Flexible nonvolatile organic ferroelectric memory transistors fabricated on polydimethylsiloxane elastomer

Organic Electronics 16 (2015) 46–53

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Flexible nonvolatile organic ferroelectric memory transistors fabricated on polydimethylsiloxane elastomer Soon-Won Jung a,⇑, Jeong-Seon Choi a,b, Jae Bon Koo a, Chan Woo Park a, Bock Soon Na a, Ji-Young Oh a, Sang Chul Lim a, Sang Seok Lee a, Hye Yong Chu a, Sung-Min Yoon b,⇑ a b

Information & Communications Core Technology Research Laboratory, Electronics Telecommunication Research Institute, Daejeon 305-700, Republic of Korea Department of Advanced Materials Engineering for Information & Electronics, Kyung Hee University, Yongin, Gyeonggi-do 446-701, Republic of Korea

a r t i c l e

i n f o

Article history: Received 2 June 2014 Received in revised form 20 August 2014 Accepted 28 August 2014 Available online 16 September 2014 Keywords: Flexible Organic ferroelectric Nonvolatile memory transistor Polydimethylsiloxane Poly(vinylidene-trifluoroethylene)

a b s t r a c t The flexible organic ferroelectric nonvolatile memory thin film transistors (OFMTs) were fabricated on polydimethylsiloxane (PDMS) elastomer substrates, in which an organic ferroelectric poly(vinylidene-trifluoroethylene) and an organic semiconducting poly(9,9-dioctylfluorene-co-bithiophene) layers were used as gate insulator and active channel, respectively. The carrier mobility, on/off ratio, and subthreshold swing of the OFMTs fabricated on PDMS showed 5  10 2 cm2 V 1 s 1, 7.5  103, and 2.5 V/decade, respectively. These obtained values did not markedly change when the substrate was bent with a radius of curvature of 0.6 cm. The memory on/off ratio was initially obtained to be 1.5  103 and maintained to be 20 even after a lapse of 2000 s. The fabricated OFMTs exhibited sufficiently encouraging device characteristics even on the PDMS elastomer to realize mechanically stretchable nonvolatile memory devices. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction One of the most important trends in modern consumer electronics is to realize highly-functional electronic devices prepared on bendable and/or rollable substrates [1–3]. These systems are composed of various types of devices to provide such functions as information processing, sensing, networking, and imaging. Therefore, nonvolatile memory elements are strongly required to save the power dissipation of these systems as well as to store the information. Various techniques have been researched and developed to exploit nonvolatile memory functions in the flexible electronic systems [4–6]. Among them, memory thin-film transistor (TFT) employing the ferro⇑ Corresponding authors. Tel.: +82 42 860 6386; fax: +82 42 860 5202 (S.-W. Jung). Tel.: +82 31 201 3617; fax: +82 31 204 8114 (S.-M. Yoon). E-mail addresses: [email protected] (S.-W. Jung), sungmin@khu. ac.kr (S.-M. Yoon). http://dx.doi.org/10.1016/j.orgel.2014.08.051 1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

electric gate insulator (GI) can be a promising device for the flexible nonvolatile memories, in which a ferroelectric-polarization-dependent field-effect induced on the semiconductor channel layer is a main origin for memory operations. A poly(vinylidene-trifluoroethylene) [P(VDFTrFE)] is the most typical organic ferroelectric material, which have been used as a ferroelectric GI for the memory TFT employing the oxide [7–8] or organic [9–10] semiconductor channels. The P(VDF-TrFE) has such merits as a relatively large remnant polarization, a good ambient stability, and a short time for polarization reversal, and hence the P(VDF-TrFE)-based organic ferroelectric memory TFTs (OFMTs) can be operated in a very sound and reproducible manner with a simple operation principle. Furthermore, considering that the reduction of process temperature is one of the most critical requirements for the flexible devices due to low glass transition temperature for most plastic substrates, low crystallization temperature (around 140 °C) of the P(VDF-TrFE) is an important benefit

