Skip to main content

Negative Differential Resistance Effect in Ru-Based RRAM Device Fabricated by Atomic Layer Deposition


In this work, Ru-based RRAM devices with atomic layer deposited AlOy/HfOx functional layer were fabricated and studied. A negative differential resistance (NDR) behavior was observed during the voltage set process, and its physical origin was explored. Based on the physics understanding of the resistive switching, the measured NDR behavior is believed to be associated with the partially unipolar reset effect, which is due to the recombination between oxygen vacancies and the thermally released oxygen ions from the RuO2 interface layer. The measured electrical characteristics and X-ray photoelectron spectroscopy (XPS) results verified the physical interpretation.


As one of the most promising emerging non-volatile memories, resistive random-access memory (RRAM) has been extensively studied regarding material optimization, performance improvement, and device integration [1,2,3,4]. Due to the significant advantages such as simple cell structure, fast operational speed, low power consumption, and incomparable miniaturization potential [5], RRAM has been widely applied in brain-inspired neuromorphic computing and reconfigurable Boolean logic [6,7,8,9,10] and also exhibits great potential for storage class memory (SCM) applications [11]. However, as indicated by the International Roadmap for Devices and Systems 2017 (, challenges including scalability, device reliability, and process compatibility are still hindering RRAM’s developments. Therefore, construction of CMOS-compatible RRAM devices with superior performance is of great significance to applications that are based on the 1T1R structure of RRAM cell [12,13,14]. According to a previous study [15], a Ru-based RRAM device shows great potential compared with Pt-based ones, whereas negative differential resistance (NDR) was demonstrated during a voltage set process. The NDR phenomena accompanied with resistive switching have been investigated in other RRAM structures, which were mainly due to the trap/detrap of electronic carriers between deeply localized states induced by implanted metal nanoparticles [16, 17], or the accumulation of defects caused junction reinstallment [18]. While the appearance of NDR in Ru-based RRAM cell under large current is still pendent, in this work, the electrical performance of a Ru-based RRAM device fabricated by atomic layer deposition (ALD) technique was evaluated. Based on the X-ray photoelectron spectroscopy (XPS) characterization and electrical measurements with different stimulus, the NDR phenomenon in the Ru-based RRAM can be explained in the framework of the oxygen vacancy conductive filament model.


The schematic diagram of the device structure and fabricated RRAM array are shown in Fig. 1a, b, respectively. The Ru/AlOy/HfOx/TiN RRAM device was fabricated on a thermally oxidized 300-nm SiO2 substrate. An 80-nm Ru thin film was deposited on a 20-nm Ti adhesion layer as a bottom electrode by DC magnetron sputtering (Angstrom Engineering NEXDEP) using a Ru metal target. SiO2 grown by plasma-enhanced chemical vapor deposition (PECVD) serves as dielectric to isolate electrodes and forms the vias. Then the resistive layer of 2 nm AlOy and 3 nm HfOx was deposited by an atomic layer deposition system (PICOSUN R200) at 300 °C with trimethylaluminum (TMA) and tetrakis[ethylmethylamino]hafnium (TEMAH) precursors. After that, 80 nm TiN was reactive sputtered using a Ti target and etched to form the top electrode patterns. Another dry etching was also performed to expose the bottom electrode for electrical contact.

Fig. 1
figure 1

a Schematic diagram of a Ru-based device. b Fabricated RRAM array

The element analysis was performed by a X-ray photoelectron spectroscopy (XPS) system (Thermo Scientific ESCALAB 250Xi) using the fresh sample without a top electrode. Prior to the measurement, surface cleaning was conducted with Ar plasma before measurement to eliminate the influence of C. The peak position of Al 2p was used for calibration. The electrical measurements were performed at room temperature by a semiconductor device analyzer (Agilent B1500A) and pulse function arbitrary generator (Agilent 81160A).

Results and Discussion

The typical IV characteristics of Ru-based RRAM devices are shown in Fig. 2a. After electroforming, a positive voltage (2.5 V) was applied for set process to switch the cell from high-resistance state (HRS) to low-resistance state (LRS) with a compliance current of 1 mA to prevent the permanent breakdown during the conductive filament (CF) formation. After the set transition, a negative voltage (− 2.3 V) was used to switch the device from LRS to HRS with a gradual decreasing current. In order to evaluate the resistance variability from device to device, 10 Ru-based RRAM cells were chosen arbitrarily. As depicted in Fig. 2b, the statistical results demonstrate the excellent uniformity of HRS and LRS with a resistance window larger than 103, which could be a promising candidate for NVM-based logic applications. Compared to the previously reported Pt-based devices [2], it is worth noting that a NDR phenomenon was observed during the set process, where the current decreases with the increasing voltage (set-phase 1) in a limited voltage scale followed by increasing to compliance current (set-phase 2).

