Ultralow Switching Energy Ni/$\hbox{GeO}_{x}$/HfON/TaN RRAM

366 IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 3, MARCH 2011

Ultralow Switching EnergyNi/GeOx/HfON/TaN RRAM

C. H. Cheng, Albert Chin, and F. S. Yeh

Abstract—Using stacked covalent-bond-dielectric GeOx onmetal–oxynitride HfON, the Ni/GeOx/HfON/TaN resistive randomaccess memory (RRAM) showed ultralow set power of 0.3 μW(0.1 μA at 3 V), reset power of 0.6 nW (−0.3 nA at −1.8 V),fast 20-ns switching time, ultralow 8-fJ switching energy (4-Voverstress), and excellent 106 cycling endurance. Such excellentperformance was reached by using hopping conduction with neg-ative temperature coefficient (TC) rather than the positive TC inmetal–oxide RRAM.

Index Terms—GeO2, HfON, hopping conduction, resistiverandom access memory (RRAM).

I. INTRODUCTION

THE charge-trapping Flash (CTF) nonvolatile memory(NVM) [1]–[3] has the lowest switching energy among

various NVMs, but the degraded endurance from 105 to 104

cycles at highly scaled device is the basic physical limitationaccording to International Technology Roadmap for Semicon-ductors (ITRS) [1]. Unfortunately, such degraded enduranceis unacceptable for high-end product. The resistive randomaccess memory (RAM) [4]–[15] shows high potential for down-scaling beyond CTF, and the simple cross-point structure ismore suitable for embedded and low-cost 3-D integration.However, the high set/reset currents and switching energyare the major drawbacks. We previously reported low-powerNi/GeOx/SrTiO3/TaN resistive RAM (RRAM) [16] using thehopping conduction mechanism [17]. In this letter, we fur-ther present better ultralow-energy RRAM device with only0.3-μW setting power, ultralow reset power of 0.6 nW, fast20-ns switching time, and 106 endurance. These were achievedin Ni/GeOx/HfON/TaN device, where HfON has been used forhigh-performance CTF [2], [3] due to its higher trap densityand deeper trap energy than HfO2. Such high-density traps arevital to reach low switching energy by defect-assisted hoppingconduction with negative temperature coefficient (TC).

Manuscript received October 27, 2010; revised November 10, 2010;accepted November 11, 2010. Date of publication January 6, 2011; date ofcurrent version February 23, 2011. This work was supported by the NationalScience Council of Taiwan. The review of this letter was arranged by Editor S.Kawamura.

C. H. Cheng and F. S. Yeh are with the Department of Electronics Engineer-ing and the Institute of Electronics, National Tsing-Hua University, Hsinchu300, Taiwan.

A. Chin is with the Department of Electronics Engineering, National ChiaoTung University, Hsinchu 300, Taiwan (e-mail: albert_achin@hotmail.com).

Color versions of one or more of the figures in this letter are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/LED.2010.2095820

II. EXPERIMENTS

The RRAM devices were integrated into very large scaleintegration backend with 200-nm SiO2 on Si substrate. The100-nm TaN was deposited by physical vapor deposi-tion (PVD). After patterning the TaN electrode, an 8.5-nmHf0.38O0.39N0.23 (HfON) with different N2/O2 was depositedon TaN/SiO2/Si using PVD. The HfOx control sample wasalso fabricated for performance comparison. Then, a 7.5-nm-thick GeOx was covered to form the GeOx/HfON. Finally, the50-nm-thick Ni was deposited and patterned as top electrode bya metal mask with an area of 11 300 μm2.

III. RESULTS AND DISCUSSION

Fig. 1(a) shows the swept I–V curve of Ni/GeOx/HfON/TaN RRAM. Good switching characteristics were mea-sured, with a large resistance window of 9 × 102 at 0.2 V. Inaddition to the forming free and self-compliance, this RRAMcan be set at an ultralow power of 0.3 μW (0.1 μA at 3 V).The lower set power than GeOx/STO device [16] may be dueto the larger band gap and deeper trap energy in HfON than inSTO, where the HfON has been used in the CTF device [2],[3] due to its deep trapping energy property. The device canbe reset from low resistance state (LRS) to high resistance state(HRS) at a very low power of only 0.6 nW (−0.3 nA at −1.8 V).The ultralow reset power is related to break the weakly linkedhopping conduction pass by injected electrons over electrode[17]. Close swept I–V curve is also obtained after 50 cyclesshown in Fig. 1(a), indicating the good switching behavior.Such low reset current can be driven by a Schottky diode atreverse bias. Using a highly defective HfON is important toreach RRAM function since the similar Ni/GeOx/HfOx/TaNdevice has no memory function, as shown in Fig. 1(b). Thegood set/reset voltage distribution in Fig. 1(c) is ascribed to theself-compliance property and low switching current by hoppingmechanism.

