Bistable Electrical Switching Characteristics and Memory Effect by

Jul 31, 2015 - In recent years, increasing attention has been paid to application of organic ... of dynamic random access memories (DRAMs) and hard-di...
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Bistable Electrical Switching Characteristics and Memory Effect by Mixing of Oxadiazole in Polyurethane Layer Yanmei Sun,†,‡ Lei Li,† Dianzhong Wen,*,† and Xuduo Bai*,§ †

HLJ Province Key Laboratories of Senior-education for Electronic Engineering, Heilongjiang University, Harbin, 150080, China Communication and Electronics Engineering Institute, Qiqihar University, Qiqihar, 161006, China § School of Chemistry and Materials Science, Heilongjiang University, Harbin, 150080, China ‡

ABSTRACT: Bistable nonvolatile memory devices with resistive switching characteristics were fabricated based on polyurethane (PU) mixing with 2-(4-tert-butylphenyl)-5-(4biphenylyl)-1,3,4-oxadiazole (PBD). The indium tin oxide/PU +PBD/aluminum device exhibited nonvolatile electrical bistable flash memory behavior with an ON/OFF state current ratio greater than 103. It has been demonstrated that the resistive switching characteristics in the memory device were strongly dependent on the treatment of the polymer blend by PBD. The additive of PBD in PU film reduced the current in the OFF-state significantly and improved the performance of device with the ON/OFF current ratio increased by 2 orders of magnitude, and ON and OFF states of the device can be maintained over 5 h without deterioration.

1. INTRODUCTION Resistive random access memory (RRAM) based on the resistive switching (RS) effect has attracted plenty of attention by virtue of its potential for the replacement of flash memory in next-generation nonvolatile memory applications.1−5 RRAM devices are usually composed of a storage layer sandwiched by two electrodes. A great many materials have been explored as storage media for RRAMs, such as inorganic and organic storage media. In general, inorganic storage media have a prominent advantage over organic ones in performance including high thermal stability,6 ultrahigh ON/OFF ratio,7 fast operation speed,8 high endurance performance,9 and good compatibility with the conventional CMOS process,9 while organic ones stand out in terms of high-mechanical flexibility,10,11 low cost, good processability,12 and the possibility for molecular design through chemical synthesis.13,14 In recent years, increasing attention has been paid to application of organic materials in memory devices on account of their excellent properties.15−17 There have been tremendous efforts focused on developing organic RRAMs, which could transcend and outperform the advantages of dynamic random access memories (DRAMs) and hard-disk drives.18,19 For now, large amounts of organic RRAMs have been fabricated using various organic materials,19−21 including small molecules like perylene imide derivative (PI),22 carbazole derivative (TCz),22 benzoquinone derivatives23 and Alq3,24,25 homogeneous polymer like functional polyimides,26,27 poly(methyl methacrylate),28 polystyrene derivatives,29 triphenylamine,30 and poly(methacrylate) derivatives,31 as well as combinations of functional components.32−34 Polyurethane (PU) attracts increasing attention since it has been synthesized. On account of the excellent physical and © XXXX American Chemical Society

chemical properties and good biological compatibility, PU has found many applications in the field ranging from automotives, rollers systems, shape memory, and films to biomedical products.35−37 Mixing PU with some micromolecules may form a continuous network of the conducting composite material. Due to these conveniences to treat PU via solution method, mixing PU with certain small molecules can effectively avoid the separation of small molecules by crystallization in the spin-coating process. Although PU has been extensively studied, to the best of our knowledge, its resistive switching behaviors have not been reported yet. In this work, we tried to modify PU layer using 2(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) with dissolved content and studied its influence on the resistive switching process. We fabricated devices with and without PBD treatment, and measured current−voltage (I−V) curves by a semiconductor device analyzer. The additive of PBD in PU film reduced the current in the OFF-state significantly. It has been demonstrated that additive of PBD in PU could enlarge the ON/OFF ratio by 2 orders of magnitude and maintain a long retention time over 5 h.

