Memristive Switching in Bi1âxSbx Nanowires - ACS Applied Materials

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Memristive Switching in Bi1-xSbx Nanowires Nalae Han, Myung Uk Park, and Kyung-Hwa Yoo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01050 • Publication Date (Web): 28 Mar 2016 Downloaded from http://pubs.acs.org on March 28, 2016

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ACS Applied Materials & Interfaces

Memristive Switching in Bi1-xSbx Nanowires Nalae Han, †, ‡ Myung Uk Park, †, ‡ and Kyung-Hwa Yoo†,* †

Department of Physics, Yonsei University, 50 Yonsei-ro, Seoul, 03722, Republic of Korea

*Corresponding author e-mail: K-H Yoo, [email protected] ‡

These authors contributed equally.

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ABSTRACT

We investigated the memristive switching behavior in bismuth-antimony alloy (Bi1-xSbx) single nanowire devices at 0.1 ≤ x ≤ 0.42. At 0.15 ≤ x ≤ 0.42, most Bi1-xSbx single nanowire devices exhibited bipolar resistive switching (RS) behavior with on/off ratios of approximately 104 and narrow variations in switching parameters. Moreover, the resistance values in the lowresistance state (LRS) were insensitive to x. On the other hand, at 0.1 ≤ x ≤ 0.15, some Bi1-xSbx single nanowire devices showed complementary RS-like behavior, which was ascribed to asymmetric contact properties. Transmission electron microscopy and elemental mapping images of Bi, Sb, and O obtained from the cross-sections of the Bi1-xSbx single nanowire devices, which were cut before and after RS, revealed that the mobile species was Sb ions, and the migration of the Sb ions to the nanowire surface brought the switch to LRS. In addition, we demonstrated that two types of synaptic plasticity, namely, short-term plasticity and long-term potentiation could be implemented in Bi1-xSbx nanowires by applying a sequence of voltage pulses with different repetition intervals.

KEYWORDS: Bi1-xSbx, memristive switching, Sb ion migration, synaptic device, nanowire device


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INTRODUCTION Bismuth-antimony alloys (Bi1-xSbx) have received considerable attention because they are some of the first materials to reveal topological insulator behavior1,2 and are some of the best available materials for thermoelectronic applications in the cryogenic temperature range.3,4 In particular, Bi1-xSbx nanowires have been extensively studied in recent decades because a lowdimensional thermoelectric material system has a higher figure of merit (Z) than the same material in a three-dimensional form due to quantum confinement effects.5,6 However, the memory effects of Bi1-xSbx alloys have never been investigated. Here, we report the memristive switching behavior in Bi1-xSbx nanowires (0.1 ≤ x ≤ 0.42). Unlike typical resistive switching (RS) materials,7-13 Bi1-xSbx nanowires at 0.15 ≤ x ≤ 0.42 exhibit bipolar RS (BRS) phenomena without a forming process that requires high-voltage applications. In addition, some Bi1-xSbx nanowires at 0.1 ≤ x ≤ 0.15 show complementary RS (CRS) - like behavior observed in CRS devices consisting of two bipolar memory cells that are antiserially connected.7-9 Electric field-induced RS phenomena have usually been studied in oxides (including NiOx, TiOx, SiOx, and VOx), perovskite oxides (including SrTiO3, and BiFeO3), and chalcogenides (including AgS2 and CuS2).10-13 These materials are initially in an insulating state, and a forming process is a prerequisite to change them to a bistable reversible state. RS is classified as either unipolar or bipolar based on the electric-polarity dependence. In the case of unipolar RS (URS), reversible changes between bistable resistance states occur with the application of a voltage with the same polarity, whereas resistance changes occur with the application of an opposite polarity voltage in the case of the BRS. Depending on the switching mechanism, these materials are also grouped into anion or cation devices.11 Most metal oxide-based devices are anion-based devices, where the oxygen anion is regarded as the mobile species. Anion motion leads to valence


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changes in the metal (cations), causing a change in the resistance of the metal oxide material. On the other hand, cation-based devices have an electrode made of an electrochemically active material (such as Cu, Ag, and an alloy of these materials) and a counter electrode made of an electrochemically inert metallic material (such as W, Pt, and Au). In most cation-based devices, the mobile species is the metallic cation, and the switching mechanisms of the devices resemble those of anion-based devices. Bi1-xSbx is not a metal oxide material and our Bi1-xSbx nanowire devices consist of two Pt electrodes. Hence, we cannot clearly classify these devices as either anion or cation-based devices. To investigate what mobile species are and where they move under electrical fields, transmission electron microscopy (TEM) and elemental mapping images were investigated for the cross-sections of the Bi1-xSbx nanowire devices before and after RS. The mobile species was found to be Sb ions. Upon application of bias voltages, the Sb ions moved to the nanowire surface and diffused into an oxide layer, leading to electrical connection between the nanowire and the Pt electrode and a transition to a low-resistance state (LRS). On the other hand, when opposite bias voltages were applied, the ions migrated back to the nanowire center and switched back to a high-resistance state (HRS). In addition, we demonstrated that the Bi1-xSbx nanowire devices could be applied to emulate synaptic plasticity that is believed to be the underlying mechanism of memory and learning.

