Spin-coated organic ferroelectric films for non-volatile memories

Phys. Status Solidi A 208, No. 2, 330–342 (2011) / DOI 10.1002/pssa.201026593 p s sa

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pplications and materials science

eature Article
Spin-coated organic ferroelectric filmsfor non-volatile memories
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Klaus Muller*,1, Karsten Henkel1, Dipankar Mandal2, Bernd Seime1, Ioanna Paloumpa1,and Dieter Schmeißer1

1 Applied Physics – Sensors, Brandenburgische Technische Universitat Cottbus (BTU), Konrad Wachsmann Allee 1, 03046 Cottbus,

Germany2 Department of Physics, Jadavpur University, Kolkata 700032, India

Received 28 April 2010, revised 30 August 2010, accepted 29 September 2010

Published online 29 October 2010

Keywords copolymers, metal–ferroelectric–insulator–semiconductor capacitors, field-effect transistors, device characterization, NEXAFS

* Corresponding author: e-mail [email protected], Phone: þ49 355 69 40 67, Fax: þ49 355 69 39 31

For low cost memory devices like ferroelectric field-effect

transistors (FeFETs) or cross bar arrays, organic materials

are important, due to their processability from solutions at

room temperature. The copolymer Poly[vinylidene fluoride

trifluoroethylene] (P[VDF/TrFE]) is a candidate for a possible

application as a ferroelectric component in such structures. In

order to improve the performance of FeFETs with P[VDF/

TrFE], we investigate metal–ferroelectric–insulator–semicon-

ductor (MFIS) capacitors. Implementing a method proposed by

Miller and McWorther, we simulate the capacitance-voltage

(C–V) characteristics of MFIS stacks and the hysteresis due to

the ferroelectric switching of the copolymer. We vary the

parameters thickness and permittivity of the insulator as well

as the as the thickness of the ferroelectric layer and predict in

that way conditions where a saturation of the polarization can

be reached. We also report on retention and cycle endurance

measurements. Furthermore, we show measurements of the

dipole alignment of ultrathin ferroelectric copolymer films,

down to 10 nm thickness. No ferroelectric threshold was found,

even for this thickness, if an adapted system of electrodes is

used. Here, we introduce the near edge X-ray absorption fine

structure (NEXAFS) method as a new tool for the character-

ization of ferroelectric dipole switching.

� 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction As a non-volatile and non-destructivememory device, ferroelectric field-effect transistors (Fe-FET) are a promising candidate [1, 2]. The FeFET could bean alternative to the recently dominating non-volatilememory, the flash memory cell [2]. In a FeFET, the classicalfunction of one transistor and one capacitor (1T1C) isintegrated into one device. The gate insulation of a FeFET isreplaced by a ferroelectric layer. A gate voltage pulse leadsto an alignment of the ferroelectric dipoles accompanied by ashift of the threshold voltage of the transistor, respectivelya ‘one’ and a ‘zero’ state of the memory.

The principle of the FeFET is shown by using perovskitetype ferroelectrics like PZT, SBT or bismuth tantalate as theferroelectric layer [1, 3–5]. The preparation of such layershas to be performed at elevated temperatures and oxygenpartial pressure. A problem of this preparation condition isthe growth of unintentional and undefined interfacial layers[1]. In consequence, a preparation of the ferroelectric layer atlower temperatures is desirable. Also in terms of a low cost

production, an investigation of organic ferroelectricmaterials spincoated from organic solvents, is promising.

As organic and ferroelectric material, the copolymerPoly(vinylidenefluoride-trifluoroethylene) P[VDF/TrFE] isinvestigated. For a content of 50–80% of the vinylidene-fluoride unit, the orthorhombic b-phase with the largestspontaneous polarization is stabilized, with a theoreticalvalue of 13mC/cm2 [6]. The copolymer is soluble in non-toxic reagents, for example 2-butanone, dimethylformamide(DMF) or cyclohexanone [7]. A preparation of organic andferroelectric thin films via spincoating from solution ispossible, also ink jet printing [8].

The polarization field of P[VDF/TrFE], is relativelyhigh, about 50 MV/m [9]. Here, a downscaling of P[VDF/TrFE] film thickness into a range below 100 nm is necessaryto come to low operation voltages of the ferroelectric device.

In order to improve the performance of FeFETs withsuch thin copolymer layers, an investigation with metal–ferroelectric–insulator–semiconductor (MFIS) capacitors


Phys. Status Solidi A 208, No. 2 (2011) 331

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S i/

P r 2 3

Si /

2 3

AlP[VDF/TrFE]

Bufferp-Si(001) Si

Pt

P(VDF/TrFE)Aga) b)

Figure 1 Set-up of the investigated MFIS stack consisting ofP[VDF/TrFE] as ferroelectric layer, as top electrode, aluminiumor indium is used (a). For the measurements of P(E) characteristicsMFM capacitors are prepared (b).

with an additional buffer layer to suppress leakage currentis possible. The advantage of an MFIS stack, instead of aMIM-structure, is the opportunity for a direct investigationof the ferroelectric behaviour in a geometry, almost identicalto the desired device, the FeFET.

The characterization of ferroelectric switching is done inthis work by performing capacitance versus voltagemeasurements (CV). An extraction of the polarization versusfield dependence (P(E)) directly from theCVmeasurement ispossible by a method proposed by Miller and McWorther[10]. By using these simulations, statements on remanentpolarization, saturation of the ferroelectric polarization orinterface reactions with the electrodes are given, as afunction of film thickness, of electrodes or the thickness of abuffer layer.

Also, a characterization of the ferroelectric retention andthe cycle behaviour is important. For Langmuir-Blodgettfilms of P[VDF/TrFE], retention measurements are per-formed by other authors [9]. Here, we present the results forspin-coated films.

Due to the high coercive field of P[VDF/TrFE] andthe necessity of a downscaling of the film, we need aninvestigation of the limits for the ferroelectrical switchingof spincoated films. Below 100 nm film thickness, adecrease of remanent polarization is observed by severalauthors. The role of electrodes on the coercive field andthe switching behaviour of copolymer films is emphasizedby different authors [11, 12]. If organic or Au electrodesinstead of Al electrodes are used, no decline of polarizationwas found.

After the description of the experimental part, the paperis organized as follows:

In Section 3, we describe the quality of our spincoatedcopolymer films in terms of the content of the b-phase,measured by FTIR, the Curie temperature and the values forsaturation polarization and coercive field, measured by anintegration of the polarization current.

