pJ-Level Energy-Consuming, Low-Voltage Ferroelectric Organic Field

Apr 24, 2019 - Ferroelectric organic field-effect transistors (Fe-OFETs) have attracted considerable attention because of their promising potential fo...
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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

pJ-Level Energy-Consuming, Low-Voltage Ferroelectric Organic Field-Effect Transistor Memories Mengjiao Pei, Jun Qian, Sai Jiang, Jianhang Guo, Chengdong Yang, Danfeng Pan, Qijing Wang, Xinran Wang, Yi Shi, and Yun Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00864 • Publication Date (Web): 24 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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pJ-Level Energy-Consuming, Low-Voltage Ferroelectric Organic Field-Effect Transistor Memories Mengjiao Pei,‡ Jun Qian,‡ Sai Jiang, Jianhang Guo, Chengdong Yang, Danfeng Pan, Qijing Wang,* Xinran Wang, Yi Shi, and Yun Li* National Laboratory of Solid-State Microstructures, School of Electronic Science and Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, Jiangsu 210093, P. R. China. AUTHOR INFORMATION Corresponding Author *E-mail:

[email protected]; [email protected].

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ABSTRACT Ferroelectric organic field-effect transistors (Fe-OFETs) have attracted considerable attention because of their promising potential for memory applications, while a critical issue is the large energy consumption as mainly caused by a high operating voltage and slow data switching. Here, we employ ultrathin ferroelectric polymer and semiconducting molecular crystals to create lowvoltage Fe-OFET memories. Devices only require a pJ-level energy consumption. The writing and erasing processes require ~1.2 and 1.6 pJ/bit, respectively, and the reading energy is ~1.9 pJ/bit (on state) and ~0.2 fJ/bit (off state). Thus, our memories only consume < 0.1% of the energy required for devices using bulk functional layers. Besides, our devices also exhibit low contact resistance and steep subthreshold swing. Therefore, we provide a strategy that opens up a path for Fe-OFETs towards emerging applications, such as wearable electronics.

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Organic electronic devices have already made significant success in the emerging applications, such as wearable technologies, that have critical requirements in low-energy consumption under battery-supplied operation.1 When using ferroelectric polymers as the gate insulators, organic transistors can be functionalized as nonvolatile memories (NVMs), possessing inherent merits of simple low-temperature fabrication from solution and nondestructive readout.2–5 However, as a long-term issue, these devices usually consume a much large energy as mainly caused by a high operating voltage and slow switching between binary data. This high energy consumption prevents the ferroelectric organic field-effect transistors (Fe-OFETs) from advancing strikingly towards circuits for wearable applications. Recently, solution-based techniques have been well demonstrated to deposit organic films with ultrathin thickness, even down to a 2D limit with good uniformity over a large area.6–9 Subsequently, for the ferroelectric layer, ultrathin thickness and reliable ferroelectric properties are potentially beneficial for low-voltage operations of memory devices. Moreover, 2D semiconducting crystals with only several molecular layers can efficiently enhance the charge injection, speeding up the data switching that corresponds to the carrier accumulation and depletion in semiconducting channels.10,11 Additionally, heat dissipation during the reading process also can be limited, contributing to good device stability. Consequently, ultrathin organic functional films are promising for the realization of ultralowenergy Fe-OFET memories under low-voltage operation but have not been previously investigated. Herein, we propose a strategy for the fabrication of Fe-OFETs using ultrathin ferroelectric polymer and semiconducting molecular crystalline films as gate dielectrics and conducting channels, respectively. Our devices exhibit a pJ-level energy consumption, that is, ~1.2, 1.6, 1.9 pJ/bit, and 0.2 fJ/bit for the writing, erasing, reading on- and off-state, respectively. Such an

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ultralow energy is attributed to a low operating voltage and a quick switching between the binary states. Therefore, our results offer a great opportunity to Fe-OFETs for ultralow-energy electronics with battery-supplied operations. We employed a hybrid dielectric layer consisting of an ultrathin P(VDF-TrFE) film on a 5-nm high-κ Al2O3 layer to ensure its application in a low-voltage transistor device with wellsuppressed leakage current (Figures S1,2). The P(VDF-TrFE) film was deposited from solution using the antisolvent-assisted crystallization technique (Figure S3).12–15 We dissolved P(VDFTrFE) in N,N-dimethylformamide (DMF) at a 0.5 wt. % concentration, where a small amount of p-anisaldehyde (~5 mg mL−1) was added as the antisolvent. The solution of ~4 μL was drop-cast onto the substrate, and a pipe attached with a flat glass plate was placed close to the upper surface of the droplet. It generated airflow that dragged the droplet to move rapidly along one direction on the substrate (Figure 1a). As the solution edge moved, P(VDF-TrFE) films were obtained after the solvent evaporated at room temperature. As shown in the optical microscopy image, the deposited P(VDF-TrFE) film can be clearly distinguished on the substrate (Figure 1b). The atomic force microscopy (AFM) measurements were performed to characterize the film morphology. The P(VDF-TrFE) layer is as thin as ~3.3 ± 0.2 nm, exhibiting a smooth surface with a root-mean-square roughness of ~0.73 nm (Figures 1c,d). The film is highly uniform over a large area of ~800 µm (Figure S4a). AFM measurements were performed at different areas taken randomly, and the root-mean-squared (RMS) roughness were all less than 1 nm (Figure S4). The ultrasmooth films can contribute to a small leakage current, which is beneficial for a good reliability in ferroelectric functional elements. Furthermore, we carried out micro-Raman spectroscopy measurements to characterize the crystalline properties of these ultrathin films. In the spectral profile, the peak at 806 cm−1 can be associated with the symmetric CF2 stretching

