JOURNAL OF SEMICONDUCTOR TECHNOLOGY AND SCIENCE, VOL.19, NO.3, JUNE, 2019 ISSN(Print) 1598-1657 https://doi.org/10.5573/JSTS.2019.19.3.287 ISSN(Online) 2233-4866

Manuscript received Nov. 17, 2018; accepted Apr. 29, 2019 School of Electrical and Electronic Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, Korea E-mail : pbe@uos.ac.kr

Characterizations of Blended PVDF/P(VDF-TrFE) Films for Ferroelectric Organic Field-Effect Transistors

Kyung-Eun Park, Min Gee kim, and Byung-Eun Park

Abstract—In this study, we fabricated blended PVDF-P(VDF-TrFE) solutions at different concentrations and formed ferroelectric films via the sol gel method. We then worked with organic ferroelectric field-effect transistors featuring ferroelectric polymer (PVDF/P(VDF-TrFE)) blended films and a poly-3-hexylthiophene(P3HT) organic channel on a TiN substrate. Finally, electrical and ferroelectric properties were compared to those of pure P(VDF-TrFE) and blended PVDF/P(VDF-TrFE) films. Index Terms—Ferroelectric, P(VDF-TrFE), PVDF, P3HT, organic FET, sol-gel method

I. INTRODUCTION

Poly(vinylidene fluoride)(PVDF) is a ferroelectric polymer whose use in organic ferroelectric devices has been widely investigated [1]. Increasing the PVDF β-phase, demands that certain processes such as stretching and poling take place [1, 2]. PVDF has a crystalline phase in at least four forms (i.e., α-, β-, γ-, and δ) [2, 10]. Generally, PVDF has a non-polar α-phase that can be transformed into a γ- or δ-phase through the application of an electric field. In the case of a γ- or δ- phase, a crystalline phase with some polarity but a stronger ferroelectric property is found in the β-phase, but it must be obtained through stretching or poling work as mentioned [1, 2].

On the other hand, poly(vinylidene fluoride-

trifluoroethylene)(P(VDF-TrFE)) which features a combination of TrFE in PVDF has been developed to resolve this problem, and is considered a material with crystallinity and ferroelectric properties superior to those in pure PVDF. This ferroelectric copolymer has been studied in various fields, including functional organic devices such as sensors, transducers, and field-effect transistors [11]. In recent P(VDF-TrFE) studies, various attempts have been made to increase the β-phase and physical stability of thin films. One such method involves the use of buffer layers, such as polyvinyl alcohol, polyvinyl pyrrolidone, and poly(methyl-methacrylate) [3]. The use of a mixture of PVDF and P(VDF-TrFE) has also been studied with the aim of maximizing the ferroelectric properties and improve the dielectric properties of P(VDF-TrFE) films [2].

Miscibility is an important factor in mixing two or more polymers [2]. Chemical studies have previously addressed this issue in experiments featuring blended PVDF/P(VDF-TrFE). According to research results, in the crystallization process, PVDF crystallizes spherically and P(VDF-TrFE) crystallizes into a stacked lamellar structure, while in the case of blended thin films, both polymers crystallize together, with a no clear phase separation. What is interesting is that the Curie transition temperature of blended thin films is lower than that of pure PVDF films [2, 5, 8]. The ability to lower the Curie temperature is advantageous in creating organic devices in which low-temperature processes are important [6, 12]. Published results show that if PVDF content is higher than that of P(VDF-TrFE), and that residual polarization is low due to the formation of the β-phase, even after heat treatment. We therefore sought to observe changes in characteristics when PVDF was added, relative to pure

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P(VDF-TrFE). In this study, we fabricated metal-ferroelectric-metal

(MFM) capacitors with TiN substrates and ferroelectric field-effect transistors (FeFETs) using poly(3-hexylthiopene) (P3HT) to compare the device characteristics of pure P(VDF-TrFE) and blended PVDF/P(VDF-TrFE) thin films. We used P3HT because it can be easily deposited in a thin film through the spin- coating method; as such, there was no need to use expensive deposition equipment. Additionally, P(VDF-TrFE) [9] which is a ferroelectric material can be manufactured through a spin-coating method. Therefore, its use was adopted to simplify the process [4, 7, 13]. This is the first time research of the PVDF/PVDF-TrFE blending films apply for Fe-OFETs..

