- Nano Express
- Open Access
Self-Rectifying Resistive Switching Memory with Ultralow Switching Current in Pt/Ta2O5/HfO2-x /Hf Stack
© The Author(s). 2017
- Received: 21 November 2016
- Accepted: 6 February 2017
- Published: 15 February 2017
In this study, we present a bilayer resistive switching memory device with Pt/Ta2O5/HfO2-x /Hf structure, which shows sub-1 μA ultralow operating current, median switching voltage, adequate ON/OFF ratio, and simultaneously containing excellent self-rectifying characteristics. The control sample with single HfO2-x structure shows bidirectional memory switching properties with symmetrical I–V curve in low resistance state. After introducing a 28-nm-thick Ta2O5 layer on HfO2-x layer, self-rectifying phenomena appeared, with a maximum self-rectifying ratio (RR) of ~4 × 103 observed at ±0.5 V. Apart from being a series resistance for the cell, the Ta2O5 rectifying layer also served as an oxygen reservoir which remains intact during the whole switching cycle.
- Leakage current issue
Presented in this work, therefore, is a material system that can provide ultralow operating current (<1 uA), sufficient ON/OFF window (~102), median operating voltages (<6 V), as well as excellent self-rectifying functionality (RR >1000). In the bilayer stack, stoichiometric Ta2O5 and anoxic HfO2-x were employed to be electrolytes, which contacted with high-work-function metal (Pt, 5.6 eV) and low-work-function metal (Hf, 3.9 eV), respectively . The method can be generally stated as one layer (in this case, HfO2-x ) and works as the RS layer by trapping and detrapping the deep trap sites while the other dielectric layer (in this case, Ta2O5) remains intact during the whole switching cycle and creates a high Schottky barrier with Pt to constitute the rectifying functionality.
The cross sectional image was observed using a UKTRA-55 field emission scanning electron microscope (FESEM); and chemical status of films were examined using an X-ray photoelectron spectroscopy (XPS, Kratos Axis UltraDLD spectrometer, Kratos Analytical-A Shimadzu group company). For the XPS measurement, the Ar + ion beam energy was set to 1 keV during the sputter-etching. As to the electrical measurements, a low-noise Keithley 4200 semiconductor characterization system was conducted at room temperature, in voltage sweep mode. Each voltage sweep began from 0 V, and the bias was applied to the TE while the BE was grounded.
When a small positive bias was applied to the Pt TE, a small amount of injected electrons, which tunneling from cathode to traps, were interfered with by the deep trap and transported through the HfO2-x via hopping mechanism, so that the current flow under this circumstance must be much lower, as schematically shown in Fig. 6b. The observation of initially low current in Fig. 3b suggests that the deep trap levels were with the trap-empty configurations under low positive bias condition, which well coincided with the HRS. As the positive voltage increased, the carrier injection became higher and the energy band of electrolytes would be tilted further, so that traps started to be filled with major injected carriers, and the others would tunnel from Hf BE to the Ec of HfO2-x layer (F-N tunneling). Actually, the subsequent emission from traps to the Ec of HfO2-x layer is essentially the Poole–Frenkel emission . At the same time, the whole system switched to LRS (as indicated in Figs. 3b and 6c). The switching back from the LRS to HRS under the negative bias could be understood as follows.
After withdrawing the positive bias and applying a negative bias to the Pt TE, the electrons in the HfO2-x traps started to detrap continually while the electron injection from the Pt TE was suppressed by the high Schottky barrier height, Fig. 6c shows the schematic diagram of this circumstance. When the traps became empty by the high negative bias, and bias was removed subsequently, the energy band diagram could be represented by Fig. 6e.
The most critical feature of the abovementioned switching mechanism is the change in the charge state of the electron traps, presumably V O with different oxidation states in HfO2-x , not the variations in their local spatial distribution or concentration. Note that it is hopeful that improve the rectifying properties of our device if its active area can scaling further.
In summary, the RRAM device with ultralow operating current (<1 uA), sufficient ON/OFF ratio (~102), median operating voltages (<6 V), as well as excellent self-rectifying properties was prepared in a simple Pt/Ta2O5/HfO2-x /Hf structure successfully. And satisfactory switching uniformity and retention performance are also demonstrated in it. In the stack, Ta2O5 layer works as a rectifier with high resistance and only slight oxygen deficiency, while HfO2-x layer plays the role of the RS layer with more oxygen deficiency, lower dielectric constant, and higher energy band gap. These abovementioned merits manifest that the prototype Pt/Ta2O5/HfO2-x /Hf devices could be used to effectively mitigate the sneak leakage in crossbar RRAM arrays.
