Epitaxy of Ferroelectric P(VDF-TrFE) Films via Removable PTFE

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Epitaxy of Ferroelectric P(VDF-TrFE) Films via Removable PTFE Templates and Its Application in Semiconducting/Ferroelectric Blend Resistive Memory Wei Xia, Christian Peter, Junhui Weng, Jian Zhang, Herbert Kliem, Yulong Jiang, and Guodong Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b01571 • Publication Date (Web): 14 Mar 2017 Downloaded from http://pubs.acs.org on March 17, 2017

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ACS Applied Materials & Interfaces

Epitaxy of Ferroelectric P(VDF-TrFE) Films via Removable PTFE

Templates

and

Its

Application

in

Semiconducting/Ferroelectric Blend Resistive Memory Wei Xia,† Christian Peter,‡ Junhui Weng,† Jian Zhang,§ Herbert Kliem,‡* Yulong Jiang,§* Guodong Zhu†*

†

Department of Materials Science, Fudan University, Shanghai, China

‡

Institute of Electrical Engineering Physics, Saarland University, Germany

§

School of Microelectronics, Fudan University, Shanghai, China

Abstract: Ferroelectric polymer based devices exhibit great potentials in low-cost and flexible electronics. To meet the requirements of both low voltage operation and low energy consumption, thickness of ferroelectric polymer films is usually required to be less than, for example, 100 nm. However, decrease of film thickness is also accompanied with the degradation of both crystallinity and ferroelectricity and also the increase of current leakage, which surely degrades device performance. Here we report one epitaxy method based on removable poly(tetrafluoroethylene) (PTFE) templates for high-quality fabrication of ordered ferroelectric polymer thin films. Experimental results indicate that such epitaxially grown ferroelectric polymer films exhibit well improved crystallinity, reduced current leakage and good resistance to electrical breakdown, implying their applications in high-performance and low voltage operated ferroelectric devices. Based on this removable PTFE template

ACS Paragon Plus Environment


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method, we fabricated organic semiconducting/ferroelectric blend resistive films which presented record electrical performance with operation voltage as low as 5 V and ON/OFF ratio up to 105. Keywords: ferroelectric polymer, epitaxy, resistive memory, removable template, semiconducting/ferroelectric blend Introduction In recent years organic materials and devices have attracted more and more attention due to their applications in low-cost and flexible electronics, among which nonvolatile memory is one of key elements. Currently the studied nonvolatile memories mainly include ferroelectric, resistive, phase-change and magnetic memories.1 Organic ferroelectric memory is usually constructed into field effect transistor structure with the copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) as ferroelectric material and organic semiconductors as active layer whose electrical properties can be modulated by the polarization states of ferroelectric layer.2 P(VDF-TrFE) is usually semi-crystalline with crystallites embedded in amorphous matrix, which surely results in large surface roughness. To meet the requirements of low operation voltage and low energy consumption, the thickness of ferroelectric layer is usually required to be less than, for example, 100 nm due to its large coercive field of ~48 MV/m. However, decrease of film thickness results in the degradation of both crystallinity and ferroelectricity in ferroelectric polymer thin films.3,4 Furthermore, large film roughness also results in more serious current leakage with decreased film thickness, as further degrades electrical performance of


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ferroelectric devices.5 Efforts have been made to enhance crystallinity and thus ferroelectricity of thin ferroelectric films by some physical confinement methods. Based on nano-imprinting technique, regular arrays of highly ordered ferroelectric polymer nanostructures are realized with enhanced crystallinity and ferroelectricity.6-8 Ferroelectric polymer nanowires and nanotubes are also obtained via porous anodic alumina oxide (AAO) templates and the nanoconfinement of AAO templates induces crystal orientation and enhanced electrical activities.9-11 Recently epitaxial growth of P(VDF-TrFE)

