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Thin Solid Films 515 (2

Organic field effect transistors with ferroelectric hysteresis

Klaus Müller ⁎, Karsten Henkel, Ioanna Paloumpa, Dieter Schmeiβer

Brandenburgische Technische Universität Cottbus, Angewandte Physik-Sensorik, 03013 Cottbus, P.O. Box 101344, Germany

Available online 18 January 2007

Abstract

The ferroelectric copolymer Poly(vinylidene fluoride trifluoroethylene) is used as insulating material for capacitor structures and organic fieldeffect transistors. For capacitors, we find the typical hysteresis in the capacitance–voltage characteristic upon increasing the voltage scan window.A writing process with adequate electric fields causes shifts in the flatband voltage. Based on these results, we fabricate organic transistors withregioregular poly(3-hexylthiophene) as organic semiconductor. The transistors are constructed in bottom gate architecture with thin layers(100 nm) of spincoated copolymer as gate insulation. The drain source current of the transistor is reversible affected by the polarized gate, whichgives opportunities for fabrication of organic nonvolatile memory elements.© 2006 Elsevier B.V. All rights reserved.

Keywords: Organic field effect transistors; Organic high-k; P(VDF-TrFE); Nonvolatile memories

1. Introduction

Electronic devices made of organic materials open a widefield for low cost and large area applications [1]. New develop-ments of integrated flexible logic circuits [2,3], flat paneldevices [4,5] or sensors [6–8] have been made. For memoryapplications, nonvolatile memories–a memory, which loses noinformation during the read out procedure–attains muchinterest. An important approach for nonvolatile and organicmemories is the resistive switching of organic semiconductorlayers containing granular metallic particles [9–11]. Anotherpossibility is the application of a ferroelectric polymer as gateinsulator for field effect transistors. Due to the ferroelectricalignment of dipoles, the threshold voltage of the transistor isaffected by a gate voltage, strong enough to switch thesedipoles. This acts as stored information. The advantage of thistechnology is that it should integrate easily into an existingproduction line using organic transistors. A well knownferroelectric polymer is Poly(vinylidene fluoride) (PVDF) orits copolymer with trifluoroethylene, P(VDF-TrFE). Thecopolymer is soluble in nontoxic reagents, a common solventis 2-butanone, and spincasted films of about 50 nm in thicknessare possible [12]. The copolymer P(VDF-TrFE) in a molar ratioof 75/25 is also usable for ink jet printing [13]. In a work before,

⁎ Corresponding author. Tel.: +49 355694067.E-mail address: Muellerk@tu-cottbus.de (K. Müller).

0040-6090/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2006.11.063

we demonstrated an improvement of the operation voltage oforganic transistors, if we use spincoated films of P(VDF-TrFE)as an organic gate isolation in a thickness of 2 μm. This is arelatively thick layer and due to the smaller electric field, nohysteresis of the transistor characteristics was found here. Theimproved operation voltage is a result of the high dielectricconstant of the copolymer (11 at 1 kHz) [14].

As ferroelectric layer in organic field effect transistors withdifferent organic semiconductors, P(VDF-TrFE) is studied byNaber, deLeeuw et al.: the workgroup present all polymertransistors with poly(p-phenylene viniylene), PCBM [15,16] orregioregular poly(3-hexylthiophene) (P3HT) as active organiclayer [17]. All these devices are processed from solution. Byusing a mixture of MEH-PPV and PCBM, the group presentedan ambipolar and ferroelectric transistor with programmablepolarity [18]. Pentacene was used as active layer in [19–21]. IfAl-electrodes are used as electrodes, the remanent polarisationseems to depend on the film thickness t of the dielectric polymerP(VDF-TrFE). As example, for films with tN200 nm apolarization field of 40 MV/m and for tb200 nm, 100 MV/mwas reported [22,23]. In addition, the switching time of theferroelectric dipoles depends on the electrodes used for thecapacitors, respectively the transistors [24].

Here, we focus on i) the behavior of the hysteresis of thecopolymer versus film thickness by using capacitor structures(metal–insulator–semiconductor) and ii) the behavior ofhysteresis in the transistor characteristics at different voltage

Fig. 1. Schematic view of our structures. For CVand transistor tests, we use structures with an oxidized Si-substrate. The P(VDF/TrFE, 70/30), is solved in 2-butanone,the films are heated 2 h at 135 °C. As top-electrode for the capacitors (left side), we use thermal evaporated Al. For the transistors (right side), we use P3HT as activelayer and thermal evaporated Au electrodes for source and drain. The channel length is 40 μm, the channel width is 1 cm.

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scan windows, respectively different field strength applied atthe gate.

