Research on Improving the Working Current of NbOx-Based Selector by Inserting a Ti Layer

Introduction

At present, the explosive growth of information has brought great challenges to the existing information storage media, which promotes the rapid development of the next-generation information storage technology with high capacity, high density, and fast read-write access. Given the low-power consumption and the high switching speed (Deng et al., 2013; Tsai et al., 2016; He et al., 2019a), resistive random access memory (RRAM) has been extensively studied in the new generation of storage technology. In order to realize a large amount of information storage, a single RRAM device should be integrated into the crossbar array structure. However, a sneak path current often occurs when operating a single RRAM device inside the crossbar array, causing a misreading of stored information (Aluguri and Tseng, 2016; Diaz Leon et al., 2017). As a result, selectors are connected in series with the RRAM as an effective solution in the crossbar array structure (Kim et al., 2012; Song et al., 2017; Alayan et al., 2017; Park et al., 2017). In terms of the selector materials, the NbO₂-based selector driven by insulator-metal transition (IMT) effect has been widely adopted because of its reliable switching performance (Li et al., 2014; Park et al., 2016; Chen et al., 2018a; Wang et al., 2018), which suppresses the sneak current effectively (Chen et al., 2018b; Chen et al., 2019). Besides, to make RRAM be driven under a large current density, it is very important to ensure the working current of the selector matches with the memory in the one-selector one-resistor (1S1R) device without any electrode between them (Yang et al., 2010; Cai et al., 2018; He et al., 2019b).

In this study, a selector device with a structure of Ti/NbO_x/Ti/Pt was designed and fabricated through the magnetron sputtering method. Meanwhile, its threshold-switching property was investigated under different compliance current (CC). Compared with our previous study (Liu et al., 2021), we found that the Ti/NbO_x/Ti/Pt device had a high durability while working under ultra-large CC of the mA level when the inserting Ti layer was introduced into the device. In addition, the threshold voltage of the Ti/NbO_x/Ti/Pt device indicated high uniformity and the OFF-state resistance. This study demonstrates that the performance of a device is improved by the inserting Ti layer, which has great potential to work in the 1S1R device with the V-point structure.

Experiments

In the study, a Ti/NbO_x/Ti/Pt selector was fabricated by the magnetron sputtering method. Briefly, an ultra-thin Ti layer, a switching layer NbO_x, and a top electrode Ti were deposited by DC, RF, and DC magnetron sputtering, respectively, to form a high-performance Ti/NbO_x/Ti/Pt structure selector. Additionally, a focused ion beam (FIB, Zeiss Crossbeam 540) scanning electron microscope (SEM) was used to study the cross section of the devices. X-ray photoelectron spectroscopy (XPS; Escalab 250Xi, Thermo Fisher Scientific) was used to characterize the NbO_x oxide composition. Electrical measurements were carried by an Agilent B1500A semiconductor parameter analyzer, in which a bottom Pt electrode was grounded and DC bias was applied to the top Ti electrode.

Results and Discussion

The cross-sectional structure of the fabricated Ti/NbO_x/Ti/Pt device was characterized by the SEM. Figure 1A illustrates a cross-sectional SEM image of the device clearly shows the deposited multilayers on the Pt/Ti/SiO₂/Si substrate. The thickness of the top electrode Ti, the switching layer NbO_x, the ultra-thin Ti layer, and the bottom electrode Pt were about 25, 58, 4, and 208 nm, respectively. The sketch of the device was shown as insets. To further study the composition of NbO_x, XPS measurement was employed and the peaks of Nb were performed, shown in Figure 1B. Based on the analysis, six diffraction peaks were found in the NbO_x film, with binding energies of 207.5 and 210.3 eV were corresponded to Nb⁵⁺, 205.6 and 208.4 eV were corresponded to Nb⁴⁺, while two other peaks located at 203.9 and 206.7 eV were corresponded to Nb²⁺, respectively (Jung et al., 2011). Based on the XPS spectrum, we can further quantify the mass proportions of Nb²⁺, Nb⁴⁺, and Nb⁵⁺ in the NbO_x thin film was about 9.22, 49.37, and 41.41%, and the molar ratio of Nb was 38.13%. The selector performance of the device was caused by the NbO₂ phase change driven by IMT; however, the presence of NbO in NbO_x might affect the performance of the device potentially.

FIGURE 1

FIGURE 1. (A) SEM image of the Ti/NbO_x/Ti/Pt device measured by crossbeam 540, sketches shown in insets. (B) XPS spectra of Nb3d of the device.

