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semiconductor for a stable multi-state memory
Journal Pre-proof A Feasible Heterostructure of P(VDF-TrFE)/Semiconductor for a Stable MultiState Memory
Qiang Wu, Jun Li, Yujie Song, Wei Ou-Yang PII:
S1566-1199(19)30518-X
DOI:
https://doi.org/10.1016/j.orgel.2019.105491
Reference:
ORGELE 105491
To appear in:
Organic Electronics
Received Date:
11 May 2019
Accepted Date:
06 October 2019
Please cite this article as: Qiang Wu, Jun Li, Yujie Song, Wei Ou-Yang, A Feasible Heterostructure of P(VDF-TrFE)/Semiconductor for a Stable Multi-State Memory, Organic Electronics (2019), https://doi.org/10.1016/j.orgel.2019.105491
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Journal Pre-proof Graphical abstract
Journal Pre-proof
A Feasible Heterostructure of P(VDF-TrFE)/Semiconductor for a Stable Multi-State Memory Qiang Wua, Jun Lia,*, Yujie Songa, and Wei Ou-Yangb,** a Department
of Electronic Science and Technology, Tongji University, Shanghai,
China; b Engineering
Research Center for Nanophotonics & Advanced Instrument, Ministry of
Education, Department of Physics, East China Normal University, Shanghai, China * Corresponding author ** Corresponding author Email Address: [email protected] (Jun Li), [email protected] (Wei OuYang)
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Journal Pre-proof A Feasible Heterostructure of P(VDF-TrFE)/Semiconductor for a Stable Multi-State Memory
Abstract Ferroelectric random-access memory is widely concerned because of its nearly infinite writing cycles and fast reading and writing ability. Since ferroelectric memory is not as dense as dynamic random-access memory and static random-access memory, which cannot store so much data in the same space, it is difficult to replace these technologies in practical applications. One possible solution to this dilemma is to increase the storage capacity per memory cell. This work uses a feasible capacitor structure of P(VDF-TrFE)/organic semiconductor heterostructure to improve the storage capacity. Both ferroelectric layer and semiconductor layer are prepared using a simple spin coating method. Due to the intervention of semiconductor, three polarization states are demonstrated owing to the splitting of a displacement current peak, where the semiconductor-dielectric transformation of the semiconductor layer plays an important role. The measurement of electrical properties shows that the ferroelectric thin film about this structure with three polarization states can be used to represent the storage 0, 1, 2, and the polarization of P1, P2, P3 are 0, 3.4 and 9.2 μC cm-2, respectively. According to the fitting results, the data retention capacity of this structure is excellent, that the residual polarizations of P1, P2 and P3 are 0, 3.2 and 9.2 μC cm-2 over 3 months, respectively.
Keywords: ferroelectric capacitors, ferroelectric random-access memories, P(VDFTrFE) films, organic semiconductors, multi-state memories
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Journal Pre-proof 1. Introduction Since the end of the 20th century, ferroelectric materials have got much attention being used in non-volatile memory devices, which could retain information when supplied power is interrupted [1-3]. Besides, the constituent ferroelectric random-access memory (FeRAM) has other advantages of low power consumption [4], low cost [5] and potentially high storage density [6]. According to the chemical composition, ferroelectric materials can be simply divided into two categories: organic and inorganic ferroelectric materials [7,8]. Among them, inorganic ferroelectric materials have been commercially available on non-volatile memory for some years [9], while organic ferroelectric materials have not yet led to any commercialization [10]. Though inorganic ferroelectric materials, such as PZT [Pb(Zr, Ti)O3] [11], SBT (SrBi2Ta2O9) [12] and HfO2 [13] have been widely used, they are often difficult and expensive to manufacture, and the toxic heavy metals contained are harmful to human health and environment [14]. In contrast, organic ferroelectric
materials
are
energy-efficient,
economically
inexpensive
and
environmentally friendly [15]. Especially, they are compatible with the next-generation flexible electronics in nature [16,17]. Therefore, research on organic ferroelectric memory is of significant importance. Poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) is one of the most promising organic ferroelectric materials with advantageous properties including a relatively large remnant polarization, a short switching time and a good thermal stability [14]. The capacitor using P(VDF-TrFE) as a dielectric can be connected to transistors to integrate into 1T1C (one transistor and one capacitor) or 2T2C (two transistors and two capacitors) configuration for non-volatile memory devices [18]. However, the performance of P(VDF-TrFE) film deteriorates with decrease of the film thickness, such as the increase of coercive electric field and leakage current [19]. Due to this 3
Journal Pre-proof inappropriate reliability, there has been no great success in reducing the feature size of the memories made of P(VDF-TrFE) compared with their counterparts of inorganic ferroelectric memories. Nevertheless, few impressive progresses have been reported on overcoming the size effect by improving the process and reducing the film thickness [2022]. If it is difficult to scale down the feature size of a ferroelectric memory, another way to solve this problem is increasing the number of memory states per memory cell [23-25]. Some efforts have been made on utilizing two or more ferroelectric films separated with an insulator, in which each film can provide two memory states. This novel approach makes the memory capacitor store 2n data (n is the number of layers of ferroelectric films) [26,27]. Although the storage capacity has been improved, the size per memory cell has been greatly increased. Others tended to use the intermediate polarization states by applying slightly different voltages in ferroelectric layers, which could improve the storage capacity per memory cell [28-30]. The intractable problem with this approach is that intermediate polarization states are not in thermodynamically stable states and easy to miswrite or misread storage data. Hence, it needs to find a feasible way to get a stable intermediate polarization state. In the present paper a simple heterostructure with a multi-step polarization switching characteristics is introduced based on a high mobility organic semiconductor of indacenodithiophene benzothiadiazole (IDT-BT) [31], and the organic ferroelectric material of P(VDF-TrFE), both of which are prepared using a simple spin-coating method. Note that different from most of previous works using a vacuum deposited organic semiconductor layer [32-35], this solution method makes the fabrication process is much feasible, compatible and possible for printed electronics. The multi-step polarization phenomenon originated from semiconductor carrier injection and depletion
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Journal Pre-proof can be used in multilevel data storage, which is described in detail below. It will be shown that the polarization states can be independently addressed by using straightforward voltage pulse. One ferroelectric capacitor with P(VDF-TrFE)/IDT-BT compound films can store three memory states (0, 1, 2) a 50% increase in areal capacity compared with conventional ferroelectric capacitors. Besides, the memory of this structure has a high resolution and a long storage time between each memory state. 2. Experimental Section The molecular structures of P(VDF-TrFE) and IDT-BT are shown in Fig. 1(a), and preparation of the metal-ferroelectric-semiconductor-metal (MFSM) capacitor, whose structure was illustrated in the inset of Fig. 1(c), was described as follows. The conductive indium tin oxide (ITO) glass substrates were cleaned for use by sonicating in an ultrasonic bath with deionized water, acetone and ethanol sequentially. Then, a certain amount of P(VDF-TrFE) (70-30 mol%, purchased from Piezotech, France) copolymer powder was dissolved in a methy-ethyl-ketone (MEK) solvent (4 wt.%), and a P(VDF-TrFE) layer with thickness of 360 nm was spin-coated (3000 rpm, 30 s) on the ITO substrates. Afterwards, the sample was annealed on a hot plate (at 135 ℃) for 1 h. Subsequently, the IDT-BT (purchased from Derthon Optoelectronic Materials Science Technology Co LTD, China) semiconductor powder was dissolved in a chlorobenzene solvent (3 ~ 4 mg ml-1) and such an organic semiconductor layer was spin-coated (2500 rpm, 30 s) on the thermally annealed P(VDF-TrFE) layer, followed by a further annealing process in vacuum (at 90 ℃) for 1 h. At last, the Au electrode (30 nm) was successively deposited by vacuum evaporation, where the process pressure and deposition rate were set to <10-4 Pa and 1 Å s-1. The metal-ferroelectric-metal (MFM) capacitor as a contrast sample
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Journal Pre-proof illustrated in the inset of Fig. 1(b) was made in the same way without the semiconductor layer and the effective area of each device was designed to 4 mm2. A source meter of Keithley 4250 was used to measure the current-voltage (I-V) characteristic. All electrical measurements were carried out by driving voltage on the top Au electrode while the bottom ITO electrode was grounded. 3. Results and Discussion Fig. 1(b-c) shows the current-voltage (I-V) characteristics of the MFM and MFSM structures measured using a source measurement unit with a current resolution of 10 fA (Keithley 2450). The result of MFM structure presents two almost symmetric current peaks with the peak position around 50 V μm-1, corresponding with our published data as well as other reports [32-35]. Previous studies have proved that the current peaks were generated by displacement current due to the polarization reversal of ferroelectric body [33,34]. Note that the two current peaks are slightly asymmetric (left current peak voltage: -17.5 V, right current peak voltage: 18.5 V) owing to the differences in the work function of the top and bottom electrodes [21,36]. The I-V curve of the MFSM structure generally shows a slightly distorted shape and a smaller peak current (corresponding to the coercive voltage) because of the involvement of a semiconducting IDT-BT layer under negative voltage sweeping. A notable finding is that the I-V curve of MFSM structure has two small peaks in the negative voltage region, whereas only one large peak in the positive region. Note that the slight reduction of coercive voltage at positive voltage regime (MFM structures: 18.5 V, MFSM structures: 18.0 V) can be explained by the decrease of the effective thickness of the P(VDF-TrFE) layer, which corresponds with the experimental observation of the slightly distorted shape and is considered to be caused by the weak interaction between semiconductor layer and