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for the flexible OFMTs. So far, for these applications, plastic substrates such as poly(ethylene naphthalate) (PEN) have only been used for the proposed OFMTs [11]. Although these general plastic substrates have features of mechanical flexibility, stretchable characteristics cannot be obtained. If the proposed OFMTs with excellent device performance can be fabricated on stretchable materials, the application fields of device are expected to be highly extended to future stretchable electronics systems such as artificial skins, wearable computer, and humanimplantable bio-medical devices. Several prototypes of nonvolatile memory devices have been embedded into the stretchable electronic applications, in which a writeonce-read-many-type organic memory [12] or nanoparticle-based resistive-change memory [13] elements were fabricated on the stretchable elastomers such as polydimethylsiloxane (PDMS). In this work, the memory TFTs composed of ferroelectric P(VDF-TrFE) GI and organic semiconducting poly(9,9-dioctylfluorene-co-bithiophene) (F8T2) were prepared on the PDMS elastomers. Although some specified device structures should be devised in order to obtain the stretchable characteristics [14], process feasibility, sound memory behaviors, and simple bending properties of the fabricated OFMTs were successfully confirmed on the PDMS substrates.

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70 °C for 10 min in a hot plate. A subsequent thermal annealing was performed in a nitrogen-purged glove box at 140 °C for 1 h to enhance the crystallinity of the ferroelectric b-phase of P(VDF-TrFE). The resultant film thickness was approximately 340 nm. Au top and bottom electrode pads were then formed through e-beam evaporation using metal shadow masks. Fig. 1a and b shows a cross-sectional schematic diagram and a photo image for the Au/P(VDF-TrFE)/Au capacitors fabricated on the PDMS substrate. The processed substrate can be also bendable as shown in Fig. 1c. On the other hand, for the fabrication of OFMTs, the following procedures were developed: source and drain electrodes were formed through e-beam evaporation of Au/Ti (70/10 nm) films using a metal shadow mask. A organic semiconductor F8T2 (American Dye Source, Inc.) was dissolved in anhydrous xylene (0.7 wt.%) and filtered with a PTFE syringe filter to remove impurities [18–20]. F8T2 films were annealed at 100 °C for 30 min to remove the solvent after spinning (2000 rpm, 1 min) in a glove box with low oxygen and moisture levels (<5 ppm). As gate dielectrics for the OFMT, the P(VDF-TrFE) film was

2. Experiment 2.1. Fabrication of organic ferroelectric memory devices on stretchable substrates We used polydimethylsiloxane (PDMS, Dow Corning’s Sylgard 184) as one of the most common elastomers. Sylgard 184 resin is composed of two parts containing vinyl (part A) and hydrosiloxane groups (part B). A cross-linked network of dimethylsiloxane group was formed by mixing two components. A cross-linker and silicone gel were mixed at a weight ratio of 1:10 [15–17]. Upon mixing, the PDMS was treated in a vacuum chamber for degassing to enhance the device characteristics. After degassing, the polymer mixtures were poured into the Petri-dish and cured at 60 °C for 2 h in a vacuum oven. The PDMS elastomers were then transferred onto a Si wafer. To prevent the transferred PDMS films from swelling, they were annealed at 200 °C for 1 h by rapid thermal process [14]. Two types of devices including memory thin-film transistors and organic ferroelectric capacitors were fabricated on the PDMS substrates. For the fabrication of the capacitors, a poly(vinylidene fluoride-trifluoroethylene) [P(VDFTrFE)] film was chosen as an organic ferroelectric thin film. The fabrication procedures can be summarized as follows; first, the bottom electrodes of Au were deposited through electron beam (e-beam) evaporation. P(VDF-TrFE) powder (70/30 mol%, Solvay SA) was dissolved in methyl-ethylketone (MEK) at a concentration of 4.5 wt.% and the solution was then filtered by a polytetrafluoroethylene (PTFE) syringe filter. P(VDF-TrFE) thin film was formed by conventional spin-coating method using a previously prepared solution at a spin rate of 2000 rpm for 10 s and dried at

Fig. 1. (a) Schematic cross-sectional diagram of the fabricated P(VDFTrFE) capacitors and (b) photograph of a PDMS substrate on which Au/ P(VDF-TrFE)/Au capacitors were fabricated. (c) A typical photograph of a PDMS substrate under a bending situation.

