PMMA nanocomposites

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Memory stabilities and mechanisms of organic bistable devices with giant memory margins based on Cu2ZnSnS4 nanoparticles/PMMA nanocomposites

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Dong Yeol Yun, Narayanasamy Sabari Arul, Dea Uk Lee, Nam Hyun Lee, Tae Whan Kim ⇑

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Department of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Republic of Korea

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a r t i c l e

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Article history: Received 3 February 2015 Received in revised form 24 April 2015 Accepted 5 May 2015 Available online xxxx Keywords: Nonvolatile memory Giant memory margin CZTS nanoparticles/PMMA nanocomposites Switchable memory devices

a b s t r a c t Organic bistable devices (OBDs) were fabricated utilizing nanocomposites made from a blend of Cu2ZnSnS4 (CZTS) nanoparticles within a polymethyl methacrylate (PMMA) matrix on a polyethylene terephthalate substrate. Energy dispersive X-ray spectroscopy profiles, X-ray diffraction patterns, and high-resolution transmission electron microscopy images showed that the polycrystalline CZTS nanoparticles were randomly distributed in the PMMA layer. The current–voltage (I–V) curves at 300 K for the fabricated OBDs showed bidirectional switchable and current hysteresis behaviors, indicative of the removal of sneak current paths without an additional layer with characteristics of diode or selector. The removal of the sneak current paths prevented the leakage current of the OBDs, resulting in an increase of the current of high conduction (ON) level. The maximum ON/low-conduction (OFF) ratio of the current bistability for the fabricated OBDs was as large as 1  109. The write–read–erase–read sequences of the OBDs showed rewritable nonvolatile memory behaviors. The ON or the OFF states could be retained for 1  105 cycles, indicative of excellent memory stability. The ON/OFF ratio of 109 was maintained after 105 cycles. The memory mechanisms of the fabricated OBDs are described on the basis of the I–V results. Ó 2015 Published by Elsevier B.V.

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1. Introduction

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Organic/inorganic nanocomposites have attracted a great deal of attention due to their promising applications in diverse electronic and optoelectronic devices [1–9]. Organic bistable devices (OBDs) based on hybrid nanocomposites containing inorganic nanoparticles (NPs) have emerged as great candidates for next-generation nonvolatile memory devices because of their excellent properties of low fabrication cost, low power consumption, high data storage density, and high mechanical flexibility [10–13]. Nonvolatile memory devices with NPs added into a polymer matrix and with a subsidiary layer with a diode function have shown some significant improvements for use as bistable memory devices [14]. While the Restriction of Hazardous Substances Directive (RoHS) prohibits the utilization of compound semiconductor materials containing Cd and Pb atoms in devices, the RoHS permits the use of CZTS as these materials are environment-friendly [15].

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Even though some studies concerning the formation and the physical properties of CZTS NPs have been carried out [16–20], investigations of enhancements of memory margins and achievement of memory stabilities for OBDs fabricated utilizing CZTS NPs blended into a PMMA layer on a flexible indium-tin-oxide (ITO)-coated polyethylene terephthalate (PET) substrate have not yet been reported. This paper reports data on significant enhancements of the memory margins and achievement of memory stabilities in OBDs utilizing CZTS NPs blended into a PMMA layer. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) pattern, energy dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) measurements were performed to investigate the microstructural properties of the CZTS NPs. Current–voltage (I–V) measurements were carried out to investigate the electrical current bistability and bidirectional switchability of the OBDs. The memory mechanisms of the OBDs are described on the basis of the I–V results.

⇑ Corresponding author. E-mail address: [email protected] (T.W. Kim). http://dx.doi.org/10.1016/j.orgel.2015.05.007 1566-1199/Ó 2015 Published by Elsevier B.V.

Please cite this article in press as: D.Y. Yun et al., Memory stabilities and mechanisms of organic bistable devices with giant memory margins based on Cu2ZnSnS4 nanoparticles/PMMA nanocomposites, Org. Electron. (2015), http://dx.doi.org/10.1016/j.orgel.2015.05.007