EXPERIMENTAL SECTION Synthesis and characterization of Bi1-xSbx nanowires: Bi1-xSbx nanowires were fabricated by electrodeposition inside the nanopores of anodized aluminum oxide (AAO) templates. AAO templates with a mean pore diameter of 80 nm were grown by a two-step anodization process.14


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For electrodeposition, a 300-nm-thick Au film was deposited onto one side of the AAO template, and the Bi1-xSbx nanowires were then electrodeposited at a constant voltage of -0.8 V in an electrolyte containing different concentrations of dimethyl sulfoxide (Fisher, 99.9%), SbCl2 of 0.05 M, and Bi(NO3)3⋅5H2O of 0.05 M (Supporting Table S1). Au/AAO, a Pt plate, and a Ag/AgCl electrode (in 3.5 M KCl solution) were used as the working, counter, and reference electrodes, respectively. After electrodeposition, the Au film was removed by ion milling and the AAO template was dissolved using NaOH of 3 M. Next, the Bi1-xSbx nanowires were collected and dispersed in ethanol. The nanowires were characterized using X-ray diffraction (XRD) spectroscopy and TEM (JEOL-KEM 2100 F) equipped with energy dispersive X-ray spectroscopy (EDS).

Fabrication of Bi1-xSbx single nanowire devices: Bi1-xSbx single nanowire devices were fabricated on a Si substrate with a 500-nm-thick thermally-grown SiO2 layer. The Pt (100 nm) electrodes were patterned by electron-beam lithography and lift-off techniques. For the Bi0.9Sb0.1 nanowrie devices with four electrodes, four 500-nm-wide electrodes were made. All electrical measurements were performed using a semiconductor parameter analyzer (Keithley 4200 SCS) and a semiconductor device analyzer (Agilent B1500A).

RESULTS AND DISCUSSION Bi1-xSbx nanowires with varying x values were fabricated by electrodeposition inside the nanopores of the AAO templates. After electrodeposition, the AAO template was dissolved, and the Bi1-xSbx nanowires were dispersed in ethanol. XRD (Figure 1(a)) and selected-area electron


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Figure 1. (a) XRD pattern indexed for rhombohedral Bi1-xSbx nanowires with x ≈ 0.15 and 0.25. (b) TEM-EDS line concentration profiles of Bi and Sb along the line drawn across the diameter of (left) Bi0.85Sb0.15 and (right) Bi0.75Sb0.25 nanowires.

(Supporting Figure 1(a)) patterns obtained from Bi1-xSbx nanowires with x ≈ 0.15 or 0.25 revealed that the nanowires had a rhombohedral crystal structure (JCPDS No. 35-0517), although they were not single-crystalline. The chemical composition of an individual nanowire was estimated using EDS within a scanning transmission electron microscope (Figure 1(b)), and


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elemental mapping images of Bi and Sb showed uniform distributions through the nanowire (Supporting Figure S1(b)).

Figure 2. (a) Representative I-V curves measured for Bi0.75Sb0.25 single nanowire devices. The inset is a field emission scanning electron microscope (FESEM) image of a Bi1-xSbx single nanowire device. (b) Endurance results and (c) cumulative probability of VSET and VRESET measured by a dc sweeping mode in a Bi0.75Sb0.25 single nanowire device for more than 300 times. (d) Representative I-V curves measured for Bi1-xSbx single nanowire devices with different x values. Cumulative probability of (e) VSET and VRESET , and (f) RLRS and RHRS measured by dc sweeping mode in 30 different Bi1-xSbx single nanowire devices with different x values.

