Article pubs.acs.org/Langmuir
Nonvolatile RRAM Cells from Polymeric Composites Embedding Recycled SiC Powders Anna De Girolamo Del Mauro,* Giuseppe Nenna, Riccardo Miscioscia, Cesare Freda, Sabrina Portofino, Sergio Galvagno, and Carla Minarini UTTP-NANO, ENEA, Piazzale E. Fermi 1, 80055 Portici, Italy ABSTRACT: Silicon carbide powders have been synthesized from tires utilizing a patented recycling process. Dynamic light scattering, Raman spectroscopy, SEM microscopy, and X-ray diffraction have been carried out to gather knowledge about powders and the final composite structure. The obtained powder has been proven to induce resistive switching in a PMMA polymer-based composite device. Memory effect has been detected in twoterminal devices having coplanar contacts and quantified by read−write− erase measurements in terms of level separation and persistence.
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INTRODUCTION
The reaction mechanism is not completely explained, but it is commonly accepted that the carbothermal reduction involves the following fundamental steps:
Silicon carbide (SiC) is a high value material used in a broad range of products and applications. Because of its intrinsic properties, SiC is one of the best choices as material for advanced applications.1−4 In fact, its properties include high hardness and strength, excellent corrosion/oxidation resistance, good high-temperature strength, and high thermal conductivity, which makes it an optimal choice as reinforcement for ceramic composites5 as well as an array of high added value products used in such industries as semiconductors, diesel particulate filters, measurement instruments, ceramic membranes, heating elements, and catalytic support productsamong many others. In recent years, these markets have grown, and they are expected to keep growing in the near future with rapid extension once more cost-effective alternatives will be available. In the microelectronic industry, SiC is one of the emerging semiconductors because of its large bandgap, ranging from 2.4 eV (3C-SiC) to 3.4 eV (2H-SiC) according to the polytype.6 SiC powders can be produced in three principal ways: direct carbonization of Si metal,7 pyrolysis of silane compounds,8 and carbothermal reduction of SiO2. The third method, the carbothermal reduction, is the cheapest one: it starts from inexpensive silicon dioxide and carbon (or precursors like carbon and silica sources) that react at temperature ranging from 1600 to 2100 °C to give SiC. The SiC phase depends on temperature synthesis; typically, at low temperature (below 1600 °C) β-phase prevails while α-SiC is the principal phase at higher temperature (up to 2100 °C). The synthesis is described by the equation9,10 SiO2 (s) + 3C(s) → SiC(s) + 2CO(g) © 2014 American Chemical Society
SiO2 (s) + C(s) → SiO(g) + CO(g)
(2)
SiO(g) + 2C(s) → SiC(s) + CO(g)
(3)
First the reaction generates the intermediate SiO(g) that in the second step reacts with carbon to form the final product. Additionally, the carbothermal reduction method allows the use of secondary raw materials as reactants to produce SiC. Within this framework, synthesis via carbothermal reduction could open a new way for high added value exploitation of the thermochemical conversion residues;11,12 in fact, this class of reactions actually plays an important role in ceramic powders synthesis,13−15 like SiC, Si3N4, TiC, NbC, etc. However, often the use of secondary raw materials introduces impurities in the product that strongly limit the range of exploitations; this effect has been not duly investigated in the literature, and few works report applications studies on the synthesized materials. The mechanical properties of the tire-derived SiC have been investigated in a previous work16 where its use was investigated in optoelectronic applications. Applications of SiC dispersions are appealing from the processability point of view because they are able to exploit the nanoscale features of the carbide powders. Such applications range from glass fabric reinforced epoxy composites with modified dielectric properties17 to electro-optical modulators;18 furthermore, they have been proposed as candidate for blue and Received: August 1, 2014 Revised: September 19, 2014 Published: September 26, 2014
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Table 1. Thickness and Roughness of SiC, PMMA, and PMMA−SiC Composite Films, Reported in Association with Mixtures and Compositions of PMMA−SiC Dispersions no.
samples
% w/w (solution)
mg/mL (solution)
mg SiC
1 2 3
SiC PMMA PMMA-67 wt % SiC
4.50 10 3.55
50.00 123.85 41.67
1000 250
ultraviolet (UV) light emitters in displays19 in substitution of silicon crystallites which are not suitable for stable and intense emission in this wavelength range. Literature experiments have studied electrical conduction properties of SiC composites obtained by using NbC, Ti, SiO2, and Y2O3 additives and hot pressing,20 but there is still a lack in the knowledge of charge transport in SiC−polymer composites. As it could be expected, to obtain electrical continuity in a composite, it is not necessary to fill any interstice in the primary phase. In fact, theoretical studies on bicomponent mixtures show that for electrical conduction it is not necessary that the primary particle surface is covered by the dispersed phase: to obtain this effect, the dispersed particles have to be rather located in favorable positions or may be occluded into interstices.21 In addition to charge percolation,22 in the determination of electrical properties of the composite the role played by the organic/inorganic interfaces should be carefully considered:18 they are indeed crucial in charge storage/charge trapping devices and photodetectors. In the field of electrically switching electronic devices, memory effects have been reported and discussed in the literature for nonorganic23−26 and organic27,28 based devices manufactured in a stacked configuration. For a variety of metal/insulator/metal (MIM) two-terminal devices structures, reversible switching behaviors have been reported28,29 exhibiting two separate conduction states. Then, many efforts have been focused on the integration of such kinds of devices in an addressable memory structure having a 2Dfashioned matrix shape to take advantage of cross-point geometry layouts.30 Switching behaviors have been observed in devices embedding nanoparticles in organic layers31 or nanoclusters in layered stacks.32,33 This class of organic devices exhibits a switching behavior associated with charge storage in charged islands34−36 within electrically active regions comprised between the electrodes. By following this approach, it has been also hypothesized that resistive random access memories (RRAM) could be fabricated by the employment of resistive switching37 devices of the kind described before, allowing to envision high data densities, data durability and persistence, and fast switching. Organic memory layouts having coplanar contacts allow to relieve from the complexity of a stacked topology design and processing and to simplify the adoption of solution processing techniques such as drop-casting and inkjet printing. Furthermore, this device configuration is useful for research purposes because it exposes the switching region to further analyses allowing to inspect the switching mechanism. This approach has been already followed in the literature by Janousch et al.38 for inorganic resistive switching devices investigations. In the present work, tire-derived SiC has been used for the preparation of PMMA-based composites to explore the electronic properties of these materials. Organic-based composite embedding SiC powders obtained from a patented recycling process39 have been demonstrated to induce resistive
mg PMMA
thickness (nm)
roughness (Rq) (nm)
2477 123.85
1100 993
102 205
switching in a polymer-based composite spun-coated on a glass substrate. Gold (Au) coplanar contacts were evaporated on PMMA−SiC composite to complete the device structure. The memory effect was detected and quantified by read−write− erase measurements in terms of level separation and persistence.
