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Resistive switching in graphene-organic device: Charge transport properties of graphene-organic device through electric field induced optical second harmonic generation and charge modulation spectroscopy
Accepted Manuscript Resistive switching in graphene–organic device: Charge transport properties of graphene-organic device through electric field induced optical second harmonic generation and charge modulation spectroscopy Mohan V. Jacob, Dai Taguchi, Mitsumasa Iwamoto, Kateryna Bazaka, Rajdeep S. Rawat PII:
S0008-6223(16)30972-1
DOI:
10.1016/j.carbon.2016.11.005
Reference:
CARBON 11449
To appear in:
Carbon
Received Date: 4 August 2016 Revised Date:
18 October 2016
Accepted Date: 3 November 2016
Please cite this article as: M.V. Jacob, D. Taguchi, M. Iwamoto, K. Bazaka, R.S. Rawat, Resistive switching in graphene–organic device: Charge transport properties of graphene-organic device through electric field induced optical second harmonic generation and charge modulation spectroscopy, Carbon (2016), doi: 10.1016/j.carbon.2016.11.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Resistive Switching in Graphene–Organic Device:
Charge transport properties of
Graphene-Organic Device through Electric Field Induced Optical Second Harmonic Generation and Charge Modulation Spectroscopy Mohan V. Jacob1*, Dai Taguchi2, Mitsumasa Iwamoto2, Kateryna Bazaka1,3 and Rajdeep 1
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S. Rawat4 Electronics Materials Lab, College of Science and Engineering, James Cook University, Townsville, QLD 4811 Australia 2
Department of Physical Electronics, Tokyo Institute of Technology, O-okayama, Meguro-ku,
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Tokyo 152-8552, Japan 3
School of Chemistry, Physics, Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4000 Australia
Natural Sciences and Science Education, National Institute of Education, Nanyang
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4
Technological University, 637616 Singapore *
Corresponding author: Tel: +61-747814379
Abstract:
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email: [email protected]
Graphene-based resistive random access memory devices is a promising non-volatile memory
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technology that combines low operation voltage and power, extremely fast write/erase speeds, excellent reliability and storage capacity of RRAM with low-cost, large area and flexibility of carbon-based technologies. However, low-cost single-step synthesis of high-quality graphene
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remains a challenge. In this paper, high quality graphene synthesized directly from sustainable carbon source (M. alternifolia oil) was used as electrode and pentacene/C60 as active layers in carbon-based RRAM. I-V measurements were used to demonstrate reproducible switching (rapid increase in current) at certain voltage which was reversible. Charge transport and accumulation was visualized using electric field induced optical second harmonic generation and charge modulation spectroscopy. Hole transport from graphene layer to the organic layer was the primary cause of the observed switching behavior.
Keywords: Graphene; Resistive random access memory; Plasma-enhanced chemical vapour deposition
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Introduction
There is growing demand for novel nonvolatile memory technology compatible with low-cost, large-area, and energy-efficient flexible electronics applications. Resistive random access memory (RRAM) has emerged as the next generation nonvolatile memory device technology
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due to its simple structure, high operation speed, high scalability, and multibit storage potential. RRAM is based on resistance change modulated by electrical stimulus, where high (HRS) and low (LRS) resistive states can be read non-destructively.
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Resistive switching characteristics have been demonstrated in a wide variety of solid-state materials, including solid electrolytes, perovskites, binary transition metal oxides, and amorphous silicon [1, 2]. Although devices based on metal oxide thin films have suitable
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combination of fast switching speed, scalability, and low-power consumption, their specific material characteristics make them unsuitable for use over large-area flexible or temperaturesensitive substrates.
RRAM devices based on carbon materials, including organic semiconductors [3], amorphous
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carbon [4] and graphene oxide [5, 6] have also been demonstrated. The advantages of using carbon-based materials include lower fabrication costs, printability, and simple device structures [7]. Furthermore, if non-volatile memory can be realized in carbon, the logic and memory devices can be integrated on a same carbon-based platform as carbon-based field
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effect transistors.
With its unrivalled combination of electrical, mechanical, thermal and optical properties [7-10],
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graphene has been investigated as a promising electrode material for organic semiconducting and memory devices [11, 12], with graphene-pentacene devices showing rectifying properties. The switching characteristics of organic RRAM devices are strongly influenced by the properties of interfaces, in particular the interface state between an electrode and an organic material as it defines the charge injection barrier, conduction mechanism, and bulk resistance [3]. However, high yield fabrication of quality graphene is expensive and is typically associated with very high process temperatures, use of hazardous chemicals, metallic catalysts, as well as lengthy and complex chemical synthesis procedures [7]. Thus, significant scientific and commercial efforts have been directed towards the development of more economical,
ACCEPTED MANUSCRIPT efficient, and environment- and human-health-friendly approaches for graphene synthesis [8, 13].
In this paper, we report on the single-step fabrication of high-quality large-area graphenes from natural non-toxic M. alternifolia oil vapor [14], and demonstrate a distinctive resistive
graphenes.
Fabricated
devices,
such
as
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switching property in graphene−organic semiconductor interfaces produced using oil-derived graphene/α–NPD/Alq3,
graphene/pentacene,
graphene/C60 and graphene/pentacene/C60 devices, are interrogated using the Electric Field Induced Second Harmonic Generation and Charge Modulation Spectroscopy to explain the
Experiments
2.1 Graphene Fabrication
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2.
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observed resistive switching phenomena.
