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Electrical switching and conduction mechanisms of nonvolatile write-once-read-many-times memory devices with ZnO nanoparticles embedded in polyvinylpyrrolidone
Organic Electronics 15 (2014) 1254–1262
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Electrical switching and conduction mechanisms of nonvolatile write-once-read-many-times memory devices with ZnO nanoparticles embedded in polyvinylpyrrolidone K. Onlaor ⇑, T. Thiwawong, B. Tunhoo Electronics and Control System for Nanodevice Research Laboratory, College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Bangkok 10520, Thailand Thailand Center of Excellence in Physics, Commission On Higher Education, Ministry of Education, 328 Si Ayutthaya Road, Bangkok 10400, Thailand Nanotec-KMITL Excellence Center On Nanoelectronic Devices, Bangkok 10520, Thailand
a r t i c l e
i n f o
Article history: Received 15 November 2013 Received in revised form 11 March 2014 Accepted 18 March 2014 Available online 1 April 2014 Keywords: PVP ZnO WORM Nanoparticle Memory device Conduction mechanism
a b s t r a c t We reported on the influence of zinc oxide nanoparticles (ZnO NPs) on the electrical bistable behavior of nonvolatile write-once-read-many-times (WORM) memory devices based on an indium-tin oxide/polyvinylpyrrolidone (PVP):ZnO NPs/aluminum (ITO/PVP:ZnO/Al) structure. The maximum ON/OFF current ratio of the nonvolatile WORM memory devices was approximately 3 103 and the devices remained in the ON state even after the applied voltage was turned off. In addition, reliability studies for response time and once write/ continuous read operations of the optimal ZnO NPs concentration are presented. The response times of both rise-time and fall-time were about 3 and 6 ls respectively. The conduction mechanisms of all voltage regions of the device were analyzed by theoretical models and electron trapping in the ZnO NPs of the electron tunneling among a PVP matrix was discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, several works have been done in the field of organic bistable or resistive switching memory devices [1–6]. The basic feature of the devices is to exhibit bistable behavior having two different conduction states, with a conductivity difference of several orders of magnitude. Resistive memory can be classified into two kinds: (1) rewritable [1] and (2) write-once-read-many-times (WORM) [7]. A similar phenomenon of two kinds has been observed in a variety of materials and device structures
⇑ Corresponding author at: Electronics and Control System for Nanodevice Research Laboratory, College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Bangkok 10520, Thailand. Tel.: +66 3298000. E-mail address: [email protected] (K. Onlaor). http://dx.doi.org/10.1016/j.orgel.2014.03.024 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
[8–18]. For the purpose of permanent data storage, WORM memory devices have been studied intensively and are particularly interesting because of their merits of simple device structure, simple fabrication process, low reading voltage, low manufacturing cost and extensive applications [7,11,18,19]. Well known examples are the smart labels and radio frequency identification (RFID) tags. These applications do not require a high memory density [20]. Among WORM memory device structures, the type comprising nanoparticles (NPs) blending with the polymer host has been subject to many studies because of the great variety of NPs and polymer species [3,4,7,10–12,21]. In addition, several conduction mechanisms of NPs blended in polymer have been proposed to explain the conductance switching, including formation of conducting filaments [18,22], formation of charge transfer complexes [23], charge-trapping [5,24], etc. However, the memory type and the switching mechanisms of the NPs blending with the polymer host
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are still a matter of debate even when made from the same polymer host materials [3,9,25,26]. Of several types of NPs used in the memory structures, zinc oxide nanoparticles (ZnO NPs) are particularly desirable because they have many promising applications for memory devices [3,4,9,21,25,26]. Among polymer species, a polyvinylpyrrolidone (PVP) polymer has been widely studied because of its good environmental stability, operation stability, easy processability and its good electrical properties [27– 32]. Here, we report on the fabrication of nonvolatile WORM memory devices with indium-tin oxide/PVP:ZnO NPs/aluminum (ITO/PVP:ZnO/Al) structure at different concentrations of ZnO NPs. The electrical characteristics of the asfabricated devices were investigated by current–voltage measurement. Our work shows that the devices exhibit maximum ON/OFF current ratio in the order of 3 103 with low reading voltage, fast response and long term stability. The conduction mechanisms of all voltage regions are proposed to explain the conductivity transition and are described on the basis of the I–V results.
2. Experimental The memory devices were ITO/PVP:ZnO/Al structure. ITO conducting glass with sheet resistance of 10 O/h is used as a substrate. Prior to spin cast, the ITO-coated glass substrates were cleaned with the following procedure that included sonication in deionized high purity water, acetone, methanol and isopropanol for 20 min. The cleaned substrates were then dried by using N2 gas. The PVP (average Mw 10,000) and ZnO NPs purchased from Aldrich Chem. Co, with average ZnO NPs size less than 35 nm were used for the storage layer in the memory devices. The PVP was dissolved in ethanol with a concentration of 30 mg/ml and kept constant to ensure comparable film thicknesses in each PVP:ZnO composition. After that, ZnO NPs were mixed with a PVP-ethanol solution at concentrations of 2, 4, 6, 8 and 10 wt%, then ultrasonically mixed to enhance the dispersion of ZnO NPs in the PVP solution. The mixed solution was spin coated on the ITO-coated glass substrates at 3000 rpm for 30 s at room temperature. After the spin coating of the mixed solution, the samples were kept at 120 °C for 60 min to remove the solvent. Finally, 100 nm-thick Al top electrode was prepared on PVP:ZnO layer by thermal evaporation technique with a base pressure of 2 106 mbar, the devices had a cross-sectional area of 4 mm2. The schematic diagram of the device and chemical structure of PVP are shown in Fig. 1. The I–V measurements were performed under dark conditions to examine the electrical properties of the fabricated devices using a precision LCR meter (Agilent E4980A). In addition, response time tests were performed with a pulse generator (RIGOL DG1022) and digital oscilloscope (RIGOL DS1102D). The pulse generator was connected to a computer with a universal serial bus (USB) using an UltraWave program for creating arbitrary waveforms. The retention characteristics were performed by using a source meter (Keithley 2410), which was controlled by a computer program (Labview). A field-emission scanning electron microscope (FE-
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Fig. 1. Schematic illustration of the ITO/PVP:ZnO/Al memory devices and chemical structure of polyvinylpyrrolidone (C6H9NO)n or PVP.
SEM, Hitachi S-4700) was used to investigate the thickness of the PVP:ZnO layer.