2. Experimental

As copolymer, we use P(VDF-TrFE) in a molar ratio of 70/30. The material is delivered as film from Piezotech S.A.,France. As solvent for spincoating, we use 2-butanone.

For the measurements of the ferroelectric hysteresis ofP(VDF-TrFE), we use capacitors with an oxidized Si-substrate(235 nm SiO2) to prevent a high amount of leakage current(Fig. 1). The P(VDF/TrFE) is solved and spincoated into filmsof a thickness from 100 nm to 1 μm. Then, we anneal thesamples for 2 h at 135 °C. This improves crystallinity [25] andthe homogeneity of the spincoated film. As top-electrode, weuse thermal evaporated aluminum, structured with a shadowmask. The measurements of capacitance versus voltage (CV)are carried out with an LCR-Meter Agilent 4284 A. The lowestfrequency of this device is 20 Hz. The voltage amplitude is25 mV and the measurements are performed with a voltagesweep of 0.75 V/min (for a sweep of ±40 V, we use 3 V/min).The thickness of the spincoated films is measured by a TaylorHobson (Talystep) profilometer.

The field effect transistors are build up on a highly n-doped Si-wafer with 50 nm thermal grown SiO2. The solution of P(VDF-TrFE) in 2-butanone is spincoated into a film of about 100 nm inthickness, estimated from spincoat conditions (4000 rpm). Thislayer is tempered for 2 h at 135 °C in air to improve crystallinity.As active layer, we use regioregular poly(3-hexylthiophene),

Fig. 2. CV characteristics for different excitation frequencies,UDUT: Gate voltage at Afrom −5 to +5 V (and back) and from −30 to +30 V(and back). The measurements alPositions of the points A and B with respect to a polarization(P)-applied field (E)-looand back, film thickness of the copolymer 300 nm.

delivered from Aldrich, Germany and Plextronics Inc. PA, USA(P3HT, average molecular weight 87.000). The organic semi-conductor is spincoated into a film of about 50 nm in thicknessfrom a solution in chloroform. The preparation and the mea-surement of the transistor characteristics are performed in a glovebox under Ar atmosphere with measured oxygen content of200 ppm. This could lead to a low on/off ratio of the transistors,but the aim of this work was to show an influence of theferroelectric polymer on the electric characteristics. Two Auelectrodes are thermally evaporated through a shadow mask,the gap between the source and drain electrode is 40 μm. Acombination of an HP power supply (E3631A) and two multi-meters (HP 34301A, PREEMA4001) is used for measuring thecurrent–voltage characteristics.

3. Results and discussion

3.1. CV measurements with capacitor structures

In Fig. 2, the typical CV characteristic of two Si/SiO2/P(VDF/TrFE) capacitors for different frequencies is shown. The filmthickness of the copolymer is here 220 nm for the high frequencymeasurement at 1 MHz (left panel of Fig. 2) and 300 nm for thelow frequency measurement at 100 Hz. As x-axis, the appliedvoltage at the gate (here the Al-electrode) is given. For themeasurement at 1 MHz, two voltage sweeps corresponding to arange of ±5 V, and ±30 V are shown. A cycle, for example±30 V, starts at −30 V in accumulation of the Si-surface, runs to+30 V and back. Due to the p-doping of the Si-substrate,

l-electrode. Left panel: Measurements at 1 MHz, for two different voltage cycles,ways start in accumulation. Thickness of the P(VDF/TrFE)-film is 220 nm Inset:p. Right panel: Measurement at 100 Hz, for a voltage sweep from −30 to +30 V

Fig. 3. Shift of flatband voltageΔVFB versus field scanwindow, (U2PVDF−U1PVDF)/t=ΔEPVDF, with a layer thickness t. The two voltagesU1,2PVDF are the voltage dropsonly over the copolymer layer.

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accumulation with a related oxide capacitance (here 57 pF)occurs for a negative gate bias. For positive gate bias, an inver-sion layer exists, with capacitance's, depending on the frequencyof the measurement. For higher frequencies, the charge carrierscannot follow the applied signal and the capacitance in inversionis smaller than for lower frequencies. The frequency of all ourfurther measurements is 1 MHz. In addition, a shift of the CVcurve depending on the voltage sweep occurs. This is clearlyvisible in Fig. 2, left panel. For a voltage sweep from −5 to +5 Vand back, the shift is also visible, but much smaller than for avoltage cycle from −30 to +30 V and back. The shift incapacitance, respectively the measured charge for the capacitor,indicates the presence of polarization charges and the ferroelec-tric behavior of the copolymer P(VDF/TrFE). The displacementof the CV characteristics is now called the hysteresis of ourdevices. The displacement of the CV curve is a result of theremanent polarization of the ferroelectric polymer. This is in-dicated by the inset of Fig. 2, the left panel. The possibility ofcharge injection and a resulting shift of the capacitance (orflatband voltage) is studied by Reece et al. [19], and was clearlyexcluded (MFIS-structures).