II. EXPERIMENTAL PROCEDURE

The materials used were further purification or purchased from the following companies: PVDF (Sigma-Aldrich, USA), P(VDF-TrFE) (Piezotech, 75/25 mol%, France), Poly(3-hexylthiopene-2,5-diyl)(P3HT; Rieke Metals, Inc., USA), N,N-Dimethylformamide(DMF; Acros Organics, Belgium), and chloroform (99.8%; Samchun, Republic of Korea).

P(VDF-TrFE) and PVDF/P(VDF-TrFE) were each prepared with a solution bearing a ratio of 3wt%; DMF was used as the solvent. Two different PVDF/P(VDF-TrFE) ratios were used and subsequently compared to a pure P(VDF-TrFE) film namely, 50:50 and 30:70. Furthermore, each solution was stirred in 12 hours. Then, also they have to the aging time during 1 day. Prepared solutions were using for ferroelectric thin films. and there were formed on the TiN substrate by spin-coating at 3000 rpm, using the prepared solution and crystallizing at 150°C in a dry oven for 1 h. P3HT was used as an organic channel layer; a solution was prepared by dissolving 1wt% in chloroform, followed by the formation of a thin film by spin-coating and annealing at 100°C in a dry oven for 1 h. All electrodes were fabricated by Au evaporation, using a thermal evaporator.

In this study, we fabricated MFM capacitors to investigate the electrical and ferroelectric properties of P(VDF-TrFE) and PVDF/P(VDF-TrFE) ferroelectric layers. To characterize the ferroelectric layer in term of such properties as remnant polarization, coercive field,

and capacitance MFM capacitors were fabricated and measured in more than five samples under each condition. In addition, we supplemented the basis of the judgment that affects the characteristics of the device, by using AFM to analyze the thin film surface of the ferroelectric layer. Finally, we fabricated Fe-FETs using P3HT and we observed certain device characteristics such as threshold voltage and memory window to investigate the possibility of eventually improving device characteristics.

III. RESULTS AND DISCUSSION

Fig. 1 is a polarization-electricfield (P-E) graph of the pure P(VDF-TrFE) and PVDF/P(VDF-TrFE) films. The P-E hysteresis loops were measured by applying an electric field until the polarization characteristics became saturated (~2 MV/cm). For pure P(VDF-TrFE) films, when the applied electric field was 2 MV/cm, the coercive field and remnant polarization were 0.79 MV/cm and 5.05 μC/cm2, respectively. The PVDF content was then increased to 30% and 50%, and changes in remnant polarization Pr were observed. When the PVDF content was 30%, the remnant polarization was 5.48 μC/cm2 and the coercive field was 0.76 MV/cm; these characteristics were either similar to those of the pure P(VDF-TrFE) thin film, or the remnant polarization tended to improve. The 50% thin blended film, on the other hand, exhibited reduced properties relative to pure P(VDF-TrFE) in terms of remnant polarization and coercive field; this appears to be due to a decrease in the

Fig. 1. P-E hysteresis properties of pure P(VDF-TrFE) and PVDF/P(VDF-TrFE) films at 30:70 and 50:50 ratios.

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crystallized β-phase [2]. Table 1 lists the remnant polarization Pr and coercive field values derived in this study.

Fig. 2 shows the C-V results. Every C-V characteristics showed the butterfly curve and Capacitance was increased when a PVDF concentration was increased from 0 to 50 wt%. Then, 50% PVDF concentration film has the smallest memory window properties. Moreover, the 30% PVDF concentration film has the largest memory window. Therefore, This C-V characteristic results were affected to Fe-OFETs characteristics as insulating properties.

As mentioned, PVDF and P(VDF-TrFE) are known to take different crystalline forms during crystallization: PVDF crystals tend to form as lamellae of spherical crystals, whereas P(VDF-TrFE) crystals form as stacked lamellar structures [1, 2]. Fig. 3. shows the surface morphology of pure P(VDF-TrFE) and blended PVDF/P(VDF-TrFE) thin films. The pure P(VDF-TrFE) thin film has a stacked lamellar structure, and as the PVDF content increases, the crystalline characteristics of PVDF and P(VDF-TrFE) coexist. Particularly, in the case of the 50:50 ratio, it was found that the P(VDF-TrFE) crystal volume was reduced. It was confirmed that the ferroelectric properties of the 50:50 sample were

attenuated in both the P-E and C-V characteristics curves. Fig. 4(a) shows the drain current-gate voltage (ID-VG)