2. DEVICE FABRICATION AND CHARACTERIZATION Two kinds of organic materials are used as the resistive layer in device: pure PU (Mw = 62000) and PU mixed by dissolved PBD (Mw = 354.44). PBD and PU were both provided by Sigma-Aldrich. ITO glass substrates (sheet resistance R□ = 6−9 Ω/□) used for the memory device was cleaned by ultraReceived: June 4, 2015 Revised: July 18, 2015

A

DOI: 10.1021/acs.jpcc.5b05337 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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respectively; the thickness of pure PU and PU+PBD composite films can be judged as 87.3 and 95.5 nm from this image, respectively. As shown in the SEM images, PBD are distributed uniformly in the PU film.

sonication with deionized water, acetone, isopropanol, and methanol for 20 min each. PBD solutions of 5 mg/mL were prepared by dissolving PBD in 1-methyl-2-pyrrolidone solvents with stirring, and filtered through a polytetrafluoroethylene (PTFE) membrane microfilter with a pore size of 0.22 μm to give a homogeneous solution. After that, 1.5 mL of the PBD solutions were mixed with 10 mL the previously filtered PU aqueous solution (6.0 wt % dispersion in H2O) and ultrasonicated for 1 h. A 250 μL mixed solution of PU+PBD was spin-coated onto the ITO glass substrate at a rotation rate of 900 rpm for 18 s and then 5000 rpm for 60 s, followed by being vacuum-dried at 70 °C overnight. As observed by PEDOT:PSS-based and SiOx-based RRAM devices,12,9 the electrodes play a key role in the switching characteristics, especially for the potential barrier between the dielectric and the top electrode for improving the device performance.38 For PBD, an n-type material, an electrode which possesses the quasi-ohmic contact characteristic to PBD and a low work function to reduce the interfacial potential barrier, such as aluminum (Al), is suggested. Therefore, Al was thermally evaporated onto the film surface at 5 × 10−6 Torr through a shadow mask to yield top electrodes with thickness around 300 nm and diameter of 200 μm. Figure 1 shows the

3. RESULTS AND DISCUSSION Generally, two probes are used in electrical measurements for RRAM. During the measurements, voltages were applied on the top electrode (Al) with the bottom electrode (ITO) grounded, and a compliance current of 0.1 A was set. Figure 3a show the I−V curves of ITO/PU/Al and ITO/PU+PBD/Al devices, respectively. There were typically four steps to finish a complete cyclic curve. In the first sweep, when the voltage was scanned from 0 to −6 V, an abrupt increase in current occurred around −0.9 V for both ITO/PU/Al and ITO/PU +PBD/Al devices, then the devices were switched from OFF state into ON state, or low resistance state (LRS). This process was defined as “SET”. In the second sweep, when the voltage was scanned from 0 to −6 V once again, the devices remained in the LRS, indicating that the resistive memory effect was nonvolatile. In the third sweep, a positive voltage sweeping from 0 to 6 V turned the devices from ON state back into OFF state, or a high resistance state (HRS) at 3.4 V for ITO/PU/Al device and 3.6 V for ITO/PU+PBD/Al device via a “RESET” process. In the fourth sweep, the devices remained in the HRS when the voltage was scanned from 0 to 6 V once again. Therefore, the devices exhibited a bipolar characteristic. It is important to note that the memory window is enlarged significantly after PBD treatment as shown in Figure 3a. Figure 3b shows the relationship between the ON/OFF current ratio and the applied voltage for both ITO/PU/Al and ITO/PU +PBD/Al devices. It is observed that the current in the OFF state obviously decreased and the ON/OFF current ratio is obviously increased by 2 orders of magnitude after PBD treatment. The ON/OFF current ratio is higher than 104 in some voltage areas. This high ON/OFF current ratio is enough to reduce the misread rate during storage procedure. The retention and endurance tests of both the LRS and HRS are very important for practical applications of nonvolatile memory devices. Figure 4a shows the result of a retention test for the ITO/PU+PBD/Al device. The resistance in ON and OFF states were measured continuously at a read voltage of 2 V. The device remained ON and OFF states for more than 5 h, and the ON/OFF current ratio maintains about 4 × 103 at 2 V. The bipolar characteristics could still be observed after the