Fig. 2
figure 2

a DC characteristics. b HRS/LRS statistical distributions of 10 Ru-based RRAM devices

The cycle-to-cycle variability of Ru-based RRAM devices was also investigated under pulse mode to study the cycling uniformity. The pulses for set (2.4 V, 15 ns) and pulse (− 3 V, 100 ns) are used to switch the device between LRS and HRS with a read voltage of 0.1 V after each pulse. As shown in Fig. 3a, the device of 1000 cycles has a uniform distribution with standard deviations of 379 Ω and 3 × 105 Ω for LRS and HRS, which results in a stable memory window larger than 100. No endurance degradation occurs even after 106 switching cycles as previously reported in Ref. [15]. In addition, the device also demonstrates an excellent retention property as shown in Fig. 3b. Both LRS and HRS resistance maintain a constant value over 105 s at 120 °C without failure.

Fig. 3
figure 3

a 1000 endurance cycles. b High temperature retention behavior of a Ru/AlOy/HfOx/TiN RRAM device

In order to explore the NDR phenomenon, the I–V curves were measured in both voltage sweep mode and current sweep mode. Figure 4a displays the forming process of five randomly selected fresh RRAM cells. The current gradually increases followed by an abruption, indicating the formation of CF, while no NDR was observed. After electroforming, set operations in different sweep modes are conducted in the same cell in order to observe the current variation, as shown in Fig. 4b. For the current-driven set, the current increases slowly with a sudden decrease in voltage, demonstrating the transition of resistance from HRS to LRS. This behavior is distinct from the characteristic that is driven by voltage, which might be due to the different stimuli-induced Joule heating across the CF. Moreover, consecutive set/reset operations under different bias were performed to investigate the intermediate resistance state in the NDR region. An appropriate stop voltage of 1 V was applied during set process, as shown in Fig. 4c, to finish the sweep at the bottom of the valley. A non-volatile resistance state was obtained after the voltage was removed, which exhibited a unipolar resistive switching behavior. Therefore, this NDR phenomenon is tentatively attributed to a second reset of the CF during the set process.

Fig. 4
figure 4

a Voltage forming of five Ru-based RRAM devices. b Voltage-driven and current-driven set processes in the same RRAM cell. c Incomplete set process with the stop voltage at the bottom of the current valley

Combined with the measurements in different modes and device fabrication processes as well as the properties of RuO2, the physical origin of the NDR phenomenon was proposed, as illustrated in Fig. 5. A previous study [19, 20] suggested that the generation and recombination of electron depleted oxygen vacancy (Vo) and oxygen ion (O2−) under different electric polarities are responsible for the conventional HfOx-based bipolar RRAM devices, which is similar to the set-phase 2 and reset processes. However, unlike the conventional set process, a second rupture of the CF takes place disconnecting the Ru electrode and CF and leading to the NDR. In general, the oxygen atoms dissociate into Vo and O2− under electric field with the drifting of the O2− to the top electrode, leaving the Vo to form the CF that is used for electron transport. But due to the Joule heating caused by the electric field, the formed RuO2 interface layer would slowly decompose at ~ 600 °C and releases O2− which could recombine with the electron depleted Vo (Vo2+) near the Ru electrode (set-phase 1) [21], resulting in a current decrease. This process can also be viewed as a partial unipolar reset process. With the further increasing voltage, the CF between TE and BE will be reconstructed by an accumulation of Vo as shown in set-phase 2 and RRAM cell switches to LRS. During reset, two processes take place simultaneously: (1) the O2− released from TiN electrode rapidly recombine with the positively charged Vo because of the enhanced capture section, (2) the O2− drifting toward BE reacts with Ru and reforms the RuO2 interface layer due to the local Joule heating [22]. At this condition, the RRAM cell switches to HRS.

Fig. 5
figure 5

Physical processes of resistive switching in Ru-based RRAM. a NDR effect (set-phase 1). b Common SET (set-phase 2) processes. c RESET process of the device

XPS analysis of the RuO2 interface layer, which was formed during the device fabrication process, also supports the proposed explanation of the NDR effect. Figure 6a shows the XPS full spectrum of the sample, which includes O 1s, Ru 3d, Al 2p, and Hf 4f core levels. The other unmarked peaks all correspond to these elements with different electron orbits. The fitting curve in Fig. 6b fits perfectly with the experimental data and is divided into four peaks, which correspond to the Ru 3d5/2 (280.01 eV for Ru and 280.75 eV for RuO2) and Ru 3d3/2 (284.3 eV for Ru and 285.26 eV for RuO2) core levels, demonstrating the coexistence of the Ru and RuO2 in the thin film [23]. The low intensity of Ru 3d5/2 peak indicates that the formed RuO2 interface layer is very thin as we have expected.