In Section 4, we give a short introduction of thesimulation method, which enables an extraction of thepolarization curve directly from the CV measurement.

The simulation is then applied for a CV investigation ofthe polarization behaviour of the copolymer for different filmthickness, with Al as the top electrode and SiO2 as the bufferlayer of the MFIS stack. These investigations are completedby a short description of our results concerning the interfacereactions of the electrodes with the copolymer, in Section 5.

A further necessary objective for very thin films of thecopolymer is the buffer layer. This buffer layer is importantfor suppression of the leakage current. Due to the resultingvoltage divider, a certain amount of the field, available forpolarization, is lost. In consequence, this buffer layer has tobe optimized in thickness and permittivity, in terms of a fullferroelectric saturation of the copolymer. This is shown inSection 6.

A characterization of the ferroelectric retention and thecycle behaviour is enabled also byCVmeasurements, shownin Section 7. The long-term experiments delivered an

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improved retention behaviour and a very good cycleendurance.

In a last part, the paper is completed by an investigationof very thin films of the copolymer, down to 10 nm. Here, weshow the NEXAFS method as a new tool for thecharacterization of ferroelectric dipole switching.

2 Experimental As copolymer, P[VDF-TrFE] in amolar ratio of 70/30 is used. The material is deliveredas film by Piezotech S.A., France. This film is dissolved in2-butanone and spin-coated onto the substrates. The spincoating is performed using a WS-400B-6NNP/LITE unitwhich is kept inside a glove box [13]. The thickness of thepolymer film is adjustable in a wide range depending onthe spin velocity and the concentration of the solution [14].The thickness of the P[VDF/TrFE] layers are determinedby a profilometer (Talystep Rank Taylor Hobson). For thispurpose, a step is created by scratching the thin film. Thethickness of ultrathin films below 50 nm is measured byAtomic Force Microscopy (CP II, Veeco).

In the presented work, a P[VDF/TrFE] film thickness inthe range of 10 nm to 1mm is investigated. In order toimprove the crystallinity of the polymer films, an annealingstep is performed for 2 h at 135 8C, for all reported samples.This step is necessary to optimize the content of theferroelectric b-phase of the copolymer [6].

Figure 1a depicts the layer sequence of the investigatedMFIS stacks. The MFIS capacitors are prepared on siliconp-type substrates with (001) orientation and a resistivity of1-20Vcm. As buffer layer, SiO2, Al2O3 and HfO2 isinvestigated. The wafers with a SiO2 buffer layer arethermally oxidized in a thickness range of 10 to 240 nm atihp Frankfurt/Oder. The substrates with 11 nm Al2O3/1 nmSiO2 or with 5 nm HfO2 are delivered by AMD Saxony,Dresden or by Dr. Niinisto from the Helsinki University. Thesize of the samples is 10� 10 mm2.

Usually, as top-electrode aluminium we use thermallyevaporated through a shadow mask with diameters between400 and 900mm. For the CV characterization of our ultrathinfilms below 100 nm, we use indium. The metal is patterned insmall pieces and pressed mechanical on the FIS-stack atroom temperature. Here, we avoid interface reactions and theformation of a small non-ferroelectric layer, as foraluminium, described in Section 6 of this work.

In order to realize direct polarization loops withoutextraction from CV measurements, MFM capacitors are


332 K. Muller et al.: Spin-coated organic ferroelectric filmsp

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100

1184

1292

848

470

PVDF/TrFE (70:30)

% T

rans

mis

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Figure 3 FT-IRspectrumofaspin-coatedfilmofP[VDF/TrFE],ona Si-wafer, solvent: 2-butanone. Film thickness: 100 nm. The peaks,marked with arrows, at 470, 848, 1184, 1292 cm�1 gives indicationsfor all-trans ferroelectric phase.

1.0

1.2

50°C70°C

100°C

c.R

T

prepared. For these experiments, a metalized (platinum)silicon wafer was covered with a P[VDF/TrFE] film of440 nm thickness, and after annealing, a silver top electrodewith an area of 4.54� 10�3 cm2 was evaporated through ashadow mask. This set-up is shown in Fig. 1b.

CV measurements are carried out by a set-up based on aAgilent 4284 LCR meter. Typically, the measurements arestarted and finished in accumulation, after driving thevoltage in the investigated range to inversion and backwards(e.g. for a p-type semiconductor: –10 to 10 V and backwardsto –10 V, this is shortly called ‘�10 V loop’). Slow DCvoltage ramps of 25 mV/s are applied using the internal biasadapter 4284U-001 of the LCR meter. This voltage isexternally controlled by a Voltcraft MXD 4660A multi-meter. The height of the AC signal is set to 25 mV (effective,i.e. RMS). For the presented data, a characteristic frequencyof 1 MHz is applied. The characteristics are measured atroom temperature and under dark condition.

Photoelectron spectroscopy is performed by usingMgKa or synchrotron radiation (undulator beamline U49/2-PGM2 at BESSY-II in Berlin/Adlershof). An EA125semispherical electron analyser made by OmicronNanoTechnology GmbH is used. For further aspects referto Ref. [16].

Also, the near edge X-ray absorption fine structure(NEXAFS) investigation is done at the BESSY-II synchro-tron [17]. In Fig. 2, the set-up of the stack for our NEXAFSmeasurements is shown. An external aluminum top-electrode is used for polarization of the copolymer. ThePMMA layer prevents leakage currents through the ultrathinlayer of the copolymer.

FTIR spectroscopy is performed by FTS 60B (BioRad),the system is described in more detail in Ref. [17].

3 Material characterization The quality of the spin-coated copolymer films has to be investigated before furthercharacterizations. First, we show the FTIR spectrum ofa spincoated copolymer film (100 nm in thickness) on asilicon-wafer as substrate (Fig. 3). The sample is annealed at135 8C for 2 h to improve crystallinity. The peaks, markedwith arrows at 470, 848, 1184, 1292 cm�1 give evidencefor the all-trans ferroelectric b-phase of the copolymer. Noindication for alternating trans-gauche conformation isobserved. For this phase, a strong absorption feature at1196 cm�1 would be present, for example Ref. [6].

The CV investigation of a ferroelectric film in differentvoltage loops leads to a shift of the flatband voltage with a

p-Si (001)AlAlPMMA. 700nmAlP[VDF/TrFE]

Figure 2 Set-up of the investigated stack for our NEXAFS inves-tigation, an external aluminum top-electrode is used for polarizationof the copolymer.


hysteresis, depending on the amplitude of the voltage [18–21]. In a further step, we now investigate, whether this CVhysteresis is controlled by the bistable ferroelectric polariz-ation with a Curie point and not by charges, which areinjected into the buffer or the polymer layer.