To characterize the electrical properties, Bi1-xSbx single nanowire devices were fabricated on a SiO2/Si (inset of Figure 2(a)). Representative I-V curves obtained from Bi1-xSbx nanowire devices at x ≈ 0.25 are shown in Figure 2(a). In contrast to conventional RS materials,7-17 they


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exhibited typical BRS behavior without an electroforming process that is required to change the HRS into a bistable reversible state. The initial resistance of the device was greater than 20 MΩ, and V was swept in the following sequence: 0 → 2 → 0 → -2 → 0 V. The device remained in the HRS at low voltages, and an abrupt transition to the LRS occurred at V ≈ 1.35 V (VSET). This LRS was maintained until the LRS switched back to the HRS at V ≈ -1.2 V (VRESET). The highto-low resistance ratio RHRS/RLRS was estimated to be over 104 (Supporting Figure S2(a)). To investigate the data-retention property of the Bi0.75Sb0.25 nanowire device, we measured I at V = 0.2 V in the LRS and HRS as a function of time at Temp = 27 and 100 °C (Supporting Figure S3). No significant changes in the resistance magnitudes were observed within 104 s for both the LRS and HRS at both temperatures. Figures 2(b) and (c) show the endurance plot and the cumulative probability of VSET and VRESET, respectively. The SET/RESET switching characteristics were obtained by measuring the I-V curves for more than 300 times. The resistance values in the LRS were highly reproducible during the repeated 300 cycles, and VSET and VRESET showed relatively narrow variations. These results suggested that the Bi0.75Sb0.25 nanowire devices could be applied for nonvolatile RS memory devices. In addition to the Bi0.75Sb0.25 nanowire device, we also fabricated Bi1-xSbx nanowire devices with different x values (0.28 ≤ x ≤ 0.42) and measured their I-V curves (Figure 2(d)). Despite the different x values, most devices exhibited similar RLRS values, although VSET and VRESET were dependent on the devices (Figure 2(e)). Figure 2(f) shows the cumulative probability graph of RLRS and RHRS measured for 30 different Bi1-xSbx nanowire devices at 0.28 ≤ x ≤ 0.42. The distribution of RHRS was somewhat broad; however, the distribution of RLRS was quite narrow, indicating that RLRS was insensitive to x. In addition, we noted that the Bi1-xSbx nanowire devices had different distances between the two electrodes; however, their RLRS values were


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similar. Because the devices had the same electrode area, these results implied that RLRS was largely determined by the

Figure 3. (a) I-V curves measured for a Bi0.85Sb0.15 single nanowire device. (b) I-V curves measured using electrodes 1 and 2 for a Bi0.9Sb0.1 single nanowire device. The inset is a FESEM image of the Bi0.9Sb0.1 single nanowire device with four electrodes. (c) I-V curves measured using electrodes 3 and 4 for a Bi0.9Sb0.1 single nanowire device. (d) I-V curves measured using electrodes 2 and 3 for a Bi0.9Sb0.1 single nanowire device.

properties of the interface between the nanowire and metal electrodes rather than by the nanowire channel.


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Figure 3(a) shows the I-V curves measured for a Bi1-xSbx single nanowire device at x ≈ 0.15. Unlike the Bi0.75Sb0.25 nanowire devices, this device yielded two abrupt electrical transitions when V was swept from 0 to 2 V. One transition was from the HRS to the LRS at V ≈ 0.8 V (VSET1) and the other was from the LRS to the HRS at V ≈ 1.7 V (VRESET1). Subsequently, when V was swept to negative voltages, two similar transitions were observed: one transition from the HRS to the LRS at VSET2 ≈ -1.1 V and the other transition from the LRS to the HRS at VRESET2 ≈ -1.64 V. This behavior was similar to the RS reported for the CRS devices.7-9 In fact, we fabricated more than 10 devices at 0.1 ≤ x ≤ 0.15. Among them, five devices exhibited CRS-like behavior with resistance ratio RHRS/RLRS ≈ 103 (Supporting Fig. S4), whereas the other devices displayed BRS behavior. To understand the origin of the CRS-like behavior, we fabricated Bi0.9Sb0.1 nanowire devices with four electrodes (insets of Figure 3). The I-V curves measured using electrodes 1-2 and 3-4 are shown in Figures 3(b) and (c), respectively. Although both I-V curves exhibited BRS behaviors, their RS parameters were different. Figure 3(d) shows the I-V curves measured with electrodes 2-3. Interestingly, these I-V curves exhibited CRS-like behavior, suggesting that the CRS-like behavior was probably ascribed to the asymmetric contacts with different RS parameters. Moreover, the CRS-like behavior were not nearly found in Bi1-xSbx nanowire devices at 0.15 ≤ x ≤ 0.42. Hence, we conjectured that asymmetric contacts might more frequently form in Bi1-xSbx nanowire devices with small x values (0.1 ≤ x ≤ 0.15) than in Bi1-xSbx nanowire devices with high x values (0.15 ≤ x ≤ 0.42) because the Sb distribution might not be uniform along a Bi1-xSbx nanowire with small x values (0.1 ≤ x ≤ 0.15).