3. Results and discussion Fig. 2 shows the current–voltage (I–V) characteristics of the ITO/PVP:ZnO/Al devices with different ZnO NPs concentrations. The curve was swept by applying a bias voltage to the bottom electrode (ITO) and grounding the top electrode (Al) with the variations of voltages across the device in a cyclical manner from +5 V to 5 V and 5 V to +5 V. In the case of ITO/PVP/Al devices, the device exhibits small hysteresis as shown in Fig. 2(a). That may have resulted from the mechanisms at the polymer/metal interface and the filament channels in the polymer layer [11,33–35]. However, the Al electrode of all devices is grown in the same conditions and was fabricated together. Fig. 3 shows a cross sectional view of FE-SEM images, the PVP:ZnO layers with different ZnO NPs concentrations are uniformly thick after deposition from the spin cast PVP:ZnO solution. The thickness of the PVP:ZnO layer of all devices is approximately 180 nm. However, ZnO NPs in the PVP matrix are difficult to observe whereas ZnO NPs contained in the films show the effects on device efficiency such as threshold voltages (Vth) and ON/OFF current ratios which will be discussed later. Thus, the switching behaviors of our devices strongly depend on the concentration of ZnO NPs in the films, which is based on a charge stored mechanism more than the effect of the PVP/Al interface, the filament channels and the films’ thickness. Interestingly, the I–V curves for the devices with ZnO NPs show a large current hysteresis behavior, indicative of memory effects in the memory devices, as shown in Fig. 2(a and b). The low-current state and the high-current state correspond to a relatively OFF state and a relatively ON state, respectively. At the initial state (curve 1), the devices initially had a low conductivity state. Subsequently the current increases gradually with the increase reverse applied bias voltages (curve 2). When the applied voltage reached Vth, the current abruptly increased, with the values of Vth in the range of 2 V to 3.5 V. The voltages were then swept from 5 V to 0 V, and the different high conductivity state emerged (curve 3). The transition from the OFF state to the ON state is equivalent to the writing process in the memory devices, in which ‘‘0’’ ? ‘‘1’’ can be switched from the low conductivity state to the high conductivity state. After the devices changed from the OFF state to the ON state, the devices could not return to the OFF state again when the
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Fig. 2. I–V characteristics of the memory devices with different ZnO NPs concentration, (a) PVP only and 2 wt% and (b) 4–10 wt%.
Fig. 3. FE-SEM images showing the cross sectional view of the PVP:ZnO films with different ZnO NPs concentration, (a) PVP only, (b) 2 wt%, (c) 4 wt%, (d) 6 wt%, (e) 8 wt% and (f) 10 wt%.
opposite bias voltages were applied (curve 4) and even when the applied voltage was turned off, which is indicative of nonvolatile WORM behavior. These results correspond to those of the WORM memory devices with ZnO NPs embedded in polymethylmethacrylate (PMMA) [3] that have been reported by Dao et al. Fig. 4(a) shows the ON/OFF current ratios for the fabricated devices with different concentrations of ZnO NPs. The relative concentrations of ZnO NPs in the films used in our devices clearly influence the electrical characteristics trends. The ON/OFF current ratios (read at ±1 V) of each concentration for both positive and negative voltage regions are measured in 20 randomly selected cells, and the data points in Fig. 4(a) are average values with standard deviations. From Fig. 2(a), the device with a ZnO NPs concentration of 2 wt% showed clearly observed ON and OFF current states. However, the average ON/OFF current ratio of the devices is smaller than that of the devices with concentrations of 4 and 6 wt%, as shown in Fig. 4(a). However, when the concentrations of the ZnO NPs are increased more than 6 wt%, the ON/OFF current ratio goes down as a consequence of the phenomenon whereby ZnO NPs in the films might be joined with the neighboring ZnO NPs, forming continuous ZnO NPs networks in the
films. This continuous ZnO NPs form allows effective transport of the charge carrier under low bias, because the electron injection probability at the electrode increases with increasing ZnO NPs content. Hence, the electric fields between the neighboring nanoparticles increase [3,36], making the OFF state current of the devices exhibit a high conductivity state, as shown in Fig. 2(b). Although, the conductivity transition from the OFF to the ON states of the devices with a concentration of 10 wt% easily occurs due to the large number of continuous ZnO NPs networks, the I–V curves for the devices exhibit the smallest ON/OFF current ratio. In other devices, however, a concentration of 2 wt% is not enough to occupy carriers for receiving the large ON/OFF current ratio. Therefore, the nonvolatile WORM memory devices with a concentration of 6 wt% show the optimal electrical switching with the highest ON/OFF current ratio (ca. 3 103). Fig. 4(b) shows the I–V characteristics with repeated scans of the optimal ZnO NPs concentration in the ITO/PVP:ZnO/Al structure, for investigating the conduction mechanisms in the next section. To examine the carrier transport mechanisms in the nonvolatile WORM memory devices prepared at ZnO NPs concentration of 6 wt%, the I–V data were fitted by using
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vice at room temperature on ln(I/V2) versus |1/V|. The electron transport in the low bias region (V < Vth) can be interpreted as direct tunneling, in which electrons tunnel through a square barrier, expressed as [39,40],
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 2d 2m /b I / V exp h
ð2Þ
where m is the effective mass and ⁄ is the reduced Planck’s constant. For the high voltage region (V > Vth), the applied voltage is enough to change the shape of the barrier to become triangular, thus described by Fowler–Nordheim (F– N) tunneling as [39,40],
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 2m /3b 4d A I / V 2 exp @ 3qhV 0
ð3Þ
The dotted line denotes the Vth at 2.2 V or denotes the transition from a direct tunneling model to the F–N tunneling model at high voltage. From the F–N tunneling region in Eq. (3), it is useful to liberalize on a logarithmic scale in terms of the voltage dependence to become Eq. (4):
ln
Fig. 4. (a) ON/OFF current ratio of the memory devices for both positive and negative voltage regions with different ZnO NPs concentration. (b) I–V curves for the optimal ZnO NPs concentration (6 wt%), showing steps of repeated scans.
" pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi # ð/b q qV=4pdei Þ ; I / T exp kb T 2
ð1Þ
where T is the temperature, /b is the barrier height, ei is the dynamic permittivity of the film, q is the electron charge, kb is the Boltzmann constant and d is the film thickness. By solving Eq. (1) in terms of V1/2, the voltage dependence can be derived as ln(I)/V1/2 [38]. A linear relationship was observed between ln(I) versus V1/2 from +5 V to 0 V, as shown in Fig. 5(a). This suggests that the current in the OFF state before the transformation to the ON state was controlled by charge injection from the electrode. Because the thermionic emission is primarily due to exciting electrons to higher energies in a metal contact, electrons can overcome the potential barrier between the insulator/metal contact [37,38]. When the reverse bias voltages were applied [curve 2 of Fig. 4(b)], two distinct voltage regions are evident from this curve; therefore two mechanisms in the low voltage and high voltage regions were needed, as shown in Fig. 6(a). Fig. 6(a) shows the tunneling characteristics across the de-
V2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 0 3 1 @ 4d 2m /b A / V 3hq
ð4Þ
In order to directly compare both tunneling transport mechanisms, the direct tunneling in Eq. (2) can be rearranged in terms of the voltage dependence to become Eq. (5),
ln
various conduction models based on the I–V curves to clarify the carrier transport and memory mechanisms of the device. In the OFF state for the positive voltage region [curve 1 of Fig. 4(b)] the data can be well fitted with the thermionic emission [20,37,38]:
I
I
V2
/ ln
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 2d 2m /b 1 : h V
ð5Þ
After the reverse bias voltages had been applied to the device, the electrons captured in the PVP:ZnO layer were increased. The current flowed via the F–N tunneling conduction mechanism so that the injected current increased sharply and induced the electrical state of the device to rapidly change from the OFF state to the ON state. In the ON state for the negative voltage region, Fig. 6(b) shows a log(I) versus log(V) plot for the I–V results of Fig. 4(b) (curve 3); the current flow was free from the influence of traps in the device. The conduction mechanism of the ON state is believed to show ohmic behavior [37]. When the forward bias voltages were applied again [curve 4 of Fig. 4(b)], the I–V curves under applied voltages from 0 V to +1.3 V in the ON state could be fitted well by using the ohmic conduction model and the F–N tunneling model at a higher applied voltage (+1.3 V–+5 V) as shown in Fig. 5(b). Hence, ohmic conduction is the dominant carrier transport mechanism for devices in the ON state. However, after the device transformation from the OFF state to the ON state, negative differential resistance (NDR) is slightly observed at +1.3 V, as shown in Fig. 4(b), leading to asymmetric bistability due to the devices being made of materials poor in conductance, such as some other polymers [41,42]. Thus, the conduction mechanisms after writing voltage of the fabricated ITO/PVP:ZnO/Al memory devices might be mainly caused by ohmic conduction mechanism (trap-states) and filamentary conduction mechanism
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Fig. 5. Experimental and fitted I–V characteristics of the memory device with a ZnO NPs concentration at 6 wt% of the forward bias voltage, (a) OFF state and (b) ON state, (curves 1 and 4 are corresponding with Fig. 4(b)).
Fig. 6. Experimental and fitted I–V characteristics of the memory device with a ZnO NPs concentration at 6 wt% of the reverse bias voltage, (a) OFF state and (b) ON state, (curves 2 and 3 are corresponding with Fig. 4(b)).
(filament channels), nevertheless the ohmic conduction is dominant over the filamentary conduction, which will be further discussed later. However, Dao et al. [3] observed that ITO/PMMA:ZnO/Al WORM memory devices are not affected by filamentary conduction; it may be dependent on the difference of conditions such as electrode thickness, film thickness, ZnO NPs size, ZnO NPs concentration and the nature of the polymers. Fig. 7 shows energy band diagrams for the ITO/ PVP:ZnO/Al devices under forward and reverse applied bias voltages. The PVP insulator layer could be a chargeblocking layer and tunneling layer because of the wide energy gap of the PVP material [43]. While ZnO material is a wide band gap n-type semiconductor, the electronic structure of conduction and valence bands of ZnO have been reported to be at 4.2 and 7.6 eV [44] below the vacuum level, respectively. The energy level difference between the work function of the electrodes and the conduction band of ZnO is much smaller than that between the work function of the electrodes and the valence band of ZnO. Thus, for both forward and reverse applied bias voltages, the difference in the energy barrier and PVP blocking layer favors electron injection over holes injection from the electrode. Hence, the switching characteristics are associated with the electron trapping mechanism by the ZnO NPs [7,25,45] and the small effect of the filamentary conduction mechanism by the local degradation of polymer films [20,46,47]. For V < Vth region [see Fig. 6(a)], a linear relationship was observed between ln(|I/V2|) versus ln|1/V|, as shown in
Fig. 8(a). In this case, electrons are injected from the ITO electrode into the localized states and the conduction band of some ZnO molecules, and then electrons tunnel among the PVP matrix [25,46,48] through the Al electrode via the direct tunneling process, which corresponds to a low conductivity state of the device [see Fig. 7(a)]. After that, if the applied bias has been large enough (V > Vth), a linear relation of ln(|I/V2|) versus |1/V| should be found, as shown in Fig. 8(b). The bands bend gradually to become triangular to allow F–N tunneling to occur across the device allowing electrons to tunnel from the localized states to the conduction band of the ZnO molecules. Then, the electrons existing at the conduction band are transported along the direction of the applied voltage through the F–N tunneling mechanism among adjacent ZnO NPs, as shown in Fig. 7(b). As a result, the electric fields between the neighboring nanoparticles increase [3,36]. Hence, the ZnO NPs act as traps, capturing electrons injected from the electrode. This results in the achievement of the writing process for the memory devices, causing the device to switch from the OFF state to the ON state. However, under electric fields, filamentary conduction or conductive channels might be formed in the device [10,22]. This might be explained by the formation of carbon-rich filaments due to local degradation of the PVP matrix [20,46,47]. Consequently, conduction mechanisms of the ON state current should depend on superposition between ohmic conduction by trap-states and filamentary conduction by filament channels. Afterwards, the bias voltages were applied in the forward direction from 0 V to +5 V. The filaments remained
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Fig. 7. Band diagrams of the memory devices during forward and reverse applied bias voltage, (a) reverse (V < Vth), (b) reverse (V > Vth), (c) forward (after writing) and (d) forward (before writing). EF is the Fermi level of the metal contacts, EVAC is the vacuum level, Vbi is the build-in potential, /M is the work function of the metals, v is the electron affinity of ZnO, and /b(PVP) is the barrier height between the metal contacts and the polymer.
Fig. 8. Experimental and linear fitting curves of the reverse bias voltage in the OFF state current, (a) V < Vth in a ln(|I/V2|) versus ln(|1/V|) plot and (b) V > Vth in a ln(|I/V2|) versus |1/V| plot.
until the voltage increased to +1.3 V. Joule heating effects in the films may exceed heat dissipation [49]. It makes the filaments rupture and the current slightly decrease, called the NDR. However, the effect of injected current from the filament channels in our devices is very small, which may be due to the dimension of filaments being much smaller when compared to the device area [47]. Fig. 7(c) shows that large applied bias voltages (V > +1.3 V) cause the bands to bend to become triangular to allow for F–N tunneling to occur from the Al to ITO electrodes; F–N tunneling conduction in the ON state might be induced by a high internal electric field after the writing process was performed. Hence, electrons tunnel from the Al electrode into the localized state and tunnel to the conduction band of ZnO molecules, and are then transported to the ITO electrode through F–N tunneling. Thus, in the ON state, forward bias voltages after the writing process have the same effect as reverse bias voltages. Therefore, after the applied voltage is turned off, the device can remain in the ON state. The energy barrier between PVP and ZnO is large enough to prevent electrons from
tunneling out of the ZnO molecule, which is indicative of nonvolatile WORM behavior. Further investigation of the transport mechanisms in the positive OFF state (thermionic emission region) is obtained from temperature-dependent measurement. Charge injection from applied bias is primarily due to thermal emission of charge carriers over the interface potential barrier, as shown in Fig. 7(d). The thermionic emission current over the barrier depends on the modified energy barrier which is modified by the image force and temperature [20,37,38]. Hence, the I–V characteristics of the optimal device were performed at various temperatures from room temperature (RT) to 393 K (RT-120 °C), as shown in Fig. 9(a). These measurements were actually performed to confirm the conduction mechanisms of the states. The current in the OFF state (curve 1) for the positive voltage region increases with increasing temperature, whereas the current ratio of the device decreases. However, the temperature shows a small effect on the current in the other states (curves 2, 3 and 4). Eq. (1) can also be solved for 1/T giving the temperature dependence of thermionic
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Fig. 10. Transient response of the WORM memory device with ZnO NPs concentration of 6 wt%, the circuit measurement method given in the top inset. Rise-time, fall-time and delay-time of device are shown in the inset (left and right).
