Controlling Resistive Switching by Using an Optimized MoS2

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage 2

Controlling resistive switching by using optimized MoS interfacial layer and the role of top electrodes on ascorbic acid sensing in TaO-based RRAM x

Jian-Tai Qiu, Subhranu Samanta, Mrinmoy Dutta, Sreekanth Ginnaram, and Siddheswar Maikap Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04090 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 25, 2019

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Controlling resistive switching by using optimized MoS2 interfacial layer and the role of top electrodes on ascorbic acid sensing in TaOx-based RRAM Jiantai Timothy Qiu┴,€, Subhranu Samanta†, Mrinmoy Dutta†, Sreekanth Ginnaram† and Siddheswar Maikap*,†,┴

€Department

Taiwan

of Biomedical Sciences, School of Medicine, Chang Gung University (CGU), Tao-Yuan 33302,

┴Division

of Gynecology/Oncology, Department of Obstetrics/Gynecology, Chang Gung Memorial Hospital (CGMH), Linkou, Tao-Yuan, 33302, Taiwan †Thin

Film Nano Tech. Lab., Department of Electronic Engineering, Chang Gung University, 59 Wen-Hwa 1st Rd., Kwei-Shan, Tao-Yuan, 33302, Taiwan

*Corresponding

authors: E-mail: [email protected]; Tel: 886-32118800 ext. 5785

ABSTRACT: Controlled resistive switching by using optimized 2 nm-thick MoS2 interfacial layer and the role of top electrodes (TEs) on ascorbic acid (AA) sensing in TaOx based RRAM platform have been investigated for the first time. Both the high-resolution transmission-electron microscope (HRTEM) image and depth profile by energy dispersive X-ray spectroscopy (EDS) confirm the presence of each layer in IrOx/Al2O3/TaOx/MoS2/TiN structure. The pristine device including the IrOx TE with 2 nm-thick interfacial layer shows highest uniform rectifying dc endurance of > 1000 cycles, large rectifying ratio (RR) of >3.2×104, and high non-linearity factor (NF) of > 700 is obtained than those of the Pt and Ru TEs. After forming, this IrOx device produces bipolar resistive switching characteristics and long program/erase (P/E) endurance of > 107 cycles at low operation current of 100 complementary resistive switching (CRS) as well as long P/E endurance of >106 cycles are obtained. Schottky barrier height modulation at a low field is observed owing to reduction-oxidation of the TE, which is evidenced through reversible AA detection. At higher field, Fowler-Nordheim (F-N) tunneling and hopping conduction are observed.

Ascorbic

acid

detection

with

a

low

concentration

of

1

pM

by

using

porous-

IrOx/Al2O3/TaOx/MoS2/TiN RRAM device directly is an additional novelty of this work, which is useful in future early diagnosis of scurvy diseases. ACS Paragon Plus Environment

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§ INTRODUCTION The resistive switching characteristics in metal-insulator-metal (MIM) structure have shown a keen attention as next generation resistive random access memory (RRAM) to change conventional non-volatile memory in future. Considering all emerging non-volatile memories, an intensive research and development on RRAM have been triggered owing to its simple structure, small feature size, low power consumption, ultrafast switching speed, impressive retention capability than others.1, 2 Several research groups have chosen various materials to possess resistive switching characteristics such as HfOx,3, 4 TaOx,5, 6 SiO2,7, 8 Al2O3 etc.9 Among them, TaOx is considered as one of the most important materials owing to its compatibility of complementary metal-oxide-semiconductor (CMOS) application, higher thermal stability up to 1000o C, high dielectric constant (k ~ 16-60), moderate Gibbs free energy (-760.5 kJ/mole at 300 K), metastable phase of TaO2, and stable phase of Ta2O5.5,

6, 10-12

Laterally, molybdenum disulfide (MoS2) as inorganic transition metal di-

chalcogenide (TMD) has drawn much attention in resistive switching memory application due to its some distinctive features like variable band gap semiconductor (Eg~1.23 eV for indirect band gap with bulk MoS2 and Eg ~ 1.8 eV for direct band gap with mono layer MoS2), high electron affinity (χMoS2~4.3 eV), wide range of dielectric constant (k~ 4-17), composed of covalently bonded S-Mo-S structures, and chemically inertness.13-15 Recently, RRAM device including MoS2 switching material has been reported by a few groups.16-20 Bessonov et al. have reported the bipolar resistive switching (BRS) by using Ag/MoOx/MoS2/Ag structure at a current compliance (CC) of 5 mA.16 Cheng et al. have executed bipolar memristive behavior in Ag/MoS2/Ag structure at a CC of 300 mA and the CRS characteristics with a current of approximately 1.5 mA.17 Shin et al. have presented multilevel resistive switching at > 1 mA current level by using Au/GO/MoS2/GO/Al structure.18 Xia et al. have demonstrated effect of different electrodes (Ag, Cu, Ti, Al) in metal/MoS2/Ti RRAM structure at a CC of 1 mA.19 Resistive switching characteristics have been studied by using MoS2 based quantum dot/nano-sphere at CC of > 1 mA.20 However, all reports contain only BRS phenomena at higher CC level of ≥1 mA and a few P/E cycles is also reported, which is one of the most primitive hindrances for practical realization. Beside the BRS characteristics, diode-like rectifying switching 7, 21

and CRS22 are also very salient features for selector cell as well as passive cross-point array application to

resolve sneak path current issue. Recently, researchers have reported rectifying and CRS characteristics in TaOx based different RRAM structures.5, 6, 23-25 Egorov et al. have reported P/E endurance up to 3×103 cycles at

