Statistical characteristics of reset switching in Cu/HfO2/Pt resistive switching memory
© Zhang et al.; licensee Springer. 2014
Received: 15 November 2014
Accepted: 11 December 2014
Published: 23 December 2014
A major challenge of resistive switching memory (resistive random access memory (RRAM)) for future application is how to reduce the fluctuation of the resistive switching parameters. In this letter, with a statistical methodology, we have systematically analyzed the reset statistics of the conductive bridge random access memory (CBRAM) with a Cu/HfO2/Pt structure which displays bipolar switching property. The experimental observations show that the distributions of the reset voltage (Vreset) and reset current (Ireset) are greatly influenced by the initial on-state resistance (Ron) which is closely related to the size of the conductive filament (CF) before the reset process. The reset voltage increases and the current decreases with the on-state resistance, respectively, according to the scatter plots of the experimental data. Using resistance screening method, the statistical data of the reset voltage and current are decomposed into several ranges and the distributions of them in each range are analyzed by the Weibull model. Both the Weibull slopes of the reset voltage and current are demonstrated to be independent of the on-state resistance which indicates that no CF dissolution occurs before the reset point. The scale factor of the reset voltage increases with on-state resistance while that of the reset current decreases with it. These behaviors are fully in consistency with the thermal dissolution model, which gives an insight on the physical mechanism of the reset switching. Our work has provided an inspiration on effectively reducing the variation of the switching parameters of RRAM devices.
Resistive random access memory (RRAM), making full use of the reversible resistive switching (RS) effect of transition metal oxide to realize information storage, has been considered as a promising technology for high-density nonvolatile memory [1–4]. Due to its easy fabrication, promising performances, and feasibility of 3-D arrays, RRAM device, with a simple metal-insulator-metal (MIM) structure, has attracted considerable attention recently [5, 6]. A majority of works have focused on exploring the underlying switching mechanism for most transition metal oxide materials in set and reset processes [7–11]. Generally, the formation and rupture of a tiny conductive filament (CF) in the metal oxides is proposed to explain the resistive switching phenomena between a high-resistance state (HRS) or off-state and a low-resistance state (LRS) or on-state. Oxygen vacancies as well as metal ions are widely accepted as playing a dominant role in the formation and disruption of filament under the influence of external stress . However, the size and location of CF in the set process and the extent of the CF dissolution during the reset process display random behaviors in RRAM devices, which causes the formation and rupture of the CF intrinsically stochastic  and results in the variation of the switching parameters and negatively affects the commercial application of RRAM [14–17]. Thus, studying the statistical characteristics of the switching parameters and deepening the understanding of the underlying physical mechanism behind the RS statistics are beneficial to the effective control and trustful forecast of the memory performance and reliability [18–23].
In this letter, we have investigated the reset statistical characteristics of the conductive bridge random access memory (CBRAM) device based on a Cu/HfO2/Pt structure connected to a transistor. The experimental results show that the reset voltage increases with on-state resistance and the reset current decreases with it, which can be well explained by the thermal dissolution model. Since the on-state resistance has strong influence on the reset switching parameters, the resistance screening method is employed to decompose the resistance into several ranges. The distributions of the reset voltage and current studied in different resistance ranges are compatible with the Weibull model. The Weibull slopes of reset voltage and current have nothing to do with the on-state resistance. The scale factor of the reset voltage linearly increases with the on-state resistance while that of the reset current decreases with it in linearity, respectively. These results are all consistent with the thermal dissolution model. Our work is of great significance on the deep understanding of the switching mechanism and the improvement of the uniformity of RRAM devices.