Fig. 9. (a) Log I–V curves of the memory devices at different temperatures. Also shown are sketches of the barrier shape in all transport regions. (b) Fitting the thermionic emission model to experimental data for current as a function of reverse temperature of the device operated in the OFF state (at +1 V). The devices prepared at ZnO NPs concentration of 6 wt% (curves 1–4 are corresponding with Fig. 4(b)).
emission as ln(I/T2)/1/T [38]. From Eqs. (2) and (3), the direct tunneling and F–N tunneling show strong dependence on the applied voltage but are independent of temperature. Fig. 9(b) shows the temperature-dependent (I–T) measurements; a linear relationship was observed between ln(I/T2) versus 1/T which was obtained from the current value at reading voltage (VR) of +1 V. Thus, experimental results supported the conclusion that the memory characteristics of the fabricated nonvolatile WORM memory devices were determined by the thermionic emission at the PVP/Al interface and the tunnel transport between ZnO NPs and the PVP matrix. When considering the filamentary conduction mechanisms, there are actually two types, which are related to metallic filaments [50,51] and carbon-rich filaments [20,46,47]. This I–T result indicates that the current state of the devices is not due to conduction by metallic filaments, unlike what is observed for metal/polymer/metal devices [50], because the metallic conduction nature of the state is increasing resistance with increasing temperature [51]. In addition, we measured pulse delay time for the nonvolatile WORM memory devices with a ZnO NPs concentration of 6 wt% at room temperature. The input and output voltage pulse was monitored using an oscilloscope, as shown in Fig. 10. In this analysis, a square voltage pulse
(3 V and 2.5 ms-width) is applied to the device in the OFF state before transition to the ON state. The voltage of Vout = 100% is approximately –0.16 V (see Fig. 10) and the current of the nonvolatile WORM memory device at 3 V pulse can be performed by measuring current through a 50 O external resistor. Hence, the current at writing pulse is approximately –3.2 mA, which corresponds to the current at 3 V in Fig. 4(b). The delay times between Vin and before rise-time and fall-time are approximately 5 and 15 ls respectively, as shown in the inset of Fig. 10 (yellow1 regions). The response times of both rise-time and fall-time are about 3 and 6 ls respectively, as shown in the inset of Fig. 10 (green regions). The time dependence of the voltage corresponding to the OFF and ON states shows an overshoot or transient response especially after switching to the ON state. The delay time and slower process might be due to the participation of slow solid state electrochemical processes. To further investigate the stability of the nonvolatile WORM memory devices before and after switching, retention time tests of the device were performed to ascertain their electrical stability in the ON and OFF states at room temperature. For retention times, retention ability is one of the key requirements for memory application [20]. Reliability of the stored conductivity state is studied for once write/continuous readout with the writing voltage pulse (2.5 ms-width) of 3 V. Fig. 11 shows the measured retention characteristics of the nonvolatile WORM memory device in the ON and OFF states; there was no significant degradation of the device in either the ON and OFF state after a continuous stress test. The OFF and ON states currents were monitored by repeating the reading process before and after the writing process. On the first scan, a reading voltage of +1 V for measuring the OFF state current was applied to the device for 1 h. After the writing voltage pulse had been applied to the device, a reading voltage of +1 V for measuring the ON state current was applied for 2 h, then the electrodes were disconnected from the device (ca. 2 h). On the second scan, after reconnecting the elec1 For interpretation of color in Fig. 10, the reader is referred to the web version of this article.
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Fig. 12. Retention times of the OFF and ON states of the WORM memory devices, under a continuous read pulse with a pulse voltage of 1 V, a pulse width (PW) of 1 s (the width of read pulse is increased to 1 s for the stability of readout) and a pulse period (PP) of 60 s. The devices prepared at ZnO NPs concentration of 6 wt%.
4. Conclusions
Fig. 11. Demonstration of data readout in a once write/continuous readout operation of the WORM memory device with a ZnO NPs concentration at 6 wt%.
trodes to the device, a reading voltage of +1 V for measuring the current state was again applied to the device. The retention characteristics show that the fabricated memory device can remain in the ON state without significant degradation of the device in the ON state current. Moreover, the current of both ON and OFF states in the negative voltage region is independent of temperature, as shown in Fig. 9(a). We can use the reading voltage at 1 V to improve the current stability from the changes of temperature. In addition, the temperature dependence of the retention for longer time is one of the most important properties in a memory device [52–54]. Fig. 12 shows the retention times of both the OFF and the ON states under different measurement temperatures and different measurement cells (see inset for pulse characteristics), all measurements were performed in air without encapsulation. The retention times of both the OFF and ON states were more than 105 s, which demonstrates that the conductivity states can retain stored data for a long period. In long period, however, the current values of the ON state decreased gradually under a high temperature because of the tunnel of the electrons out of the trap states at a high temperature. While the OFF state was influenced by the thermionic emission at the PVP/Al interface, the current increases with increasing temperature. This result indicates that the stored conductivity state (ON state) is rather stable up to 333 K. Therefore, the ITO/PVP:ZnO/Al structure showed the important characteristics of nonvolatile WORM memory to be a candidate for practical permanent data storage.
Nonvolatile WORM memory devices of ITO/PVP:ZnO/Al structure were investigated, and it was observed that the bistability depends on the existence of ZnO NPs. The device exhibited two distinctive states of conductivity with the maximum ON/OFF current ratio 3 103 at reading voltage of +1 V. The conduction mechanisms of the nonvolatile WORM memory devices with optimal ZnO NPs contained are described on the basis of the I–V results, which were determined by the thermionic emission at the PVP/Al interface and the tunnel transport between ZnO NPs and the PVP matrix. Moreover, the conduction mechanisms of the device were confirmed by temperature dependent current–voltage characteristics. In addition, reliability studies for response time and once write/continuous read operations of the optimal structure are presented. It is shown that fast response and stored conductivity states are stable. Acknowledgments This work has partially been supported by Prof. Dr. Jiti Nukeaw and the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network. This work was financially supported by King Mongkut’s Institute of Technology Ladkrabang (KMITL) research fund. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.03.024. References [1] L.P. Ma, J. Liu, Y. Yang, Appl. Phys. Lett. 80 (2002) 2997. [2] L. Ma, Q. Xu, Y. Yang, Appl. Phys. Lett. 84 (2004) 4908. [3] Toan T. Dao, T.V. Tran, K. Higashimine, H. Okada, D. Mott, S. Maenosono, H. Murata, Appl. Phys. Lett. 99 (2011) 233303. [4] F. Li, T.W. Kim, W. Dong, Y.H. Kim, Thin Solid Films 517 (2009) 3916.