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EXPERIMENTAL SECTION
Synthesis of Silicon Carbide. SiC powder was synthesized from tire pyrolysis char and commercial silicon dioxide (Carlo ERBA silica gel 60 230−400 mesh ASTM). The carbon matrix was milled into particles up to 1 mm in diameter, mixed with silica gel in a mixer drum for 2 h, fixing the powder weight ratio SiO2/C at 1.514 wt %. After being grounded, samples were maintained under ambient conditions. The reactions were conducted at 1650 °C in a high-temperature tubular furnace (Nabertherm 120/300/1800), with residence time of 1.5 h, in an argon atmosphere (flow rate of 50 L/h). Samples of about 20 g were treated in alumina boats. The product yields were very close to the theoretical value (nearly 35%). SiC powder was purified from the residual carbon by treating the samples at 700 °C in oxidizing atmosphere and from the residual silica by dissolving the samples in hydrofluoric acid 50% in weight solution and by subsequently washing the residues with deionized water. Considering the nature of the precursors (waste-derived materials), metallic impurities (like iron, zinc, and chromium) could be present in the final product. To get a high purity ceramic compound, a method for the purification from metals was set up: basically the ceramic precursors or the ceramic compound itself were subject to a combination of purification steps conducted with chelating agents and basic solutions; the process was able to drastically reduce the metal content and was recently patented by the authors.40 Preparation of PMMA/SiC Composites. Before preparing composite with PMMA, SiC powder was charged and sealed in an agate vial together with five agate balls. The ball-milling experiments were performed in a high-energy planetary ball mill (Retsch PM 100) at a rotation speed of 5 s−1 in an air atmosphere for 20 min. Then, SiC powder was dispersed in chlorobenzene at concentrations of up to 50 mg/mL. The dispersion of the ceramic powder was sonicated for 10 min in a ultrasonic bath at 100 W at ambient temperature, and it was left to stand undisturbed until no visible sign of aggregation was detectable. Polymer solution was prepared by dissolving PMMA (Mw ≅ 120 000, Aldrich) in chlorobenzene at a concentration of 123.85 mg/mL. Once completely dissolved, SiC dispersion was added to the polymeric solution and dispersed by physical agitation through ultrasonication treatment. PMMA−SiC composite film was deposited by spin-coating onto glass substrates. Glass substrates were precleaned with detergent in boiling water for 2 h in an ultrasonic bath and successively rinsed with isopropyl alcohol and finally dried under nitrogen flow. All films were cured in oven at 100 °C for 2 h (Table 1). From Table 1, it is observed that the thickness of the films decreases when the roughness increases. Characterization. The dispersions were characterized by dynamic light scattering (DLS) with a HPPS 3.1 system from Malvern Instruments (Worcestershire, UK) at different aging times to evaluate the size distribution and time stability of the SiC aggregates in the suspensions. The film thicknesses and the roughness were measured by KLA Tencor P-10 surface profiler. UV−vis optical transmittance analysis of layers deposited on glass substrates were carried out by using PerkinElmer Lambda 900 spectrophotometer. Phase identi12422
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fication of these samples was performed by the X-ray diffraction (XRD) method on an X’Pert MPD diffractometer using nickel-filtered Cu Kα radiation in the range of 2θ = 20°−80°. The morphology and element content of the sintered samples were observed by scanning electron microscopy (SEM, Model: LEO 1530) and EDX (energy dispersive X-ray) spectral analysis. Raman measurements have been carried out on the samples with a Renishaw inVia Reflex Raman spectrometer. The Raman spectra were recorded at room temperature with an optical microscope using the 514 nm line of an Ar+ laser (Laser Physics) as excitation source. The electrical conductivity of the films was evaluated by carrying out current−voltage (I−V) measurements. For this analysis, an interdigitated Au structure was evaporated on top of the spin-coated films on glass substrates.
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RESULTS AND DISCUSSION Figure 1 shows particle size distribution measured by the DLS, conducted suspending the SiC powder in chlorobenzene. The
Figure 2. XRD Patterns of pure PMMA, Td-SiC, and PMMA-67 wt % SiC.
Figure 1. Dynamic light scattering pattern of the SiC in chlorobenzene (A) and PMMA−SiC composite material (B).
average size obtained for SiC was in the range 200−1000 nm, with an average particle diameter of 450 nm and with a polydispersity index of 0.25 (Figure 1A). Instead, the size of SiC particles in PMMA matrix (Figure 1B) was found to be around 430 nm with a polydispersity index of 0.17. It is observed that the average sizes of the SiC particles in both samples are similar, and this indicates a good dispersion of SiC in the polymer matrix. The coating of the SiC particles with PMMA matrix was first identified through analyzing the peaks in the XRD patterns of the SiC and PMMA−SiC composite. The XRD patterns of the SiC and PMMA−SiC composite are presented in Figure 2. Xray diffraction patterns reveal that the SiC produced is highly crystalline, with the signals at diffraction angle 2θ equal to 35.7°, 41.5°, 60.0°, and 71.8° which can be attributed both to α and β phases, the signal at 75.5° attributed to β form only, and the signals at 33.7° and 38.1° attributed to α-SiC (Figure 2). As expected under these experimental conditions, β-phase prevails on α-SiC. It is also apparent from Figure 2 that the X-ray pattern of PMMA matrix shows broad bands peaking, indicating its amorphous nature and peaks characteristics of crystalline SiC. These results indicate that the SiC nanoparticles are coated with PMMA. Figure 3 shows the Raman spectrum of synthesized SiC and of PMMA−SiC composite recorded at room temperature. The peaks centered at 796 and 966 cm−1 could be attributed to the SiC TO and LO mode, respectively;41 the structure of the peaks suggests the presence of α and β phases.
Figure 3. Raman spectra of the Td-SiC and of the PMMA−SiC composite.
SiC embedded in PMMA was also examined by SEM. The SEM images of SiC purified and PMMA−SiC composite containing 67 wt % SiC are shown in Figure 4. For SiC powder, the analysis reveals that there is a broad particle size distribution in the powder, on average ranging from 100 to 500 nm. In any case the larger particles found are below 1 μm. In addition, the SEM micrographs reveal the presence of a very low amount of whiskers and bamboo-like fibers; these structures are usually produced by other synthesis processes (for example CVD), but they could be found as byproduct of the SiC powders produced by carbothermal synthesis42,43 when the reaction is conducted at high temperature and in the presence of metallic impurities (i.e., Fe, Zn, etc.). The fibers can grow according to several secondary gas−gas reactions, like the following: SiO(g) + 3CO(g) → SiC(s) + 2CO2 (g) 3SiO(g) + CO(g) → SiC(s) + 2SiO2 (s)
However, the quantity of the fibers detected by SEM analysis is so low to not significantly affect the properties of the sample. 12423
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Figure 4. SEM micrograph of synthesized SiC (A) and PMMA−SiC composite (B).
Figure 5. EDX patterns for Td-SiC (spot 1) and PMMA−SiC composite (spot 2).