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Several models have been proposed for the driving mechanism in RS involving an interface-type conducting path, such as electrochemical migration of oxygen vacancies,7,

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trapping of

Figure 4. Elemental mapping images of (first column) Bi, (second column) Sb, and (third column) O, and (fourth column) merged images of Bi and O, and (fifth column) merged images of Sb and O measured for cross sections of Bi0.75Sb0.25 single nanowire devices in the (a) initial state, (b) LRS, and (c) HRS. The scale bar is 25 nm.

charge carriers,19 and a Mott transition induced by carriers doped at the interface.13 To explore the mechanism of BRS in Bi1-xSbx nanowire devices, we measured elemental mapping images of Bi, Sb, and O for a cross-section of the Bi0.75Sb0.25 nanowire device that was cut across the nanowire before or after RS. In the initial state, Bi and Sb were uniformly distributed across the


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nanowire, and the nanowire surface was found to be surrounded by a native oxide layer grown under ambient conditions (Figure 4(a), Supporting Figure S5). However, the elemental mapping images obtained from the Bi1-xSbx nanowire in the LRS revealed that the concentration of Sb was higher at the nanowire surface than at the nanowire center (Figure 4(b)). Furthermore, the diameter of the Sb distribution became larger than that of the Bi distribution and comparable with that of the O distribution. These observations suggested the notion that Sb ions migrated to the nanowire surface and diffused into the oxide layer, resulting in switching to the LRS. Similar experiments were repeated with another Bi1-xSbx nanowire device. Similarly, the Sb concentration was enhanced at the nanowire surface, indicating that switching to the LRS could be attributed to the diffusion of Sb ions into the oxide layer (Supporting Figure S6). Figure 4(c) shows elemental mapping images for the cross-section of a Bi0.75Sb0.25 nanowire that was cut after switching to the LRS and then back to the HRS. Similar to the initial state, the relatively uniform distribution of Sb was recovered, and this was probably due to the migration of Sb ions to the nanowire center under opposite bias voltages. However, compared with the initial state, more Sb ions were found in the oxide layer. To further investigate how the Sb distribution was related to the RS, a Bi1-xSbx nanowire device with four electrodes was fabricated. The nanowire segment between electrodes 1 and 2 was switched to the LRS, whereas the nanowire segment between electrodes 3 and 4 was switched to the LRS and then back to the HRS (Figure 5(a)). To compare the Sb and Bi distributions in the LRS and HRS, this device was cut along the nanowire. Figure 5(b) shows the TEM image measured for the Bi1-xSbx nanowire between electrodes 1 and 2 in the LRS. Small clusters were observed immediately under the Pt electrodes. Elemental mapping images revealed that Sb was rich in the clusters (Figure 5(b)). However, these clusters disappeared in the Bi1-xSbx


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nanowire in the HRS between electrodes 3 and 4 (Figure 5(c)). For confirmation, similar experiments were carried out using another Bi1-xSbx nanowire device (Supporting Figure S7). As expected, the Sb concentration near the Pt electrodes increased for the nanowire in the LRS, although small clusters

Figure 5. (a) Schematics of the Bi0.75Sb0.25 single nanowire device with four electrodes. The nanowire segment between electrodes 1 and 2 is in the LRS, and the nanowire segment between electrodes 3 and 4 is in the HRS. To examine the TEM and elemental mapping images, the nanowire device is cut along the nanowire, as indicated by the dashed line (white). (b) TEM image obtained from the cross section of the Bi0.75Sb0.25 nanowire in the LRS. The lower first column shows an enlarged TEM image of the region denoted by a red box. The lower second, third, and fourth columns are elemental mapping images of Bi and Sb, and merged images of Bi and Sb, respectively, for the region denoted by the yellow box in the enlarged TEM image. (c) TEM images obtained from the cross section of the Bi0.75Sb 0.25 nanowire in the HRS. The scale bar is 50 nm.


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were not clearly observed under the Pt electrodes. On the other hand, the Bi concentration was almost unchanged by the RS. These results verified that the RS behavior was caused by the electromigration of the Sb ions. A model is proposed based on the above results, as shown in Figure 6. Initially, the device is in the HRS due to the oxide layer. However, the application of V > VSET induces the Sb ions to migrate to the nanowire surface and to diffuse into the oxide layer, leading to the formation of an electrical bridge between the Pt electrodes and the nanowire. As a result, the device switches from the HRS to the LRS. When |V| > |VRESET| with opposite polarity is applied, the Sb ions migrate toward the nanowire center; therefore, the Sb atomic bridges are broken, and switching to the HRS occurs. To investigate whether these RS phenomena are a result of an electric-field or a thermal effect, such as Joule heating, the time dependence of I at various bias voltages was studied with a current compliance of 10 µA (Supporting Figure S8). The transition from the HRS to the LRS could be achieved even with V=0.01, 0.05, or 0.1 V (

Memristive Switching in Bi1âxSbx Nanowires - ACS Applied Materials

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