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Speed Up Ferroelectric Organic Transistor Memories by Using Two-dimensional Molecular Crystalline Semiconductors Lei Song, Yu Wang, Qian Gao, Yu Guo, Qijing Wang, Jun Qian, Sai Jiang, Bing Wu, Xinran Wang, Yi Shi, Youdou Zheng, and Yun Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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ACS Applied Materials & Interfaces

Speed Up Ferroelectric Organic Transistor Memories by Using Two-dimensional Molecular Crystalline Semiconductors Lei Song,‡ Yu Wang, ‡ Qian Gao, Yu Guo, Qijing Wang, Jun Qian, Sai Jiang, Bing Wu, Xinran Wang, Yi Shi*, Youdou Zheng, and Yun Li* National Laboratory of Solid-State Microstructures, School of Electronic Science and Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China KEYWORDS: 2D molecular crystals, ferroelectric organic field-effect transistor memory, nonvolatile, high-speed organic transistor memory, solution processed

ABSTRACT: Ferroelectric organic field-effect transistors (Fe-OFETs) have attracted intensive attention because of their promising potentials in non-volatile memory devices. The quick switching between binary states is a significant fundamental feature in evaluating Fe-OFET memories. Here, we employ 2D molecular crystals via a solution-based process as the conducting channels in transistor devices, in which ferroelectric polymer acts as the gate dielectric. A high carrier mobility of up to 5.6 cm2 V−1 s−1 and a high on/off ratio of 106 are obtained. In addition, the efficient charge injection by virtue of the ultrathin 2D molecular crystals is beneficial in achieving rapid operations in the Fe-OFETs; devices exhibit short

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switching time of ~2.9 and ~3.0 ms from the on- to the off-state and from the off- to the on-state, respectively. Consequently, the presented strategy is capable of speeding up Fe-OFET memory devices by using solution-processed 2D molecular crystals.

1. INTRODUCTION Organic field-effect transistors (OFETs), in which the polymeric ferroelectric of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] acts as the gate insulator, have been of great interest for their promising applications in non-volatile memory device, because of their advantages, such as simple processing and excellent stability.1−8 Improved operating speed in programming and reading is important for the development of ferroelectric OFETs (Fe-OFETs). Nevertheless, using P(VDF-TrFE) as an insulator layer to fabricate Fe-OFET memories generally induces a slower switching behavior between the two conductance states than that in inorganic memories.9,10 This issue can be addressed by enhancing the charge carrier mobility of the channel materials.11−14 In our previous work, we reported an improved operating speed with a switching time of tens of milliseconds when switching between data states in Fe-OFETs.15 However, the value of the charge carrier mobility should exceed the limit of organic semiconducting materials to improve the operating speed further. Note that switching between the binary states of Fe-OFETs causes charge accumulation or depletion in the channel.9,10 Therefore, the contact resistance during charge injection is also a key factor in the improvement of the operation speed of Fe-OFETs.16,17 Contact resistance generally includes the resistance regarded with the injection of charge carrier at the metal/semiconductor interface and the access resistance from the metal/semiconductor interface to the semiconducting region. Our recent work revealed that the conducting channel of 2D molecular crystals is necessary in reducing contact


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resistance, which occurs when the access resistance significantly decreases because the ultrathin crystals only have several molecular layer structures.18 Therefore, 2D semiconducting molecular crystals are used to guarantee efficient charge injections in Fe-OFETs and thus realize highspeed memory devices.18−21 Herein, we propose a strategy for the fabrication of high-speed Fe-OFETs by using solutionprocessed 2D molecular crystals as semiconducting channels. The devices yield high carrier mobility up to ~5.6 cm2 V−1 s−1 and short switching time to only several milliseconds when switching between binary conductance states. The enhanced operation speed of our Fe-OFETs is attributed to the use of 2D molecular crystals as channel materials because their molecular layer structures ensure the efficient charge injection from metal contacts. Therefore, our results confirm that this approach can contribute to the realization of high-speed Fe-OFET memory devices. 2. EXPERIMENTAL DETAILS Fabrication of Fe-OFET Memories. Thermal evaporation of Au through a shadow mask at a deposition speed of ~0.1 Å/s was used for the formation of gate electrodes with a thickness of 30 nm on pre-cleaned SiO2/Si substrate. Then, the polymeric ferroelectric of P(VDF-TrFE) (the ratio between VDF and TrFE was 70:30) was used as received from Solvay, Inc. It was dissolved in a methyl ethyl ketone solution with the concentration of 3 wt.%. The deposition of P(VDFTrFE) was carried out in a nitrogen-filled glovebox under the room temperature. The solution was spin-coated on the substrate at 500 rpm for 5 s and 1500 rpm for 30 s. The sample was softbaked on a hotplate at 90 °C for 30 min, and then was annealed at 130 °C for 2 h. After that, a P(VDF-TrFE) film with a thickness of ~300 nm in polycrystalline phase were obtained. The small-molecule semiconductor dioctylbenzothienobenzothiophene (C8-BTBT) was received from