I-V measurements were used to characterize the electrical properties of the Ti/NbO_x/Ti/Pt device, as shown in Figure 2A. Under 5 mA CC, the current of the device suddenly increased at positive threshold voltage (V_th+) of 0.75 V when a positive I-V sweeps from 0 to 2 V, indicating the device state switching from OFF state to ON state. In opposite, the positive hold voltage (V_hold+) was measured when the voltage reverse swept from 2 to 0 V; the current decreased rapidly at 0.75 V and the device recovered from ON state to OFF state. Similarly, V_th− and V_hold− were measured at −0.85 V and −0.85 V in the negative voltage region, respectively. The four voltage values (V_th+, V_hold+, V_th−, and V_hold−) increase along with the increase of CC, while the device still kept threshold-switching property even when CC is up to 100 mA. As a comparison, the working current of the other research was low to a high microamps level (Wang et al., 2018; Lee et al., 2020; Gao et al., 2017; Park et al., 2018; Luo et al., 2019), whereas the working current of the Ti/NbO_x/Ti/Pt device has been increased by 10 times or more. This indicated that the Ti/NbO_x/Ti/Pt device has the great potential to drive RRAM under ultra-large working current.

FIGURE 2

FIGURE 2. (A) I-V curve of the Ti/NbO_x/Ti/Pt device under different compliance current (CC). (B) Voltage distribution of the device under different CC. (C) The CV value of the device under different CC.

100 cycling of the I-V curves were further employed to analyze the stability of the voltages. Figure 2B demonstrated a statistical distribution of device voltage under different CC, illustrating that the voltages were largely increased as CC rose. Meanwhile, the voltages of the device were limited to a small fluctuation range when CC from 5 mA up to 50 mA, which showed remarkable stability. The voltages fluctuation range of the device was slightly increased when CC was increased to 80 and 100 mA, which still showed enough stability. This demonstrated that the device has very stably switching voltages at the mA level of CC. Furthermore, given the mean value was quite different, the coefficient of variation (CV) was used to evaluate the voltage distribution, which was defined as the ratio of standard deviation (σ) with respect to mean value (μ). The CV of the device increased with the increase in the CC, as shown in Figure 2C. The CV of the device was smaller than 5% for working under different CC. The smaller the value of the CV, the weaker the voltage fluctuation, which indicated the consistency of the voltage was better. The drift of voltage of the Ti/NbO_x/Ti/Pt device was smaller and exhibited a more reasonable uniformity, which illustrated that the four voltages have outstanding consistency under the mA level CC except 100 mA.

Furthermore, in order to further explore the stability of the Ti/NbO_x/Ti/Pt device, the OFF-state resistance distribution was carefully examined. Figure 3A shows the OFF-state resistances of the device from 1 to 100 cycles under different CC. The OFF-state resistance of the device decreased along with the increase in CC from 5 to 100 mA. Besides, OFF-state resistances kept stable during 1–100 cycling under 5–80 mA CC. However, when CC was increased to 100 mA, the resistance decreased as the cycling increased, demonstrated a degraded device performance. Therefore, the Ti/NbO_x/Ti/Pt device illustrated a remarkable stability of OFF-state resistance under tens of the mA level CC, thus it has a great potential to work under ultra-large current density. In order to confirm the performance of the device, the selectivity, as one of the most important performance parameter, was studied. In Figure 3B, the selectivity average value of the device was increased from 11 to 53 as CC increased. The selectivity statistics distribution of the device exhibited remarkable stability distributions when CC was increased from 5 to 80 mA, except for the slight poor trend when CC was 100 mA. The stability of selectivity could inhibit the sneak path current more effectively in crossbar arrays (Zhou et al., 2014). Therefore, the selectivity distribution of the Ti/NbO_x/Ti/Pt device demonstrated that had has the potential to suppress sneak path current in crossbar arrays.

FIGURE 3

FIGURE 3. (A) Resistance distribution at 1/2 V_read of the Ti/NbO_x/Ti/Pt device in 100 cycles. (B) Selectivity distribution of the device under different CC.