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2 mA current level by using Al2O3 interfacial layer in TiN/Ta2O5/Al2O3/TiN RRAM stack.26 Bai et al. have demonstrated multi-level performances in Pt/AlOδ/Ta2O5-x/TaOy/Pt stack.27 Interface interactions/modifications and their impact on resistive switching for both conductive bridging and valance change memory have been discussed in several recent papers.28, 29 Especially the role of graphene (similar to MoS2 is a 2D material) and a-C modified interfaces on the switching characteristics has been also reported. There for first time has been suggested that the interface barrier (Graphene) is blocking the oxygen reaction.28 There is a further review on effects of interface modifications and their effects on RRAMs.30 In this study, we are using the Al2O3 barrier layer. Therefore, a simultaneous study of rectifying and BRS/CRS characteristics in a TaOx based single RRAM platform are demanded, which is very tough to achieve. Another key challenge in RRAM is the proper selection of top electrode material.31, 32 Among different electrode metals, IrOx is promising one owing to its inertness (Gibbs free energy of -183.75 kJ mol-1 at 300 K) than the Ru (Gibbs free energy of -285.7 kJ mol-1 at 300 K)33, higher work function (θIrOx~5.6 eV), good thermal stability, better conductivity, superior reductionoxidation capability with splendid porosity which make it able to sense divergent bio-analytes.9,

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Additionally, effect of another two TE metals say, Pt (θPt ~5.65 eV and Gibbs free energy of -164.4 kJ mol-1 at 300 K) 35, 36 and Ru (θRu ~4.71 eV )35 is also examined. In addition, ascorbic acid (AA) i.e., vitamin C is a water soluble essential vitamin in human body.37 AA is also referred as antioxidant, which helps to form cartilage, blood vessel, healthy skin, muscles in our body. Deficiency of AA causes scurvy like degenerative diseases. However, higher amount of vitamin C is harmful and causes diarrhea, nausea, vomiting, headache, etc. Hence, it is also important to maintain the normal range of AA (0.6-2 mg/dL) in our body.37 Some researchers have applied other techniques like differential pulse voltammetry,38 transmittance colorimetric,39, 40 fluorescence,41 and electrochemical impedance spectroscopy (EIS)42 method to detect AA within sensing range of 2.6 pM – 300 µM. However, low concentration detection of AA with short detection time is very important to diagnostic of human diseases at early stage. Hence, our additional novel achievement is to detect AA through simple IrOx/Al2O3/TaOx/MoS2/TiN resistive switching memory directly at lower concentration (1 pM) with reduced sample volume as well. In this scenario, we have presented the controlled resistive switching by optimized MoS2 interfacial layer and the role of top electrodes (IrOx, Pt, Ru) in TaOx based RRAM. Transmission electron microscopy (TEM) image of IrOx/Al2O3/TaOx/MoS2/TiN device with a size of 0.4 ×0.4 µm2 confirms the presence of each


