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Mechanism of Non-Volatile Resistive Switching in ZnO/ #-FeO Core-Shell Heterojunction Nanorod Arrays 2

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Debasish Sarkar, and Ashutosh Kumar Singh J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 24 May 2017 Downloaded from http://pubs.acs.org on May 30, 2017

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The Journal of Physical Chemistry

Mechanism of Non-Volatile Resistive Switching in ZnO/α-Fe2O3 Core-Shell Heterojunction Nanorod Arrays Debasish Sarkar*, †, ┴ and Ashutosh K. Singh*, ‡, ┴ †

Solid State and Structural Chemistry Unit, Indian Institute of Science, Bengaluru 560012, India.

‡

Large Area Device Laboratory, Centre for Nano and Soft Matter Sciences, Jalahalli, Bengaluru 560013, India.

ABSTRACT Non-volatile resistive switching based resistive-random-access-memory (RRAM) is evolving rapidly among various other nanoscaled-semiconductor technologies. In this article, resistive switching mechanism in a solution-route-processed ZnO/α-Fe2O3 core-shell n-n heterojunction nanorods (NRs) is investigated for the first time. As fabricated nanostructured electrode shows resistive switching with compelling ON/OFF ratio at a significantly small reverse bias voltage (0.55 V). Moreover, this core-shell nanorod based resistive-switch exhibits an excellent timeretention (with relaxation constant (α) ~ -0.0065 even after ~103 s) and endurance (with a minute change in switching potential after 100 switching cycles). Resistive switching in this core-shell nanorods system arises due to the tuning of band-alignment at the heterojunction interface governed by fast and reversible migration of charge/ionic species on either side of the interface under reverse-bias condition, facilitating electron tunneling across the interface as supported by experimental observations, together with highly non-linear dependency of the drift velocities of oxygen-vacancies on applied potential bias. Such understanding behind the high-degree and energy-efficient non-volatile resistive switching in ZnO/α-Fe2O3 core-shell NRs make them a potential candidate in engineering next-generation nano-heterostructure based RRAM devices. __________________________ * Author to whom correspondence should be addressed. E-mail: [email protected], Tel: +91 (080) 2293 2945; E-mail: [email protected], Tel: +91 (080) 2308 4242. ┴ Authors contributed equally to this work 1 ACS Paragon Plus Environment


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1. INTRODUCTION Nanostructured heterojunctions with synergistic combination of different functional materials have gained immense research and technological interest due to their wide variety of tunable properties, important for developing modern momory, opto-electronic and sensor devices.1-3 In particular, transition metal oxides (TMOs), specially ZnO and α-Fe2O3 and their composites have been investigated meticulously because of their omnipresent nature and having wide assortment of physical and chemical properties.2, 4 Interestingly, both of the materials have shown potential as an active component in developing resistive switching based resistive-random-access-memory (RRAM) with advantages like high-density integration, high endurance, high-speed operation, non-volatility and low-power consumption.5,

6, 7

Recent findings suggest that heterojunction

devices comprising two oxide layes may offer improved performance over single-layer based devices, including cycling and scaling potential.1,

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Therefore, it is envisaged that the

heterojunction of ZnO and α-Fe2O3 would provide some unique features in resistive-switching (RS) governed by their intrinsic properties. Different models have been proposed for TMO based resistive switches or ‘memristors’ with basic metal/insulator/metal (MIM) design, including alteration of bulk resistivity due to defects or trapped charges, modification of metal/insulator interface due to the same defects or trapped carriers and formation of conducting filamentary path within the electrode materials under applied electric field.5,

9, 10, 11

In the light of the foregoing, ZnO and α-Fe2O3 are eye-

catching as insulating layer in MIM design because of their wide band gap and ‘intrinsically-selfdoping’ tendency by different native interstitial or vacancy point defects.12-14 Furthermore, these point defects should provide an additional advantage in terms of rapid response time as they have to drift only very short distances (a few angstroms) under applied electric field.15 It is feasible 2 ACS Paragon Plus Environment