Plasma-enhanced chemical vapour deposition (PECVD) was used for the fabrication of graphene. The experimental system consists of a custom made quartz tube reactor with heater wound on the quartz tube. A variable voltage controller was used to obtain the temperature of 800 °C. The base pressure of 0.05 mbar was initially obtained. H2 gas was flown at the rate of
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30 sccm and the system pressure was adjusted to 0.20 mbar. The RF energy from the Navio RF Generator operating at 13.56 MHz frequency through the Navio Matching Network was coupled to the reactor using capacitive coupling. The system was optimized for an input RF energy of 500 W and reactor tube temperature of 800 °C. Silicon substrates with 100 nm oxide
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layer were pretreated with plasma for 1 min for surface preparation and cleaning. The vapors of
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Melaleuca alternifolia oil were then introduced into the system using a flow controller.
M. alternifolia oil is a non-synthetic source of carbon. M. alternifolia essential oil is a plantderived precursor rich in carbon compounds such as terpinen-4-ol (C10H18O, 30–48%), γterpinene (C10H16, 10–28%), α-terpinene (C10H16, 5–13%). and 1,8-cineole (C10H18O, 0–15%). The oil of M. alternifolia is volatile, with a flash point of 64°C and vapor pressure of 15.7 Torr, which allows the monomer vapors to be produced under rotary vacuum and room temperature conditions. This makes the oil and its constituents highly suited to vapor-based deposition methods [15-17].
2.2 Graphene - Organic Device Fabrication Different graphene/organic semiconductor structures were fabricated to understand the
ACCEPTED MANUSCRIPT interface charge accumulation/ transport properties of graphene. Layers of C60, pentacene, α−NPD, Alq3, and Al and Ag electrode were deposited onto the graphene layer using vacuum evaporation technique, at a pressure of 10-5 Torr and deposition rate of around 1 nm/min.
2.3 Current−Voltage Measurement
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The current−voltage (I−V) measurement was carried out using Keithley 2400 source meter. DC ramp voltage was applied to the graphene electrode with reference to the grounded Al/Ag electrode. The voltage sweeping rate was approximately 0.1 V/s.
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2.4 Electric Field Induced Optical Second Harmonic Generation (EFISHG) Measurement The EFISHG measurement was performed to directly probe carrier processes in multilayer devices. EFISHG is material-dependent and hence the SHG will depend on the interface
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material and the wavelength of the laser used. EFISHG has been used extensively to selectively probe electrostatic field in a single layer within multilayer devices [18, 19]. In the graphene/pentacene/C60/Al device, injected charges accumulate at the pentacene/C60 interface. Consequently, under DC voltage application, electrostatic field in the C60 layer ( ) is given as
+
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=
(1)
where is the C60 layer thickness, (=1.2 nF) and (=1.8 nF) are capacitance of pentacene and C60 layers, respectively, as an equivalent circuit element in the terms of the Maxwell-
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Wagner effect model [20]. Here, we define the positive electric field as one that points from the graphene to Al electrode in the layers. The SHG light is emitted from the C60 layer with the laser pulse irradiation (laser wavelength: 1000 nm [18]). The intensity of the SHG light is
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proportional to the square of (= ) as (2)
The electric field in the pentacene layer (thickness, ) is obtained by using the relationship = +
(3)
Consequently we can analyze carrier processes ( = − where and are dielectric constant of pentacene and C60, respectively) based on Eqs. (1-3). Noteworthy that using double-layer structure device is very helpful to discuss the space charge field effect (2nd term of eq.(1)) due to accumulated charge and the polarity of transport carriers.
ACCEPTED MANUSCRIPT 2.5 Charge Modulation Spectroscopy Charge modulation spectroscopy (CMS) was carried out using experimental procedures described in our previous study [21]. White light from a halogen lamp was used as a probing light. Briefly, the light was focused on samples with a spot size of 10 µm from silica/graphene
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side. The reflected signal was analyzed by a spectrometer and a high sensitivity cooled charge coupled device (CCD) image sensor. DC voltages were applied to graphene devices, by using Keithley 2400 source meter. CMS records a modulation spectrum − / where is a reflectance spectrum with varied voltage in referenced to spectrum at a reference voltage
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. In present CMS measurement, = −10 V was used. Figures S1 and S2 show the experimental system we used for Electric Field Induced Second Harmonic Generation and
3.
Results and Discussion
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Charge Modulation Spectroscopy.
In order to develop graphene-based resistive devices, high-quality large-area graphene samples were produced from Melaleuca alternifolia essential oil following the procedure outlined in [14]. The oil-derived samples exhibited the signature Raman peaks of graphene. The vertical
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orientation and long reactive edge of large-area defect-free graphene sheets can be effectively utilized in many environmentally-sustainable electronics applications [22-24].
Figure 1 shows the examples of RRAM device structures implemented to study the switching
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property of graphene and the corresponding energy band diagrams for these devices. The organic materials used to separate graphene and aluminum electrode are selectively chosen to
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create different charge transfer conditions from graphene to aluminum electrode.
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characteristics.
3.1 Current–Voltage Characteristics
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Figure 1. Various graphene−organic semiconductor structures used to study the I−V
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Figure 2. Representative I−V characteristics of graphene/α−NPD/Alq3/Al devices. Inset: a sharp increase in current is observed at 3.15V.
Figure 2 shows representative I−V characteristics of graphene/α–NPD/Alq3/Al devices. The voltage was increased gradually and the current shows a sharp increase at 3.15V (see inset in Figure 2), exhibiting the switching behavior. The work function of graphene and gold are
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graphene/pentacene/C60/Al
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graphene/pentacene/C60/Ag. The thickness of pentacene and C60 layers are 200nm. The inset shows the rapid change in current for a very small change in voltage.