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Nippon Kayaku Co. It was dissolved in a mixture solvent with anisole (Sigma-Aldrich) and immiscible p-anisaldehyde (0.5 wt.%) that served as a good solvent and antisolvent, respectively. Different PMMA (Sigma-Aldrich MW = 996 k) concentrations (0.01 wt.%, 0.05 wt.%, 0.08 wt.%, 0.1 wt.%, 0.25 wt.%, 0.5 wt.%, and 1.0 wt.%) were simultaneously dissolved in the solution. The preparation of 2D C8-BTBT crystals were performed under the ambient condition. A droplet of the solution was casted onto the P(VDF-TrFE) surface; and then, an airflow produced by a mechanical pump led to a movement of the droplet at ~6 mm s−1. And C8-BTBT crystals with 2D features were deposited during this solution-based process. Finally, the source and drain electrodes with thicknesses of 30 nm were formed using thermal evaporation of Au through a shadow mask. The channel width and length for our Fe-OFETs were 1000 and 250 µm, respectively. Fabrication of Au/PMMA/P(VDF–TrFE)/Au Capacitor. The bottom electrode of Au with a thickness of 30 nm was thermally evaporated on the pre-cleaned SiO2/Si substrate. Then, the P(VDF-TrFE) layer was spin-coated on the sample. A double layer structure near the solution edge emerged during the evaporation-driven flow of the good solvent, with PMMA underneath the C8-BTBT. The solvent of cyclohexane was used to remove the C8-BTBT and to expose the underneath PMMA/P(VDF-TrFE) surface. Finally, the top electrode was deposited using thermal evaporation under the same conditions (see Figure S1 in the Supporting Information). Film Characterizations. The atomic force microscopy (AFM) measurements of P(VDFTrFE), C8-BTBT molecular crystals with various PMMA concentrations were performed using a scanning probe microscope (SPA-400) controlled by SPI (4000 probe station, Seiko Instruments, Inc.).


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Electrical Characterizations. A KEYSIGHT B1500A semiconductor device analyzer was used for the electrical characterizations of our devices under the ambient conditions. An AGILENT E4980A precision LCR parameter analyzer was employed in the C-V measurements. The C/G versus voltage frequency was tested using a HIOKI IM3570 impedance analyzer. 3. Results and Discussion 3.1. Solution-based Deposition and Characterizations of 2D Molecular Crystals on P(VDF-TrFE). We used C8-BTBT for the p-type semiconducting channels in our OFETs.22−24 The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of C8-BTBT are −5.4 eV and −1.6 eV, respectively. In our previous work, we successfully fabricated 2D molecular crystals with large-area coverage on the SiO2/Si substrate and a ultra-smoothness at the atomic level via the floating-coffee-ring-driven assembly.18 To deposit 2D molecular crystals on the ferroelectric polymer P(VDF-TrFE) surface, we used a mixture of C8-BTBT and poly(methyl methacrylate) (PMMA), rather than sole C8-BTBT, in the anisole solution with a small amount of antisolvent p-anisaldehyde (0.5 wt.%) (Figure 1a). The addition of a small amount of PMMA in the solution is important to ensure that 2D molecular crystals are successfully deposited onto the rough ferroelectric layer, because the vertical phase separation during the solution-base process can form a double layer structure with ~10 nm of PMMA underneath the C8-BTBT.25 Figure 1b shows the optical microscopy image of deposited 2D molecular crystals with a high uniformity over a large size of ~800 µm on top of the ferroelectric P(VDF-TrFE) layer. An annealing treatment at ~130 °C was necessary for the formation a P(VDF-TrFE) layer with a highly crystalline phase that enhances its ferroelectric property (Figures 1c and 1d) and results in a relatively high roughness of ~5.5 nm. The AFM measurements were employed for the


