J.F. Kang

Analytic model of endurance degradation and its practical applications for operation scheme optimization in metal oxide based RRAM

An analytic model for the endurance degradation of metal oxide based RRAM is presented for the first time. The endurance degradation behaviors under various operation modes can be predicted by the model, which were verified by the measured data in different devices. Furthermore, the 106 endurance for all 4-level resistance states in HfOX-based RRAM is demonstrated by using the proposed optimization operation scheme for multi-level data storage based on the model prediction. Guided by the model, a dynamic self-recovery operation scheme is developed to achieve more than 2 orders endurance enhancement at a high switching speed (10 ns).

Understanding the intermediate initial state in TiO2−δ/La2/3Sr1/3MnO3 stack-based bipolar resistive switching devices

Bipolar resistive switching (RS) behavior was observed in epitaxially deposited TiO2−δ/La2/3Sr1/3MnO3 stacks. An intermediate initial state of this device demonstrated that the original resistive state would switch to either the higher or the lower state, depending only on the electrical polarity of the initial measurement. As the analogue intermediate states were obtained by device fabrication, the direct observation of these switching states was possible, knowing the microscopic physical origin of the RS effect. Based on x-ray photoelectron spectroscopy analysis, an understanding of this interesting phenomenon was proposed, well supporting the oxygen vacancy formation and recombination mechanism.

Unipolar resistive switching and mechanism in Gd-doped-TiO2-based resistive switching memory devices

Gd-doped-TiO2-based resistive switching random-access memory (RRAM) devices with MOM structure of top metal electrode (W, Ti)/Gd-doped-TiO2/Pt/Ti/SiO2/Si were fabricated, and resistive switching characteristics were investigated. The W/Gd-doped-TiO2/Pt RRAM device showed typical unipolar resistive switching behavior irrespective of the bias polarity on the W top electrode. However, no unipolar switching was observed in the Ti/Gd-doped-TiO2/Pt devices when positive bias was applied on the Ti top electrode. A new resistive switching model was proposed to explain the unipolar switching behavior in the W/Gd-doped-TiO2/Pt RRAM device, based on the formation and destruction of conducting filaments which result from pinning and unpinning effects of surface trap states of oxide film on the Fermi level of metal electrode. The model clearly elucidates the mechanism of the low resistance state (LRS) and the high resistance state (HRS) as well as the switching behavior between LRS and HRS in the W/Gd-doped-TiO2/Pt RRAM device. The model also indicates the important role of the metal electrode material in unipolar resistive switching of oxide-based RRAM devices.

Identification and application of current compliance failure phenomenon in RRAM device

A new current sweep measurement method is introduced to investigate the details of the set process. The electrode-dependent current compliance failure phenomenon is investigated in oxide-based resistive switching memory. The results indicate that the amount of conductive filaments formed in the oxide layer is the critical factor that influences the set behavior (sudden or slowly) and causes the compliance failure. The different switching behaviors measured by the current sweep mode could be used as a criterion for distinguishing the RRAM devices from the applications of 1D1R or multilevel storage.

Multi-level resistive switching characteristics correlated with microscopic filament geometry in TMO-RRAM

For the first time, we report that the resistive switching behaviors of the multi-level resistance states can be influenced by the operation mode. Simulations reveal that this is due to the different geometry of the conductive filament in the multi-level resistance states stemmed from the different operation mode. Based on this understanding, improved stability and uniformity of the medium resistance states during switching process are predicted and experimentally verified.

Gd doping improved resistive switching characteristics of TiO2-based resistive memory devices

2Theta (degree)

Multilevel resistive switching characteristics in Ag/SiO 2 /Pt RRAM devices

Ag/SiO2/Pt-based resistive random access memory (RRAM) devices were fabricated and investigated. Multilevel resistive switching (RS) phenomenon was observed in Ag/SiO2/Pt devices under different operation modes. Good endurance and retention characteristics of RRAM device with four resistance states were achieved. The possible mechanism of multilevel RS was discussed.

Current Compliance-Free Resistive Switching in Nonstoichiometric CeOx Films for Nonvolatile Memory Application

The RS behaviors of stoichiometric and nonstoichiometric CeOx films were studied. Current compliance-free resistive switching was achieved in the nonstoichiometric CeOx film, which are helpful to remove the limitation of current compliance to simplify RRAM circuits design.

Large low-field magnetoresistance observed in heterostructure multilayers

The behaviour of resistivity versus temperature in a series of (LCMO)/(GCMO) multilayers has been studied. The resistivity peak temperature of the multilayers decreases with decreasing thickness of the LCMO layers. Large low-field magnetoresistance (LFMR) of % is obtained at 78.5 K in heterostructure LCMO/GCMO multilayers when a magnetic field is swept within Oe. This result suggests that the interface strain between LCMO and GCMO layers may influence the magnetic transport properties and a suitable film structure could be used to increase the LFMR.

Insights into resistive switching characteristics of TaO x -RRAM by Monte-Carlo simulation

An atomistic Monte-Carlo simulator is developed for TaO_x-based resistive switching random access memory (RRAM) including both the generation & recombination effect of oxygen vacancy defects with oxygen ions and the phase change effect between Ta₂O₅ and TaO₂. Using the developed simulation tool, the resistive switching characteristics of the bi-layered Ta₂O₅/TaO_x RRAM are investigated. The typical self-compliance behavior measured in the bi-layered Ta₂O₅/TaO_x RRAM is reproduced by considering the interaction between Ta₂O₅ and TaO_x, indicating that TaO_x layer plays a critical role to the self-compliance behavior.