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2. Experimental details

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An overall schematic of the OBDs device fabrication processes is shown in Fig. 1. The detailed preparation processes and the structural properties of the CZTS NPs are described elsewhere [21]. CZTS NPs were synthesized by a hydrothermal method at 180 °C for 6 h using Cu, Zn, Sn chloride precursors along with thiourea as a precipitating agent. The obtained precipitate was centrifuged, washed, and dried in vacuum oven at 80 °C for 4 h (Fig. 1(a–c)). Subsequently, the blended solution containing 3.3-wt.% of synthesized CZTS NPs was mixed with PMMA in an N,N-dimethylformamide (DMF) solvent by using ultrasonication, as shown in Fig. 1(d). Meanwhile, the ITO-coated PET substrates were ultra-sonicated in methanol, DI water, and isopropyl alcohol for 15 min each. The cleaned substrates were blown dry by using N2 gas to evaporate the existing solvents on the substrate (Fig. 1(e)). The blended solution was dropped onto ITO-coated PET substrates and was spin-coated sequentially at 500 rpm for 5 s, 3000 rpm for 40 s, and 500 rpm for 5 s (Fig. 1(f)). After having baked the samples at 90 °C for 30 min to remove the remaining solvents and improve the uniformity of the CZTS NPs blended into the PMMA layer on the ITO-coated PET substrates, Al top electrodes with thicknesses of 140 nm and widths of 300 lm were deposited on the PMMA layers containing the CZTS NPs by using thermal evaporation through a metal mask at a pressure of 1  106 Torr (Fig. 1(g)). The thickness of the PMMA layer containing the CZTS NPs, as determined using a-step equipment, was approximately 160 nm. The OBDs based on the nanocomposites are sandwiched between two Al electrodes, and the final memory device with a bidirectional selector is shown in Fig. 1(h). EDX and HRTEM measurements (model: JEM 2100F) were done at 200 kV to investigate the chemical compositions and the morphologies of the CZTS nanoparticles. XRD patterns were measured by using a Rigaku D/MAX-2500 V diffractometer with Cu Ka radiation, which was operated at a scanning rate of 5°/min over a 2h range between 20° and 90°. The I–V and the write–read–erase– read characteristics were investigated by using a 4140B pA

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meter/DC voltage source and an Agilent 3250A 80-MHz function/arbitrary waveform generator, respectively.

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3. Results and discussion

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Fig. 2 shows plane-view TEM images, a SAED pattern, an EDS spectrum, and an XRD curve for synthesized CZTS NPs. The formation of the CZTS nanospheres is clearly shown in Fig. 2(a), and the inset of Fig. 2(a) depicts the SAED of the synthesized CZTS NPs. The SAED pattern shows the reflection of (1 1 2), (2 2 0), and (3 1 2) planes of the CZTS NPs, which is in reasonable agreement with XRD results. The CZTS nanospheres compose of triangle geometry CZTS NPs, indicative of the existence of the grain boundaries, as shown in Fig. 2(b), can be assigned to the kesterite phase of CZTS [21,22]. The crystallite size of the CZTS NPs in the nanospheres is approximately 4 nm, which is consistent with Scherrer equation for the X-ray diffraction pattern. The energy peaks of the EDX spectrum show that the synthesized NPs contain elemental Cu, Zn, Sn, and S, indicative of the formation of CZTS, as shown in Fig. 2(c). The atomic stoichiometries of the synthesized CZTS compositions are, on average, 26.1% Cu, 25.4% Zn, 14.1% Sn, and 34.4% S. The crystalline structure of the synthesized CZTS NPs characterized by the XRD curve is shown in Fig. 2(d). The XRD curve of the CZTS NPs exhibits three broad peaks, 28.5°, 47.6°, and 56.3°, which correspond to the diffractions from the (1 1 2), (2 2 0), and (3 1 2) planes of the CZTS NPs [21]. The HRTEM and XRD results are in reasonable agreement with those reported for polycrystalline and stoichiometric CZTS in the tetragonal phase [23]. The CZTS NPs structured by polycrystalline provide more trap sites than that of crystal for the capture of carriers [24,25]. The I–V curves at 300 K for the Al/CZTS NPs blended into PMMA layer/ITO/PET devices are shown in Fig. 3. The applied voltage across the device was varied in a sequence of 0, 4, 0, 4, and 0 V, as shown in Fig. 3(a) and (b). The I–V curves below low voltage (0 to 0.4 and 0 to 0.4 V) show bidirectional diode characteristics, indicative of the preventions of the number of sneak current paths [14]. The I–V curves above low voltages both forward and reverse

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Fig. 1. Schematic diagrams of the fabrication processes for the OBDs: (a) chloride precursors of Cu, Zn, Sn, and thiourea (CH4N2S) were added to ethylene glycol (C2H6O2) and stirred well; (b) the solution was maintained at 180 °C for 6 h and then allowed to cool to room temperature, after which it was washed and dried at 60 °C to obtain the CZTS nanoparticles (NPs); (c) photographs of the CZTS NPs; (d) blending of CZTS NPs and PMMA in N,N-dimethylformamide (DMF); (e) ITO-coated PET substrates; (f) blended CZTS:PMMA layer formation on the ITO layer by using a spin-coating technique; (g) Al-electrode deposition on CZTS:PMMA/ITO/PET sheets by using a thermal evaporation process; (h) final memory device with a bidirectional selector.