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characterizations the morphological properties of the 2D molecular crystals deposited from the mixture solution of C8-BTBT and 0.1 wt.% PMMA. Figures 1e and f show the morphology and phase images of the 2D C8-BTBT crystals, respectively. Both images exhibit the 2D crystals because the grains of the P(VDF-TrFE) layer are clearly visible. The yellow dotted line in Figure 1f marks the boundary of P(VDF-TrFE) with and without C8-BTBT coverage. Moreover, the AFM images show that the C8-BTBT molecular crystals are smoothly spread out on the crystalline P(VDF-TrFE) layer and yield a good C8-BTBT/PMMA surface. Besides, the measured thickness was ~8.2 nm, which indicates the trilayer molecular structure of the 2D C8BTBT molecular crystals. Furthermore, high-resolution AFM characterizations were performed to further examine the crystalline properties of the 2D C8-BTBT crystals. Figures 1g–h show the obtained image and the corresponding fast Fourier transform (FFT), revealing that the upper molecular layer has an oblique unit cell geometry that exhibits lattice constants of a = 6.19 ± 0.13 Å, b = 8.12 ± 0.11 Å, and θ = 79.5 ± 1.5°. The lattice observed in the image is slightly curved rather than straight, which is due to the relatively large roughness of the P(VDF-TrFE) surface underneath. Thus, our 2D C8-BTBT molecular crystals were deposited well on P(VDFTrFE), which ensures a good interface between the semiconductor and the dielectric layers. 3.2. Fe-OFET Memory Devices using 2D C8-BTBT Crystals. We fabricated a bottom-gate top-contact architecture for the Fe-OFET device with 2D C8-BTBT molecular crystals (Figure 2a). Figure 2b shows a typical transfer curve with the gate voltage (VG) sweeping from ~15 V to 15 V and then backwards, exhibiting a current hysteresis with a clockwise direction. And the the leakage current of the P(VDF-TrFE)-PMMA dielectric is as low as 10−11 A. Hence, such a charge-switching behavior mainly results from the dipolar polarization in the P(VDF-TrFE) layer rather than the charge trapping in the conducting channel.26 In addition, we performed a


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capacitance versus voltage measurement on a capacitor with an Au-PMMA/P(VDF-TrFE)-Au structure. As shown in Figure S1 in the Supporting Information, the capacitor exhibits a butterfly-shaped hysteresis, which is due to the ferroelectric polarization in the P(VDF-TrFE) layer. Furthermore, we used the transfer curves with VG sweeping in a reverse manner for the estimation of carrier mobility, in order to exclude the dipole switching in the ferroelectric.15,27 The extracted charge carrier showed a high mobility of 5.6 cm2 V−1 s−1; and the gate capacitance was measured at a low-voltage-frequency of 20 Hz. Therefore, when VG was set to ~1.8 V, our FET device exhibited two states, namely, the off- and on-states with different electrical conductions. The on/off ratio and memory window estimated from the transfer curve were ~106 and ~12 V, respectively, which indicate good memory performance. Moreover, a decrease in the curve of capacitance/conduction versus frequency from ~2000 Hz indicates a short reading time of ~0.5 ms (Figure 2c). Especially in the measurement where a gate voltage was added to switch between the binary states, the device exhibited short switching time of ~2.9 and ~3.0 ms from the on- to the off-state and from the off- to the on-state, respectively (Figure 2d and e). And the value of the delay time was estimated as the period in which the change of the drain current from one conduction state with a stable current to the other. Such short switching time represents a quick operation speed. To the best of our knowledge, our devices possess record-high values of both operation speed and carrier mobility for Fe-OFET memories (see Table S1 in the Supporting Information).3,14,15 Furthermore, we performed the retention measurements of the currents at the on- and off-states for 106 s (Figure 2f). The currents were both well maintained during the retention test, which indicates the good non-volatile property of our Fe-OFET memories. Besides, the Fe-OFET memory device maintained a good performance after 100