characteristic curves of the FeFETs, from using P3HT as a organic channel layer on a TiN substrate. Gate voltage was applied at strength ranging from -15V to 15V; the drain voltage value of -10V was used to evaluate the device characteristics. First, in the case of pure P(VDF-TrFE) thin films, the on-current and off-current were approximately 1.63×10-6 and 2.32×10-7, respectively. On the other hand, the on/off ratio of the blended film was improved. For a sample of the 30:70, it became current in the on state when improved to about 1.18×10-5; if the current was off, there was a similar level for pure P(VDF-TrFE) film, of 2.12×10-7. Sample with a 50:50 ratio was found to have the most pronounced on/off ratio characteristics with an on-current of 1.01×10-5 and an off-current of 4.04×10-8. Furthermore, the field effect mobility (mFET) can be extracted from the Eq. (1)

( )212DS FE FET GS TH

WI C V VL

m= - (1)

where IDS is the drain to source current, CFE is the capacitance of the Ferroelectric insulator per unit area, and VGS is the gate-to-source voltage. The extracted hole mobility was increased from 0.005 cm2/V·s to 0.06 cm2/V·s when PVDF concentration was increased

Table 1. Summary of remnant polarization Pr and coercive field

P(VDF-TrFE)

PVDF/PVDF-TrFE 30:70

PVDF/PVDF-TrFE 50:50

Pr (μC/cm2) 5.05 5.48 4.08 Coercive field

(MV/cm) 0.79 0.76 0.65

Fig. 2. C-V characteristics of pure P(VDF-TrFE) and blended PVDF/P(VDF-TrFE) films, blended at 30:70 and 50:50.

Fig. 3. AFM images of surfaces of (a) Pure P(VDF-TrFE), (b)PVDF/P(VDF-TrFE) 30:70, (c) PVDF/P(VDF-TrFE) 50:50.The image size is 20 x 20 mm.

290 KYUNG-EUN PARK et al : CHARACTERIZATIONS OF BLENDED PVDF/P(VDF-TrFE) FILMS FOR FERROELECTRIC …

from 0 to 50 wt%. In addition, we used extrapolation to calculate the

threshold voltage and memory window, the core characteristics of FeFET devices; the results thereof are shown in Fig. 4(b). According to the calculated results, the memory window has a pattern similar to that of the 4-5V level overall, but the 50:50 samples tend to have a slightly smaller memory window than those in other experimental conditions.

IV. CONCLUSIONS

In this experiment, we conducted a characterization experiment with an organic FeFET device, using blended PVDF/P(VDF-rFE) thin films. First, blended PVDF/P(VDF-TrFE) thin films were fabricated and compared with pure P(VDF-TrFE) thin films. Then, a

FeFET using P3HT was fabricated, and the device characteristics were observed and compared. The blended film was found to be more electrically stable than the pure P(VDF-TrFE) film. Especially, for the 30:70 sample, both the ferroelectric and electrical characteristics of the FeFET were found to be superior to those of the pure P(VDF-TrFE) film.

ACKNOWLEDGMENTS

This work was supported by the 2017 Research Fund of the University of Seoul.

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Fig. 4. (a) Drain current – gate voltage curve, (b) (-ID)1/2-VG curve of FeFETs.

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Kyung-Eun Park received the M.S. degree in Electrical and Computer Engineering from the University of Seoul, Seoul, Korea, in 2013. Currently, he is pursuing his Ph.D. degree in Electrical and Electronics Engineering, Tokyo Institute of

Technology, Yokohama, Japan.

Min Gee Kim received the B.Eng. and M.Eng degrees in Electrical and Computer Engineering from the University of Seoul, Seoul, Korea, in 2015 and 2017, respectively. He is currently pursuing his Ph.D. degree in Electrical and Electronic

Engineering, Tokyo Institute of Technology, Yokohama, Japan.

Byung-Eun Park received the B.S. and M.S. degrees in University of Seoul, Seoul, Korea, and the Ph.D. degree in applied engineering from the Tokyo Institute of Technology, Tokyo, Japan, in 1987, 1994, and 1999, respectively. He is a professor

at the School of Electrical and Computer Engineering, University of Seoul, Seoul, Korea. Prior to joining University of Seoul in 2004, he was an assistant professor in Tokyo Institute of Technology, and joined the R&D Association for Future Electron Devices, Japan. He has over 200 journal publications and International presentations, and has about 300 patents including pending. He has co-authored book "Ferroelectric-Gate Field Effect Transistor Memories -Device physics and Applications" by Springer in 2016. He is also interested in semiconductor devices, display devices, and solar cell.