Статьи на перевод PVDF_P(VDF-TrFE) / The Intrinsic Coercive Field for P(VDF-TrFE) Thin-Films with Different Thicknesses

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Integrated Ferroelectrics, 133:81–87, 2012

Copyright © Taylor & Francis Group, LLC

ISSN: 1058-4587 print / 1607-8489 online

DOI: 10.1080/10584587.2012.673985

The Intrinsic Coercive Field for P(VDF-TrFE) Thin-Films with Different Thicknesses

X. B. LIU, A. Q. JIANG, AND T. A. TANG

State Key Lab of ASIC and System, Fudan University, Shanghai, 200433, China

Generally, the organic ferroelectric P(VDF-TrFE) thin film is partially crystallized with a mixture of ferroelectric crystallites, non-crystalline molecules, and additional non-ferroelectric crystallites, e.g., Trifluoroethylene, in the copolymers. Therefore, it is almost impossible for the domains to switch under a normal field without incurring of charge injection across these non-ferroelectric phases. In this paper, we use an equivalent-circuit description of the film consisting of a non-ferroelectric layer in series with a ferroelectric layer. Based on the polynomial fitting of polarization switching/nonswitching results, we extract the intrinsic coercive field across the ferroelectric layer in the films with different thicknesses.

Pacs Numbers: 77.22.Ej; 77.80.Fm; 77.55.+f

Index Terms: Intrinsic coercive field; P(VDF-TrFE) thin-film

I. Introduction

Poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer thin films have attracted a great deal of attention in organic flexible electronics, such as non-volatile memories, transducers and display elements [1–3]. The material ferroelectricity originates from molecular dipoles associated with positively charged hydrogen atoms and negatively charged fluorine atoms. The all-trans conformation of chain molecular and their parallel packing cause an alignment of all molecular dipoles in one direction to induce a large polarization. Polarization reversal occurs above a coercive field through the rotation of molecules about their chain axes. For decades, many efforts had been put in the investigation of the intrinsic coercive field of P(VDF-TrFE) thin films. The intrinsic coercive field had been realized in two-dimensional Langmuir-Blodgett polymer films with outstanding crystallinity [4]. A semi-empirical law of coercive field increasing with decreasing of film thickness had been used successfully in polymer thin films down to one nanometer accounting for depolarization corrections [5]. It may, however, be noted that the organic ferroelectric P(VDF-TrFE) thin film is partially crystallized with a mixture of ferroelectric crystallites, non-crystalline molecules, and additional non-ferroelectric crystallites, e.g., Trifluoroethylene, in the copolymers. Beside that, it is suspected that, during the top-electrode deposition, thin passive layers are formed between the ferroelectric and electrodes. Therefore, it’s necessary to investigate the intrinsic coercive field of P(VDFTrFE) thin films after taking into consideration of the non-ferroelectric contribution in the

Received in final form August 1, 2011.Corresponding author. E-mail: aqjiang@fudan.edu.cn

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films. Recently, we used an electrical equivalent circuit description of the P(VDF-TrFE) thin films, in which the non-ferroelectric parts can behave as a linear capacitor in series with a ferroelectric capacitor. By a polynomial-term fit of polarization switching/non-switching dependence on the applied voltage, we extract the intrinsic coercive voltage across the ferroelectric layer in the P(VDF-TrFE) thin film [6]. In this article, we extract the intrinsic coercive field of different thick films without determining the thickness of the equivalent ferroelectric layer of the films.

II. Experrimental

P(VDF-TrFE) (70/30) copolymer was dissolved in DEC (diethyl carbonate) solution with the concentration of 2.0, 2.5, 3.0 and 3.5 wt%. The films were fabricated by spin-coating technology on platinized-Si substrates with the film thickness of 40, 60,120 and 200 nm, as detailed elsewhere [7], and then the film were annealed at 135◦C for 2 hours. Al top electrodes were evaporated on the film through shadow mask at room temperature with each electrode area of 3.14 × 10−3 cm2. Polymer film thicknesses were determined using a spectroscopic ellipsometers (SOPRA, GES5E).

Various function-step voltage pulses with rise times of 8 and 12 ns were supplied by Agilent 33250A and 8114A waveform generators for domain switching investigation between the voltage ranges of 0–10 V and 0–100 V, respectively. Transient voltages across the film and all in-series resistors with the total resistance of RL (including the internal resistance of the voltage source) were simultaneously monitored by an LC WR 6200A oscilloscope interfaced with a remote computer. Domain switching and non-switching charge densities of Psw and Pnsw are measured through time integrations of switching and nonswitching currents under pulse traces of negative-positive and positive-positive sequences, respectively, whereas the remanent polarization is defined as Pr = (Psw-Pnsw)/2. Polarization versus voltage (P-V) hysteresis loops were performed on a Radiant Technologies Precision Analyzer (Premium II) with a triangular wave form.