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that different mechanisms may co-exist or different mechanisms may dominate in different systems, however, in case of semiconductor heterojunctions one could expect an additional contribution from the junction-interface towards RS behavior, because the overall band alignment at the heterojunction interface would govern the directional charge-transport in MIM structure.2, 16 Thus, careful tuning of band alignment at the interface between two semiconductors through external bias voltage could also result in RS phenomena in such systems. Though there are very few reports investigating the RS mechanism in different composite nanostructures based on ZnO and Fe2O3, namely bi-layered Pt-Fe2O3-core-shell-nanoparticle/γFe2O3-nanoparticle assembly,14 TiN/SiO2/Fe electrode structure,17 ZnO/SrTiO3 heterojunction,2 ZnO–TaOx–p-GaN heterostructure,16 etc, however, there has been no such report on the synthesis and RS properties of ZnO/α-Fe2O3 core-shell nanorod (NR) heterojunction arrays so far. Here, we have reported a facile synthesis of high-density ZnO NRs on flexible stainless-steel (SS) substrate and coated those NRs with α-Fe2O3 nanoparticles to form the core-shell nanoheterostructure. Such distinct nano-rod geometry of this electrode material with core-shell design shows an excellent non-volatile RS behavior at a significantly low reverse bias voltage along with high endurance of switching potential that can be ascribed to the interfacial band engineering at the heterojunction interface in conjunction with the reversible migration of charged point defects across the interface. 2. EXPERIMENTAL PROCEDURES Growth of ZnO NR arrays on SS substrate First of all, vertically aligned NRs of ZnO were grown on a polished SS substrate using a seedmediated hydrothermal synthesis procedure reported elsewhere.18 First step comprises the

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synthesis of highly dispersed ZnO nanocrystal seeds by reducing zinc acetate dihydrate (0.01M) with sodium hydroxide (0.03M) in ethanol solution at 60°C with continuous stirring for 2 hrs. The nanocrystals were then drop-casted on properly cleaned and polished SS foils for several times followed by intermediate drying at 150°C for 15 min to increase particle adhesion on the substrate surface. After that, the substrates were placed horizontally inside a teflon-lined stainless-steel autoclave containing zinc nitrate dihydrate (0.025M) and hexamethylenetetramine (0.025M) followed by heating at 90°C for 6 hrs in an oven for the growth ZnO NR arrays. The substrate with white ZnO layer was taken out from the autoclave, copiously cleaned with water and ethanol followed by drying overnight in an oven at 60°C. Finally, ZnO NRs were annealed at 350°C for 3 hrs in H2 atmosphere. Formation of ZnO/α-Fe2O3 core-shell NR heterojunction arrays Further, deposition of α-Fe2O3 nanoparticles on ZnO NRs was performed using spin coating technique where ethanol solution of ferric chloride hexahydrate (0.05M) was drop-casted onto ZnO NR array and spin dried at 3000 rpm for 30s followed by heating at 250°C for 5 min to improve particle adhesion at NR surface. After repeating the procedure for 5 times, the white colored substrate turned yellow indicating the formation of FeOOH, which was then annealed in N2 atmosphere at 400°C for 1 h to completely convert FeOOH into α-Fe2O3 with a visible change in substrate color from yellow to brick-red. Materials characterization and electrical measurement technique Morphology, crystal structure and composition of the electrode materials were investigated using Field-Emission Scanning Electron Microscope (FESEM, FEI, Quanta FEG 650), Transmission


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Electron Microscope (TEM, FEI TECNAI G2 TF20ST), XRD (XRD, PANalytical Empyrean Xray Diffractometer) and X-ray photo-electron spectroscopy (XPS, AXIS, ULTRA). All electrical measurements were carried out on Keithley 4200 SCS and 236 source meter. Contacts were taken from sample with the help of probe station Cascade Microtech EPS 150 TRIAX. The resistive switching measurements were performed by sweeping the voltage within ±1 V with a compliance current fixed at 100 mA to prevent the unrecoverable breakdown of the metal oxide layer. 3. RESULTS AND DISCUSSION Morphology and crystallography

