VDF/TrFE

In general, the polymers evaluated and discussed here are poled piezoelectric films of 30 pm PVDF from MSI and 28 pm P(VDF-TrFE) from... 

Sample details for MISSE-6, VUV exposure only are two MSI PVDF bimorphs and two Ktech TrFE copolymer bimorphs with electrodes for actuation on either side. Passive sample are a combination of solution cast 80 20 P(VDF-TrFE), and 50 50 VF2-TrFE films, MSI homo PVDF, Ktech TrFE and preirradiated materials. Different coating thicknesses were applied. 

As ferroelectric material we use poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). This copolymer is soluble in non-toxic reagents, for example 2-butanone. The preparation of organic and ferroelectric thin films via spin coating from solution is possible [13]. The polarisation field of P(VDF-TrFE) is relatively high, about 50 MV/m [14]. Here, a downscaling of the P(VDF-TrFE) film thickness into a range below 100 nm is necessary in order to use small bias voltages for polarisation. 

We present an XPS study of the interfaces P(VDF-TrFE)/Al and P(VDF-TrFE)/PEDOT PSS. As copolymer, we used P(VDF-TrFE) in a molar ratio of 70 30. The material was delivered as film from Piezotech SA, France. As solvent for spin coating, we used 2-butanone. We prepared different concentrations (as wt% P(VDF-TrFE) in 2-butanone) to investigate the dependence on layer thickness. After spin-coating at 6000 rpm, the film was annealed at 135 °C for 2 h to improve the crystallinity of the aXX-trans conformation, the so called P-phase [21]. The thickness of the spin-coated film was measured using a Taylor Hobson (Talystep) profilometer. 

The speetroseopie eharaeterisation of the P(VDF-TrFE)/Al interfaee was performed in top and bottom eleetrode geometries, the investigation of the P(VDF-TrFE)/PEDOT PSS interfaee only in bottom electrode geometry a silicon wafer, covered with a film of spin-coated PEDOT PSS and then an ultrathin layer of P(VDF-TrFE). 

For the measurements of the ferroelectric hysteresis of P(VDF-TrFE) via the flatband shift, we used capacitors with oxidised p-type (-lO cm ) silicon substrate (100-235 nm Si02) to prevent large amoimts of leakage current. The copolymer film was prepared as described above. We used films of thickness fiom 100 nm to 1 pm. The structiues were prepared in top electrode geometry , with thermal evaporated aluminium, patterned via a shadow mask. The measiuements of capacitance versus voltage (CV) were carried out with an Agilent 4284A LCR meter at a frequency of 1 MHz with sweep rates fiom... 

As an example for depth profiling of interfaces, an XPS investigation of the ferroelectric copolymer P(VDF-TrFE) is presented. For a memory application, low operation voltages and a downscaling of the film thickness are necessary. For thin films of the copolymer, interface phenomena become important. We compare the two interfaces P(VDF-TrFE)/Al and P(VDF-TrFE)/PEDOT PSS. For the organic electrode, a better functionality of P(VDF-TrFE) is reported, as described above. 

Figure 21.5 Cls XPS spectram of a P(VDF-TrFE) film, after thermal evaporation of aluminium (solid line), compared with the spectrum of the bulk film (dashed line). The intensity after deposition of A1 is normalised with reference to the CF2 peak of the bulk film.

First, we show an XPS analysis of the P(VDF-TrFE)/Al interfaee, in top electrode geometry. Figure 21.5 shows an XPS spectrum of a P(VDF-TrFE) film after evaporation of a thin aluminium layer (around 1 nm). Compared to the spectrum of the bulk P(VDF-TrFE) film without aluminium, we find the following modifications (i) the relative intensity between CH2 and CF2 peaks is modified towards lower fluorine content, approximately from CH2/CF2 = 0.8 before and CH2/CF2 = 1.0 after evaporation (ii) a small shift of CH2, CFH and CF2 and (iii) a new peak at lower binding energies. This is a clear indication of a surface reaction. 

Figure 21.6 Cls XPS spectra of a P(VDF-TrFE) film, spin-coated onto an Al/Si substrate (bottom electrode structure). Thick solid curve just after spin coating. Dashed curve after thermal armeahng at 135 °C for 2 h. The concentration for spin coating is 0.1 wt% P(VDF-TrFE) in 2-butanone. Spectra are corrected for 0.4 eV and normalised with reference to the bulk film, also shown (thin solid curve).

For investigations of the P(VDF-TrFE)/PEDOT PSS interface, samples in bottom electrode geometry were prepared. Thin films of P(VDF-TrFE) are spin-coated on PEDOT PSS, as described before. Figure 21.7 summarises the Cls spectra of four samples with thin films of P(VDF-TrFE), spin-coated in different concentrations in 2-butanone, compared with a pure PEDOT PSS spectrum (No. 1). As revealed from the figure, no additional structure near the CH2 feature occurs. 

Like aluminium, a possible interface reaction should have an influence on the relative intensities, also (for example CF2 and CH2 of P(VDF-TrFE)). Here an analysis of peak attenuation is helpful. As can be seen, also in Figure 21.7, the intensity of the PEDOT-related signal at -284.8 eV is attenuated proper-... 

