Tunable Power Switching in Nonvolatile Flexible Memory Devices

Aug 28, 2014 - Fax: 787 764 2571., *E-mail: [email protected]. ... GOZNs-based RRAM devices having top metal aluminum electrodes were fabricated o...
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Tunable Power Switching in Nonvolatile Flexible Memory Devices Based on Graphene Oxide Embedded with ZnO Nanorods Geetika Khurana, Pankaj Misra,* Nitu Kumar, and Ram S. Katiyar* Institute of Functional Nanomaterials and Department of Physics, University of Puerto Rico, Rio Piedras, San Juan, Puerto Rico 00936-8377, United States S Supporting Information *

ABSTRACT: The growing demand for portable and bendable nonvolatile memory systems has motivated extensive research in the field of flexible resistive random access memory (RRAM) devices. This study investigated the resistive switching and flexibility behavior of zinc oxide nanorods (ZNs) incorporated graphene oxide (GO) sheets. GOZNs-based RRAM devices having top metal aluminum electrodes were fabricated on flexible indium tin oxide (ITO) coated polyethylene terephthalate (ITOPET) substrate. The devices having the structure Al/GOZNs/ITOPET showed typical bipolar resistive switching characteristics with switching voltages lower than those of Al/GO/ITOPET devices. The significant (∼50%) decrement in operating voltages in the case of GOZNs-based RRAM was attributed to enhanced concentration of oxygen vacancies into the GO matrix due to the incorporation of ZNs, which was supported by X-ray photoelectron spectroscopy studies. These memory devices showed repeatable and reliable switching characteristics having an on/off ratio of ∼100, lower switching voltages, good retention properties up to ∼104 s, and endurance performance over 200 cycles. The resistance ratio of the GOZNs RRAM devices was maintained almost constant even for the extreme bending radius of 4 mm and mechanical flexing test over 103 cycles, indicating excellent flexibility. These GOZNs-based RRAM devices showed great potential for use in future flexible nonvolatile memory devices. is still a need to further investigate its utility in flexible RRAM devices. Flexible RRAM devices are one of the categories of RRAM devices, which have recently received much attention as promising memories for bendable systems.22−26 The inherit merits of low cost, low weight, excellent portability, and userfriendly interfaces when compared with conventional rigid silicon technology has increased the demand for flexible memory.27 The flexible substrates used for this type of memory could not withstand high temperatures, which leads to the need for advanced materials. In this context, GO is readily oxidizable and water-soluble, which makes it so easier to be transferred in thin film form on any substrates at room or moderate temperatures. This property of GO supported the deposition of GO films at room temperatures on flexible plastic substrates,

1. INTRODUCTION Resistive random access memory (RRAM), which operates on the change of resistance state of the material, has attracted considerable attention because of its potential applications in memory devices.1,2 The general attributes, such as simple structure of metal−insulator−metal (MIM), high speed, long retention, and endurance have led to RRAM devices being highly investigated memory devices in the present era.3−6 The resistive switching phenomenon has been observed in various oxide materials, including transition metal oxides, perovskite oxides, organic materials, and carbon-based materials.7−15 Among carbon-based materials, graphene has excellent physical and transport properties.16,17 Metallic natured graphene and its oxide, graphene oxide (GO), which is insulating or semiconducting, have potential applications in RRAM devices.18,19 Recently reported GO-based RRAM devices have shown excellent switching characteristics with the potential to be established as future nonvolatile memory devices,20,21 but there © 2014 American Chemical Society

