- Nano Express
- Open Access
Analysis on the Filament Structure Evolution in Reset Transition of Cu/HfO2/Pt RRAM Device
© The Author(s). 2016
- Received: 31 March 2016
- Accepted: 13 May 2016
- Published: 25 May 2016
The resistive switching (RS) process of resistive random access memory (RRAM) is dynamically correlated with the evolution process of conductive path or conductive filament (CF) during its breakdown (rupture) and recovery (reformation). In this study, a statistical evaluation method is developed to analyze the filament structure evolution process in the reset operation of Cu/HfO2/Pt RRAM device. This method is based on a specific functional relationship between the Weibull slopes of reset parameters’ distributions and the CF resistance (R on). The CF of the Cu/HfO2/Pt device is demonstrated to be ruptured abruptly, and the CF structure of the device has completely degraded in the reset point. Since no intermediate states are generated in the abrupt reset process, it is quite favorable for the reliable and stable one-bit operation in RRAM device. Finally, on the basis of the cell-based analytical thermal dissolution model, a Monte Carlo (MC) simulation is implemented to further verify the experimental results. This work provides inspiration for RRAM reliability and performance design to put RRAM into practical application.
- Conductive filament (CF)
- Structure evolution
- Monte Carlo simulator
With conventional flash memories approaching their technical and physical limits, there will be severe problems in the scaling of solid-state memory [1–4]. A great amount of research attention has been focused on the next generation memory devices. Resistive random access memory (RRAM), with the reversible and reproducible resistive switching (RS) phenomena induced by applied electric field has been extensively studied due to its potential applications in high density memory  and neuromorphic electronic systems [6–9]. The electrochemical metallization (ECM)-based RRAM with an active metal electrode such as Ag or Cu is referred to as programmable metallization cell (PMC) or conductive bridge RAM (CBRAM), which is an important type of RRAM device. The RS phenomena of the PMC are attributed to the oxidation of the active anode metal into cations, the transport of these cations, and their reduction on the cathode or in the RS layer [10–12]. Via the above redox process, the nanoscale conductive filaments (CFs) are formed in the set process and ruptured in the reset process in the RS layer [13–21]. The confinement of the resistive switching phenomenon to a nanometric filament has been widely demonstrated by conductive AFM [22–24] and cross-sectional TEM [25, 26]. However, the filamentary switching has the stochastic nature, similar to the dielectric breakdown, which has ever been a serious obstacle to boost RRAM into practical applications [27–30]. Studying the statistics of the RS parameters [31, 32] is also significant to discover the filament annihilation/reconstruction information and guide us to improve the uniformity. The Weibull distribution has been often used to analyze the statistics of electron devices. Because the initial filament width or on-state resistance (R on) has a significant impact on the reset transition process and there is an analytical correlationship between the Weibull slopes (β) of reset parameters’ distributions and CF size or R on [33, 34], this relationship could be made use of to analyze the filament microstructure evolution. At the reset point where the current is the maximum in the reset I–V curve, when β the Weibull slope changes with R on, the degradation of the CF structure has occurred, and the reset transition inclines to be abrupt . On the contrary, the CF just starts to dissolve at the reset point and the reset switching tends to be gradual  when β the Weibull slope is a constant, independent on R on.
In this paper, the annihilation behavior of the filament in Cu/HfO2/Pt PMC device is investigated according to the above mentioned statistical evaluation method. The Weibull slopes (β V and β I ) of our Cu/HfO2/Pt CBRAM device decrease with R on, so the filament dissolution or the reset transition is abrupt. A Monte Carlo method is utilized to simulate and capture the experimental results. The controllable abrupt reset operation will bring great benefits to the reliable binary operation of RRAM. Our work has great significance in providing inspiration for RRAM performance and reliability design to put RRAM into practical application.
As the abrupt reset behavior has the advantages to the reliable binary operation of RRAM, it is important to control the reset transition. Some methods can be used to get the abrupt reset switching. For example, utilizing current sweep [37, 38] operation in a single RRAM cell or using gate voltage sweep operation in a 1T1R structure , the reset transition can be implemented to preset well-controlled abrupt switching characteristics. By combining the above method with the approaches of increasing resistances such as introducing a barrier layer, it is expected to achieve the abrupt and low-power set/reset operation.
The detailed microstructure evolution before the reset point in the CF of Cu/HfO2/Pt RRAM devices has been analyzed. The Weibull slopes of our device change with the different on-resistance or CF size. This result indicates that dissolution has just finished at the reset point. The obvious Joule heat generation in the wide CF may be the underlying reason for the drastic CF dissolution. To model the experimental results, a Monte Carlo simulator has been established and the simulated results are fully in consistency with those of the experiment.
The authors declare that they have no competing interests.
MZ and SL did the statistical data analysis. SL, JS, EM and MZ interpreted the results. MZ and SL designed the samples and carried out the RRAM fabrication. ZM and SL drafted the manuscript. YL, QL, HL, EM, JS, and ML participated in the manuscript writing and discussion of results. All authors critically read and contributed to the manuscript preparation. All authors read and approved the final manuscript.
This work was supported by the National Natural Science Foundation of China (NSFC) under Grant Nos. 61322408, 61521064, 61574169, 61334007, 61274091, 61422407,61522048, 61474136, 61574166, and 61376112, the National High Technology Research Development Program of China under Grant No. 2014AA032900, the Beijing Training Project For The Leading Talents in S & T under Grant No. ljrc201508, the Opening Project of Key Laboratory of Microelectronics Devices and Integrated Technology, Institute of Microelectronics of Chinese Academy of Sciences, the Spanish Ministry of Science and Technology under contract TEC2012-32305 (partially funded by the EU FEDER program), and the DURSI of the Generalitat de Catalunya under contract 2014SGR384.
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