Figure 1. FESEM images of high density well-aligned ZnO NRs (a) and ZnO/α-Fe2O3 core-shell NR arrays (b); (c) TEM image of a single core-shell NR showing non-uniform shell layer, inset shows the High-Resolution TEM image; (d) XRD pattern for ZnO/α-Fe2O3 core-shell NR arrays,



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showing crystallinity of component materials along with the SS substrate and (e) Oxygen 1s core-level XPS spectrum collected from as-synthesized ZnO NR arrays. Figure 1a shows the field-emission Scanning Electron Microscope (FESEM, FEI, Quanta FEG 650) image of the high density vertically aligned ZnO NRs grown on a polished SS substrate through hydrothermal method. The NRs are hexagonal in shape due to hexagonal Wurtzite crystal structure of ZnO with smooth surface and sharp tip because of nucleation and growth procedures. The average length and diameter of these NRs are found to be ~3.5 ± 0.2 µm and 130 ± 20 nm, respectively. However, the surface morphology of these NRs changes significantly after their surface modification with α-Fe2O3 nanoparticles, as can be seen from the Figure 1b. The surface of the ZnO/α-Fe2O3 core-shell NRs is very rough because of non-uniform deposition of iron oxide and can be confirmed further through the transmission electron microscope (TEM, FEI TECNAI G2 TF20ST) image of an individual core-shell NR, as shown in Figure 1c. The average thickness of α-Fe2O3 shell is found to be almost 6 ± 2 nm. HRTEM image, as shown in the inset of Figure 1c, reveals aligned lattice fringes of ZnO core oriented along [0001] direction with lattice spacing of 0.26 nm corresponding to (002) crystal planes of ZnO.4,

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Figure 1d shows the X-ray diffraction (XRD, PANalytical Empyrean X-ray

Diffractometer) data for the core-shell NR array, where the peaks can be identified as different characteristic peaks of Wurtzite ZnO (JCPDS # 36-1451),19, 20 rhombohedral α-Fe2O3 (JCPDS #89-0597)20 and the SS substrate. Core-level X-ray photo-electron spectra (XPS, AXIS, ULTRA) of Fe 2p and Zn 2p for the ZnO/α-Fe2O3 core-shell NRs (Figure S1, Supporting Information) indicate the presence of Fe3+ and Zn2+ species. Furthermore, O 1s core-level spectrum collected from as-synthesized ZnO NRs provides insight about different oxygen species present in ZnO lattice. As shown in Figure 1e, the asymmetric O 1s spectrum can be


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deconvoluted into three Gaussian peaks centered around 529.7, 531 and 532.2 eV that are assigned to different charged oxygen species, like O −2 ions in Wurtzite ZnO lattice, O −2 ions in oxygen deficient regions within ZnO matrix and chemisorbed or dissociated oxygen species on the surface regions or OH, respectively.5, 21 As will be discussed later in the manuscript, different oxygen species along with oxygen vacancies have significant role in controlling electronic transport across the ZnO/α-Fe2O3 heterojunction interface. Electrical transport behavior and resistive switching (RS) mechanism

Figure 2. (a) Schematic diagram of the electrical measurement technique using probe-station with probe-tip connected to the core-shell NRs; (b) Linear plot of I-V characteristics of the ZnO/α-Fe2O3 core-shell NR heterojunction within ±0.5 V, inset shows the semi-log plot of the same I-V curve. Figure 2a shows the schematic of the electrical measurements technique of ZnO/α-Fe2O3 core-shell heterojunction NR electrode. All electrical measurements were carried out on Keithley 4200 SCS and 236 source meter. Contacts were taken from sample with the help of probe station Cascade Microtech EPS 150 TRIAX. The resistive switching measurements were performed by 7 ACS Paragon Plus Environment