thin

films

was

also

developed

on

friction-transferred

poly(tetrafluoroethylene) (PTFE) templates.12-14 Such epitaxially grown P(VDF-TrFE) films show ordered orientation of strip-like crystallites and enhanced crystallinity and thermal stability of ferroelectric phase. However, for this PTFE-based epitaxial growth, thin PTFE layer is always attached to ferroelectric film, forming bi-layer structure, which surely not only degrades apparent electrical activities but also limits its applications in some fields whose fabrication process is not compatible with this epitaxy technique. Furthermore, film roughness of such epitaxially grown films is still expected to be further decreased in order to lower leakage current. Here we report the development of removable PTFE template technique so that the negative influence of PTFE layer on electrical properties of ferroelectric polymer thin films can be eliminated. Due to the physical confinement of smooth PTFE template, the epitaxially grown P(VDF-TrFE) films present much decreased surface roughness and thus lowered current leakage. Such epitaxially grown ferroelectric thin films are expected to be used in



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high-performance and low-voltage-operated ferroelectric devices. As an example, we applied this removable PTFE template method to the fabrication of high-performance semiconducting/ferroelectric blend resistive devices. Semiconducting/ferroelectric resistive films are firstly reported by K. Asadi et al.15 Blend films are fabricated from the blending solution of both organic semiconductor and ferroelectric P(VDF-TrFE). During the evaporation of organic solvent, phase separation occurs resulting in discrete semiconducting filaments which connect both top and bottom electrodes and continuous ferroelectric phase whose polarization states modulate the conductivity of semiconducting

phase,

thus

forming

resistive

property.

Due

to

spinodal-decomposition-induced phase separation in semiconducting/ferroelectric blend films, usually large surface roughness is inevitable resulting in low fabrication yield and degraded device performance.16 Though some measures, such as side chain modification,17 high-temperature wire-bar coating16 and temperature-controlled spin coating18, have been taken to improve film roughness, most of reported blend devices are still operated by a voltage larger than 10 V18-23 and resistive switching property, especially the OFF-state current, should be further improved. Here based on this removable

PTFE

template

method,

we

successfully

fabricated

semiconducting/ferroelectric blend resistive films with operation voltage as low as 5 V and ON/OFF as high as 105. This high ON/OFF ratio is a record value for low-voltage operated semiconducting/ferroelectric resistive films. Experimental 1. Materials: PTFE rods, 70/30 P(VDF-TrFE) and p-type organic semiconductor


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F8T2 (poly[(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(bithiophene)]) were bought from NICHIAS, Kunshan Hisense Electronic Co. and American Dye Source, respectively, and were used as received. 2. Fabrication of epitaxially grown P(VDF-TrFE) films via removable PTFE templates. Fabrication process includes four steps, which are briefly illuminated in Figure 1a. The epitaxially grown films fabricated via removable PTFE templates are labeled as P(VDF-TrFE)/PTFEoff films. Step 1. Fabrication of PTFE templates. PTFE templates are fabricated by friction transfer method which is developed by Wittmann and Smith24 and usually used for high quality fabrication of highly ordered block25 and semiconducting26 thin films. Recently its application is extended to the epitaxial growth of ferroelectric polymer films12-14. Fabrication of PTFE templates is briefly introduced here and more details can refer to our previous work.14 PTFE templates were deposited onto polished silicon substrates by pressing a PTFE rod with a pressure P1 of about 1.0 MPa and sliding it against the silicon substrate with a rate between 0.2-0.8 mm/s, during which the temperature of silicon substrates was kept at 130 oC. Here we select polished Si as substrates because polishing results in extremely low surface roughness which makes sure the effective contact between PTFE template and P(VDF-TrFE) films deposited on another substrates as described below. Thickness of PTFE layers is about 5-20 nm, dependent on the pressure, temperature and friction rate. Step 2. Fabrication of P(VDF-TrFE) thin films. P(VDF-TrFE) films were deposited by spin-coating method from its solution in cyclohexanone onto cleaned