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Journal Pre-proof ferroelectric layer during the device fabrication [33,37]. For convenience, the peak centered at -17.5 V is named for peak A, and the other peak at -27.0 V is denoted as peak B. The starting voltage of peak A (-10.0 V) is symmetric with the starting voltage of the peak under positive bias (10.0 V). In Fig. 3 (a), I-V curves of more voltage ranges are measured, and it is found that even at higher scanning voltage, the split current peak still exists and has a high consistency. This indicates that the splitting current peak has initial stability, and its retention ability will be further discussed latter. We note that Naber et al. [38] also made MIS (metal-insulator-semiconductor, the insulator layer is constituted by P(VDF-TrFE/P3HT)) diodes with similar structures, while they did not obtain the profiled curves of three current peaks. This may be due to differences in semiconductor materials and thickness. The split of the current peak indicates that the charge compensation for the polarization can be divided into two steps. The reason is the Schottky barrier between the semiconductor and the metal electrode and the concentration of carriers in the semiconductor. We will elaborate on this in the following paragraphs. Fig. 2 illustrates the cause of current peak generation and splitting. There are six key states for explaining unique current peaks of the MFSM structure, which are noted in the I-V curve of Fig. 2(a). Fig. 2(b-g) display polarization states in the ferroelectric film and charge distribution in the organic semiconducting layer corresponding to the six states in Fig. 2(a). After negative voltage bias the polarization direction in the ferroelectric layer is fully upward for the State 1 shown in Fig. 2(b). When the voltage bias is positive, the contact barrier between the semiconductor layer and the metal is extremely low as the IDT-BT is a p-type high-mobility semiconductor [39,40]. In other words, the IDT-BT layer at this moment works as a part of the electrode and the positive charges can easily penetrate through the semiconductor from the Au electrode sketched in Fig. 2(c) [20]. The current peak marked with the State 2 resembles that of the MFM structured capacitor.
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Journal Pre-proof Furthermore, when the voltage returns to 0 V (the State 3), some compensating charges are separated and stored at the P(VDF-TrFE)/IDT-BT interface (Fig. 2(d)). While a small negative bias is applied, the contact barrier for electron injection between the semiconductor layer and the metal is high, and the carriers in the semiconductor cannot be replenished. Therefore, the interface charges act as carriers (Fig. 2(e)), and the compensating charges on the P(VDF-TrFE)/IDT-BT interface cause the peak A (State 4). Afterwards, the IDT-BT layer could be carrier-depleted and should work as an insulating layer (Fig. 2(f)) at State 5. As the voltage increases further, the voltage applied to P(VDFTrFE) layer reaches the coercivity again (Fig. 2(g)), and then the polarization reversal produces a new peak (State 6). This semiconductor-dielectric conversion process caused by carrier transport in the MFSM devices makes the current peak split. Actually, our earlier results [41] have partially shown that the splitting current summit gradually merges when the device is exposed to strong light, which provides experimental basis for this semiconductordielectric conversion mechanism. When the device is irradiated by an external light source, the carriers in the semiconductor will be excited. More sufficient carriers can gather at the interface to promote the polarization inversion of the ferroelectric layer, and when the number of carriers is large enough, the splitting current peak will disappear. These indicate that the number of carriers in the semiconductor is indeed the main factor affecting the ferroelectric inversion process of MFSM structure. In short, when the applied voltage is positive, the semiconductor is working as an electrode. While the applied voltage is negative, the semiconductor becomes dielectric after the carriers are exhausted. This semiconductor-dielectric transformation mechanism results in the peculiar current peaks in the MFSM structured capacitor and makes the
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Journal Pre-proof polarization of P(VDF-TrFE) being multistate. The feasible operation of multistate polarization can be used in multi-storage. Ferroelectric capacitors can be used in memory devices because of different polarization states driven by applied voltage. Normally, a ferroelectric film has only two polarization states which can be associated to the Boolean 0 and 1 bit. However, the polarization behavior, encountered in ferroelectric capacitors, requires a certain area of ferroelectric [42,43]. There is an applicable way to overcome the disadvantage of FeRAM’s large scale, that is increasing the storage capacity of the individual memory cell. Fig. 3(b) demonstrates the feasibility of MFSM structured capacitors for improving storage capacitor. Three polarization states can be accessed by different applied voltage ranges. At a scanning voltage ranging from -30 V to +30 V, the P(VDF-TrFE) layer can be completely polarized and has maximum polarization charges with three current peaks. From -20 V to +30 V scanning voltage, the P(VDF-TrFE) layer can be partially polarized. Because the semiconductor acts as an insulator at -20 V and small negative voltage cannot make P(VDF-TrFE) completely reversed. The P(VDF-TrFE) does not occur polarization reversal at the range from 0 V to +30 V. The appearance of recorded current peaks for different voltage ranges with different polarization charges proves the possibility of the tristate memory of MFSM structured capacitors. For convenience, hereafter we use P1, P2, and P3 for the three polarization states, which can be associated to memory states 0, 1, and 2. To demonstrate a multilevel writing and reading process, Fig. 4(a) shows an example of a pulse sequence that was used to individually write and read each polarization state. Before a writing process, a positive unipolar triangular pulse of +30 V is applied to preset polarization state. P1, P2, and P3 were respectively given by 0 V, -20 V, and -30 V of
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Journal Pre-proof unipolar triangular applied voltage pulse to set respective polarization states. A reading process is simulated by applying a unipolar triangular pulse of +30 V, which is the same as the preset voltage pulse. Therefore, the preset process is only needed at the first time, and the reading process causes a destructive reading that must be rewritten after each reading which means it have a destructive read out. Fig. 4(b) shows the I-V curves of three individual writing processes to get three different polarization states P1, P2, and P3. The essence is that the semiconductor intervention leads to polarization process of negative applied voltage in two steps. The current response corresponds to Fig. 3(b) and confirms that it is feasible to get P1, P2, and P3 separately. The writing process curve would show a wide voltage margin of the memory for stable operation. Fig. 4(c) shows the reading current of three polarization states at 0 to +30 V voltage pulse. And the charge of reading current peaks corresponds to that of writing current peaks, which means memory states can be controlled by writing voltage. The results presented above suggest that multiple polarization states originated from two-step polarization reversal at negative voltage of MSFM structured capacitor are attainable for tristate memory devices. According to the principle previously analyzed, the thicker the semiconductor is, the higher the voltage corresponding to peak B will be, that is, the larger the distance between peak A and peak B will be. However, too thick semiconductor will lead to depolarization of ferroelectric layer, so it is very important to choose the right semiconductor thickness [44]. The sizes of peak A and peak B are controlled by the carrier concentration of the semiconductor. The higher the concentration is, the higher the peak A is and the smaller the peak B is, and vice versa. It is therefore possible to exploit the fact that the semiconductor layer can control the size and location of the two split current peaks, which means the electrical properties of memory is controllable. Note that currently the problems associated with the devices are high
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Journal Pre-proof operating voltage. Nevertheless, this is only related to the characteristics of P(VDF-TrFE) material itself whose thickness can be greatly reduced by using a molecular layer deposition technique of Langmuir-Blodgett [45, 46]. Since the polarization is divided into two steps, the distinguishing abilities and retention properties could be the most serious reliability concern of this memory. Fig. 5(a) shows polarization charge of each state and the measuring device is the same as in Fig. 4. The polarizations of P1, P2 and P3 states obtained by calculating the peak area in the illustration are 0, 3.4 and 9.2 μC cm-2, respectively. The polarizations of P3 is nearly three times of that of P2 which meets the requirement of memory with a large discrimination. Fig. 5(b) shows the retention properties. Measurements were performed for retention time from 10 to 104. Due to the destructive reading process of ferroelectric devices, the data need to be rewritten after each measurement and the retention time is calculated from each writing process. The points in the graph indicate the measured data, and the dashed line shows an extrapolation based on the data measured. At atmospheric environment, the P2 remaining after 107 s (above 3 months) are expected to be 3.0 μC cm-2, and the polarization of P1 and P3 are almost constant. This may be attributed to the fact that P2, which is not fully polarized, is not in a thermodynamically stable state and depolarized more easily. In general, this structure of ferroelectric capacitors is feasible for three memory states data storage both in distinguish abilities and retention properties. 4. Conclusions A P(VDF-TrFE)/IDT-BT heterostructure thin film is presented to study the feasibility of stable multi-state and destructive read-out storage and related physical mechanism. This special thin film is used in capacitors with three polarization states as the multi-level memory for high density data storage, where the principle of three
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Journal Pre-proof polarization states is semiconductor-dielectric transformation of the semiconductor layer. The feasibility of this ferroelectric memory is demonstrated by the actual operating voltage, and the data resolution and retention ability are studied in detail. Three polarization states (0, 3.4 and 9.2 μC cm-2) are developed in this work and the retention time is expected to reach 107 s. The ferroelectric capacitance is 50% higher than that of traditional memories, which provides a solution to the scale problem of FeRAM. The experimental demonstration of stable multi-state storage and the behind physical mechanism of semiconductor-dielectric transformation in the heterostructure of P(VDFTrFE)/semiconductor shed light on the feasible multi-state memory.
