Byungwook Yoo

High-mobility bottom-contact n -channel organic transistors and their use in complementary ring oscillators

The electrical characteristics of bottom-contact organic field-effect transistors fabricated with the air-stable n-type semiconductor N,N′-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) are described. The mobility, threshold voltage, subthreshold swing, and Ion∕Ioff ratio(VDS=40V, VG=0∼40V) are 0.14cm2∕Vs, 1.6V, 2.0V/decade, and 1.2×103, respectively. The effect of electrode/dielectric surface treatment on these devices is also examined, with a combination of 1-octadecanethiol and hexamethyldisilazane. Organic complementary five-stage ring oscillators were fabricated using pentacene and PDI-8CN2, and operated at an oscillation frequency of 34kHz and a propagation delay per stage of 3μs.

Controlled Deposition of a High‐Performance Small‐Molecule Organic Single‐Crystal Transistor Array by Direct Ink‐Jet Printing

Ink-jet printed small-molecule organic single-crystal transistors are realized by using selective surface energy modification, precise control of volume density of ink droplets on spatially patterned areas, and a co-solvent system to control solvent evaporation properties. The single-crystal formation in bottom-contact-structured transistors via direct printing is expected to permit high-density array fabrication in large-area electronics.

Nanoscale n-channel and ambipolar organic field-effect transistors

N-channel and ambipolar organic field-effect transistors (OFETs) with a few tens of nanometer channel length were fabricated and characterized. N,N′-bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI-8CN2) was employed as the active semiconductor and yielded a linear regime electron mobility of 2.3×10−3cm2∕Vs at 5×105V∕cm in an OFET with channel length of 15nm. An ambipolar heterostructure transistor consisting of thin layers of PDI-8CN2 and pentacene was fabricated with channel length of about 23nm. Field-effect hole and electron mobilities of 9.2×10−3 and 4.0×10−3cm2∕Vs, respectively, are obtained at 5×105V∕cm. These results represent the shortest channel length n-channel and ambipolar organic transistors that have been fabricated.

Selective and localized laser annealing effect for high-performance flexible multilayer MoS2 thin-film transistors

AbstractWe report the use of ultra-short, pulsed-laser annealed Ti/Au contacts to enhance the performance of multilayer MoS2 field effect transistors (FETs) on flexible plastic substrates without thermal damage. An analysis of the temperature distribution, based on finite difference methods, enabled understanding of the compatibility of our picosecond laser annealing for flexible poly(ethylene naphthalate) (PEN) substrates with low thermal budget (< 200 °C). The reduced contact resistance after laser annealing provided a significant improvement in transistor performance including higher peak field-effect mobility (from 24.84 to 44.84 cm2·V−1·s−1), increased output resistance (0.42 MΩ at Vgs − Vth = 20 V, a three-fold increase), a six-fold increase in the self-gain, and decreased sub-threshold swing. Transmission electron microscopy analysis and current-voltage measurements suggested that the reduced contact resistance resulted from the decrease of Schottky barrier width at the MoS2-metal junction. These results demonstrate that selective contact laser annealing is an attractive technology for fabricating low-resistivity metal-semiconductor junctions, providing important implications for the application of high-performance two-dimensional semiconductor FETs in flexible electronics.

Germanium nanowire transistors with ethylene glycol treated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) contacts

Germanium nanowires (Ge NWs) were synthesized via the supercritical fluid-liquid-solid (SFLS) process, followed by surface passivation with isoprene. The Ge NWs were then drop cast from ethanol suspension onto SiO2∕Si substrates. Conductivity-enhanced poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) with ethylene glycol treatment was employed as the source-drain electrodes. The field-effect mobility of Ge nanowire field-effect transistors was as high as 7.0cm2∕Vs, with a p-type response similar to Pt-electrode devices previously reported for SFLS-grown Ge NWs. The organic based contacts provide a potential platform for inexpensive production of flexible nanowire devices.

Direct measurement of carrier drift velocity and mobility in a polymer field-effect transistor

An electronic method to measure the drift velocity and mobility of charge carriers in polymer thin film transistor has been developed. The measurement is based on the movement of a packet of carriers injected into the channel. This technique can be used to explore trap states and therefore obtain a comprehensive understanding of charge transport in these materials. Drift mobility of 0.52cm2∕Vs is obtained from the transit time which is a factor of 3 higher than the field-effect transistor mobility.