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substrates. Here P(VDF-TrFE) films for XRD and AFM characterizations are deposited on polished silicon substrates while those films for electrical measurements are deposited on metalized glass substrates. Metalized glass substrates were prepared by pre-deposition of Al electrodes via vacuum thermal evaporation. Thickness of P(VDF-TrFE) films is modulated by solution concentration from 1% to 8% by weight in order to obtain ferroelectric films with thickness from less than 100 nm to several microns. Step 3. Epitaxy of P(VDF-TrFE) films via removable PTFE templates. Epitaxial growth of P(VDF-TrFE) films was realized by pressing P(VDF-TrFE) films closely onto pre-prepared PTFE template. P(VDF-TrFE)/substrate closely contacted with PTFE/Si under a pressure P2 of about 1.2 MPa to make sure the perfect contact. PDMS with thickness of about 100 µm was coated on glass substrate. Then PDMS layer was pressed against PTFE/Si to realize uniform pressure distribution on the whole sample surface. This sandwich structure was put onto a heat stage with pre-set temperature of ~170 oC, higher than the melting point 149 oC of P(VDF-TrFE), for epitaxial growth. After pre-set time of two hours, the temperature of heat stage was cooled down to room temperature and then the pressure was removed. Finally PDMS/glass and PTFE/Si were sequentially peeled off with tweezers from P(VDF-TrFE)/substrate. Due to the super-hydrophobic property of PTFE layer, the removal of PTFE from P(VDF-TrFE) is feasible and don’t result in obvious damage to ferroelectric film. Step 4. Deposition of top electrodes. For samples to be electrically measured,


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Al top electrodes were vacuum deposited to form effective electrode area of 0.25 mm2. Fabrication of control samples. Two kinds of control samples are also prepared. One is P(VDF-TrFE) films directly deposited on silicon substrates for AFM and XRD characterizations or Al-coated glass substrates for electrical measurements, while the other is epitaxially grown P(VDF-TrFE) films directly on PTFE templates. The fabrication process of the latter is shown in Figure 1b. P(VDF-TrFE) films are directly spin coated on PTFE templates and then annealed at 130 oC for four hours to realize epitaxy of ferroelectric films. Note that here PTFE templates cannot be removed from P(VDF-TrFE) films. Such a bi-layer structure is labeled as P(VDF-TrFE)/PTFE films. 3. Fabrication of semiconducting/ferroelectric blend resistive memory devices. Organic p-type semiconductor F8T2 and ferroelectric P(VDF-TrFE) are used to form the blend resistive memory devices. P(VDF-TrFE) powder was dissolved in tetrahydrofuran (THF) solvent to obtain a 1.0 wt. % solution. Then F8T2 was added into P(VDF-TrFE)/THF solution to get the blend solutions with 5:95 weight ratio between F8T2 and P(VDF-TrFE). Then via temperature-controlled spin coating technique,18 blend solution was spin coated at 50 °C onto Ag-deposited glass substrates to form the blend films. Thickness of these blend films is about 65 nm. Similar to the fabrication of epitaxially grown P(VDF-TrFE) films via removable PTFE templates, F8T2/P(VDF-TrFE) blend films were also closely pressed onto PTFE templates for two hours under pressure of 1.2 MPa and temperature of 170 oC. Then PTFE template was carefully removed. Thus epitaxial F8T2/P(VDF-TrFE)



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blend

films

were

obtained.

For

simplification

these

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epitaxially

grown

F8T2/P(VDF-TrFE) films are labeled as blend/PTFEoff. Control samples were also prepared from 3.0 wt. % F8T2/ P(VDF-TrFE) blend solution which was directly spin coated on Ag-deposited glass substrates and then annealed at 135 oC for two hours. For simplification these F8T2/P(VDF-TrFE) films are labeled as blendnoPTFE. To make electrical measurements of both samples, Ag top electrodes were vacuum deposited via hard mask to form sandwiched resistive devices with effective electrode area of 0.25 mm2 for each memory cell. 4. Structural and electrical characterizations. Structural characterizations were made by X-ray diffraction (XRD, D8, Bruker Inc.), atomic force microscope (AFM, Dimension Edge, Bruker) and scanning electron microscope (SEM, XL30FEG, Philips). Ferroelectric property of P(VDF-TrFE) films was determined by polarization-electric field (P-E) hysteresis loops via home-made Sawyer-Tower measurement system. The Sawyer-Tower system is constructed with a signal generator including a voltage signal smoothing and power operational amplifier (Apex PA98), a reference capacitor of 10 nF, at least 100 times higher than the capacitance of the samples to be tested, an instrumentation amplifier with an input resistance of about 1016 Ohm (TI INA116) and a 16-bit data acquisition (NI PCI-6251). This home-made Sawyer-Tower system can operate with output voltage up to 130 V and frequency between 0.1 Hz to 10 kHz. Dielectric spectrum measurements were performed on Agilent 4294A precision impedance analyzer. Current-voltage (I-V) and retention measurements were performed by a sourcemeter


