16083
2007, 111, 16083-16087 Published on Web 10/05/2007
Photopatternable Organosiloxane-Based Inorganic-Organic SiO2-ZrO2 Hybrid Dielectrics for Organic Thin Film Transistors Sunho Jeong,† Seonghee Lee,† Dongjo Kim,† Hyunjung Shin,‡ and Jooho Moon*,† Department of Materials Science and Engineering, Yonsei UniVersity, Seoul, 120-749 Korea, and School of AdVanced Materials Engineering, Kookmin UniVersity, Seoul, 136-702 Korea ReceiVed: August 1, 2007; In Final Form: September 21, 2007
A photopatternable and solution-processable thin gate dielectric for organic thin-film transistors has been fabricated here using an organosiloxane-based organic-inorganic hybrid material. Incorporation of a UVsensitive functional group allowed us to directly obtain a high-resolution patterned gate dielectric using conventional photolithography. Uniform distribution of ZrO2 nanoclusters increased the dielectric constant of the hybrid material from which the hydroxyl groups were removed by low-temperature heat treatment. Coplanar-type organic thin-film transistors utilizing the hybrid dielectrics showed a low threshold voltage and nearly no shift in the threshold voltage due to their high capacitance and the absence of the hydroxyl groups.
Introduction Organic thin-film transistors (OTFTs) have received considerable attention as of late because of their flexibility, light weight, low cost, and easy processability. OTFTs have been considered potential candidates for a wide variety of applications, including large-area displays and low-end electronic devices, such as smart cards and electronic identification tags, as well as large-area sensing devices.1-4 The performance of OTFTs has improved significantly in the past decade and has already reached a level comparable to that of hydrogenated amorphous-silicon transistors.5 However, little work has been reported on gate dielectrics, a critical factor in the electrical performance of OTFTs, relative to the impressive advances that have been made with OTFTs. Several organic gate dielectrics, such as polyvinylphenol, polymethacrylate, polyimide, polyvinyl alcohol, benzo-cyclobutene, and parylene, have been investigated.6-10 Using these materials, gate dielectrics with requisite dielectric properties and flexibility can be fabricated on large area substrates using economical and easy methods (e.g., a solution-processable method). However, most organic dielectrics have the inevitable shortcoming with respect to the fabrication of transistors that is gate dielectrics should be patterned to access either the gate electrode in a bottom-gate configuration or the source/drain electrode in a top-gate configuration, so as to be applicable to active-matrix displays and integrated circuits. Photolithography, which involves photoresist patterning, gate dielectric etching, and photoresist removal, has been employed to pattern the gate dielectric. However, this process is rather complicated and relatively expensive. Photopatternable gate dielectrics are considered a viable alternative material that can simplify complicated procedures, thereby reducing the manu* To whom correspondence should be addressed. Phone: +82-21232855. Fax: +82-365-5882. E-mail:
[email protected]. † Yonsei University. ‡ Kookmin University.
10.1021/jp0761463 CCC: $37.00
facturing costs of large-area organic electronic applications. The research on photoimagiable gate dielectrics has been restricted to a few materials, such as polyvinylphenol, polyimide, acrylbased polymer, and sol-gel derived siloxane-based hybrid polymer.11-17 In particular, it is beneficial for the sol-gel dielectric material that the chemical structure can be tailored at the molecular level by controlling the precursor chemistry. However, the sol-gel dielectrics reported so far suffer from critical shortcomings such as a high threshold voltage around -15 V due to a low capacitance below 3 nF/cm2 and/or significant threshold voltage shift.12,16 In this study, we present sol-gel derived photopatternable organosiloxane-based dielectrics with a relatively high capacitance and nearly zero threshold voltage shift. These characteristics are accomplished by incorporating ZrO2 with a high dielectric constant (k) into thin dielectric films and by completely eliminating hydroxyl groups. The mechanism underlying the photopatternability was also investigated and the electrical performances of transistors based on hybrid dielectrics were analyzed. Experimental Methods Photosensitive organic-inorganic hybrid precursor solutions were prepared from a combination of 3-methacryloxypropyltrimethoxysilane [H2CdC(CH3)CO2(CH2)3Si(OCH3)3] (MEMO, Aldrich, purity 98%) and zirconium propoxide [Zr(OCH(CH3)2)4] (Zr(OPr)4, Aldrich, 70% in 1-propanol). MEMO was pre-hydrolyzed with 1 molar equiv H2O in the presence of HCl as a catalyst to reduce the chemical reactivity difference between MEMO and Zr(OPr)4. Zr(OPr)4 and methacrylic acid [H2Cd C(CH3)CO2H] (MMA, Aldrich, purity 98%) were introduced into the pre-hydrolyzed MEMO solution at a molar ratio of MEMO:Zr(OPr)4:MMA ) 8:2:4. The addition of water then completed the hydrolysis and condensation. After 24 h of agitation, a photoinitiator, 1-hydroxycyclohexylphenylketone © 2007 American Chemical Society
16084 J. Phys. Chem. C, Vol. 111, No. 44, 2007
Letters
Figure 1. (a) Schematic diagram showing the chemical structure of a photopatternable organosiloxane-based organic-inorganic hybrid precursor synthesized via hydrolysis and condensation reactions using MEMO and Zr(OPri)4 and (b) schematic diagram showing the mechanism of photopolymerization propagated by interaction between a photodecomposed initiator and a methacryl group.
[HOC6H10COC6H5] (IRGACURE 184, Aldrich, purity 99%), was added at a molar ratio of the mixed precursor:IRGACURE ) 20:1. The synthesized precursor solution was diluted by different ratios to control the thickness of the hybrid dielectric film. The resulting solution was filtered through a 0.2 µm membrane (PTFE, Advantec MFS) prior to spin-coating. The heavily doped silicon substrate was used as a substrate and cleaned by a wet method using trichloroethylene, acetone, isopropyl alcohol, methyl alcohol, and deionized water. Surface native oxide on the Si substrate (a few angstrom-thick) was not removed since its presence did not cause a variation in the electrical properties of the hybrid dielectrics. The sol was then spin-coated and prebaked at 90 °C to evaporate the solvent and improve adhesion to the substrate. For the generation of patterns, the hybrid dielectric layer was selectively irradiated using a UV lamp (350W Hg, λ
