- Email: [email protected]
ITO device
Materials Science in Semiconductor Processing 91 (2019) 246–251
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Resistive switching behaviors mediated by grain boundaries in one longitudinal Al/MoS2&PVP/ITO device ⁎
Na Baia, Min Xua, Cong Hua, Yaodong Maa, Qi Wanga, Deyan Hea, Jing Qia, , Yingtao Lia,b,
T ⁎⁎
a Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China b Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Resistive switching Threshold switching Switching mechanism MoS2
Nonvolatile memory devices based on 2D layered transition metal dichalcogenides have inspired tremendous amounts of research interests in recent years. In this work, resistive switching (RS) devices based on hybrid molybdenum disulphide and Polyvinylpyrrolidone (MoS2&PVP) is explored. One longitudinal Al/MoS2&PVP/ ITO device exhibits three different modes of I-V characteristics, i.e. typical bipolar RS, asymmetrical bipolar RS and threshold switching (TS), which can be explained by the formation and rupture of conducting filaments (CFs) consisting of sulfur vacancy filaments in MoS2 nanoflakes and sulfur ions filaments in PVP. These CFs form from three different kinds of grain boundaries (GBs), intersecting-GB, bisecting-GB, and bridge-GB in which GBs attach one electrode, parallel to the two electrodes, and attach both electrodes, respectively, corresponding to the three modes of I-V characteristics. These three kinds of GBs coexist in one device and the CF forms randomly from one kind of them during sweeping loops. The mechanism was experimentally proved by our results of the increasing probability of typical bipolar RS in device with one layer of PVP thin film inserted into the interface of MoS2&PVP and ITO and other researchers’ results in all-surface MoS2 single-layer MoS2 devices.
1. Introduction With the technology development in the information age, highperformance memories are urgently needed for large data storages. Among all memories, resistive random access memory (RRAM) has inspired tremendous amounts of research interests over the past years with simple structure, high speed, low power, excellent scalability, high density data and longer data retention time [1–5]. The RRAM is based on the electrically stimulated change of the resistance, i.e. RS, in a metal–insulator–metal (MIM) sandwich structure [4,6], in which the applied voltage between two electrodes leads to the switching between high resistive state (HRS) and low resistive state (LRS). The information can be stored in these two states. The MoS2 is a layered transition metal dichalcogenides with weak interlayer van der Waals coupling, from which the monolayer or fewlayers nanoflakes can be obtained by mechanical exfoliation [7], liquidbased exfoliation [8,9], chemical vapor deposition [10] and lithiumintercalation [11,12]. With the number of layers decreasing, MoS2 varies from an indirect band gap to a direct band gap, the band gap
from 1.2 eV to 1.8 eV [13–15]. MoS2 nanoflakes show good photoelectrochemical stability [16], large specific surface areas [5,17], and unique physicochemical/electronic properties [15]. MoS2 is the most promising materials for energy conversion and storage as well [18]. It has also been utilized in fabricating RS devices [5,9,11,19–24]. Typical bipolar RS phenomenon was observed in devices utilizing poly(N-vinylcarbazole)-covalently grafted MoS2 nanosheets [5], 2H-MoS2-PVP [9], MoS2/GO [11], MoOx/MoS2 [19], chiral MoS2 [20], MoS2-PVA [21], 1T Phase MoS2 [23] and PMMA- MoS2 [24] as switching layers. Write-once-read-many-times memory effect was also observed in 1T@ 2H-MoS2-PVP [9] devices. Most interestingly, three kinds of switching behavior, i.e. typical bipolar RS, asymmetrical bipolar RS, which shows only LRS during negative voltage sweep, and TS, were observed in different devices with planar structure of Au/monolayer MoS2/Au [25]. In this work, coexistence of these three kinds of switching behavior in one longitudinal Al/MoS2&PVP/ITO device was observed. The phenomena can be explained by the formation and rupture of CFs consisting of sulfur vacancies in MoS2 nanoflakes and sulfur ions in PVP, which form randomly from three kinds of GB, i.e. intersecting-GB,
⁎
Corresponding author. Corresponding author at: Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China. E-mail addresses: [email protected] (J. Qi), [email protected] (Y. Li). ⁎⁎
https://doi.org/10.1016/j.mssp.2018.11.024 Received 25 September 2018; Received in revised form 13 November 2018; Accepted 16 November 2018 1369-8001/ © 2018 Published by Elsevier Ltd.
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
shows that MoS2 powder was a layered materials. The X-ray diffraction (XRD) pattern (Fig. S1b, Supporting Information) indicates that the obtained MoS2 powder is pure hexagonal MoS2-2H, which was also proved by Raman spectra (Fig. S1c, Supporting Information). The Raman results also indicate that the MoS2 powder is bulk material. The characterizations of MoS₂&PVP are shown in Fig. 2. Fig. 2a is the morphology and thickness information of MoS₂ nanoflakes obtained by Atomic Force Microscope (AFM). The nanoflakes are butterfly-like. The height is between 0.8 nm and 18.4 nm, and the average height of which is under 10 nm. Inset shows the layer number distribution of butterfly-like MoS2 nanoflakes, which was obtained from the thickness of MoS2 nanoflakes shown in Fig. 2a divided by 0.8 nm because the thickness of a MoS2 monolayer is normally about 0.8 nm [26]. These nanoflakes are from 1 layer to 23 layers and sixty percent of them are less than 15 layers. The photoluminescence (PL) spectra (Fig. S2a, Supporting Information) and the UV–vis diffuse reflectance spectra (Fig. S2b, Supporting Information) of MoS2&PVP also show that the MoS2 nanoflakes are few-layers and the band gap is 1.52 eV [8,27]. From the optical transmittance and spectrum show in Fig. S2c and Fig. S2d, Supporting Information, it can be obtained that the MoS2&PVP/ ITO/PET device is transparent and the optical transmittance is about 80%. The transmission electron microscopy (TEM) image and the corresponding selected area electron diffraction (SAED) pattern are shown in Fig. 2b and c, respectively. These results indicate that the MoS2 nanoflakes are hexagonal single crystal with only a few layers. The highresolution TEM (HRTEM) image is shown in Fig. 2d, from which it can be known that the interplanar distances are both 0.27 nm, which match well with the plane distance of d(100) and d(010) [28–30], respectively. The corresponding fast Fourier transform (FFT) image in inset reveals the highly crystalline property of MoS2. To investigate the switching behaviors of Al/MoS2&PVP/ITO memory device, the electrical properties of the device was obtained by applying electrical stresses on top Al electrode. Three different kinds of electroforming-free switching behavior, i.e. typical bipolar RS, asymmetrical bipolar RS, and TS, can be randomly observed in one device at size of 200 µm in diameter, at the current compliance of 10 µA during the voltage sweeping, as shown in Fig. 3. Fig. 3a is a typical bipolar RS. When the voltage is swept from 0 V to 3 V, the current increased abruptly at 1.17 V, which switches the device from HRS to LRS and corresponds to the SET process. When the voltage is swept back from 3 V to 0 V, the device kept at LRS even after the power is off (the applied voltage is 0 V). For the subsequent negative sweep from 0 V to −3 V, the current decreases abruptly at −0.98 V and the device switches from LRS to HRS, and this corresponds to the RESET process. After RESET, the device keeps at HRS until next SET process. Fig. 3b shows the asymmetrical bipolar RS. The device is SET at 1.47 V during the voltage sweep of 0 V ~3 V ~0 V and RESET during the sweep of 0 V ~− 3–0 V. The most obvious difference between typical bipolar RS and asymmetrical bipolar RS is negative voltage sweeps; (e), (f) TS for the positive and negative voltage sweeps, respectively. that there is no abrupt current change in asymmetrical bipolar RS when the negative swept from 0 V to −3 V and back to 0 V, i.e. no particular RESET voltage. However, the device is switched back to HRS after the voltage sweeping from −3 V back to 0 V, and keeps at HRS until next SET process. Both HRS and LRS can be maintained when the power is off for typical bipolar RS
bisecting-GB, and bridge-GB, during voltage sweeping loops. This switching mechanism is similar to the one proposed by Vinod K. Sangwan et al. [25], and further proved by our experiment results of the increased probability of typical bipolar RS and the decreased probability of TS in Al/MoS2&PVP/PVP/ITO device. The results of this paper offer another way to improve the stability of the MoS2-based resisitive devices, which may provide possible application in neuromorphic computation. 2. Experiment 2.1. Preparation of MoS2&PVP nanocomposites The MoS2 bulk were synthesized through a hydrothermal method. First, 1.56 g CS(NH₂) and 1.21 g Na₂MoO₄•2H₂O were dissolved in 60 ml deionized water under room temperature. Second, the final solution was transferred into a 100 ml Teflon-lined stainless steel autoclave and maintained at 220 °C for 30 h. Third, the solution was cooled down to room temperature, and washed with deionized water and ethyl alcohol for five times. Then, the solution was dried in vacuum to get the desired MoS₂ powder. Finally, MoS2 powders were grounded for 2 h in a mortar in order to get small size MoS2 (Fig. 1a). The 40 mg small size MoS2 powder and 160 mg PVP was mixed and dispersed in 8 ml ethyl alcohol. The MoS2 nanoflakes were obtained from the mixture of bulk MoS2 powder and PVP in ethyl alcohol by sonication for 14 h. The stable dispersion solution of MoS2&PVP (Fig. 1b) was obtained by collecting the supernatant of the mixture centrifuging at 4000 rpm for 40 min. The solution of PVP was prepared by the same method. PVP of 160 mg was dispersed in 8 ml ethyl alcohol and sonicated for 14 h. The stable solution of PVP was obtained by collecting the supernatant of the mixture centrifuging at 4000 rpm for 40 min. 2.2. Fabrication of memory device The devices have the sandwich structure of the Al top electrode, MoS2&PVP resistive layer and the ITO bottom electrode coated on the PET with size of 1 × 1 cm2. ITO/PET substrate had been cleaned with acetone, ethyl alcohol and deionized water for 15 min, respectively. Then, the MoS2&PVP solution was spin-coated onto the ITO at 500 rpms for 15 s and 2000 rpms for 20 s. The film was annealed in air at 70 °C for 30 mins. Finally, Al top electrode was evaporated by E-beam evaporator to fabricate the Al/MoS2&PVP/ITO device (Fig. 1c). Al/MoS2&PVP/ PVP/ITO device was fabricated by the same method. The PVP solution was spin-coated onto the ITO at 500 rpms for 15 s and 2000 rpms for 20 s and was annealed in air at 70 °C for 30 mins before MoS2&PVP spin-coating. 3. Result and discussion Fig. 1d shows the scanning electron microscope (SEM) image of the cross section for the MoS2&PVP/ITO layers. It can be observed that the thickness of the MoS2&PVP and ITO layers are ~70 nm and ~185 nm, respectively. The SEM image of MoS2 powder (Fig. S1a, Supporting Information)
Fig. 1. The schematic preparation process of Al/MoS2&PVP/ITO devices. (a) The MoS2 powder. (b) MoS2&PVP nanosheets. (c) The device structure. (d) The SEM image of the cross section for the MoS2&PVP/ITO layers. 247
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
Fig. 2. Characterizations of MoS₂&PVP nanocomposites. (a) AFM image, inset: the layer number distribution. (b) The TEM image. (c) The SEAD image. (d) HRTEM image. Inset of (d): FFT image.