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The switching behavior is confirmed in the devices with a double layer of pentacene and C60 (Figure 3). The current increased by 4 orders in magnitude, which is very high compared to similar reported (2 orders in magnitude) work [4]. Typically, it is possible to have charge accumulation at the pentacene/C60 interface. In graphene/pentacene/C60/Al structure, it is
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possible for the electrons to be injected from Al electrode, with C60 promoting the electron injection. However, due to the higher work function of Ag, the electron injection in
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graphene/pentacene/C60/Ag device will be smaller. Devices with different work functions will enable understanding the electron/hole injection and charge transport through the different layers on the resistive switching.
Both devices showed switching during the positive cycle, whereas the switching is not prominent in the negative cycle for graphene/pentacene/C60/Ag device. The current through the device during the high resistive state was in the order of nanoampere whereas during the low resistive state was of the milliampere order. The relative change in the order of resistance from HRS to LRS state is 105 or higher. This is extremely high transition compared to that reported [5, 25] for many other graphene/graphene oxide RRAM devices. The switching property is
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month.
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Figure 4. Representative I–V characteristics of graphene/pentacene/Al and graphene/C60/Al devices.
To confirm that the resistive transition is not due to charge accumulation, I–V characteristics of graphene/pentacene(200 nm)/Al and graphene/C60(200 nm)/Al devices are shown in Figure 4. In the case of pentacene device, the switching voltage was observed at ~3V. In some cycles the
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ONSET current is smaller but the fluctuation is within the experimental error of 1 µA. In the case of C60, the device does not exhibit any switching and the response is purely ohmic. The graphene electrodes have edge states that result in transport channels close to the Fermi level that strongly affect the conductance at switching voltages [26]. According to the energy
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diagrams of these devices shown in Fig.1, energy structures around the Fermi level of graphene
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against C60 and pentacene are notably different. Figures 5 (a) and 5 (b) show the cyclic response of the graphene/pentacene/C60/Al device and the ability to retain HRS/LRS, respectively. The switching behavior was repeated over 100 cycles and the current level was retained over 400 s.
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Figure 5. a) The switching cycle of a representative graphene/pentacene/C60/Al device. The
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switch can be RESET by removing the voltage source (V=0) b) The current at High and Low resistive states are constant over the period.
It has been previously demonstrated in Graphene Oxide devices that resistive switching can take place in the double-ended GO devices, where the carbon-carbon bonds within the carbon cycle would stretch and relax with the absorption and desorption of oxygen atoms, facilitated
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by a filament concentrated electrical field. This was attributed to the increased resistance in the GO as more oxygen containing chemical groups are added to the material, due to higher oxidation ratio and the longer hopping distance of electrons [27]. In carbon devices, the RESET can be facilitated by Joule-heating-induced rupture of the conductive filaments as in
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the case of carbon-based memories or a-C:H memories. In order to understand the switching phenomenon in graphene Electric Field Induced Second Harmonic Generation (EFISHG) and
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Charge Modulation Spectroscopy (CMS) were used. 3.2 EFISHG measurement
In order to directly analyze switching behavior in the graphene device, we employed the electric field induced optical SHG measurement (Fig S1) which selectively probes electric field in the layer of the multilayer device [15]. In the SHG measurement, we probed electric field in the C60 layer selectively, by choosing a laser wavelength of 1000 nm [18]. Figure 6 (a) shows the I−V characteristics of the graphene/pentacene/C60/Al device. The I-V characteristics exhibited hysteresis loops in positive and negative voltage regions. In positive voltage region, the device switches from LRS to HRS1 at = +7.8 V with increasing voltage, whereas the
ACCEPTED MANUSCRIPT device switches from HRS1 to HRS2 at = 5.3 V with decreasing the voltage. In negative voltage region, the device changes from HRS to LRS at =-2.8 V. Figure 6b illustrates the corresponding SHG intensity variation during one voltage cycle. The SHG exhibited hysteresis loops in a way similar to those in the I-V characteristics. In the positive voltage region, the SHG increased with increasing the voltage in the region < ′ (= +6.8 V) whereas the SHG
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further increases in the region > ′ . On the other hand, during the voltage reversal from Vs2 from V′s2 the SHG intensity is relatively stable, before the SHG intensity begins to drop sharply after ′ .
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In order to discuss carrier injection property of graphene/pentacene contact, we converted the SHG intensity to the electric field in semiconducting pentacene layer (for details please refer to the Methodology section). Figure 6c displays the obtained electric field in pentacene where the
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hysteresis loop due to the switching of the device is clearly seen. With increasing the voltage 0 < < ′ , positive charges + and negative charges − are induced on graphene/pentacene contact and Al electrode, respectively. These charges form electric field in the pentacene layer, and assist hole injection from the graphene/pentacene contact and electron
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injection from the Al electrode. As a result, the device is in LRS.