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sweeping the voltage within ±1 V with a compliance current fixed at 100 mA to prevent the unrecoverable breakdown of the metal oxide layer. The room temperature I-V curve is plotted in Figure 2b within the potential window between -0.5V to 0.5V. As can be seen from the Figure 2b, the curve shows diode-like rectification behavior with a rectification ratio in excess of 10 (ratio between forward and reverse currents) at an applied bias of 0.5 V. Inset of the figure shows the same I-V plot in semi-log scale further confirms the rectification behavior of the heterojunction NRs. Such rectification behavior within this potential window can be ascribed to the suppressed current in the reverse biased n-n heterojunction NR electrode and will be discussed later. As we increase the measuring potential window beyond ± 0.5 V, the ZnO/α-Fe2O3 coreshell heterojunction NRs exhibit unique ‘memristive’ behavior in electrical transport. Figure 3a shows the typical resistive switching behavior of the present n-n heterojunction NR electrode plotted in semi-log scale within the potential window of ±1 V. It is interesting to note that this NR heterojunction electrode demonstrates resistive switching, i.e. hysteresis behavior or electrical bi-stability in current response only in reverse bias condition (scan steps 1 to 3), though no hysteresis appears during forward potential bias condition (scan steps 4 and 5). For regular resistive switching devices, an electro-forming procedure is necessary to put the device in switchable state. However, as observed during reverse bias, the device remains in highresistance-state (HRS) or ‘OFF’ state below a certain reverse voltage. When the voltage crosses ~ -0.55 V, a sudden increase in current reaching the compliance level could be observed, thus setting the device into the low-resistance state (LRS) or ‘ON’ state. The ON/OFF ratio is found to be ~ 20 at an applied reverse bias of -0.55 V, large enough for meeting requirements for practical memory applications.22, 23


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As the top-probe/α-Fe2O3/bottom-probe and top-probe/ZnO/bottom-probe contacts show ohmic I-V characteristics in electrical transport (shown in Figure S2, Supporting Information), therefore, such unique bi-stable resistive switching as demonstrated above for this heterojunction NRs, can only be attributed to arise from the ZnO/α-Fe2O3 heterojunction interface. Particularly, reversible migration of ionized defects, like charged oxygen vacancies (VO+), chemisorbed oxygen ions ( O −x ads ) under the electrical or thermal influence have been found to be responsible behind the resistive switching mechanism in many other heterojunction devices.2, 24-26 Here also, the role of such ionized defects near the ZnO/α-Fe2O3 heterojunction interface could be reiterated in realizing the unique switching behavior. In general, solution-processed ZnO contains intrinsic oxygen vacancies that have low formation energy and remain positively ionized.12, 20 Moreover, the asymmetric O 1s core-level spectra taken from ZnO NR arrays further confirms the presence of ample amount of oxygen vacancies in the ZnO lattice.5 These ionized oxygen defects can be moved throughout the material by applying positive or negative voltages; thus constitute the transfer line also for electron transport. It is also well established that oxygen vacancies can be incorporated into α-Fe2O3 lattice by annealing under inert atmosphere during its conversion from iron oxy-hydroxide, as mentioned in the synthesis part.27 Furthermore, equilibrium band alignment at the ZnO/α-Fe2O3 heterojunction interface also plays significant role in the RS mechanism as discussed below. Figure 3b depicts the schematic of relative band positions and interfacial band alignment of ZnO and α-Fe2O3 before and after the formation of n-n heterojunction interface. According to some earlier literatures, donor concentrations in ZnO and nitrogen-treated α-Fe2O3 are found to be in the order of 1020 and 1019 cm-3, respectively.12, 13, 27 Now, α-Fe2O3 has a work function (φ) of 5.88 eV which is higher as compared to ZnO with φ = 5.2 eV and electron affinity (χ) of α9 ACS Paragon Plus Environment


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Fe2O3 (4.78 eV) is also higher than that of ZnO (4.3 eV).20, 28 Therefore, when the junction is formed, electrons from the Fermi level (EF) of ZnO migrate towards the α-Fe2O3 until the Fermi levels on both sides equalize which would push the conduction band (EC) of α-Fe2O3 at some higher energy level in relation to the conduction band position in ZnO (a ‘staggered’ type II band configuration at the interface) because of lower energy difference between the conduction band and Fermi level in the latter, as depicted in the related Figure 3b. Thus, in equilibrium, an energy barrier is generated at the heterojunction interface which further controls electron transfer on either side of the interface.