with increases in voltage bias, as shown in Fig. 4a. For low applied voltage of HRS state, log I vs. log V is linear with a slope of 1.02 (~1), which corresponds to Ohmic conducting mechanism described as
and asymmetrical bipolar RS, which are suitable for nonvolatile memory application [31]. Fig. 3c is I–V characteristics of TS. When the voltage is swept from 0–3 V, the current increases at 1.35 V suddenly. The device is switched to LRS and kept at LRS during the voltage sweeping of 3 V ~ 0 V. However, the LRS cannot be maintained after the power is off. After the sweeping of 3–0 V, whether the voltage is swept from 0 V to −3 V or from 0 V to 3 V, the device is at HRS until next SET process. The behavior during the sweep of 0 V–3 V–0 V is similar to that during the sweep of 0 V–3 V–0 V. This kind of switching cannot be utilized in nonvolatile memory [32]. And the device undergoes a transition from the HRS to LRS at high voltage and goes back to HRS at a low voltage which can be utilized as a selection device [33]. However, the pure MoS2 film has no RS behavior at low bias sweep [5,11]. and no switching phenomenon was observed in the pure pvp film-based device either [8], as shown in Fig. S3, Supporting information. To understand the conduction mechanisms of the three kinds of switching behavior, double-logarithmic I-V curves were plotted, as shown in Fig. 4a–f. The slopes of the double-logarithmic I-V curve in positive voltage sweep for typical bipolar RS vary from ~1 to ~2 to ~5
J = Vexp
{ −kTΔE } ae
(1)
where ΔEae is the activation energy of electrons, k is the Boltzmann constant, and T is the temperature [34]. As the applied voltage increases, the slope of the fitting line varies from 1.02 to 1.72 (~ 2). The slope of ~ 2 can be described by SCLC theory [8, [34] where the relation of current and the applied voltage is I ∝ V 2 . As the applied voltage further increases to 0.49 V, the slope varies to 5.12, which can be described by trap-filled SCLC (I ∝ V a, a > 2 ) [35,36]. Deep traps in SCLC are formed because of the energy level difference between PVP and MoS2 nanosheets according to the energy band diagram of the Al/ MoS2&PVP/ITO device shown in Fig. S4, Supporting Information. And when the applied voltage increases to 1.17 V, SET process occurred and the device is switched to LRS. At LRS, the slopes of the fitting line are 1.21 and 1.58 at low and high voltage respectively, which indicates that the device is controlled by SCLC at high voltage and Ohmic conduction
Fig. 3. I-V Characteristics of Al/PVP&MoS2/ITO device. (a) Typical bipolar RS, (b) asymmetrical bipolar RS, and (c) TS. 248
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
Fig. 4. The double-logarithmic I-V curves for (a), (b) the typical bipolar RS mode for the positive and negative voltage sweeps; (c), (d) the asymmetrical bipolar RS for the positive and negative voltage sweeps; (e), (f) TS for the positive and negative voltage sweeps, respectively.
and SCLC theory. The slopes at LRS are from about 1–2 for positive sweep and about 1 for negative sweep, indicating that Ohmic conduction and SCLC dominate for positive sweep and Ohmic conduction dominates for negative sweep. The positive SET and negative SET processes occur at 1.35 V and 1.11 V, respectively. According to the above conduction mechanism of SCLC and Ohmic conduction, the three kinds of switching behavior in Al/MoS2&PVP/ITO device can be explained by the formation and rupture of CFs [25] consisting of sulfur vacancies in MoS2 nanoflakes and sulfur ions in PVP. The CFs in typical bipolar RS, asymmetrical bipolar RS, and TS form randomly from three different kinds of GBs defect states, respectively, i.e. intersecting-GBs, bisecting-GBs, and bridge-GBs in which GBs of the MoS2 nanoflakes attach one electrode, parallel to the two electrodes, and attach both electrodes. Higher concentration of sulfur vacancies are distributed around the GBs [25]. The three GBs coexist in one device and compete with each other during the formation of CFs because MoS2 nanoflakes are uniformly dispersed in PVP and the direction of GBs and MoS2 nanoflakes are randomly. If the CFs form from intersecting-GBs, the device exhibits typical bipolar RS behavior. If the CFs form from bisecting-GB, the device exhibits asymmetrical bipolar RS behavior. And if the CFs form from bridge-GB, the device shows TS behavior. Similar results were observed in all-surface single-layer MoS2 devices by
at low voltage. The slopes of the LRS double-logarithmic I-V curve of typical bipolar RS mode in negative voltage sweeps shown in Fig. 4b vary from 1.42 (~1) to 1.68 (~2) with increases in voltage bias, indicating Ohmic conducting and SCLC theory dominating. When the applied voltage increases to 0.98 V, RESET process occurred and the device is switched to HRS. At HRS, the slopes of the fitting line are 1.64 and 1.24 at high and low voltage respectively, which indicates that the device is still controlled by SCLC and Ohmic conduction. Fig. 4c and d are the double-logarithmic I-V curve of asymmetrical bipolar RS. The slopes in both positive and negative voltage sweeps vary from ~1 to ~2 with increases in voltage bias. The HRS of device is also controlled by SCLC and Ohmic conduction, according to the slopes of 1.75 (~2) and 1.15 (~1) shown in Fig. 4c. SET process occurs at 1.47 eV. Log I vs. log V is linear with slopes of 0.97 (~1) and 0.99 (~1) at LRS, corresponding to Ohmic conducting mechanism, which indicates that the charges were trapped in MoS2. The double-logarithmic I-V curves shown in Fig. 4d indicate that the device is controlled by SCLC because the slopes are 1.94 (~2) in whole negative voltage sweeps. The doublelogarithmic I- V curves of threshold RS in both positive and negative voltage sweeps are shown in Fig. 4e and f. The slopes vary from ~1 to ~2 with increases in voltage bias at HRS for both positive and negative voltage sweeps. The conduction is still controlled by Ohmic conduction 249
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
Fig. 5. Schematics of switching mechanism for three kinds of switching behavior in one Al/ MoS2&PVP/ITO device. (a) Initial state and HRS for the device. The intersecting-GB, bisecting-GB and bridge-GB coexist in one device. (b) LRS for typical bipolar RS. The CFs form from intersecting-GB. (c) LRS of asymmetrical bipolar RS. The CFs form from bisecting-GB. (d) LRS of TS. The CFs form from bridge-GB.