On the other hand, in the region > ′ , hole injection is enhanced which is indicated by significant reduction in rate of increase (or rather saturation) in electric field E1 across the pentacene layer. Accordingly, holes excessively accumulate at the pentacene/C60 interface,
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resulting in the saturation of electric field in the pentacene layer. That is, the electric field in the pentacene layer = 1/ ∙ / + ∙ linearly increases as the voltage increases
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if the excess charge accumulation is negligible, while the electric field = 1/ ∙ / + ∙ − 1/ ∙ / + saturates if hole injection dominates and the relationship of = (>0) is satisfied. It can be noticed that as the voltage excursion continues onwards (in the region > ′ ) then at = the device switches from LRS to HRS1 at (as seen in Figure 6a) indicating the sudden and strong change in carrier injection level but E1 across the pentacene layer continues to increase/change linearly (as seen in Figure 6c). This result evidenced that a change of electric field at the contact never gives rise to the switching behavior but the change of injection property of graphene/pentacene contact is a cause of the switching behavior.
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ACCEPTED MANUSCRIPT Figure 6. Representative I−V characteristics during SHG measurement (a) and SHG measurement (b) of graphene/pentacene/C60/Al devices. (c) Electric field in the pentacene layer of graphene/pentacene/C60/Al device. The switching behavior from HRS1 to HRS2 is similar to this switching behavior from LSR to
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HRS1. In the region > during the voltage reversal, the electric field in pentacene layer linearly decreases with decreasing voltage, during which the device switches from HRS1 to HRS2. In other words, while the current level discontinuously switches from 3.8×10-4 A to 1.3×10-4 A at = , but electric field E1 across the pentacene layer changes only linearly and
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continuously. This result again evidenced that the change of injection property of graphene/pentacene contact is a cause of the switching. These SHG results clearly indicated that carrier injection process at the graphene/pentacene contact governs the switching behavior
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of the graphene/pentacene/C60/Al device. It is noteworthy that we used a double layer device with a structure of graphene/pentacene/C60/Al device, because the accumulated charge at the double-layer interface contributes to the electric field across the pentacene layer, and we can discuss the switching behavior in association with the polarity of accumulated charges. Results of Figure 6 evidently showed that holes injected from the graphene electrode are a source of
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switching behavior. On the other hand, based on the examination of the I-V behavior for singlelayer devices with structures of graphene/pentacene /Al and graphene/C60/Al, it is difficult to definitely state whether injected holes are a main source of switching property of these devices or not, and therefore we can only assume that since holes are a main source on the basis of
Charge Modulation Spectroscopy
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results of the double-layer devices, e.g., graphene/pentacene/C60/Al device.
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Figure 7. The transmission spectra measured at the graphene/pentacene/C60/Al interface using
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Charge Modulation Spectroscopy.
By using Charge Modulation Spectroscopy (Fig S2), we further studied carrier process of graphene device. CMS is an optical method to probe energy state of carrier injected into organic devices. Figures 7 and 8 show the CMS spectra for two graphene samples with different
electrode
system,
i.e.,
graphene/pentacene/C60/Al
device
and
graphene/pentacene/C60/Ag device, respectively. The I−V of these two samples showed
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switching in a similar way as shown in Figure 3. With increasing voltage the device switched from HRS to LRS at around Vs=4 V. However, CMS spectra were different for the two devices. The graphene device with Al electrode showed ∆T/T decrease in the wavelength region at λ = 680 nm whereas the device with Ag electrode showed no CMS absorption peak.
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Here, the CMS absorption peak at λ = 680 nm is identified as electron injection into C60 molecules [19]. Consequently, CMS shows that the electron injection depends on electrode (Al
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or Ag), but never governs switching behavior. This fact also supports the idea that hole injection from the graphene/pentacene interface governs the switching behavior of the device.
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Figure 8. The transmission spectra measured at the graphene/pentacene/C60/Ag interface using Charge Modulation Spectroscopy.
Conclusions
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4.
In summary, we have demonstrated that graphene fabricated from sustainable carbon source, the essential oil of M. alternifolia, is a promising electrode material for the development of large-area, flexible and low-cost nonvolatile resistive memory devices. This was confirmed by high repeatability and the magnitude of transition from HRS to LRS in all graphene-based
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devices studies in this work. By investigating the transport and accumulation of charges at the interface of pentacene/C60 layer in graphene-based switching devices, as well as the switching behavior dependence on the device structure and the composition of organic layers, we showed that the hole transport from graphene layer to the organic layer was the primary cause of the
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observed switching behavior.
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Acknowledgements
Authors acknowledge the financial support of Grant-in-Aid for Scientific Research (S) (Grant No. 22226007) from the Japan Society for the Promotion of Science (JSPS), Japan, and Australian Research Council (DE130101550). MJ also acknowledges the JCU RIB grant.
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D. Taguchi, L. Zhang, J. Li, M. Weis, T. Manaka, and M. Iwamoto, "Analysis of Carrier Transients in Double-Layer Organic Light Emitting Diodes by Electric-FieldInduced Second-Harmonic Generation Measurement," The Journal of Physical Chemistry C, vol. 114, pp. 15136-15140, 2010/09/09 2010. T. Manaka, S. Kawashima, and M. Iwamoto, "Charge modulated reflectance topography for probing in-plane carrier distribution in pentacene field-effect transistors," Applied Physics Letters, vol. 97, p. 113302, 2010. B. F. Machado and P. Serp, "Graphene-based materials for catalysis," Catalysis Science & Technology, vol. 2, pp. 54-75, 2012. D. E. Lobo, P. C. Banerjee, C. D. Easton, and M. Majumder, "Miniaturized Supercapacitors: Focused Ion Beam Reduced Graphene Oxide Supercapacitors with Enhanced Performance Metrics," Advanced Energy Materials, pp. n/a-n/a, 2015. D. H. Seo, Z. J. Han, S. Kumar, and K. Ostrikov, "Supercapacitors: StructureControlled, Vertical Graphene-Based, Binder-Free Electrodes from Plasma-Reformed Butter Enhance Supercapacitor Performance (Adv. Energy Mater. 10/2013)," Advanced Energy Materials, vol. 3, pp. 1250-1250, 2013. S. Ki Hong, J. Eun Kim, S. O. Kim, and B. Jin Cho, "Analysis on switching mechanism of graphene oxide resistive memory device," Journal of Applied Physics, vol. 110, pp. -, 2011. I. A. Pshenichnyuk, P. B. Coto, S. Leitherer, and M. Thoss, "Charge Transport in Pentacene–Graphene Nanojunctions," The Journal of Physical Chemistry Letters, vol. 4, pp. 809-814, 2013/03/07 2013. H. Tian, H.-Y. Chen, B. Gao, S. Yu, J. Liang, Y. Yang, et al., "Monitoring Oxygen Movement by Raman Spectroscopy of Resistive Random Access Memory with a Graphene-Inserted Electrode," Nano Letters, vol. 13, pp. 651-657, 2013/02/13 2013.