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modes in α-phase P(VDF-TrFE) (Figure 1e).16 The negligible peak of β-phase at 840 cm−1 in the Raman spectrum is attributed to the pure α-polymorph structure of our ultrathin P(VDF-TrFE) films. The intensity, position, and shape of the obtained spectra peak are all consistent with the results in our previous work.17 The result is further confirmed by grazing-incident X-ray diffraction (GIXRD) measurements. As illustrated in Figure 1f, the ultrathin ferroelectric films yield typical peaks at 2θ = 17.12° and 20.12°, corresponding to the diffractions in planes (020) and (110), respectively. Thus, the solution-processed ultrathin P(VDF-TrFE) films with high uniformity are well-crystallized in the α-polymorph structure.18

Figure 1. (a) Schematic illustration of the solution-based deposition of ultrathin P(VDF-TrFE) films on an Al2O3 substrate. (b) Optical microscopy and (c) AFM height images of the P(VDFTrFE) film. (d) AFM surface topography image of the P(VDF-TrFE) film. (e) Raman spectrum of the sample in Figure 1b. (f) 2θ scan image taken from GIXRD measurements.

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Encouraged by the well-crystallized P(VDF-TrFE) films on Al2O3, we next explored the ferroelectric properties. Electric fields of variable amplitude (among ±40, ±80, and ±120 mV/nm) were applied to induce polarization switching. Full hysteresis loops were achieved as the electric field increased to ±120 MV/m. Clear phase hysteresis and a butterfly-like amplitude shape can be observed in Figures 2a,b. The coercive fields are approximately +58 and −53 MV/m, which are comparable to the values in bulk flims.19 After poling the left- and right-hand regions with external stimulus, electric fields of ±120 MV/m, the resulting domain structures were visualized by a PFM tip scanning over the entire prepoled areas. A large phase difference of ~180° and a notable mechanical strain of ~36 pm can be observed (Figures 2c,d), indicating that the polarized states of the ultrathin P(VDF-TrFE) film can be switched up or down. Furthermore, we evaluated the dielectric properties in the capacitor structure of Au/P(VDFTrFE)/Al2O3/n++ Si. The measured capacitance was 0.7 μF/cm2 at a voltage frequency of 10 Hz. A small change of only ~0.15 μF/cm2 occurred as the applied voltage frequency increased from 10 Hz to 1 MHz (Figure 2f), which is attributed to the low surface trap density of the Al2O3 layer.20 Thus, the hybrid dielectric materials exhibit good dielectric properties, which are favorable for applications in transistor memories.

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Figure 2. Polarization properties of the ultrathin P(VDF-TrFE) film. (a) Local PFM phase curves with an unambiguous 180° phase shift. The insets show the molecular structures corresponding to the two polarization states. (b) Local PFM amplitude curves. The insets show the stretched and compressed configurations of the ultrathin film. (c) PFM out-of-plane phase images and (d) amplitude images of the ultrathin film. (e) Dependence of capacitance versus the frequency of gate voltage. The insert is the schematic illustration of the compound dielectrics.

Ultrathin semiconducting crystalline films featuring highly ordered conjugated molecule packing through van der Waals forces in few-layered structures are of considerable interest as a promising

class

of

materials

for

high-performance

electronic

applications.