Optically transparent thin-film transistors based on 2D multilayer MoS2 and indium zinc oxide electrodes

We report on optically transparent thin film transistors (TFTs) fabricated using multilayered molybdenum disulfide (MoS2) as the active channel, indium tin oxide (ITO) for the back-gated electrode and indium zinc oxide (IZO) for the source/drain electrodes, respectively, which showed more than 81% transmittance in the visible wavelength. In spite of a relatively large Schottky barrier between MoS2 and IZO, the n-type behavior with a field-effect mobility (μ(eff)) of 1.4 cm(2) V(-1) s(-1) was observed in as-fabricated transparent MoS2 TFT. In order to enhance the performances of transparent MoS2 TFTs, a picosecond pulsed laser was selectively irradiated onto the contact region of the IZO electrodes. Following laser annealing, μ(eff) increased to 4.5 cm(2) V(-1) s(-1), and the on-off current ratio (I(on)/I(off)) increased to 10(4), which were attributed to the reduction of the contact resistance between MoS2 and IZO.

Enhancement-mode operation of multilayer MoS2 transistors with a fluoropolymer gate dielectric layer

Enhancement-mode multilayer molybdenum disulfide (MoS2) field-effect transistors (FETs), which are an immensely important component toward low-power electronics based on a two-dimensional layered semiconductor, are demonstrated using the fluoropolymer CYTOP as a gate dielectric. The fabricated devices exhibit threshold voltage (VTH) of ∼5.7 V with field-effect mobility (μFE) of up to 82.3 cm2/V s, and the characteristics are compared with the depletion-mode characteristics of MoS2 FETs with the cross-linked Poly(4-vinylphenol) gate dielectric (VTH ∼ −7.8 V). UV photoelectron spectroscopy analysis indicates that increased surface potential due to the surface dipole effect of the fluorine group influences the positive VTH shift.

Fluorous-inorganic hybrid dielectric materials for solution-processed electronic devices

We report the synthesis and characterisation of fluorous-inorganic hybrid dielectric (FIHD) materials processable in highly fluorinated orthogonal solvents for printed electronic devices. FIHD materials were prepared successfully via ligand exchange reactions between organic ligands on the surfaces of nanomaterials and highly fluorinated carboxylic acids. When hafnium oxide (HfO2) or zirconium oxide (ZrO2) nanoparticles stabilized with trioctylphosphine oxide (TOPO) were treated with perfluoro-3,6,9-trioxatridecanoic acid in HFE-7500 at 130 °C, the modified surface characteristics of the nanoparticles resulted in excellent solubilities in the fluorous solvent. The dielectric constant of HfO2 and ZrO2 nanoparticles modified with fluorous acid was ca. 4.4 and 4.3 at 1 KHz, respectively, which is significantly higher than that of fluoropolymers. Top-gate organic thin film transistors (OTFTs) were fabricated using solution-processed organic semiconductors and HfO2-based FIHD materials. The hole mobilities of the OTFTs produced were as high as 0.08 cm2 V−1 s−1 (Vds = −40 V) and the on/off ratio reached 3.3 × 106 when 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (diF-TESADT) was employed as the semiconductor layer. These device performances demonstrate that FIHD materials can be useful components for general printed electronic devices processed with soluble organic electronic materials.

High performance and transparent multilayer MoS2 transistors: Tuning Schottky barrier characteristics

Various strategies and mechanisms have been suggested for investigating a Schottky contact behavior in molybdenum disulfide (MoS2) thin-film transistor (TFT), which are still in much debate and controversy. As one of promising breakthrough for transparent electronics with a high device performance, we have realized MoS2 TFTs with source/drain electrodes consisting of transparent bi-layers of a conducting oxide over a thin film of low work function metal. Intercalation of a low work function metal layer, such as aluminum, between MoS2 and transparent source/drain electrodes makes it possible to optimize the Schottky contact characteristics, resulting in about 24-fold and 3 orders of magnitude enhancement of the field-effect mobility and on-off current ratio, respectively, as well as transmittance of 87.4 % in the visible wavelength range.