For the PMMA−SiC composite, the SiC particles are completely embedded in the polymer matrix (Figure 4B). To identify the distribution of SiC in PMMA, EDX analysis was done to reveal the existing elements in the composite (Figure 5). In particular, Figure 5 shows the EDX patterns for spot 1 (SiC particle) and spot 2 (matrix phase). While for spot 1, the EDX analysis clearly shows Si and C peaks detected on the SiC particles, for spot 2, the EDX analysis exhibits Si, C, and O peaks (the presence of O peak identifies the polymer matrix material). To manufacture the devices, PMMA−SiC composite was spin-coated on a glass substrate, and after the deposition of the dispersion, Au-interdigitated contacts were evaporated. The scheme and the layout of the contacts of the substrate are reported in Figure 6. Geometry has been evaluated by processing 10× microscope images calibrated by profilometric measurements. The electric performances of the devices were studied by current−voltage (I−V) measurements. Figure 7A shows the I−V characteristics of the composite-based device PMMA−SiC, recorded by sweeping the voltage between −25 and 25 V. After the first voltage sweep ranging from 0 to 25 V, voltage cycles have been repeated 10 times. In Figure 7B the same voltage scans are reported in a log−log scale to compare for positive biases the slopes of the two different states. In particular, each I−V characteristic changes its slope from a near-ohmic regime for low biases to a space-charge-limited current (SCLC) regime for high voltages. The last cycle is reported in Figure 8A where we can observe the bistable behavior and two distinct states. In detail, each state is characterized by a rectifying I−V response. The electrical response is switched to its complementary curve by exceeding a defined voltage bias and going through a negative differential
Figure 6. Contacts geometry on glass substrate and their characteristics.
resistance (NDR) branch. After the state is switched, the device acts again as a rectifying diode but in the opposite direction. Symmetrically, to switch to the initial uncomplemented state, the device has to be reversely biased to get over a negative threshold and its associate NDR region. As can be observed in Figure 8A and more schematically in Figure 8B, the device has the dynamic response of a nonvolatile, bistable resistive memory. Its behavior can be defined bipolar because two opposite voltage levels are necessary to switch the device between its conducting states.44 The slight variations in the current values of the negative and 12424
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Figure 7. I−V hysteresis curves up to the stabilization (A) and the relative log−log plot of the positive branch (B).
Figure 8. I−V hysteresis curves after the stabilization (A) and schematic representation of the device operation (B).
appearance. The OFF state is partially characterized by the ohmic conduction (exponent 1.09). In detail, as a consequence of the I−V analysis, we assume that the OFF state is determined by the presence of charged submicrometric particles in the composite. The charge trapped by the particles induces a partial space charge effect which inhibits the charge injection from the electrodes and limits charge transport. On the contrary, the ON state corresponds to uncharged polymer/particles interface.35 According to this theory, the initial ON state is observed below the NDR region because higher bias should be applied to restore the same charge level in the particles. By biasing the device at voltages over VP+ (and vice versa below VP−), the behavior is switched between two symmetric rectifying conditions. Similarities to resistive switching in planar contacts, memories can be observed in the literature for nonlinear memristive devices.46 A similar switching mechanism has been observed and explained by Asadi et al.47 for interpenetrating networks of different polymers where the diode poling procedure modified the charge injection at the contacts interface. In the cited work,47 a bistable rectifying resistive switch has been demonstrated by depositing the poorly injecting contacts on a mixture of a high-bandgap polymer and an organic semiconductor forming an interpenetrated network. The metal−polymer interfaces were switched by bias application between two rectifying states having opposite polarity. In the referred case, the switching phenomenon was operated by the accumulated charge and to its consequent band bending. Once bands were persistently bended at the contacts interface, the injection barrier was lowered enough to switch the junction to an ohmic behavior.
positive branches are due to the very low difference in the current pathway through the network of SiC islands between the electrodes changing the polarities. In the last cycles the curves become more stable with a NDR effect arising from 15 to 25 V. It is known that NDR shows in many nanocompositebased organic memories.31,45 Therefore, the two states of the device can be named after the current magnitudes measured in a given bias condition. In particular, the device is in its forward state (namely ON state) if it acts as a forwardly-biased rectifying junction; conversely, it is in the reverse state (OFF state) if it is behaving as a diode connected in a direction opposite to the bias polarity (a reversely biased junction). When the device is in the forward state and it is forwardbiased, by keeping the voltage below the NDR region, it persists in its state as required for nonvolatile memories. When a forward voltage threshold (VP+) is overcome, the device goes through the NDR area and switches off to the reverse state. The diode behavior, common in tunnel diodes, should be observable after the NDR region. Such bias condition cannot be reached because the applied voltages were not high enough. The forward state is then recovered by applying a negative voltage, and the transition is induced by the same mechanism. In such case, when reverse biased, the device switches to the reverse (complemented) state when bias goes under a second threshold voltage (VP−) as shown in Figure 8A,B. In the ON state, in the forward bias branch of the log−log plot (Figure 7B), between 3 and 14 V a power law characteristic (I ∼ Vm) has been detected, and its exponent has been estimated from a linear fit to be about 1.42. This value allows to associate a trap charge limited current (TCLC) model to this region. The accumulated charge limits the current flow until reaching too filled pathways and determining the NDR 12425
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Figure 9. Read−write−erase cycles. Vread = +5 V, Vprogram = +20 V, Verase = −20 V.
Likewise to Asadi’s device,47 the system acts as two nonvolatile bistable diodes (Figure 8B inset) connected with the anode of the first at the cathode of the second in a ring configuration which respects the geometric symmetry breaking the electrical characteristic symmetry by means of interface poling. As a consequence, the two rectifying states are toggled reversibly. Then, we suggest that in our device the poling procedure defined by a bias point over the switching voltage Vp is able to modify the Au/composite interfaces to switch from an injecting contact to a blocking contact and vice versa, according to the voltage pulse polarity. Material-specific and interface poling effects can coexist in the same device, and further investigations should be done to quantify which amount of each behavior is responsible of the experimental traces. In the obtained RRAM cell two logic levels are distinguishable: the forward state having high conductance in forward bias and a reverse state having low conductance in the same bias conditions. The cell reading is performed nondestructively when the reading bias is kept between −10 and +10 V. Programming is accomplished by boosting the bias potential to a writing condition characterized by high fields. In detail, the reverse state can be written by bringing the bias at V > 16 V while the forward state is written into the cell when the bias potential is below −15 V. To establish the workability of such devices and their capability to store the information RWE (read−write−erase), several cycles are performed.48,49 In this case, as it is possible to observe from Figure 9, we performed the readings at 5 V while the change in the states was accomplished using a bias of +20 V (to store the off status) or −20 V (to store the on status). Again, the transients during the status storage show how long the trapped charge inhibits the current flow before the final status is reached and persisted. In this case, to obtain the reverse state persistence, it is necessary to hold the programming voltage for about 25 s. Such a long time to store information is expected for a coplanar structure because of the large area between gaps and the large contacts surface providing numerous charge storage sites. The adoption of the device as a test case is then justified because it allows appreciating storage transients and induces complementary rectifying switching as suggested by Linn et al.50 The difference in the reading current between the reverse state and the forward state is about 0.14 μA. Such a difference is about an order of magnitude between the two persisted states.