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Organic Electronics journal homepage: www.elsevier.com/locate/orgel
Electrical switching and conduction mechanisms of nonvolatile write-once-read-many-times memory devices with ZnO nanoparticles embedded in polyvinylpyrrolidone K. Onlaor ⇑, T. Thiwawong, B. Tunhoo Electronics and Control System for Nanodevice Research Laboratory, College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Bangkok 10520, Thailand Thailand Center of Excellence in Physics, Commission On Higher Education, Ministry of Education, 328 Si Ayutthaya Road, Bangkok 10400, Thailand Nanotec-KMITL Excellence Center On Nanoelectronic Devices, Bangkok 10520, Thailand
a r t i c l e
i n f o
Article history: Received 15 November 2013 Received in revised form 11 March 2014 Accepted 18 March 2014 Available online 1 April 2014 Keywords: PVP ZnO WORM Nanoparticle Memory device Conduction mechanism
a b s t r a c t We reported on the influence of zinc oxide nanoparticles (ZnO NPs) on the electrical bistable behavior of nonvolatile write-once-read-many-times (WORM) memory devices based on an indium-tin oxide/polyvinylpyrrolidone (PVP):ZnO NPs/aluminum (ITO/PVP:ZnO/Al) structure. The maximum ON/OFF current ratio of the nonvolatile WORM memory devices was approximately 3 103 and the devices remained in the ON state even after the applied voltage was turned off. In addition, reliability studies for response time and once write/ continuous read operations of the optimal ZnO NPs concentration are presented. The response times of both rise-time and fall-time were about 3 and 6 ls respectively. The conduction mechanisms of all voltage regions of the device were analyzed by theoretical models and electron trapping in the ZnO NPs of the electron tunneling among a PVP matrix was discussed. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Recently, several works have been done in the field of organic bistable or resistive switching memory devices [1–6]. The basic feature of the devices is to exhibit bistable behavior having two different conduction states, with a conductivity difference of several orders of magnitude. Resistive memory can be classified into two kinds: (1) rewritable [1] and (2) write-once-read-many-times (WORM) [7]. A similar phenomenon of two kinds has been observed in a variety of materials and device structures
⇑ Corresponding author at: Electronics and Control System for Nanodevice Research Laboratory, College of Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Chalongkrung Road, Bangkok 10520, Thailand. Tel.: +66 3298000. E-mail address: [email protected] (K. Onlaor). http://dx.doi.org/10.1016/j.orgel.2014.03.024 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.
[8–18]. For the purpose of permanent data storage, WORM memory devices have been studied intensively and are particularly interesting because of their merits of simple device structure, simple fabrication process, low reading voltage, low manufacturing cost and extensive applications [7,11,18,19]. Well known examples are the smart labels and radio frequency identification (RFID) tags. These applications do not require a high memory density [20]. Among WORM memory device structures, the type comprising nanoparticles (NPs) blending with the polymer host has been subject to many studies because of the great variety of NPs and polymer species [3,4,7,10–12,21]. In addition, several conduction mechanisms of NPs blended in polymer have been proposed to explain the conductance switching, including formation of conducting filaments [18,22], formation of charge transfer complexes [23], charge-trapping [5,24], etc. However, the memory type and the switching mechanisms of the NPs blending with the polymer host
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are still a matter of debate even when made from the same polymer host materials [3,9,25,26]. Of several types of NPs used in the memory structures, zinc oxide nanoparticles (ZnO NPs) are particularly desirable because they have many promising applications for memory devices [3,4,9,21,25,26]. Among polymer species, a polyvinylpyrrolidone (PVP) polymer has been widely studied because of its good environmental stability, operation stability, easy processability and its good electrical properties [27– 32]. Here, we report on the fabrication of nonvolatile WORM memory devices with indium-tin oxide/PVP:ZnO NPs/aluminum (ITO/PVP:ZnO/Al) structure at different concentrations of ZnO NPs. The electrical characteristics of the asfabricated devices were investigated by current–voltage measurement. Our work shows that the devices exhibit maximum ON/OFF current ratio in the order of 3 103 with low reading voltage, fast response and long term stability. The conduction mechanisms of all voltage regions are proposed to explain the conductivity transition and are described on the basis of the I–V results.
2. Experimental The memory devices were ITO/PVP:ZnO/Al structure. ITO conducting glass with sheet resistance of 10 O/h is used as a substrate. Prior to spin cast, the ITO-coated glass substrates were cleaned with the following procedure that included sonication in deionized high purity water, acetone, methanol and isopropanol for 20 min. The cleaned substrates were then dried by using N2 gas. The PVP (average Mw 10,000) and ZnO NPs purchased from Aldrich Chem. Co, with average ZnO NPs size less than 35 nm were used for the storage layer in the memory devices. The PVP was dissolved in ethanol with a concentration of 30 mg/ml and kept constant to ensure comparable film thicknesses in each PVP:ZnO composition. After that, ZnO NPs were mixed with a PVP-ethanol solution at concentrations of 2, 4, 6, 8 and 10 wt%, then ultrasonically mixed to enhance the dispersion of ZnO NPs in the PVP solution. The mixed solution was spin coated on the ITO-coated glass substrates at 3000 rpm for 30 s at room temperature. After the spin coating of the mixed solution, the samples were kept at 120 °C for 60 min to remove the solvent. Finally, 100 nm-thick Al top electrode was prepared on PVP:ZnO layer by thermal evaporation technique with a base pressure of 2 106 mbar, the devices had a cross-sectional area of 4 mm2. The schematic diagram of the device and chemical structure of PVP are shown in Fig. 1. The I–V measurements were performed under dark conditions to examine the electrical properties of the fabricated devices using a precision LCR meter (Agilent E4980A). In addition, response time tests were performed with a pulse generator (RIGOL DG1022) and digital oscilloscope (RIGOL DS1102D). The pulse generator was connected to a computer with a universal serial bus (USB) using an UltraWave program for creating arbitrary waveforms. The retention characteristics were performed by using a source meter (Keithley 2410), which was controlled by a computer program (Labview). A field-emission scanning electron microscope (FE-
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Fig. 1. Schematic illustration of the ITO/PVP:ZnO/Al memory devices and chemical structure of polyvinylpyrrolidone (C6H9NO)n or PVP.
SEM, Hitachi S-4700) was used to investigate the thickness of the PVP:ZnO layer.
