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.
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.
Results and Discussion
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.
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