By observing the conductance−voltage plots in Figure 9 and comparing it to Figure 8A, charge storage begins at biases lower than the beginning of the NDR region; that is why the reading voltage has to be comprised between VT− and VT+ to guarantee from destructive readings in RWE cycles. In addition, to avoid incomplete writings, voltages have to overcome the VP+ and VP− thresholds (Figure 10). This mechanism is due to the
Figure 10. Comparison between small signal conductance acquired at zero bias at f = 100 Hz and dc differential conductance.
trap−detrapping time that changes in reason for the applied frequency both in bulk and at the interfaces.49 In particular at zero frequency we will reach the maximum in the applied voltage to observe the switching, as stated by the zero-crossing points in the differential resistance plots in Figure 10. We tried also other wt %, and we found the best results in terms of hysteresis amplitude and stability using 67% of SiC powders. In particular, it seems that particle distribution is even more effective on the device performance respect to the powder concentration. The interface characterized by a more uniform distribution of SiC particles could have better and more reproducible hysteresis loops. For lower concentrations, with the appearance of bigger badly connected clusters, we found an insulating behavior that takes place lowering the current also of 2 orders of magnitude. Hysteresis cycles were more unstable and less pronounced. For higher concentrations we found a “too” connected network, obtaining a general increase in the current magnitude but also a reduction in terms of hysteresis cycles amplitude. Moreover, the loop’s stability was difficult to reach, and several cycles were necessary to stabilize the current 12426
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(7) Esposito, L.; Sciti, D.; Piancastelli, A.; Bellosi, A. Microstructure and properties of porous b-SiC templated from soft woods. J. Eur. Ceram. Soc. 2004, 24, 533−540. (8) Zhang, Y.; Wang, N.; He, R.; Chen, X.; Zhu. Synthesis of SiC nanorods using floating catalyst. J. Solid State Commun. 2001, 118, 595−598. (9) Han, H.-W.; Liu, H.-S. Characterization of vapour deposited products in furnace tube during SiC synthesis from carbonized rice hulls. Ceram. Int. 1999, 25, 631−637. (10) Koc, R.; Cattamanchi, S. V. Synthesis of beta silicon carbide powders using carbon coated fumed silica. J. Mater. Sci. 1998, 33, 2537−2549. (11) Qian, J.-M.; Wang, J. P.; Qiao, G.-J.; Jin, Z.-H. Preparation of porous SiC ceramic a woodlike microstructure by sol-gel and carbothermal reduction processing. J. Eur. Ceram. Soc. 2004, 24, 3251−3259. (12) Qian, J.-M.; Wang, J. P.; Jin, Z.-H. Preparation of biomorphic SiC ceramic by carbothermal reduction of oak wood charcoal. Mater. Sci. Eng., A 2004, 371, 229−235. (13) Wang, J.; Ishida, R.; Takarada, T. Carbothermal reactions of quartz and kaolinite with coal char. Energy Fuels 2000, 14, 1108−1114. (14) Weimer, A. W. In Carbide, Nitride and Boride Materials. Synthesis and Processing; Chapman & Hall: London, 1997. (15) Alcalà, M. D.; Criado, J. M.; Real, C. Influence of the experimental conditions and the grinding of the starting materials on the structure of silicon nitride synthesised by carbothermal reduction. Solid State Ionics 2001, 141−142, 657−661. (16) Magnani, G.; Brentari, A.; Galvagno, S.; Sico, G. Toughening of silicon carbide-based materials. Acta Fracturae IGF XXII 2013, 383− 388. (17) Suresha, B.; Chandramohan, G.; Renukappa, N. M.; Siddaramaiah. Influence of silicon carbide filler on mechanical and dielectric properties of glass fabric reinforced epoxy composites. J. Appl. Polym. Sci. 2009, 111, 685−691. (18) Kassiba, A.; Bouclé, J.; Makowska-Janusik, M.; Errien, N. Some fundamental and applicative properties of [polymer/nano-SiC] hybrid nanocomposites. J. Phys. (Conf. Ser.) 2007, 79, 1−10. (19) Fan, J. Y.; Wu, X. L.; Chu, P. K. Low-dimensional SiC nanostructures: Fabrication, luminescence, and electrical properties. Prog. Mater. Sci. 2006, 51, 983−1031. (20) Frajkorová, F.; Hnatko, M.; Lencés, Z.; Sajgalík, P. Electrically conductive silicon carbide with the addition of Ti NbC. J. Eur. Ceram. Soc. 2012, 32 (10), 2513−2518. (21) Kusy, R. P. Influence of particle size ratio on the continuity of aggregates. J. Appl. Phys. 1977, 48, 5301−5305. (22) Kirkpatrick, S. Percolation and conduction. Rev. Mod. Phys. 1973, 45, 574−588. (23) Lee, W.; Park, J.; Son, M.; Lee, J.; Jung, S.; Kim, S.; Park, S.; Shin, J.; Hwang, H. Excellent state stability of Cu/SiC/Pt programmable metallization cells for nonvolatile memory applications. IEEE Electron Device Lett. 2011, 32, 680−682. (24) Lee, W.; Siddik, M.; Jung, S.; Park, J.; Kim, S.; Shin, J.; Lee, J.; Park, S.; Son, M.; Hwang, H. Effect of Ge2Sb2Te5 thermal barrier on reset operations in filament-type resistive memory. IEEE Electron Device Lett. 2011, 32, 1573−1574. (25) Zhong, L.; Reed, P. A.; Huang, R.; de Groot, C. H.; Jiang, L. Resistive switching of Cu/SiC/Au memory devices with a high ON/ OFF ratio. Solid-State Electron. 2014, 94, 98−102. (26) Zhong, L.; Jiang, L.; Huang, R.; de Groot, C. H. Nonpolar resistive switching in Cu/SiC/Au non-volatile resistive memory devices. Appl. Phys. Lett. 2014, 104, 093507. (27) Kang, S. H.; Crisp, T.; Kymissis, I.; Bulovíc, V. Memory effect from charge trapping in layered organic structures. Appl. Phys. Lett. 2004, 85 (20), 4666−4668. (28) Park, J.-G.; Nam, W.-S.; Seo, S.-H.; Kim, Y.-G.; Oh, Y.-H.; Lee, G.-S.; Paik, U.-G. Multilevel nonvolatile small-molecule memory cell embedded with Ni nanocrystals surrounded by a NiO tunneling barrier. Nano Lett. 2009, 9 (4), 1713−1719.
levels in the RWE characteristic, leading to a further reduction in the logic levels separation.
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CONCLUSIONS Nonvolatile RRAM cells were produced from polymeric composites embedding SiC powders obtained by the tire recycling process. Tire-derived SiC has been analyzed by DLS, X-ray diffraction, SEM imaging, and Raman spectroscopy. The SiC powders were embedded in PMMA-based composites preparations to explore the electronic properties of these materials. Memory effect with NDR has been observed. To investigate the evidence of RRAM mechanism, I−V, RWE, and G−V cycles have been performed. The reported complementary rectifying switching mechanism allowed reaching about 1 order of magnitude in current difference between the two stable current states. We suggested the explanation of the memory behavior to be found in bias-induced interface modification effects. In this framework, metal−composite barriers are modulated persistently by bias conditions; therefore, charge injection modifications reflected in a transition between a rectifying and an ohmic state. This hypothesis has to be confirmed by further work necessary to inspect the relationship between surface morphology of the compound and the electrical switching performances. This result can be considered remarkable if compared to the simplicity and the extension of the geometry and allowed us to envision more performing architectures obtainable by redesigning topologies in order to not disclose the underlying physics but to take advantage by integration and switching times.
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AUTHOR INFORMATION
Corresponding Author
*E-mail [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The present research has been supported by the Italian Ministry of Education, University and Research (MIUR) through the National Project entitled SMARTAGS (PON02_00556_3420580) and partially by the Seventh Framework Programme (FP7) 2007−2013, in the frame of the TyGRe Project (Contract No. 226549).
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(49) Huang, S.; Banerjee, S.; Oda, S. Temperature and frequency dependencies of charging and discharging properties in MOS memory based on nanocrystalline silicon dot. Mater. Res. Soc. Symp. Proc. 2002, 715. (50) Linn, E.; Rosezin, R.; Kügeler, C.; Waser, R. Complementary resistive switches for passive nanocrossbar memories. Nat. Mater. 2010, 9, 403.