The authors thank the Instrumental Analysis Center of Shanghai Jiao Tong University and Key Laboratory of Microelectronics Devices & Integrated Technology, Institute of Microelectronics of Chinese Academy of Sciences.
There is no funding source for this work.
HM and JF designed the experiments. HM and TG carried out the experiments. HM wrote the manuscript. JF and HL provided suggests for the experimental results analyzing and helped amend the manuscript. XX, QL, TG, and PY helped for the electrical tests and developed relevant analysis tools. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- Chang SH, Lee SB, Jeon DY, Park SJ, Kim GT, Yang SM, Chae SC, Yoo HK, Kang BS, Lee MJ, Noh TW (2011) Oxide double-layer nanocrossbar for ultrahigh-density bipolar resistive memory. Adv Mater 23:4063View ArticleGoogle Scholar
- Pan F, Gao S, Chen C, Song C, Zeng F (2014) Recent progress in resistive random access memories: materials, switching mechanisms, and performance. Mater Sci Eng RRep 83:1View ArticleGoogle Scholar
- Yoon JH, Song JS, Yoo IH, Seok JY, Yoon KJ, Kwon DE, Park TH, Hwang CS (2014) Highly uniform, electroforming-free, and self-rectifying resistive memory in the Pt/Ta2O5/HfO2-x/TiN structure. Adv Func Mater 24:5086View ArticleGoogle Scholar
- Li YT, Jiang XY, Tao CL (2013) A self-rectifying bipolar rram device based on Ni/HfO2/N(+)-Si structure. Modern Phys Lett B 28:389Google Scholar
- Wang YF, Hsu CW, Wan CC, Wang IT, Lai WL, Chou CT, Lee YJ, Hou TH (2014) Homogeneous barrier modulation of Ta2O5/TiO2 bilayers for ultra-high endurance three-dimensional storage-class memory. Nanotechnology 25:165202View ArticleGoogle Scholar
- Kwon JY, Park JH, Kim TG (2015) Self-rectifying resistive-switching characteristics with ultralow operating currents in SiOxNy/AlN bilayer devices. Appl Phys Lett 106:223506View ArticleGoogle Scholar
- Michaelson HB (1977) The work function of the elements and its periodicity. J Appl Phys 48:4729View ArticleGoogle Scholar
- Song WD, Ying JF, He W, Zhuo VY-Q, Ji R, Xie HQ, Ng SK, Serene LG-NG, Jiang Y (2015) Nano suboxide layer generated in Ta2O5 by Ar+ ion irradiation. Appl Phys Lett 106:031602View ArticleGoogle Scholar
- Kruchinin VN, Aliev VSH, Perevalov TV, Islamov DR, Gritsenko VA, Prosvirin IP, Cheng CH, Chin A (2015) Nanoscale potential fluctuation in non-stoichiometric HfOx and low resistive transport in RRAM. Microelectronic Eng 147:165View ArticleGoogle Scholar
- Tyapi P (2011) Ultrathin Ta2O5 film based photovoltaic device. Thin Solid Film 519:2355View ArticleGoogle Scholar
- Cho B, Song S, Ji Y, Lee T (2010) Electrical characterization of organic resistive memory with interfacial oxide layers formed by O2 plasma treatment. Appl Phys Lett 97:063305View ArticleGoogle Scholar
- Zeng W, Bowen KH, Li J, Dabkowska I, Gutowski M (2005) Electronic structure differences in ZrO2 vs HfO2. J Phys Chem A 109:11521View ArticleGoogle Scholar
- Lai BC-M, Kung N-H, Lee JY-M (1999) A study on the capacitance–voltage characteristics of metal-Ta2 O5-silicon capacitors for very large scale integration metal-oxide-semiconductor gate oxide applications. J Appl Phys 85:4087View ArticleGoogle Scholar
- Gavartin JL, Ramo DM, Shluger AL, Bersuker G, Lee BH (2006) Negative oxygen vacancies in HfO2 as charge traps in high-k stacks. Appl Phys Lett 89:082908View ArticleGoogle Scholar
- Wong H-SP, Lee HY, Yu S, Chen YS, Wu Y, Chen PS, Lee B, Chen FT, Tsai MJ (2012) Metal–oxide RRAM. Proc IEEE 100:1951View ArticleGoogle Scholar