generation. The device is RESET back to HRS as shown in Fig. 5a. If the CFs form from bridge-GB, the device exhibits TS as shown in Fig. 3c. The bridge-GB are high resistive because the sulfur vacancies around bridge-GB are very sparse. When the voltage sweeps from 0 to 3 V, the sulfur vacancies are generated and migrate toward the bridge-GB, resulting in decrease of the resistance of the device. When the voltage reaches 1.38 V, the density of sulfur vacancies reaches maximum and the device is SET to LRS, as shown in Fig. 5d. When the voltage sweeps from 3 V to 0 V, the rupture of CFs is caused by the Joule heating, which causes the HRS [25,32], as shown in Fig. 5a. It is similarwhen the voltage sweeps from 0 V to − 3 V and − 3–0 V. The above switching mechanism was proved experimentally by Sangwan et al. [25] in allsurface MoS2 single layer device. To prove that it is the same mechanism controlling the switching behavior in our MoS2&PVP longitudinal devices, a layer of PVP was inserted between MoS2&PVP and ITO bottom electrodes to increase the probability of intersecting-GBs. The structure of the device is Al/MoS2&PVP/PVP/ITO. The typical I-V characteristics of Al/MoS2&PVP/PVP/ITO devices is shown in Fig. S5, Supporting Information. In this case, to trigger repeatable typical bipolar RS behavior in the fresh device, an electroforming process with positive voltage on Al electrode and the same current compliance (ICC = 10 μA) are required. The probability of three kinds of switching behavior were counted and analyzed. The results are presented in Table 1. From the table it can be observed that the probabilities of typical bipolar RS, asymmetrical bipolar RS and TS are 16%, 24%, and 60%, respectively for Al/MoS2&PVP/ITO device, while they are 60%, 14% and 26% for Al/MoS2&PVP/PVP/ITO device. The probability of typical bipolar RS increases from 16% to 60% while that of TS decreases from 60% to 26% after inserting a layer of PVP. These results indicate that the probability of intersecting-GB increases while the probability of bisecting-GB decreases by inserting a PVP layer. Technically, the probability of RS should increase to at least 86% and that of TS should decrease to 0%. However, in the procedure of the experiment, it is very difficult to insert a layer of pure PVP film into device because MoS2 nanoflakes and PVP are very easy to be mixed together during spincoating. The results of probability statics indicate that the mechanism of GB-assisted CFs formation and rupture does control the switching behavior in our case.
Sangwan et al. [25]. Fig. 5 is the switching mechanism schematic of the three modes of switching behavior. at initial states and HRS, intersecting-GBs, bisecting-GBs and bridge-GBs coexist in one device. The sulfur vacancies in MoS2 have been shown to accumulate near GBs and the electrons donated by dangling bonds should render the MoS2 regions nears GBs more conductive [25]. The self-assembled structures of the PVP matrix create interconnected polymer chains, which provide the long-range ion transport pathways [37], as shown in Fig. 5a. During voltage sweep, sulfur ions can be pulled out from their equilibrium positions and migrate in MoS2 nanoflakes and PVP chains, which is similar to the generation and migration of oxygen vacancies in the ZnO material devices [38]. Sulfur ions being pulled out from their equilibrium positions results in the generation of sulfur vacancies in MoS2 nanoflakes. When the voltage sweeps from 0 to 3 V, sulfur ions are pulled out from their equilibrium positions and migrate in MoS2 and PVP continually. The density of sulfur vacancies and the migration of sulfur ions increase with the increase of sweeping voltage. If the CFs form from intersectingGB, when the sweep voltage increases to 1.17 V, as shown in Fig. 3a, the density of sulfur vacancies reaches the saturation density, sulfur ions migrate into PVP, which result in the connecting of the GBs and the bottom electrode by the CFs consisting of sulfur vacancies in MoS2 nanoflakes and sulfur ions in PVP, as shown in Fig. 5b. And the device is switched from HRS to LRS, which is corresponding to SET process. After that, the density keeps at the saturation value until negative voltage is applied. The sulfur ions in PVP act as charge traps, so currents at LRS for both positive and negative are controlled by Ohmic conduction and SCLC. When the voltage sweeps from 0 to −3 V, the trapped charges are detrapped, and the sulfur ions in PVP are pulled back into the equilibrium positions in MoS2 nanoflakes, and the CFs fracture, which switches the device from LRS to HRS and is corresponding to the RESET process. The device goes back to the initial state shown in Fig. 5a. If the CFs form from bisecting-GB, the device exhibits the asymmetrical bipolar RS shown in Fig. 3b. When the sweep voltage increases to 1.47 V, the density of sulfur vacancies reaches the maximum, and the sulfur ions drift (driven by the electric field) and diffuse (driven by the concentration gradient) obeying the dynamic and nonlinear relation to form sulfur-ion-rich and sulfur-vacancy-rich regions on either side of the bisecting-GB [25]. The CFs consisting of sulfur vacancies in MoS2 and sulfur ions in PVP form and the device is SET from HRS to LRS as shown in Fig. 5c. Under the negative bias, the sulfur ions drift and diffuse from sulfur-ion-rich region back to sulfur-vacancy-rich region. When the rate of sulfur ions drift is equal to that of sulfur vacancies generate, the filaments will not rupture and the device still maintains high conductive, which causes LRS and HRS overlap to each other in the reset section. With the decrease of voltage, the rate of sulfur ions drift greatly increases and exceeds the rate of sulfur vacancies
Table 1 The results of probability statics.
250
Device structure
Typical bipolar RS
Asymmetrical bipolar RS
TS
Al/MoS2&PVP/ITO/PET Al/MoS2&PVP/PVP/ITO/ PET
16% 60%
24% 14%
60% 26%
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
4. Conclusion
metal dichalcogenide nanobelts, Adv. Mater. 26 (2014) 6250–6254. [13] H.S. Lee, S.W. Min, Y.G. Chang, M.K. Park, T. Nam, H. Kim, J.H. Kim, S. Ryu, S. Im, MoS2 nanosheet phototransistors with thickness-modulated optical energy gap, Nano Lett. 12 (2012) 3695–3700. [14] J.S. Kim, H.W. Yoo, H.O. Choi, H.T. Jung, Tunable volatile organic compounds sensor by using thiolated ligand conjugation on MoS2, Nano Lett. 14 (2014) 5941–5947. [15] T.P. Nguyen, S. Choi, J.-M. Jeon, K.C. Kwon, H.W. Jang, S.Y. Kim, Transition metal disulfide nanosheets synthesized by facile sonication method for the hydrogen evolution reaction, J. Phys. Chem. C 120 (2016) 3929–3935. [16] B. Peng, P.K. Ang, K.P. Loh, Two-dimensional dichalcogenides for light-harvesting applications, Nano Today 10 (2015) 128–137. [17] H. Liu, D. Su, R. Zhou, B. Sun, G. Wang, S.Z. Qiao, Highly ordered mesoporous MoS2 with expanded spacing of the (002) crystal plane for ultrafast lithium ion storage, Adv. Energy Mater. 2 (2012) 970–975. [18] Y. Xue, Q. Zhang, W. Wang, H. Cao, Q. Yang, L. Fu, Opening two-dimensional materials for energy conversion and storage: a concept, Adv. Energy Mater. (2017) 1602684. [19] A.A. Bessonov, M.N. Kirikova, D.I. Petukhov, M. Allen, T. Ryhanen, M.J. Bailey, Layered memristive and memcapacitive switches for printable electronics, Nat. Mater. 14 (2015) 199–204. [20] C. Tan, X. Qi, Z. Liu, F. Zhao, H. Li, X. Huang, L. Shi, B. Zheng, X. Zhang, L. Xie, Z. Tang, W. Huang, H. Zhang, Self-assembled chiral nanofibers from ultrathin lowdimensional nanomaterials, J. Am. Chem. Soc. 137 (2015) 1565–1571. [21] M.M. Rehman, G.U. Siddiqui, J.Z. Gul, S.W. Kim, J.H. Lim, K.H. Choi, Resistive switching in all-printed, flexible and hybrid MoS2-PVA nanocomposite based memristive device fabricated by reverse offset, Sci. Rep. 6 (2016) 36195. [22] X.N. Zhao, Z.Y. Fan, H.Y. Xu, Z.Q. Wang, J.Q. Xu, J.G. Ma, Y.C. Liu, Reversible alternation between bipolar and unipolar resistive switching in Ag/MoS2/Au structure for multilevel flexible memory, J. Mater. Chem. C 6 (2018) 7195–7200. [23] P. Cheng, K. Sun, Y.H. Hu, Memristive behavior and ideal memristor of 1T phase MoS2 nanosheets, Nano Lett. 16 (2016) 572–576. [24] S.M. Shinde, G. Kalita, M. Tanemura, Fabrication of poly(methyl methacrylate)MoS2/graphene heterostructure for memory device application, J. Appl. Phys. 116 (2014). [25] V.K. Sangwan, D. Jariwala, I.S. Kim, K.S. Chen, T.J. Marks, L.J. Lauhon, M.C. Hersam, Gate-tunable memristive phenomena mediated by grain boundaries in single-layer MoS2, Nat. Nanotechnol. 