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(B2902A, Agilent). In retention measurements, reading voltage was applied only during reading operation, otherwise reading voltage was removed to minimize the influence of reading voltage on stored information. Results and discussion In Figure 1a is shown the fabrication process of epitaxially grown P(VDF-TrFE) films, labeled as P(VDF-TrFE)/PTFEoff, via removable PTFE templates, while in Figure 1b is shown the process for epitaxial P(VDF-TrFE) films on PTFE templates which forms P(VDF-TrFE)/PTFE bilayer films. Note that here the influencing factors on epitaxial growth include pressure, temperature of heat stage and the contact between PTFE and P(VD-TrFE) layers. Perfect contact between PTFE and P(VDF-TrFE) layers is required to make sure effective epitaxial growth in large scale. All these factors can be optimized from structural and electrical measurements. For the fabrication of P(VDF-TrFE)/PTFEoff films, the temperature of heat stage during epitaxy should be between both melting points of P(VDF-TrFE) (~149 oC) and PTFE (~329 oC).14 Temperature far below 149 oC usually results in failure of epitaxy.

Figure

1.

Schematic

diagram

for

the

fabrication


processes

of

(a)


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P(VDF-TrFE)/PTFEoff films and (b) P(VDF-TrFE)/PTFE bilayer films. AFM measurements were first performed to determine morphologies of PTFE and P(VDF-TrFE) films fabricated by various processes. In Figure 2 are shown these AFM results. PTFE template, fabricated according to the friction transfer method, shows highly oriented structure with ridge-like stripes parallel to the friction direction (Figure 2a). From section analysis in Figure 2f, maximum vertical distance between the highest and the lowest points is about 9.1 nm resulting in low root mean square (RMS) roughness of 1.3 nm. XRD analysis in our previous work also indicates that such PTFE templates are single crystal like.14 In Figure 2b and 2c are shown the AFM morphologies of control samples which were directly spin coated on silicon substrates and annealed at 135 oC (below melting point) and 170 oC (above melting point), respectively. The surface of 135 oC-annealed P(VDF-TrFE) film (Figure 2b) is covered with disordered rod-like crystallites, corresponding to RMS roughness of 8.1 nm, and its XRD spectrum (Figure 3a) shows a characteristic peak at about 19.8o, corresponding to the superimposition of (200) and (110) reflections of polar ferroelectric β-phase.14,27 As for the film annealed at 170 oC (Figure 2c), much higher than its melting point of ~149 oC, the film melts and re-crystallizes to much low degree of crystallinity (Figure 3a), resulting in characteristic morphology of long and straight ridges disorderly spreading on film surface and thus rough surface with RMS roughness as high as 13.6 nm. Work from other groups have proved that such re-crystallized films nearly completely lose their ferroelectricity due to the rotation of b-axis to the direction perpendicular to the electric field.28,29 In Figure 2d is shown the


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AFM morphology of P(VDF-TrFE)/PTFE films where P(VDF-TrFE) was directly epitaxially grown on PTFE template at 135 oC. Film surface is covered with parallel stripe-like crystallites with the length direction perpendicular to the friction direction of PTFE template. RMS roughness is about 5.78 nm, lower than that of 135 o