Received: July 9, 2014 Revised: August 27, 2014 Published: August 28, 2014 21357

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continuously for 2 h. Then, this mixture was stirred at room temperature with an addition of excess of distilled water to the solution. Hydrogen peroxide (H2O2, 30 wt %) was then added slowly while the mixture was being stirred until the gas evolution stopped. The resultant mixture was stirred for another 15 min and then filtered through a nylon membrane. Repeated washing was done by distilled water and 5% HCl solution until the filtrate was neutral. Sonication and centrifugation was done to finally get a colloidal suspension of GO. 2.2. Synthesis of ZnO Nanorods. To synthesize ZNs, we followed the chemical route,36 using precursors zinc nitrate and sodium hydroxide (NaOH). Zinc nitrate solution of 0.5 M was added dropwise to NaOH solution (1 M) with constant stirring at room temperature. Stirring was continued for a few hours until the formation of a white precipitate. The precipitate was then filtered and washed with distilled water. Then, the powdered sample was dried at 60 °C. 2.3. Fabrication of the Device. The powdered ZNs were then added to the GO solution in the ratio of 10:1, 5:1, 3:1, and 2:1 (GO:ZNs) with constant stirring at room temperature for about 4 h. This final solution of GOZNs was then coated to commercially available indium tin oxide (ITO) coated polyethylene terephthalate (ITOPET) substrates by a spin coating process to get the desired thickness of the films. Then, these films were allowed to dry completely overnight in ambient conditions, and then top electrodes of aluminum were deposited by thermal evaporation method through a shadow mask having a diameter of 200 μm. Thus, the device structure formed was Al/GOZNs/ITOPET.

which cannot bear high temperatures. The incorporation of nanomaterials into oxides has been proven favorable for improving the resistive switching characteristics of the systems.28−30 Although there are reports on resistive switching characteristics of GO thin films incorporated with oxide nanomaterials grown on rigid substrates, similar studies using flexible substrates are scanty. Among all other nanostructured oxides, zinc oxide (ZnO) is one of the most attractive materials because of its easy and mature growth methodology, rugged structure, and chemical stability.31,32 Among all nanostructures of ZnO, one-dimensional ZnO nanorods (ZNs) grown vertically were used as guided filament for resistive switching in a recent report.33 However, we preferred to hybridize ZNs in a horizontal direction on GO sheets to maximize the contact area between the nanorods and the GO sheets.34 The consequence of this was observed in significant reduction in switching voltages in comparison to that of GO alone. Notably, while achieving the reliable and repeatable switching characteristics at lower switching voltages, the GOZNs-based devices did not sacrifice their other merits such as good endurance, retention properties, and excellent flexibility behavior. Such tunability can be realized in these cost-effective simple structured GOZNs-based devices, whereas similar results have been recently observed in oxide-based memory devices having complex structures fabricated using advanced techniques.35 In this study, GO and ZNs were synthesized separately by chemical routes and then mixed together in a particular proportion by stirring continuously at room temperature to prepare a homogeneous dispersion. Using this dispersion, the GOZNs thin films were deposited on flexible substrates by a spin coating method and their resistive switching characteristics were studied. The schematic of the devices is shown in Figure 1a. Figure 1b shows the FESEM picture for a ZnO nanorod

3. CHARACTERIZATIONS The morphological characterization was carried out using a field emission scanning electron microscopy instrument (FESEM, JEOL JSM-7500F SEM). For X-ray photoelectron spectroscopy (XPS), Al Kα (1486.6 eV) radiation from a monochromatized X-ray source operating at 350 W was used. Current−voltage (I−V) measurements were done using a Keithley 2401 instrument in top−bottom configuration. 4. RESULTS AND DISCUSSION Figure 2 shows the typical current−voltage characteristics of the device with and without ZNs incorporated into the GO matrix. Initially, the Al/GOZNs/ITOPET devices were in high resistance state (HRS). A current compliance of 2 mA was applied to avoid the breakdown of the device. In the very first cycle, a large positive bias voltage around 5 V with current compliance of 2 mA was applied to activate these devices. This activation is called the forming process (not shown here). After the forming cycle, positive voltage was applied and gradually increased on the top electrode and current started increasing with the voltage, but at 2.1 V, there was a sudden increase in the current and the device attained low resistance state (LRS). This is the SET process, and now the device is in the ON state. The device remained in LRS even after the removal of the voltage, showing nonvolatility of the device. As the negative polarity of the voltage was applied to the device, first the current increased with voltage, but at −2 V, the current decreased sharply and the device switched to HRS. This is the RESET process, and the device goes to the OFF state. Now, to investigate the effect of ZNs addition into the GO matrix, we fabricated an Al/GO/ITOPET device following the

