A series of high-k, ultrathin copolymer gate dielectrics were synthesized from 2-cyanoethyl acrylate (CEA) and di(ethylene glycol) divinyl ether (DEGDVE) monomers by a free radical polymerization via a one-step, vapor-phase, initiated chemical vapor deposition (iCVD) method. The chemical composition of the copolymers was systematically optimized by tuning the input ratio of the vaporized CEA and DEGDVE monomers to achieve a high dielectric constant (k) as well as excellent dielectric strength. Interestingly, DEGDVE was nonhomopolymerizable but it was able to form a copolymer with other kinds of monomers. Utilizing this interesting property of the DEGDVE cross-linker, the dielectric constant of the copolymer film could be maximized with minimum incorporation of the cross-linker moiety. To our knowledge, this is the first report on the synthesis of a cyanide-containing polymer in the vapor phase, where a high-purity polymer film with a maximized dielectric constant was achieved. The dielectric film with the optimized composition showed a dielectric constant greater than 6 and extremely low leakage current densities (<3 × 10 A/cm in the range of ±2 MV/cm), with a thickness of only 20 nm, which is an outstanding thickness for down-scalable cyanide polymer dielectrics. With this high-k dielectric layer, organic thin-film transistors (OTFTs) and oxide TFTs were fabricated, which showed hysteresis-free transfer characteristics with an operating voltage of less than 3 V. Furthermore, the flexible OTFTs retained their low gate leakage current and ideal TFT characteristics even under 2% applied tensile strain, which makes them some of the most flexible OTFTs reported to date. We believe that these ultrathin, high-k organic dielectric films with excellent mechanical flexibility will play a crucial role in future soft electronics.

Advances in device technology have been accompanied by the development of new types of materials and device fabrication methods. Considering device design, initiated chemical vapor deposition (iCVD) inspires innovation as a platform technology that extends the application range of a material or device. iCVD serves as a versatile tool for surface modification using functional thin film. The building of polymeric thin films from vapor phase monomers is highly desirable for the surface modification of thermally sensitive substrates. The precise control of thin film thicknesses can be achieved using iCVD, creating a conformal coating on nano-, and microstructured substrates such as membranes and microfluidics. iCVD allows for the deposition of polymer thin films of high chemical functionality, and thus, substrate surfaces can be functionalized directly from the iCVD polymer film or can selectively gain functionality through chemical reactions between functional groups on the substrate and other reactive molecules. These beneficial aspects of iCVD can spur breakthroughs in device fabrication based on the deposition of robust and functional polymer thin films. This review describes significant implications of and recent progress made in iCVD-based technologies in three fields: electronic devices, surface engineering, and biomedical applications.

Organic-inorganic hybrid dielectrics have attracted considerable attention for improving both the dielectric constant ( k) and mechanical flexibility of the gate dielectric layer for emerging flexible and wearable electronics. However, conventional solution-based hybrid materials, such as nanocomposite and self-assembled nanodielectrics, have limitations in the dielectric quality when the thickness is deep-scaled, which is critical to realizing high-performance flexible devices. This study proposes a novel vapor-phase synthesis method to form an ultrathin, homogeneous, high- k organic-inorganic hybrid dielectric. A series of hybrid dielectrics is synthesized via initiated chemical vapor deposition (iCVD) in a one-step manner, where 2-hydroxyethyl methacrylate and trimethylaluminum are used as the monomer and inorganic precursor, respectively. The thickness and composition are effectively controlled to form a uniform, defect-free hybrid dielectric. As a result, the synthesized hybrid dielectric has a high- k value as high as 7 and exhibits a low leakage current density of less than 3 × 10 A/cm at 2 MV/cm, even with an equivalent oxide thickness of less than 5 nm. Furthermore, the dielectric layer shows exceptional chemical stability without any degradation in its dielectric performance and a smooth surface morphology. The dielectric layer also has good flexibility, maintaining its excellent dielectric performance under a tensile strain of up to 2.6%. Organic thin-film transistors with the developed hybrid dielectric as the gate dielectric achieved hysteresis-free transfer characteristics, with an operating voltage of up to 4 V and excellent mechanical flexibility as well. The hybrid dielectric synthesized via the iCVD process is a promising candidate for high-performance, low-power flexible electronics.