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layer. The pristine device shows successive self-rectifying diode-like current -voltage (I-V) characteristics of > 1000 cycles. High work function difference between top TE and titanium-nitride (TiN) bottom electrode (BE) helps in producing asymmetric I-V characteristics. The MoS2 interfacial layer plays the role as surface confined quantum well (SCQW) and controls electronic injection/transmission. This structure executes both BRS and CRS characteristics. Reduction-oxidation of the TEs is responsible for Schottky barrier height modulation as well as resistive switching mechanism is confirmed though AA detection with a low concentration of 1 pM in RRAM device. Additionally, it is important to diagnosis human diseases at early stage by using TaOx-based RRAM platform. § EXPERIMENTAL SECTION Memory device fabrication To fabricate the RRAM devices, TiN BE was deposited in following a Ti layer on SiO2 film. The SiO2 layer was grown on Si 8 inch wafer. The thicknesses of TiN, Ti and SiO2 layers were 40 nm, 160 nm, and 200 nm, respectively. Then, the SiO2 layer with a thickness of 150 nm was deposited on BE. Next, via-holes having ranges from 0.4×0.4 to 4×4 µm2 were patterned by photolithography method in following dry etching process. Then, nine (9) pieces are cut from the wafer where each piece was contained four dies. Later on, molybdenum disulfide (MoS2) of having thickness 2 nm for three pieces and 4 nm for another three pieces were deposited by radio frequency (RF) sputtering process. During deposition of MoS2, a ceramic target was used. Chamber vacuum level, argon (Ar) gas flow rate, and power during deposition were set at 6 mTorr, 10 sccm, and 100 watt, respectively. After that, TaOx as switching material (SM) was deposited using E-gun evaporator. Purity of tantalum-pentoxide (Ta2O5) granules was 99.99% and kept in the graphite crucible. The SM thickness was 5 nm. Ejection rate was 0.1Å/sec. High vacuum level of approximately 6×10-6 Torr was unchanged during deposition. Then, aluminum-oxide (Al2O3) with a thickness of approximately 2 nm was deposited by RF sputtering process. Argon (Ar) gas flow rate, deposition power, and chamber pressure were fixed at 25 sccm, 80 watt, and 0.03 Torr, respectively. Then, iridium-oxide (IrOx), platinum (Pt), or ruthenium (Ru) as a TE was deposited by RF sputtering method. Each metal target with purity of 99.99% was used. The thickness of TE was approximately 100 nm. Deposition power of 50 watt, pressure of 20 mTorr, and Ar gas flow rate of 25 sccm were kept constant for the Pt or Ru electrode. During IrOx deposition, oxygen (O2) and Ar gas flow rate were the same 15 sccm. To obtain RRAM devices, lift-off process was carried out finally, namely ACS Paragon Plus Environment

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IrOx/Al2O3/TaOx/TiN (S1), IrOx/Al2O3/MoS2(2nm)/TaOx/TiN (S2), IrOx/Al2O3/MoS2(4 nm)/TaOx/TiN (S3), Pt/Al2O3/MoS2(2 nm)/TaOx/TiN (S4), and Ru/Al2O3/MoS2(2nm)/TaOx/TiN (S5) structures. For fair comparison, nine structures are tabulated in Table-S1 (supporting Information). Electrical measurements were carried out by using B1500A precision semiconductor analyzer in our lab. During entire measurement process, bias was applied on the TE keeping BE grounded. A schematic view of the via-hole device is shown in Figure 1a. The same RRAM device was also used for detection of ascorbic acid. TRIS buffer and AA solution preparation Ascorbic acid was purchased from Sigma Aldrich. To prepare TRIS buffer solution, first we dissolved 0.2422 gm TRIS base according to its molecular weight into 100 mL de-ionized (DI) water and it produces 20 mM concentration solution. Then we adjusted the pH value of pH 7.4 by using dilute HCl solution and it was measured by pH meter. After that 20 mM ascorbic acid stock solution was prepared by dissolving 0.0352 gm into 10 mL TRIS buffer (pH 7.4) solution. Then required solutions were made from 1 pM to 1 µM. A small sample volume of 1 µL was used by 0.1-10 µL micropipette, as shown in Figure 1a. A B1500A precision semiconductor system was used to measure the current versus time (I-t) characteristics.

§ RESULTS AND DISCUSSION Schematic diagram of ascorbic acid detection by using porous-IrOx/Al2O3/TaOx/MoS2/TiN (S2) RRAM device is shown in Figure 1a. Table S1 shows different structures with IrOx, Pt, and Ru top electrodes and the thickness of MoS2 is varied from 2 to 4 nm (supporting information). Cross-sectional transmission electron microscope (TEM) image displays structural view of the pristine S2 device with a via-hole size of 0.4 × 0.4 µm2, as shown in Figure S1 (supporting information). The IrOx film or electrode shows porous, as shown in Figure S2 (supporting information). Layer-by-layer structure is clearly visualized from cross-sectional HRTEM image inside the via-hole region (Figure 1b). The thickness of Al2O3, TaOx as SM and MoS2 layer are approximately 2.2 nm, 7.2 nm, and 2.3 nm, respectively. Another native oxide layer i.e., TiNxOy of ~2.5 nm thickness is observed. Elementary composition of S2 stack is confirmed by TEM image and EDS line profile in Figure 1c and d, which is measured from TE to BE direction. Figure 1d exhibits the presence of Ir, O, Al, Ta, Mo, S, Ti, and N elements by using EDS line profile. The mass percentage of Al is 10% in Al2O3 layer. Significant amount of Mo (28%) and S (10%) is observed at bottom interface, which assures the formation of


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MoS2 layer. Due to inert MoS2 layer, amount of O at the BE interface becomes higher than top interface (25 % vs 15%). Before forming, this RRAM device shows diode characteristics.

(a)

TiN

IrOx

120 100

Mass (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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(d)

TE

80

N Al Ti Ir

O S Mo Ta

BE

60 40 20 0 0

10 20 30 40 Distance (nm)

50

Figure 1. (a) Schematic view of IrOx/Al2O3/TaOx/MoS2/TiN RRAM structure for detection of ascorbic acid. (b) HRTEM image of IrOx/Al2O3/TaOx/MoS2/TiN memory device (S2) having via-hole size of 0.4 ×0.4 µm2. (c) TEM image for elemental profiles of IrOx/Al2O3/TaOx/MoS2/TiN device. The depth profile is measured along green line, indicated on TEM image. (d) EDS depth profiles of each layer of S2 device from (c).