Figure 1. (a) Schematic for the Al/GOZNs/ITOPET device. (b) FESEM picture showing ZnO nanorod on the surface of GO sheets. The inset shows the ZNs in a flowerlike pattern.

resting on the surface of wrinkled GO sheets. The lengths of ZNs lie in the range of 1−4 μm, and the diameters in the range of 100−200 nm. The inset of Figure 1b shows the flowerlike pattern of as-grown ZNs.

2. EXPERIMENTAL SECTION 2.1. Synthesis of Graphene Oxide Suspension. Graphene oxide was synthesized following the method provided in a previous report.21 In brief, highly oriented pyrolytic graphite (HOPG, 2 g) was oxidized using potassium permanganate (KMnO4, 7 g) in the presence of concentrated H2SO4 (50 mL) in an ice bath. KMnO4 was added very slowly to the solution, maintaining a low temperature less than 5 °C. After addition of whole KMnO4, the mixture was stirred 21358

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was increased to half that of GO (ratio 2:1), the device showed instability in switching behavior with a large window of SET voltage ranging from 1−4.8 V and different on/off ratios, which might be due to the excess generation of oxygen vacancies. Hence, according to our study of the devices with various compositions, the devices with ratio of 3:1(named GOZNs) showed the best results, which are discussed in the present contribution. These studies reveal that the introduction of ZNs is favorable in lowering the switching voltages and advantageous in the field of low-power electronics. So far, various models such as trapping−detrapping of charge carriers and formation−rupture of conductive filament have been proposed to explain the phenomena for resistive switching. The mechanism behind the formation of conducting filaments varies from one system to another. Some of the possible mechanisms for formation of conducting filaments are reported as being due to oxygen vacancies37,38 and metallic nanobridges.39 In Al/GOZNs/ITOPET devices, we propose that the conducting filament formation during the SET process is due to the oxygen vacancies. We have deposited top Al electrodes through a thermal evaporation method, which is generally well-known as one of the most readily oxidizable metals. With the presence of Al top electrode, oxygen atoms in GO diffuse toward and react with Al because of the existence of an oxygen concentration gradient and the high oxidation tendency of Al.27 This process will lead to the formation of a new interfacial Al oxide layer13 and also induces the oxygen vacancies in the region of GO. In our case, with the application of positive polarity on the top electrode, these oxygen vacancies are deeply inserted into the film and form the conducting filaments during the SET process. While the reversal of polarity results in repelling back these oxygen vacancies and breaking the conductive path during the RESET process. This phenomenon is responsible for the resistive switching behaviors in the Al/GOZNs/ITOPET as well as Al/GO/ITOPET devices. But with the incorporation of ZNs into the GO matrix, we observed a significant reduction in the switching voltages as stated above and also can be seen in Figure 2. The reason for this can be understood by the role of oxygen desorption and release at the interface of GO matrix and ZNs, which facilitates the formation and rupture of conducting channels on the application of suitable bias. This proposed phenomenon based on oxygen vacancies is well-supported by

Figure 2. Typical I−V switching characteristics in device Al/GOZNs/ ITOPET and the inset showing the I−V characteristics for Al/GO/ ITOPET device.