Figure 3. (a) Semi-log plot of a I-V switching loop within the potential window of ±1 V with a compliance current of 100 mA, for the ZnO/α-Fe2O3 core-shell heterojunction NR arrays, 10 ACS Paragon Plus Environment

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scanning sequence and directions are marked by the arrows; (b) Schematic diagram of equilibrium band positions before and after the formation of ZnO/α-Fe2O3 heterojunction interface along with the state of the junction under reverse electrical bias; and (c) Best-fitting of LRS current with the electron tunneling equation I ≈ wnα sinh(βV ) , where, α, β are fitting constants and w is the state variable for the memsirtor. When the n-n heterojunction is reverse biased, i.e. positive voltage in ZnO side, oxygen vacancies (VO+) in ZnO lattice are pushed towards the interface; while negative voltage in αFe2O3 side injects electrons towards the interface, though could not cross the interface because of the built-in energy barrier. The conductive paths are blocked on both sides of the junction suppressing the diffusion current across the interface (Figure 2b and 3a) below a certain reverse bias voltage. On further increasing reverse bias, the migrating oxygen vacancies (VO+) get trapped at the interface forming a high-doping region in the ZnO side of the interface. Electrons, on the other hand, form another high doping region on the α-Fe2O3 side of the interface. Thus the effective barrier energy and depletion layer width reduce significantly with the increase in reverse bias voltage.3,

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When the barrier becomes sufficiently thin enough, large amount of

electrons from α-Fe2O3 side can tunnel through the barrier and set up the conducting path across the interface, thus putting the device in low-resistance-state (LRS) or ‘ON’ state leading to a sudden increase of current. Due to the high density of charged or ionic species across the interface, tunneling persists and the LR state is non-volatile. Under forward bias condition, when α-Fe2O3 side becomes positively biased with respect to negative ZnO side, built-in electric field across the interface is reduced because of the reverse migration of trapped charges away from the interface, which results in the further reduction of the depletion layer. In such situation, electrons injected to the conduction band of ZnO can easily move to the conduction band of α-Fe2O3 11 ACS Paragon Plus Environment


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because of the favorable band alignment across the interface, leading to a high forward current. The system returns to the high-resistance-state (HRS), i.e. RESET condition, with decreasing forward bias voltage followed by the full recovery of depletion layer. Therefore, the system SET in during reverse bias, while RESET only during forward bias voltage. In successive cycles, fast and reversible migration of the charged species on either side of the heterojunction interface keep changing the device resistance from HR to LR states and vice-versa. Fluctuations in the current response can be ascribed to the non-uniform distribution of electric field on randomly-oriented NRs during measurements. In another mechanism of resistive switching, different native defects, like oxygen vacancies (VO+) and surface-adsorbed oxygen ions on both sides of the interface, can align to form extended filaments under reverse bias, connecting top and bottom electrodes throughout and thus conduct electricity to switch ‘ON’ the device in LR state.14 Whereas, upon forward biasing, the charged oxygen vacancies are reduced by the injected electrons, thereby breaking the transfer line or conductive filaments and led to the HR state. In this regard, the current fluctuation during potential sweeping can also be understood in terms of partially formed filaments that are unstable at low potential bias. The I-V characteristics of ZnO seed-layer/αFe2O3 nanoparticle heterojunction interface (as shown in the Supporting Information, Figure S3) does not show any sign of switching behavior under reverse bias which might be due to the electron scattering at the ZnO grain boundaries together with undefined heterojunction interface suppressing electron tunneling phenomena. Now, resistive switches or mesristors usually operate as dynamical resistors which changes states with the time integral of the applied current or voltage and the experimental current-voltage characteristics for a memristor having rectification behavior

can

be

simply

reproduced

with

the

equation

given

as:9

I = wnα sinh( βV ) + χ (exp(γ V ) − 1) , where the first term represents a flux-controlled memory