10 (2015) 403–406. [26] H. Li, Z.Y. Yin, Q.Y. He, X. Huang, G. Lu, D.W.H. Fam, A.I.Y. Tok, Q. Zhang, H. Zhang, Fabrication of single- and multilayer MoS2 film-based field-effect transistors for sensing no at room temperature, Small 8 (2012) 63. [27] K.F. Mak, C. Lee, J. Hone, J. Shan, T.F. Heinz, Atomically thin MoS2: a new directgap semiconductor, Phys. Rev. Lett. 105 (2010) 136805. [28] C. Kisielowski, Q.M. Ramasse, L.P. Hansen, M. Brorson, A. Carlsson, A.M. Molenbroek, H. Topsoe, S. Helveg, Imaging MoS2 nanocatalysts with singleatom sensitivity, Angew. Chem. Int. Ed. Engl. 49 (2010) 2708–2710. [29] L. Yang, X. Cui, J. Zhang, K. Wang, M. Shen, S. Zeng, S.A. Dayeh, L. Feng, B. Xiang, Lattice strain effects on the optical properties of MoS2 nanosheets, Sci. Rep. 4 (2014) 5649. [30] A. Sanne, R. Ghosh, A. Rai, M.N. Yogeesh, S.H. Shin, A. Sharma, K. Jarvis, L. Mathew, R. Rao, D. Akinwande, S. Banerjee, Radio frequency transistors and circuits based on CVD MoS2, Nano Lett. 15 (2015) 5039–5045. [31] Y. Wang, Q. Liu, S. Long, W. Wang, Q. Wang, M. Zhang, S. Zhang, Y. Li, Q. Zuo, J. Yang, M. Liu, Investigation of resistive switching in Cu-doped HfO2 thin film for multilevel non-volatile memory applications, Nanotechnology 21 (2010) 045202. [32] Y. Chao Yang, F. Pan, F. Zeng, Bipolar resistance switching in high-performance Cu/ ZnO: Mn/Pt nonvolatile memories: active region and influence of Joule heating, New J. Phys 12 (2010) 023008. [33] Y.J. Baek, Q. Hu, J.W. Yoo, Y.J. Choi, C.J. Kang, H.H. Lee, S.H. Min, H.M. Kim, K.B. Kim, T.S. Yoon, Tunable threshold resistive switching characteristics of PtFe2O3 core-shell nanoparticle assembly by space charge effect, Nanoscale 5 (2013) 772–779. [34] P. Chowdhury, H.C. Barshilia, N. Selvakumar, B. Deepthi, K.S. Rajam, A.R. Chaudhuri, S.B. Krupanidhi, The structural and electrical properties of TiO2 thin films prepared by thermal oxidation, Phys. B: Condens. Matter 403 (2008) 3718–3723. [35] H.-S. Lee, S.G. Choi, H.-J. Choi, S.-W. Chung, H.-H. Park, A study of resistive switching property in Pr0.7Ca0.3MnO3, CaMnO3, and their bi-layer films, Thin Solid Films 529 (2013) 347–351. [36] D. Maa, I.A. Hu, B. Hub, F.E. Karaszb, X. Jingc, L. Wangc, F. Wangc, Determination of electron mobility in a blue-emitting alternating block copolymer by spacecharge-limited current measurements, Solid State Commun. 112 (1999) 251–254. [37] K. Krishnan, T. Tsuruoka, C. Mannequin, M. Aono, Mechanism for conducting filament growth in self-assembled polymer thin films for redox-based atomic switches, Adv. Mater. 28 (2016) 640–648. [38] C. Hu, Q. Wang, S. Bai, M. Xu, D. He, D. Lyu, J. Qi, The effect of oxygen vacancy on switching mechanism of ZnO resistive switching memory, Appl. Phys. Lett. 110 (2017) 073501.
Three different modes of I-V characteristics, i.e. typical bipolar RS, asymmetrical bipolar RS and TS, were observed in one Al/MoS2&PVP/ ITO RS device. The conduction mechanism for both HRS and LRS of all three kinds of switching behavior can be explained by Ohmic conduction and SCLC theory. The switching behaviors are resulted in by the formation and rupture of CFs consisting of sulfur vacancies in MoS2 nanoflakes and sulfur ions in PVP, which form from three different kinds of GBs, intersecting-GBs, bisecting-GBs, and bridge-GBs, respectively. The switching mechanism is proven by the statistical results of the probabilities of typical bipolar RS and TS in devices with and without PVP layer between MoS2&PVP and ITO bottom electrodes. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (No. lzujbky-2017-185, lzujbky-2018-114 and lzujbky-2018-115), National Natural Science Foundation of China (No. 61874051), National Natural Science Foundation of China (No. 61404064 and U1732136), National Natural Science Foundation of China (Nos. 61774079 and 61664001), Opening Project of Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mssp.2018.11.024. References [1] H.T. Sun, Q. Liu, C.F. Li, S.B. Long, H.B. Lv, C. Bi, Z.L. Huo, L. Li, M. Liu, Direct observation of conversion between threshold switching and memory switching induced by conductive filament morphology, Adv. Funct. Mater. 24 (2014) 5679–5686. [2] P.Y. Gu, F. Zhou, J. Gao, G. Li, C. Wang, Q.F. Xu, Q. Zhang, J.M. Lu, Synthesis, characterization, and nonvolatile ternary memory behavior of a larger heteroacene with nine linearly fused rings and two different heteroatoms, J. Am. Chem. Soc. 135 (2013) 14086–14089. [3] S. Ambrogio, S. Balatti, D.C. Gilmer, D. Ielmini, Analytical modeling of oxide-based bipolar resistive memories and complementary resistive switches, IEEE Trans. Electron. Devices 61 (2014) 2378–2386. [4] C. Pan, Y. Ji, N. Xiao, F. Hui, K. Tang, Y. Guo, X. Xie, F.M. Puglisi, L. Larcher, E. Miranda, L. Jiang, Y. Shi, I. Valov, P.C. McIntyre, R. Waser, M. Lanza, Coexistence of grain-boundaries-assisted bipolar and threshold resistive switching in multilayer hexagonal boron nitride, Adv. Funct. Mater. 27 (2017) 1604811. [5] F. Fan, B. Zhang, Y. Cao, Y. Chen, Solution-processable poly(N-vinylcarbazole)covalently grafted MoS2 nanosheets for nonvolatile rewritable memory devices, Nanoscale 9 (2017) 2449–2456. [6] X.-Y. Xu, Z.-Y. Yin, C.-X. Xu, J. Dai, J.-G. Hu, Resistive switching memories in MoS2 nanosphere assemblies, Appl. Phys. Lett. 104 (2014) 033504. [7] J. Wu, H. Li, Z. Yin, H. Li, J. Liu, X. Cao, Q. Zhang, H. Zhang, Layer thinning and etching of mechanically exfoliated MoS2 nanosheets by thermal annealing in air, Small 9 (2013) 3314–3319. [8] J. Liu, Z. Zeng, X. Cao, G. Lu, L.H. Wang, Q.L. Fan, W. Huang, H. Zhang, Preparation of MoS2-polyvinylpyrrolidone nanocomposites for flexible nonvolatile rewritable memory devices with reduced graphene oxide electrodes, Small 8 (2012) 3517–3522. [9] P. Zhang, C. Gao, B. Xu, L. Qi, C. Jiang, M. Gao, D. Xue, Structural phase transition effect on resistive switching behavior of MoS2-polyvinylpyrrolidone nanocomposites films for flexible memory devices, Small 12 (2016) 2077–2084. [10] Y. Gong, Z. Lin, G. Ye, G. Shi, S. Feng, Y. Lei, A.L. Elías, N. Perea-Lopez, R. Vajtai, H. Terrones, Z. Liu, M. Terrones, P.M. Ajayan, Te-assisted low-temperature synthesis of MoS and WS monolayers, ACS Nano (2015). [11] Z. Yin, Z. Zeng, J. Liu, Q. He, P. Chen, H. Zhang, Memory devices using a mixture of MoS₂ and graphene oxide as the active layer, Small 9 (2013) 727–731. [12] X. Hong, J. Liu, B. Zheng, X. Huang, X. Zhang, C. Tan, J. Chen, Z. Fan, H. Zhang, A universal method for preparation of noble metal nanoparticle-decorated transition
251
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Resistive switching behaviors mediated by grain boundaries in one longitudinal Al/MoS2&PVP/ITO device ⁎
Na Baia, Min Xua, Cong Hua, Yaodong Maa, Qi Wanga, Deyan Hea, Jing Qia, , Yingtao Lia,b,
T ⁎⁎
a Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China b Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Resistive switching Threshold switching Switching mechanism MoS2
Nonvolatile memory devices based on 2D layered transition metal dichalcogenides have inspired tremendous amounts of research interests in recent years. In this work, resistive switching (RS) devices based on hybrid molybdenum disulphide and Polyvinylpyrrolidone (MoS2&PVP) is explored. One longitudinal Al/MoS2&PVP/ ITO device exhibits three different modes of I-V characteristics, i.e. typical bipolar RS, asymmetrical bipolar RS and threshold switching (TS), which can be explained by the formation and rupture of conducting filaments (CFs) consisting of sulfur vacancy filaments in MoS2 nanoflakes and sulfur ions filaments in PVP. These CFs form from three different kinds of grain boundaries (GBs), intersecting-GB, bisecting-GB, and bridge-GB in which GBs attach one electrode, parallel to the two electrodes, and attach both electrodes, respectively, corresponding to the three modes of I-V characteristics. These three kinds of GBs coexist in one device and the CF forms randomly from one kind of them during sweeping loops. The mechanism was experimentally proved by our results of the increasing probability of typical bipolar RS in device with one layer of PVP thin film inserted into the interface of MoS2&PVP and ITO and other researchers’ results in all-surface MoS2 single-layer MoS2 devices.