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cycles (see Figure S2 in the Supporting Information) and thus revealed the good stability of our Fe-OFET memories. 3.3. Influences of PMMA on Fe-OFETs. As mentioned above, the addition of PMMA into the solution containing the small-molecule semiconductor of C8-BTBT is critical to the formation of 2D C8-BTBT crystals on the rough surface of P(VDF-TrFE). To investigate the role of PMMA, we deposited 2D C8-BTBT molecular crystals from the mixture solutions on the crystalline P(VDF-TrFE) layer using different PMMA concentrations (from 0.08 wt.% to 1.0 wt.%). The solvent of cyclohexane was used to rinse the samples to remove the C8-BTBT layer only. The AFM characterizations on this surface showed that the PMMA surface was exposed after rinsing (Figures 3a–j). The polycrystalline P(VDF-TrFE) surface initially has a large rootmean-square (RMS) roughness of ~5.5 nm. The deposition of C8-BTBT from the solution without PMMA onto the P(VDF-TrFE) surface resulted in no field-effect performance coupling with a large gate leakage current. This is due to the penetration of semiconducting molecules into the gate dielectric underneath, which leads to the formation of conducting paths from the gate to the source/drain electrodes. However, the addition of PMMA filled the voids in the P(VDF-TrFE) and thus reduced the values of RMS roughness at the PMMA/P(VDF-TrFE) surface compared with those of pure P(VDF-TrFE). The RMS roughness of the PMMA/P(VDF-TrFE) surface was reduced from 3.6 to 2.7 nm by increasing the PMMA concentration from 0.08 wt.% to 0.1 wt.%. Particularly, in the sample deposited from the solution with 0.1 wt.% PMMA, the grain boundaries among the P(VDF-TrFE) crystalline grains were entirely filled by PMMA, thus forming a much smooth surface. A further increase in the PMMA concentration resulted in larger surface roughness and degraded the device performance. The RMS roughness rebounded from


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3.3 to 3.8 nm when the PMMA concentration continuously increased from 0.25 wt.% to 1.0 wt.% (Figures 3e–j). Subsequently, to study the influence of the PMMA concentration on the memory device performance, the average device performance out of 10 samples for each concentration was evaluated. The estimated values of carrier mobility and memory window of the devices with the PMMA concentration of 0.08 wt.% are 0.3 ± 0.1 cm2 V−1 s−1 and 2 ± 0.3 V, respectively (Figure 4a). Devices can hardly exhibit any memory performance with clockwise hysteresis when the PMMA concentration becomes lower. And the Fe-OFETs fabricated from the solution with 0.1 wt.% PMMA yielded much enhanced electrical performance, exhibiting a high carrier mobility of 2.9 ± 0.3 cm2 V−1 s−1 and a large memory window of 12 ± 2 V (Figure 4b). Further increase in the PMMA concentration resulted in the degradation in both the carrier mobility and memory window (Figure 4c-e). Note that the carrier mobility shows a similar change with that of the PMMA/P(VDF-TrFE) surface roughness (Figure 4f). The results thus indicate that the P(VDFTrFE) surface with PMMA coverage is critical for the electrical performance of our Fe-OFET memories. According to our previous results, the PMMA inserted between the P(VDF-TrFE) and C8-BTBT

layers

can

efficiently

suppress

the

polarization

fluctuation

at

the

semiconductor/ferroelectric interface, and thus enhances the charge transport at the conducting region.15 Finally, we examined the operation speeds of our Fe-OFET memories with different PMMA concentrations by measuring the switching time as the devices switch between the binary states. As shown in Figure 5a, the Fe-OFET memory using 2D C8-BTBT crystals deposited from the solution with a 0.1 wt.% PMMA concentration presents a rapid operation, exhibiting the shortest switching time of ~2.9 and ~3.0 ms from on- to off-state and from off- to on-state, respectively.


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Further increase in the PMMA concentration generally cause longer switching time between binary states (Figure 5b-d). Table 1 summarizes the PMMA/P(VDF-TrFE) surface roughness, memory window, carrier mobility, and the switching time between binary states for the FeOFETs fabricated using different PMMA concentrations. In comparison, in our previous work, Fe-OFETs using 30-nm-thick polycrystalline C8-BTBT yielded a charge carrier mobility of 4.6 cm2 V−1 s−1, which is similar as that obtained in this work; and the switching time between the binary conductance states was ~30 ms. Note that the charge accumulation or depletion in the channel is determinant for the switching between the binary states in a transistor memory.28 Thus, the enhancement in the operation speed in our devices is attributed to the ultrathin thickness of 2D molecular crystals that can ensure an efficient charge injection from the metal contact to the conducting channel by greatly reducing the access resistance.29,30 4. CONCLUSION In conclusion, we proposed a novel Fe-OFET using 2D molecular crystals and P(VDF-TrFE) as the conducting channel and the ferroelectric layer, respectively. The 2D molecular crystals were deposited from a mixture solution of C8-BTBT and PMMA. Investigations show that the addition of PMMA into the solution is beneficial in improving the surface morphology of the ferroelectric layer and the charge carrier transport at the semiconducting channel. The Fe-OFET memory device exhibits a high charge carrier mobility of 5.6 cm2 V−1 s−1 and a quick switching with only several milliseconds of switching time between the binary states. The improvement in the operation speed of Fe-OFETs is mainly attributed to the application of 2D molecular crystals that inherently possess low access resistance. Moreover, since P(VDF-TrFE) is the most commonly used ferroelectric polymer for Fe-OFET memories, our results provide a promising strategy for the future development in the field of Fe-OFETs; also, we anticipate our work to