3. Results and discussion Fig. 2 shows the current–voltage (I–V) characteristics of the ITO/PVP:ZnO/Al devices with different ZnO NPs concentrations. The curve was swept by applying a bias voltage to the bottom electrode (ITO) and grounding the top electrode (Al) with the variations of voltages across the device in a cyclical manner from +5 V to 5 V and 5 V to +5 V. In the case of ITO/PVP/Al devices, the device exhibits small hysteresis as shown in Fig. 2(a). That may have resulted from the mechanisms at the polymer/metal interface and the filament channels in the polymer layer [11,33–35]. However, the Al electrode of all devices is grown in the same conditions and was fabricated together. Fig. 3 shows a cross sectional view of FE-SEM images, the PVP:ZnO layers with different ZnO NPs concentrations are uniformly thick after deposition from the spin cast PVP:ZnO solution. The thickness of the PVP:ZnO layer of all devices is approximately 180 nm. However, ZnO NPs in the PVP matrix are difficult to observe whereas ZnO NPs contained in the films show the effects on device efficiency such as threshold voltages (Vth) and ON/OFF current ratios which will be discussed later. Thus, the switching behaviors of our devices strongly depend on the concentration of ZnO NPs in the films, which is based on a charge stored mechanism more than the effect of the PVP/Al interface, the filament channels and the films’ thickness. Interestingly, the I–V curves for the devices with ZnO NPs show a large current hysteresis behavior, indicative of memory effects in the memory devices, as shown in Fig. 2(a and b). The low-current state and the high-current state correspond to a relatively OFF state and a relatively ON state, respectively. At the initial state (curve 1), the devices initially had a low conductivity state. Subsequently the current increases gradually with the increase reverse applied bias voltages (curve 2). When the applied voltage reached Vth, the current abruptly increased, with the values of Vth in the range of 2 V to 3.5 V. The voltages were then swept from 5 V to 0 V, and the different high conductivity state emerged (curve 3). The transition from the OFF state to the ON state is equivalent to the writing process in the memory devices, in which ‘‘0’’ ? ‘‘1’’ can be switched from the low conductivity state to the high conductivity state. After the devices changed from the OFF state to the ON state, the devices could not return to the OFF state again when the
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Fig. 2. I–V characteristics of the memory devices with different ZnO NPs concentration, (a) PVP only and 2 wt% and (b) 4–10 wt%.
Fig. 3. FE-SEM images showing the cross sectional view of the PVP:ZnO films with different ZnO NPs concentration, (a) PVP only, (b) 2 wt%, (c) 4 wt%, (d) 6 wt%, (e) 8 wt% and (f) 10 wt%.
opposite bias voltages were applied (curve 4) and even when the applied voltage was turned off, which is indicative of nonvolatile WORM behavior. These results correspond to those of the WORM memory devices with ZnO NPs embedded in polymethylmethacrylate (PMMA) [3] that have been reported by Dao et al. Fig. 4(a) shows the ON/OFF current ratios for the fabricated devices with different concentrations of ZnO NPs. The relative concentrations of ZnO NPs in the films used in our devices clearly influence the electrical characteristics trends. The ON/OFF current ratios (read at ±1 V) of each concentration for both positive and negative voltage regions are measured in 20 randomly selected cells, and the data points in Fig. 4(a) are average values with standard deviations. From Fig. 2(a), the device with a ZnO NPs concentration of 2 wt% showed clearly observed ON and OFF current states. However, the average ON/OFF current ratio of the devices is smaller than that of the devices with concentrations of 4 and 6 wt%, as shown in Fig. 4(a). However, when the concentrations of the ZnO NPs are increased more than 6 wt%, the ON/OFF current ratio goes down as a consequence of the phenomenon whereby ZnO NPs in the films might be joined with the neighboring ZnO NPs, forming continuous ZnO NPs networks in the
films. This continuous ZnO NPs form allows effective transport of the charge carrier under low bias, because the electron injection probability at the electrode increases with increasing ZnO NPs content. Hence, the electric fields between the neighboring nanoparticles increase [3,36], making the OFF state current of the devices exhibit a high conductivity state, as shown in Fig. 2(b). Although, the conductivity transition from the OFF to the ON states of the devices with a concentration of 10 wt% easily occurs due to the large number of continuous ZnO NPs networks, the I–V curves for the devices exhibit the smallest ON/OFF current ratio. In other devices, however, a concentration of 2 wt% is not enough to occupy carriers for receiving the large ON/OFF current ratio. Therefore, the nonvolatile WORM memory devices with a concentration of 6 wt% show the optimal electrical switching with the highest ON/OFF current ratio (ca. 3 103). Fig. 4(b) shows the I–V characteristics with repeated scans of the optimal ZnO NPs concentration in the ITO/PVP:ZnO/Al structure, for investigating the conduction mechanisms in the next section. To examine the carrier transport mechanisms in the nonvolatile WORM memory devices prepared at ZnO NPs concentration of 6 wt%, the I–V data were fitted by using
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vice at room temperature on ln(I/V2) versus |1/V|. The electron transport in the low bias region (V < Vth) can be interpreted as direct tunneling, in which electrons tunnel through a square barrier, expressed as [39,40],
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 2d 2m /b I / V exp h
ð2Þ
where m is the effective mass and ⁄ is the reduced Planck’s constant. For the high voltage region (V > Vth), the applied voltage is enough to change the shape of the barrier to become triangular, thus described by Fowler–Nordheim (F– N) tunneling as [39,40],
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 2m /3b 4d A I / V 2 exp @ 3qhV 0
ð3Þ
The dotted line denotes the Vth at 2.2 V or denotes the transition from a direct tunneling model to the F–N tunneling model at high voltage. From the F–N tunneling region in Eq. (3), it is useful to liberalize on a logarithmic scale in terms of the voltage dependence to become Eq. (4):
ln
Fig. 4. (a) ON/OFF current ratio of the memory devices for both positive and negative voltage regions with different ZnO NPs concentration. (b) I–V curves for the optimal ZnO NPs concentration (6 wt%), showing steps of repeated scans.
" pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi # ð/b q qV=4pdei Þ ; I / T exp kb T 2
ð1Þ
where T is the temperature, /b is the barrier height, ei is the dynamic permittivity of the film, q is the electron charge, kb is the Boltzmann constant and d is the film thickness. By solving Eq. (1) in terms of V1/2, the voltage dependence can be derived as ln(I)/V1/2 [38]. A linear relationship was observed between ln(I) versus V1/2 from +5 V to 0 V, as shown in Fig. 5(a). This suggests that the current in the OFF state before the transformation to the ON state was controlled by charge injection from the electrode. Because the thermionic emission is primarily due to exciting electrons to higher energies in a metal contact, electrons can overcome the potential barrier between the insulator/metal contact [37,38]. When the reverse bias voltages were applied [curve 2 of Fig. 4(b)], two distinct voltage regions are evident from this curve; therefore two mechanisms in the low voltage and high voltage regions were needed, as shown in Fig. 6(a). Fig. 6(a) shows the tunneling characteristics across the de-
V2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1 0 3 1 @ 4d 2m /b A / V 3hq
ð4Þ
In order to directly compare both tunneling transport mechanisms, the direct tunneling in Eq. (2) can be rearranged in terms of the voltage dependence to become Eq. (5),
ln
various conduction models based on the I–V curves to clarify the carrier transport and memory mechanisms of the device. In the OFF state for the positive voltage region [curve 1 of Fig. 4(b)] the data can be well fitted with the thermionic emission [20,37,38]:
I
I
V2
/ ln
pffiffiffiffiffiffiffiffiffiffiffiffiffiffi! 2d 2m /b 1 : h V
ð5Þ
After the reverse bias voltages had been applied to the device, the electrons captured in the PVP:ZnO layer were increased. The current flowed via the F–N tunneling conduction mechanism so that the injected current increased sharply and induced the electrical state of the device to rapidly change from the OFF state to the ON state. In the ON state for the negative voltage region, Fig. 6(b) shows a log(I) versus log(V) plot for the I–V results of Fig. 4(b) (curve 3); the current flow was free from the influence of traps in the device. The conduction mechanism of the ON state is believed to show ohmic behavior [37]. When the forward bias voltages were applied again [curve 4 of Fig. 4(b)], the I–V curves under applied voltages from 0 V to +1.3 V in the ON state could be fitted well by using the ohmic conduction model and the F–N tunneling model at a higher applied voltage (+1.3 V–+5 V) as shown in Fig. 5(b). Hence, ohmic conduction is the dominant carrier transport mechanism for devices in the ON state. However, after the device transformation from the OFF state to the ON state, negative differential resistance (NDR) is slightly observed at +1.3 V, as shown in Fig. 4(b), leading to asymmetric bistability due to the devices being made of materials poor in conductance, such as some other polymers [41,42]. Thus, the conduction mechanisms after writing voltage of the fabricated ITO/PVP:ZnO/Al memory devices might be mainly caused by ohmic conduction mechanism (trap-states) and filamentary conduction mechanism
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Fig. 5. Experimental and fitted I–V characteristics of the memory device with a ZnO NPs concentration at 6 wt% of the forward bias voltage, (a) OFF state and (b) ON state, (curves 1 and 4 are corresponding with Fig. 4(b)).