Dioctylbenzothienobenzothiophene (C8-BTBT), a p-type small-molecule semiconductor, is used

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due to its high carrier mobility, facile solubility, and good crystallinity.21,22 We dissolved C8BTBT (0.5 wt. %) and poly(methyl methacrylate) (PMMA) (0.1 wt. %) in a mixed solvent of anisole and p-anisaldehyde (0.5 wt. %). The antisolvent-assisted crystallization technique was adopted, which ensured the obtained crystalline films with a large area of hundreds of micrometers.11,15 During the deposition process, the PMMA film was formed between C8-BTBT and P(VDF-TrFE) films through vertical phase separation.23–25 This film can improve the morphology of the ferroelectric film, which is potentially beneficial for deposition of high quality C8-BTBT films. Besides, the PMMA layer can be used to suppress the polarization fluctuation at the semiconductor/dielectric interface, improving the charge transport in semiconducting channels.10,11 The deposited C8-BTBT films exhibited good uniformity with a small surface roughness (~4.2 Å) over a large area of >200 μm on the surface of the P(VDFTrFE) film (Figure 3a). Furthermore, the layer-defined films were confirmed by the AFM characterizations. The steps between two adjacent molecular layers are distinguishable in the height image (Figure 3b). The measured thickness is ~5.3 nm, which indicates the bilayer molecular structure of the C8-BTBT molecular crystalline films.15 To further assess the crystalline properties of our bilayer C8-BTBT semiconductors, high-resolution AFM (HR-AFM) characterizations were performed. Figure 3c shows the herringbone-type molecular packing of C8-BTBT and the corresponding fast Fourier transform patterns, which reveal that the upper C8BTBT unit cell geometry is oblique with lattice constants of a = 6.20 ± 0.51 Å, b = 6.24 ± 0.34 Å, and θ = 79.5 ± 4.6°. Besides, in the GIXRD measurements, peaks can be clearly observed at 2θ = 6.17° and 9.15° for the sample, indicating that the C8-BTBT film is highly crystalline (Figure S5).26 Compared with the bulk semiconducting channels, the high-quality ultrathin molecular crystalline films with morphological uniformity and highly ordered molecular packing enable

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overall optimization of the device performance, such as enhanced contact injection and carrier mobility.27–31

Figure 3. (a) Optical microscopy image of an ultrathin C8-BTBT molecular crystalline film deposited on the surface of P(VDF-TrFE). (b) AFM morphology image of the C8-BTBT film. (c) Morphology images of the HRAFM characterizations showing the molecular packing of 2D C8BTBT crystals on the P(VDF-TrFE) layer. The inset shows the corresponding fast Fourier transforms.

The Fe-OFET devices were fabricated under a bottom-gate top-contact architecture with the ultrathin crystalline P(VDF-TrFE) and C8-BTBT semiconducting layers using transferred Au pads as the source and drain electrodes (Figure 4a).32 Typical transfer characteristics of the device are shown in Figure 4b, operating in the linear region with a drain voltage (Vd) of −1 V. As denoted by arrows, the counterclockwise hysteresis is consistent with the polarization switching between spontaneous polarization states in the ferroelectric layer. Because of the good ferroelectric properties of ultrathin crystalline P(VDF-TrFE) films, hysteresis was obtained even at a low sweeping voltage of ±5 V. And the subthreshold swing (SS) of ~41 and 97 mV/decade

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were extracted from the upper and lower curves, respectively, which were much smaller than those of the Fe-OFET devices with bulk P(VDF-TrFE) of ~300 nm in our previous work.11 Furthermore, we estimated the charge carrier mobility in the linear region with the gate voltage (Vg) sweeping from −5 to −2 V to avoid the switching process in the ferroelectric layers.33 A high carrier mobility of 2.4 cm2 V−1 s−1 was obtained (the gate capacitance was 0.7 μF/cm2, as extracted at an applied voltage with a frequency of 10 Hz). In addition, the transfer curve was highly symmetric to Vg = 0 V. Thus, the device can maintain its highest on/off ratio of ~105 at Vg = 0 V, which is a significant fundamental feature desired in NVMs. The output characteristics show good saturation behaviors for the drain currents (Id) (Figure 4c). A linear increase in Id was also observed in the small range of Vd, indicating nearly ohmic contact with efficient charge injection from the metal contact to the conducting channels, which is guaranteed by the virtue of few-layered structures of ultrathin molecular crystalline films. Then, we estimated the contact resistance using the modified transmission-line method (M-TLM) (Figure S6).34 The value of 303 Ω cm (Vg − Vth = −3 V) is much lower than that in Fe-OFETs using bulk functional films, indicating less energy dissipation for the Joule heating.10 Subsequently, when Vg was applied to switch between the binary states, the fast charge accumulation and depletion occurred in the semiconducting region due to the efficient charge injection and high carrier mobility. Thus, the device exhibited short writing and erasing times of ~1.1 and 1.4 ms, respectively (Figures 4d,e). To the best of our knowledge, these results represent a record rapid operation speed for FeOFET memories.11 Furthermore, we performed retention measurements of the currents in the onand off-states (Figure 4f). A small gate voltage pulse of −1 V was applied every several seconds to refresh the on state, because decreasing the ferroelectric layer thickness can lead to a strong depolarization field that debilitates the spontaneous polarization.35 And the currents of the binary