C-annealed films shown in Figure 2b. Our previous work further indicates that on

such PTFE templates even higher annealing temperature also results in epitaxial growth of ferroelctric films as long as the temperature is below the melting point of PTFE template.14 Most interesting is the epitaxial P(VDF-TrFE) films fabricated via removable PTFE templates (Figure 2e). Different from P(VDF-TrFE)/PTFE films shown in Figure 2d, PTFE template is removed from P(VDF-TrFE)/PTFEoff film after the epitaxial process. P(VDF-TrFE)/PTFEoff film still presents obvious characteristic of epitaxial growth: parallel stripe-like crystallites with length direction perpendicular to the friction direction of PTFE template.13 P(VDF-TrFE)/PTFEoff film shows extremely smooth roughness with RMS roughness of only 0.98 nm and maximum vertical distance of only 3.5 nm in section analysis of Figure 2f. XRD analysis in Figure 3a indicates much enhanced degree of crystallinity, compared to those films without epitaxial process. Enhanced crystallinity should be attributed mostly to the influence of epitaxial growth.12,14 Since some work reported that one-dimensional physical confinement would also result in the enhancement of crystallinity,30 we verified the possible influence of one-dimensional confinement in epitaxial growth process. Polished Si substrate without PTFE template was directly pressed on



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P(VDF-TrFE) film under a pressure of 1.2 MPa and temperature of 170 oC. After the removal of Si substrate, AFM morphology of the P(VDF-TrFE) film is just like the morphology of re-crystallized film shown in Figure 2c and XRD analysis also indicates much low crystallinity just like that of re-crystallized film (Figure 3a). Both AFM and XRD measurements prove that here physical confinement shows ignorable influence on crystallinity of P(VDF-TrFE)/PTFEoff films. Totally fifteen P(VDF-TrFE)/PTFEoff films were prepared for AFM and XRD measurements. All these films show similar epitaxial structure with parallel stripe-like crystallites and improved crystallinity as long as the annealing temperature is higher than the melting point of P(VDF-TrFE).

Figure 2. (a-e) AFM morphologies and (f) section analyses of (a) PTFE template and (b-e) P(VDF-TrFE) films fabricated via different processes. (b, c) P(VDF-TrFE) films which were directly deposited on silicon substrates and then annealed at (b) 135 oC and (c) 170 oC, respectively. (d) P(VDF-TrFE)/PTFE film which was directly


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deposited on PTFE template and then annealed at 135 oC. (e) P(VDF-TrFE)/PTFEoff film which was fabricated via removable PTFE template at 170 oC. Arrows in (a), (d) and (e) indicated the friction direction during the fabrication of PTFE templates. All P(VDF-TrFE) films were spin coated from 3.0 wt. % solution resulting in film thickness of about 270 nm. Electrical measurements of such P(VDF-TrFE)/PTFEoff films were conducted. Ferroelectric property was characterized by polarization-electric field (P-E) hysteresis loops (Figure 3b). P(VDF-TrFE)/PTFEoff films present typical rectangle-like P-E hysteresis with remanent polarization Pr of about 0.073 C/m2 and coercive field of 45 MV/m at 10 Hz. This Pr value is much larger than those of 0.064 C/m2 14 and even 0.017 C/m2

13

obtained from P(VDF-TrFE)/PTFE films where the existence of PTFE

layer degrades the apparent ferroelectric property. Current leakage of P(VDF-TrFE)/PTFEoff films was tested by current-voltage (I-V) measurements shown in Figure 3c. We also determined the leakage of one control P(VDF-TrFE) film which was directly deposited on metalized glass substrate and then annealed at 135 oC. Both films present ferroelectric switching peaks at about 47 MV/m. This value is well consistent with the coercive field of 45 MV/m obtained from the P-E hysteresis loops in Figure 3b. As for control film, with the further increase of the applied electric field to about 98 MV/m, leakage current dramatically increases implying electrical breakdown. However, P(VDF-TrFE)/PTFEoff film can retain leakage current as low as 0.19 A/m2 under the high electric field of 400 MV/m. Obviously P(VDF-TrFE)/PTFEoff film shows much low current leakage and excellent



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resistance to electrical breakdown. Totally ten P(VDF-TrFE)/PTFEoff films were prepared for leakage measurement, in which eight films show obviously reduced current leakage similar to that shown in Figure 3c. From

the

measurement

of

the

real

part

of

permittivity

of

such

P(VDF-TrFE)/PTFEoff films, their typical dielectric constant was determined as a function of frequency and shown in Figure 3d. P(VDF-TrFE)/PTFEoff films exhibit large relative dielectric constant εr of, for example, 15.7 at 103 Hz and with increased frequency εr gradually decreases to 11.9 at 106 Hz.