Figure 21.7 Cls spectra of samples with thin films ofP(VDF-TrFE), spin-coated in different concentrations in 2-hutanone (for the concentrations (wt%) indicated).

Figure 21.8 Relative intensities of the PEDOT PSS-attributed Cls feature versus P(VDF-TrFE) concentration in 2-butanone solution, as ///Jedot-The line is drawn as a guide for the eyes.

In Figure 21.9, a typical CV characteristic of a Si/lOOnm SiO2/110nm P(VDF-TrFE) capacitor is shown. The CV plot shows sections of the 10 V measurements at room temperature (RT) and at 100 °C. The inset shows the flatband shift AV b (memory window) versus temperature. 

Figure 21.9 Top schematic of our capacitors. Bottom capacitance-voltage characteristic of a 100 nm Si02/110 nm P(VDF-TrFE) sample at room temperature and at 100 °C. The inset shows the flathand voltage shifts dependent on temperature.

In a next step, we study the flatband shift, the polarisation as a function of copolymer film thickness. For the measurements, a relatively thick Si02 buffer layer of 235 nm was used and we reveal a clear thickness dependence of the ferroelectric polarisation [32]. We calculated AiJpvDF s the voltage drop only over the P(VDF-TrFE) layer. For accumulation, negative voltages, the voltage is divided into two parts Ui = 17ipvdf + For depletion, we have to cal-... 

We further recognise a significant reduction of polarisation for the P(VDF-TrFE) thickness below 100 nm, while between 200 nm and 950 nm the dependence is very similar. Generally it has been postulated that the coercive field increases with decreasing of its thickness [38]. For P(VDF-TrFE) a range of 70-100 nm has been proposed as a critical thickness due to reduced crystal-... 

Figure 21.10 Calculated fixed charges versus the electric field change of the ferroelectric layer inside one CV loop for Si02/P(VDF-TrFE) stacks with different thickness of the ferroelectric layer. Thickness of buffer layer is 235 nm. Thickness of P(VDF-TrFE) is as indicated.

Unity [39] or due to interface interactions [14]. It must be pointed out here that a critical thickness is found for electrodes made of aluminium, not for PEDOT PSS [14], Our data are consistent in this context. In Section 21.3.3 we already showed reactive interactions between P(VDF-TrFE) and aluminium, not for the P(VDF-TrFE)/PEDOT PSS interface. This becomes even more important when the thickness of P(VDF-TrFE) film is further downscaled. 

Based on these results, we build up OFETs with a ferroelectric gate insulation, consisting of P(VDF-TrFE). 

We compare two devices the ferroelectric OFET of Figure 21.12 and a similar OFET, without the ferroelectric P(VDF-TrFe) copolymer, for f/pg = -50 V, without gate bias. 

Under radiation damage-free eonditions, we show a clear indication for a surface reaction of P(VDF-TrFE) with A1 electrodes, not only for evaporated aluminium as top electrode, but also, at room temperature, for the aluminium as bottom electrode. In contrast, for PEDOT PSS, the XPS measiuements indicate a layer-by-layer structure of PEDOT PSS/P(VDF-TrFE) without any interface modification. This could be the reason for lower relaxation times, higher switching frequencies and, in consequence, a better field dependenee of the ferroelectric polarisation, if we choose PEDOT PSS as material for the electrode, especially for thin films of the eopolymer. 

The ferroelectric hysteresis of P(VDF-TrFE) is directly investigated by MIS capacitors and OFETs. By using MIS capacitors, a systematic shift of flatband voltage is observed, after applieation of different voltage scan windows. The MIS structures are built up as Al/P(VDF-TrFE)/Si02/Si sandwieh strueture. The dependence of the remanent polarisation on thickness of the eopolymer shows an elevated polarisation voltage for a copolymer film thiekness below 100 nm, obviously due to the above mentioned interface reaction between the eopolymer and aluminium. 

Legrand et al. [100] reported F wideline NMR measurements on a 70/30 mol% VDF-TrFE random copolymer, made in order to study molecular motion both below and above the ferroelectric transition temperature, Tc. The samples consisted of semicrystalline copolymer films of 0.51 mm thickness, with biaxial orientation of the crystalline axis. The samples were rolled (without poling) at 70°C, with a draw ratio of 300%. The F resonance was chosen, rather than the proton resonance, because the abundance ratio of F to nuclei is 1.4. In addition, the F free-induction decay (FID) lasts longer than that of the proton, which decreases the influence of spectrometer deadtime. FID analyses were made assuming a simple superposition of two... 

Fig. 18.18. Temperature dependence of the proton relaxation parameters for poled and drawn VDF/TrFE (52/48 mol%) copolymer, showing the ferroelectric transition. The filled triangles correspond to the case where three components for Tip can be resolved. [Reproduced with permission from Ref. 99.]...

In this paper we have addressed the issue of piezoelectric performance of PVDF and copolymers of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)) over temperature ranges simulating the LEO environment, and examined the effects of radiation (gamma and vacuum ultraviolet) and atomic oxygen. 

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