wileyonlinelibrary.comand portable devices, the device performance must be substantially improved. To this end, high-performance organic gate dielectric layers with high mechanical stability and large-area processability are urgently requested. [ 1 ] However, OTFTs made of polymer gate dielectrics are suffering from high operating voltage, typically exceeding 20 V, [ 3,4 ] due to the gate dielectrics with the thickness often greater than 100 nm to ensure low gate leakage current. [ 4 ] Thermally crosslinked polymers including poly(4vinylphenol) (PVP), [ 5 ] Cytop, [ 6 ] benzocyclobutene (BCB), [ 7 ] and polyimides (PIs) [ 8 ] have been adopted for gate dielectrics, but only few of them could successfully reduce the operating voltage of OTFTs down to 10 V by reducing the thickness of dielectric layer. Moreover, the high annealing temperature required to induce the crosslinking of polymer gate dielectrics is also problematic, which may cause damage to OTFTs and limit their application to thermally vulnerable substrates.Besides reducing the operating voltage of OTFTs, controlling the interface between dielectric and semiconductor is another critical factor to optimize the performance of OTFTs. [ 9,10 ] Applying selfassembled monolayers (SAMs) [ 10 ] or plasma treatment [ 11 ] had been widely applied to optimize the fi lm morphology of semiconductors on the surface of gate dielectrics. However, most of these SAM treatment procedures require specifi c surface coupling reaction, and had been applied mostly onto inorganic dielectric layers rather than polymer dielectrics, mostly due to the lack of target surface functionalities on most of the polymer dielectrics and/or a reliable, damage-free coupling reaction applicable onto the polymer dielectrics.Recently, we have proposed initiated chemical vapor deposition (iCVD) as a new deposition method to form ultrathin (<10 nm) crosslinked polymer dielectrics with high density. [ 12 ] iCVD is a well-established dry process, which can deposit highly pure polymer thin fi lms in mild process temperature (10-40 °C) and pressure (in the order of 100 mTorr). [ 13 ] Especially, an organosilicon polymer, poly(1,3,5-trivinyl-1,3,5-trimethyl cyclotrisiloxane) (pV3D3), exhibited an extremely low leakage current (less than 10 −9 A cm −2 at 3 MV cm −1 ) even with ultralow thickness (≈6 nm), and chemical/mechanical robustness. Nevertheless, Tailoring the surface of the dielectric layer is of critical importance to form a good interface with the following channel layer for organic thin fi lm transistors (OTFTs). Here, a simple surface treatment method is applied onto an ultrathin (<15 nm) organosilicon-based dielectric layer via the initiated chemical vapor deposition (iCVD) to make it compatible with organic semiconductors without degrading its insulating property. A molecular-thin oxide capping layer is formed on a 15 nm thick poly(1,3,5-trimetyl-1,3,5-trivinyl cyclotrisiloxane) (pV3D3) by a brief oxygen plasma treatment. The capping layer greatly enhances the thermal stability of the diel...

This work demonstrates that threshold voltage (VT) of organic thin‐film transistors (OTFTs) can be controlled systematically by introducing new copolymer dielectrics with electropositive functionality. A series of homogeneous copolymer dielectrics are polymerized from two monomers, 1,3,5‐trimethyl‐1,3,5‐trivinyl cyclotrisiloxane (V3D3) and 1‐vinylimidazole (VI), via initiated chemical vapor deposition. The chemical composition of the copolymer dielectrics is exquisitely controlled to tune the VT of C60 OTFTs. In particular, all the copolymer dielectrics demonstrated in this work exhibit extremely low leakage current densities (lower than 2.5 × 10−8 A cm−2 at ±3 MV cm−1) even with a thickness less than 23 nm. Furthermore, by introducing an ultrathin pV3D3 interfacial layer (about 3 nm) between the copolymer dielectrics and C60 semiconductor, the high mobility of the C60 OTFTs (about 1 cm2 V−1 s−1) remains unperturbed, showing that VT can be controlled independently by tuning the composition of the copolymer dielectrics. Coupled with the ultralow dielectric thickness, the independent VT controllability allows the VT to be aligned near 0 V with sub‐3 V operating voltage, which enables a substantial decrease of device power consumption. The suggested method can be employed widely to enhance device performance and reduce power consumption in various organic integrated circuit applications.