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ACCEPTED MANUSCRIPT Supplementary Material
Figures S1 and S2 show the experimental system we used for Electric Field Induced Second Harmonic Generation and Charge Modulation Spectroscopy.
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EFISHG
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CMS
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Figure S1: optical setup for EFISHG measurement.
Figure S2: optical setup for CMS measurement.
S0008-6223(16)30972-1
DOI:
10.1016/j.carbon.2016.11.005
Reference:
CARBON 11449
To appear in:
Carbon
Received Date: 4 August 2016 Revised Date:
18 October 2016
Accepted Date: 3 November 2016
Please cite this article as: M.V. Jacob, D. Taguchi, M. Iwamoto, K. Bazaka, R.S. Rawat, Resistive switching in graphene–organic device: Charge transport properties of graphene-organic device through electric field induced optical second harmonic generation and charge modulation spectroscopy, Carbon (2016), doi: 10.1016/j.carbon.2016.11.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT Resistive Switching in Graphene–Organic Device:
Charge transport properties of
Graphene-Organic Device through Electric Field Induced Optical Second Harmonic Generation and Charge Modulation Spectroscopy Mohan V. Jacob1*, Dai Taguchi2, Mitsumasa Iwamoto2, Kateryna Bazaka1,3 and Rajdeep 1
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S. Rawat4 Electronics Materials Lab, College of Science and Engineering, James Cook University, Townsville, QLD 4811 Australia 2
Department of Physical Electronics, Tokyo Institute of Technology, O-okayama, Meguro-ku,
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Tokyo 152-8552, Japan 3
School of Chemistry, Physics, Mechanical Engineering, Queensland University of Technology, Brisbane, QLD 4000 Australia
Natural Sciences and Science Education, National Institute of Education, Nanyang
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4
Technological University, 637616 Singapore *
Corresponding author: Tel: +61-747814379
Abstract:
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email: [email protected]
Graphene-based resistive random access memory devices is a promising non-volatile memory
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technology that combines low operation voltage and power, extremely fast write/erase speeds, excellent reliability and storage capacity of RRAM with low-cost, large area and flexibility of carbon-based technologies. However, low-cost single-step synthesis of high-quality graphene
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remains a challenge. In this paper, high quality graphene synthesized directly from sustainable carbon source (M. alternifolia oil) was used as electrode and pentacene/C60 as active layers in carbon-based RRAM. I-V measurements were used to demonstrate reproducible switching (rapid increase in current) at certain voltage which was reversible. Charge transport and accumulation was visualized using electric field induced optical second harmonic generation and charge modulation spectroscopy. Hole transport from graphene layer to the organic layer was the primary cause of the observed switching behavior.
Keywords: Graphene; Resistive random access memory; Plasma-enhanced chemical vapour deposition
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Introduction
There is growing demand for novel nonvolatile memory technology compatible with low-cost, large-area, and energy-efficient flexible electronics applications. Resistive random access memory (RRAM) has emerged as the next generation nonvolatile memory device technology
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due to its simple structure, high operation speed, high scalability, and multibit storage potential. RRAM is based on resistance change modulated by electrical stimulus, where high (HRS) and low (LRS) resistive states can be read non-destructively.
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Resistive switching characteristics have been demonstrated in a wide variety of solid-state materials, including solid electrolytes, perovskites, binary transition metal oxides, and amorphous silicon [1, 2]. Although devices based on metal oxide thin films have suitable
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combination of fast switching speed, scalability, and low-power consumption, their specific material characteristics make them unsuitable for use over large-area flexible or temperaturesensitive substrates.
RRAM devices based on carbon materials, including organic semiconductors [3], amorphous
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carbon [4] and graphene oxide [5, 6] have also been demonstrated. The advantages of using carbon-based materials include lower fabrication costs, printability, and simple device structures [7]. Furthermore, if non-volatile memory can be realized in carbon, the logic and memory devices can be integrated on a same carbon-based platform as carbon-based field
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effect transistors.
With its unrivalled combination of electrical, mechanical, thermal and optical properties [7-10],
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graphene has been investigated as a promising electrode material for organic semiconducting and memory devices [11, 12], with graphene-pentacene devices showing rectifying properties. The switching characteristics of organic RRAM devices are strongly influenced by the properties of interfaces, in particular the interface state between an electrode and an organic material as it defines the charge injection barrier, conduction mechanism, and bulk resistance [3]. However, high yield fabrication of quality graphene is expensive and is typically associated with very high process temperatures, use of hazardous chemicals, metallic catalysts, as well as lengthy and complex chemical synthesis procedures [7]. Thus, significant scientific and commercial efforts have been directed towards the development of more economical,
ACCEPTED MANUSCRIPT efficient, and environment- and human-health-friendly approaches for graphene synthesis [8, 13].