For this purpose, CV loops at elevated temperaturesare performed. Here, a common laboratory oven is used.Figure 4 depicts the CV measurements at different tempera-tures up to 100 8C on a 110 nm P[VDF/TrFE]/100 nm SiO2

stack. We observe a decreasing of the flatband voltagewindow, while the hysteresis totally vanishes at tempera-tures around 100 8C, pointing out the phase transition fromthe ferroelectric to the paraelectric phase. The investigationis done with two different samples, in both cases, the flatbandvoltage shift is reduced to almost zero values at ca. 100 8C.It is important to note that the hysteresis appears again,when a CV is measured afterwards at RT. In consequence,this temperature should be the Curie temperature of thecopolymer. The literature value for the Curie temperature isin this range, for example reported by Furukawa et al. [6]. Inconclusion, the flatband shift is due to a ferroelectrichysteresis and no caused by injected charges.

-10 -5 0 5 100.6

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RT

C/C

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Voltage [V]Figure 4 CV curves of a 110 nm P[VDF/TrFE]/100 nm SiO2 stackon p-Si(001) at RT (thick solid line), 50 8C (dashed line), 70 8C(dotted line), and 100 8C (thin solid line).

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A)Figure 5 Polarization–fielddependenceofaP[VDF/TrFE] layerasa result of an IV measurement on a Ag/440 nm P[VDF/TrFE]/PtMFM stack. The solid lines show the as-measured result, while thedotted lines show the corrected values due to the electrical con-duction at higher fields.

In order to get information about the polarization loopparameters, namely the saturated polarization charge and thecoercive field, our films are analysed by a fast current–voltage (IV) loop. For this purpose, a MFM capacitor on ametalized (platinum) silicon wafer consisting of 440 nmP[VDF/TrFE] and a silver top electrode is prepared (seeFig. 1b). The IV curve on this sample is plotted in Fig. 5.

The curve shows distinct peaks at �21 (�0.5) Vindicating a polarization reversal due to the alignment ofthe VDF oligomers in the direction of the applied field [22].The peaks correspond to a field strength of approximately48 MV/m, which is in a good agreement with other findingsfor the coercive field reported in the literature [6, 23]. In orderto calculate the P(E) dependence of the P[VDF/TrFE] film,the IV curve is integrated taking into account the sweep rate.

However, the conduction current flowing in the back-ground of the IV gives an overestimated value of thepolarization charge. This is due to a superposition ofdielectric and dipole switching contributions, and due toleakage currents in the course of the IVmeasurement. Takinginto account this nonlinear electric conduction at higherfields as visible in the IV data in Fig. 5, our data are correctedby the subtraction of a background line [22]. The uncorrectedand corrected IV and polarization curves are presented in ourFig. 5 as solid and dotted lines, respectively. The value of thepolarization charge estimated from the corrected hysteresiscurve is 8.6mC/cm2, which is consistent with literature data[22]. The knowledge of these quantitative values for thepolarization charge (8.6mC/cm2) and for the coercive field(48 MV/m) are important for a fitting procedure of the CV-data, according to the model of Miller and McWorther,which is described in the next section.

4 Extraction of P(E) directly from CVmeasurements In order to extract the importantparameters of the ferroelectric behaviour from the CVmeasurement, we need a fitting procedure.

We use the model of Miller and McWorther [10] in orderto describe the CV characteristic of a MFIS stack. The area

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capacitance of the MFIS system (C00MFIS) can be written as

follows:

1

CMFIS

¼ tPVDFe0ePVDF

þ tbufe0ebuf

þ 1

CS

; (1)

where e0, ePVDF and ePVDF is the permittivity of vacuum, theferroelectric and the buffer, tPVDF and tbuf are the thicknessof the ferroelectric and the buffer, and C00

S is the areadepletion capacitance [10]. The gate voltage Vg can than bewritten as

Vg ¼ fS�sS

Cstack

þ VMS�PðEPVDFÞtPVDFe0eFPDF

(2)

where VMS is the work function difference between themetal and the semiconductor, FS and sS are the bandbending and surface charge of the semiconductor [24],C00

stack is the insulator capacitance calculated by the two firstterms in Eq. (1), and P is the dipole polarization, whichdepends on the electric field EPVDF in the ferroelectric andits history. The saturated polarization loop P�

sat is describedby

P�sat ¼ �PStanh

�EPVDF�Ec

2D

� �; (3)

where PS is the saturation polarization, Ec is the coercivefield and D is a hysteresis loop parameter, which is afunction of the ratio of the remanent and saturatedpolarization. The dipole polarization is calculated bynumerical integration taking into account the history ofthe polarization. For further details refer to Ref. [10].

In a more simplified method we normalize the flatbandvoltage window to the thickness of the polymer which gives avalue of a surface charge density N with q as the elementarycharge:

P � N ¼ DVFBe0ePVDFqtPVDF

: (4)

This method is applied in Section 8, ‘Ultrathin films,NEXAFS Investigation’ as estimation of the polarizationbehaviour of P[VDF/TrFE] layers below 50 nm.

5 Layer thickness dependence As application ofthe fitting procedure, we present some data of the influence ofthe layer thickness on the polarization behaviour of theP[VDF/TrFE] layers. First, we discuss two samples withdifferent P[VDF/TrFE] thickness of 95 and 950 nm. Thesubstrate is an oxidized p-Si(001) wafer with 240 nm SiO2. Adoping level of 9� 1014 cm�3 is determined on a referencesample without P[VDF/TrFE]. The contact area of allsamples is equal (0.63� 0.01 mm2). These values and theparameters for the dielectric constant of the semiconductoreS¼ kSi¼ 11.9, and T¼ 300 K are used as fixed numbers inthe calculations regarding these samples. The whole seriesconsists of six samples with different thickness [18]. As topelectrode, evaporated aluminium is used.



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a)

b)

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±10 V ±20 V ±30 V ±40 V

-14 -12 -10 -8 -6 -4 -2 020

25

30

Voltage / V

Cap

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950 nm

Figure 6 CV measurements on MFIS samples with 95 nm (a) and950 nm (b) P[VDF/TrFE]. As substrate p-Si(001) is used which isthermally oxidized (240 nm SiO2). The following CV cycles areshown: �10 V (circles), �20 V (triangles),�30 V (diamonds), and�40 V (stars). The measurement data are depicted by the symbols,while additionally, the fitted lines are shown by solid lines.