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resistor and α sinh( β V ) can be taken as approximation for its ON state, which is essentially electron tunneling process through a thin interfacial barrier; α and β are junction-specific constants. Moreover, w is a state variable of the memristor, proportional to the time integral of the applied bias voltage with normalized values between 0 (OFF) and 1(ON). Now, n = 1 means the drift velocities of the oxygen vacancies are directly proportional to the applied filed.9 Second term approximates the rectification behavior of the system with χ and γ as fitting parameters. However, in present system the voltage dependence of current during ‘ON’ state is found to perfectly follow the equation typical for electron tunneling process with the values of w, n, α and β calculated to be 0.73 (±0.003), 3.99 (±0.02), 1.62 (±0.06) and 1.21 (±0.012), respectively. Most importantly, n ~ 4 is further an evidence of non-linear dependency of oxygen-vacancy drift velocity on the applied potential bias and also supporting the proposed electron tunneling mechanism behind resistive switching.2, 9 However, the value of n is somewhat lower than that for other TiO2-x based devices where a value ranging from 14 to 22 has been reported.9

(a)

Bias voltage = -0.5V (b)

102

10-1

HR State

2

101

101

RS ratio ~ t 100 10

0

1

10

Current (A)

10

RS ratio

Resistance (Ohm)

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-0.0065

10

2

10-2

3

10

Time (s)

LR State

100

101 102 Time (s)

1st Cycle

10-3

103

-1.0

100th Cycle

-0.5

0.0 0.5 Voltage (V)

1.0

Figure 4. (a) Retention data of the ZnO/α-Fe2O3 heterojunction NR electrode during ‘ON’ and ‘OFF’ states, measured at a bias voltage of -0.5 V, inset figure shows the variation of RS ratio 13 ACS Paragon Plus Environment


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with time; (b) showing the semi-log plot of 1st and 100th I-V loop during the endurance test for the ZnO/α-Fe2O3 heterojunction NR electrode. The resistance-retention data for the ZnO/α-Fe2O3 heterojunction NR electrode is plotted in Figure 4a which shows that the resistance, in both HR and LR states, remains almost constant within the measurement duration of up to 103 seconds, indicating non-volatile nature of HR and LR states. Nearly constant resistance in LR state can be attributed to the high retention/stability of the accumulated or trapped charges across the interface. In fact, results suggest that the charge/ionic species do not diffuse away from the interface in time, as observed in case of other heterojunction electrode showing decay in resistance in the course of reaching equilibrium.2 Moreover, the variation of RS ratio with time is also depicted in the inset of Figure 4a. In earlier reports of heterostructure switches, relaxation of current after switching to the LRS followed the Curie-von Schweidler law as R (i ) = R0t α with α < 1.22,

29

However, in the present system the

value of α as calculated from the slope of the straight line log (RS ratio) vs log (t) is found to be 0.0065 (with standard error of 7.6 × 10-5), i.e. RS ratio (t) = 18.5 × t

-0.0065

, further representing

excellent time retention which is far better than other heterostructure junctions reported elsewhere.22, 29 Endurance performance for the electrode has been investigated during 100 RS cycles and the 1st and 100th cycles are depicted in Figure 4b. The switching potential is found to vary during endurance measurements; however, the variation remains within 10 % of the initial value (-0.55 V), which further confirms excellent stability of the switching potential and nonvolatile nature of the LR and HR states. The consistency in retention and endurance performance and more importantly, low-switching-voltage (

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