Acknowledgements This work was financially supported by National Natural Science Foundation of China under Grant Nos. 61504042, 61504098, and 61771198, Natural Science Foundation of Shanghai under Grant No. 17ZR1447000, and the Shanghai “Yang-Fan Plan” for Young Scientists, China under Grant No. 14YF1403400.
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Fig. 1. (a) Molecular structures of P(VDF-TrFE) and IDT-BT. Switching current loops (voltage range: -30 V to 30 V) for capacitors with different structures: (b) MFM structure; (c) MFSM structure (The IDT-BT solution concentration is 3 mg ml-1).
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Fig. 2. Illustrations of current peak generation and splitting under certain voltage bias conditions. (a) I-V curve of the MFSM structured capacitor with six states denoted using circled dots (the arrows indicate the trend of curve). Diagrams of ferroelectric polarization state and carrier motion under different voltages: (b) 0 V indicated position (1) in the I-V curve, (c) +18 V indicated position (2) in I-V curves, (d) 0 V indicated position (3) in I-V curves, (e) -17.5 V indicated position (4) in I-V curves, (f) -20 V indicated position (5) in I-V curves and (g) -27 V indicated position (6) in I-V curves.
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Fig. 3. I-V curves of MFSM structured capacitors at different scanning voltage regimes. (a) The black square indicates that the voltage scanning range is -40 V to +40 V, red circle demonstrates -50 V to + 50 V, blue triangle points out -60 V to +60 V and pink inverted triangle shows -70 V to + 70 V. (b) The black square indicates that the voltage scanning range is -30 V to +30 V, red circle indicates -20 V to +30 V, and blue triangle indicates 0 V to +30 V. The IDT-BT solution concentration of the device in (a) is 4 mg ml-1, while the concentration in (b) is 3 mg ml-1.
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Fig. 4. (a) Schematic diagram of obtaining different polarization states by setting triangular voltage pulses. Before every writing operation, the devices need to give a +30 V preset voltage pulse. Three different polarization states are written by 0 V, -20 V and 30 V. Reading pulse is the same as the preset pulse. (b) Polarization current of three different polarization states by writing sweep voltage. (c) Polarization current of three different polarization states at 0 to +30 V reading sweep voltage. Here, the concentration of IDT-BT solution is 3 mg ml-1.
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Fig. 5. (a) Integrated polarization charges obtained after reading each polarization state. Inset shows the current-time curves for three different polarization states. (b) Retention properties of three polarization states.
(All pictures print in black and white.)
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List of changes 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
28. 29. 30. 31.
Remove “(FeRAM)”, “by researchers”, “(DRAM)”, “(SRAM)” in P2L5 to P2L8. Update “by” to “using a”, “due” to “owing” in P2L13 to P2L14. Update “polarization” to “polarizations” in P2L20. Add “and” and “respectively” in P2L21.” Update “concentrated” to “made” in P4L8. Add “, both of which are prepared …and possible for printed electronics.” In P4L21 to P4L24. Update “which increase 50% capacity in the same area compared” to “a 50% increase in areal capacity compared” in P5L4. Add “The molecular structures of P(VDF-TrFE) and IDT-BT are shown in Fig. 1(a),” in P5L8. Update “Preparation” to “and preparation” in P5L8 to P5L9. Add “by sonicating” in P5L11 to P5L12. Add experimental details: “(3000 rpm, 30 s)”, “(purchased from Derthon Optoelectronic Materials Science Technology Co LTD, China)”, “(3 ~ 4 mg ml-1)”, “(2500 rpm, 30 s)” in P5L15 to P5L20. Add “illustrated in the inset of Fig. 1(b)” in P5L24. Update “mm-2” to “mm2” in P6L2. Update “Fig. 1” to “Fig. 1(b-c)” in P6L7. Remove “metal-ferroelectric-metal (”, “)”, “metal-ferroelectric-semiconductor-metal (” and “)” in P6L7 to P6L8. Update “mm-1” to “μm-1” in P6L11. Add references “35” in P6L12. Add discussion about genetic cause and stability of splitting peak (“In Fig. 3(a), I-V curves of … in semiconductor materials and thickness.”) in P7L5 to P7L12. Update “showed” to “shown” in P7L21. Add “the” in P7L23. Add discussion about the influence of illumination (“This semiconductor-dielectric conversion … ferroelectric inversion process of MFSM structure.”) in P8L12 to P8L21. Add “To demonstrate a multilevel writing and reading process,” in P9L22. Add destructive reading comments (“Therefore, the preset process is only … it has a destructive read out.”) in P10L3 to P10L5. Add “However, too thick semiconductor will lead to depolarization of ferroelectric layer, so it is very important to choose the right semiconductor thickness [44].” in P10L18 to P10L20. Add “that the semiconductor layer can control the size and location of the two split current peaks,” in P10L23 to P10L24. Add “and the measuring device is the same as in Fig. 4.” in P11L6 to P11L7. Add “Due to the destructive reading process of ferroelectric devices, the data need to be rewritten after each measurement and the retention time is calculated from each writing process.” In P11L11 to P11L13. Add “destructive read-out” in P11L22. Add “is expected to” in P12L5. Revise the authors of [20] (“W. Ou-Yang, M. Weis, T. Manaka, M. Iwamoto”) in P14L4 Revise the authors of [32] (“J. Li, D. Taguchi, T. Manaka, M. Iwamoto,”) in P15L5.