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prepared. The solution preparation, coating, and heattreatment conditions for the P(VDF-TrFE) films were the exactly same as those for the fabrication of the capacitors, as mentioned above. Gate electrodes of Al (100 nm) were evaporated on polymer dielectrics using a metal shadow mask. Finally, the polymer gate stacks composed of F8T2 and P(VDF-TrFE) were removed away by oxygen plasma except for the gate area using the Al gate electrodes as hard masks. The plasma power, oxygen partial pressure, and process time were chosen as 100 W, 100 sccm, and 3 min, respectively, which were optimized to minimize the undesirable crack formation into the PDMS [14]. Fig. 2 shows the device cross-section of fabricated OFMT, and illustrates the molecular structures of the F8T2 semiconductor and P(VDF-TrFE) gate dielectric. 2.2. Electrical characterization for the fabricated devices The capacitance–voltage (C–V) characteristics and the ferroelectric polarization–electric field (P–E) hysteretic behaviors of the fabricated P(VDF-TrFE) capacitors were evaluated using an impedance analyzer (HP4194A) and a ferroelectric tester (Precision LC, Radiant Technologies, Inc.). The transfer and output characteristics of the fabricated OFMTs were measured using a semiconductor parameter analyzer (Agilent Technologies, B1500A). The

device parameters such as carrier mobility (lTFT), threshold voltage (VTh), and subthreshold swing (SS) were calculated in the saturation regime using the standard formalism for the field-effect transistors [21]. All measurements were carried out at room temperature in a dark box.

3. Results and discussions 3.1. Electrical characteristics of P(VDF-TrFE) capacitors prepared on PDMS substrate First, the basic characteristics of the fabricated Au/340nm-thick P(VDF-TrFE)/Au capacitors were measured. Fig. 3a shows the variations in capacitance at zero bias as a function of frequency. The capacitance values monotonically decreased from 32 to 27 nF/cm2 when the measurement frequency was varied from 1 kHz to 1 MHz, which was considered to result from the limitation of polarization response time for the dipole alignment in the P(VDF-TrFE) with high dielectric permittivity. Consequently, the dielectric constant for the P(VDF-TrFE) thin film prepared on the PDMS substrate was calculated to be in the range from 12.4 to 10.5 at the measurement frequency range. It was noticeable that the dielectric constant of 10.5 could be obtained even at a high frequency of 1 MHz, which was similarly

Fig. 2. (a) Cross-sectional schematic configuration of a top-gate/bottom contact organic memory TFT, and molecular structures of the organic semiconductor F8T2 and ferroelectric gate dielectric [P(VDF–TrFE)]. (b) Microscopic images of the proposed and fabricated organic memory TFT on a PDMS substrate. The gate length (L) and width (W) of the TFT shown in (b) were 20 and 60 lm, respectively.

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Fig. 4a and b shows the P–E characteristics of the Au/ P(VDF-TrFE)/Au capacitor before and after the bending event. The capacitor size and signal frequency were 3.14  10 4 cm2 and 10 Hz, respectively. The applied electric field was varied from 0.5 to 1.9 MV/cm. The radius of curvature was set as 6.85 mm for the measurements under bending conditions. Without bending situation, the remnant polarization (Pr) and coercive field (Ec) were estimated to be approximately 9.0 lC/cm2 and 415 kV/cm, respectively. Furthermore, the saturation behaviors of the ferroelectric polarization with an increase in the electric field were observed to be sufficiently good. These obtained results reflect the general trends of P(VDF-TrFE) capacitors and indicate that the P(VDF-TrFE) thin film showed good ferroelectric characteristics. On the other hand, the same capacitor showed a small degree of variations in P–E characteristics under the bending situation, in which Pr and Ec were measured to be 8.7 lC/cm2 and 475 kV/cm, respectively. These trends are similar to those obtained for the P(VDF-TrFE) capacitors prepared on poly(ethylene naphthalate) substrate [23]. As for the increase in Ec, further investigations should be performed to identify the feasible origins. For organic ferroelectric capacitors to be used in nonvolatile memory applications, long lifetime in operation would be absolutely required. The polarization fatigue is one of the most important long-term device reliabilities, which is defined as the reduction in the amount of remnant polarization with repeated switching or usage cycles.