same process as stated above in experimental details, except the incorporation of ZNs, and studied its I−V characteristics. In the Al/GO/ITOPET device, as the positive voltage was applied to the top electrode, current started increasing, and at 3.9 V, the current value jumps to the limit of compliance current and the device switched to LRS. As the polarity of the voltage on the top electrode was reversed, first the current again started increasing with voltage, but at a voltage of −3.5 V, current abruptly decreased. These measurements clearly showed that SET and RESET voltages were drastically decreased to approximately half in the device containing ZNs in comparison to those in the device without the ZNs. To further explore the effect of ZNs concentration in GO matrix on switching characteristics, we have also studied the I− V characteristics of all other compositions (10:1, 5:1, and 2:1). The results are provided in the Supporting Information. The ratio 10:1 showed characteristics comparable to those of pristine GO device with similar values of SET and RESET voltages. On the other hand, the device with ratio 5:1 showed I−V characteristics with a slight decrement in SET and RESET voltages in comparison to those of the pristine GO device and the device with a ratio of 10:1. However, as the ratio of ZNs

Figure 3. (a) Comparative XPS spectra of GO and GOZNs for C 1S peak. (b) XPS spectra of ZNs and GOZNs showing O 1S peak resolved into two components O1 and O2. (c) Zn2p spectra of ZNs and GOZNs samples. 21359

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Figure 4. (a) Al/GOZNs/ITOPET device with no potential. (b) Filament formation during SET process when +ve voltage was applied on top electrode. (c) Breaking of filament during RESET process when −ve voltage was applied on top electrode.

Figure 5. Temperature dependence of the Al/GOZNs/ITOPET device in (a) LRS and (b) HRS.

Figure 3c, which has been attributed to the presence of excess oxygen.40 Hence, on the basis of the XPS study, we propose that ZNs enter into the GO matrix and extract some oxygen from GO sheets, rendering GO matrix sp2 hybridized and hence comparatively less resistive, whereas ZNs acquire excess oxygen content and hence have highly resistive behavior. Moreover, the introduction of ZNs into the GO matrix was done in such a way that rods should lie horizontal in the plane of GO sheets; this provided more surface area of the nanorods to be in contact with GO sheets, which expedited the oxygen transfer from GO matrix to ZNs. Therefore, in Al/GOZNs/ ITOPET devices with excess oxygen containing ZNs, resistance imparted by ZNs dominates the resistance of the whole system. On the basis of the above discussions, the reason for the lowering in switching voltages in Al/GOZNs/ITOPET system can be well-explained with the help of Figure 4a−c. Figure 4a depicts the schematic for the fabricated device, when no voltage was applied. As discussed, the native aluminum oxide layer was formed during the deposition of top Al electrodes by extracting oxygen from the top layer of GO. Also, the oxygen was extracted by ZNs from the GO matrix, resulting in more oxygen vacancies in GO matrix. Hence, the oxygen vacancy concentration in GOZNs was higher than that in GO devices. With the application of a positive polarity of electric field on the top electrode, as shown in Figure 4b, the oxygen vacancies start drifting into the GOZNs matrix from the top to bottom electrode and form the conductive path during the SET process, rendering the device in LRS. In order to form a similar conducting channel in GO, sufficient concentration of oxygen vacancies are required, which can be generated by application of higher voltages and/or current compliance during the SET process. It is worth

the XPS studies of these samples. Figure 3a−c shows the XPS graph for C 1s peak, O 1s peak and Zn 2p spectra for GO, ZNs, and GOZNs samples. Figure 3a shows the XPS data for C 1s peak of GO and GOZNs samples. This figure clearly shows that the C 1s graph of GO contains sp2 and C−O−C peaks, whereas with the introduction of ZNs into the GO matrix, the C−O−C peak disappeared and only the sp2 peak dominates in the GOZNs spectra. These observations showed that the oxygen content has diminished with a missing C−O−C peak, and more oxygen vacancies have been introduced into the GO matrix with the incorporation of ZNs. GO has become comparatively less resistive with its dominant sp2 character. Figure 3b shows the O 1s peak for ZNs and GOZNs samples. The O 1s peak for both the samples has deconvoluted into two components, O1 and O2. The O1 peak related to O−Zn bonding is located at 530.6 eV, and O2 for surface oxy peak is at a higher energy of 532.8 eV for ZNs sample. ZNs are well-known for chemisorbed oxygen at its periphery, and the fitted O2 peak for O 1s in Figure 3b is a signature for the surface oxy peak. However, these O1 and O2 peaks for the GOZNs sample were also found to be slightly shifted toward lower energy. Additionally, we observed a noticeable increment in the intensity of the O2 peak in the GOZNs sample as compared to that of ZNs. These observations of the O 1s spectra are clear indications of excess absorption of oxygen by ZNs, which was released by GO matrix resulting in enhanced concentration of oxygen vacancies in GO matrix with sp2 dominance. The absorption of additional oxygen by ZNs results in shifting of O 1s peak toward lower binding energy, as can be seen in Figure 3b.40 Further, the shift in the Zn 2p peak for the GOZNs sample in comparison to that for the ZNs sample toward lower energy can also be clearly observed in 21360