100 150 200 250 900 950 10000.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98∆EPVDF

30 MV/m

15 MV/m

P r/Ps

tPVDF [nm]

Figure 7 Pr/Ps ratioversus thethicknessadaptedasfixedparameterfor the fitting of the model to the CV measurements. The valueDEPVDF is the field strenght across the ferroelectric layer. It has tobe pointed out, that the Pr/Ps ratio is here a parameter for the curvefitting, for the case of application of the coercive field of 48 MV/m.The saturated polarization charge Ps is set to 8.6mC/cm2, as meas-ured by a MIM structure.

0 200 400 600 800 100068

101214161820

Reference (SiO2)1/C

" *10

00 [m

2 /F]

tPVDF [nm]

Figure 8 Reciprocal accumulation capacitance in dependence onthe P[VDF/TrFE] thickness. The reference value is determined at aAl/240 nm SiO2/Si MIS stack.

In Fig. 6 the CV measurements of these two samples areplotted. Different voltage loops of �5, �10, �15, �20,�25, �30 and �40 V are applied, which are partially shownin the diagrams of Fig. 6. A clear expansion of the flatbandvoltage shift DVFB can be observed in all samples when theapplied voltage window is increased. Moreover, DVFB ishigher when the P[VDF/TrFE] thickness becomes higher.

The solid lines, drawn in Fig. 6 represent the fittingresult, according to the above mentioned method after Millerand McWorter. The agreement for the accumulation range isgood in the range of the flatband point, the discrepancies inthe range of depletion are not covered by this simulation,they could be the result of interface states of the substrate, forexample.

A further important result of the fitting procedure for allmeasured samples of this batch (with 240 nm SiO2) isrepresented in Fig. 7. We recognize a significant reduction ofthe polarization charge for the P[VDF/TrFE] thicknessbelow 150 nm, while between 200 and 950 nm the polariz-ation only slightly increases. This drop of remanentpolarization is significant for aluminium electrodes,either as top or bottom electrode, as reported by severalauthors [11, 12].

By X-ray photoelectron spectroscopy an interfacialreaction on both Al/P[VDF/TrFE] and P[VDF/TrFE]/Alstacks accompanied by the formation of a ‘dead layer’ forferroelectric switching, is shown by our group [13, 17]. It isshown that the deposition temperature is not the driving force


of this interaction, because in the case of the P[VDF/TrFE]/Al stack the polymer is spin-coated on top at roomtemperature, while in the case of the Al/P[VDF/TrFE] stackthe aluminum film is thermally evaporated.

An additional confirmation for the above mentionedXPS data is the electrical characterization: by analysing theaccumulation capacitance in the CV data, we show theformation of an interfacial layer, too, and moreover weestimate its thickness. The reversed area accumulationcapacitance 1/C00

MFIS of the MFIS stack can be written as

1

CMFIS

¼ 1

CI

þ 1

CIL

þ 1

e0ePVDFtPVDF (5)

According to Eq. (5) we perform a linear fit to the data ofthe reversed accumulation capacitance in dependence on thepolymer thickness (Fig. 8).

In agreement with other reported values in Refs. [6, 25,26] we obtain from the slope of this fit a kPVDF value of8.7� 0.4.

Furthermore, we observe, that the extrapolated intersec-tion of the fit with the y-axis at tPVDF¼ 0 differs from the

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Thickness t FE [nm]

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±20 V

Pola

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P d [µC

/cm

2 ]

Field EPVDF [MV/m]

Sat. MFM 10 nm Al2O3 10 nm SiO2 100 nm SiO2

Figure 9 Calculation of the voltage divider in Eq. (5) in depend-ence on the ferroelectric thickness tFE (a); and simulation of P(E)curves of stacks with 100 nm P[VDF/TrFE] (b) for MFIS systems,which use the following buffer layers: 100 nm SiO2 (thick solidlines), 10 nm SiO2 (dotted lines) and 10 nm Al2O3 (dashed lines). Forcomparison, calculations ona stackwitha conventional ferroelectric(PZT, kFE¼ 500) (dashed-dotted line in part (a)) and on a MFMcapacitor with the same thickness of the ferroelectric layer (thinsolid line in part (b)) are shown. The following parameters are used:Ecoer¼ 48 MV/m, Ps¼ 8.6mC/cm2, Pr/Ps¼ 0.95, kPVDF¼ 8.7,kSiO2¼ 3.9, kAl2O3¼ 9, pB¼ 1� 10�15 cm�3, eS¼ kSi¼ 11.9, andT¼ 300 K.

reference value of the buffer layer capacitance (dotted line inFig. 8). The reference reversed capacitance 1/C00

I of thebuffering SiO2 layer displays a value of 6814 m2/F, while theintersection of the fit delivers a value of 7736� 266 m2/F.Therefore, the term þ1/C00

IL is added in Eq. (5) representingthe capacitance of an interfacial layer. Calculating thedifference between the reference and the intersection valuesand assuming a permittivity value of approximately 4 for andegenerated P[VDF/TrFE] layer without the ferroelectric b-phase, [26] we estimate a thickness of the interfacial layer of32� 10 nm. It is important to note that this interfacial layer isin particular dominating when the thickness of the polymer isin the same order of magnitude. Here the overall capacitanceis influenced and the polarization is strongly screened.

Naber et al. [27] reported on 60-155 nm thick spin-coated P[VDF/TrFE] films with gold electrodes whichshowed always the same values of remanent polarizationPr, and coercive field Ecoer and switching time. Herethe coercive field was 47 MV/m, while in Ref. [28] for a60 nm P[VDF/TrFE] LB film with Al electrodes a value of150 MV/m is found. In our recent collaboration we foundalso no thickness dependence of Ecoer in spincoated filmsusing silver electrodes [29, 30]. This strengthens theconclusion that the thickness dependence reported in theliterature is characteristic to the electrode interface but not tothe P[VDF/TrFE] itself.

6 Buffer layer optimization A layer shrinking of theferroelectric is essential for a low voltage operation of amemory device based on MFIS like structures. However, in aMFIS stack a part of the programming voltage drops over thebuffering insulator, which is necessary for the hindrance ofdepolarization causing leakage currents. Again for lowvoltage operation, these programming losses should be assmall as possible.

The ratio a of the voltage drop over the ferroelectriclayer (VPVDF) to the programming voltage (Vp) according to

www

a ¼ VPVDF

Vp

¼ 1

1 þ ePVDFtIeItPVDF

; (6)

where eI, tI is the dielectric constant and thickness of theinsulator.

