Conduction Mechanism and Improved Endurance in HfO2-Based RRAM with Nitridation Treatment
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A nitridation treatment technology with a urea/ammonia complex nitrogen source improved resistive switching property in HfO2-based resistive random access memory (RRAM). The nitridation treatment produced a high performance and reliable device which results in superior endurance (more than 109 cycles) and a self-compliance effect. Thus, the current conduction mechanism changed due to defect passivation by nitrogen atoms in the HfO2 thin film. At a high resistance state (HRS), it transferred to Schottky emission from Poole-Frenkel in HfO2-based RRAM. At low resistance state (LRS), the current conduction mechanism was space charge limited current (SCLC) after the nitridation treatment, which suggests that the nitrogen atoms form Hf–N–Ox vacancy clusters (Vo +) which limit electron movement through the switching layer.
KeywordsHfO2-based RRAM Nitridation Endurance Space charge limit current
Fourier-transform infrared spectroscopy
High resistance state
Low resistance state
Resistive random access memory
Space charge limited current
Recently, resistance random access memory (RRAM) composed of an insulating layer sandwiched by two electrodes has been widely studied as a promising candidate for next-generation nonvolatile memory due to its superior properties such as simple structure, low power consumption, high-speed operation (< 300 ps), and nondestructive readout [1, 2, 3, 4, 5, 6, 7, 8, 9]. Although most RRAM devices have many properties superior to nonvolatile memory, the high operation current of RRAM and performance degradation are major issues in nonvolatile memory in terms of the application of portable electronic products.
The Pt/HfO2/TiN structure can supply a conduction path which induces a resistive switching behavior [10, 11, 12, 13, 14, 15, 16, 17, 18, 19]. However, the defects of amorphous HfO2 will increase the number of leakage paths, leading to power consumption and joule heating degradation. In this work, the resistive switching layer of HfO2 was treated by a solution with a urea/ammonia complex nitrogen source as the nitridation treatment to enhance its electrical switching properties.
Results and Discussion
An electroforming process is required to activate all of the RRAM devices using a DC bias with a compliance current of 10 μA, as shown in Fig. 1a. After the forming process, the electrical current-voltage (I-V) properties of the HfO2-based RRAM were compared at initial and after the nitridation treatment. At LRS, the current was obviously reduced compared to that of untreated HfO2 thin film, as shown in Fig. 1b. The current reduction can be attributed to the defects passivated by the NH3 molecule in the treatment solution. We found that HRS distribution is much more stable after the nitridation treatment, as in the inset of Fig. 1b. The resistance states are extracted with a reading voltage of 0.1 V during the 100 sweep cycles with DC operation (inset of Fig. 1b). The resistance on/off ratio was slightly reduced after the nitridation treatment. Interestingly, a self-compliance resistive switching property was observed in these HfO2-based RRAM devices after the treatment, as shown in Fig. 1c. After more than 103 sweep cycles, a repeatable self-protective characteristic of the device without hard breakdown was observed. The retention time was evaluated at 85 °C and remained stable even after 104 s both in HRS and LRS.
In contrast, HfO2-based RRAM exhibited the Schottky emission mechanism according to the linear relationship between ln(I/T2) and the square root of the applied voltage (V1/2) of HRS, as shown in Fig. 4c [23, 24]. This is due to the decrease in defects and dangling bonds, as bonds become passivated by nitrogen atoms after the nitridation treatment. In addition, we also analyzed the current conduction mechanism with and without treatment at LRS in HfO2-based RRAM. On LRS, the carrier transport mechanism of the untreated HfO2-based RRAM was dominated by ohmic conduction, where current decreases with increasing temperature, as shown in Fig. 4e. After nitridation treatment, the current conduction mechanism transfers to space charge limited current (SCLC) with a slope of 1.52. The I-V curve is not relative to temperature, with a linear relationship between ln(I) and the square of the applied voltage V2 on LRS, as shown in Fig. 4f .
In summary, a self-compliance resistive switching property was observed in a Pt/HfO2/TiN RRAM device after the nitridation treatment. Endurance times reached 109 cycles and a retention time of more than 104 s was achieved at 85 °C. Due to the smaller electron negativity of the nitrogen atom when compared to the oxygen atom, the dipole of the Hf–N bond is smaller than that of the Hf–O bond, which creates a lower dielectric constant region. During the SET process, the Hf–N–Ox vacancy clusters (Vo+) form the conductive path. The insulating Hf–N–Ox vacancy clusters (Vo+) protect the device from hard breakdown and perform a self-compliance property.
This work was performed at National Science Council Core Facilities Laboratory for Nano-Science and Nano-Technology in Kaohsiung-Pingtung area and supported by the Ministry of Science and Technology of the Republic of China under Contract No. MOST-106-2112-M-110-008-MY3 and MOST-106-2119-M-110-003.
Availability of data and materials
All data are fully available without restriction.
FYY, YTT, and WCC carried out the sample preparation and the measurements. CCS, MHW, and HXZ participated in the discussion. ND, TCC, KCC, HW, HQ, and SMS supervised the project. All the authors have read and approved the final manuscript.
Fang-Yuan Yuan is a doctor of the Institute of Microelectronics, Beijing, Tsinghua University. Ning Deng and Huaqiang Wu are professors of the Institute of Microelectronics, Beijing, Tsinghua University and Tsinghua National Laboratory for Information Science and Technology (TNList). He Qian is a professor of the Institute of Microelectronics, Beijing, Tsinghua University and head of Tsinghua National Laboratory for Information Science and Technology (TNList). Chih-Cheng Shih and Wen-Chung Chen are doctors of the Department of Materials and Optoelectronic Science, Kaohsiung, National Sun Yat-Sen University. Yi-Ting Tseng is a doctor of the Department of Physics, Kaohsiung, National Sun Yat-Sen University. Ting-Chang Chang is a professor of the Department of Physics, Kaohsiung, National Sun Yat-Sen University and Advanced Optoelectronics Technology Center, Taiwan, National Cheng Kung University. Kuan-Chang Chang is an assistant professor of the School of Electronic and Computer Engineering, Shenzhen, Peking University. Simon M. Sze is professor of the Department of Electronics Engineering, Hsinchu, National Chiao Tung University.
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