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Figure 6. Conduction properties of the Al/GOZNs/ITOPET device in the (a) LRS and (b) HRS regions.

Figure 7. (a) Endurance properties and (b) retention characteristics of the Al/GOZNs/ITOPET resistive memory device.

mentioning here that the oxygen ions chemisorbed on the surface of ZNs also get detached during the SET process and move toward the Al/GOZNs interface. Whereas during the RESET process, when the field was reversed, as shown in Figure 4c, the oxygen vacancies start moving back to the Al/ GOZNs interface, initiating the conducting filament rupture process. However, the onset of conducting filament rupture was hastened by the back movement of oxygen ions, which were detached from ZNs during the SET process, into the GOZNs matrix because of replenishment of the oxygen vacancies transiting the device in HRS. Some of these oxygen ions are again captured by ZNs maintaining the initial chemical state of the GOZNs system and the system gets ready for the next SETRESET cycle. Therefore, the lowering of the SET and RESET voltages in the Al/GOZNs/ITOPET compared to Al/GO/ ITOPET device can be attributed to enhanced oxygen vacancies because of the incorporation of ZNs, which assisted the formation and rupture of conducting filament during the resistance switching process. To further understand the switching mechanism in the Al/ GOZNs/ITOPET memory device, resistance was measured as a function of temperature in LRS and HRS. As these flexible plastic substrates cannot withstand high temperatures, the behavior of the resistances with temperature was measured in a narrow window ranging from 245 to 350 K. In the LRS region, the device showed linear behavior obeying Ohm’s law in the temperature range of 245−350 K as shown in Figure 5a. The linear graph was fitted by the following equation:

R(T ) = R o[1 − α(T − To)]

(1)

Where, Ro is the resistance at temperature To and α is the temperature coefficient of resistance. The value of α was calculated to be ∼4.2 × 10−3 K−1 by fitting the experimental data. This value of α is typical for conduction in metallic nanowires, indicating the presence of metal-like conductive paths in the device. This observation supported the above discussion of formation of conducting filaments by agglomeration of oxygen vacancies between top and bottom electrodes in such a way that electrons can percolate as in a metallic nanowire. While in the HRS when the conducting channel was broken, the resistance was found to be decreasing with increasing temperature, as shown in Figure 5b, resulting in semiconducting or insulating behavior in HRS.21 To investigate the current conduction mechanism prevailing in the device, I−V plots were redrawn in log−log scale as shown in Figure 6a,b. In the LRS shown in Figure 6a, the curve is linear having a slope ∼1 in low- and high-voltage regimes, which showed that here conduction obeys Ohm’s law, which supports the observation of conductive channels formed between the top and bottom electrode during LRS. However, when the device was in the HRS as shown in Figure 6b, the curve has been plotted in two voltage regimes. The I−V curve was found to be linear in the low-voltage regime as shown in Figure 6b perhaps because of leftover tiny conducting channels formed during the SET process, whereas in the high-voltage region, the behavior of the curve was nonlinear, as shown in inset of Figure 6b. This nonlinear I−V behavior in the HRS at 21361

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Figure 8. Measurements on Al/GOZNs/ITOPET RRAM device. (a) Flexibility test for various bending radii. (b) Mechanical bending endurance of device at bending radius of 6 mm.