Figure 1. Chemical structure of PU and PBD and schematic of the RRAM device.

schematic of the RRAM device and chemical structure of PU and PBD. The electrical characterization of the memory device was performed with a Keithley 4200-SCS semiconductor parameter analyzer. The cross-section scanning electron microscopic (SEM) images of the pure PU and PU+PBD composite films before the evaporation of Al electrode are shown in Figure 2: from top to bottom the composition is glass, ITO film, and storage layer,

Figure 2. Cross-section SEM images: (a) Pure PU (without mixing PBD) films; (b) PU+PBD composite films. B

DOI: 10.1021/acs.jpcc.5b05337 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. (a) I−V curves of ITO/PU/Al (red line) device and ITO/PU+PBD/Al (blue line) device. (b) Relationship between the ON/OFF ratio of current and the applied voltage for both ITO/PU/Al and ITO/PU+PBD/Al devices.

Figure 4. (a) Retention performance of ITO/PU+PBD/Al device. (b) Endurance performance of ITO/PU+PBD/Al device.

However, the maximum ON/OFF ratio with the stable switching property was observed in the PU:PBD mixture of 10:1.5 (volume ratio). Further increasing or decreasing PBD contents resulted in smaller ON/OFF ratio, few cycling times, and poor retention behavior in spite of similar switching property. To acquire insight into the resistive switching phenomenon, we investigated the previous I−V curves of the ITO/PU+PBD/ Al and ITO/PU/Al memory devices on a log−log scale, in which the slope of the fitted line contains the transport mechanism for both ON and OFF states. Figure 6 shows the double logarithmic plot and its linear fitting of the I−V curve for the set and reset process. As shown in Figure 6, at LRS of the devices, the log I−log V curve has a linear region with a slope of 0.98, 1.02, and 1.05. The slopes were very close to 1. This means that the relationship between current and voltage of the LRS obeys the well-known Ohm’s law. Then, for the HRS of the devices, the log I−log V plot shows an Ohmic conduction behavior at the low voltage region, namely, a linear proportion (with the slope of 1.09 and 1.07 for set process, and 1.09 and 1.13 for reset process), for current to voltage gradually changes to the Child’s law region (with the slope of 2.08 and 2.03 for set process, and 2.05, 2.11, and 2.02 for reset process) at the high voltage region, for which the current is proportional to the square of the voltage.39 In terms of the trap-controlled space charge limited conduction (SCLC) conduction, the I−V curve first follows Ohm’s law at low bias and then Child’s law at high bias. Indeed, for the LRS (as shown in Figure 6), Ohmic behavior (I ∝ V) is followed by Child’s law behavior (I ∝ V2), suggesting a change from the trap-unfilled SCLC conduction to the trap-filled SCLC conduction. This probably occurs due to the creation of trap states at the metal−organic semiconductor interface. Here, Al atoms diffused into the organic layer at the time of thermal evaporation of the top Al electrode leading to the formation of an impurity band for current conduction. Hence, potential band bending occurs at the Al-semiconductor

retention test. The endurance performance of the device in both ON and OFF states was measured at 2 V pulse (2 ms in period, 1 ms in duration width), as shown in Figure 4b. The device exhibited little degradation over 3 × 104 cycles with ON/OFF current ratio of over 4 × 103. Write−read−erase−reread (WRER) measurements were performed in air in order to investigate the rewritability of ITO/PU+PBD/Al device. The write, read, erase, and reread voltages are set as −3, −0.5, 5, and 2 V, respectively, as shown in Figure 5. A voltage pulse of −3 V writes the ON state with