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states were both well maintained during the retention test, showing a high on/off ratio of ~103 after 6000 s. In addition, the devices maintained a high on/off ratio of 103 after cycling 100 times (Figure S8), revealing high stability. The measurements were performed in vacuum to efficiently reduce the external influences, considering that the air-stability of two-dimensional organic semiconducting films is still a major obstacle in development of OFET devices.36–38

Figure 4. (a) Schematic illustration of a Fe-OFET with a bottom-gate top-contact structure. (b) Typical transfer curve of the Fe-OFET memory. (c) Output characteristics at different gate voltages. Inset shows a linear increase in the drain currents in a small range of the drain voltage, indicating a nearly ideal ohmic contact in our devices. (d) Switching behaviors between the binary states of the Fe-OFET memory from the on- to off-state and (e) from the off- to on-state. (f) Retention measurements of the currents in the on- and off-states.

In addition to continuous improvement in the performance of Fe-OFET NVMs, a reduction in their energy consumption is also critical for further development and remains a great

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challenge.2,10,39–45 In our low-voltage memory devices, the high carrier mobility, rapid switching between binary states, and ultrathin thickness of the semiconducting layer are all beneficial for achieving ultralow-energy-consumption operations. Therefore, we estimated the energy consumption for the writing and erasing processes of our ferroelectric memory devices, which can be expressed as 𝐸𝑤/𝑒 = 𝑈𝑤/𝑒 × 𝐼𝑔 × 𝑡𝑤/𝑒,

(1)

where Uw/e is the amplitude of the Vg applied for writing or erasing, Ig is the leakage current flowing through the gate electrode, and tw/e is the time to complete one operation. The energy consumed during the reading process is described as 𝐸𝑟 = 𝑉𝑑 × 𝐼𝑑 × 𝑡𝑟,

(2)

where tr is the time to read a storage state. More than ten individual OFET memories were fabricated and measured, and these devices yielded an average energy consumption of ~1.2 and ~1.6 pJ/bit for the writing and erasing processes, respectively, which is < 0.1% of the energy required for devices using bulk functional layers (Figure 5). Moreover, the curve of capacitance/conduction versus frequency decreased from ~6000 Hz, indicating a short reading time of ~0.16 ms (Figure S7). Hence, we applied a voltage of −0.5 V for such a short period of time at the drain electrode for the reading process (Figure S9). The average consumed energy was estimated to be as low as ~1.9 pJ/bit and ~0.2 fJ/bit for reading the on- and off-states, respectively. The disparity between the two reading processes is due to the high on/off ratio at Vg = 0 V. Therefore, our Fe-OFET memories with high performance are promising candidates for emerging organic memory applications in next-generation organic electronic devices.

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Figure 5. The estimated energy consumption of the writing (blue), erasing (red), and reading on(green) and off-states (purple) processes of our Fe-OFET memory devices. The amplitude of the operating voltage was 5 V for the writing and erasing processes. The Vd applied for reading processes was −0.5 V. For comparison, the energy consumptions estimated from literature (sphere) are illustrated (Table S1). The extracted drain current during reading processes in different devices are normalized with the same width to length ratio as our devices.

In conclusion, we demonstrated pJ-level-consumption, low-voltage Fe-OFET memories by using ultrathin P(VDF-TrFE) and semiconducting molecular crystalline films. The devices

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exhibit a low operating voltage of 5 V, a high charge carrier mobility of 2.4 cm2 V−1 s−1, and rapid switching between binary states. Only ~1.2 and ~1.6 pJ/bit are necessary for the writing and erasing processes, respectively; the reading energy is ~1.9 pJ/bit (on state) and ~0.2 fJ/bit (off state). Our results provide an effective strategy to realize energy-efficient memory devices at a pJ level, which is preferable for applications with high energy consumption requirements, such as wearable electronic devices.

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ASSOCIATED CONTENT Supporting Information Experimental methods, detailed information of the hybrid dielectrics and memory devices, summary of the estimated energy consumption of conventional Fe-OFETs using bulk functional layers in literature and our results. AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This study was supported partially by NSFJS under Grant No. BK20170075, NSFC under Grant Nos. 61774080, 61574074, and 51861145202, Doctoral Fund of Ministry of Education of China. M.J.P. and J.Q. contributed equally to this work. M.J.P., J.Q., and Y.L. conceived, designed and organized the experiments. M.J.P., J.Q., and S.J. were responsible for preparation of the samples. M.J.P., J.Q., and J.H.G. performed micro-Raman measurements. M.J.P., J.Q., and Q.J.W. performed

grazing-incident

XRD

measurements.

M.J.P.

and

J.Q.

performed

PFM

characterizations. M.J.P., J.Q., and Y.L. analyzed the experimental data and co-wrote the paper. All authors reviewed and revised the manuscript. REFERENCES (1)

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