(29) Cölle, M.; Buchel, M.; de Leeuw, D. M. Switching and filamentary conduction in non-volatile organic memories. Org. Electron. 2006, 7 (5), 305−312. (30) Scott, J. C.; Bozano, L. D. Nonvolatile memory elements based on organic materials. Adv. Mater. 2007, 19 (11), 1452−1463. (31) Nenna, G.; Masala, S.; Bizzarro, V.; Re, M.; Pesce, E.; Minarini, C.; Di Luccio, T. Particle size dependence of resonant-tunneling effect induced by CdS nanoparticles in a poly(N-vinylcarbazole) polymer matrix. J. Appl. Phys. 2012, 112, 044508. (32) Miscioscia, R.; Vacca, P.; Nenna, G.; Fasolino, T.; La Ferrara, V.; Tassini, P.; Minarini, C.; Della Sala, D. Analysis of effects induced by metallic interlayers in organic leds. IEEE Trans. Electron Devices 2009, 56 (9), 1912−1918. (33) Yang, Y.; Ouyang, J.; Ma, L.; Jia-Hung Tseng, R.; Chu, C.-W. Electrical switching and bistability in organic thin films and devices. Adv. Funct. Mater. 2006, 16, 1001−1014. (34) Okamoto, K.; Araki, Y.; Ito, O.; Fukuzumi, S. A dramatic elongation of the lifetime of charge-separated state by complexation with yttrium triflate in ferrocene−anthraquinone linked dyad. J. Am. Chem. Soc. 2004, 126, 56−57. (35) Ipe, B. I.; Thomas, K. G.; Barazzouk, S.; Hotchandani, S.; Kamat, P. V. Photoinduced charge separation in a fluorophore−gold nanoassembly. J. Phys. Chem. B 2002, 106, 18−21. (36) Peng, H.; Ran, C.; Yu, X.; Zhang, R.; Liu, Z. Scanning-tunnelingmicroscopy based thermochemical hole burning on a new chargetransfer complex and its potential for data storage. Adv. Mater. 2005, 17, 459−464. (37) Yang, J. J.; Strukov, D. B.; Stewart, D. R. Memristive switching mechanism for metal/oxide/metal nanodevices. Nat. Nanotechnol. 2008, 3, 429−433. (38) Janousch, M.; Meijer, G. I.; Staub, U.; Delley, B.; Karg, S. F.; Andreasson, BP. Role of oxygen vacancies in Cr-doped SrTiO3 for resistance-change memory. Adv. Mater. 2007, 19, 2232−2235. (39) Galvagno, S.; Casu, S.; Dinoi, A.; Bezzi, G. Procedimento per la trasformazione del granulato di pneumatico in Carburo di Silicio (SiC), 2008, Italian Patent no. RM2003A000121. (40) Galvagno S., Portofino S., Freda C., Magnani G., Donatelli A., Morgana M., Iovane P., De Girolamo Del Mauro A. Method of purification from metals for the preparation of high purity ceramic materials, 2013, Italian Patent no.RM2013A000277. (41) Nakashima, S.; Higashihira, M.; Maeda, K.; Tanaka, H. Raman scattering characterization of polytype in silicon carbide ceramics: Comparison with X-ray diffraction. J. Am. Ceram. Soc. 2003, 86 (5), 823−829. (42) Vyshnyakova, K.; Yushin, G.; Pereselentseva, L.; Gogotsi, Y. Formation of porous SiC ceramics by pyrolysis of wood impregnated with silica. J. Appl. Ceram. 2006, 3 (6), 485−490. (43) Galvagno, S.; Portofino, S.; Casciaro, G.; Casu, S.; d’Aquino, L.; Martino, M.; Russo, A.; Bezzi, G. Synthesis of beta silicon carbide powders from biomass gasification residue. J. Mater. Sci. 2007, 42, 6878−6886. (44) Yang, J. J.; Strukov, D. B.; Stewart, D. R. Memristive devices for computing. Nat. Nanotechnol. 2013, 8, 13−24. (45) Son, D.-I.; Kim, J.-H.; Park, D.-H.; Choi, W. K.; Li, F.; Ham, J. H.; Kim, T. W. Nonvolatile flexible organic bistable devices fabricated utilizing CdSe/ZnS nanoparticles embedded in a conducting poly-Nvinylcarbazole polymer layer. Nanotechnology 2008, 19, 055204− 055209. (46) Kumar, R.; Pillai, R. G.; Pekas, N.; Wu, Y.; McCreery, R. L. Spatially resolved Raman spectroelectrochemistry of solid-state polythiophene/viologen memory devices. J. Am. Chem. Soc. 2012, 134, 14869−14876. (47) Asadi, K.; De Leeuw, D. M.; De Boer, B.; Blom, P. W. M. Organic non-volatile memories from ferroelectric phase-separated blends. Nat. Mater. 2008, 7, 547−550. (48) Chen, C.-H.; Liu, C.-M.; Wei, K.-H.; Jeng, U.-S.; Su, C.-H. Nonvolatile organic field-effect transistor memory comprising sequestered metal nanoparticles in a diblock copolymer film. J. Mater. Chem. 2012, 22, 454−461. 12428
dx.doi.org/10.1021/la503060v | Langmuir 2014, 30, 12421−12428
Nonvolatile RRAM Cells from Polymeric Composites Embedding Recycled SiC Powders Anna De Girolamo Del Mauro,* Giuseppe Nenna, Riccardo Miscioscia, Cesare Freda, Sabrina Portofino, Sergio Galvagno, and Carla Minarini UTTP-NANO, ENEA, Piazzale E. Fermi 1, 80055 Portici, Italy ABSTRACT: Silicon carbide powders have been synthesized from tires utilizing a patented recycling process. Dynamic light scattering, Raman spectroscopy, SEM microscopy, and X-ray diffraction have been carried out to gather knowledge about powders and the final composite structure. The obtained powder has been proven to induce resistive switching in a PMMA polymer-based composite device. Memory effect has been detected in twoterminal devices having coplanar contacts and quantified by read−write− erase measurements in terms of level separation and persistence.