In this paper, we report on the single-step fabrication of high-quality large-area graphenes from natural non-toxic M. alternifolia oil vapor [14], and demonstrate a distinctive resistive
graphenes.
Fabricated
devices,
such
as
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switching property in graphene−organic semiconductor interfaces produced using oil-derived graphene/α–NPD/Alq3,
graphene/pentacene,
graphene/C60 and graphene/pentacene/C60 devices, are interrogated using the Electric Field Induced Second Harmonic Generation and Charge Modulation Spectroscopy to explain the
Experiments
2.1 Graphene Fabrication
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2.
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observed resistive switching phenomena.
Plasma-enhanced chemical vapour deposition (PECVD) was used for the fabrication of graphene. The experimental system consists of a custom made quartz tube reactor with heater wound on the quartz tube. A variable voltage controller was used to obtain the temperature of 800 °C. The base pressure of 0.05 mbar was initially obtained. H2 gas was flown at the rate of
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30 sccm and the system pressure was adjusted to 0.20 mbar. The RF energy from the Navio RF Generator operating at 13.56 MHz frequency through the Navio Matching Network was coupled to the reactor using capacitive coupling. The system was optimized for an input RF energy of 500 W and reactor tube temperature of 800 °C. Silicon substrates with 100 nm oxide
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layer were pretreated with plasma for 1 min for surface preparation and cleaning. The vapors of
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Melaleuca alternifolia oil were then introduced into the system using a flow controller.
M. alternifolia oil is a non-synthetic source of carbon. M. alternifolia essential oil is a plantderived precursor rich in carbon compounds such as terpinen-4-ol (C10H18O, 30–48%), γterpinene (C10H16, 10–28%), α-terpinene (C10H16, 5–13%). and 1,8-cineole (C10H18O, 0–15%). The oil of M. alternifolia is volatile, with a flash point of 64°C and vapor pressure of 15.7 Torr, which allows the monomer vapors to be produced under rotary vacuum and room temperature conditions. This makes the oil and its constituents highly suited to vapor-based deposition methods [15-17].
2.2 Graphene - Organic Device Fabrication Different graphene/organic semiconductor structures were fabricated to understand the
ACCEPTED MANUSCRIPT interface charge accumulation/ transport properties of graphene. Layers of C60, pentacene, α−NPD, Alq3, and Al and Ag electrode were deposited onto the graphene layer using vacuum evaporation technique, at a pressure of 10-5 Torr and deposition rate of around 1 nm/min.
2.3 Current−Voltage Measurement
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The current−voltage (I−V) measurement was carried out using Keithley 2400 source meter. DC ramp voltage was applied to the graphene electrode with reference to the grounded Al/Ag electrode. The voltage sweeping rate was approximately 0.1 V/s.
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2.4 Electric Field Induced Optical Second Harmonic Generation (EFISHG) Measurement The EFISHG measurement was performed to directly probe carrier processes in multilayer devices. EFISHG is material-dependent and hence the SHG will depend on the interface
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material and the wavelength of the laser used. EFISHG has been used extensively to selectively probe electrostatic field in a single layer within multilayer devices [18, 19]. In the graphene/pentacene/C60/Al device, injected charges accumulate at the pentacene/C60 interface. Consequently, under DC voltage application, electrostatic field in the C60 layer ( ) is given as
+
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=
(1)
where is the C60 layer thickness, (=1.2 nF) and (=1.8 nF) are capacitance of pentacene and C60 layers, respectively, as an equivalent circuit element in the terms of the Maxwell-
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Wagner effect model [20]. Here, we define the positive electric field as one that points from the graphene to Al electrode in the layers. The SHG light is emitted from the C60 layer with the laser pulse irradiation (laser wavelength: 1000 nm [18]). The intensity of the SHG light is
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proportional to the square of (= ) as (2)
The electric field in the pentacene layer (thickness, ) is obtained by using the relationship = +
(3)
Consequently we can analyze carrier processes ( = − where and are dielectric constant of pentacene and C60, respectively) based on Eqs. (1-3). Noteworthy that using double-layer structure device is very helpful to discuss the space charge field effect (2nd term of eq.(1)) due to accumulated charge and the polarity of transport carriers.
ACCEPTED MANUSCRIPT 2.5 Charge Modulation Spectroscopy Charge modulation spectroscopy (CMS) was carried out using experimental procedures described in our previous study [21]. White light from a halogen lamp was used as a probing light. Briefly, the light was focused on samples with a spot size of 10 µm from silica/graphene
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side. The reflected signal was analyzed by a spectrometer and a high sensitivity cooled charge coupled device (CCD) image sensor. DC voltages were applied to graphene devices, by using Keithley 2400 source meter. CMS records a modulation spectrum − / where is a reflectance spectrum with varied voltage in referenced to spectrum at a reference voltage
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. In present CMS measurement, = −10 V was used. Figures S1 and S2 show the experimental system we used for Electric Field Induced Second Harmonic Generation and
3.
Results and Discussion
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Charge Modulation Spectroscopy.