Please cite this article in press as: D.Y. Yun et al., Memory stabilities and mechanisms of organic bistable devices with giant memory margins based on Cu2ZnSnS4 nanoparticles/PMMA nanocomposites, Org. Electron. (2015), http://dx.doi.org/10.1016/j.orgel.2015.05.007

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Fig. 2. (a) Plane-view transmission electron microscopy (TEM) image of the synthesized CZTS NPs blended into a PMMA layer. The inset of the Fig. 1(a) presents a selected area electron diffraction pattern of the CZTS NPs. (b) The high-resolution TEM image, (c) the energy dispersive X-ray spectroscopy spectrum, and (d) the X-ray diffraction curve of the synthesized CZTS NPs show the compound structure materials and lattice planes, respectively.

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voltages (0.5 to 3.8 and 0.5 to 2 V) exhibit current hysteresis behaviors, indicative of the achievement of excellent memory characteristics with high conductivity (ON) and low conductivity (OFF) states at the same voltage [14]. The I–V curves show switchable characteristics, which depend on the magnitude of the applied voltage like a diode, and nonvolatile memory characteristic above switching characteristic regions. When the applied voltage is larger in positive voltages or is smaller in negative voltages than the switching voltage, the I–V curves for the OBDs depict nonvolatile memory characteristics and transit from the OFF to the ON states around 3.8 V or from the ON to the OFF states around 2 V, resulting in a writing or an erasing process, respectively. Even though the fluctuations, such as terraced different current states in negative voltage, are shown in Fig. 3(b) due to the influences of the low resistance of the nanocomposite layer, the I–V curves of the OBDs are enough to show the memory performance, such as ON and OFF states. Because the ON state is significantly related to sneak current paths from neighbor cells due to the low resistance cells existing in a device, the I–V curves of the nonvolatile memory devices should have switchable characteristics to remove sneak current paths. The removal of the sneak current paths decreases a leakage current originating from neighbor cells. The maximum ON/OFF current ratio for the OBDs at voltages above the switching voltage is approximately 1  109, indicative of a giant memory margin. Such a characteristic can clearly distinguish the digit 1 from the digit 0 in the memory bit, resulting in a dramatic decrease in the memory reading error. When the I–V curves are transited, the ON state or the OFF state is maintained at voltages above the switching voltage until an erasing or a writing voltage is applied to the devices. Even though the operating mechanisms for the diode in OBDs have not been clearly clarified yet, the Schottky diode mechanisms of the I–V data is described based on other literatures [14,26]. In bidirectional switching characteristics, the initial states for the devices are depicted by having bidirectional Schottky diodes at interfaces between both electrodes and a PMMA layer containing CZTS NPs. When the positive voltages, which are above a switching voltage of 0.5 V, are applied to the device, the oxygen vacancies, which

are existed in a PMMA layer containing CZTS NPs and are generated in the CZTS NPs by an external electric field, move to a bottom electrode, indicating that the oxygen vacancies are heaped up and that an energy barrier around interfaces between a PMMA layer containing CZTS NPs and a bottom electrode decreases [26]. These results cause that the Schottky barrier disappears, as shown in processes 1 and 2 of Fig. 3. When the negative voltages, which are below a switching voltage of 0.5 V, are applied, the phenomenon about heaping the oxygen vacancies up the bottom electrode occurs in the opposite direction, indicative of the processes 3 and 4 of Fig. 3 [26]. The appearance of a giant memory margin for the OBDs is dominantly due to the existence of the quaternary elementary compositions of the CZTS NPs and the grain boundaries of a CZTS polycrystalline structure [27]. When the quaternary CZTS NPs are formed, the number of trap sites for the CZTS NPs is much larger than it is for single, binary, or ternary NPs. The high ON/OFF ratio of the I–V curves is substantially increased due to the many carrier trap sites in the CZTS NPs [27]. The I–V curves for the devices without CZTS NPs show no memory characteristics with low currents of approximately 1  1013 A, as shown in the inset of Fig. 4(a), indicating that the bidirectional switchable characteristics and the large current hysteresis properties of the OBDs are due to the existence of the CZTS NPs. The write–read–erase–read performances of the OBDs are shown in Fig. 4. The sequence of the input voltage pulses was set as write (+5 V), read (+1 V), erase (5 V), and read (+1 V), as shown in Fig. 4(a). One cycle of the write–read–erase–read sequence pulses was 1 ms. Fig. 4(b) shows that the output current corresponding to the input voltage pulses can clearly distinguish the ON current state or the OFF current state at the same reading voltage after an applied writing or erasing voltage. The ratio between the ON current and the OFF current states at the same reading voltage is approximately 1  109. The memory stability for the Al/CZTS NPs blended into PMMA layer/ITO/PET devices at room temperature was evaluated by using a cycling stress test, as shown in Fig. 5. After a writing voltage of +5 V or an erasing voltage of 5 V was applied to the devices, the