1. Introduction With the technology development in the information age, highperformance memories are urgently needed for large data storages. Among all memories, resistive random access memory (RRAM) has inspired tremendous amounts of research interests over the past years with simple structure, high speed, low power, excellent scalability, high density data and longer data retention time [1–5]. The RRAM is based on the electrically stimulated change of the resistance, i.e. RS, in a metal–insulator–metal (MIM) sandwich structure [4,6], in which the applied voltage between two electrodes leads to the switching between high resistive state (HRS) and low resistive state (LRS). The information can be stored in these two states. The MoS2 is a layered transition metal dichalcogenides with weak interlayer van der Waals coupling, from which the monolayer or fewlayers nanoflakes can be obtained by mechanical exfoliation [7], liquidbased exfoliation [8,9], chemical vapor deposition [10] and lithiumintercalation [11,12]. With the number of layers decreasing, MoS2 varies from an indirect band gap to a direct band gap, the band gap
from 1.2 eV to 1.8 eV [13–15]. MoS2 nanoflakes show good photoelectrochemical stability [16], large specific surface areas [5,17], and unique physicochemical/electronic properties [15]. MoS2 is the most promising materials for energy conversion and storage as well [18]. It has also been utilized in fabricating RS devices [5,9,11,19–24]. Typical bipolar RS phenomenon was observed in devices utilizing poly(N-vinylcarbazole)-covalently grafted MoS2 nanosheets [5], 2H-MoS2-PVP [9], MoS2/GO [11], MoOx/MoS2 [19], chiral MoS2 [20], MoS2-PVA [21], 1T Phase MoS2 [23] and PMMA- MoS2 [24] as switching layers. Write-once-read-many-times memory effect was also observed in 1T@ 2H-MoS2-PVP [9] devices. Most interestingly, three kinds of switching behavior, i.e. typical bipolar RS, asymmetrical bipolar RS, which shows only LRS during negative voltage sweep, and TS, were observed in different devices with planar structure of Au/monolayer MoS2/Au [25]. In this work, coexistence of these three kinds of switching behavior in one longitudinal Al/MoS2&PVP/ITO device was observed. The phenomena can be explained by the formation and rupture of CFs consisting of sulfur vacancies in MoS2 nanoflakes and sulfur ions in PVP, which form randomly from three kinds of GB, i.e. intersecting-GB,
⁎
Corresponding author. Corresponding author at: Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, China. E-mail addresses: [email protected] (J. Qi), [email protected] (Y. Li). ⁎⁎
https://doi.org/10.1016/j.mssp.2018.11.024 Received 25 September 2018; Received in revised form 13 November 2018; Accepted 16 November 2018 1369-8001/ © 2018 Published by Elsevier Ltd.
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
shows that MoS2 powder was a layered materials. The X-ray diffraction (XRD) pattern (Fig. S1b, Supporting Information) indicates that the obtained MoS2 powder is pure hexagonal MoS2-2H, which was also proved by Raman spectra (Fig. S1c, Supporting Information). The Raman results also indicate that the MoS2 powder is bulk material. The characterizations of MoS₂&PVP are shown in Fig. 2. Fig. 2a is the morphology and thickness information of MoS₂ nanoflakes obtained by Atomic Force Microscope (AFM). The nanoflakes are butterfly-like. The height is between 0.8 nm and 18.4 nm, and the average height of which is under 10 nm. Inset shows the layer number distribution of butterfly-like MoS2 nanoflakes, which was obtained from the thickness of MoS2 nanoflakes shown in Fig. 2a divided by 0.8 nm because the thickness of a MoS2 monolayer is normally about 0.8 nm [26]. These nanoflakes are from 1 layer to 23 layers and sixty percent of them are less than 15 layers. The photoluminescence (PL) spectra (Fig. S2a, Supporting Information) and the UV–vis diffuse reflectance spectra (Fig. S2b, Supporting Information) of MoS2&PVP also show that the MoS2 nanoflakes are few-layers and the band gap is 1.52 eV [8,27]. From the optical transmittance and spectrum show in Fig. S2c and Fig. S2d, Supporting Information, it can be obtained that the MoS2&PVP/ ITO/PET device is transparent and the optical transmittance is about 80%. The transmission electron microscopy (TEM) image and the corresponding selected area electron diffraction (SAED) pattern are shown in Fig. 2b and c, respectively. These results indicate that the MoS2 nanoflakes are hexagonal single crystal with only a few layers. The highresolution TEM (HRTEM) image is shown in Fig. 2d, from which it can be known that the interplanar distances are both 0.27 nm, which match well with the plane distance of d(100) and d(010) [28–30], respectively. The corresponding fast Fourier transform (FFT) image in inset reveals the highly crystalline property of MoS2. To investigate the switching behaviors of Al/MoS2&PVP/ITO memory device, the electrical properties of the device was obtained by applying electrical stresses on top Al electrode. Three different kinds of electroforming-free switching behavior, i.e. typical bipolar RS, asymmetrical bipolar RS, and TS, can be randomly observed in one device at size of 200 µm in diameter, at the current compliance of 10 µA during the voltage sweeping, as shown in Fig. 3. Fig. 3a is a typical bipolar RS. When the voltage is swept from 0 V to 3 V, the current increased abruptly at 1.17 V, which switches the device from HRS to LRS and corresponds to the SET process. When the voltage is swept back from 3 V to 0 V, the device kept at LRS even after the power is off (the applied voltage is 0 V). For the subsequent negative sweep from 0 V to −3 V, the current decreases abruptly at −0.98 V and the device switches from LRS to HRS, and this corresponds to the RESET process. After RESET, the device keeps at HRS until next SET process. Fig. 3b shows the asymmetrical bipolar RS. The device is SET at 1.47 V during the voltage sweep of 0 V ~3 V ~0 V and RESET during the sweep of 0 V ~− 3–0 V. The most obvious difference between typical bipolar RS and asymmetrical bipolar RS is negative voltage sweeps; (e), (f) TS for the positive and negative voltage sweeps, respectively. that there is no abrupt current change in asymmetrical bipolar RS when the negative swept from 0 V to −3 V and back to 0 V, i.e. no particular RESET voltage. However, the device is switched back to HRS after the voltage sweeping from −3 V back to 0 V, and keeps at HRS until next SET process. Both HRS and LRS can be maintained when the power is off for typical bipolar RS
bisecting-GB, and bridge-GB, during voltage sweeping loops. This switching mechanism is similar to the one proposed by Vinod K. Sangwan et al. [25], and further proved by our experiment results of the increased probability of typical bipolar RS and the decreased probability of TS in Al/MoS2&PVP/PVP/ITO device. The results of this paper offer another way to improve the stability of the MoS2-based resisitive devices, which may provide possible application in neuromorphic computation. 2. Experiment 2.1. Preparation of MoS2&PVP nanocomposites The MoS2 bulk were synthesized through a hydrothermal method. First, 1.56 g CS(NH₂) and 1.21 g Na₂MoO₄•2H₂O were dissolved in 60 ml deionized water under room temperature. Second, the final solution was transferred into a 100 ml Teflon-lined stainless steel autoclave and maintained at 220 °C for 30 h. Third, the solution was cooled down to room temperature, and washed with deionized water and ethyl alcohol for five times. Then, the solution was dried in vacuum to get the desired MoS₂ powder. Finally, MoS2 powders were grounded for 2 h in a mortar in order to get small size MoS2 (Fig. 1a). The 40 mg small size MoS2 powder and 160 mg PVP was mixed and dispersed in 8 ml ethyl alcohol. The MoS2 nanoflakes were obtained from the mixture of bulk MoS2 powder and PVP in ethyl alcohol by sonication for 14 h. The stable dispersion solution of MoS2&PVP (Fig. 1b) was obtained by collecting the supernatant of the mixture centrifuging at 4000 rpm for 40 min. The solution of PVP was prepared by the same method. PVP of 160 mg was dispersed in 8 ml ethyl alcohol and sonicated for 14 h. The stable solution of PVP was obtained by collecting the supernatant of the mixture centrifuging at 4000 rpm for 40 min. 2.2. Fabrication of memory device The devices have the sandwich structure of the Al top electrode, MoS2&PVP resistive layer and the ITO bottom electrode coated on the PET with size of 1 × 1 cm2. ITO/PET substrate had been cleaned with acetone, ethyl alcohol and deionized water for 15 min, respectively. Then, the MoS2&PVP solution was spin-coated onto the ITO at 500 rpms for 15 s and 2000 rpms for 20 s. The film was annealed in air at 70 °C for 30 mins. Finally, Al top electrode was evaporated by E-beam evaporator to fabricate the Al/MoS2&PVP/ITO device (Fig. 1c). Al/MoS2&PVP/ PVP/ITO device was fabricated by the same method. The PVP solution was spin-coated onto the ITO at 500 rpms for 15 s and 2000 rpms for 20 s and was annealed in air at 70 °C for 30 mins before MoS2&PVP spin-coating. 3. Result and discussion Fig. 1d shows the scanning electron microscope (SEM) image of the cross section for the MoS2&PVP/ITO layers. It can be observed that the thickness of the MoS2&PVP and ITO layers are ~70 nm and ~185 nm, respectively. The SEM image of MoS2 powder (Fig. S1a, Supporting Information)
Fig. 1. The schematic preparation process of Al/MoS2&PVP/ITO devices. (a) The MoS2 powder. (b) MoS2&PVP nanosheets. (c) The device structure. (d) The SEM image of the cross section for the MoS2&PVP/ITO layers. 247
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
Fig. 2. Characterizations of MoS₂&PVP nanocomposites. (a) AFM image, inset: the layer number distribution. (b) The TEM image. (c) The SEAD image. (d) HRTEM image. Inset of (d): FFT image.