Journal Pre-proof 32. Add references: “[28] K. Asadi, P.W.M. Blom, D.M. de Leeuw, Conductance switching in organic ferroelectric field-effect transistors, Appl. Phys. Lett. 99 (2011) 053306. [29] B. Kam, X. Li, C. Cristoferi, E.C.P. Smits, A. Mityashin, S. Schols, J. Genoe, G. Gelinck, P. Heremans, Origin of multiple memory states in organic ferroelectric field-effect transistors, Appl. Phys. Lett. 101 (2012) 033304. [35] L. Hu, S. Dalgleish, M.M. Matsushita, H. Yoshikawa, K. Awaga, Storage of an electric field for photocurrent generation in ferroelectric-functionalized organic devices, Nat. Commun. 5 (2014) 3279. [38] R.C.G. Naber, J. Massolt, M. Spijkman, K. Asadi, P.W.M. Blom, D.M. de Leeuw, Origin of the drain current bistability in polymer ferroelectric field-effect transistors, Appl. Phys. Lett. 90 (2007) 113509. [41] M. Weis, J. Li, D. Taguchi, T. Manaka, M. Iwamoto, Effect of Photogenerated Carriers on Ferroelectric Polarization Reversal, Appl. Phys. Ex. 4 (2011) 121601. [44] K. Asadi, J. Wildeman, P.W.M. Blom, D.M.d. Leeuw, Retention Time and Depolarization in Organic Nonvolatile Memories Based on Ferroelectric Semiconductor Phase-Separated Blends, IEEE T. Electron Dev. 57 (2010) 3466-3471.” 33. Add fig. 1(a) in P17L1. 34. Add “(a) Molecular structures of P(VDF-TrFE) and IDT-BT.” and “(The IDT-BT solution concentration is 3 mg ml-1)” in P17L3 to P17L5. 35. Add fig. 3(a) in P19L1. 36. Update “(a) The black square indicates … while the concentration in (b) is 3 mg ml-1.” in P19L4 to P19L9. 37. Add “Here, the concentration of IDT-BT solution is 3 mg ml-1.” in P20L8 to P20L9.
Journal Pre-proof Dear reviewer, Poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) is one of the most promising organic ferroelectric materials with advantageous properties, and it could be used in ferroelectric random-access memory (FeRAM) as a dielectric. While the performance of P(VDF-TrFE) film deteriorates with decrease of the film thickness, such as the coercive electric field and leakage current. Due to this unreliability, truly practical miniaturized ferroelectric memory has not been developed to date. Another way to solve this problem of scaling down the feature size of a ferroelectric memory is increasing the number of memory states per memory cell, which means the devices of the same size can store more information. Here we use a heterostructure of P(VDF-TrFE)/semiconductor to realize a stable multi-state memory. There are three main highlights: 1. We research the transfer mechanism of carriers in P(VDF-TrFE)/organic semiconductor heterostructure capacitors, and propose a concept of semiconductor-dielectric conversion. This mechanism makes the polarization states of P(VDF-TrFE) become multi-state, and the retention time is extremely long. 2. The stability and repeatability of this the polarization polymorphism phenomenon have been proved by experiments. And according to this phenomenon, we study the performance of this heterostructure as multi-state memory. 3. The semiconductor layer and ferroelectric layer was prepared by using a spin-
Journal Pre-proof coting method, which is different from most of previous works using a vacuum deposited way.
Qiang Wu, Jun Li, Yujie Song, Wei Ou-Yang PII:
S1566-1199(19)30518-X
DOI:
https://doi.org/10.1016/j.orgel.2019.105491
Reference:
ORGELE 105491
To appear in:
Organic Electronics
Received Date:
11 May 2019
Accepted Date:
06 October 2019
Please cite this article as: Qiang Wu, Jun Li, Yujie Song, Wei Ou-Yang, A Feasible Heterostructure of P(VDF-TrFE)/Semiconductor for a Stable Multi-State Memory, Organic Electronics (2019), https://doi.org/10.1016/j.orgel.2019.105491
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof Graphical abstract
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A Feasible Heterostructure of P(VDF-TrFE)/Semiconductor for a Stable Multi-State Memory Qiang Wua, Jun Lia,*, Yujie Songa, and Wei Ou-Yangb,** a Department
of Electronic Science and Technology, Tongji University, Shanghai,
China; b Engineering
Research Center for Nanophotonics & Advanced Instrument, Ministry of
Education, Department of Physics, East China Normal University, Shanghai, China * Corresponding author ** Corresponding author Email Address: [email protected] (Jun Li), [email protected] (Wei OuYang)
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Journal Pre-proof A Feasible Heterostructure of P(VDF-TrFE)/Semiconductor for a Stable Multi-State Memory
Abstract Ferroelectric random-access memory is widely concerned because of its nearly infinite writing cycles and fast reading and writing ability. Since ferroelectric memory is not as dense as dynamic random-access memory and static random-access memory, which cannot store so much data in the same space, it is difficult to replace these technologies in practical applications. One possible solution to this dilemma is to increase the storage capacity per memory cell. This work uses a feasible capacitor structure of P(VDF-TrFE)/organic semiconductor heterostructure to improve the storage capacity. Both ferroelectric layer and semiconductor layer are prepared using a simple spin coating method. Due to the intervention of semiconductor, three polarization states are demonstrated owing to the splitting of a displacement current peak, where the semiconductor-dielectric transformation of the semiconductor layer plays an important role. The measurement of electrical properties shows that the ferroelectric thin film about this structure with three polarization states can be used to represent the storage 0, 1, 2, and the polarization of P1, P2, P3 are 0, 3.4 and 9.2 μC cm-2, respectively. According to the fitting results, the data retention capacity of this structure is excellent, that the residual polarizations of P1, P2 and P3 are 0, 3.2 and 9.2 μC cm-2 over 3 months, respectively.
Keywords: ferroelectric capacitors, ferroelectric random-access memories, P(VDFTrFE) films, organic semiconductors, multi-state memories
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Journal Pre-proof 1. Introduction Since the end of the 20th century, ferroelectric materials have got much attention being used in non-volatile memory devices, which could retain information when supplied power is interrupted [1-3]. Besides, the constituent ferroelectric random-access memory (FeRAM) has other advantages of low power consumption [4], low cost [5] and potentially high storage density [6]. According to the chemical composition, ferroelectric materials can be simply divided into two categories: organic and inorganic ferroelectric materials [7,8]. Among them, inorganic ferroelectric materials have been commercially available on non-volatile memory for some years [9], while organic ferroelectric materials have not yet led to any commercialization [10]. Though inorganic ferroelectric materials, such as PZT [Pb(Zr, Ti)O3] [11], SBT (SrBi2Ta2O9) [12] and HfO2 [13] have been widely used, they are often difficult and expensive to manufacture, and the toxic heavy metals contained are harmful to human health and environment [14]. In contrast, organic ferroelectric
materials
are
energy-efficient,
economically
inexpensive
and
environmentally friendly [15]. Especially, they are compatible with the next-generation flexible electronics in nature [16,17]. Therefore, research on organic ferroelectric memory is of significant importance. Poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) is one of the most promising organic ferroelectric materials with advantageous properties including a relatively large remnant polarization, a short switching time and a good thermal stability [14]. The capacitor using P(VDF-TrFE) as a dielectric can be connected to transistors to integrate into 1T1C (one transistor and one capacitor) or 2T2C (two transistors and two capacitors) configuration for non-volatile memory devices [18]. However, the performance of P(VDF-TrFE) film deteriorates with decrease of the film thickness, such as the increase of coercive electric field and leakage current [19]. Due to this 3
Journal Pre-proof inappropriate reliability, there has been no great success in reducing the feature size of the memories made of P(VDF-TrFE) compared with their counterparts of inorganic ferroelectric memories. Nevertheless, few impressive progresses have been reported on overcoming the size effect by improving the process and reducing the film thickness [2022]. If it is difficult to scale down the feature size of a ferroelectric memory, another way to solve this problem is increasing the number of memory states per memory cell [23-25]. Some efforts have been made on utilizing two or more ferroelectric films separated with an insulator, in which each film can provide two memory states. This novel approach makes the memory capacitor store 2n data (n is the number of layers of ferroelectric films) [26,27]. Although the storage capacity has been improved, the size per memory cell has been greatly increased. Others tended to use the intermediate polarization states by applying slightly different voltages in ferroelectric layers, which could improve the storage capacity per memory cell [28-30]. The intractable problem with this approach is that intermediate polarization states are not in thermodynamically stable states and easy to miswrite or misread storage data. Hence, it needs to find a feasible way to get a stable intermediate polarization state. In the present paper a simple heterostructure with a multi-step polarization switching characteristics is introduced based on a high mobility organic semiconductor of indacenodithiophene benzothiadiazole (IDT-BT) [31], and the organic ferroelectric material of P(VDF-TrFE), both of which are prepared using a simple spin-coating method. Note that different from most of previous works using a vacuum deposited organic semiconductor layer [32-35], this solution method makes the fabrication process is much feasible, compatible and possible for printed electronics. The multi-step polarization phenomenon originated from semiconductor carrier injection and depletion