Please cite this article in press as: D.Y. Yun et al., Memory stabilities and mechanisms of organic bistable devices with giant memory margins based on Cu2ZnSnS4 nanoparticles/PMMA nanocomposites, Org. Electron. (2015), http://dx.doi.org/10.1016/j.orgel.2015.05.007

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Fig. 3. Current–voltage (I–V) curves of the Al/CZTS NPs blended into PMMA layer/ ITO/PET devices under (a) forward applied voltages and (b) reverse applied voltages. The inset of (a) presents the I–V curves for the Al/PMMA layer/ITO/PET reference device.

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retention stress of the ON state or the OFF state was measured at the same reading voltage of +1 V. The ON/OFF ratio of 109 was

Fig. 4. Write–read–erase–read results for Al/CZTS NPs blended into PMMA layer/ ITO/PET devices. The input voltage and the output current are shown.

retained after 105 cycles. Even though some fluctuations appeared due to the electrical cycling stress, no significant degradation of the OBDs in either the OFF state or the ON state after 105 cycles of continuous stress was observed, indicative of the information storage stability of the OBDs. The storage stability of the fabricated OBDs originated from the stable nanocomposites of the CZTS NPs blended into a PMMA layer. The memory mechanisms for the OBDs are described on the basis of the I–V curves. The I–V data have been fitted to investigate the carrier transport mechanisms, and the fitted I–V data for the OBDs are shown in Fig. 6. The fitting of the data has been performed by using thermionic emission (0–0.4 V), as shown in the inset of Fig. 6, Ohmic conduction (0.45–1.35 V), and space-charge-limited current (SCLC; OFF states between 1.4 and 4 V) due to charge trapping [28–30]. Because a slight current in the Schottky diodes ranges flows via a thermal energy, the Schottky diode ranges are well fitted by thermionic emission [31]. The Ohmic and the SCLC mechanisms are well fitted above the Schottky diode ranges. The a, which is an exponential fitting parameter for the I–V data, is used for the Ohmic conduction and the SCLC related to the equation of J / Va. The carrier transport in the ON state exhibited SCLC effects until the switching voltage had returned, indicative of the appearance of a switching characteristic.

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4. Summary and conclusions

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OBDs utilizing CZTS NPs blended into a PMMA layer were fabricated on ITO-coated PET substrates by using a spin-coating method. HRTEM images showed that the polycrystalline CZTS NPs were randomly distributed in the PMMA layer. The I–V curves for the OBDs showed bidirectional switchable behaviors at low voltages and nonvolatile memory characteristics above the bidirectional switching voltage in positive and negative voltages. The maximum ON/OFF current ratio for the OBDs was as large as 1  109. When the OBDs were operated with a write (+5 V)-read (+1 V)-erase (5 V)-read (+1 V) sequence, they showed a rewritable nonvolatile memory behavior. The ON and the OFF states could be retained for as many as 1  105 cycles, indicative of excellent memory stability. The memory mechanisms of the fabricated OBDs were described on the basis of the theoretical fittings of the I–V curves. These results demonstrate the feasibility of fabricating nonvolatile and bidirectional switchable memory devices without the need for an additional diode or switching layer.

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Fig. 5. Current as a function of the number of cycles for the ON and the OFF states of the Al/CZTS NPs blended into PMMA layer/ITO/PET devices at a reading voltage of 1 V.

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Fig. 6. Fitted I–V data at 300 K for the Al/CZTS NPs blended into PMMA layer/ITO/ PET devices. While the OFF state could be fitted by using the thermionic emission, Ohmic conduction, and space-charge-limited-current (SCLC) mechanisms, the ON state could be fitted by using only the SCLC mechanism. The inset presents the thermionic emission mechanism.

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Acknowledgment

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This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2013-016467).

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