Figure 5. WRER sequence of the ITO/PU+PBD/Al device.

the high current value of almost 10−1 A. Then, an applied voltage pulse of −0.5 V reads the device as in the ON state with a current level of more than 10−2 A. A 5 V voltage pulse performs the erase process of the OFF state with the smaller current value of 10−4 A, and this is again read by the voltage pulse of 2 V as an OFF state with the current value of 10−5 A. The devices exhibit a highly stable characteristic with wellresolved states in terms of operating a repeated write−read− erase−reread (WRER) sequence test. In the WRER cycles, the LRS can be restored to the HRS successfully. We also investigated various contents of PBD, attempting to find their effects on the characteristics of the I−V curves. C

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Figure 6. Linear fitting and corresponding slopes for the I−V curves of ITO/PU/Al and ITO/PU+PBD/Al devices. (a) Set process. (b) Reset process.

Figure 7. (a) UV−vis absorption spectra of PU film. (b) C−V sweeps of the PU film.

Figure 8. (a) UV−vis absorption spectra of PBD solution. (b) C−V sweeps of the PBD solution.

heterojunction due to the trapped electrons. These findings are found to be consistent with that reported earlier.40 It can be concluded from the analysis above that the conductive behaviors of LRS and HRS are totally different. The high conductivity of LRS follows Ohm’s law, while the poor conductivity of HRS complies with SCLC theory. Figure 7a shows the UV−vis absorption spectrum for PU film. A distinct absorbance band centered at around 447.5 nm was observed on the spectrum of the PU, with the absorption edge extending to a wavelength (ledge) of 552 nm, based on which the optical energy band gap (Eg) of the PU was calculated to be 2.25 eV. Cyclic voltammetry (C−V) sweeps of the PU film are shown in Figure 7b, C−V data was collected using a CHI 611B electrochemical analyzer. A three-electrode cell based on an ITO glass working electrode, an Ag/AgCl reference electrode, and a Pt wire counter electrode was purged with nitrogen. The electrochemical properties of the PU films were detected under 0.1 M anhydrous acetonitrile solution containing tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte. A scan rate of 100 mV s−1 was used. The Eonset was the onset oxidation potential and Eferrocene was the external standard potential of the ferrocene/ferrocenium ion couple.

The UV−vis absorption spectra of PBD solution are given in Figure 8a, and the absorption peak at 364 nm and adjoining pronounced shoulder are observed, with the absorption band edge to be about 390 nm, based on which the Eg of PBD was calculated to be 3.18 eV. According to the C−V data of PBD solution as shown in Figure 8b, the PBD shows oxidation behavior with the first oxidation peak at 2.13 eV. Based on the above experimental results, the HOMO and LUMO energy levels of PU and PBD were estimated, and the results are listed in Table 1. The HOMO and LUMO energy levels of the PU and PBD as well as the work function of Al top and ITO bottom electrodes were considered here to understand the memory behavior of ITO/PU+PBD/Al devices. Figure 9 shows the relative energy levels of ITO, PU, PBD, and Al in the memory device. The HOMO energy levels of PU and PBD are −5.40 and −6.55 eV, respectively, while the LUMO energy levels are −3.15 and −3.37 eV, respectively. The high electron affinity of PBD coming from its oxadiazole groups indicates the strong accepting ability of PBD. On the other hand, the low-lying HOMO of PU with the urethane group acts as weak donor in this blend system. The stability of conductive charge transfer complexes may depend on the recombination ability of the separated holes/electrons referred to donor− acceptor strength and effective charge delocalization. D

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carrier density caused by this double injection mechanism, the current density increases rapidly to switch the device to the ON states. The effective electron delocalization in the PBD and hole delocalization in the conducting PU chains can stabilize the charge transfer states of the PU+PBD composite, leading to a nonvolatile nature of the memory device. However, the application of a reverse negative bias to the device can extract electrons from the PBD and consequently reduce the PU, returning the composite to the initial OFF states.