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Journal Pre-proof can be used in multilevel data storage, which is described in detail below. It will be shown that the polarization states can be independently addressed by using straightforward voltage pulse. One ferroelectric capacitor with P(VDF-TrFE)/IDT-BT compound films can store three memory states (0, 1, 2) a 50% increase in areal capacity compared with conventional ferroelectric capacitors. Besides, the memory of this structure has a high resolution and a long storage time between each memory state. 2. Experimental Section The molecular structures of P(VDF-TrFE) and IDT-BT are shown in Fig. 1(a), and preparation of the metal-ferroelectric-semiconductor-metal (MFSM) capacitor, whose structure was illustrated in the inset of Fig. 1(c), was described as follows. The conductive indium tin oxide (ITO) glass substrates were cleaned for use by sonicating in an ultrasonic bath with deionized water, acetone and ethanol sequentially. Then, a certain amount of P(VDF-TrFE) (70-30 mol%, purchased from Piezotech, France) copolymer powder was dissolved in a methy-ethyl-ketone (MEK) solvent (4 wt.%), and a P(VDF-TrFE) layer with thickness of 360 nm was spin-coated (3000 rpm, 30 s) on the ITO substrates. Afterwards, the sample was annealed on a hot plate (at 135 ℃) for 1 h. Subsequently, the IDT-BT (purchased from Derthon Optoelectronic Materials Science Technology Co LTD, China) semiconductor powder was dissolved in a chlorobenzene solvent (3 ~ 4 mg ml-1) and such an organic semiconductor layer was spin-coated (2500 rpm, 30 s) on the thermally annealed P(VDF-TrFE) layer, followed by a further annealing process in vacuum (at 90 ℃) for 1 h. At last, the Au electrode (30 nm) was successively deposited by vacuum evaporation, where the process pressure and deposition rate were set to <10-4 Pa and 1 Å s-1. The metal-ferroelectric-metal (MFM) capacitor as a contrast sample
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Journal Pre-proof illustrated in the inset of Fig. 1(b) was made in the same way without the semiconductor layer and the effective area of each device was designed to 4 mm2. A source meter of Keithley 4250 was used to measure the current-voltage (I-V) characteristic. All electrical measurements were carried out by driving voltage on the top Au electrode while the bottom ITO electrode was grounded. 3. Results and Discussion Fig. 1(b-c) shows the current-voltage (I-V) characteristics of the MFM and MFSM structures measured using a source measurement unit with a current resolution of 10 fA (Keithley 2450). The result of MFM structure presents two almost symmetric current peaks with the peak position around 50 V μm-1, corresponding with our published data as well as other reports [32-35]. Previous studies have proved that the current peaks were generated by displacement current due to the polarization reversal of ferroelectric body [33,34]. Note that the two current peaks are slightly asymmetric (left current peak voltage: -17.5 V, right current peak voltage: 18.5 V) owing to the differences in the work function of the top and bottom electrodes [21,36]. The I-V curve of the MFSM structure generally shows a slightly distorted shape and a smaller peak current (corresponding to the coercive voltage) because of the involvement of a semiconducting IDT-BT layer under negative voltage sweeping. A notable finding is that the I-V curve of MFSM structure has two small peaks in the negative voltage region, whereas only one large peak in the positive region. Note that the slight reduction of coercive voltage at positive voltage regime (MFM structures: 18.5 V, MFSM structures: 18.0 V) can be explained by the decrease of the effective thickness of the P(VDF-TrFE) layer, which corresponds with the experimental observation of the slightly distorted shape and is considered to be caused by the weak interaction between semiconductor layer and
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Journal Pre-proof ferroelectric layer during the device fabrication [33,37]. For convenience, the peak centered at -17.5 V is named for peak A, and the other peak at -27.0 V is denoted as peak B. The starting voltage of peak A (-10.0 V) is symmetric with the starting voltage of the peak under positive bias (10.0 V). In Fig. 3 (a), I-V curves of more voltage ranges are measured, and it is found that even at higher scanning voltage, the split current peak still exists and has a high consistency. This indicates that the splitting current peak has initial stability, and its retention ability will be further discussed latter. We note that Naber et al. [38] also made MIS (metal-insulator-semiconductor, the insulator layer is constituted by P(VDF-TrFE/P3HT)) diodes with similar structures, while they did not obtain the profiled curves of three current peaks. This may be due to differences in semiconductor materials and thickness. The split of the current peak indicates that the charge compensation for the polarization can be divided into two steps. The reason is the Schottky barrier between the semiconductor and the metal electrode and the concentration of carriers in the semiconductor. We will elaborate on this in the following paragraphs. Fig. 2 illustrates the cause of current peak generation and splitting. There are six key states for explaining unique current peaks of the MFSM structure, which are noted in the I-V curve of Fig. 2(a). Fig. 2(b-g) display polarization states in the ferroelectric film and charge distribution in the organic semiconducting layer corresponding to the six states in Fig. 2(a). After negative voltage bias the polarization direction in the ferroelectric layer is fully upward for the State 1 shown in Fig. 2(b). When the voltage bias is positive, the contact barrier between the semiconductor layer and the metal is extremely low as the IDT-BT is a p-type high-mobility semiconductor [39,40]. In other words, the IDT-BT layer at this moment works as a part of the electrode and the positive charges can easily penetrate through the semiconductor from the Au electrode sketched in Fig. 2(c) [20]. The current peak marked with the State 2 resembles that of the MFM structured capacitor.
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Journal Pre-proof Furthermore, when the voltage returns to 0 V (the State 3), some compensating charges are separated and stored at the P(VDF-TrFE)/IDT-BT interface (Fig. 2(d)). While a small negative bias is applied, the contact barrier for electron injection between the semiconductor layer and the metal is high, and the carriers in the semiconductor cannot be replenished. Therefore, the interface charges act as carriers (Fig. 2(e)), and the compensating charges on the P(VDF-TrFE)/IDT-BT interface cause the peak A (State 4). Afterwards, the IDT-BT layer could be carrier-depleted and should work as an insulating layer (Fig. 2(f)) at State 5. As the voltage increases further, the voltage applied to P(VDFTrFE) layer reaches the coercivity again (Fig. 2(g)), and then the polarization reversal produces a new peak (State 6). This semiconductor-dielectric conversion process caused by carrier transport in the MFSM devices makes the current peak split. Actually, our earlier results [41] have partially shown that the splitting current summit gradually merges when the device is exposed to strong light, which provides experimental basis for this semiconductordielectric conversion mechanism. When the device is irradiated by an external light source, the carriers in the semiconductor will be excited. More sufficient carriers can gather at the interface to promote the polarization inversion of the ferroelectric layer, and when the number of carriers is large enough, the splitting current peak will disappear. These indicate that the number of carriers in the semiconductor is indeed the main factor affecting the ferroelectric inversion process of MFSM structure. In short, when the applied voltage is positive, the semiconductor is working as an electrode. While the applied voltage is negative, the semiconductor becomes dielectric after the carriers are exhausted. This semiconductor-dielectric transformation mechanism results in the peculiar current peaks in the MFSM structured capacitor and makes the