Figure 9a illustrates this ratio in dependence on theferroelectric layer thickness for stacks with high (PZT) andlow (P[VDF/TrFE]) permittivity ferroelectric as well as withdifferent buffer layer thickness and permittivity.

It depicts that the thickness of the ferroelectric layercould be enlarged in order to realize a high value of this ratio,but this contradicts the low voltage operation. In the oppositedirection, this ratio is reduced by the polymer layershrinking. Thus, the buffer layer should be adapted eitherby the reduction of the buffer layer thickness (dotted line inFig. 9a) or by the increasing of its permittivity value (dashedline in Fig. 9a). In the latter one the calculation is done with ak-value of Al2O3 of 9 while for SiO2 a k-value of 3.9 is

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normally used. Additionally, Fig. 9a emphasizes, that for thecase, in which a buffer layer is needed, a ferroelectric with arelatively small permittivity is preferable in terms of thisvoltage divider optimization (compare the dashed-dottedline to the thick solid line). From this point of view, P[VDF/TrFE] with its k-value in the range of around 10 is apromising material.

Secondly, when optimizing the buffer layer, theachievement of the fully saturated polarization of theferroelectric should be preferred in order to get a MFISmemory stack with long data retention. However, the simplemodel of the voltage divider regarding Eq. (5) does notconsider the polarization inside the ferroelectric layer.Hence, we performed simulations of P(E) curves using theprocedure of Miller and McWorther with the same bufferlayer thickness and materials as used in Fig. 8. Assuming,that � 20 V CV loops are performed, we calculate theresulting P(E) loop on a 100 nm P[VDF/TrFE] layer with abuffer layer of 100 nm SiO2, 10 nm SiO2, or 10 nm Al2O3.For comparison the saturation loop of a pure 100 nm thickP[VDF/TrFE] MFM capacitor is shown in Fig. 9b too. Theparameters which are used for the calculations are given in



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acc

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130 nm P[VDF/TrFE]11 nm Al2O31 nm SiO2

Voltage [V]

Figure 10 CV measurements in �10 V (filled squares) and �20 V(open triangles) input voltage windows on MFIS stacks with120� 25 nm P[VDF/TrFE] and 50 nm SiO2 (a), 10 nm SiO2 (b),or 11 nm Al2O3/1 nm SiO2 (c) buffer layer. The fits to the data aredepicted with dotted (�10 V) and solid lines (�20 V). The param-eters of the fits are given in the text.

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Figure 11 Influence of the SiO2 buffer layer thickness on theestimated surface charge densityN (a), as well as on the polarizationat themaximalnegative (b)andatzerofield (c)achievedbythefittingprocedure for MFIS stacks with 120� 25 nm P[VDF/TrFE]. TheSiO2 thickness is labelled in the graphs. The data are analysed afterthe highest negative voltage (a, b) and in the full loop (c) of the CVmeasurement. For further details refer to the text.

the figure caption. They are similar to above determinedvalues.

In Fig. 9b, we recognize that the SiO2 buffer layershrinking enlarges the voltage drop over the ferroelectric atthe same input voltage and leads therefore to increasedpolarization values, but not to a complete saturation. TheSiO2 thickness range which is used in these calculations isinvestigated in CV measurements too.

These theoretical predictions are tested in our furtherinvestigations by CV. In the CV measurements reported hereinput voltage loops between � 5 V and � 20 V were applied.In Fig. 10, � 10 V and � 20 V measurements on MFISstacks with two different thickness of the SiO2 buffer layerand with the Al2O3 buffer are shown. Furthermore the fittedcurves using the model introduced above are added in thediagrams. As parameters for the fitting procedure are usedEc¼ 48 MV/m, Ps¼ 8.6mC/cm2 as determined above.These values are kept constant inside each sample.Furthermore, eSiO2¼ 3.9, eAl2O3¼ 9, eHfO2¼ 20, eSi¼ 11.9,the copolymer thickness and T¼ 300 K are used as fixed


parameters. For the fitting of the CV curves, the dopingconcentration of the Si-substrate is chosen in a range of1� 1015 cm�3 to 1� 1016 cm�3, the permittivity of P[VDF/TrFE] is chosen in a range of 5.2 to 8, respectively. Also, theremanent polarization Pr and the initial flatband voltage isvaried. The fitting is done in a way that the calculated curvematches to the data at the flatband capacitance.

In our data we recognize qualitatively that the thinnerSiO2 buffer layer will lead to a higher hysteresis, due to ahigher field on the ferroelectric. This is valid for a bufferlayer thickness up to 30 nm. For 10 nm SiO2, the fieldexceeds the critical field for breakdown as discussed below.

Therefore, firstly, in Fig. 11a the calculated surfacecharge density N calculated via Eq. (4) for the samples with

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Figure 12 Resultsof the fittings to theCVdata on MFISstackswithapproximately 100 nm P[VDF/TrFE] with different buffer layerthickness (a) or material (b, c) as depicted in the diagrams. Theresults are regarding the electric field in the buffer layer (a, c) and thepolarization at zero field in the ferroelectric. The data are analysed inthe negative going branch (a, dV/dt< 0) and in the full loop (b, c) ofthe CV measurement.

SiO2 buffer layer is presented in dependence on the maximalnegative input voltage, i.e. only the influence of the highnegative voltage onto that branch of the CV characteristicfrom accumulation to inversion is taken into account. Forthe illustration of the improvement of the voltage divider thedata are plotted versus the input voltage. Generally, weobserve in Fig. 11a that by the decreasing of the buffer layerthickness higher polarization values are achieved. Secondly,the analysis of the fitted CV curves in Fig. 10 delivers similardependencies which are shown in Fig. 11b. However, anunderestimation of the polarization by the method via thesurface charge density N can be argued, because thisestimation does not take into account the polarizationhistory. The meaning of ‘History of CV measurements’ isas follows: Application of a negative bias leads to a shift ofthe CV curve towards positive voltages and application of apositive bias leads to a shift of theCV curve towards negativevoltages. If we come from �30 V, for example, but measureat þ2 V, the curve is already shifted towards negativevoltages here. This is a problem of the measurement and amemory window DV, directly measured from the CV-plotgives always a lower surface charge than the real polarizationcharge. The fit takes into account this history. The factorbetween the values achieved by the fitting procedure andthe estimated values N is in the range of 5 to 10. In addition,the plot of N in Fig. 11a may suggest, that a saturation of thepolarization starts to occur in the samples with 10 nm and30 nm SiO2. But, in the plot of the polarization valueachieved by the fitting procedure according to Miller andMcWorther (Fig. 11b) this trend is not yet observed, hereonly the line of the 100 nm P[VDF/TrFE]/10 nm SiO2 sampleshows a weak curvature.