Table 1. Optical and Electrochemical Properties of PU and PBD

PU PBD

λmax (nm)

λonset (nm)

Ega (eV)

Eonset (eV)

HOMOb (eV)

LUMOc (eV)

447.5 364

552 390

2.25 3.18

0.98 2.13

−5.40 −6.55

−3.15 −3.37

a

Eg is estimated from the UV−vis absorption edge wavelength (ledge) using the Planck equation: band gap = (1240/λonset).41 bHOMO energy levels were determined from the C−V onset ionization potential (Eonset) using ferrocene as the external reference (4.8 eV below the vacuum level): EHOMO = −[Eonset − Eferrocene + 4.8)] (eV). Eferrocene is determined to be 0.38 V vs Ag/AgCl. cELOMO is determined from the equation: ELOMO = EHOMO + Eg.42

4. CONCLUSION In summary, the resistive switching behavior of the organic bistable devices fabricated utilizing PU+PBD layer by using a spin coating method were investigated. The ITO/PU+PBD/Al device exhibited nonvolatile electrical bistable flash memory behavior with an ON/OFF state current ratio greater than 103. The device performance was substantially enhanced when the storage layer was mixed with PBD. The loss of PU and charge traps induced by PBD reduced the current in OFF-state significantly. As a result, the additive of PBD increased the ON/ OFF ratio by 2 orders of magnitude. Moreover, the large ON/ OFF ratio is stable for over 5 h without significant degradation.

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Figure 9. Relative energy levels of ITO, PU, PBD, and Al in the memory device.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +86 0451 86608413. Fax: +86 0451 86608413. *E-mail: [email protected]. Tel: +86 0451 86608131. Fax: +86 0451 86608413.

The resistive switching mechanism of the ITO/PU/Al memory devices can be attributed to the charge trapping and detrapping in the PU backbones. When a negative voltage is applied to the Al electrode with sufficient amplitude, the traps will become fully filled with holes. The subsequently injected holes from the ITO electrode can migrate more freely in the polymer thin film and convert the memory device from OFF state to ON state. On the other hand, the OFF state is achieved by removing the negative charge with the application of a reverse electric field. In this case, the injected charges are extracted from PU film, and the filled traps are detrapped to regenerate the potential well for charge carrier hopping and switch the device back to OFF state.43 Inevitably, the additive of PBD into PU will induce the loss of PU in some extent. Furthermore, charge traps induced by the high electron affinity of PBD will decrease the current of OFF state. PBD molecules may be obstacles that occupy some position of PU and block the charge transportation, so the current of ITO/PU+PBD/Al device in OFF state is reduced significantly. In sum, there are typically two main functions of PBD: induce charge traps to capture electrons, and form charge transfer with weak donor PU in this blend system. It has already been demonstrated that PU and PBD can act as donors and acceptors, respectively. Therefore, the electrical switching behavior for the ITO/PU+PBD/Al device might be attributed to the electric-field-induced charge transfer between PU and PBD. The device is in the OFF state primarily. When the negative bias is applied, the charge carriers (holes and electrons) begin to be injected into the device and are trapped near the corresponding electrodes as well as at the PU+PBD interfaces in the composite film, and cause the formation of space charge layers. The space charge layer screens the electric field and limits further carrier injection. Once the applied bias approaches the switched-on voltage, the traps are filled by holes and the Al electrode starts to inject electrons into the vacancies in the LUMO of PBD. Due to the enhancement in charge

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the support of the National Science Foundation of China (Grant no. 61204127 and 21372067), Doctoral Fund of Ministry of Education of China (20132301110001) and the Natural Science Foundation of Heilongjiang Province, China (Grant No. A2015010).

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DOI: 10.1021/acs.jpcc.5b05337 J. Phys. Chem. C XXXX, XXX, XXX−XXX