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Journal Pre-proof polarization of P(VDF-TrFE) being multistate. The feasible operation of multistate polarization can be used in multi-storage. Ferroelectric capacitors can be used in memory devices because of different polarization states driven by applied voltage. Normally, a ferroelectric film has only two polarization states which can be associated to the Boolean 0 and 1 bit. However, the polarization behavior, encountered in ferroelectric capacitors, requires a certain area of ferroelectric [42,43]. There is an applicable way to overcome the disadvantage of FeRAM’s large scale, that is increasing the storage capacity of the individual memory cell. Fig. 3(b) demonstrates the feasibility of MFSM structured capacitors for improving storage capacitor. Three polarization states can be accessed by different applied voltage ranges. At a scanning voltage ranging from -30 V to +30 V, the P(VDF-TrFE) layer can be completely polarized and has maximum polarization charges with three current peaks. From -20 V to +30 V scanning voltage, the P(VDF-TrFE) layer can be partially polarized. Because the semiconductor acts as an insulator at -20 V and small negative voltage cannot make P(VDF-TrFE) completely reversed. The P(VDF-TrFE) does not occur polarization reversal at the range from 0 V to +30 V. The appearance of recorded current peaks for different voltage ranges with different polarization charges proves the possibility of the tristate memory of MFSM structured capacitors. For convenience, hereafter we use P1, P2, and P3 for the three polarization states, which can be associated to memory states 0, 1, and 2. To demonstrate a multilevel writing and reading process, Fig. 4(a) shows an example of a pulse sequence that was used to individually write and read each polarization state. Before a writing process, a positive unipolar triangular pulse of +30 V is applied to preset polarization state. P1, P2, and P3 were respectively given by 0 V, -20 V, and -30 V of
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Journal Pre-proof unipolar triangular applied voltage pulse to set respective polarization states. A reading process is simulated by applying a unipolar triangular pulse of +30 V, which is the same as the preset voltage pulse. Therefore, the preset process is only needed at the first time, and the reading process causes a destructive reading that must be rewritten after each reading which means it have a destructive read out. Fig. 4(b) shows the I-V curves of three individual writing processes to get three different polarization states P1, P2, and P3. The essence is that the semiconductor intervention leads to polarization process of negative applied voltage in two steps. The current response corresponds to Fig. 3(b) and confirms that it is feasible to get P1, P2, and P3 separately. The writing process curve would show a wide voltage margin of the memory for stable operation. Fig. 4(c) shows the reading current of three polarization states at 0 to +30 V voltage pulse. And the charge of reading current peaks corresponds to that of writing current peaks, which means memory states can be controlled by writing voltage. The results presented above suggest that multiple polarization states originated from two-step polarization reversal at negative voltage of MSFM structured capacitor are attainable for tristate memory devices. According to the principle previously analyzed, the thicker the semiconductor is, the higher the voltage corresponding to peak B will be, that is, the larger the distance between peak A and peak B will be. However, too thick semiconductor will lead to depolarization of ferroelectric layer, so it is very important to choose the right semiconductor thickness [44]. The sizes of peak A and peak B are controlled by the carrier concentration of the semiconductor. The higher the concentration is, the higher the peak A is and the smaller the peak B is, and vice versa. It is therefore possible to exploit the fact that the semiconductor layer can control the size and location of the two split current peaks, which means the electrical properties of memory is controllable. Note that currently the problems associated with the devices are high
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Journal Pre-proof operating voltage. Nevertheless, this is only related to the characteristics of P(VDF-TrFE) material itself whose thickness can be greatly reduced by using a molecular layer deposition technique of Langmuir-Blodgett [45, 46]. Since the polarization is divided into two steps, the distinguishing abilities and retention properties could be the most serious reliability concern of this memory. Fig. 5(a) shows polarization charge of each state and the measuring device is the same as in Fig. 4. The polarizations of P1, P2 and P3 states obtained by calculating the peak area in the illustration are 0, 3.4 and 9.2 μC cm-2, respectively. The polarizations of P3 is nearly three times of that of P2 which meets the requirement of memory with a large discrimination. Fig. 5(b) shows the retention properties. Measurements were performed for retention time from 10 to 104. Due to the destructive reading process of ferroelectric devices, the data need to be rewritten after each measurement and the retention time is calculated from each writing process. The points in the graph indicate the measured data, and the dashed line shows an extrapolation based on the data measured. At atmospheric environment, the P2 remaining after 107 s (above 3 months) are expected to be 3.0 μC cm-2, and the polarization of P1 and P3 are almost constant. This may be attributed to the fact that P2, which is not fully polarized, is not in a thermodynamically stable state and depolarized more easily. In general, this structure of ferroelectric capacitors is feasible for three memory states data storage both in distinguish abilities and retention properties. 4. Conclusions A P(VDF-TrFE)/IDT-BT heterostructure thin film is presented to study the feasibility of stable multi-state and destructive read-out storage and related physical mechanism. This special thin film is used in capacitors with three polarization states as the multi-level memory for high density data storage, where the principle of three
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Journal Pre-proof polarization states is semiconductor-dielectric transformation of the semiconductor layer. The feasibility of this ferroelectric memory is demonstrated by the actual operating voltage, and the data resolution and retention ability are studied in detail. Three polarization states (0, 3.4 and 9.2 μC cm-2) are developed in this work and the retention time is expected to reach 107 s. The ferroelectric capacitance is 50% higher than that of traditional memories, which provides a solution to the scale problem of FeRAM. The experimental demonstration of stable multi-state storage and the behind physical mechanism of semiconductor-dielectric transformation in the heterostructure of P(VDFTrFE)/semiconductor shed light on the feasible multi-state memory.
Acknowledgements This work was financially supported by National Natural Science Foundation of China under Grant Nos. 61504042, 61504098, and 61771198, Natural Science Foundation of Shanghai under Grant No. 17ZR1447000, and the Shanghai “Yang-Fan Plan” for Young Scientists, China under Grant No. 14YF1403400.
References: [1] C.A.P. de Araujo, J.D. Cuchiaro, L.D. McMillan, M.C. Scott, J.F. Scott, Fatigue-free ferroelectric capacitors with platinum electrodes, Nature 374 (1995) 627. [2] K. Asadi, D.M. de Leeuw, B. de Boer, P.W.M. Blom, Organic non-volatile memories from ferroelectric phase-separated blends, Nat. Mater. 7 (2008) 547. [3] X. Liu, R. Liang, G. Gao, C. Pan, C. Jiang, Q. Xu, J. Luo, X. Zou, Z. Yang, L. Liao, Z.L. Wang, MoS2 Negative-Capacitance Field-Effect Transistors with Subthreshold Swing below the Physics Limit, Adv. Mater. 30 (2018) 1800932. [4] C.-H. Chiu, C.-W. Huang, Y.-H. Hsieh, J.-Y. Chen, C.-F. Chang, Y.-H. Chu, W.-W. Wu, In-situ TEM observation of Multilevel Storage Behavior in low power FeRAM device, Nano Energy 34 (2017) 103-110.
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Journal Pre-proof [5] D. Lee, S.M. Yang, T.H. Kim, B.C. Jeon, Y.S. Kim, J.-G. Yoon, H.N. Lee, S.H. Baek, C.B. Eom, T.W. Noh, Multilevel Data Storage Memory Using Deterministic Polarization Control, Adv. Mater. 24 (2012) 402-406. [6] V. Garcia, M. Bibes, Inside story of ferroelectric memories, Nature 483 (2012) 279. [7] L.W. Martin, A.M. Rappe, Thin-film ferroelectric materials and their applications, Nat. Rev. Mater. 2 (2016) 16087. [8] S. Horiuchi, Y. Tokura, Organic ferroelectrics, Nat. Mater. 7 (2008) 357. [9] Scott, J.F., 2000. Ferroelectric memories. Springer-Verlag. [10]M. Mai, S. Ke, P. Lin, X. Zeng, Ferroelectric polymer thin films for organic electronics, J. Nanomater. 16 (2015) 181-181. [11]M.T. Ghoneim, M.A. Zidan, M.Y. Alnassar, A.N. Hanna, J. Kosel, K.N. Salama, M.M. Hussain, Thin PZT-Based Ferroelectric Capacitors on Flexible Silicon for Nonvolatile Memory Applications, Adv. Electron. Mater. 1 (2015) 1500045. [12]J. Li, F. Li, Z. Xu, S. Zhang, Multilayer Lead-Free Ceramic Capacitors with Ultrahigh Energy Density and Efficiency, Adv. Mater. 30 (2018) 1802155. [13]N. Gong, T. Ma, Why Is FE–HfO2 More Suitable Than PZT or SBT for Scaled Nonvolatile 1-T Memory Cell? A Retention Perspective, IEEE Electron. Device L. 37 (2016) 1123-1126. [14]R.C.G. Naber, K. Asadi, P.W.M. Blom, D.M. de Leeuw, B. de Boer, Organic Nonvolatile Memory Devices Based on Ferroelectricity, Adv. Mater. 22 (2010) 933945. [15]Y. Noda, T. Yamada, K. Kobayashi, R. Kumai, S. Horiuchi, F. Kagawa, T. Hasegawa, Few-Volt Operation of Printed Organic Ferroelectric Capacitor, Adv. Mater. 27 (2015) 6475-6481. [16]X. Chen, X. Han, Q.-D. Shen, PVDF-Based Ferroelectric Polymers in Modern Flexible Electronics, Adv. Electro. Mater. 3 (2017) 1600460. [17]K. Nakayama, W. Ou-Yang, M. Uno, I. Osaka, K. Takimiya, J. Takeya, Flexible airstable three-dimensional polymer field-effect transistors with high output current density, Org. Electron. 14 (2013) 2908-2915. [18]H. Ishiwara, M. Okuyama, Y. Airmot, 2004. Ferroelectric random access memories: fundamentals and applications. Springer-Verlag.