Fig. 6
figure 6

XPS spectra of a Ru/AlOy/HfOx thin film and b Ru 3d core level. The RuO2 thin film between Ru and AlOy forms during ALD process [18]


In this study, the CMOS-compatible Ru/AlOy/HfOx/TiN RRAM device was fabricated. The excellent switching performance was achieved with uniform device-to-device resistance and a large resistance window as well as a good high-temperature retention property. Based on the electrical measurements and XPS analysis, the NDR phenomenon in the set process can be explained by the recombination of oxygen vacancies and oxygen ions released from the RuO2 interface layer due to the electric-induced Joule heating.



Atomic layer deposition


Conductive filament


High-resistance state


Low-resistance state


Negative differential resistance


Plasma enhanced chemical vapor deposition


Resistive random-access memory


Storage class memory


X-ray photoelectron spectroscopy


  1. Chen HY, Brivio S, Chang CC, Frascaroli J, Hou TH, Hudec B, Liu M, Lv HB, Molas G, Sohn J, Spiga S, Teja VM, Vianello E, Wong HSP (2017) Resistive random access memory (RRAM) technology: from material, device, selector, 3D integration to bottom-up fabrication. J Electroceram 39:21–38

    Article  Google Scholar 

  2. Gao B, Chen B, Liu R, Zhang FF, Huang P, Liu LF, Liu XY, Kang JF, Chen HY, Yu SM, Wong HSP (2014) 3-D cross-point array operation on AlOy/HfOx-based vertical resistive switching memory. IEEE Trans Electron Devices 61(5):1377–1381

    Article  CAS  Google Scholar 

  3. Yu SM, Chen HY, Gao B, Kang JF, Wong HSP (2013) HfOx-based vertical resistive switching random access memory suitable for bit-cost-effective three-dimensional cross-point architecture. ACS Nano 7(3):2320–2325

    Article  CAS  Google Scholar 

  4. Wang XH, Wu HQ, Gao B, Li XY, Deng N, Qian H (2018) Thermal stability of HfOx-based resistive memory array: a temperature coefficient study. IEEE Electron Device Lett 39(2):192–195

    Article  CAS  Google Scholar 

  5. Pan F, Gao S, Chen C, Song C, Zeng F (2014) Recent progress in resistive random access memories: materials, switching mechanisms, and performance. Mat Sci Eng R 83:1–59

    Article  Google Scholar 

  6. Yu SM (2018) Neuro-inspired computing with emerging nonvolatile memory. Proc IEEE 106(2):260–285

    Article  CAS  Google Scholar 

  7. Kang JF, Huang P, Gao B, Li HT, Chen Z, Zhao YD, Liu C, Liu LF, Liu XY (2016) Design and application of oxide-based resistive switching devices for novel computing architectures. IEEE J Electron Devices Soc 4(5):307–313

    Article  CAS  Google Scholar 

  8. Han RZ, Huang P, Zhao YD, Chen Z, Liu LF, Liu XY, Kang JF (2017) Demonstration of logic operations in high-performance RRAM crossbar array fabricated by atomic layer deposition technique. Nanoscale Res Lett 12:37

    Article  Google Scholar 

  9. Huang P, Kang JF, Zhao YD, Chen SJ, Han RZ, Zhou Z, Chen Z, Ma WJ, Li M, Liu LF, Liu XY (2016) Reconfigurable nonvolatile logic operations in resistance switching crossbar array for large-scale circuits. Adv Mater 28(44):9758–9764

    Article  CAS  Google Scholar 

  10. Wu HQ, Yao P, Gao B, Wu W, Zhang QT, Zhang WQ, Deng N, Wu D, Wong HSP, Yu SM, Qian H (2017) Device and circuit optimization of RRAM for neuromorphic computing. 2017 IEEE International Electron Devices Meeting, San Francisco, pp 11–15 1

    Google Scholar 

  11. Luo Q, Xu XX, Gong TC, Lv HB, Dong DN, Ma HL, Yuan P, Gao JF, Liu J, Yu ZA, Li JF, Long SB, Liu Q, Liu M (2017) 8-layers 3D vertical RRAM with excellent scalability towards storage class memory applications. 2017 IEEE International Electron Devices Meeting, San Francisco, pp 2–7 1

    Google Scholar 

  12. Su YT, Chang TC, Tsai TM, Chang KC, Chu TJ, Chen HL, Chen MC, Yang CC, Huang HC, Lo I, Zheng JC, Sze SM (2017) Suppression of endurance degradation by applying constant voltage stress in one-transistor and one-resistor resistive random access memory. Jpn J Appl Phys 56:010303