higher field was found to follow the Poole−Frenkel (P−F) emission model. According to P−F emission model ln(J /E) ∝ q3/2(πεrε0)−1/2 (rkT )−1E1/2

show no noticeable degradation up to 1000 times of repeated bending. These measurements performed on the Al/GOZNs/ ITOPET device show excellent results in flexibility and mechanical endurance and indicate the device is suitable for flexible memory applications. Our studies show that ZNs embedded GO-based resistive switching devices have potential for future flexible nonvolatile memory applications.

(2)

where J is the current density, E the electric field, T the temperature, q the electronic charge, εr the dynamic dielectric constant of the material, ε0 the permittivity of free space, and k Boltzmann’s constant, respectively. Here, r is a parameter ranging between 1 and 2, which depends on the position of the Fermi level.41 A linear fit to ln(J/E) versus E1/2 has been observed as shown in the inset of Figure 6b, confirming P−F emission to be the dominant mechanism in HRS in the highervoltage regime. To investigate the practical applications of the resistive memory device, its endurance and retention properties were observed as a function of the number of cycles and time as shown in panels a and b of Figure 7, respectively. As shown in Figure 7a, over more than 200 stress cycles the device performed well, maintaining almost the same resistance ratio of ∼100 (read at 0.1 V) between the two states, which is suitable for practical device applications. Retention characteristics of the device in Figure 7b also showed excellent behavior with no degradation of the ratio between the two resistance states over around ∼104 seconds at a read voltage of 0.1 V. These measurements showed that the device has good endurance and retention properties, which indicate the stability and repeatability of the device are favorable for future nonvolatile memory applications. The flexibility and mechanical endurance are the key parameters to test the performance of flexible electronic devices. To confirm the feasibility of our Al/GOZNs/ITOPET device for flexible memory applications, mechanical flexibility tests were conducted. In the flexibility test, the substrate was bent to various bending radii and the resistance was measured in both states as a function of the bending radius, as shown in Figure 8a. The substrate was bent from its flat position up to an extreme bending radius of 4 mm, and surprisingly the device maintained an almost constant resistance ratio between the LRS and HRS at this extreme bending. The reliability in mechanical endurance of the device was tested by continuously flexing the device a number of times from its flat position to the curvature having a radius of 6 mm, and the resistance was measured in both states as a function of bending cycles, as shown in Figure 8b. The resistances in the HRS as well as LRS

5. CONCLUSIONS The flexible and resistive switching characteristics of the Al/ GOZNs/ITOPET device were investigated. The inherit merits, such as stable resistance ratio between the two states having lower switching voltages with good retention and endurance properties, were favorable in terms of its performance as memory device. A significant lowering in operating voltages in Al/GOZNs/ITOPET as compared to that in Al/GO/ITOPET was achieved by the introduction of ZNs into the GO matrix, resulting in enhanced oxygen vacancies into the system. The formation and rupture of conductive filaments upon the application of suitable bias demonstrates the mechanism for the resistive switching process. The device performance was commendable during the mechanical endurance and flexibility tests, even at extreme bending. These well-executed low-power nonvolatile RRAM devices based on GOZNs have the requisite characteristics to become one of the leading memory devices for flexible systems.

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ASSOCIATED CONTENT

S Supporting Information *

Current−voltage switching characteristics of the GOZNs (10:1, 5:1, and 2:1) devices. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: 787 751 4210. Fax: 787 764 2571. *E-mail: [email protected]. Tel: 787 751 4210. Fax: 787 764 2571. Notes

The authors declare no competing financial interest. 21362

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ACKNOWLEDGMENTS The authors acknowledge financial support from DOE (Grant DE-FG02-ER46526). G.K. acknowledges (NSF Grant EPS01002410) for fellowship. The authors acknowledge contributions by Dr. Esteban Fachini, University of Puerto Rico for XPS measurements.

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