Figure 2a shows the asymmetric rectifying I–V characteristics of a pristine S2 device under positive bias on the IrOx TE. Voltage sweep directions are indicated by arrows 1→4. Device switches to higher current from lower current at +2 V and current level gets self-saturated at ~ 4.2 µA. Hence, external current limitation is not needed to protect the device from permanent breakdown; however, operation voltage is limited to ±2 V. This self-current compliance property will facilitate to mitigate current overshoot problem and will be better alternate of additional selector cell (i.e., one transistor-one resistor (1T1R) or one selector-one resistor (1S1R)).


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In reverse bias, it returns to low current with a value of 16.8 pA at -2 V. Analogous I-V hysteresis behavior is observed for the S1, S3, D1, S4, D2, S5, D4 devices except D3 structure, as shown in Figure S3 (supporting information). The forward current (IF) values at Vread of +2 V are found to be 6.2, 4.2, 2.4, 1.42, 0.27, 0.06, 64, 23, and 10 µA for the S1, S2, S3, D1, S4, D2, D3, S5, and D4 devices, respectively. The IF value is decreased with increasing MoS2 thickness for each TE, which is owing to thicker stack layer. On the other hand, the reverse current (IR) values at Vread of -2 V are found to be 0.57 nA, 16.8 pA, 35 pA, 0.14 nA, and 0.56 µA for the S1, S2, S3, S4 and S5 devices, respectively. The IR values in both polarities show high for the Ru TE devices (D3, S5, D4) than those of the IrOx (S1, S2, S3) and Pt (D1, S4, D2) TEs. These asymmetric I-V characteristics are observed owing to the difference of work function in between TEs and BE (5.6 eV for IrOx, 5.65 eV for Pt, and 4.71 eV for Ru vs. 4.5 eV for TiN). Therefore, Schottky barrier height (ΦSB) values for both the IrOx and Pt TEs are higher than the Ru TE as well as the IR value of Ru TE is higher. In addition, IF value decreases with increasing the Gibbs free energy value in an order of RuO23.2 ×104), NF (> 700) and lowest ƞ (1.85) values are obtained from the S2 devices. This is owing to TiNxOy/MoS2/TaOx quantum well formation, which acts as SCQW. Sun et al. have also reported SCQW in MoS2 thin film.48 This n-type MoS2 layer in the S2 structure confines extra electrons which is coming from TE and suppresses the IR than S1 (16.8 pA vs. 0.57 nA at Vread of -2 V) remarkably. Wu et al. have mentioned MoS2 as triboelectric electron-acceptor layer in their triboelectric nanogenerators.49 As a result, the RR value increases enormously in the S2 device (3.2 × 104 vs. 5.1 × 103) with excellent controllable uniformity. If the thicker MoS2 (4 nm) is inserted in the S3 structure, then thickness factor dominates. The RR value of 5.8×103 and NF value of 160 get reduced. In our present observation, minimum ƞ value in the S2 devices deviates from ideal value slightly (1 vs 1.85). This is owing to inhomogeneous Schottky barrier formation or higher series resistance effect of thicker oxide layer.47, 50 In order to check the BRS phenomena, forming voltage (Vform) is applied to all devices having area of 4×4 µm2. Typical I-V forming and switching characteristics of the S2 devices at a low CC of 30 µA are shown in Figure 2d and e. Due to insertion and increasing thickness of MoS2 layer, average Vform amplitude increases -2.9>ACS Paragon Plus Environment

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3.4>-4 V for the S1, S2, and S3 devices, respectively (Figure 2d). Hence, ten pristine devices for each structure have been chosen and plotted their Vform values. In contrast, the S5 device needs lower Vform value of -2.7 V as compares to the S2 (-3.4 V) and S4 (-3.3 V) devices. This is owing to lower Gibbs free energy of RuO2. Hence, Ru TE has a higher probability to react with oxygen as well as defect generates higher level. Though previous research suggests that it is possible to obtain work function of IrOx ranging from 4.23 to 5.6 as compared to metallic Ir (5.27), here we consider higher work function of IrOx leading to higher forming voltage in S1, S2 and S3 devices.34,

