell. Based on this mechanism, we propose an intentional refresh introduction that enables a reduction in the standard deviation (σ) of current values. The activation energy (E<inf>A</inf>) associated with oxygen vacancies (V<inf>o</inf>s) in ReRAM was investigated using low-frequency-noise spectroscopy, revealing continuous variation of E<inf>a</inf>.