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help advance solution-processed 2D semiconducting molecular crystals in electronic applications.


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Figure 1. (a) Schematic illustration for the solution-based deposition of 2D C8-BTBT molecular crystals on the P(VDF-TrFE) layer. (b) Optical microscopy image of a 2D C8-BTBT molecular crystal deposited on the surface of annealed P(VDF-TrFE). Scale bar is 125 µm. (c-d) Atomic force microscopy (AFM) morphology and phase images of the P(VDF-TrFE) film. (e-f) 2D C8BTBT crystals deposited from a mixture solution with PMMA on P(VDF-TrFE). The yellow dotted line in (f) marks the boundary between the regions with and without the coverage of 2D C8-BTBT crystals. Scale bars in (c-f) are all for 500 nm in length. (g-h) Morphology and phase images of the high-resolution AFM characterizations, showing the molecular packing of 2D C8BTBT crystals on the P(VDF-TrFE) layer. Inset in (h) shows the corresponding fast Fourier transforms. Scale bars in (g-h) are both for 1 nm.


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Figure 2. (a) Schematic illustration of a Fe-OFET with a bottom-gate top-contact structure. (b) Typical transfer curve of the Fe-OFET with 0.1 wt.% PMMA. Inset shows the optical microscopy image of a fabricated Fe-OFET (scale bar is 400 µm). (c) Dependence of capacitance divided by the channel conductance versus the frequency of gate voltage. (d-e) Switching behaviors between the binary states of the Fe-OFET memory from on- to off-state and from offto on-state, respectively. (f) Retention measurements of the currents at the on- and off-states.


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Figure 3. (a-j) Atomic force microscopy (AFM) morphology and phase images of PMMA/P(VDF-TrFE) films with different PMMA concentrations on the SiO2/Si substrate. Scale bars are all for 500 nm in length.


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Figure 4. Transfer characteristics of Fe-OFETs with different PMMA concentrations, (a) 0.08 wt.%, (b) 0.1 wt.%, (c) 0.25 wt.%, (d) 0.5 wt.%, and (e) 1.0 wt.%. (f) Summary of the values of carrier mobilities and the RMS roughnesses of PMMA/P(VDF-TrFE) surface at different PMMA concentrations.


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Figure 5. Switching behaviors between the binary states of our Fe-OFET memories with different PMMA concentrations of (a) 0.1 wt.%, (b) 0.25 wt.%, (c) 0.5 wt.%, and (d) 1.0 wt.%.


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Table 1. Summary of the performances of Fe-OFET memories under different PMMA concentrations.

memory window (V)

carrier mobility (cm2V-1s-1)

3.6

2 ± 0.3

0.3 ± 0.1

0.1

2.7

12 ± 2

0.25

3.3

0.5 1

PMMA concentration (wt.%)

PMMA/P(VDF-TrFE) roughness (nm)

0

5.5

0.01

4.9

0.05

4.2

0.08

switching time (ms) on to off

off to on

2.9 ± 0.3

2.9

3.0

9±2

1.1 ± 0.2

3.9

3.9

3.5

8±1

0.4 ± 0.1

3.9

4.9

3.8

5±1

0.2 ± 0.1

3.8

4.0


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ASSOCIATED CONTENT Supporting Information. Capacitance versus bias voltage measurement of an Au/PMMA/P(VDF-TrFE)/Au sample. Programming cycles of Fe-OFETs. This material is available free of charge at

http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected]. Author Contributions ‡These authors contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. ACKNOWLEDGMENTS This study was supported partially by 973 projects under Grant Nos. 2013CBA01600 and 2013CB932900, NSFC under Grant Nos. 61574074, and Open Partnership Joint Projects of NSFC-JSPS Bilateral Joint Research Projects under Grant No. 61511140098. REFERENCES (1)

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Speed up Ferroelectric Organic Transistor ... - ACS Publications

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