with increases in voltage bias, as shown in Fig. 4a. For low applied voltage of HRS state, log I vs. log V is linear with a slope of 1.02 (~1), which corresponds to Ohmic conducting mechanism described as
and asymmetrical bipolar RS, which are suitable for nonvolatile memory application [31]. Fig. 3c is I–V characteristics of TS. When the voltage is swept from 0–3 V, the current increases at 1.35 V suddenly. The device is switched to LRS and kept at LRS during the voltage sweeping of 3 V ~ 0 V. However, the LRS cannot be maintained after the power is off. After the sweeping of 3–0 V, whether the voltage is swept from 0 V to −3 V or from 0 V to 3 V, the device is at HRS until next SET process. The behavior during the sweep of 0 V–3 V–0 V is similar to that during the sweep of 0 V–3 V–0 V. This kind of switching cannot be utilized in nonvolatile memory [32]. And the device undergoes a transition from the HRS to LRS at high voltage and goes back to HRS at a low voltage which can be utilized as a selection device [33]. However, the pure MoS2 film has no RS behavior at low bias sweep [5,11]. and no switching phenomenon was observed in the pure pvp film-based device either [8], as shown in Fig. S3, Supporting information. To understand the conduction mechanisms of the three kinds of switching behavior, double-logarithmic I-V curves were plotted, as shown in Fig. 4a–f. The slopes of the double-logarithmic I-V curve in positive voltage sweep for typical bipolar RS vary from ~1 to ~2 to ~5
J = Vexp
{ −kTΔE } ae
(1)
where ΔEae is the activation energy of electrons, k is the Boltzmann constant, and T is the temperature [34]. As the applied voltage increases, the slope of the fitting line varies from 1.02 to 1.72 (~ 2). The slope of ~ 2 can be described by SCLC theory [8, [34] where the relation of current and the applied voltage is I ∝ V 2 . As the applied voltage further increases to 0.49 V, the slope varies to 5.12, which can be described by trap-filled SCLC (I ∝ V a, a > 2 ) [35,36]. Deep traps in SCLC are formed because of the energy level difference between PVP and MoS2 nanosheets according to the energy band diagram of the Al/ MoS2&PVP/ITO device shown in Fig. S4, Supporting Information. And when the applied voltage increases to 1.17 V, SET process occurred and the device is switched to LRS. At LRS, the slopes of the fitting line are 1.21 and 1.58 at low and high voltage respectively, which indicates that the device is controlled by SCLC at high voltage and Ohmic conduction
Fig. 3. I-V Characteristics of Al/PVP&MoS2/ITO device. (a) Typical bipolar RS, (b) asymmetrical bipolar RS, and (c) TS. 248
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
Fig. 4. The double-logarithmic I-V curves for (a), (b) the typical bipolar RS mode for the positive and negative voltage sweeps; (c), (d) the asymmetrical bipolar RS for the positive and negative voltage sweeps; (e), (f) TS for the positive and negative voltage sweeps, respectively.
and SCLC theory. The slopes at LRS are from about 1–2 for positive sweep and about 1 for negative sweep, indicating that Ohmic conduction and SCLC dominate for positive sweep and Ohmic conduction dominates for negative sweep. The positive SET and negative SET processes occur at 1.35 V and 1.11 V, respectively. According to the above conduction mechanism of SCLC and Ohmic conduction, the three kinds of switching behavior in Al/MoS2&PVP/ITO device can be explained by the formation and rupture of CFs [25] consisting of sulfur vacancies in MoS2 nanoflakes and sulfur ions in PVP. The CFs in typical bipolar RS, asymmetrical bipolar RS, and TS form randomly from three different kinds of GBs defect states, respectively, i.e. intersecting-GBs, bisecting-GBs, and bridge-GBs in which GBs of the MoS2 nanoflakes attach one electrode, parallel to the two electrodes, and attach both electrodes. Higher concentration of sulfur vacancies are distributed around the GBs [25]. The three GBs coexist in one device and compete with each other during the formation of CFs because MoS2 nanoflakes are uniformly dispersed in PVP and the direction of GBs and MoS2 nanoflakes are randomly. If the CFs form from intersecting-GBs, the device exhibits typical bipolar RS behavior. If the CFs form from bisecting-GB, the device exhibits asymmetrical bipolar RS behavior. And if the CFs form from bridge-GB, the device shows TS behavior. Similar results were observed in all-surface single-layer MoS2 devices by
at low voltage. The slopes of the LRS double-logarithmic I-V curve of typical bipolar RS mode in negative voltage sweeps shown in Fig. 4b vary from 1.42 (~1) to 1.68 (~2) with increases in voltage bias, indicating Ohmic conducting and SCLC theory dominating. When the applied voltage increases to 0.98 V, RESET process occurred and the device is switched to HRS. At HRS, the slopes of the fitting line are 1.64 and 1.24 at high and low voltage respectively, which indicates that the device is still controlled by SCLC and Ohmic conduction. Fig. 4c and d are the double-logarithmic I-V curve of asymmetrical bipolar RS. The slopes in both positive and negative voltage sweeps vary from ~1 to ~2 with increases in voltage bias. The HRS of device is also controlled by SCLC and Ohmic conduction, according to the slopes of 1.75 (~2) and 1.15 (~1) shown in Fig. 4c. SET process occurs at 1.47 eV. Log I vs. log V is linear with slopes of 0.97 (~1) and 0.99 (~1) at LRS, corresponding to Ohmic conducting mechanism, which indicates that the charges were trapped in MoS2. The double-logarithmic I-V curves shown in Fig. 4d indicate that the device is controlled by SCLC because the slopes are 1.94 (~2) in whole negative voltage sweeps. The doublelogarithmic I- V curves of threshold RS in both positive and negative voltage sweeps are shown in Fig. 4e and f. The slopes vary from ~1 to ~2 with increases in voltage bias at HRS for both positive and negative voltage sweeps. The conduction is still controlled by Ohmic conduction 249
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
Fig. 5. Schematics of switching mechanism for three kinds of switching behavior in one Al/ MoS2&PVP/ITO device. (a) Initial state and HRS for the device. The intersecting-GB, bisecting-GB and bridge-GB coexist in one device. (b) LRS for typical bipolar RS. The CFs form from intersecting-GB. (c) LRS of asymmetrical bipolar RS. The CFs form from bisecting-GB. (d) LRS of TS. The CFs form from bridge-GB.