Fig. 3. (a) Capacitance-frequency (at an applied voltage of 10 V) and (b) capacitance–voltage (at an applied frequency of 1 MHz) characteristics of the P(VDF-TrFE) capacitor fabricated on a PDMS substrate, and (c) gate leakage current density–voltage characteristics of the P(VDF-TrFE) capacitor.

obtained as previous reports [18,22]. Fig. 3b shows the C–V characteristics measured at a frequency of 1 MHz and small signal amplitude of 20 mV. A series of voltage sweep with various amplitudes (from ±10 to ±40 V) were applied to the gate electrodes. The voltage-dependent capacitance variations showed typical butterfly-shaped hysteresis behaviors owing to the polarization reversal in the film. This result clearly indicates that the P(VDF-TrFE) film prepared on the PDMS substrate exhibits ferroelectric natures. The current–voltage (I–V) characteristics of the P(VDF-TrFE) film were also evaluated, as shown in Fig. 3c. The leakage current density of the film at room temperature was on the order of 10 7 A/cm2 at the applied voltage of ±40 V. A marked dielectric breakdown was not observed within the measured electric field.

Fig. 4. A typical P–E characteristics of the Au/P(VDF-TrFE)/Au capacitors with the size of 3.14  10 4 cm2 at the signal frequency of 10 Hz. (a) Initial, and (b) after the bending event.

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The fatigue behaviors of Au/P(VDF-TrFE)/Au capacitor before and after the bending events are shown in Fig. 5. The devices are stressed at a frequency of 10 Hz and an electric field of ±1 MV/cm, high enough to cause dipole switching in every fatigue cycle. It was found that the P(VDF-TrFE) capacitors retained 70% of the initial polarization even after 6  104 cycles. These data are consistent with previously reported fatigue behaviors [24–30]. An internal electric field created by the trapped charges acts opposite to the external electric field, causing an apparent increase in the switching voltage for the ferroelectric domains in P(VDF-TrFE) [31].

3.2. Organic ferroelectric transistor characteristics Based on the basic ferroelectric properties of the P(VDFTrFE) capacitors fabricated on the PDMS substrate, the device characteristics of the fabricated OFMTs were extensively investigated. Fig. 6a shows the drain current–gate voltage (ID–VG) transfer characteristics at various sweep ranges in VG, which were measured with a double sweep mode of forward and reverse directions in VG at VD of 10 V. The gate length (L) and width (W) of the measured device were 20 and 60 lm, respectively. As can be seen in figure, we could obtain sufficiently good device performances, in which a carrier mobility, on/off ratio, and subthreshold swing (SS) were as high as 5  10 2 cm2 V 1 s 1, 7.5  103, 2.5 V/decade, respectively. Considering that the most organic transistors using an F8T2 channel were reported to have a lsat of 1  10 2 cm2 V 1 s 1 [19], the obtained device characteristics were sufficiently encouraging. Clockwise hysteresis obtained in each transfer curve originated from the ferroelectric field effect. The width of the memory window gradually increased from 4 to 11 V when the VG sweep range increased from ±7.5 to ± 20 V. It could be noticeable that the memory window was confirmed to be as wide as 4 V even when the VG sweep was ±7.5 V. This suggests that our OFMTs can be operated at a relatively low voltage. The operating voltage of an OTFT