    Article  Google Scholar 

  13. Liu HT, Lv HB, Yang BH, Xu XX, Liu RY, Liu Q, Long SB, Liu M (2014) Uniformity improvement in 1T1R RRAM with gate voltage ramp programming. IEEE Electron Device Lett 35(12):1224–1226

    Article  Google Scholar 

  14. Wu W, Wu HQ, Gao B, Deng N, Yu SM, Qian H (2017) Improving analog switching in HfOx-based resistive memory with a thermal enhanced layer. IEEE Electron Device Lett 38(8):1019–1022

    Article  CAS  Google Scholar 

  15. Feng YL, Huang P, Zhou Z, Zhu DB, Han RZ, Ding XX, Liu LF, Liu XY, Kang JF (2018) Ru-based oxide resistive random access memory for BEOL-compatible novel NVM applications. 2018 14th IEEE International Conference on Solid-State and Integrated Circuit Technology (ICSICT), Qingdao

    Book  Google Scholar 

  16. Wu FC, Si SY, Shi T, Zhao XL, Liu Q, Liao L, Lv HB, Long SB, Liu M (2018) Negative differential resistance effect induced by metal ion implantation in SiO2 film for multilevel RRAM application. Nanotechnology 59:054001

    Article  Google Scholar 

  17. Sun HT, Liu Q, Long SB, Lv HB, Banerjee W, Liu M (2014) Multilevel unipolar resistive switching with negative differential resistance effect in Ag/SiO2/Pt device. J Appl Phys 116:154509

    Article  Google Scholar 

  18. Guo T, Sun B, Zhou Y, Zhao HB, Lei M, Zhao Y (2018) Overwhelming coexistence of negative differential resistance effect and RRAM. Phys Chem Chem Phys 20:20635–20640

    Article  CAS  Google Scholar 

  19. Huang P, Liu XY, Chen B, Li HT, Wang YJ, Deng YX, Wei KL, Zeng L, Gao B, Du G, Zhang X, Kang JF (2013) A physics-based compact model of metal-oxide-based RRAM DC and AC operations. IEEE Trans Electron Devices 60(12):4090–4097

    Article  CAS  Google Scholar 

  20. Gao B, Wu HQ, Wu W, Wang XH, Yao P, Xi Y, Zhang WQ, Deng N, Huang P, Liu XY, Kang JF, Chen HY, Yu SM, Qian H (2017) Modeling disorder effect of the oxygen vacancy distribution in filamentary analog RRAM for neuromorphic computing. 2017 IEEE International Electron Devices Meeting, San Francisco, p 4 4.1

    Google Scholar 

  21. Campbell PF, Ortner MH, Anderson CJ (1961) Differential thermal analysis and thermogravimetric analysis of fission product oxides and nitrates to 1500°C. Anal Chem 33(1):58–61

    Article  CAS  Google Scholar 

  22. Steeves MM (2011) Electronic transport properties of ruthenium and ruthenium dioxide thin films, The University of Maine

    Google Scholar 

  23. Song YH, Chen YL, Chi Y, Liu CS, Ching WL, Kai JJ, Chen RS, Huang YS, Carty AJ (2003) Deposition of conductive Ru and RuO2 thin films employing a pyrazolate complex [Ru(CO)3(3,5-(CF3)2-pz)]2 as the CVD source reagent. Chem Vap Depos 9(3):162–169

    Article  CAS  Google Scholar 

Download references


This work was supported in part by the National Key Research and Development Program (2017YFB0405602) and the National Natural Science Foundation of China (61421005, 61874006, 61334007, and 61834001).


This work was supported in part by the National Key Research and Development Program (2017YFB0405602) and the National Natural Science Foundation of China (61421005, 61874006, 61334007, and 61834001).

Availability of Data and Materials

The datasets supporting the conclusions of this manuscript are included within the manuscript.

Author information

Authors and Affiliations



YF fabricated the RRAM array, carried out the electrical measurements with the assistance of ZZ and XD, and drafted the manuscript under the instruction of JK, LL, PH, and XL. JK and LL supervised the work and finalized the manuscript. All authors read and approved the final manuscript.

Corresponding authors

Correspondence to Peng Huang or Lifeng Liu.

Ethics declarations

Competing Interests

The authors declare that they have no competing interests.

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Rights and permissions

Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (, 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.

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Feng, Y., Huang, P., Zhou, Z. et al. Negative Differential Resistance Effect in Ru-Based RRAM Device Fabricated by Atomic Layer Deposition. Nanoscale Res Lett 14, 86 (2019).

Download citation

  • Received:

  • Accepted:

  • Published:

  • DOI:


  • Negative differential resistance
  • Ruthenium
  • RRAM
  • Atomic layer deposition