generation. The device is RESET back to HRS as shown in Fig. 5a. If the CFs form from bridge-GB, the device exhibits TS as shown in Fig. 3c. The bridge-GB are high resistive because the sulfur vacancies around bridge-GB are very sparse. When the voltage sweeps from 0 to 3 V, the sulfur vacancies are generated and migrate toward the bridge-GB, resulting in decrease of the resistance of the device. When the voltage reaches 1.38 V, the density of sulfur vacancies reaches maximum and the device is SET to LRS, as shown in Fig. 5d. When the voltage sweeps from 3 V to 0 V, the rupture of CFs is caused by the Joule heating, which causes the HRS [25,32], as shown in Fig. 5a. It is similarwhen the voltage sweeps from 0 V to − 3 V and − 3–0 V. The above switching mechanism was proved experimentally by Sangwan et al. [25] in allsurface MoS2 single layer device. To prove that it is the same mechanism controlling the switching behavior in our MoS2&PVP longitudinal devices, a layer of PVP was inserted between MoS2&PVP and ITO bottom electrodes to increase the probability of intersecting-GBs. The structure of the device is Al/MoS2&PVP/PVP/ITO. The typical I-V characteristics of Al/MoS2&PVP/PVP/ITO devices is shown in Fig. S5, Supporting Information. In this case, to trigger repeatable typical bipolar RS behavior in the fresh device, an electroforming process with positive voltage on Al electrode and the same current compliance (ICC = 10 μA) are required. The probability of three kinds of switching behavior were counted and analyzed. The results are presented in Table 1. From the table it can be observed that the probabilities of typical bipolar RS, asymmetrical bipolar RS and TS are 16%, 24%, and 60%, respectively for Al/MoS2&PVP/ITO device, while they are 60%, 14% and 26% for Al/MoS2&PVP/PVP/ITO device. The probability of typical bipolar RS increases from 16% to 60% while that of TS decreases from 60% to 26% after inserting a layer of PVP. These results indicate that the probability of intersecting-GB increases while the probability of bisecting-GB decreases by inserting a PVP layer. Technically, the probability of RS should increase to at least 86% and that of TS should decrease to 0%. However, in the procedure of the experiment, it is very difficult to insert a layer of pure PVP film into device because MoS2 nanoflakes and PVP are very easy to be mixed together during spincoating. The results of probability statics indicate that the mechanism of GB-assisted CFs formation and rupture does control the switching behavior in our case.
Sangwan et al. [25]. Fig. 5 is the switching mechanism schematic of the three modes of switching behavior. at initial states and HRS, intersecting-GBs, bisecting-GBs and bridge-GBs coexist in one device. The sulfur vacancies in MoS2 have been shown to accumulate near GBs and the electrons donated by dangling bonds should render the MoS2 regions nears GBs more conductive [25]. The self-assembled structures of the PVP matrix create interconnected polymer chains, which provide the long-range ion transport pathways [37], as shown in Fig. 5a. During voltage sweep, sulfur ions can be pulled out from their equilibrium positions and migrate in MoS2 nanoflakes and PVP chains, which is similar to the generation and migration of oxygen vacancies in the ZnO material devices [38]. Sulfur ions being pulled out from their equilibrium positions results in the generation of sulfur vacancies in MoS2 nanoflakes. When the voltage sweeps from 0 to 3 V, sulfur ions are pulled out from their equilibrium positions and migrate in MoS2 and PVP continually. The density of sulfur vacancies and the migration of sulfur ions increase with the increase of sweeping voltage. If the CFs form from intersectingGB, when the sweep voltage increases to 1.17 V, as shown in Fig. 3a, the density of sulfur vacancies reaches the saturation density, sulfur ions migrate into PVP, which result in the connecting of the GBs and the bottom electrode by the CFs consisting of sulfur vacancies in MoS2 nanoflakes and sulfur ions in PVP, as shown in Fig. 5b. And the device is switched from HRS to LRS, which is corresponding to SET process. After that, the density keeps at the saturation value until negative voltage is applied. The sulfur ions in PVP act as charge traps, so currents at LRS for both positive and negative are controlled by Ohmic conduction and SCLC. When the voltage sweeps from 0 to −3 V, the trapped charges are detrapped, and the sulfur ions in PVP are pulled back into the equilibrium positions in MoS2 nanoflakes, and the CFs fracture, which switches the device from LRS to HRS and is corresponding to the RESET process. The device goes back to the initial state shown in Fig. 5a. If the CFs form from bisecting-GB, the device exhibits the asymmetrical bipolar RS shown in Fig. 3b. When the sweep voltage increases to 1.47 V, the density of sulfur vacancies reaches the maximum, and the sulfur ions drift (driven by the electric field) and diffuse (driven by the concentration gradient) obeying the dynamic and nonlinear relation to form sulfur-ion-rich and sulfur-vacancy-rich regions on either side of the bisecting-GB [25]. The CFs consisting of sulfur vacancies in MoS2 and sulfur ions in PVP form and the device is SET from HRS to LRS as shown in Fig. 5c. Under the negative bias, the sulfur ions drift and diffuse from sulfur-ion-rich region back to sulfur-vacancy-rich region. When the rate of sulfur ions drift is equal to that of sulfur vacancies generate, the filaments will not rupture and the device still maintains high conductive, which causes LRS and HRS overlap to each other in the reset section. With the decrease of voltage, the rate of sulfur ions drift greatly increases and exceeds the rate of sulfur vacancies
Table 1 The results of probability statics.
250
Device structure
Typical bipolar RS
Asymmetrical bipolar RS
TS
Al/MoS2&PVP/ITO/PET Al/MoS2&PVP/PVP/ITO/ PET
16% 60%
24% 14%
60% 26%
Materials Science in Semiconductor Processing 91 (2019) 246–251
N. Bai et al.
4. Conclusion
metal dichalcogenide nanobelts, Adv. Mater. 26 (2014) 6250–6254. [13] H.S. Lee, S.W. Min, Y.G. Chang, M.K. Park, T. Nam, H. Kim, J.H. Kim, S. Ryu, S. Im, MoS2 nanosheet phototransistors with thickness-modulated optical energy gap, Nano Lett. 12 (2012) 3695–3700. [14] J.S. Kim, H.W. Yoo, H.O. Choi, H.T. Jung, Tunable volatile organic compounds sensor by using thiolated ligand conjugation on MoS2, Nano Lett. 14 (2014) 5941–5947. [15] T.P. Nguyen, S. Choi, J.-M. Jeon, K.C. Kwon, H.W. Jang, S.Y. Kim, Transition metal disulfide nanosheets synthesized by facile sonication method for the hydrogen evolution reaction, J. Phys. Chem. C 120 (2016) 3929–3935. [16] B. Peng, P.K. Ang, K.P. Loh, Two-dimensional dichalcogenides for light-harvesting applications, Nano Today 10 (2015) 128–137. [17] H. Liu, D. Su, R. Zhou, B. Sun, G. Wang, S.Z. Qiao, Highly ordered mesoporous MoS2 with expanded spacing of the (002) crystal plane for ultrafast lithium ion storage, Adv. Energy Mater. 2 (2012) 970–975. [18] Y. Xue, Q. Zhang, W. Wang, H. Cao, Q. Yang, L. Fu, Opening two-dimensional materials for energy conversion and storage: a concept, Adv. Energy Mater. (2017) 1602684. [19] A.A. Bessonov, M.N. Kirikova, D.I. Petukhov, M. Allen, T. Ryhanen, M.J. Bailey, Layered memristive and memcapacitive switches for printable electronics, Nat. Mater. 