For a quantitative characterization of the hysteresis, we usethe shift of the flatband capacitance. The flatband capacitanceper unit area

CWHFFB ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiq2e0espkT

r¼ e0es

LDebye¼ 7� 10−5F=m2

is calculated after Sze [26] and is indicated in Fig. 2, left panel.The concentration of the p-dopant was estimated from ameasurement without P(VDF/TrFE). In approximation for deepdepletion, this value is calculated as 1.3×1015 cm−3 for ourwafers. The calculated flatband capacitance corresponds to arelated flatband voltage, also shown in the figure. Then, thehysteresis of the device can be calculated as shift of flatbandvoltage.

In a next step, we study the influence of layer thickness onthe flatband shift, respectively the polarization of the P(VDF/TrFE) layers. Fig. 3 shows the shift of flatband voltage ΔVFB

versus the field amplitude ΔEPVDF. Here, ΔEPVDF is givenas x-axis. We calculated ΔEPVDF as voltage drop only over theP(VDF/TrFE)-layer. For accumulation, respectively negativevoltages, the voltage is divided into tow parts, U1=U1PVDF+U1SiO2

. For depletion, we have to calculate an additionalvoltage drop over the depletion layer: U2=U2PVDF+U2SiO2

+U2D. The field, applied only at the P(VDF/TrFE)-layer iscalculated as field amplitude (U2PVDF−U1PVDF) / t=ΔEPVDF,with a layer thickness t.

First, the flatband shift is proportional to the electric fieldstrength applied. The variation of flatband shift versus ΔUDUT

is nearly linear. For the remanent polarization, which isproportional to the flatband shift, a saturation at relativelyhigh field strength is to be due. For our samples, even at highapplied fields, no saturation occurs.

Second, the voltage necessary for switching of ferroelectricdipoles, depends clearly on the film thickness of the P(VDF/TrFE) film. For thinner films, we need a higher polarization

field than for thicker films. This result is consistent with severalother publications [22,23]. Here, a decline of the remanentpolarization for a critical film thickness of about 70–100 nmwas reported and has been attributed to a reduction ofcrystallinity, as determinated by X-ray diffraction measure-ments [27]. The ferroelectric behavior is a result of thecrystallized polymer, the β-phase, which is stabilized especiallyin the copolymer [25]. Furthermore, thermally evaporated Alreacts with the surface of the polymer, and/or could building upa thin layer of Al2O3. A “dead layer” of reaction products notavailable for ferroelectricity, could be formed by preparingmetal electrodes. An alternative for metal electrodes wassuggested by Naber et al. [12]: in this work, PEDOT:PSS(poly(3,4-ethylenedioxidythiophene):poly(styrenesulfonicacid)was used as polymeric electrode and an improvement of i)saturation fields and ii) switching times was reported.

Based on these results, we build up organic field effecttransistors with a ferroelectric gate insulation, consisting ofP(VDF/TrFE).

3.2. Field effect transistors

In Fig. 4, we show experiments with two field effect devices,based on Si/SiO2 substrate with 235 nm oxide thickness. Herewe use P3HT from Aldrich. Due to an amount of 200 ppmoxygen inside our glove box, an ohmic current was measuredwithout applied gate voltage. The residual current with appliedgate voltage is a superposition of both, the amount ofaccumulation charge and the ohmic current [28]. The on/offratio of these devices is in the range of 5, due to this fact. Theinfluence of a ferroelectric hysteresis is obvious, nevertheless.We show in the left panel of Fig. 4 a measurement of the sourcedrain current without applied gate voltages, but after an appliedgate voltage pulse of + or −73 V for 2 min. As reference, wemeasure a device without the copolymer. Here, a small shift ismeasured, also, but the switching of the device with thecopolymer as additional gate isolation is approximately onemagnitude higher than for this device. Here we confirm, that arealization of an “on” and “off” state seems to be possible. As

Fig. 4. Left panel: Measurement of the source drain current without applied gate voltages, but after an applied gate voltage pulse of + or −73 V for 2 min. Afterapplication of negative voltage pulses, the channel resistance increases. After application of positive voltage pulses, the channel resistance decreases. Right panel:comparison to a similar device, but without the P(VDF/TrFe) copolymer, measurement for UDS=−50 V, without gate bias.

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the figure demonstrates, after application of negative voltagepulses, the channel resistance increases. After application ofpositive voltage pulses, the channel resistance decreases, whenmeasured at zero voltage gate bias again. This behavior ismeasured in several devices and is also found for devices, basedon Si/SiO2 substrates with 50 nm oxide thickness and cleanedP3HT, delivered by Plextronics, USA. A transfer characteristicof such a device is shown in Fig. 5.