Figure 11c depicts the difference of the polarization atzero field versus the electric field over the P[VDF/TrFE]layer resulting from the fits to theCV data in both branches ofthe CV loops. We find that the expected further increase ofthe polarization by buffer layer shrinking is not fulfilledanymore when a SiO2 layer thickness of 10 nm is combinedwith 100 nm P[VDF/TrFE].

Here a screening of the electric field can be argued forthe decreased values of the polarization. In Ref. [13] aninterfacial layer formation between P[VDF/TrFE] and thealuminum electrode leading to screening effects is inten-sively discussed. When the buffer layer is shrunken in orderto increase the voltage drop over the ferroelectric, it can beassumed that the voltage drop over the interfacial layer isenlarged too. Therefore, the discussed effect of the chargeinjection may increase and the polarization is partiallyscreened.

In addition, the electric field in the buffer layer isachieving values near the breakdown field. Figure 12a showsthe calculated electric field in the buffer layer at negative gatevoltages resulting from the fitting. In the MFIS stack with the10 nm SiO2 buffer layer the field exceeds values of 10 MV/cm. Regarding this critical field a maximal polarization valueof 3.5mC/cm2 is mentioned as a limit for the controllabilityof a MFIS stack with a SiO2 buffer layer [1], because at

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higher polarization the field in the buffering insulatorexceeds the breakdown field. Then the hindrance of leakagecurrents is not fulfilled anymore and the polarization may bedestroyed.

Therefore, two different effects can cause the strongpolarization collapse in the 10 nm SiO2 buffer sample: (i) thecharge injection into the degenerated interfacial layer and(ii) the breakthrough of the insulator.

The buffer layer thickness shrinking is leading toimproved values of the polarization, however, the saturationcondition cannot be fulfilled. Therefore, a high-k materialmay substitute the SiO2 layer in order to optimize the inputvoltage divider and to realize polarization values closer to thesaturation condition. In Fig. 10c we observe an increasedmemory window in the stack consisting of 11 nm Al2O3. In



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Figure 13 Section of a � 5V CV loop on a Al/100 nm P[VDF/TrFE]/30 nm SiO2/Si MFIS stack and illustration of the HCS andLCS (a). Part (b) depicts retention measurements of the HCS at a gatevoltage of �1 V after applying different programming pulses asgiven in the legend. The dotted line is showing a comparison for aLangmuir Blodgett 70/30 P[VDF/TrFE] film in a Au/36 nm P[VDF/TrFE]/10 nm SiO2/Si stack taken from literature [9].

the fitted data we recognize an improvement in thepolarization values (Fig. 12b) as well as a reduction of thefield in the buffer layer (Fig. 12c) when a 11nm Al2O3/1nmSiO2 buffer stack is inserted into the MFIS sequence. Inaddition we tried to further improve these values by using athinner layer with a higher permittivity. However in oursample with 5 nm HfO2 buffer layer we observe high leakagecurrents which influence strongly the polarization. Although,the results achieved by the fitting exhibit a lower field in theinsulator (Fig. 12c), they show also worse polarization valuescompared to the Al2O3 sample (Fig. 12b). Nevertheless, webelieve that a HfO2 layer with a better leakage behaviour willfurther improve the polarization performance[14, 15].

Summarizing, we find that the buffer layer thicknessshrinking leads to the predicted increase of the polarization,however the discussed limit of approximately 3.5mC/cm2,which is controllable with a SiO2 buffer without a break-through of the insulator, is confirmed by these investigations.In the 10 nm SiO2/100 nm P[VDF/TrFE] stack a strongcollapse of the polarization value is found. Therefore, besidethis buffer layer shrinking, alternatively, high-k materialsshould be used as buffer layer in MFIS stacks consisting ofP[VDF/TrFE] as ferroelectric material. First improvementsare demonstrated by the introduction of a 11 nm Al2O3 /1 nmSiO2 buffer stack.

7 Retention and cycle behaviour In this part wetest the long term stability of our MFIS stacks with P[VDF/TrFE] as ferroelectric layer [19]. In a memory device it isimportant how long an information can be saved (retention)and how often (cycles) a memory cell can be programmed.

Retention investigations are performed at a Al/100 nmP[VDF/TrFE]/30 nm SiO2/Si stack using capacitance-timemeasurements at a fixed bias voltage after applyingprogramming pulses. This thickness combination is chosenfor this investigation because at a lower SiO2 thickness weobserve a partial collapse of the polarization.

In order to define the fixed bias value, a� 5 VCV cycle isperformed before retention measurements. Figure 13aindicates, that after the application of a negative voltagepulse the CV curve is moving towards positive voltages,while after a positive pulse it moves to negative direction.Thus, at a voltage of �1 V two different states of thecapacitance can be distinguished, which can be defined as ahigh capacitance state (HCS) and a low capacitance state(LCS), or in terms of a memory cell as ‘1’ and ‘0’.

Further C(t) measurements are performed at this fixedbias voltage value after applying proper programmingpulses. It has to be pointed out that this voltage is not onlyapplied when the data points are taken, but is kept constant atthe whole retention measurement. Therefore, this proceduremay stress the stack.

Figure 13b summarizes measurements after applyingnegative pulses (HCS) of different amplitude and duration. Along time measurement on a 36 nm P[VDF/TrFE]/10 nmSiO2/Si stack prepared by Langmuir-Blodgett techniquetaken from literature [9] is added for comparison (dashed


curve). After applying �30 V for 5 min we find a sufficientretention of the HCS in our sample (solid curve). It has to bestated that while finding the right programming pulse thepulse duration is increased successively at�20 V up to 5 minand no improvement is found. Afterwards the voltage isincreased to �30 V keeping the 5 min pulse duration. Here,we have to point out, that for such a stack geometry with arelatively thick copolymer (100 nm) and buffer layer (30 nm)a pulse of �30 V is not to high: In Fig. 11b, we extract for30 nm SiO2 a non-saturated curve for�20 V. We believe thatonce the proper write voltage is found the pulse duration canbe decreased, as switching times below 1 ms are reported forFeFET devices with P[VDF/TrFE] as ferroelectric layer.