In order to develop graphene-based resistive devices, high-quality large-area graphene samples were produced from Melaleuca alternifolia essential oil following the procedure outlined in [14]. The oil-derived samples exhibited the signature Raman peaks of graphene. The vertical
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orientation and long reactive edge of large-area defect-free graphene sheets can be effectively utilized in many environmentally-sustainable electronics applications [22-24].
Figure 1 shows the examples of RRAM device structures implemented to study the switching
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property of graphene and the corresponding energy band diagrams for these devices. The organic materials used to separate graphene and aluminum electrode are selectively chosen to
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create different charge transfer conditions from graphene to aluminum electrode.
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characteristics.
3.1 Current–Voltage Characteristics
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Figure 1. Various graphene−organic semiconductor structures used to study the I−V
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Figure 2. Representative I−V characteristics of graphene/α−NPD/Alq3/Al devices. Inset: a sharp increase in current is observed at 3.15V.
Figure 2 shows representative I−V characteristics of graphene/α–NPD/Alq3/Al devices. The voltage was increased gradually and the current shows a sharp increase at 3.15V (see inset in Figure 2), exhibiting the switching behavior. The work function of graphene and gold are
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graphene/pentacene/C60/Al
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graphene/pentacene/C60/Ag. The thickness of pentacene and C60 layers are 200nm. The inset shows the rapid change in current for a very small change in voltage.
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The switching behavior is confirmed in the devices with a double layer of pentacene and C60 (Figure 3). The current increased by 4 orders in magnitude, which is very high compared to similar reported (2 orders in magnitude) work [4]. Typically, it is possible to have charge accumulation at the pentacene/C60 interface. In graphene/pentacene/C60/Al structure, it is
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possible for the electrons to be injected from Al electrode, with C60 promoting the electron injection. However, due to the higher work function of Ag, the electron injection in
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graphene/pentacene/C60/Ag device will be smaller. Devices with different work functions will enable understanding the electron/hole injection and charge transport through the different layers on the resistive switching.
Both devices showed switching during the positive cycle, whereas the switching is not prominent in the negative cycle for graphene/pentacene/C60/Ag device. The current through the device during the high resistive state was in the order of nanoampere whereas during the low resistive state was of the milliampere order. The relative change in the order of resistance from HRS to LRS state is 105 or higher. This is extremely high transition compared to that reported [5, 25] for many other graphene/graphene oxide RRAM devices. The switching property is
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Figure 4. Representative I–V characteristics of graphene/pentacene/Al and graphene/C60/Al devices.
To confirm that the resistive transition is not due to charge accumulation, I–V characteristics of graphene/pentacene(200 nm)/Al and graphene/C60(200 nm)/Al devices are shown in Figure 4. In the case of pentacene device, the switching voltage was observed at ~3V. In some cycles the
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ONSET current is smaller but the fluctuation is within the experimental error of 1 µA. In the case of C60, the device does not exhibit any switching and the response is purely ohmic. The graphene electrodes have edge states that result in transport channels close to the Fermi level that strongly affect the conductance at switching voltages [26]. According to the energy
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diagrams of these devices shown in Fig.1, energy structures around the Fermi level of graphene
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against C60 and pentacene are notably different. Figures 5 (a) and 5 (b) show the cyclic response of the graphene/pentacene/C60/Al device and the ability to retain HRS/LRS, respectively. The switching behavior was repeated over 100 cycles and the current level was retained over 400 s.
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Figure 5. a) The switching cycle of a representative graphene/pentacene/C60/Al device. The
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switch can be RESET by removing the voltage source (V=0) b) The current at High and Low resistive states are constant over the period.
It has been previously demonstrated in Graphene Oxide devices that resistive switching can take place in the double-ended GO devices, where the carbon-carbon bonds within the carbon cycle would stretch and relax with the absorption and desorption of oxygen atoms, facilitated
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by a filament concentrated electrical field. This was attributed to the increased resistance in the GO as more oxygen containing chemical groups are added to the material, due to higher oxidation ratio and the longer hopping distance of electrons [27]. In carbon devices, the RESET can be facilitated by Joule-heating-induced rupture of the conductive filaments as in
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the case of carbon-based memories or a-C:H memories. In order to understand the switching phenomenon in graphene Electric Field Induced Second Harmonic Generation (EFISHG) and
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Charge Modulation Spectroscopy (CMS) were used. 3.2 EFISHG measurement
In order to directly analyze switching behavior in the graphene device, we employed the electric field induced optical SHG measurement (Fig S1) which selectively probes electric field in the layer of the multilayer device [15]. In the SHG measurement, we probed electric field in the C60 layer selectively, by choosing a laser wavelength of 1000 nm [18]. Figure 6 (a) shows the I−V characteristics of the graphene/pentacene/C60/Al device. The I-V characteristics exhibited hysteresis loops in positive and negative voltage regions. In positive voltage region, the device switches from LRS to HRS1 at = +7.8 V with increasing voltage, whereas the
ACCEPTED MANUSCRIPT device switches from HRS1 to HRS2 at = 5.3 V with decreasing the voltage. In negative voltage region, the device changes from HRS to LRS at =-2.8 V. Figure 6b illustrates the corresponding SHG intensity variation during one voltage cycle. The SHG exhibited hysteresis loops in a way similar to those in the I-V characteristics. In the positive voltage region, the SHG increased with increasing the voltage in the region < ′ (= +6.8 V) whereas the SHG
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further increases in the region > ′ . On the other hand, during the voltage reversal from Vs2 from V′s2 the SHG intensity is relatively stable, before the SHG intensity begins to drop sharply after ′ .