Results and discussion
Figure 1b presents several I-V curves of the Cu/HfO2/Pt RRAM device. The metal-CF-type Cu/HfO2/Pt devices are operated in a bipolar mode. The point with the maximum value of current is defined as the reset point at which the voltage and current are recorded as the Vreset and Ireset. We find that after the reset point, a series of current jumps occur during the reset process and the device finally switches to HRS. Through linear fitting to the reset I-V curve at the low-voltage region before the reset point, the on-state resistance of the 1T1R structure (Ron_total) is obtained, which is a sum of the LRS resistance of RRAM cell (Ron) and the source-drain resistance of the transistor (RDS). Ron is then got through correcting Ron_total by RDS. RDS is usually in the order of several hundreds of ohms, which is comparable to Ron, so it should not be neglected during the reset process of RRAM device in 1T1R structure. Figure 1c shows a tested transfer characteristic curve (IDS - VG curve) of the transistor with the source-drain voltage fixed to be 0.05 V. Through fitting the curve according to the output characteristic of the transistor with the equation , the intrinsic values of WunCox/2 L and VT are obtained, where un is the electronic mobility, W is the gate width, L is the gate length, and Cox is the capacitance of the gate oxide. Based on the output characteristic curve (IDS - VDS curve) of the transistor in the reset operation under VG = 3.3 V, the source-drain resistance is available to be 300 Ω in average according to RDS = VDS/IDS. Here, IDS is the measured current flowing through the transistor and the RRAM device, and VDS is calculated from the above equation using the abstracted WunCox/2 L and VT value and the measured IDS values.
The above experiment and thermal dissolution model results both demonstrated that the statistical spread of reset parameters Vreset and Ireset are intrinsically limited by the on-state resistance Ron, so controlling the value and distribution of Ron is very critical to acquire high uniformity of reset switching. In the previous works, some methods have been proposed to reduce the variation of the off-state resistance (Roff) and on-state resistance (Ron), through which the uniformity of the set and reset switching parameters also have been improved and the set and reset events are controlled in certain ranges. These methods include: 1) doping impurities in the switching layer , selecting the electrode materials , and inserting interface layers  so as to effectively control the concentration and distribution of defects such as charge traps, metal ions, and oxygen vacancies etc.; 2) introducing the electric field-concentrating initiators (e.g., nanocrystals) on the bottom electrode to enhance the local electric field and reduce the random growth of filaments ; and 3) utilizing optimized operation methods . In our recent work, we have also paid attention to the new pulse operation with regard to its role in improving the switching uniformity. Different from the traditional single pulse operation method in which only one wide pulse is applied in each switching cycle, a novel width/height-adjusting pulse operation method is proposed for RRAM. This method utilizes a series of pulses with the width or height increased gradually until a set or reset switching process is completely finished and no excessive stress is produced. The new operation method can exactly control the final resistance and significantly improve the uniformity, stability, and endurance of RRAM device. Additionally, we are also focusing on optimizing the device structure by etching the substrate into a cone shape on which the RRAM device is fabricated. The optimized structure can control the formation and rupture of the conductive filament in the oxide layer at the tip of the cone shape due to the high-generated electric field in this position. Thus, the variation of the switching parameters can be significantly reduced and the uniformity of these parameters will be improved.
In summary, we have studied the reset statistics of the CBRAM device with a Cu/HfO2/ Pt structure. The experimental results show that the reset voltage increases with on-state resistance and reset current decreases with it. The distributions of the reset voltage and current observed in different resistance ranges divided by ‘resistance screening method’ are compatible with the Weibull model. The Weibull slopes of reset voltage and current are constant, independent of the on-state resistance. The scale factors of the reset voltage increases and that of the reset current decreases with the on-state resistance in linearity, respectively. These results are in agreement with the thermal dissolution model. Our work is helpful in revealing the physics of the switching mechanism and giving guidelines to improve the uniformity of RRAM devices.
This work was supported by National Natural Science Foundation of China (NSFC) (Grant Nos. 61322408, 61221004, 61422407, 61334007, and 61274091), ‘973’ Program (Grant No. 2011CBA00602) and ‘863’ Program (Grant No. 2014AA032900) of Ministry of Science and Technology of China, Chinese Academy of Sciences Visiting Professorship for Senior International Scientists (Grant No. 2011T2G23), the Spanish Ministry of Science and Technology under contract TEC2012-32305 (partially funded by the FEDER program of the European Union), and the DURSI of the Generalitat de Catalunya under contract 2014SGR384. Jordi Suñé also acknowledges the ICREA Academia award.
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