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INTRODUCTION
The reaction mechanism is not completely explained, but it is commonly accepted that the carbothermal reduction involves the following fundamental steps:
Silicon carbide (SiC) is a high value material used in a broad range of products and applications. Because of its intrinsic properties, SiC is one of the best choices as material for advanced applications.1−4 In fact, its properties include high hardness and strength, excellent corrosion/oxidation resistance, good high-temperature strength, and high thermal conductivity, which makes it an optimal choice as reinforcement for ceramic composites5 as well as an array of high added value products used in such industries as semiconductors, diesel particulate filters, measurement instruments, ceramic membranes, heating elements, and catalytic support productsamong many others. In recent years, these markets have grown, and they are expected to keep growing in the near future with rapid extension once more cost-effective alternatives will be available. In the microelectronic industry, SiC is one of the emerging semiconductors because of its large bandgap, ranging from 2.4 eV (3C-SiC) to 3.4 eV (2H-SiC) according to the polytype.6 SiC powders can be produced in three principal ways: direct carbonization of Si metal,7 pyrolysis of silane compounds,8 and carbothermal reduction of SiO2. The third method, the carbothermal reduction, is the cheapest one: it starts from inexpensive silicon dioxide and carbon (or precursors like carbon and silica sources) that react at temperature ranging from 1600 to 2100 °C to give SiC. The SiC phase depends on temperature synthesis; typically, at low temperature (below 1600 °C) β-phase prevails while α-SiC is the principal phase at higher temperature (up to 2100 °C). The synthesis is described by the equation9,10 SiO2 (s) + 3C(s) → SiC(s) + 2CO(g) © 2014 American Chemical Society
SiO2 (s) + C(s) → SiO(g) + CO(g)
(2)
SiO(g) + 2C(s) → SiC(s) + CO(g)
(3)
First the reaction generates the intermediate SiO(g) that in the second step reacts with carbon to form the final product. Additionally, the carbothermal reduction method allows the use of secondary raw materials as reactants to produce SiC. Within this framework, synthesis via carbothermal reduction could open a new way for high added value exploitation of the thermochemical conversion residues;11,12 in fact, this class of reactions actually plays an important role in ceramic powders synthesis,13−15 like SiC, Si3N4, TiC, NbC, etc. However, often the use of secondary raw materials introduces impurities in the product that strongly limit the range of exploitations; this effect has been not duly investigated in the literature, and few works report applications studies on the synthesized materials. The mechanical properties of the tire-derived SiC have been investigated in a previous work16 where its use was investigated in optoelectronic applications. Applications of SiC dispersions are appealing from the processability point of view because they are able to exploit the nanoscale features of the carbide powders. Such applications range from glass fabric reinforced epoxy composites with modified dielectric properties17 to electro-optical modulators;18 furthermore, they have been proposed as candidate for blue and Received: August 1, 2014 Revised: September 19, 2014 Published: September 26, 2014
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Table 1. Thickness and Roughness of SiC, PMMA, and PMMA−SiC Composite Films, Reported in Association with Mixtures and Compositions of PMMA−SiC Dispersions no.
samples
% w/w (solution)
mg/mL (solution)
mg SiC
1 2 3
SiC PMMA PMMA-67 wt % SiC
4.50 10 3.55
50.00 123.85 41.67
1000 250
ultraviolet (UV) light emitters in displays19 in substitution of silicon crystallites which are not suitable for stable and intense emission in this wavelength range. Literature experiments have studied electrical conduction properties of SiC composites obtained by using NbC, Ti, SiO2, and Y2O3 additives and hot pressing,20 but there is still a lack in the knowledge of charge transport in SiC−polymer composites. As it could be expected, to obtain electrical continuity in a composite, it is not necessary to fill any interstice in the primary phase. In fact, theoretical studies on bicomponent mixtures show that for electrical conduction it is not necessary that the primary particle surface is covered by the dispersed phase: to obtain this effect, the dispersed particles have to be rather located in favorable positions or may be occluded into interstices.21 In addition to charge percolation,22 in the determination of electrical properties of the composite the role played by the organic/inorganic interfaces should be carefully considered:18 they are indeed crucial in charge storage/charge trapping devices and photodetectors. In the field of electrically switching electronic devices, memory effects have been reported and discussed in the literature for nonorganic23−26 and organic27,28 based devices manufactured in a stacked configuration. For a variety of metal/insulator/metal (MIM) two-terminal devices structures, reversible switching behaviors have been reported28,29 exhibiting two separate conduction states. Then, many efforts have been focused on the integration of such kinds of devices in an addressable memory structure having a 2Dfashioned matrix shape to take advantage of cross-point geometry layouts.30 Switching behaviors have been observed in devices embedding nanoparticles in organic layers31 or nanoclusters in layered stacks.32,33 This class of organic devices exhibits a switching behavior associated with charge storage in charged islands34−36 within electrically active regions comprised between the electrodes. By following this approach, it has been also hypothesized that resistive random access memories (RRAM) could be fabricated by the employment of resistive switching37 devices of the kind described before, allowing to envision high data densities, data durability and persistence, and fast switching. Organic memory layouts having coplanar contacts allow to relieve from the complexity of a stacked topology design and processing and to simplify the adoption of solution processing techniques such as drop-casting and inkjet printing. Furthermore, this device configuration is useful for research purposes because it exposes the switching region to further analyses allowing to inspect the switching mechanism. This approach has been already followed in the literature by Janousch et al.38 for inorganic resistive switching devices investigations. In the present work, tire-derived SiC has been used for the preparation of PMMA-based composites to explore the electronic properties of these materials. Organic-based composite embedding SiC powders obtained from a patented recycling process39 have been demonstrated to induce resistive
mg PMMA
thickness (nm)
roughness (Rq) (nm)
2477 123.85
1100 993
102 205
switching in a polymer-based composite spun-coated on a glass substrate. Gold (Au) coplanar contacts were evaporated on PMMA−SiC composite to complete the device structure. The memory effect was detected and quantified by read−write− erase measurements in terms of level separation and persistence.
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EXPERIMENTAL SECTION
Synthesis of Silicon Carbide. SiC powder was synthesized from tire pyrolysis char and commercial silicon dioxide (Carlo ERBA silica gel 60 230−400 mesh ASTM). The carbon matrix was milled into particles up to 1 mm in diameter, mixed with silica gel in a mixer drum for 2 h, fixing the powder weight ratio SiO2/C at 1.514 wt %. After being grounded, samples were maintained under ambient conditions. The reactions were conducted at 1650 °C in a high-temperature tubular furnace (Nabertherm 120/300/1800), with residence time of 1.5 h, in an argon atmosphere (flow rate of 50 L/h). Samples of about 20 g were treated in alumina boats. The product yields were very close to the theoretical value (nearly 35%). SiC powder was purified from the residual carbon by treating the samples at 700 °C in oxidizing atmosphere and from the residual silica by dissolving the samples in hydrofluoric acid 50% in weight solution and by subsequently washing the residues with deionized water. Considering the nature of the precursors (waste-derived materials), metallic impurities (like iron, zinc, and chromium) could be present in the final product. To get a high purity ceramic compound, a method for the purification from metals was set up: basically the ceramic precursors or the ceramic compound itself were subject to a combination of purification steps conducted with chelating agents and basic solutions; the process was able to drastically reduce the metal content and was recently patented by the authors.40 Preparation of PMMA/SiC Composites. Before preparing composite with PMMA, SiC powder was charged and sealed in an agate vial together with five agate balls. The ball-milling experiments were performed in a high-energy planetary ball mill (Retsch PM 100) at a rotation speed of 5 s−1 in an air atmosphere for 20 min. Then, SiC powder was dispersed in chlorobenzene at concentrations of up to 50 mg/mL. The dispersion of the ceramic powder was sonicated for 10 min in a ultrasonic bath at 100 W at ambient temperature, and it was left to stand undisturbed until no visible sign of aggregation was detectable. Polymer solution was prepared by dissolving PMMA (Mw ≅ 120 000, Aldrich) in chlorobenzene at a concentration of 123.85 mg/mL. Once completely dissolved, SiC dispersion was added to the polymeric solution and dispersed by physical agitation through ultrasonication treatment. PMMA−SiC composite film was deposited by spin-coating onto glass substrates. Glass substrates were precleaned with detergent in boiling water for 2 h in an ultrasonic bath and successively rinsed with isopropyl alcohol and finally dried under nitrogen flow. All films were cured in oven at 100 °C for 2 h (Table 1). From Table 1, it is observed that the thickness of the films decreases when the roughness increases. Characterization. The dispersions were characterized by dynamic light scattering (DLS) with a HPPS 3.1 system from Malvern Instruments (Worcestershire, UK) at different aging times to evaluate the size distribution and time stability of the SiC aggregates in the suspensions. The film thicknesses and the roughness were measured by KLA Tencor P-10 surface profiler. UV−vis optical transmittance analysis of layers deposited on glass substrates were carried out by using PerkinElmer Lambda 900 spectrophotometer. Phase identi12422
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fication of these samples was performed by the X-ray diffraction (XRD) method on an X’Pert MPD diffractometer using nickel-filtered Cu Kα radiation in the range of 2θ = 20°−80°. The morphology and element content of the sintered samples were observed by scanning electron microscopy (SEM, Model: LEO 1530) and EDX (energy dispersive X-ray) spectral analysis. Raman measurements have been carried out on the samples with a Renishaw inVia Reflex Raman spectrometer. The Raman spectra were recorded at room temperature with an optical microscope using the 514 nm line of an Ar+ laser (Laser Physics) as excitation source. The electrical conductivity of the films was evaluated by carrying out current−voltage (I−V) measurements. For this analysis, an interdigitated Au structure was evaporated on top of the spin-coated films on glass substrates.