14 (2015) 199–204. [20] C. Tan, X. Qi, Z. Liu, F. Zhao, H. Li, X. Huang, L. Shi, B. Zheng, X. Zhang, L. Xie, Z. Tang, W. Huang, H. Zhang, Self-assembled chiral nanofibers from ultrathin lowdimensional nanomaterials, J. Am. Chem. Soc. 137 (2015) 1565–1571. [21] M.M. Rehman, G.U. Siddiqui, J.Z. Gul, S.W. Kim, J.H. Lim, K.H. Choi, Resistive switching in all-printed, flexible and hybrid MoS2-PVA nanocomposite based memristive device fabricated by reverse offset, Sci. Rep. 6 (2016) 36195. [22] X.N. Zhao, Z.Y. Fan, H.Y. Xu, Z.Q. Wang, J.Q. Xu, J.G. Ma, Y.C. Liu, Reversible alternation between bipolar and unipolar resistive switching in Ag/MoS2/Au structure for multilevel flexible memory, J. Mater. Chem. C 6 (2018) 7195–7200. [23] P. Cheng, K. Sun, Y.H. Hu, Memristive behavior and ideal memristor of 1T phase MoS2 nanosheets, Nano Lett. 16 (2016) 572–576. [24] S.M. Shinde, G. Kalita, M. Tanemura, Fabrication of poly(methyl methacrylate)MoS2/graphene heterostructure for memory device application, J. Appl. Phys. 116 (2014). [25] V.K. Sangwan, D. Jariwala, I.S. Kim, K.S. Chen, T.J. Marks, L.J. Lauhon, M.C. Hersam, Gate-tunable memristive phenomena mediated by grain boundaries in single-layer MoS2, Nat. Nanotechnol. 10 (2015) 403–406. [26] H. Li, Z.Y. Yin, Q.Y. He, X. Huang, G. Lu, D.W.H. Fam, A.I.Y. Tok, Q. Zhang, H. Zhang, Fabrication of single- and multilayer MoS2 film-based field-effect transistors for sensing no at room temperature, Small 8 (2012) 63. [27] K.F. Mak, C. Lee, J. Hone, J. Shan, T.F. Heinz, Atomically thin MoS2: a new directgap semiconductor, Phys. Rev. Lett. 105 (2010) 136805. [28] C. Kisielowski, Q.M. Ramasse, L.P. Hansen, M. Brorson, A. Carlsson, A.M. Molenbroek, H. Topsoe, S. Helveg, Imaging MoS2 nanocatalysts with singleatom sensitivity, Angew. Chem. Int. Ed. Engl. 49 (2010) 2708–2710. [29] L. Yang, X. Cui, J. Zhang, K. Wang, M. Shen, S. Zeng, S.A. Dayeh, L. Feng, B. Xiang, Lattice strain effects on the optical properties of MoS2 nanosheets, Sci. Rep. 4 (2014) 5649. [30] A. Sanne, R. Ghosh, A. Rai, M.N. Yogeesh, S.H. Shin, A. Sharma, K. Jarvis, L. Mathew, R. Rao, D. Akinwande, S. Banerjee, Radio frequency transistors and circuits based on CVD MoS2, Nano Lett. 15 (2015) 5039–5045. [31] Y. Wang, Q. Liu, S. Long, W. Wang, Q. Wang, M. Zhang, S. Zhang, Y. Li, Q. Zuo, J. Yang, M. Liu, Investigation of resistive switching in Cu-doped HfO2 thin film for multilevel non-volatile memory applications, Nanotechnology 21 (2010) 045202. [32] Y. Chao Yang, F. Pan, F. Zeng, Bipolar resistance switching in high-performance Cu/ ZnO: Mn/Pt nonvolatile memories: active region and influence of Joule heating, New J. Phys 12 (2010) 023008. [33] Y.J. Baek, Q. Hu, J.W. Yoo, Y.J. Choi, C.J. Kang, H.H. Lee, S.H. Min, H.M. Kim, K.B. Kim, T.S. Yoon, Tunable threshold resistive switching characteristics of PtFe2O3 core-shell nanoparticle assembly by space charge effect, Nanoscale 5 (2013) 772–779. [34] P. Chowdhury, H.C. Barshilia, N. Selvakumar, B. Deepthi, K.S. Rajam, A.R. Chaudhuri, S.B. Krupanidhi, The structural and electrical properties of TiO2 thin films prepared by thermal oxidation, Phys. B: Condens. Matter 403 (2008) 3718–3723. [35] H.-S. Lee, S.G. Choi, H.-J. Choi, S.-W. Chung, H.-H. Park, A study of resistive switching property in Pr0.7Ca0.3MnO3, CaMnO3, and their bi-layer films, Thin Solid Films 529 (2013) 347–351. [36] D. Maa, I.A. Hu, B. Hub, F.E. Karaszb, X. Jingc, L. Wangc, F. Wangc, Determination of electron mobility in a blue-emitting alternating block copolymer by spacecharge-limited current measurements, Solid State Commun. 112 (1999) 251–254. [37] K. Krishnan, T. Tsuruoka, C. Mannequin, M. Aono, Mechanism for conducting filament growth in self-assembled polymer thin films for redox-based atomic switches, Adv. Mater. 28 (2016) 640–648. [38] C. Hu, Q. Wang, S. Bai, M. Xu, D. He, D. Lyu, J. Qi, The effect of oxygen vacancy on switching mechanism of ZnO resistive switching memory, Appl. Phys. Lett. 110 (2017) 073501.
Three different modes of I-V characteristics, i.e. typical bipolar RS, asymmetrical bipolar RS and TS, were observed in one Al/MoS2&PVP/ ITO RS device. The conduction mechanism for both HRS and LRS of all three kinds of switching behavior can be explained by Ohmic conduction and SCLC theory. The switching behaviors are resulted in by the formation and rupture of CFs consisting of sulfur vacancies in MoS2 nanoflakes and sulfur ions in PVP, which form from three different kinds of GBs, intersecting-GBs, bisecting-GBs, and bridge-GBs, respectively. The switching mechanism is proven by the statistical results of the probabilities of typical bipolar RS and TS in devices with and without PVP layer between MoS2&PVP and ITO bottom electrodes. Acknowledgements This work was supported by the Fundamental Research Funds for the Central Universities (No. lzujbky-2017-185, lzujbky-2018-114 and lzujbky-2018-115), National Natural Science Foundation of China (No. 61874051), National Natural Science Foundation of China (No. 61404064 and U1732136), National Natural Science Foundation of China (Nos. 61774079 and 61664001), Opening Project of Key Laboratory of Microelectronic Devices & Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.mssp.2018.11.024. References [1] H.T. Sun, Q. Liu, C.F. Li, S.B. Long, H.B. Lv, C. Bi, Z.L. Huo, L. Li, M. Liu, Direct observation of conversion between threshold switching and memory switching induced by conductive filament morphology, Adv. Funct. Mater. 24 (2014) 5679–5686. [2] P.Y. Gu, F. Zhou, J. Gao, G. Li, C. Wang, Q.F. Xu, Q. Zhang, J.M. Lu, Synthesis, characterization, and nonvolatile ternary memory behavior of a larger heteroacene with nine linearly fused rings and two different heteroatoms, J. Am. Chem. Soc. 135 (2013) 14086–14089. [3] S. Ambrogio, S. Balatti, D.C. Gilmer, D. Ielmini, Analytical modeling of oxide-based bipolar resistive memories and complementary resistive switches, IEEE Trans. Electron. Devices 61 (2014) 2378–2386. [4] C. Pan, Y. Ji, N. Xiao, F. Hui, K. Tang, Y. Guo, X. Xie, F.M. Puglisi, L. Larcher, E. Miranda, L. Jiang, Y. Shi, I. Valov, P.C. McIntyre, R. Waser, M. Lanza, Coexistence of grain-boundaries-assisted bipolar and threshold resistive switching in multilayer hexagonal boron nitride, Adv. Funct. Mater. 27 (2017) 1604811. [5] F. Fan, B. Zhang, Y. Cao, Y. Chen, Solution-processable poly(N-vinylcarbazole)covalently grafted MoS2 nanosheets for nonvolatile rewritable memory devices, Nanoscale 9 (2017) 2449–2456. [6] X.-Y. Xu, Z.-Y. Yin, C.-X. Xu, J. Dai, J.-G. Hu, Resistive switching memories in MoS2 nanosphere assemblies, Appl. Phys. Lett. 104 (2014) 033504. [7] J. Wu, H. Li, Z. Yin, H. Li, J. Liu, X. Cao, Q. Zhang, H. Zhang, Layer thinning and etching of mechanically exfoliated MoS2 nanosheets by thermal annealing in air, Small 9 (2013) 3314–3319. [8] J. Liu, Z. Zeng, X. Cao, G. Lu, L.H. Wang, Q.L. Fan, W. Huang, H. Zhang, Preparation of MoS2-polyvinylpyrrolidone nanocomposites for flexible nonvolatile rewritable memory devices with reduced graphene oxide electrodes, Small 8 (2012) 3517–3522. [9] P. Zhang, C. Gao, B. Xu, L. Qi, C. Jiang, M. Gao, D. Xue, Structural phase transition effect on resistive switching behavior of MoS2-polyvinylpyrrolidone nanocomposites films for flexible memory devices, Small 12 (2016) 2077–2084. [10] Y. Gong, Z. Lin, G. Ye, G. Shi, S. Feng, Y. Lei, A.L. Elías, N. Perea-Lopez, R. Vajtai, H. Terrones, Z. Liu, M. Terrones, P.M. Ajayan, Te-assisted low-temperature synthesis of MoS and WS monolayers, ACS Nano (2015). [11] Z. Yin, Z. Zeng, J. Liu, Q. He, P. Chen, H. Zhang, Memory devices using a mixture of MoS₂ and graphene oxide as the active layer, Small 9 (2013) 727–731. [12] X. Hong, J. Liu, B. Zheng, X. Huang, X. Zhang, C. Tan, J. Chen, Z. Fan, H. Zhang, A universal method for preparation of noble metal nanoparticle-decorated transition
251