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In order to discuss carrier injection property of graphene/pentacene contact, we converted the SHG intensity to the electric field in semiconducting pentacene layer (for details please refer to the Methodology section). Figure 6c displays the obtained electric field in pentacene where the
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hysteresis loop due to the switching of the device is clearly seen. With increasing the voltage 0 < < ′ , positive charges + and negative charges − are induced on graphene/pentacene contact and Al electrode, respectively. These charges form electric field in the pentacene layer, and assist hole injection from the graphene/pentacene contact and electron
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injection from the Al electrode. As a result, the device is in LRS.
On the other hand, in the region > ′ , hole injection is enhanced which is indicated by significant reduction in rate of increase (or rather saturation) in electric field E1 across the pentacene layer. Accordingly, holes excessively accumulate at the pentacene/C60 interface,
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resulting in the saturation of electric field in the pentacene layer. That is, the electric field in the pentacene layer = 1/ ∙ / + ∙ linearly increases as the voltage increases
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if the excess charge accumulation is negligible, while the electric field = 1/ ∙ / + ∙ − 1/ ∙ / + saturates if hole injection dominates and the relationship of = (>0) is satisfied. It can be noticed that as the voltage excursion continues onwards (in the region > ′ ) then at = the device switches from LRS to HRS1 at (as seen in Figure 6a) indicating the sudden and strong change in carrier injection level but E1 across the pentacene layer continues to increase/change linearly (as seen in Figure 6c). This result evidenced that a change of electric field at the contact never gives rise to the switching behavior but the change of injection property of graphene/pentacene contact is a cause of the switching behavior.
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ACCEPTED MANUSCRIPT Figure 6. Representative I−V characteristics during SHG measurement (a) and SHG measurement (b) of graphene/pentacene/C60/Al devices. (c) Electric field in the pentacene layer of graphene/pentacene/C60/Al device. The switching behavior from HRS1 to HRS2 is similar to this switching behavior from LSR to
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HRS1. In the region > during the voltage reversal, the electric field in pentacene layer linearly decreases with decreasing voltage, during which the device switches from HRS1 to HRS2. In other words, while the current level discontinuously switches from 3.8×10-4 A to 1.3×10-4 A at = , but electric field E1 across the pentacene layer changes only linearly and
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continuously. This result again evidenced that the change of injection property of graphene/pentacene contact is a cause of the switching. These SHG results clearly indicated that carrier injection process at the graphene/pentacene contact governs the switching behavior
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of the graphene/pentacene/C60/Al device. It is noteworthy that we used a double layer device with a structure of graphene/pentacene/C60/Al device, because the accumulated charge at the double-layer interface contributes to the electric field across the pentacene layer, and we can discuss the switching behavior in association with the polarity of accumulated charges. Results of Figure 6 evidently showed that holes injected from the graphene electrode are a source of
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switching behavior. On the other hand, based on the examination of the I-V behavior for singlelayer devices with structures of graphene/pentacene /Al and graphene/C60/Al, it is difficult to definitely state whether injected holes are a main source of switching property of these devices or not, and therefore we can only assume that since holes are a main source on the basis of
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Figure 7. The transmission spectra measured at the graphene/pentacene/C60/Al interface using
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Charge Modulation Spectroscopy.
By using Charge Modulation Spectroscopy (Fig S2), we further studied carrier process of graphene device. CMS is an optical method to probe energy state of carrier injected into organic devices. Figures 7 and 8 show the CMS spectra for two graphene samples with different
electrode
system,
i.e.,
graphene/pentacene/C60/Al
device
and
graphene/pentacene/C60/Ag device, respectively. The I−V of these two samples showed
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switching in a similar way as shown in Figure 3. With increasing voltage the device switched from HRS to LRS at around Vs=4 V. However, CMS spectra were different for the two devices. The graphene device with Al electrode showed ∆T/T decrease in the wavelength region at λ = 680 nm whereas the device with Ag electrode showed no CMS absorption peak.
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Here, the CMS absorption peak at λ = 680 nm is identified as electron injection into C60 molecules [19]. Consequently, CMS shows that the electron injection depends on electrode (Al
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or Ag), but never governs switching behavior. This fact also supports the idea that hole injection from the graphene/pentacene interface governs the switching behavior of the device.
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Figure 8. The transmission spectra measured at the graphene/pentacene/C60/Ag interface using Charge Modulation Spectroscopy.
Conclusions
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In summary, we have demonstrated that graphene fabricated from sustainable carbon source, the essential oil of M. alternifolia, is a promising electrode material for the development of large-area, flexible and low-cost nonvolatile resistive memory devices. This was confirmed by high repeatability and the magnitude of transition from HRS to LRS in all graphene-based
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devices studies in this work. By investigating the transport and accumulation of charges at the interface of pentacene/C60 layer in graphene-based switching devices, as well as the switching behavior dependence on the device structure and the composition of organic layers, we showed that the hole transport from graphene layer to the organic layer was the primary cause of the
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Acknowledgements
Authors acknowledge the financial support of Grant-in-Aid for Scientific Research (S) (Grant No. 22226007) from the Japan Society for the Promotion of Science (JSPS), Japan, and Australian Research Council (DE130101550). MJ also acknowledges the JCU RIB grant.
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Figures S1 and S2 show the experimental system we used for Electric Field Induced Second Harmonic Generation and Charge Modulation Spectroscopy.
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Figure S1: optical setup for EFISHG measurement.
Figure S2: optical setup for CMS measurement.