To uncover the high sustainability of the Ti/NbO_x/Ti/Pt device under large CC, the models of initial ON and OFF working states are presented in Figure 4. The initial working state of the model is shown in Figure 4A. The oxygen ions in the NbO_x film were controlled and diffused into the Ti film; hence, a TiO_x film was formed, given its ability to absorb oxygen ions, meanwhile, created more defects, that is, oxygen vacancy in the NbO_x film (Samanta et al., 2019). Then, driven by a forming voltage, the diffusion of oxygen ions in NbO_x leads to the formation of Nb₂O₅ near the top of the Ti electrode and NbO₂ near the bottom of the Pt electrode. The conducting filament played by oxygen vacancy could permanently form in the Nb₂O₅ region. In a forward scanning process, the phase of NbO₂ changed from insulating to metallic phase when the DC bias voltage reached V_th+, and the device changed from the OFF state to ON state, as shown in Figure 4B. At this time, the oxygen vacancy formed by oxygen absorption effect of ultra-thin Ti films could also play an auxiliary role in the stability of the conductive filaments to withstand higher working current. Meanwhile, the NbO, which performed conductively (Liu et al., 2012), was mixed into NbO₂ and Nb₂O₅, and then made that the device could withstand more current pass through, resulting in outstanding sustainability under large CC. Moreover, with conductive properties, NbO could also play an active role in the stable conductive channel formed by NbO₂ and conductive filaments, which reduced the randomness of the conductive channels and therefore improved the stability of the threshold voltage and OFF-state resistance, and in the subsequent reverse scanning process, the device switched from ON state to OFF state and was triggered by the phase transition of NbO₂ from the metallic phase to the insulating phase, as shown in Figure 4C. The NbO with conductive properties also had an auxiliary role in forming a path for leakage of current when NbO₂ was in the insulating phase state. In general, because of the existence of an ultra-thin Ti film led to form high conductive properties NbO, the Ti/NbO_x/Ti/Pt device could stay stably under ultra-large CC, thus demonstrated an outstanding property of switching, and provided an effective method to enhance the selector performance.

FIGURE 4

FIGURE 4. (A)The model as an initial state of the Ti/NbO_x/Ti/Pt device; (B) and (C) schematic diagram of the ON state and OFF state of the device.

Conclusion

In summary, the Ti/NbO_x/Ti/Pt selector device was prepared by the magnetron sputtering method on the Pt/SiO₂/Si substrate, and the device showed its excellent threshold-switching properties under ultra-large CC. Furthermore, the threshold voltages and OFF-state resistance display high consistency as well as selectivity, thus better accords with the practical application requirements of the ultra-large operating current device. Besides, a schematic analysis indicated that the share current effect of conductive properties of NbO contributes to the stability of the Ti/NbO_x/Ti/Pt device. This study was provided for improving the work current of selector devices and indicated that the Ti/NbO_x/Ti/Pt selector device had a great potential in driving the RRAM device.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material; further inquiries can be directed to the corresponding author.

Author Contributions

CL demonstrated all experiments in the study and wrote the manuscript. GM and HW conducted deep review, editing, guidance, and supervision. All authors have read and approved the article for publication.

Funding

This work was supported in part by the National Key R and D Program under Grant No. 2017YFB0405600, the National Science and Technology Major Project under Grant No. 2017ZX02301, Science and Technology Major Project of Hubei under Grant No. 2020AAA005, Hubei Key Laboratory of Advanced Memories, and the Scientific Research Project of Education Department of Hubei Province No. Q20181009.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

References

Alayan, M., Vianello, E., Navarro, G., Carabasse, C., Barbe, S., La. Verdy, A., et al. (2017). In-Depth Investigation of Programming and Reading Operations in RRAM Cells Integrated with Ovonic Threshold Switching (OTS) Selectors. IEEE Int. Electron Devices Meet. 2 IEDM17, 32–35. doi:10.1109/iedm.2017.8268311

CrossRef Full Text | Google Scholar

Aluguri, R., and Tseng, T.-Y. (2016). Notice of Violation of IEEE Publication Principles: Overview of Selector Devices for 3-D Stackable Cross Point RRAM Arrays. IEEE J. Electron. Devices Soc. 4, 294–306. doi:10.1109/JEDS.2016.2594190

CrossRef Full Text | Google Scholar

Cai, H., Ma, G., He, Y., Liu, C., and Wang, H. (2018). A Remarkable Performance of CH 3 NH 3 PbI 3 Perovskite Memory Based on Passivated Method. Org. Electron. 58, 301–305. doi:10.1016/j.orgel.2018.04.025