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The effect of work function increment of IrOx layer has also been

observed in AA sensing as explained later. Accounting the combination of work function, defects, and Gibbs free energy, the leakage current as well as Vform values of all the devices can be explained in Figure S5 (supporting information). The S2 devices produce better device-to-device uniformity with minimal Vform deviation margin (-3.4 to -3.8 V). This helps also controllable resistive switching by optimized 2 nm-thick MoS2 interfacial layer as well. The switching characteristics of all devices are also shown in Figure S5 (supporting information). Voltage sweeping directions are marked by arrows 1→4. Ehen negative bias is applied on the TE, soft breakdown of TaOx SM occurs at Vform of -3.5 V and device switches to low resistance state (LRS) from initial resistance state (IRS). In reverse VRESET bias (+Ve), it returns to high resistance state (HRS). Except S2, the RESET currents are quite high (76 µA vs. ≥200 µA) in forming cycle. Therefore, the BRS characteristics at lower CC of 30 µA are not easy to obtain from all devices except S2. For comparison, IV switching characteristics at a CC of 200 µA have been described. A typical I-V hysteresis characteristic of the S2 device with a size of 0.4×0.4 µm2 is exhibited at a CC of 30 µA (Figure S6a, supporting information). A small VSET/VRESET value of -2.4/+1.5 V is required. The RESET current of 37 µA is sufficiently low for the reproducible resistive switching. Furthermore, a partial SET tendency is noticeable at VSET2 before the completion of RESET process in +Ve bias regime (Figure 2e). It indicates the possibility to obtain CRS characteristics with proper tuning of RESET bias or CC, which has been elaborated later. To find the reason behind the typical BRS, the conduction mechanism has been investigated by I-V curve fitting (Figure S6a, supporting information). The linear fitted of ln (I) vs V1/2 curve assures the Schottky conduction of LRS and HRS currents, as shown in Figure S6b (supporting information). Using eq. (1), the ΦSB values of the LRS and HRS currents are found to be 0.28 eV and 0.38 eV, respectively. The εd values for LRS and HRS are 7.3 and 5.01, respectively. The values of n are consistent with previously reported values of TiO2 (2.16 to 2.7).52, 53 At


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high field regime, Figure 2f shows well fitted ln(J/E2) vs 1/E curve of HRS current that Fowler-Nordheim (FN) current conduction mechanism by using equation (2),43

(2) where, ΦF-N is tunneling barrier height,

is the reduced Planck’s constant, q is the electronic charge, and mox is

the tunneling effective mass of electron in MoS2 layer. Considering mox = 0.45 m0,54 the calculated ΦF-N from eq. (2) is 0.73 eV above the critical electric field (EC) of ˃ 5 MV/cm. These results are consistent with our previous reports.6, 55 To obtain reliable RRAM, a long program/erase endurance of more than 107 cycles is attained and a pulse width of 100 ns is applied (Figure 2g). Typical input pulse wave sequence appeared on the measurement probe is shown in Figure S6c (supporting information). Programing and erasing energy are found to be low 9 pJ and 15 pJ, respectively. Additionally, a stable retention characteristic of >104 sec with considerable HRS/LRS ratio of more than 10 establishes non-volatility feature (Figure S6d, supporting information). The memory characteristics are conducted with lower current level of 30 µA having Vread of 0.1 V. There is no report by using TaOx/MoS2 based RRAM at a low operation current of 108 cycles and device kept at SET condition, the cross-sectional TEM image in ex-situ was performed, as shown in Figure S7. The programming current of 100 µA with a small P/E pulse width of 100 ns is applied. Figure 3 shows the HRTEM image from marked region ‘A’. The thicknesses of Al2O3 and TaOx layers are 1.3 nm and 9.1 nm, respectively. The Al2O3 interfacial layer is reduced from 2.2 nm (Figure 1c) to 1.3 nm. Both the MoS2 and TiNxOy layers with a thickness of approximately 7.1 nm show poly-crystalline. On the other hand, both TaOx and Al2O3 layers are amorphous. Basically, total thickness is increased to 17.4 nm as compared to the pristine device (14.2 nm). This suggests that the TaOx layer is migrated towards Al2O3 layer. Therefore, reduced Al2O3/TaOx interface is observed. On the other hand, thickness of TiNxOy layer is increased, which is evidenced the O2- ions migration towards the BE under negative SET operation and reacts with TiN BE. This is possible conductive filament formation/rupture under external bias, however further study is also needed. On the other hand, electronic charge transports from TE to BE through the oxygen vacancy conducting filament and device sets at LRS. Additional interfacial MoS2 barrier layer captures electron and prevents the reaction of O2- ions with TiN, which results the restricted size/no of filaments during SET operation. On the other hand, the oxygen vacancy sinks into the TE as well as reduction (M0, M = Ir, Pt, Ru) occurs. In reverse bias, O2- ions repel back, which oxidize conducting filament in TaOx layer and the devices reset to HRS as well as the TE oxidation (Mz+, z = 1, 2, 3, -) occurs. Inert MoS2 layer repulses O2- ions with ease to control RESET process, while it is not easy for the S1, D1, and D3 devices because a reactive residual TiOx layer remains at the TiN/TaOx interface permanently. This enhances RESET current and discards the possibility of BRS at lower CC of 30 µA. In spite of inserting MoS2 interfacial layer in the S5 and D4 devices; they are incapable to produce BRS at 30 µA because of thicker MoS2 layer. The Ru TE is more reactive with oxygen than the Pt electrode. Therefore, Pt creates additional space charge layer by repelling O2- ions and subsequent electric field drops across this layer. This enhances filament diameter as well as the RESET current is abruptly increased in the S4 and D2 devices. Conversely in S2, the IrOx TE along with 2 nm-thick MoS2 interfacial layer changes to reduction-oxidation under SET/RESET is optimized structure to produce controllable resistive switching at lower CC as well as best electrical performances are obtained. In all structures, 2 nm-thick Al2O3 layer plays the role as interfacial barrier layer or blocking layer as we have ACS Paragon Plus Environment