Figure 3. (a) XRD, (b) P-E hysteresis, (c) current leakage and (d) dielectric spectrum analyses of P(VDF-TrFE) films fabricated via varied processes. Inset in Figure c is the enlarged leakage curve of P(VDF-TrFE)/PTFEoff film. Film thickness in Figure a and c is about 270 nm, while film thickness in Figure b and d is about 510 nm.


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Note that here we only discuss the decreases of surface roughness and leakage current and the enhancement of film crystallinity for P(VDF-TrFE)/PTFEoff thin films. In fact, imprint effect may also influence the electrical performance of ferroelectric polymer thin films. Imprint summarizes time dependent effects of ferroelectric polarization which tends to resist polarization reversal and thus degrade ferroelectric performance in practical applications.31-33 However, mechanisms of imprint effect are much complicated in ferroelectric polymer films. Some work indicate that imprint effect can be understood according to interface screening mechanism in which there exists amorphous and non-ferroelectric layers at ferroelectric/electrode interfaces.31-32 From this viewpoint, imprint-induced degradation of ferroelectricity should get more serious with the decrease of film thickness. However, there are also some other work which attributes imprint effect to bulk phenomenon. Thus imprint effect is independent of film thickness.33 Obviously further work should be done to well understand imprint effect especially in epitaxially grown ferroelectric films in order to judge whether or to what extent imprint effect results in degradation of ferroelectricity in these epitaxial films. Since P(VDF-TrFE)/PTFEoff films possess those advantages of enhanced crystallinity, low current leakage and good resistance to electric breakdown, they are expected to be used for high performance ferroelectric devices, especially devices requiring low voltage operation and low energy consumption. As an example, here we report the application of removable PTFE template method for fabrication of low voltage operated organic semiconducting/ferroelectric blend resistive films. Organic



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p-type semiconductor F8T2 and ferroelectric P(VDF-TrFE) are used to construct F8T2/P(VDF-TrFE) blend resistive films. In Figure 4 are shown the typical morphologies of F8T2/P(VDF-TrFE) blend films with and without removable PTFE-based epitaxial process. From both AFM and SEM images in Figure 4b, blendnoPTFE film (i.e. F8T2/P(VDF-TrFE) blend films without epitaxial process) shows characteristic morphology of semiconducting/ferroelectric blends: discrete disk-like F8T2 phase embedded in ferroelectric matrix of disordered needle-like crystallites. RMS roughness of blendnoPTFE film is about 11.6 nm. Note that in the AFM image many pinholes are randomly distributed on the whole film surface. These pinholes result in large leakage current and thus degrade device performance and even result in electrical failure. Furthermore, with the decrease of film thickness, pinhole-induced leakage current exhibits more serious negative influence on resistive performance, such as large OFF-state current and even electrical breakdown. That is why here we choose a 270 nm thick control blend film whose resistive property is compared with that from a 65 nm thick blend/PTFEoff film (i.e. F8T2/P(VDF-TrFE) blend films fabricated via removable PTFE-based epitaxial process). Both AFM and SEM morphologies of blend/PTFEoff film are shown in Figure 4a. We can still observe the characteristic structure of semiconducting/ferroelectric resistive films: discrete disk-like semiconducting phase embedded in continuous needle-like ferroelectric matrix. However, most interesting is that film surface is covered with parallel aligned needle-like ferroelectric crystallites among which disk-like semiconducting domains are randomly distributed. Film surface shows few pinholes. RMS roughness of the


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blend/PTFEoff film is only 6.69 nm, much lower than that of blendnoPTFE film shown in Figure 4b. Both decreased number of pinholes and reduced surface roughness are of great advantages to inhibit leakage current and thus enhance resistive property. Improved crystallinity of ferroelectric phase may also enhance the control of polarization states on conductivity of semiconducting phase.