CrossRef Full Text | Google Scholar

Chen, A., Ma, G., He, Y., Chen, Q., Liu, C., Wang, H., et al. (2018a). Research on Temperature Effect in Insulator-Metal Transition Selector Based on NbOx Thin Films. IEEE Trans. Electron. Devices. 65, 5448–5452. doi:10.1109/TED.2018.2873638

CrossRef Full Text | Google Scholar

Chen, C.-K., Lin, C.-Y., Chen, P.-H., Chang, T.-C., Shih, C.-C., Tseng, Y.-T., et al. (2018b). The Demonstration of Increased Selectivity During Experimental Measurement in Filament-Type Vanadium Oxide-Based Selector. IEEE Trans. Electron. Devices. 65, 4622–4627. doi:10.1109/TED.2018.2862917

CrossRef Full Text | Google Scholar

Chen, A., Ma, G., Zhang, Z., Lin, C. Y., Lin, C. C., Chang, T. C., et al. (2019). Multi‐Functional Controllable Memory Devices Applied for 3D Integration Based on a Single Niobium Oxide Layer. Adv. Electron. Mater. 6, 1900756–1900764. doi:10.1002/aelm.201900756

CrossRef Full Text | Google Scholar

Deng, Y., Huang, P., Chen, B., Yang, X., Gao, B., Wang, J., et al. (2013). RRAM Crossbar Array with Cell Selection Device: A Device and Circuit Interaction Study. IEEE Trans. Electron. Devices. 60, 719–726. doi:10.1109/TED.2012.2231683

CrossRef Full Text | Google Scholar

Diaz Leon, J. J., Norris, K. J., Yang, J. J., Sevic, J. F., and Kobayashi, N. P. (2017). A Niobium Oxide-Tantalum Oxide Selector-Memristor Self-Aligned Nanostack. Appl. Phys. Lett. 110, 103102–103106. doi:10.1063/1.4977945

CrossRef Full Text | Google Scholar

Gao, L., Holbert, K., and Yu, S. (2017). Total Ionizing Dose Effects of Gamma-Ray Radiation on NbOx Based Selector Devices for Crossbar Array Memory. IEEE Trans. Nucl. Sci. 64, 1. doi:10.1109/TNS.2017.2700434

CrossRef Full Text | Google Scholar

He, Y., Ma, G., Cai, H., Liu, C., Zhang, H., and Wang, H. (2019a). High Performance and Mechanism of the Resistive Switching Device Based on lead Halide Thin Films. J. Phys. D: Appl. Phys. 52, 135103–135110. doi:10.1088/1361-6463/aafe5e

CrossRef Full Text | Google Scholar

He, Y., Ma, G., Zhou, X., Cai, H., Liu, C., Zhang, J., et al. (2019b). Impact of Chemical Doping on Resistive Switching Behavior in Zirconium-Doped CH3NH3PbI3 Based RRAM. Org. Electron. 68, 230–235. doi:10.1016/j.orgel.2019.02.025

CrossRef Full Text | Google Scholar

Jung, K., Kim, Y., Park, Y. S., Jung, W., Choi, J., Park, B., et al. (2011). Unipolar Resistive Switching in Insulating Niobium Oxide Film and Probing Electroforming Induced Metallic Components. J. Appl. Phys. 109, 054514. doi:10.1063/1.3552980

CrossRef Full Text | Google Scholar

Kim, S., Liu, X. J., Park, J., Jung, S., Lee, W., Woo, J., et al. (2012). Ultrathin (<10nm) Nb₂O₅/NbO₂ Hybrid Memory With Both Memory and Selector Characteristics for High Density 3D Vertically Stackable RRAM Applications. IEEE Symp. VLSI Technol. (VLSIT). Honolulu, HI, USA, 155–156.

Google Scholar

Lee, S., Yoo, J., Park, J., and Hwang, H. (2020). Understanding of the Abrupt Resistive Transition in Different Types of Threshold Switching Devices from Materials Perspective. IEEE Trans. Electron. Devices. 67, 2878–2883. doi:10.1109/TED.2020.2997670

CrossRef Full Text | Google Scholar

Li, S., Liu, X., Nandi, S. K., Venkatachalam, D. K., and Elliman, R. G. (2014). Temperature Dependence of Threshold Switching in NbOx Thin Films. In Conference On Optoelectronic And Microelectronic Materials & Devices. Perth, WA, Australia, 14-17, 138–217. doi:10.1109/COMMAD.2014.7038673