In Fig. 14a, the retention of both the HCS and the LCS isreported, where at first the HCS and then the LCS isinvestigated for approximately 5 days. Here the data arerecorded in an interval of 30 s. In the very beginning of themeasurement both states are changing their value ratherstrongly, but after approximately 3 h the values stabilize andthe difference between the two states is stable, but is stillslightly decreasing. After 5 days we are still able todistinguish these two states, the difference in the capacitanceis 27 pF, while it is 29 pF after three days and 32 pF after oneday. The increase of the capacitance in the LCS might besupported by the negative sign of the permanent appliedfixed bias voltage, which is opposite to the necessaryprogramming pulse of this state. In contrast, the HCS isfound to be very stable for more than 19 days (Fig. 14b).

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Figure 14 (a) Retention measurements of the HCS and LCS at agate voltage of �1 V on a Al/100 nm P[VDF/TrFE]/30 nm SiO2/SiMFIS stack during 5 days. Part (b) shows the HCS during approx-imately 20 days.

102 103 104 105 106 107 1080.5

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Figure 15 Ratio of capacitances at HCS and LCS (squares) takenfrom Fig. 14 versus time, and extrapolated tendency line (dottedline).

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Figure 16 Applied cycles in the cycle endurance measurements.

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c -30 V, 1 s

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Figure 17 Cycle endurance measurements performed on a MFISstack consisting of 100 nm P[VDF/TrFE] and 30 nm SiO2, wherepulses of �30 V and 35 V are applied with a pulse duration of 1 s.

In order to analyse some tendency of the retentionbehaviour, the ratio of the HCS and LCS capacitance arelinearly fitted and extrapolated along the time axis (Fig. 15)[18]. At the start of this investigated time scale (3 h) the ratioof the capacitances of HCS and LCS is 1.59, after 5 days itdisplays a value of 1.34. We calculate a ratio of still 1.25 and1.1 after 27 and 291 days, respectively. A ratio of one isachieved after 4.3 years.

These investigations were only done on a partiallyoptimized layer sequence in terms of the layer thickness ofthe SiO2 buffer (30 nm). For this layer thickness, the fieldstrength over the ferroelectric material is maximal, withoutbreakdown of the buffer (insulator). We believe, that furtherimprovements can be achieved by using an optimizedsequence of the buffer (like high-kmaterials, grown by ALD)and ferroelectric layers with higher degree of b-phase orother electrode materials without interface reactions as well.

Finally, we test the cycle endurance of our MFIS stackswith P[VDF/TrFE] as ferroelectric layer. Using the samearguments as in the retention investigations, again weprepare stacks with a 30 nm SiO2 buffer layer. Here twostacks with 50 nm or 100 nm P[VDF/TrFE] are explored.In addition a reference MIS sample with 30 nm SiO2 isinvestigated in order to check if the HCS and LCS areinfluenced by charge injections into the buffering insulator.

We perform cycles as shown in Fig. 16. The voltage ofthe measurement (Vmeasure) is defined before in an initial CVloop as in the retention investigations. A positive voltagepulse (Vhigh) is applied for a certain time (thigh). After that, the

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voltage is set back toVmeasure and after a defined waiting time(twait1) the capacitance is measured (meas. 1). Then, a similarnegative voltage pulse is established with the parametersVlow, tlow and twait2, and the capacitance is recorded again(meas. 2). Afterwards, this full cycle is repeated severaltimes.

As voltages for the programming of the HCS and LCS weuse the same values as in the retention measurement abovefor the 100 nm P[VDF/TrFE]/ 30 nm SiO2 stack. But here thepulse duration is set to only 30, 10 or 1 s in both pulses(thigh ¼ tlow). The waiting time in all measurements reportedhere is defined as 2 s. The value of the capacitances in allpresented cycle measurements is normalized to the accumu-lation capacitance of the stack determined in the CV loop.

Figure 17 displays the cycle measurements performed ona MFIS stack consisting of 100 nm P[VDF/TrFE] and 30 nm



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Figure 18 The F1s NEXAFS data of a 10 nm thick P[VDF-TrFE]film, after an external field polarization. The data are taken in TFYmode. The geometry of the test set-up is shown in Fig. 2. The data aretaken with the incoming electric-field vector aligned as follows: thefield vectorE// corresponds to an angleQof 508, and E? correspondstoQ¼ 108. The angleQ is indicated in part (a). The linear dichroism,expressed as P¼ (IE// � IE?)/(IE//þ IE?) in %, is shown in (b).

SiO2. Very good cycle endurance results are demonstratedon a MFIS stack with P[VDF/TrFE] and 30 nm SiO2. Evenbetter values are expected for comprehensively optimizedlayers. An improvement due to a defined alignment ofdipoles is expected for full saturation of the P[VDF/TrFE],not possible with a 30 nm SiO2, shown in Section 6 (bufferlayer optimization). Here, we propose high-k materials likeHfO2 for the buffer. Further, inert electrodes instead ofaluminium should be used to prevent surface reactions andthe degeneration of the ferroelectric polymer.

8 Ultrathin films, NEXAFS investigation A very-elegant method of demonstrating the existence of ferro-electric dipoles within a thin layer is to use linear dichroismin the X-ray absorption data. The intensity of an absorptionband depends on the relative alignment of the linear-polarized incoming light of synchrotron radiation and thelinear dipoles within the layer. In a corresponding exper-iment, the angle of incidence has to be varied and the changein intensity is attributed to the relative alignment of thepolarization vectors [31].

We apply the technique of linear dichroism by using thetotal fluorescence yield (TFY) to determine the near edgeX-ray absorption fine structure (NEXAFS) data [17, 32–34].This has the advantage that the detected signal is bulksensitive and not restricted to the surface near region (as inthe case of total electron yield (TEY) spectra).

For the P[VDF/TrFE] system, we have demonstratedthat the linear dichroism signal is only for the C–F bonds andnot for the C–C and C–H bonds of the polymer [17]. We havededuced this from a careful analysis of the contributions inthe C1s NEXAFS signals and later on we have just focusedon the measurements of the F1s signal [32]. This isreasonable as the ferroelectric dipoles are associated withthe asymmetric atomic arrangement of the fluorine bondsalong the C–C backbone of the polymer chain.