Figure 4. AFM and SEM characterizations of F8T2/P(VDF-TrFE) blend films fabricated on metalized glass substrates (a) via removable PTFE templates and (b) without epitaxial process. Film thicknesses are 65 m and 270 nm for Figure a and b, respectively. Current-voltage measurements were also conducted on both films to determine their resistive performance. For the control blendnoPTFE film with thickness of 270 nm,



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I-V curve in Figure 5b shows obvious butterfly shape indicating good resistive property with ON/OFF ratio of 1.7×103 at 8 V. Sharp increase of current occurs at 13 V and -13.6 V, corresponding to an averaged electric field of 49.3 MV/m, which is obviously due to polarization switching of ferroelectric phase. Epitaxially grown blend/PTFEoff film with thickness of 65 nm also presents good resistive performance as shown in Figure 5a. The operation voltage is as low as 5 V resulting in butterfly-shape resistive curve with ON/OFF ratio as high as 2.7×105 at 1.5 V. resistive switching occurs at 3.4 and -2.6 V, corresponding to an averaged electric field of 46.2 MV/m, well consistent with the coercive field of ferroelectric phase. Obviously the mechanism of resistive switching in such low-voltage operated devices is still due to the polarization modulation on conductivity of semiconducting phase. Furthermore, this large ON/OFF ratio up to 105 is a record value for reported low-voltage operated semiconducting/ferroelectric blend devices. We note that a ON/OFF ratio in the order of 105 is also reported in PFO/P(VDF-TrFE) blend system, however the optimized thickness of such blend films is about 150 nm, corresponding to operation voltage usually larger than 10 V.21 As for low-voltage operated PFO/P(VDF-TrFE) blend films, the reported ON/OFF is only about 103-104.16,34 Though both films shown in Figure 5a and 5b have much different thicknesses, we still put both I-V curves together in Figure 5c in order to exhibit the advantages of removable PTFE-based epitaxy for device performance. ON-state current from blend/PTFEoff film is a little larger than that from the blendnoPTFE film. For example, corresponding to an electrical field of 20 MV/m, ON-state current is 1.6 µA and 1.1


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µA for blend/PTFEoff and blendnoPTFE films, respectively. This small increase of ON-state current in blend/PTFEoff film is most possibly due to its smaller thickness rather than the improvement of epitaxial process. However, obvious difference is shown for OFF-state current obtained from both films. Corresponding to applied field of 20 MV/m, OFF-state current of 31.8 pA for blend/PTFEoff film is about 40 times lower than that of 1.3 nA for blendnoPTFE film. This obviously inhibited OFF-state current should be mostly due to improved surface roughness and reduced pinholes due to the removable PTFE-based epitaxy as mentioned above. Totally five blend/PTFEoff samples were prepared for resistive measurements, in which four samples show characteristic butterfly-shape I-V curve indicating good resistive performance. Retention property of blend/PTFEoff film was also measured and shown in Figure 5d. To minimize the influence of reading voltage on stored information and also to imitate actual operation of memory devices,22 in retention measurements reading voltage was applied only during reading operation. The film was pre-poled by +5 V (-5 V) voltage for 15 s in order to switch the current to ON (OFF) state and then +1.5 V reading voltage was applied at pre-set time to detect the change in ON-state (OFF-state) current with time. In the whole retention measurement period, OFF-state current keeps nearly constant of 11.3 pA while ON-state current slightly decreases from 3.61 to 2.49 µA, corresponding to a decrease of ON/OFF ratio from 3.19×105 to 2.2×105. Good retention performance verifies nonvolatile memory function of such low-voltage operated blend films. Mechanism of resistive switching in semiconducting/ferroelectric blend films has



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been well accepted through comprehensive experimental researches15-23 and theoretical

simulations35.

Here

we

only

give

a

brief

explanation.