CrossRef Full Text | Google Scholar

Liu, C., Ma, G., Chen, A., Zhao, X., Zhou, C., and Wang, H. (2021). Research on the Ability of Over Current in Ti/NbOx/Pt-Based Selector. J. Mater. Sci. Mater. Electron. 32, 12822–12827. doi:10.1007/s10854-020-03670-7

CrossRef Full Text | Google Scholar

Liu, X., Md. Sadaf, S., Son, M., Park, J., Shin, J., Lee, W., et al. (2012). Co-Occurrence of Threshold Switching and Memory Switching in P_t/NbOx/P_t Cells for Crosspoint Memory Applications. IEEE Electron. Device Lett. 33, 236–238. doi:10.1109/LED.2011.2174452

CrossRef Full Text | Google Scholar

Luo, Q., Zhang, X., Yu, J., Wang, W., Gong, T., Xu, X., et al. (2019). Memory Switching and Threshold Switching in a 3D Nanoscaled NbOX System. IEEE Electron. Device Lett. 40, 718–721. doi:10.1109/LED.2019.2904279

CrossRef Full Text | Google Scholar

Park, J., Cha, E., Karpov, I., and Hwang, H. (2016). Dynamics of Electroforming and Electrically Driven Insulator-Metal Transition in NbOx Selector. Appl. Phys. Lett. 108, 232101–232105. doi:10.1063/1.4953323

CrossRef Full Text | Google Scholar

Park, J., Hadamek, T., Posadas, A. B., Cha, E., Demkov, A. A., and Hwang, H. (2017). Multi-Layered NiOy/NbOx/NiOy Fast Drift-free Threshold Switch with High Ion/Ioff Ratio for Selector Application. Sci. Rep. 7, 4068–4076. doi:10.1038/s41598-017-04529-4

PubMed Abstract | CrossRef Full Text | Google Scholar

Park, J., Yoo, J., Song, J., Sung, C., and Hwang, H. (2018). Hybrid Selector with Excellent Selectivity and Fast Switching Speed for X-point Memory Array. IEEE Electron. Device Lett. 39, 1171–1174. doi:10.1109/LED.2018.2845878

CrossRef Full Text | Google Scholar

Samanta, S., Panpan, Z., Han, K., Xiao, G., Chakraborty, S., Li, Y., et al. (2019). Impact of Ti Interfacial Layer on Resistive Switching Characteristics at Sub-µa Current Level in SiOx-Based Flexible Cross-Point RRAM. In 2019 IEEE International Conference on Flexible and Printable Sensors and Systems. Glasgow, United Kingdom: FLEPS, 18936085. doi:10.1109/FLEPS.2019.8792259

CrossRef Full Text | Google Scholar

Song, J., Woo, j., Lim, S., Chekol, S. A., and Hwang, H. (2017). Self-Limited CBRAM With Threshold Selector for 1S1R Crossbar Array Applications. IEEE Electron. Device Lett. 38, 1532–1535. doi:10.1109/LED.2017.2757493

CrossRef Full Text | Google Scholar

Tsai, T.-M., Chang, K.-C., Chang, T.-C., Zhang, R., Wang, T., Pan, C.-H., et al. (2016). Resistive Switching Mechanism of Oxygen-Rich Indium Tin Oxide Resistance Random Access Memory. IEEE Electron. Device Lett. 37, 408–411. doi:10.1109/LED.2016.2532883

CrossRef Full Text | Google Scholar

Wang, Z., Kumar, S., Wong, H.-S. P., and Nishi, Y. (2018). Effect of Thermal Insulation on the Electrical Characteristics of NbOx Threshold Switches. Appl. Phys. Lett. 112, 073102–073106. doi:10.1063/1.5015941

CrossRef Full Text | Google Scholar

Yang, R., Li, X. M., Yu, W. D., Gao, X. D., Shang, D. S., and Chen, L. D. (2010). Endurance Improvement of Resistance Switching Behaviors in the La0.7Ca0.3MnO3 Film Based Devices with Ag-Al alloy Top Electrodes. J. Appl. Phys. 107, 063707. doi:10.1063/1.3309473

CrossRef Full Text | Google Scholar

Zhou, J. T., Kuk-Hwan Kim, K., and Wei Lu, W. (2014). Crossbar RRAM Arrays: Selector Device Requirements during Read Operation. IEEE Trans. Electron. Devices. 61, 1369–1376. doi:10.1109/TED.2014.2310200

CrossRef Full Text | Google Scholar