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Journal Pre-proof [19]T. Furukawa, T. Nakajima, Y. Takahashi, Factors governing ferroelectric switching characteristics of thin VDF/TrFE copolymer films, IEEE T. Dielec. El In. 13 (2006) 1120-1131. [20]W. Ou-Yang, M. Weis, T. Manaka, M. Iwamoto, Effect of an Upward and Downward Interface Dipole Langmuir–Blodgett Monolayer on Pentacene Organic Field-Effect Transistors: A Comparison Study, J. Appl. Phys. 51 (2012) 024102. [21]A. Gerber, M. Fitsilis, R. Waser, T.J. Reece, E. Rije, S. Ducharme, H. Kohlstedt, Ferroelectric field effect transistors using very thin ferroelectric polyvinylidene fluoride copolymer films as gate dielectrics, J. Appl. Phys. 107 (2010) 124119. [22]J. Ryu, K. No, Y. Kim, E. Park, S. Hong, Synthesis and Application of Ferroelectric Poly(Vinylidene
Fluoride-co-Trifluoroethylene)
Films
using
Electrophoretic
Deposition, Sci. Rep. 6 (2016) 36176. [23]L. Baudry, I. Lukyanchuk, V.M. Vinokur, Ferroelectric symmetry-protected multibit memory cell, Sci. Rep. 7 (2017) 42196. [24]R. Dan, O. Masanori, Control of analog ferroelectric states by small dc-bias in conjunction with fluctuating waveforms, J. Phys. D. Appl. Phys. 42 (2009) 085410. [25]A. Ghosh, G. Koster, G. Rijnders, Multistability in Bistable Ferroelectric Materials toward Adaptive Applications, Adv. Funct. Mater. 26 (2016) 5748-5756. [26]W.Y. Kim, H.-D. Kim, T.-T. Kim, H.-S. Park, K. Lee, H.J. Choi, S.H. Lee, J. Son, N. Park, B. Min, Graphene–ferroelectric metadevices for nonvolatile memory and reconfigurable logic-gate operations, Nat. Commun. 7 (2016) 10429. [27]G.A. Boni, L.D. Filip, C. Chirila, I. Pasuk, R. Negrea, I. Pintilie, L. Pintilie, Multiple polarization states in symmetric ferroelectric heterostructures for multi-bit nonvolatile memories, Nanoscale 9 (2017) 19271-19278. [28]K. Asadi, P.W.M. Blom, D.M. de Leeuw, Conductance switching in organic ferroelectric field-effect transistors, Appl. Phys. Lett. 99 (2011) 053306. [29]B. Kam, X. Li, C. Cristoferi, E.C.P. Smits, A. Mityashin, S. Schols, J. Genoe, G. Gelinck, P. Heremans, Origin of multiple memory states in organic ferroelectric field-effect transistors, Appl. Phys. Lett. 101 (2012) 033304. [30]D. Zhao, I. Katsouras, K. Asadi, W.A. Groen, P.W.M. Blom, D.M. de Leeuw, Retention of intermediate polarization states in ferroelectric materials enabling memories for multi-bit data storage, Appl. Phy. Lett. 108 (2016) 232907.
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Journal Pre-proof [31]X. Zhang, H. Bronstein, A.J. Kronemeijer, J. Smith, Y. Kim, R.J. Kline, L.J. Richter, T.D. Anthopoulos, H. Sirringhaus, K. Song, M. Heeney, W. Zhang, I. McCulloch, D.M. DeLongchamp, Molecular origin of high field-effect mobility in an indacenodithiophene–benzothiadiazole copolymer, Nat. Commun. 4 (2013) 2238. [32]J. Li, D. Taguchi, T. Manaka, M. Iwamoto, Two-Step Polarization Reversal Process in Pentacene/Poly(vinylidene fluoride–trifluoroethylene) Double-Layer Capacitor: Reduced Coercive Field, J. Appl. Phys. 51 (2012) 02BK07. [33]J. Li, D. Taguchi, W. OuYang, T. Manaka, M. Iwamoto, Interaction of interfacial charge and ferroelectric polarization in a pentacene/poly(vinylidene fluoridetrifluoroethylene) double-layer device, Appl. Phys. Lett. 99 (2011) 063302. [34]X. Cui, D. Taguchi, T. Manaka, W. Pan, M. Iwamoto, Study of effect of inserted pentacene layer in ITO/P(VDF-TrFE)/α-NPD/Au capacitor using electric-fieldinduced optical second-harmonic generation and displacement current, Org. Electron. 15 (2014) 537-542. [35]L. Hu, S. Dalgleish, M.M. Matsushita, H. Yoshikawa, K. Awaga, Storage of an electric field for photocurrent generation in ferroelectric-functionalized organic devices, Nat. Commun. 5 (2014) 3279. [36]J. Li, W. Ou-Yang, M. Weis, Electric-field enhanced thermionic emission model for carrier injection mechanism of organic field-effect transistors: understanding of contact resistance, J. Phys. D. Appl Phys. 50 (2016) 035101. [37]W. Ou-Yang, N. Mitoma, T. Kizu, X. Gao, M.-F. Lin, T. Nabatame, K. Tsukagoshi, Controllable film densification and interface flatness for high-performance amorphous indium oxide based thin film transistors, Appl. Phys. Lett. 105 (2014) 163503. [38]R.C.G. Naber, J. Massolt, M. Spijkman, K. Asadi, P.W.M. Blom, D.M. de Leeuw, Origin of the drain current bistability in polymer ferroelectric field-effect transistors, Appl. Phys. Lett. 90 (2007) 113509. [39]T. Matsumoto, W. Ou-Yang, K. Miyake, T. Uemura, J. Takeya, Study of contact resistance of high-mobility organic transistors through comparisons, Org. Electron. 14 (2013) 2590-2595. [40]F. Huang, A. Liu, H. Zhu, Y. Xu, F. Balestra, G. Ghibaudo, Y. Noh, J. Chu, W. Li, Reliable Mobility Evaluation of Organic Field-Effect Transistors with Different Contact Metals, IEEE Electron Dev. Lett. 40 (2019) 605-608.
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Journal Pre-proof [41] M. Weis, J. Li, D. Taguchi, T. Manaka, M. Iwamoto, Effect of Photogenerated Carriers on Ferroelectric Polarization Reversal, Appl. Phys. Ex. 4 (2011) 121601. [42]R. Waser, A. Rüdiger, Pushing towards the digital storage limit, Nat. Mater. 3 (2004) 81. [43]M. Alexe, C. Harnagea, D. Hesse, U. Gösele, Polarization imprint and size effects in mesoscopic ferroelectric structures, Appl. Phys. Lett. 79 (2001) 242-244. [44]K. Asadi, J. Wildeman, P.W.M. Blom, D.M.d. Leeuw, Retention Time and Depolarization in Organic Nonvolatile Memories Based on Ferroelectric Semiconductor Phase-Separated Blends, IEEE T. Electron Dev. 57 (2010) 34663471. [45]W. Ou-Yang, M. Weis, X. Chen, T. Manaka, M. Iwamoto, Study of phase transition of two-dimensional ferroelectric copolymer P(VDF-TrFE) Langmuir monolayer by Maxwell displacement current and Brewster angle microscopy, J Chem Phys. 131 (2009) 104702. [46]X. Chen, W. Ou-Yang, M. Weis, D. Taguchi, T. Manaka, M. Iwamoto, Reduction of Hysteresis in Organic Field-Effect Transistor by Ferroelectric Gate Dielectric, Jpn. J. Appl. Phys. 49 (2010) 021601.