Fig. 6. Experimental and fitted I–V characteristics of the memory device with a ZnO NPs concentration at 6 wt% of the reverse bias voltage, (a) OFF state and (b) ON state, (curves 2 and 3 are corresponding with Fig. 4(b)).
(filament channels), nevertheless the ohmic conduction is dominant over the filamentary conduction, which will be further discussed later. However, Dao et al. [3] observed that ITO/PMMA:ZnO/Al WORM memory devices are not affected by filamentary conduction; it may be dependent on the difference of conditions such as electrode thickness, film thickness, ZnO NPs size, ZnO NPs concentration and the nature of the polymers. Fig. 7 shows energy band diagrams for the ITO/ PVP:ZnO/Al devices under forward and reverse applied bias voltages. The PVP insulator layer could be a chargeblocking layer and tunneling layer because of the wide energy gap of the PVP material [43]. While ZnO material is a wide band gap n-type semiconductor, the electronic structure of conduction and valence bands of ZnO have been reported to be at 4.2 and 7.6 eV [44] below the vacuum level, respectively. The energy level difference between the work function of the electrodes and the conduction band of ZnO is much smaller than that between the work function of the electrodes and the valence band of ZnO. Thus, for both forward and reverse applied bias voltages, the difference in the energy barrier and PVP blocking layer favors electron injection over holes injection from the electrode. Hence, the switching characteristics are associated with the electron trapping mechanism by the ZnO NPs [7,25,45] and the small effect of the filamentary conduction mechanism by the local degradation of polymer films [20,46,47]. For V < Vth region [see Fig. 6(a)], a linear relationship was observed between ln(|I/V2|) versus ln|1/V|, as shown in
Fig. 8(a). In this case, electrons are injected from the ITO electrode into the localized states and the conduction band of some ZnO molecules, and then electrons tunnel among the PVP matrix [25,46,48] through the Al electrode via the direct tunneling process, which corresponds to a low conductivity state of the device [see Fig. 7(a)]. After that, if the applied bias has been large enough (V > Vth), a linear relation of ln(|I/V2|) versus |1/V| should be found, as shown in Fig. 8(b). The bands bend gradually to become triangular to allow F–N tunneling to occur across the device allowing electrons to tunnel from the localized states to the conduction band of the ZnO molecules. Then, the electrons existing at the conduction band are transported along the direction of the applied voltage through the F–N tunneling mechanism among adjacent ZnO NPs, as shown in Fig. 7(b). As a result, the electric fields between the neighboring nanoparticles increase [3,36]. Hence, the ZnO NPs act as traps, capturing electrons injected from the electrode. This results in the achievement of the writing process for the memory devices, causing the device to switch from the OFF state to the ON state. However, under electric fields, filamentary conduction or conductive channels might be formed in the device [10,22]. This might be explained by the formation of carbon-rich filaments due to local degradation of the PVP matrix [20,46,47]. Consequently, conduction mechanisms of the ON state current should depend on superposition between ohmic conduction by trap-states and filamentary conduction by filament channels. Afterwards, the bias voltages were applied in the forward direction from 0 V to +5 V. The filaments remained
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Fig. 7. Band diagrams of the memory devices during forward and reverse applied bias voltage, (a) reverse (V < Vth), (b) reverse (V > Vth), (c) forward (after writing) and (d) forward (before writing). EF is the Fermi level of the metal contacts, EVAC is the vacuum level, Vbi is the build-in potential, /M is the work function of the metals, v is the electron affinity of ZnO, and /b(PVP) is the barrier height between the metal contacts and the polymer.
Fig. 8. Experimental and linear fitting curves of the reverse bias voltage in the OFF state current, (a) V < Vth in a ln(|I/V2|) versus ln(|1/V|) plot and (b) V > Vth in a ln(|I/V2|) versus |1/V| plot.
until the voltage increased to +1.3 V. Joule heating effects in the films may exceed heat dissipation [49]. It makes the filaments rupture and the current slightly decrease, called the NDR. However, the effect of injected current from the filament channels in our devices is very small, which may be due to the dimension of filaments being much smaller when compared to the device area [47]. Fig. 7(c) shows that large applied bias voltages (V > +1.3 V) cause the bands to bend to become triangular to allow for F–N tunneling to occur from the Al to ITO electrodes; F–N tunneling conduction in the ON state might be induced by a high internal electric field after the writing process was performed. Hence, electrons tunnel from the Al electrode into the localized state and tunnel to the conduction band of ZnO molecules, and are then transported to the ITO electrode through F–N tunneling. Thus, in the ON state, forward bias voltages after the writing process have the same effect as reverse bias voltages. Therefore, after the applied voltage is turned off, the device can remain in the ON state. The energy barrier between PVP and ZnO is large enough to prevent electrons from
tunneling out of the ZnO molecule, which is indicative of nonvolatile WORM behavior. Further investigation of the transport mechanisms in the positive OFF state (thermionic emission region) is obtained from temperature-dependent measurement. Charge injection from applied bias is primarily due to thermal emission of charge carriers over the interface potential barrier, as shown in Fig. 7(d). The thermionic emission current over the barrier depends on the modified energy barrier which is modified by the image force and temperature [20,37,38]. Hence, the I–V characteristics of the optimal device were performed at various temperatures from room temperature (RT) to 393 K (RT-120 °C), as shown in Fig. 9(a). These measurements were actually performed to confirm the conduction mechanisms of the states. The current in the OFF state (curve 1) for the positive voltage region increases with increasing temperature, whereas the current ratio of the device decreases. However, the temperature shows a small effect on the current in the other states (curves 2, 3 and 4). Eq. (1) can also be solved for 1/T giving the temperature dependence of thermionic
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Fig. 10. Transient response of the WORM memory device with ZnO NPs concentration of 6 wt%, the circuit measurement method given in the top inset. Rise-time, fall-time and delay-time of device are shown in the inset (left and right).