For

semiconducting/ferroelectric blend system, electrode and semiconductor materials should be carefully selected so as to build up a potential barrier with proper height at semiconductor/electrode interface.21 F8T2 has a HOMO (highest occupied molecular orbital) level of -5.5 eV.36 Thus Ag is selected as electrode material with work function of 4.26 eV, resulting in a potential barrier of 1.24 V between F8T2/Ag interface. For the as-prepared blend film, this large barrier limits the charge injection through the interface and charge transport is injection limited, resulting in low current. Once ferroelectric phase is polarized, one polarization state can effectively reduce the barrier height at F8T2/Ag interface and thus charge transport is switched from injection limited to space charge limited current, inducing large current. The other opposite polarization state cannot decrease the barrier height and thus carrier transport is still injection limited. Resistive property is realized in such blend films. Usually short retention time of logic states is characteristic of ferroelectric nonvolatile memories. T. P. Ma et al. believe that two main causes should contribute to the short retention time in ferroelectric transistors. One is depolarization of ferroelectric film and the other is charge leakage and trapping tending to screen polarization charges.37 Retention has been comprehensively characterized in ferroelctric polymer-based nonvolatile field effect transistor memories. Some work indicate that depolarization greatly degrades retention property and even results in the loss of one polarization state right after the removal of electric field.38-39 However,


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there are also some other experiments implying that depolarization field is not an important factor for polarization retention.40-41 Charge injection and thus the resulting shift in threshold voltage may contribute to the short retention time.41-42 Till now only seldom work has been reported for the study of instability of resistive states in semiconducting/ferroelectric blend films. K. Asadi et al. inserted an additional polyphenylenevinylene-type

semiconductor

layer

between

poly(3-hexylthiophene)/P(VDF-TrFE) blend film and Ag electrode and found that ON-state was stable only when an electric field larger than the coercive field was applied.43

V.

Khikhlovskyi

et

al.

studied

data

retention

in

Poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl)](F8BT)/P (VDF-TrFE) blend films and found that partial depolarization in ferroelectric phase was the main mechanism for imperfect retention.44 Theoretical simulation indicated that depolarization, occurring in close vicinity to semiconductor/ferroelectric interface, was driven by energy minimization.44



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Figure 5. (a-c) resistive switching and (d) retention properties of F8T2/P(VDF-TrFE) blend films. Results in Figure a and d are obtained from epitaxially grown blend film. The curve in Figure b is obtained from F8T2/P(VDF-TrFE) film directly coated on metalized glass substrate and annealed at 135 oC without epitaxial process. In Figure c is the comparison of resistive property from both blend films shown in Figure a and b. Arrows in Figure c indicate the direction of voltage sweeping. Conclusion In conclusion, here we report the high-quality fabrication of ferroelectric P(VDF-TrFE) polymer thin films via removable PTFE templates. Surfaces of P(VDF-TrFE)/PTFEoff films are covered with highly-ordered and parallel stripe-like crystallites with enhanced crystallinity, reduced roughness and decreased current leakage, which are of great advantages for applications in high performance


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ferroelectric

devices.

As

an

example,

low-voltage

operated

organic

semiconducting/ferroelectric blend resistive films are fabricated via removable PTFE template method. Due to the decrease of surface roughness and the number of pinholes, such blend films present record resistive performance with operating voltage as low as 5 V and ON/OFF ratio up to 105. This removable PTFE template method is also expected to be used for high-quality fabrication of other organic materials, such as organic semiconductors. AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected] (Prof. H. Kliem), [email protected] (Prof.

Y. Jiang), [email protected] (Prof. G. Zhu) ACKNOWLEDGMENT W. Xia, J. Weng and G. Zhu would like to thank the support from NSAF (U1430106) and STCSM (13NM1400600). References (1) Pan, F.; Gao, S.; Chen, C.; Song, C.; Zeng, F. Recent Progress in Resistive Random Access Memories: Materials, Switching Mechanisms, and Performance. Mater. Sci. Eng., R 2014, 83, 1-59. (2) Naber, R.; Tanase C.; Blom, P.; Gelinck, G.; Marsman, A.; Touwslager, F.; Setayesh S.; de Leeuw, D. High-Performance Solution-Processed Polymer Ferroelectric Field-Effect Transistors. Nat. Mater. 2005, 4, 243-248. (3) Xia, F.; Razavi, B.; Xu, H.; Cheng, Z.; Zhang, Q. M. Dependence of Threshold



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Epitaxy of Ferroelectric P(VDF-TrFE) Films via Removable PTFE

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