This is in contrast to the results shown by Naber et al. [17] fortransistors with P3HT as the active semiconductor. Thesetransistors show an on/off ratio of about 1000. Here, thesituation is reversed: after application of negative gate voltage,the channel resistance is lowered, and after application ofpositive gate voltages the channel resistance increases. Thereason for this difference could be the number of intrinsiccharge carriers. In the case of a very small ohmic current, thepolarization charge itself causes a charge accumulation insidethe channel. After application of a negative gate bias theremanent polarization of the ferroelectric polymer acts like anexternal field (for example at zero gate bias) and cause anaccumulation layer with higher currents than before. For the

Fig. 5. Transfer characteristic of an Au/P3HT/PVDF/SiO2/Si device at constant drwindows), which are defined as follows: for example, a sweep of ±10 V starts at 0 Vpanel shows the magnified area at positive gate voltages, for 3 voltage sweeps, as IDS

1/2

threshold voltage of the transistor.

above mentioned preparation conditions, the amount of ohmiccurrent is relatively high and the remanent polarization,especially at zero gate bias, is not strong enough to build up anoticeable accumulation layer. Here, the oriented dipoles insidethe ferroelectric polymer, should act as a trap for the positivecharge carriers, when a negative gate voltage is applied before.The result is a lowered current, as measured.

In Fig. 5, we show a transfer characteristic of an Au/P3HT/PVDF/SiO2/Si device (Ids versus gate bias). The drain sourcevoltage is constant at −5 V. Like in our CV measurements, weapply different gate voltage sweeps. For example, a sweep of±10 V starts at 0 V, runs to +10 V, to −10 Vand back to 0 V. Theramp rate is 10 V/min. The right panel shows the magnified areaat positive gate voltages. The characteristic is parabolic,demonstrating that the field effect induced accumulation chargeis saturated (saturation regime of the transistor). For the transfercharacteristics, plotted as Ids

1/2 (Ugate), the intersection of thecurves with the x-axis gives directly the threshold voltage of thetransistor. The slope of the Ids

1/2 (Ugate)-plot is directlyproportional to the square root of the mobility: d

ffiffiffiffiffiffiffiIDS

p=dUGS ¼

ðl0C=2l2Þ1=2 [28]. If we use a measured isolator capacitance per

ain source voltage of −5 V. We show different voltage sweeps (voltage scan, runs to +10 V, to −10 Vand back to 0 V. The ramp rate is 10 V/min. The rightversus Ugate plot. The intersection of the curves with the x-axis gives directly the

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unit area of 28.7 nF/cm2 at 20 Hz, the resulting value formobility is (3.9±0.28)×10−4 cm2/Vs. This is a typical value forP3HT [29].

In the transfer characteristic, we observe a shift of thresholdvoltage, which can be distinguished into two parts: i) a shift ofthe transfer characteristic itself. This could be due to anaccumulation of residual polarization charges or due to anoxidation of the organic semiconductor (pO2 is about 200 ppm).ii) A shift of threshold voltage in the course of a voltage sweep(from positive to negative voltages). This shift, here called asΔUth, could be due to the ferroelectric hysteresis. For voltagesweeps of about ±10 Vor smaller, ΔUth is equal to zero. This isa further evidence for ferroelectric behavior: here, the fieldstrength applied is not strong enough for polarization. In thecase of charge injection, a shift of threshold ΔUth should be notequal zero at ±10 V.

If we take a look at the source drain current with no gatevoltage applied, the measurements in Fig. 4 are confirmed. Foran amplitude of ±40 V for example, we start at 0 V (gate),(Fig. 5, right panel), then, after application of +40 V and back,the channel current at 0 V (gate) is increased, the channelresistance is decreased. This is the same observation as before(Fig. 4).

4. Summary

In summary, we show by using MIS-capacitors, a systematicshift of flatband voltage, if we apply different voltage scanwindows. The MIS structures are build up as Al/P(VDF/TrFE)/SiO2/Si sandwich structure. The shift is attributed to aferroelectric hysteresis of the copolymer. The dependence ofthe remanent polarization on thickness of the copolymer wasconfirmed. If we use an Al top-electrode, the remanentpolarisation decreases with increasing field strength.

Organic transistors with the copolymer P(VDF-TrFE) asferroelectric gate insulation, P3HT as active and organicsemiconductor are fabricated. We show that by application ofmoderate gate voltages, a programmable on and off state of thedrain source current is possible. This bistable drain sourcecurrent is a result of the ferroelectric alignment of the dipolesinside the copolymer, which causes a shift of the thresholdvoltage, like the measured shift of flatband voltage for MISstructures.

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