Статьи на перевод PVDF_P(VDF-TrFE) / Characterization of Poly(vinylidene fluoride-trifluoroethylene) 50-50

Characterization of Poly(vinylidene fluoride-trifluoroethylene) 50/50 Copolymer Films as a Gate Dielectric

Soojin Wi, N. Senthilkumar and Shi-Woo Rhee*

Laboratory for Advanced Molecular Processing (LAMP) Department of Chemical Engineering,

Pohang University of Science and Technology (POSTECH), Pohang 790-784, Korea

* Author to whom correspondence should be addressed e-mail: srhee@postech.ac.kr

Submitted to Journal of the Electrochemical Society

Dec.21, 2006

Abstract

Thin films of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) 50/50 copolymer were prepared by spin coating on p-Si substrate. Thermal behavior of the film was observed by measuring the film thickness with ellipsometry as a function of the temperature and abrupt volume expansion was observed at 130-150ºC. Capacitancevoltage (C-V) and current-voltage (I-V) behavior of the aluminum/P(VDF-TrFE)/p-Si MIS (metal-insulator-semiconductor) structures were studied and dielectric constant of the P(VDF-TrFE) film was measured to be about 15.3 at optimum condition. No hysteresis was observed in the C-V curve for films as deposited and annealed (70-200ºC). Films annealed at temperatures higher than the volume expansion temperature showed substantial surface roughness due to the crystallization. Flat band voltage (VFB) of the MIS structure with as deposited films was about -0.3 V and increased up to -2.0 V with annealing. This suggested that positive charges were generated in the film. Electronic properties of the annealed P(VDF-TrFE) film at above melting temperature were degraded substantially with larger shift in flat band voltage, low dielectric constant and low breakdown voltage. Organic thin film transistor with pentacene active layer and P(VDF-TrFE) as a gate dielectric layer showed a mobility of 0.31 cm2/V s and threshold voltage of -0.45 V.

Introduction

The development of organic thin film transistors using polymers as a gate dielectric and organic semiconducting materials has raised interest for their use in the field of electronics.1-5 The use of polymeric materials is most attractive for low cost devices fabricated by printing techniques on large area, flexible substrates.2,6-10 There are numerous papers about the polymeric thin films as a gate dielectric in organic thin film transistors (OTFTs).11-20 The dielectric material needs to have very high resistivity (greater than 1012 Ω cm) and high breakdown strength (greater than 0.2 MV/cm) to prevent the leakage between gate metal and semiconductor channel. It should have the highest possible dielectric constant (greater than 4) to have enough capacitance for channel current flow.21 High dielectric constant insulators result in low switching voltage of the OTFTs. Polymeric dielectric films are deposited by spin coating and the surface roughness should be as low as possible (less than 10 nm in root mean square roughness) because it has an effect on the morphology of the organic semiconductor grown on it and also on the mobility of the carrier. The film should be easily formed at low temperature with high thermal stability and the thermal expansion coefficient should match the substrate.

Poly(vinylidene fluoride-trifluoroethylene) copolymer [P(VDF-TrFE): -(CH2- CHF)m-(CHF-CF2)n-] is one of the promising polymeric materials and draws much attention as a high dielectric constant polymeric insulator and as a ferroelectric material.11-16,22-24 P(VDF-TrFE) has a dielectric constant of 12 – 40 which is much greater than most other polymers (3 – 4) due to the high polarity from fluorine with high electronegativity.

In this paper, thin films of poly(vinylidene fluoride-trifluoroethylene) copolymer (P(VDF-TrFE)) were made with spin coating and the electronic characteristics such as capacitance-voltage (C-V) and current-voltage (I-V) behavior of the aluminum/P(VDF- TrFE)/p-Si structure (also called MPS: metal-polymer-semiconductor) were measured. Pentacene is used as a semiconductor, due to its high field effect mobility and its ability to form ordered films on various types of substrates. This is easily soluble in common organic solvents and can be fabricated at low temperatures.18,25 The study of C-V and I-V characteristics of MIS structure gives useful information about the properties of the insulator and insulator-semiconductor interface.26 These measurements give information about charges in the insulating films.27,28 The effect of annealing on the P(VDF-TrFE) films prepared by spin coating technique on silicon was studied. X-ray diffraction (XRD), atomic force microscopy (AFM) and ellipsometry were used to characterize P(VDFTrFE) film. Also organic thin film transistor with pentacene active layer and P(VDFTrFE) film as a gate dielectric was made and its characteristics were measured.