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reported earlier in Ir/Al2O3/TaOx/TiN stack.6 As we have mentioned in Figure 2e, that proper tuning of both VRESET and CC generates the CRS characteristics as well. The device exhibits consecutive CRS of > 100 cycles within -1.8/+1.8 V at self-CC (Figure 4a). Voltage sweeping direction of the CRS characteristics is indicated by using arrows 1→6. The states are denoted by using ‘0’,

ON

VRESET2 VRESET1

3 4

VSET1 -5

10

0

10 -2

12

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1

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-1 0 1 Voltage (V)

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'0' state : 0.28 eV '1' state : 0.24 eV

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FSB at

0.0 0.4 0.8 1.2 1.6 1/2 V VRESET2 VRESET1 ON -3 (d) ON 10 5 2

(c)

At high field regime of '1' state Hopp. dist. : 0.57 nm

-16

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All 100th

S2

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VSET1 0 all 100 th 1

3

VSET2

4 0 6

10 -3 -2 -1 0 1 2 Voltage (V)

3

Figure 4. (a) Repeatable I-V characteristics of complementary resistive switching (CRS) at self CC; (b) Schottky emission and (c) hopping conduction fitting at CRS; (d) CRS endurance at CC of 1 mA for the S2 devices.

‘ON’, ‘1’ symbol. The S2 device switches to ‘ON’ states from ‘0’ state or ‘1’ state at average bias of -1 V (VSET1) /+1 V (VSET2) on both polarities and this reaches to average ~810/790 µA current level. Then ‘ON’ state switches back to ‘1’ state and ’0’ state at -1.8 V (VRESET2) and +1.8 V (VRESET1), respectively. The current values of ‘0’/ ‘1’ states at ½ Vread are found to be 10.6/42.7 µA and 8.6/40 µA in –Ve and +Ve bias, respectively. A small dispersion between ‘0’ and ‘1’ states (~4) with acceptable NF value of ˃19 is obtained,


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which is comparable with other reported results.58-60 However, this CRS performance of the S2 device with recent papers is shown in Table2.6, 17, 22, 56-60 Conversely, the S1 device shows only 33 CRS cycles

Table 2: Comparison of CRS characteristics in recent reported papers.6, 17, 56-62

Device IrOx/Al2O3/Ta2O5/MoS2/TiN [This article] Ir/Al2O3/TaOx/TiN6 IrOx/HfO2/Al2O3/TiOx/TiN56 Ag/MoS2/Ag/MoS2/Ag17 Pt/HfO2/Ti/HfO2/Pt57 Pt/Ta2O5/Ta/Ta2O5/Pt58 Pt/SiOx/TiN59 Pt/TiO2/Pt60 Pt/SiO2/GeSe/Cu22

Type single

Operating voltage (V) -1.8/1.8

Current 1 mA

Cycles 102

single single antiserial antiserial antiserial single single antiserial

-2.7/2.5 -4/4 -1/1 -1/1 -1.5/1.5 -2/2 -2/2 -1.8/1.8

300 μA 400 µA ~1.3 mA 1 mA ~250μA 5 mA 6 mA 100 successive CRS cycles with reduced current lag ~1.3 between those two states. It is owing to the same residual filament diameter in both (‘0’ and ‘1’) states. However, further study is required to evident switching mechanism through filament formation/rupture and to improve the cycle-to-cycle CRS uniformity. Another novel approach of AA detection directly by using RRAM devices with IrOx, Pt, or Ru TE has been presented here, which is co-related with reduction-oxidation (redox) of the TEs. First, I-V sweeping characteristics are performed in TRIS buffer (pH 7.4) solution applying symmetric external bias on both polarities until this reaches its saturation current limit prior to the breakdown of SM (Figure 5a). Voltage sweepings are directed by path1→path4. Higher current follows the path 1 and path 3, which are owing to reduction. On the other hand, lower current follows the path 2 and path 4, which are owing to oxidation of the TE. To execute above test, 1µL aqueous solution of TRIS buffer has been dropped over via-hole of the device


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by micropipette (Figure 1a). This TRIS buffer solution changes its surface potential by redox, which leads to the current conduction modulation. At invariant condition, I-V characteristics are also tested for the Pt and Ru

Current (A)