■
RESULTS AND DISCUSSION Figure 1 shows particle size distribution measured by the DLS, conducted suspending the SiC powder in chlorobenzene. The
Figure 2. XRD Patterns of pure PMMA, Td-SiC, and PMMA-67 wt % SiC.
Figure 1. Dynamic light scattering pattern of the SiC in chlorobenzene (A) and PMMA−SiC composite material (B).
average size obtained for SiC was in the range 200−1000 nm, with an average particle diameter of 450 nm and with a polydispersity index of 0.25 (Figure 1A). Instead, the size of SiC particles in PMMA matrix (Figure 1B) was found to be around 430 nm with a polydispersity index of 0.17. It is observed that the average sizes of the SiC particles in both samples are similar, and this indicates a good dispersion of SiC in the polymer matrix. The coating of the SiC particles with PMMA matrix was first identified through analyzing the peaks in the XRD patterns of the SiC and PMMA−SiC composite. The XRD patterns of the SiC and PMMA−SiC composite are presented in Figure 2. Xray diffraction patterns reveal that the SiC produced is highly crystalline, with the signals at diffraction angle 2θ equal to 35.7°, 41.5°, 60.0°, and 71.8° which can be attributed both to α and β phases, the signal at 75.5° attributed to β form only, and the signals at 33.7° and 38.1° attributed to α-SiC (Figure 2). As expected under these experimental conditions, β-phase prevails on α-SiC. It is also apparent from Figure 2 that the X-ray pattern of PMMA matrix shows broad bands peaking, indicating its amorphous nature and peaks characteristics of crystalline SiC. These results indicate that the SiC nanoparticles are coated with PMMA. Figure 3 shows the Raman spectrum of synthesized SiC and of PMMA−SiC composite recorded at room temperature. The peaks centered at 796 and 966 cm−1 could be attributed to the SiC TO and LO mode, respectively;41 the structure of the peaks suggests the presence of α and β phases.
Figure 3. Raman spectra of the Td-SiC and of the PMMA−SiC composite.
SiC embedded in PMMA was also examined by SEM. The SEM images of SiC purified and PMMA−SiC composite containing 67 wt % SiC are shown in Figure 4. For SiC powder, the analysis reveals that there is a broad particle size distribution in the powder, on average ranging from 100 to 500 nm. In any case the larger particles found are below 1 μm. In addition, the SEM micrographs reveal the presence of a very low amount of whiskers and bamboo-like fibers; these structures are usually produced by other synthesis processes (for example CVD), but they could be found as byproduct of the SiC powders produced by carbothermal synthesis42,43 when the reaction is conducted at high temperature and in the presence of metallic impurities (i.e., Fe, Zn, etc.). The fibers can grow according to several secondary gas−gas reactions, like the following: SiO(g) + 3CO(g) → SiC(s) + 2CO2 (g) 3SiO(g) + CO(g) → SiC(s) + 2SiO2 (s)
However, the quantity of the fibers detected by SEM analysis is so low to not significantly affect the properties of the sample. 12423
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Figure 4. SEM micrograph of synthesized SiC (A) and PMMA−SiC composite (B).
Figure 5. EDX patterns for Td-SiC (spot 1) and PMMA−SiC composite (spot 2).
For the PMMA−SiC composite, the SiC particles are completely embedded in the polymer matrix (Figure 4B). To identify the distribution of SiC in PMMA, EDX analysis was done to reveal the existing elements in the composite (Figure 5). In particular, Figure 5 shows the EDX patterns for spot 1 (SiC particle) and spot 2 (matrix phase). While for spot 1, the EDX analysis clearly shows Si and C peaks detected on the SiC particles, for spot 2, the EDX analysis exhibits Si, C, and O peaks (the presence of O peak identifies the polymer matrix material). To manufacture the devices, PMMA−SiC composite was spin-coated on a glass substrate, and after the deposition of the dispersion, Au-interdigitated contacts were evaporated. The scheme and the layout of the contacts of the substrate are reported in Figure 6. Geometry has been evaluated by processing 10× microscope images calibrated by profilometric measurements. The electric performances of the devices were studied by current−voltage (I−V) measurements. Figure 7A shows the I−V characteristics of the composite-based device PMMA−SiC, recorded by sweeping the voltage between −25 and 25 V. After the first voltage sweep ranging from 0 to 25 V, voltage cycles have been repeated 10 times. In Figure 7B the same voltage scans are reported in a log−log scale to compare for positive biases the slopes of the two different states. In particular, each I−V characteristic changes its slope from a near-ohmic regime for low biases to a space-charge-limited current (SCLC) regime for high voltages. The last cycle is reported in Figure 8A where we can observe the bistable behavior and two distinct states. In detail, each state is characterized by a rectifying I−V response. The electrical response is switched to its complementary curve by exceeding a defined voltage bias and going through a negative differential
Figure 6. Contacts geometry on glass substrate and their characteristics.
resistance (NDR) branch. After the state is switched, the device acts again as a rectifying diode but in the opposite direction. Symmetrically, to switch to the initial uncomplemented state, the device has to be reversely biased to get over a negative threshold and its associate NDR region. As can be observed in Figure 8A and more schematically in Figure 8B, the device has the dynamic response of a nonvolatile, bistable resistive memory. Its behavior can be defined bipolar because two opposite voltage levels are necessary to switch the device between its conducting states.44 The slight variations in the current values of the negative and 12424
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Figure 7. I−V hysteresis curves up to the stabilization (A) and the relative log−log plot of the positive branch (B).