In order to analyse the linear dichroism of the F1s signalwe used the difference in the relative intensities for differentincident angles to calculate the polarization degree in thethin films. Figure 18 shows this analysis for two angles ofincidence. The field vector E// corresponds to an incidentangleQ of 508 for the synchrotron beam, and E? correspondsto Q¼ 108. The angle Q is indicated as inset in Fig. 18a.Now, the linear dichroism is expressed as ratioP¼ (IE//� IE?)/(IE//þ IE?) in %. This calculation is shownin Fig. 18b.

First, we demonstrated that ferroelectric dipoles exist,even in pristine (non-polarized) ultrathin films with athickness of 10 nm, without application of an external field[32]. The degree P of polarization is of the order of 30% inthis case.

Next, we applied an external electric field to the ultrathinfilm using a plate condenser. The corresponding data aredisplayed in Fig. 18 and we found a strong difference in theintensity of the F1s absorption band depending on thepolarization of the incoming light. The NEXAFS signal ofsuch an ultrathin film has changed upon the external


polarization. We find values for the linear dichroism P ofabove 70% after polarization, which indicates that theexternal field has caused a significant reorientation ofthe ferroelectric dipoles. A detailed analysis of the relativeintensities before and after application of the external field isgiven in Ref. [32].

The NEXAFS investigation shows ferroelectric switch-ing, even for ultrathin films with 10 nm thickness. A similaragreement is obtained for our MIS structures with theferroelectric P[VDF-TrFE]. In Fig. 19a, we show the CVmeasurement with a p-Si/SiO2 substrate of a 10 nm thickP[VDF-TrFE] film, as determined from AFM measurementsAs inert top electrode without interface reactions, indiumwas used instead of aluminum. With indium, a preparationis possible at room temperture, without evaporation. Here,possible interface reactions and degradation of the ferro-electric switching should be suppressed.

The figure shows two voltage loops of þ5 V and þ15 V.In both experiments, we realize a clear hysteresis. Thishysteresis of the CV line indicates the presence ofpolarization charges and hence, of a ferroelectric alignment.In contrast, a measurement of a p-Si/SiO2/In stack withoutthe P[VDF/TrFE] layer shows almost no hysteresis shift atall (shown as inset of Fig. 19a). An estimation of the

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Figure 19 CVmeasurements on a Si/SiO2 (30 nm) substrate. Dataare obtained from two different bias windows of (a) þ5 V and (b)þ15 V, as inset: Reference without P[VDF/TrFE] (a). Fixed polar-ization chargeN, after Eq. (4), versus film thickness is plotted in (b).

polarization charge is possible with a calculation of fixedsurface chargesN, according to Eq. (4), show in Fig. 19b. Wedetermined these values from our CV measurements asdisplayed in Fig. 19a. Evidently, there is no significantchange of the obtained polarization charges when the filmthickness is reduced from 150 nm down to 10 nm. For thinP[VDF/TrFE] layers, the memory window DV has to bemuch smaller while the surface charge remains the same,according to Eq. (4).

To conclude this chapter, we have been able todemonstrate that a fundamental threshold for ferroelectricswitching does not exist down to at least a 10 nm thickness ofa spin-coated P[VDF/TrFE] copolymer film. A prerequisitefor these observations seems to be an adapted system ofelectrodes without interactions at the interfaces.

9 Summary and outlook It is demonstrated that thehysteresis in the CV loop of a MFIS system is due to theferroelectric switching as revealed by measurements athigher temperatures and on reference samples. The operationvoltage can be reduced by an adapted system, where thebuffer layer (thickness, permittivity) and the ferroelectricthickness is optimized.

Interfacial reactions between Al and P[VDF/TrFE] areobserved in the spectroscopic and electrical data. Thereforethe choice of a subsequent electrode material avoidinginterfacial reactions is essential for the further thickness

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reduction of the polymer in order to allow low voltageoperation of such a memory cell.

Long term experiments delivered an improved retentionbehaviour and a very good cycle ability. A retention of atleast 5 days in both states of capacitance could be achieved inmeasurements. Extrapolated data suggest a retention up to291 days, when a minimal difference of 10% between bothstates is assumed. More than 105 cycles are applied without asignificant change in the capacitance. These investigationsare done only on a partially optimized layer sequence, evenbetter values are expected for comprehensively optimizedlayers [19]. Optimization should be possible for fullsaturation of the P[VDF/TrFE], not possible for SiO2 asbuffer layer. As alternative buffer, we propose high-kmaterials like Al2O3 or HfO2. Further, inert electrodesinstead of aluminium should be used to prevent surfacereactions and the degeneration of the ferroelectric polymer.

The application of the simulation proposed by Miller andMcWorther is a very useful tool in order to discuss theparameters of the buffer and the ferroelectric layer, and,hence, to further optimize these stacks. Here, we observe avery good matching of the predicted and measured results.

Furthermore, by using an adapted system of electrodeswe could show, that there exists no fundamental threshold forthe ferroelectric switching of ultra-thin spin-coated P[VDF/TrFE] films down to a thickness of at least 10 nm. This is anadditional prerequisite for systems with reduced operationvoltages. Currently, we are working to learn more about thespontaneous polarization of the ferroelectric dipoles inultrathin films.

Additionally, the results of this work about the formationof an interfacial non-ferroeletric layer as well as of thethickness dependent polarization partially motivated discus-sions of screening effects of non-ferroelectric layers onto thepolarization. More Information is available in Ref. [30].

The copolymer Poly[vinylidene fluoride trifluoroethy-lene] (P[VDF/TrFE]) is a very promising candidate for non-volatile memory applications. Due to the possibility of apreparation of spin coated films at room temperature, a lowcost approach could be realized. Also, the integration in fullorganic approach is possible by using of organic conductorslike Poly(3,4-ethylenedioxythiophene) poly(styrenesulfo-nate) (PEDOT:PSS) as material for electrodes, not only forcross bar arrays, but also for ferroelectric field-effecttransistors, based on organic semiconductors or graphene.

Acknowledgements The authors gratefully acknowledgethe Deutsche Forschungsgemeinschaft (DFG) for financial fundingduring the priority programme 1157 (Schm 745/11-1-2). For waferswith HfO2 and Al2O3 we gratefully acknowledge Dr J. Niinisto fromHelsinki University and Professor Dr E. Zschech from FraunhoferIZFP Dresden. For P(E) measurements with MIM structures weacknowledge Professor Dr P. Muller, Dr I. Lazareva and Dr I. Kovalfrom the University of Erlangen. Further, it is a pleasure toacknowledge the skilful experimental assistance of G. Beuckert,P. Hoffmann, and of the Berliner Elektronenspeicherring-Gesellschaft fur Synchrotronstrahlung (BESSY).



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