Gate Dielectric Dependent on Pentacene Growth and Electrical Stability in OTFTs
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0965-S06-35
Gate Dielectric Dependent on Pentacene Growth and Electrical Stability in OTFTs Kim Chang Su, Jo Sung Jin, Lee Sung Won, and Baik Hong Koo Materials Science and Technology, Yonsei University, Seodaemungu Shinchondong 134, Seoul, 120-749, Korea, Republic of
ABSTRACT We report on the effects of modifying the gate dielectrics by spin coating of HMDS, PVP and PVA which cause different surface energy and surface roughness owing to the different functional groups. In changing the surface state with applying various surface treatments, I-V and C-V measurements of the OTFTs were performed. The PVP-coated OTFTs, which has smoother and lesser amount of OH-groups on the gate dielectric surface, showed enhanced pentacene growth and nearly free hysteresis behavior than that of the HMDS and PVA-coated OTFTs. INTRODUCTION Organic thin film transistors (OTFTs) have received interest in recent years for their potential as a low-cost alternative to amorphous silicon thin film transistors for electronic applications [1-3]. The role of gate dielectric is critical in OTFTs. This is especially true in the case of "bottom gate" architectures in which the organic semiconductor is deposited on top of the gate dielectric and the surface characteristics of the gate dielectric strongly influence the quality of the gate dielectric/organic semiconductor interface, significantly affecting the performance of the device [4-8]. For this reason, the precise control of the surface of the gate dielectric is considered to be one of the key issues for the realization of stable and reliable OTFTs. EXPERIMENT In this study, four types of gate dielectrics with different surface conditions after surface treatments were prepared; a) As-prepared CeO2-SiO2 dielectric [9], b) Hexamethyldisilazane (HMDS)-treated CeO2-SiO2 dielectric, c) 45nm Poly-4-vinylphenol (PVP)-coated CeO2-SiO2 dielectric and d) 200nm Polyvinylalcohol (PVA)-coated CeO2-SiO2 dielectric. DISCUSSION The surface state changed from OH-rich surface to CH3, C6H5 and CH3OH-terminated surface by HMDS, PVP and PVA treatment, respectively. The surface energy and surface roughness of the various-treated CeO2-SiO2 dielectrics are listed in Table 1. Before surface treatment, the water contact angle of as-prepared CeO2-SiO2 dielectric was around 11o, indicating hydrophilic surface (>70 mJ/m2). After non-polar films treatment, contact angles were increased, revealing change of surface from hydrophilic to more hydrophobic surface chemistry. The measured surface energies were 41.32 mJ/m2, 26.32 mJ/m2 and 44.15 mJ/m2 for HMDS,
PVP, PVA, respectively. Especially, after PVP coating, the surface shows dramatically reduced OH-group density known from the lowest surface energy. And, the surface roughness of the gate dielectric is also an important parameter that affects device performance and the morphology of the deposited organic semiconductor films. Increased roughness leads to valleys in the channel region, which may act as carrier traps and various scattering [10,11]. Although the surface Table
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