-6

10

100.0n Path 1

Path 3

-7

Path 4

10

Path 2

± 0.2V ± 0.4V ± 0.6V ± 0.8V ± 1V ± 1.2V

-8

10

S2

Current (A)

(a)

-6 -12 -18 -1

10

1

3

5

7

10 10 10 10 AA concentration (pM)

S2 Buffer 1 pM 10 pM

40.0n 20.0n

Current change (nA)

0

Vread: -0.2 V

60.0n

0.0

sensitivity : -0.79 nA/pM

6

(b)

80.0n

-9

10 -1.5-1.0-0.5 0.0 0.5 1.0 1.5 Sweeping voltage (V) 12 (c) S2

Current change (nA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 1 2 3 4 5 6 7 8 Response time (sec)

40 (d) S4 35 sensitivity : -0.37 nA/pM 30 25 20 15 10 5 0 -1 1 3 5 7 10 10 10 10 10 AA concentration (pM)

Figure 5. Current-voltage characteristics of pristine (a) S2 at TRIS buffer aqueous solution with pH value of 7.4. Current vs response time curves of the (b) S2 devices at buffer and different ascorbic acid concentrations of 1 and 10 pM. Current change vs AA concentrations ranging of 1 pM to 1 µM for the (c) S2 and (d) S4 devices.

TEs (Figure S10, supporting information). Next, AA detection test has been conducted with respect to TRIS buffer solution. The S2, S4, and S5 devices exhibit the current versus time (I-t) response curve in buffer and different AA concentrations of 1 pM and 10 pM (Figure 5b and Figure S11, supporting information). Total response time is 5 s at time interval of 10 ms and saturated sensing current level corresponds to each AA concentration, which is clearly visible. All current states are read at a Vread of -0.2 V. It is clearly visualized that, saturated current level first increases in 1 pM AA concentration with respect to TRIS buffer current and then reduces at higher concentration of AA. A double step reaction takes place during AA sensing (Figure 1a).


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At 1 pM concentration, AA reduces first the TE from higher to lower oxidation states (Mz+ → M0). For example, the Ir4+ changes to Ir3+/Ir0 oxidation state.6 At higher AA concentration (10 pM), this lower oxidation state is oxidized to higher oxidation state (M0 → Mz+) or Ir0 state changes to Ir3+/Ir4+ (Ir0 ↔ Ir3+ + 3é; Ir0 ↔ Ir4+ + 4é). The oxidation state increment leads to increase work function of IrOx (5.6 eV).34 This oxidizing current is detected, which is decreased with elevated AA concentration. Therefore, the ΦSB value at the TE/Al2O3 interface increases gradually as well as the sensing current decreases with increasing AA concentration. Turkusic et al. have reported similar phenomena to detect AA on MnO2 bulk modified screen printed electrode.67 It is worthy to be noted that, the reversible saturated current decreases consistently from 1 pM to 1 µM only in the S4 devices (Figure 5d) than the S2 (Figure 5c) and S5 devices (Figure S12, supporting information). In Figure 5c, the saturation of ascorbic acid after 1000 pM is observed. Ascorbic acid is chemically reacted on porous IrOx surface sites. It is expected that the ascorbic acid molecules are reacted fully on IrOx surface sites up to a concentration of 1000 pM. Beyond this concentration, there is no empty surface sites to react further unless it is cleaned by DI water. Therefore, the ascorbic acid sensing is saturated after 1000 pM. However, further investigation is necessary to explore the reaction dynamics in future. The sensitivity value of the S2 sensors shows highest -0.79 nA/pM at low concentration range of 1 to 10 pM, while the sensitivity values of -0.37 and -0.26 nA/pM are found to be for the Pt and Ru electrodes, respectively. The sensitivity is calculated with concentrations from 1 to 10 pM. The linear range are found to be 1 to 1 nM, 1 pM to 1 µM, and 1 pM to 1 nM for the S2, S4, and S5 devices, respectively. On the other hand, the S4 sensor can measure long range. Therefore, both S2 and S4 sensors are good for application because both Ir and Pt electrodes have good redox properties. For low concentration measurement, IrOx electrode is one of the best candidates to detect ascorbic acid. The AA detection technique using TaOx/MoS2 RRAM structure is a novel approach and lower sensing limit of 1 pM is superior to other reported results,38-42 as tabulated in Table 3. Detection of lower AA concentration (1 pM) will be benefitted for early stage diagnosis as well as to reduce sample volume to know health status in emergency case. Overall, optimized IrOx/Al2O3/TaOx/MoS2(2 nm)/TiN RRAM structure is very promising for nanoscale non-volatile memory applications as well as 1 pM ascorbic acid detection is also very important for diagnosis of human diseases at early stage.


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Table 3: Comparison of ascorbic acid detection with other reported results in literature.