Fig. 5. Normalized remnant polarization as a function of the number of cycles. Fatigue test was carried out by a bipolar triangular field with amplitude of 1 MV/cm and frequency of 10 Hz. The variations in remnant polarizations were compared between before (black dotted line) and after (red dotted line) the bending events. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. (a) Sets of ID–VG transfer curves of the fabricated organic memory TFT fabricated on the PDMS substrate when the VG sweep ranges were varied. Indicated arrows describe the direction of transfer curves in a double sweep of VG. (b) Variations of transfer characteristics of the same device from the 1st to 7th sweeps in VG. The VD was set to be 10 V.

depends on the dielectric constant and the thickness of the gate dielectric. In this study, the use of P(VDF-TrFE) with a high permittivity was also suggested to be desirable in reducing the operation voltage for the proposed OFMTs. The gate leakage currents could be suppressed at lower than 10 10 A, even though the device was fabricated on a PDMS substrate by performing low-temperature processes of below 140 °C. It would be more desirable to scale the operation voltage for the proposed OFMTs. Introduction of thinner films can be a simple solution for lowering the operation voltage. However, too an aggressive scaling of the film thickness may cause a remarkable increase in leakage current. On the other hand, introduction of an inorganic insulating layer can be a good solution because they can be formed with only a few nanometers thick. Thus, the applied voltage can be fully exploited for memory device operation [32–33]. As described above, it would be very difficult to achieve an extremely low power operation by only reducing the film thickness. Therefore, novel and unique methodologies to obtain a high-quality P(VDF-TrFE) thin film with a thinner thickness would be required for the scaling in operation voltage. It was confirmed that the transfer characteristics did not change at each repetitive measurements with VG sweeps, as shown in Fig. 6b. This is also an important point to accurately evaluate, considering the fact that the

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transfer curves of this kind of memory TFT are subject to markedly be fluctuated if the fabrication processes are not optimized for the device. Although only seven-times repetitive measurements of transfer characteristics cannot guarantee the endurance in device performance, the undesirable variations in device characteristics during the repetitive operations could be easily examined even by performing only several times successive sweeps. Therefore, it can be concluded that the proposed OFMT was well fabricated on the PDMS substrate without any critical damages caused by fabrication processes. Fig. 7a shows sets of transfer curves when the sweep range of VG increased only to the negative direction at a fixed 20 V in the positive side. In these series of measurements, the memory windows were slightly increased toward only the positive direction, while the turn-on voltage (Von) remained the same. This resulted from the fact that the off-program operations were ensured in each measurement by a sufficient positive voltage to program the off-state. On the contrary, Fig. 7b shows variations of transfer curves when the sweep range of VG increased only to positive direction at a fixed 20 V in the negative side. The memory windows were slightly increased toward only the negative direction, while the Von remained the same. Consequently, we can control and design the operation voltage and corresponding nonvolatile memory behaviors

by exploiting the operation schemes shown in Fig. 6a and b [22]. Fig. 8a shows a photograph of the processed PDMS substrate on which Al (gate)/P(VDF-TrFE)/F8T2/Au (S/D)/ PDMS TFTs were fabricated. The substrate was bendable, and a series of electrical measurements could be performed by setting up a proving system, as shown in Fig. 8b. Fig. 8c shows the bending durability by measuring the memory transfer characteristics when the substrate was bent with the radius of curvature of 0.6 cm. As can be seen in figure, the OFMT did not experience very marked variations in its device behaviors. The change in memory window of the OFMT was approximately 1 V at most. These results indicate that the proposed OFMT fabricated on the PDMS can be utilized under bending for any flexible device [23,34–38]. In this work, the bending radius could not be reduced to smaller state owing to the limitations of substrate size and machine specification. However,

Fig. 7. Sets of ID–VG transfer curves for the fabricated organic memory TFT with ferroelectric P(VDF-TrFE) gate dielectric. Indicated arrows describe the direction of transfer curves in a double sweep of VG. (a) Transfer curve variations when only the negative side of the VG sweep range is decreased from 6 to 20 V while maintaining the positive side at 20 V. (b) Transfer curve variations when only the positive side of the VG sweep range is increased from 2 to 20 V while maintaining the negative side at 20 V.

