Croconic Acid Thin Film Formation for Ferroelectric Gate OFETs
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Croconic Acid Thin Film Formation for Ferroelectric Gate OFETs Shun-ichiro Ohmi, Kazuaki Takayama, and Hiroshi Ishiwara Tokyo Institute of Technology, J2-72, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. ABSTRACT Pentacene-based ferroelectric gate transistors with croconic acid (CrA) thin film was fabricated for the first time. The memory window (MW) of 1.9 V was obtained from the capacitance-voltage (C-V) characteristics of Al/CrA(50 nm)/SiO2/Si(100) metal-ferroelectricinsulator-semiconductor (MFIS) diode, where the deposition temperature of CrA was room temperature (RT). Butterfly type C-V characteristics was observed for Al/CrA(50 nm)/Al/SiO2/ Si(100) metal-ferroelectric-metal (MFM) diode. Furthermore, a pentacene-based p-type organic field-effect transistor (OFET) with CrA gate insulator was fabricated, and clockwise hysteresis loop was observed in ID-VG characteristic, which is attributed to the ferroelectric properties of CrA gate insulator. INTRODUCTION Pentacene-based organic field-effect transistors (OFETs) have attracted much attention owing to their possibilities in flexible devices applications [1-6]. In order to realize organic integrated circuits, not only the OFETs but the non-volatile memories should be introduced. Among the various non-volatile memories, ferroelectric field-effect transistors (FeFETs) have clear advantages such as scalability, low power, fast read/write speed, and so on. Therefore, organic FeFET is promising for the organic electronics. One of the issue of the organic ferroelectrics is thin film formation. P(VDF-TrFE) is a typical organic ferroelectric material and shows the excellent ferroelectric properties [7, 8]. However, it is a polymer material and the typical reported thicknesses are 50-100 nm because it is formed by spin-coating method. Therefore, thin film formation is required to realize the low-voltage operation. Recently, it has been reported that the croconic acid (CrA) of 250 m-thick bulk, which is a low molecular mass organic material, shows the ferroelectric property (remanent polarization, Pr: 21 C/cm2) at the room temperature [9]. The molecular mass is 142.07 which is smaller than that of pentacene. Therefore, thin film formation by evaporation would be expected. Furthermore, the polarization is caused by the photon transfer between the molecules, which leads to the lower coercive electric field (Ec: 15 kV/cm). These properties seems to be suitable for low-voltage operation. In this paper, we investigated the ferroelectric properties of CrA thin films formed by evaporation method. Furthermore, ferroelectric gate OFETs with CrA thin films were fabricated for the first time [10]. EXPERIMENTAL DETAILS Figure 1 shows the schematics of metal-ferroelectric-insulator-semiconductor (MFIS) diode (Fig. 1(a)), metal-ferroelectric-metal (MFM) diode (Fig. 1(b)) and ferroelectric gate OFET
Al Croconic Acid SiO2 n-Si(100) Al
Al Croconic Acid Al SiO2 + n -Si(100) Al
(a)
(b)
Au
Au
Pentacene Croconic Acid SiO2 + n -Si(100) Al
(c)
Figure 1. Schematics of (a) MFIS d
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