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Journal Pre-proof Figure
Fig. 1. (a) Molecular structures of P(VDF-TrFE) and IDT-BT. Switching current loops (voltage range: -30 V to 30 V) for capacitors with different structures: (b) MFM structure; (c) MFSM structure (The IDT-BT solution concentration is 3 mg ml-1).
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Fig. 2. Illustrations of current peak generation and splitting under certain voltage bias conditions. (a) I-V curve of the MFSM structured capacitor with six states denoted using circled dots (the arrows indicate the trend of curve). Diagrams of ferroelectric polarization state and carrier motion under different voltages: (b) 0 V indicated position (1) in the I-V curve, (c) +18 V indicated position (2) in I-V curves, (d) 0 V indicated position (3) in I-V curves, (e) -17.5 V indicated position (4) in I-V curves, (f) -20 V indicated position (5) in I-V curves and (g) -27 V indicated position (6) in I-V curves.
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Fig. 3. I-V curves of MFSM structured capacitors at different scanning voltage regimes. (a) The black square indicates that the voltage scanning range is -40 V to +40 V, red circle demonstrates -50 V to + 50 V, blue triangle points out -60 V to +60 V and pink inverted triangle shows -70 V to + 70 V. (b) The black square indicates that the voltage scanning range is -30 V to +30 V, red circle indicates -20 V to +30 V, and blue triangle indicates 0 V to +30 V. The IDT-BT solution concentration of the device in (a) is 4 mg ml-1, while the concentration in (b) is 3 mg ml-1.
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Fig. 4. (a) Schematic diagram of obtaining different polarization states by setting triangular voltage pulses. Before every writing operation, the devices need to give a +30 V preset voltage pulse. Three different polarization states are written by 0 V, -20 V and 30 V. Reading pulse is the same as the preset pulse. (b) Polarization current of three different polarization states by writing sweep voltage. (c) Polarization current of three different polarization states at 0 to +30 V reading sweep voltage. Here, the concentration of IDT-BT solution is 3 mg ml-1.
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Fig. 5. (a) Integrated polarization charges obtained after reading each polarization state. Inset shows the current-time curves for three different polarization states. (b) Retention properties of three polarization states.
(All pictures print in black and white.)
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List of changes 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.
28. 29. 30. 31.
Remove “(FeRAM)”, “by researchers”, “(DRAM)”, “(SRAM)” in P2L5 to P2L8. Update “by” to “using a”, “due” to “owing” in P2L13 to P2L14. Update “polarization” to “polarizations” in P2L20. Add “and” and “respectively” in P2L21.” Update “concentrated” to “made” in P4L8. Add “, both of which are prepared …and possible for printed electronics.” In P4L21 to P4L24. Update “which increase 50% capacity in the same area compared” to “a 50% increase in areal capacity compared” in P5L4. Add “The molecular structures of P(VDF-TrFE) and IDT-BT are shown in Fig. 1(a),” in P5L8. Update “Preparation” to “and preparation” in P5L8 to P5L9. Add “by sonicating” in P5L11 to P5L12. Add experimental details: “(3000 rpm, 30 s)”, “(purchased from Derthon Optoelectronic Materials Science Technology Co LTD, China)”, “(3 ~ 4 mg ml-1)”, “(2500 rpm, 30 s)” in P5L15 to P5L20. Add “illustrated in the inset of Fig. 1(b)” in P5L24. Update “mm-2” to “mm2” in P6L2. Update “Fig. 1” to “Fig. 1(b-c)” in P6L7. Remove “metal-ferroelectric-metal (”, “)”, “metal-ferroelectric-semiconductor-metal (” and “)” in P6L7 to P6L8. Update “mm-1” to “μm-1” in P6L11. Add references “35” in P6L12. Add discussion about genetic cause and stability of splitting peak (“In Fig. 3(a), I-V curves of … in semiconductor materials and thickness.”) in P7L5 to P7L12. Update “showed” to “shown” in P7L21. Add “the” in P7L23. Add discussion about the influence of illumination (“This semiconductor-dielectric conversion … ferroelectric inversion process of MFSM structure.”) in P8L12 to P8L21. Add “To demonstrate a multilevel writing and reading process,” in P9L22. Add destructive reading comments (“Therefore, the preset process is only … it has a destructive read out.”) in P10L3 to P10L5. Add “However, too thick semiconductor will lead to depolarization of ferroelectric layer, so it is very important to choose the right semiconductor thickness [44].” in P10L18 to P10L20. Add “that the semiconductor layer can control the size and location of the two split current peaks,” in P10L23 to P10L24. Add “and the measuring device is the same as in Fig. 4.” in P11L6 to P11L7. Add “Due to the destructive reading process of ferroelectric devices, the data need to be rewritten after each measurement and the retention time is calculated from each writing process.” In P11L11 to P11L13. Add “destructive read-out” in P11L22. Add “is expected to” in P12L5. Revise the authors of [20] (“W. Ou-Yang, M. Weis, T. Manaka, M. Iwamoto”) in P14L4 Revise the authors of [32] (“J. Li, D. Taguchi, T. Manaka, M. Iwamoto,”) in P15L5.
Journal Pre-proof 32. Add references: “[28] K. Asadi, P.W.M. Blom, D.M. de Leeuw, Conductance switching in organic ferroelectric field-effect transistors, Appl. Phys. Lett. 99 (2011) 053306. [29] B. Kam, X. Li, C. Cristoferi, E.C.P. Smits, A. Mityashin, S. Schols, J. Genoe, G. Gelinck, P. Heremans, Origin of multiple memory states in organic ferroelectric field-effect transistors, Appl. Phys. Lett. 101 (2012) 033304. [35] L. Hu, S. Dalgleish, M.M. Matsushita, H. Yoshikawa, K. Awaga, Storage of an electric field for photocurrent generation in ferroelectric-functionalized organic devices, Nat. Commun. 5 (2014) 3279. [38] R.C.G. Naber, J. Massolt, M. Spijkman, K. Asadi, P.W.M. Blom, D.M. de Leeuw, Origin of the drain current bistability in polymer ferroelectric field-effect transistors, Appl. Phys. Lett. 90 (2007) 113509. [41] M. Weis, J. Li, D. Taguchi, T. Manaka, M. Iwamoto, Effect of Photogenerated Carriers on Ferroelectric Polarization Reversal, Appl. Phys. Ex. 4 (2011) 121601. [44] K. Asadi, J. Wildeman, P.W.M. Blom, D.M.d. Leeuw, Retention Time and Depolarization in Organic Nonvolatile Memories Based on Ferroelectric Semiconductor Phase-Separated Blends, IEEE T. Electron Dev. 57 (2010) 3466-3471.” 33. Add fig. 1(a) in P17L1. 34. Add “(a) Molecular structures of P(VDF-TrFE) and IDT-BT.” and “(The IDT-BT solution concentration is 3 mg ml-1)” in P17L3 to P17L5. 35. Add fig. 3(a) in P19L1. 36. Update “(a) The black square indicates … while the concentration in (b) is 3 mg ml-1.” in P19L4 to P19L9. 37. Add “Here, the concentration of IDT-BT solution is 3 mg ml-1.” in P20L8 to P20L9.
Journal Pre-proof Dear reviewer, Poly (vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) is one of the most promising organic ferroelectric materials with advantageous properties, and it could be used in ferroelectric random-access memory (FeRAM) as a dielectric. While the performance of P(VDF-TrFE) film deteriorates with decrease of the film thickness, such as the coercive electric field and leakage current. Due to this unreliability, truly practical miniaturized ferroelectric memory has not been developed to date. Another way to solve this problem of scaling down the feature size of a ferroelectric memory is increasing the number of memory states per memory cell, which means the devices of the same size can store more information. Here we use a heterostructure of P(VDF-TrFE)/semiconductor to realize a stable multi-state memory. There are three main highlights: 1. We research the transfer mechanism of carriers in P(VDF-TrFE)/organic semiconductor heterostructure capacitors, and propose a concept of semiconductor-dielectric conversion. This mechanism makes the polarization states of P(VDF-TrFE) become multi-state, and the retention time is extremely long. 2. The stability and repeatability of this the polarization polymorphism phenomenon have been proved by experiments. And according to this phenomenon, we study the performance of this heterostructure as multi-state memory. 3. The semiconductor layer and ferroelectric layer was prepared by using a spin-
Journal Pre-proof coting method, which is different from most of previous works using a vacuum deposited way.