Sensing electrode

Sensing method

Minimum detection

IrOx [This work] Exfoliated flexible graphite paper (e-FGP)38

Reduction-oxidation in RRAM device

1 pM

DPV

0.01 µM

Ag NPs39

Transmittance colorimetric

10.5 µM

Monolayer MnO2 nanosheet40

Transmittance colorimetric

62.8 nM

Carbon dots (CDs)41

Fluorescence

300 µM

Copper catalyzed42

EIS

2.6 pM

§ CONCLUSION In conclusion, a complete study of resistive switching characteristics (rectifying, BRS, CRS) by inserting optimized MoS2 interfacial layer and switching mechanism through ascorbic acid detection in TE/Al2O3/TaOx/MoS2/TiN structure have been reported. Effects of porous IrOx, Pt, or Ru TE have been investigated. Structural and elemental composition of the S2 device is confirmed by HRTEM image and EDS characterization, respectively. Prior to the forming, the S2 device exhibits diode-like highly uniform rectifying endurance of > 1000 cycles with large RR of > 3.2×104, higher NF of >700 at lower self-CC level of 5 µA within ±2 V. Low work function of Ru TE makes devices unable to produce superior rectifying I-V. After forming, the S2 device shows BRS with long P/E endurance of more than 107 cycles at low P/E current of 30/50 µA and a small P/E pulse width of 100 ns is applied. A proper tuning of VRESET produces successive CRS endurance of >100 cycles with P/E endurance of >106 cycles. Here, MoS2 layer acts as SCQW, which confines electron and provides controllable switching. Linear fitting of I-V curves discloses the charge transportation mechanism. Schottky barrier height modulation at low field is confirmed by reduction-oxidation under SET/RESET as well as evolution of switching mechanism through AA detection is investigated. Hopping conduction and the F-N tunneling at high field have been investigated. A simple and novel technique of AA detection directly up to 1 pM by using TaOx-based RRAM platform is one of the key aspects of this


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work. Moreover, this IrOx/Al2O3/TaOx/MoS2/TiN RRAM device is highly propitious for non-volatile memory and this device will be also useful to diagnostic of scurvy disease at early stage.

KEYWORDS. Resistive switching, MoS2, TaOx, ascorbic acid, sensing

ASSOCIATED CONTENT Supporting Information.

Table containing different structures; TEM image of IrOx/Al2O3/TaOx/MoS2/TiN (S2) via-hole RRAM device with a size of 0.4 × 0.4 µm2; planar-view TEM image of porous IrOx layer; rectifying diode characteristics for the S1, S2, S3, D1, S4, D2, D3, S5, D4 devices; I-V fitting characteristics for the S1, S2, S3, S4, S5 devices; forming and SET/RESET I-V characteristics of the S1, S3, D1, S4, D2, D3, S5, and D4 devices; resistive switching characteristics at a low current of 30 µA, Schottky fitting, applied input pulse on the device for P/E endurance test, and data retention; TEM, HAADF, and EDS analysis of the S2 device after long P/E endurance; CRS characteristics of the S1, S4 and S5 devices; activation energy evaluation by Arrhenius plot; I-V characteristics of the S4 and S5 devices; . I-t characteristics of TRIS buffer, 1 pM and 10 pM AA concentrations for the S4 and S5 devices; current change versus AA concentration ranging from 1 pM to 1 µM for the S5 devices. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACS Paragon Plus Environment

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Funding Sources Ministry of Science and Technology (MOST), Taiwan under contract numbers: MOST-104-2221-E182-075, MOST-107-2221-E-182-041; Chang Gung Memorial Hospital (CGMH), Linkou under contract number: CMRPD2E0091. Notes The authors declare that they have no competing interests.

ACKNOWLEDGMENTS This work was supported by Ministry of Science and Technology (MOST), Taiwan under contract numbers: MOST-104-2221-E-182-075, MOST-107-2221-E-182-041, and Chang Gung Memorial Hospital (CGMH), Linkou under contract number: CMRPD2E0091. The authors are grateful to MSSCORP Co. Ltd., Hsinchu, Taiwan for HRTEM supporting of our pristine and stressed devices. The authors are also grateful to Electro-Optical Research Laboratory (EOL), Industrial Technology Research Institute (ITRI), Hsinchu, Taiwan for the via-hole patterning.

ABBREVIATIONS Top electrodes, (TEs); Ascorbic acid, AA; High-resolution transmission-electron microscope, HRTEM; Surface confined quantum well, SCQW; Two dimensional, 2D; amorphous carbon, a-C; Energy dispersive Xray spectroscopy, EDS; Rectifying Ratio, RR; Non-linearity factor, NF; Program/erase, P/E; Complementary resistive switching, CRS; Fowler-Nordheim, F-N; CRS; Metal-insulator-metal, MIM; Transition metal dichalcogenide, TMD; Resistive random access memory, RRAM; Electrochemical impedance spectroscopy, EIS; Bottom electrode, BE; Radio frequency, RF; Switching material, SM; One transistor-one resistor, 1T1R; One selector-one resistor, 1S1R. ■ REFERENCES

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Controlling Resistive Switching by Using an Optimized MoS2

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