Fig. 8. (a) A typical photograph of a PDMS substrate on which the device fabrication process was completed. The thickness and size of the PDMS substrate employed in this work were 300 lm and 2  2 cm2, respectively. (b) Photograph of the electrical evaluation when the processed PDMS substrate was bent with bending radius of 0.6 cm. (c) Variations of the transfer characteristics and memory behaviors of the fabricated organic memory TFT under substrate bending with bending radius of 0.6 cm.

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further investigations would be necessary for the practical applications, such as cyclic bending tests with large number of bending at smaller radius of curvature. The response time and memory retention characteristics of the fabricated OFMTs were also investigated, which are very important for actually employing the nonvolatile memory components embedded into the electronic systems. The required voltage pulse width for the stable programming event is directly related to the programming speed of the memory transistor. We confirmed variations in the initially obtained ID values for programming conditions as a function of the program pulse width. If the memory margin is required to be no less than 10, the minimum duration of program voltage pulse could be estimated to be 200 ms. These relatively long switching times can be explained by combining two factors of (1) the formation of a fully depletion layer in the semiconducting channel and (2) the RC time constant generated by the product of the S/D channel resistance and gate capacitance [31]. The variations in the programmed drain currents (ID) in onand off-states were measured as a function of retention time, as shown in Fig. 9. For the on-programming operations, a 20-V-high pulse was first applied to the gate terminal to initialize ferroelectric polarization in the P(VDFTrFE) gate insulator, followed by the application of a 20-V-high programming pulse. Then, ID was measured at a VD of 10 V with keeping VG at 5 V for read-out operation. For the off-programming operation, the pulse amplitudes were reversely applied to the case of onprogramming. When the pulse durations for programming events were fixed at 990 ms, the programmed current ratio between on- and off-states was initially obtained to be approximately 1.5  103 and decreased to approximately 20 after a lapse of 2000 s. Although it was sufficiently encouraging to confirm the memory on/off ratio of higher than 1-orders-of magnitude for the fabricated OFMT even after a lapse of 2000 s, the programming and retention behaviors should be much more improved for future practical applications. Considering the feasible applications employing the proposed flexible memory TFT, it would

not always be necessary to guarantee years-of retention time. However, the several-days stability of the stored data surely expands the application fields of the proposed flexible nonvolatile OFMTs. 4. Conclusion In this work, we proposed and demonstrated a flexible organic nonvolatile memory TFT employing a ferroelectric copolymer gate insulator and organic semiconductor active channel as a memory component for flexible-type electronic devices. First, the capacitor structure was designed to be Au/P(VDF-TrFE)/Au/PDMS. Sound ferroelectric characteristics of the fabricated flexible P(VDF-TrFE) capacitors were well confirmed, which was important in that they could be obtained even on a PDMS substrate at a temperature of as low as 140 °C. The memory characteristics of the fabricated flexible OFMTs with a L/W of 20/ 60 lm were also then evaluated, in which an 11-V memory window and a 4-orders-of magnitude on/off ratio, were successfully obtained. These characteristics did not experience very marked degradations under bending with a radius curvature of 0.6 cm. We can conclude from the obtained results that our proposed OFMT can be a suitable candidate for an embeddable memory device to realize flexible low-cost electronic applications. However, as future work, the bending characteristics of the device will be more systematically investigated, which provides useful insight into design of flexible memory TFTs with excellent performance. The enhancements in programming and retention behaviors are also demanding for various flexible applications. Acknowledgments This research was funded by the MSIP (Ministry of Science, ICT & Future Planning), Korea in the ICT R&D Program (The core technology development of light and space adaptable energy-saving I/O platform for future advertising service). References

Fig. 9. Variations in ID programmed into both on- and off-states with the lapse of memory retention time. The inset shows the programming pulse sequences for the evaluation of memory retention behaviors of ferroelectric memory TFT. The first and second pulses of each sequence for the on- and off operations correspond to initialization and programming pulses, respectively. The on and off states were programmed by applying the voltage pulses of 20 and 20 V, respectively. The pulse width was set to 990 ms.

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