Fig. 9. (a) Log I–V curves of the memory devices at different temperatures. Also shown are sketches of the barrier shape in all transport regions. (b) Fitting the thermionic emission model to experimental data for current as a function of reverse temperature of the device operated in the OFF state (at +1 V). The devices prepared at ZnO NPs concentration of 6 wt% (curves 1–4 are corresponding with Fig. 4(b)).
emission as ln(I/T2)/1/T [38]. From Eqs. (2) and (3), the direct tunneling and F–N tunneling show strong dependence on the applied voltage but are independent of temperature. Fig. 9(b) shows the temperature-dependent (I–T) measurements; a linear relationship was observed between ln(I/T2) versus 1/T which was obtained from the current value at reading voltage (VR) of +1 V. Thus, experimental results supported the conclusion that the memory characteristics of the fabricated nonvolatile WORM memory devices were determined by the thermionic emission at the PVP/Al interface and the tunnel transport between ZnO NPs and the PVP matrix. When considering the filamentary conduction mechanisms, there are actually two types, which are related to metallic filaments [50,51] and carbon-rich filaments [20,46,47]. This I–T result indicates that the current state of the devices is not due to conduction by metallic filaments, unlike what is observed for metal/polymer/metal devices [50], because the metallic conduction nature of the state is increasing resistance with increasing temperature [51]. In addition, we measured pulse delay time for the nonvolatile WORM memory devices with a ZnO NPs concentration of 6 wt% at room temperature. The input and output voltage pulse was monitored using an oscilloscope, as shown in Fig. 10. In this analysis, a square voltage pulse
(3 V and 2.5 ms-width) is applied to the device in the OFF state before transition to the ON state. The voltage of Vout = 100% is approximately –0.16 V (see Fig. 10) and the current of the nonvolatile WORM memory device at 3 V pulse can be performed by measuring current through a 50 O external resistor. Hence, the current at writing pulse is approximately –3.2 mA, which corresponds to the current at 3 V in Fig. 4(b). The delay times between Vin and before rise-time and fall-time are approximately 5 and 15 ls respectively, as shown in the inset of Fig. 10 (yellow1 regions). The response times of both rise-time and fall-time are about 3 and 6 ls respectively, as shown in the inset of Fig. 10 (green regions). The time dependence of the voltage corresponding to the OFF and ON states shows an overshoot or transient response especially after switching to the ON state. The delay time and slower process might be due to the participation of slow solid state electrochemical processes. To further investigate the stability of the nonvolatile WORM memory devices before and after switching, retention time tests of the device were performed to ascertain their electrical stability in the ON and OFF states at room temperature. For retention times, retention ability is one of the key requirements for memory application [20]. Reliability of the stored conductivity state is studied for once write/continuous readout with the writing voltage pulse (2.5 ms-width) of 3 V. Fig. 11 shows the measured retention characteristics of the nonvolatile WORM memory device in the ON and OFF states; there was no significant degradation of the device in either the ON and OFF state after a continuous stress test. The OFF and ON states currents were monitored by repeating the reading process before and after the writing process. On the first scan, a reading voltage of +1 V for measuring the OFF state current was applied to the device for 1 h. After the writing voltage pulse had been applied to the device, a reading voltage of +1 V for measuring the ON state current was applied for 2 h, then the electrodes were disconnected from the device (ca. 2 h). On the second scan, after reconnecting the elec1 For interpretation of color in Fig. 10, the reader is referred to the web version of this article.
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Fig. 12. Retention times of the OFF and ON states of the WORM memory devices, under a continuous read pulse with a pulse voltage of 1 V, a pulse width (PW) of 1 s (the width of read pulse is increased to 1 s for the stability of readout) and a pulse period (PP) of 60 s. The devices prepared at ZnO NPs concentration of 6 wt%.
4. Conclusions
Fig. 11. Demonstration of data readout in a once write/continuous readout operation of the WORM memory device with a ZnO NPs concentration at 6 wt%.
trodes to the device, a reading voltage of +1 V for measuring the current state was again applied to the device. The retention characteristics show that the fabricated memory device can remain in the ON state without significant degradation of the device in the ON state current. Moreover, the current of both ON and OFF states in the negative voltage region is independent of temperature, as shown in Fig. 9(a). We can use the reading voltage at 1 V to improve the current stability from the changes of temperature. In addition, the temperature dependence of the retention for longer time is one of the most important properties in a memory device [52–54]. Fig. 12 shows the retention times of both the OFF and the ON states under different measurement temperatures and different measurement cells (see inset for pulse characteristics), all measurements were performed in air without encapsulation. The retention times of both the OFF and ON states were more than 105 s, which demonstrates that the conductivity states can retain stored data for a long period. In long period, however, the current values of the ON state decreased gradually under a high temperature because of the tunnel of the electrons out of the trap states at a high temperature. While the OFF state was influenced by the thermionic emission at the PVP/Al interface, the current increases with increasing temperature. This result indicates that the stored conductivity state (ON state) is rather stable up to 333 K. Therefore, the ITO/PVP:ZnO/Al structure showed the important characteristics of nonvolatile WORM memory to be a candidate for practical permanent data storage.
Nonvolatile WORM memory devices of ITO/PVP:ZnO/Al structure were investigated, and it was observed that the bistability depends on the existence of ZnO NPs. The device exhibited two distinctive states of conductivity with the maximum ON/OFF current ratio 3 103 at reading voltage of +1 V. The conduction mechanisms of the nonvolatile WORM memory devices with optimal ZnO NPs contained are described on the basis of the I–V results, which were determined by the thermionic emission at the PVP/Al interface and the tunnel transport between ZnO NPs and the PVP matrix. Moreover, the conduction mechanisms of the device were confirmed by temperature dependent current–voltage characteristics. In addition, reliability studies for response time and once write/continuous read operations of the optimal structure are presented. It is shown that fast response and stored conductivity states are stable. Acknowledgments This work has partially been supported by Prof. Dr. Jiti Nukeaw and the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network. This work was financially supported by King Mongkut’s Institute of Technology Ladkrabang (KMITL) research fund. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.03.024. References [1] L.P. Ma, J. Liu, Y. Yang, Appl. Phys. Lett. 80 (2002) 2997. [2] L. Ma, Q. Xu, Y. Yang, Appl. Phys. Lett. 84 (2004) 4908. [3] Toan T. Dao, T.V. Tran, K. Higashimine, H. Okada, D. Mott, S. Maenosono, H. Murata, Appl. Phys. Lett. 99 (2011) 233303. [4] F. Li, T.W. Kim, W. Dong, Y.H. Kim, Thin Solid Films 517 (2009) 3916.
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