Figure 8. I−V hysteresis curves after the stabilization (A) and schematic representation of the device operation (B).
appearance. The OFF state is partially characterized by the ohmic conduction (exponent 1.09). In detail, as a consequence of the I−V analysis, we assume that the OFF state is determined by the presence of charged submicrometric particles in the composite. The charge trapped by the particles induces a partial space charge effect which inhibits the charge injection from the electrodes and limits charge transport. On the contrary, the ON state corresponds to uncharged polymer/particles interface.35 According to this theory, the initial ON state is observed below the NDR region because higher bias should be applied to restore the same charge level in the particles. By biasing the device at voltages over VP+ (and vice versa below VP−), the behavior is switched between two symmetric rectifying conditions. Similarities to resistive switching in planar contacts, memories can be observed in the literature for nonlinear memristive devices.46 A similar switching mechanism has been observed and explained by Asadi et al.47 for interpenetrating networks of different polymers where the diode poling procedure modified the charge injection at the contacts interface. In the cited work,47 a bistable rectifying resistive switch has been demonstrated by depositing the poorly injecting contacts on a mixture of a high-bandgap polymer and an organic semiconductor forming an interpenetrated network. The metal−polymer interfaces were switched by bias application between two rectifying states having opposite polarity. In the referred case, the switching phenomenon was operated by the accumulated charge and to its consequent band bending. Once bands were persistently bended at the contacts interface, the injection barrier was lowered enough to switch the junction to an ohmic behavior.
positive branches are due to the very low difference in the current pathway through the network of SiC islands between the electrodes changing the polarities. In the last cycles the curves become more stable with a NDR effect arising from 15 to 25 V. It is known that NDR shows in many nanocompositebased organic memories.31,45 Therefore, the two states of the device can be named after the current magnitudes measured in a given bias condition. In particular, the device is in its forward state (namely ON state) if it acts as a forwardly-biased rectifying junction; conversely, it is in the reverse state (OFF state) if it is behaving as a diode connected in a direction opposite to the bias polarity (a reversely biased junction). When the device is in the forward state and it is forwardbiased, by keeping the voltage below the NDR region, it persists in its state as required for nonvolatile memories. When a forward voltage threshold (VP+) is overcome, the device goes through the NDR area and switches off to the reverse state. The diode behavior, common in tunnel diodes, should be observable after the NDR region. Such bias condition cannot be reached because the applied voltages were not high enough. The forward state is then recovered by applying a negative voltage, and the transition is induced by the same mechanism. In such case, when reverse biased, the device switches to the reverse (complemented) state when bias goes under a second threshold voltage (VP−) as shown in Figure 8A,B. In the ON state, in the forward bias branch of the log−log plot (Figure 7B), between 3 and 14 V a power law characteristic (I ∼ Vm) has been detected, and its exponent has been estimated from a linear fit to be about 1.42. This value allows to associate a trap charge limited current (TCLC) model to this region. The accumulated charge limits the current flow until reaching too filled pathways and determining the NDR 12425
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Figure 9. Read−write−erase cycles. Vread = +5 V, Vprogram = +20 V, Verase = −20 V.
Likewise to Asadi’s device,47 the system acts as two nonvolatile bistable diodes (Figure 8B inset) connected with the anode of the first at the cathode of the second in a ring configuration which respects the geometric symmetry breaking the electrical characteristic symmetry by means of interface poling. As a consequence, the two rectifying states are toggled reversibly. Then, we suggest that in our device the poling procedure defined by a bias point over the switching voltage Vp is able to modify the Au/composite interfaces to switch from an injecting contact to a blocking contact and vice versa, according to the voltage pulse polarity. Material-specific and interface poling effects can coexist in the same device, and further investigations should be done to quantify which amount of each behavior is responsible of the experimental traces. In the obtained RRAM cell two logic levels are distinguishable: the forward state having high conductance in forward bias and a reverse state having low conductance in the same bias conditions. The cell reading is performed nondestructively when the reading bias is kept between −10 and +10 V. Programming is accomplished by boosting the bias potential to a writing condition characterized by high fields. In detail, the reverse state can be written by bringing the bias at V > 16 V while the forward state is written into the cell when the bias potential is below −15 V. To establish the workability of such devices and their capability to store the information RWE (read−write−erase), several cycles are performed.48,49 In this case, as it is possible to observe from Figure 9, we performed the readings at 5 V while the change in the states was accomplished using a bias of +20 V (to store the off status) or −20 V (to store the on status). Again, the transients during the status storage show how long the trapped charge inhibits the current flow before the final status is reached and persisted. In this case, to obtain the reverse state persistence, it is necessary to hold the programming voltage for about 25 s. Such a long time to store information is expected for a coplanar structure because of the large area between gaps and the large contacts surface providing numerous charge storage sites. The adoption of the device as a test case is then justified because it allows appreciating storage transients and induces complementary rectifying switching as suggested by Linn et al.50 The difference in the reading current between the reverse state and the forward state is about 0.14 μA. Such a difference is about an order of magnitude between the two persisted states.
By observing the conductance−voltage plots in Figure 9 and comparing it to Figure 8A, charge storage begins at biases lower than the beginning of the NDR region; that is why the reading voltage has to be comprised between VT− and VT+ to guarantee from destructive readings in RWE cycles. In addition, to avoid incomplete writings, voltages have to overcome the VP+ and VP− thresholds (Figure 10). This mechanism is due to the
Figure 10. Comparison between small signal conductance acquired at zero bias at f = 100 Hz and dc differential conductance.
trap−detrapping time that changes in reason for the applied frequency both in bulk and at the interfaces.49 In particular at zero frequency we will reach the maximum in the applied voltage to observe the switching, as stated by the zero-crossing points in the differential resistance plots in Figure 10. We tried also other wt %, and we found the best results in terms of hysteresis amplitude and stability using 67% of SiC powders. In particular, it seems that particle distribution is even more effective on the device performance respect to the powder concentration. The interface characterized by a more uniform distribution of SiC particles could have better and more reproducible hysteresis loops. For lower concentrations, with the appearance of bigger badly connected clusters, we found an insulating behavior that takes place lowering the current also of 2 orders of magnitude. Hysteresis cycles were more unstable and less pronounced. For higher concentrations we found a “too” connected network, obtaining a general increase in the current magnitude but also a reduction in terms of hysteresis cycles amplitude. Moreover, the loop’s stability was difficult to reach, and several cycles were necessary to stabilize the current 12426
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levels in the RWE characteristic, leading to a further reduction in the logic levels separation.
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CONCLUSIONS Nonvolatile RRAM cells were produced from polymeric composites embedding SiC powders obtained by the tire recycling process. Tire-derived SiC has been analyzed by DLS, X-ray diffraction, SEM imaging, and Raman spectroscopy. The SiC powders were embedded in PMMA-based composites preparations to explore the electronic properties of these materials. Memory effect with NDR has been observed. To investigate the evidence of RRAM mechanism, I−V, RWE, and G−V cycles have been performed. The reported complementary rectifying switching mechanism allowed reaching about 1 order of magnitude in current difference between the two stable current states. We suggested the explanation of the memory behavior to be found in bias-induced interface modification effects. In this framework, metal−composite barriers are modulated persistently by bias conditions; therefore, charge injection modifications reflected in a transition between a rectifying and an ohmic state. This hypothesis has to be confirmed by further work necessary to inspect the relationship between surface morphology of the compound and the electrical switching performances. This result can be considered remarkable if compared to the simplicity and the extension of the geometry and allowed us to envision more performing architectures obtainable by redesigning topologies in order to not disclose the underlying physics but to take advantage by integration and switching times.
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AUTHOR INFORMATION
Corresponding Author
*E-mail [email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The present research has been supported by the Italian Ministry of Education, University and Research (MIUR) through the National Project entitled SMARTAGS (PON02_00556_3420580) and partially by the Seventh Framework Programme (FP7) 2007−2013, in the frame of the TyGRe Project (Contract No. 226549).
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