III. Result and Dissution

A. Electric Equivalent Circuit Description of Non-Ferroelectric Layer Capacitor in Series with a Ferroelectric Layer Capacitor

What the non-ferroelectric impurities and interfacial passive layers can behave equivalently as a linear capacitor (denoted as Ci) in series with a ferroelectric capacitor (denoted as Cf) had been manifested [6]. The equivalent circuit is as sketched in Fig. 1.

The capacitor of the whole film Ct can be described by an in-series capacitor formula

The dependence of Pnsw-Vappl is roughly described by

where Vappl is the applied voltage.

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Figure 1. Equivalent circuit description of the P(VDF-TrFE) film composed of Ci and Cf.

The intrinsic voltage Vcf (the voltage across Cf only) and equivalent non-ferroelectric

Where V0 is the distribution voltage, S is the area of electrode and P0 is constant polarization.

Figure 2. Hysteresis loops of P(VDF-TrFE) thin films with thickness of 40 nm, 60 nm, 120 nm and 200 nm at 100 Hz.

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Figure 3. Domain switching and non-switching currents of P(VDF-TrFE) thin films with thickness of 40 nm, 60 nm, 120 nm and 200 nm.

However, it’s impossible to determine the value of df in the P(VDF-TrFE) thin films. Due to this difficulty, we multiply both sides of Eq.4 by an intrinsic voltage of Vcf, and get,

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B. Intrinsic Coercive Field Extraction from P(VDF-TrFE) Thin Films

P-V hysteresis loops at 100 Hz in the films with the thickness of 40 nm, 60 nm, 120 nm and 200 nm show a nearly square-like shape, as shown in Fig. 2. Figure 3 shows switching and non-switching current transients of the capacitor preset negatively or positively on the top electrode under Vappl increasing from 12 V to 26 V. For the large resistance in series with the circuit (RL = 1 MΩ), the domain switching time is in the same order of P-V hysteresis loop characterization time of 10 ms.

Figure 5. Non-switching polarizations of P(VDF-TrFE) thin films with thickness of 40 nm, 60 nm, 120 nm and 200 nm.

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Figure 6. The capacitance of the whole polymer films Ct and non-ferroelectric capacitances Ci derived from Pnsw-Vappl and Psw-Vappl dependence.

From time integrations of switching and non-switching currents, we get the switching polarizations and non-switching polarizations of P(VDF-TrFE) thin films with thicknesses of 40 nm, 60 nm, 120 nm and 200 nm as shown in Figs. 4 and 5. The solid-lines in Fig. 4 are the best fitting the data according to Eq. 3 from which the intrinsic voltages Vcf and equivalent non-ferroelectric capacitances Ci were determined. With the film thickness increasing from 40 nm to 200 nm, Vcf increases from 3.0 V to 8.4 V smaller than that from 3.21 V to 12.1 V observed from the apparent coercive voltages of the whole films. The capacitors of the whole film Ct were extracted from the best fitting line of nonswitching polarizations in Fig. 5 according to Eq. 2. Ct and Ci of P(VDF-TrFE) thin films were plotted in Fig. 6. Unexpectedly, the capacitors of the whole film Ct keep constantly with the increasing of the film thickness, this may be due to the higher crystallinity in thick films [7], which induced the thickening of ferroelectric layer coincide with that the

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non-ferroelectric capacitances Ci have a linear increasing from 6.4 nF to 14 nF with film thickness.

Due to the fact that non-ferroelectric impurity phases and interfacial passive layers can behave equivalently as a linear capacitor Ci in series with a ferroelectric layer capacitor Cf, the ferroelectric layer can be seen as a PVDF layer with high crystallinity with relative dielectric permittivity εf = 10 [8]. From Eq. 6, we get the intrinsic coercive field of the polymer films with thickness of 40 nm, 60 nm, 120 nm and 200 nm as shown in Fig. 7. The intrinsic coercive fields decrease with the increase of the film thickness but have larger values reported in Ref. 4. However, the coercive fields are more close to the results calculated from the LG theory [9]. A precise dielectric permittivity of the ferroelectric layer is required for the accurate determination of the coercive field.

IV. Conclusions

In summary, we use an equivalent-circuit description of the film as a non-ferroelectric layer in series with a ferroelectric layer. The intrinsic coercive fields had been extracted from the polymer films without requirement of the unknown thickness of the equivalent ferroelectric layer of the films. Only requirement from our extraction is the accurate determination of the relative dielectric permittivity of the ferroelectric layer.

Acknowledgments

This work was funded by Shanghai Key Program (grant 1052nm07600) and the Program for Professor of Special Appointment (Eastern Scholar) at Shanghai.

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