To analyze the record low switching power, we have mea-sured the temperature-dependent I–V characteristics at bothHRS and LRS. As shown in Fig. 2, the very small HRS currentfitted well by the Schottky emission via high work-functionNi electrode, where the same mechanism also governed thelow leakage current of metal–insulator–metal capacitors usedfor dynamic RAM [18]. The current at LRS follows the initialohmic conduction to space-charge-limited current at highervoltage, indicating the current conduction of LRS to be trapcontrolled [19].

To study the potential mechanism, we have measured thetemperature-dependent current at LRS and HRS. As shown

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CHENG et al.: ULTRALOW SWITCHING ENERGY Ni/GEOx/HfON/TaN RRAM 367

Fig. 1. Swept I–V curves of (a) Ni/GeOx/HfON/TaN, (b) Ni/HfON/TaNand control Ni/GeOx/HfOx/TaN, and (c) distribution of Ni/GeOx/HfON/TaNdevices. The arrows indicate the bias sweeping directions.

Fig. 2. I–V curves of the HRS and LRS of Ni/GeOx/HfON/TaN RRAMby fitting with Schottky emission and space-charge-limited current conductionmechanism, respectively.

Fig. 3. Extracted activation energy and band diagram of Ni/GeOx/HfON/TaN RRAM. The inset is the temperature-dependent HRS current.

Fig. 4. (a) Switching response and (b) endurance characteristics ofNi/GeOx/HfON/TaN RRAM devices. A 4-MΩ resistor was connected to theRRAM for switching response measurements.

in Fig. 3, the increasing HRS current at high temperatureis predicted well by Schottky emission mechanism [18] withhigher thermal energy to overcome the Schottky barrier height.A negative TC is measured up to 150 ◦C and opposite tothe positive TC in conventional metal–oxide RRAM [4]. Anactivation energy of 0.4 eV is close to that of negative TCin highly defective Si governed by hopping conduction [17],which suggests LRS mechanism to be related by hoppingvia defects in Ni/GeOx/HfON/TaN. This is supported fromthe lower defect formation energy and much more oxygen-deficient centers in GeO2 than those in SiO2 [20]. Such hop-ping conduction and negative TC were reported previously inGeO2 [21]. The oxygen vacancy can be formed at a very lowtemperature to degrade the GeO2/Ge MOSFET [22] but is veryuseful for RRAM. The hopping conduction provides a largeinternal resistance [17] to reach a small self-compliance set

368 IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 3, MARCH 2011

TABLE ICOMPARISON OF DEVICE INTEGRITY DATA FOR RRAM DEVICES

current, which is quite different from the large leakage currentin conventional RRAM ruled by metallic filament.

Endurance is another bottleneck for unipolar RRAM.Fig. 4(a) and (b) shows the switching response and endurancecharacteristics, respectively. Fast response on a fixed 4-MΩexternal resistor is measured at applied 20-ns 4-V overstressedset pulse. Stable switching to 106 cycles is obtained, due tothe fast 20-ns time and ultralow 8-fJ switching energy (4-Vset pulse) with less stress to RRAM. Such ultralow energy isclose to the 3-fJ energy (E = Q×ΔV ) of Flash memory at500 electrons storage and ±20-V switching.

Table I compares various RRAM devices. This device hasimportant merits of the lowest switching power, fast 20-nsswitching time, and 106 cycling endurance.

IV. CONCLUSION

The Ni/GeOx/HfON/TaN RRAM shows excellent perfor-mance of ultralow 0.3-μW switching power, fast 20-ns switch-ing time, excellent 106 cycling endurance, and usefulness inembedded and 3-D integration.

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