Experimental

Poly(vinylidene fluoride-trifluoroethylene) 50/50 copolymer (P(VDF-TrFE) was obtained from Solvay Korea. Molecular structure of the polymer is shown in Figure 1 and the polymer was used without further purification to form the insulator layer. 2-Butanone solvent was used to make P(VDF-TrFE) solution and details of the experimental conditions were summarized in Table 1. The solution was spun on boron doped Si (100) substrate (p-type Si) at room temperature to prepare thin films. Before deposition, the surface preparations of silicon wafers were done by degreasing it with organic solvents

such as trichloroethylene (TCE), ethanol and then followed by a rinse in deionized (DI) water. The solution concentration was between 0.4g-0.6g polymer/cc solvent. 0.25 ml of the solution was introduced at the center of the substrate and spinning was about a minute. For the electronic characterization, 300 nm thick polymer film was made. After spin coating process, the samples were dried in the vacuum chamber for 3 to 4 hours to evaporate the solvent remained in the film and then the films were annealed at 70-200ºC for 60 minutes in argon (Ar) ambient. The bottom metal contact was formed by evaporating aluminum (Al) on the back side of the Si substrate and the top metal contact was formed by evaporating Al over the polymer film surface by using a suitable metal mask (dot size, 5x10-3 cm2). Small angle X-ray diffractometer was used to examine the structure of the as grown and annealed films. Spectroscopic ellipsometry (SE, J.A. Woolam Co., Model WVASE32) was used to measure the thickness of the films coated on Si substrate. Temperature variation of the film thickness was also measured with ellipsometry to identify the thermal behavior of the film. The surface morphology of the as grown and annealed films was investigated with SEM and AFM. The capacitancevoltage (C-V) measurements were performed using HP 4284A. The C-V measurements were carried out at a voltage range of -4 V to 4 V at 1 MHz. The current-voltage (I-V) characteristics were studied by using HP 5270A. Surface energy of the film was obtained by measuring the contact angle with water with static contact angle goniometry. Pentacene film was deposited with organic molecular beam deposition (OMBD) at 70˚C

and 2 x 10-6 torr to make 300 Å film at a deposition rate of 0.3 Å/s. Gold was evaporated to make a source and drain contact.

Results and Discussion

Figure 2 shows the thickness of the film as a function of the spin speed and the concentration. With thicker solution, the film thickness was larger. With higher spin speed, film became thinner and the film thickness was saturated when the spin speed was greater than 5000 rpm. Figure 3 shows the effect of the solution concentration on the film thickness at the spin speed of 3000 rpm. The film thickness is almost linearly proportional to the concentration. For the electronic characterization, 300 nm thick film was made at the spin speed of 4000 rpm with 0.5 g/cc solution.

The glass transition temperature and melting point of the film reported by the manufacturer were -22ºC and 158.5ºC, respectively. Figure 4 shows the in-situ measured film thickness with spectroscopic ellipsometry as a function of the temperature. Temperature was raised at a rate of 2ºC/min. For P(VDF-TrFE) copolymer, the glass transition temperature is low and at room temperature, the polymer is at quasi-crystal state above the glass transition temperature. The volume was rapidly increased at around 130 – 150ºC and it is believed to be from the melting. The melting of the thin film seems to occur below the melting of the bulk material. Annealing is a process related with stress relief and local structural rearrangement of polymer chains and it is expected that annealing effect can be different below or above the melting temperature. To identify the annealing effect on the dielectric and capacitance-voltage behavior of the film, investigations were carried out for various annealing temperatures ranging from 70 to 200ºC.

The atomic force micrographs of as grown and annealed P(VDF-TrFE) copolymer films are shown in Figure 5. Figure 5(a) is the as grown film surface and (b), (c) are films

annealed below the melting point of 130 - 150ºC. Both as grown and annealed films exhibited round grains of crystalline phase and the root mean square surface roughness is about 1.5 - 2.5 nm which is about one order of magnitude greater than amorphous polymer films.31 Films annealed at 150 and 200ºC showed a surface morphology of high surface roughness formed by melting.

X-ray diffraction spectrum of as grown P(VDF-TrFE) copolymer films and films subjected to various annealing temperatures ranging from 70 to 200ºC are shown in Figure 6. All the films (as deposited film and annealed films) were observed to be α- phase with peak at 18.8˚ with paraelectric properties as mentioned in previous x-ray

studies.22,29,30 The peak intensity of α-phase is increased with annealing and then decreased at high temperature annealing. As confirmed in Figure 8, dielectric constant of the film is directly related with the crystallization of α-phase. α-phase disappeared with annealing at above the melting temperature and it is believed that dielectric constant is more closely related with this phase range.

