Synthesis of high-k ultrathin polymer gate dielectrics for thin-film transistors (TFTs)

Abstract

A series of ultrathin, high-k copolymer gate dielectric films was demonstrated. The copolymer films were syn-thesized from 2-cyanoethyl acrylate (CEA) and di(ethylene glycol) divinyl ether (DEGDVE) monomers by a free radical polymerization reaction via a vapor-phase method, initiated chemical vapor deposition (iCVD). The chemical composition of the copolymers was systematically optimized by tuning the input ratio of the vaporized CEA and DEGDVE monomers in order to achieve a high dielectric constant (k) as well as excellent dielectric strength. The DEGDVE crosslinker matrix provides a reinforced insulating performance to the polymer dielectric layer, whereas the high dielectric constant stems from the CEA moiety with high dipole moment. The dielectric film with the optimized composition showed the dielectric constant of 6.2 and also extremely low leakage current densities ($<3.2 x 10^{-8} A/cm^2$ in the range of $\pm$2 MV/cm), whose thickness was only 20 nm. Such an excellent dielectric property was maintained even with the applied tensile strain up to 2.2%. With the high-k dielectric layer, organic TFTs (OTFTs) and oxide TFTs were fabricated, which commonly showed hysteresis-free transfer characteristics with the operating voltage less than 5 V . Furthermore, the flexible OTFTs on a plastic substrate retained their low gate leakage current and ideal TFT characteristics even in bent condition with the 2% of the applied tensile strain. These ultrathin, high-k organic dielectric films with excellent dielectric performance and mechanical flexibility will play a crucial role in future soft electronics.