Figure 7 shows the capacitance-voltage characteristics of the aluminum/P(VDF- TrFE)/p-Si MIS (metal-insulator-semiconductor) structure. Accumulation, depletion and inversion regions are observed for both as grown and annealed films. C-V measurements were performed with a small superimposed AC signal at a frequency of 1 MHz on top of a preselected DC voltage. The voltage was swept forward from negative to positive. At negative voltage, it showed maximum capacitance with accumulation of holes in the p type silicon at the polymer-silicon interface. As the voltage increases, depletion and inversion region were clearly observed with the decrease of the capacitance because now the total capacitance comes from the serial coupling of the capacitance from the polymer

dielectric and the silicon layer. The ideal MOS capacitor has zero flat band voltage if we neglect the work function difference between aluminum and silicon and assume that there is no mobile or fixed charge in the dielectric film and in the interface. In the aluminum/P(VDF-TrFE)/p-Si, the flat band voltage was shifted in the negative direction, which indicates that positive fixed charges exist in the polymer film.27 The shift was about -0.3 V for films as-deposited and about -2.0 V for films annealed at 200ºC. It seems that as we increased the annealing temperature, more chemical bonds were broken in the film and positive charges were formed. Similar C-V behavior (with a VFB shift value of - 8.0 V to -12.0 V) was reported by Lee and Muraka27 for Cu/fluorinated polyimide/SiO2/Si capacitor structure and by Na and Rhee31 for Al/PMMA (poly(methyl methacrylate))/p-Si structure (VFB of about -15 to -30 V) and Al/CEP(cyanoethyl pullulan)/p-Si (VFB of about -1.0 V). P(VDF-TrFE) is thermally more stable than PMMA or polyimide like CEP. It is expected that threshold voltage of the device made with P(VDF-TrFE) will be much lower. This result shows that electrical characterization of MIS structure is quite effective to identify the characteristics of the polymer film.

The dielectric constant of the polymer film was obtained from the maximum capacitance and plotted in Figure 8 as a function of the annealing temperature. As grown P(VDF-TrFE) films showed a relative dielectric constant value of about 13.8. Films annealed at 100ºC showed the maximum dielectric constant of about 15.3 whereas films annealed above the melting temperature showed a decrease in dielectric constant with the increase of annealing temperature. It seems that formation of paraelectric α-phase affects the dielectric constant but ferroelectric β-phase does not.

Figure 9 shows the current-voltage characteristics of 300 nm thick P(VDF-TrFE) films. For as deposited P(VDF-TrFE) films, the leakage current level is low at 10-9-10-6

A/cm2 up to 1.5 MV/cm. The leakage current increased up to the order of 10-6 when the film was annealed at 70 and 100ºC and up to 10-5 when annealed at 150 and 200ºC but they are stable in the field strength range tried in the measurement. The surface energy of the P(VDF-TrFE) films was measured to be 51±0.4 mJ/m2 regardless of the annealing temperature. P(VDF-TrFE) films contain polar groups like -C-F and is more hydrophilic with high surface energy. Annealing did not affect surface energy very much, which indicates that annealing did change the bulk property due to the rotation and twisting of the polymer chain but did not affect the surface state much. Organic thin film transistor with pentacene active layer and P(VDF-TrFE) as a gate dielectric layer showed a mobility of 0.31 cm2/V s and threshold voltage of -0.45 V. The threshold voltage is quite low because P(VDF-TrFE) has high dielectric constant and low flat band voltage shift.

Conclusions

P(VDF-TrFE) thin films were obtained by spin coating and AFM studies revealed that as grown and annealed films showed surface roughness greater than amorphous films due to crystallization. The XRD spectrum of as grown and films subjected to various annealing temperatures showed α phase and this phase content was maximum at 100ºC annealing. Dielectric constant was maximum at an annealing temperature of 100ºC. Capacitance-voltage measurements of the Al/P(VDF-TrFE)/p-Si MIS(metal-insulator- semiconductor) structure showed good depletion behavior. Flat band voltage (VFB) of aluminum/P(VDF-TrFE)/p-Si structure with as deposited films was about -0.3 V and

increased up to -2.0 with annealing. This suggested that positive charge was generated in the film. For as deposited P(VDF-TrFE) films, the leakage current level is low at 10-9-10- 6 A/cm2 up to 1.5 MV/cm. The leakage current increased up to the order of 10-6 when the film was annealed at 70 and 100ºC and up to 10-5 when annealed at 150 and 200ºC but they are stable in the field strength range carried out in the measurement. The study of electrical characterization, C-V and I-V characteristics of MIS structure provides useful information about the properties of the insulator and insulator-semiconductor interface. Moreover P(VDF-TrFE) copolymers possess more thermal stability when compared with other polymers. Organic thin film transistor with pentacene active layer and P(VDFTrFE) as a gate dielectric layer showed a mobility of 0.31 cm2/V s and threshold voltage of -0.45 V. The threshold voltage is quite low because P(VDF-TrFE) has high dielectric constant and low flat band voltage shift.

Acknowledgements

This work was supported by grant No. RTI04-01-04 from the Regional Technology Innovation Program of the Ministry of Commerce, Industry and Energy (MOCIE), and the Korea Science and Engineering Foundation (KOSEF) through the National Research Laboratory Project.