Junyeon Kwon

Giant Photoamplification in Indirect‐Bandgap Multilayer MoS2 Phototransistors with Local Bottom‐Gate Structures

Local-gate multilayer MoS2 phototransistors exhibit a photoresponsivity of up to 342.6 A W(-1) , which is higher by 3 orders of magnitude than that of global-gate multilayer MoS2 phototransistors. These simulations indicate that the gate underlap is critical for the enhancement of the photoresponsivity. These results suggest that high photoresponsivity can be achieved in indirect-bandgap multilayer MoS2 phototransistors by optimizing the optoelectronic design.

Highly Crystalline CVD-grown Multilayer MoSe2 Thin Film Transistor for Fast Photodetector

Hexagonal molybdenum diselenide (MoSe2) multilayers were grown by chemical vapor deposition (CVD). A relatively high pressure (>760 Torr) was used during the CVD growth to achieve multilayers by creating multiple nuclei based on the two-dimensional crystal growth model. Our CVD-grown multilayer MoSe2 thin-film transistors (TFTs) show p-type-dominant ambipolar behaviors, which are attributed to the formation of Se vacancies generated at the decomposition temperature (650 °C) after the CVD growth for 10 min. Our MoSe2 TFT with a reasonably high field-effect mobility (10 cm2/V · s) exhibits a high photoresponsivity (93.7 A/W) and a fast photoresponse time (τrise ~ 0.4 s) under the illumination of light, which demonstrates the practical feasibility of multilayer MoSe2 TFTs for photodetector applications.

High‐Mobility Transistors Based on Large‐Area and Highly Crystalline CVD‐Grown MoSe2 Films on Insulating Substrates

Large-area and highly crystalline CVD-grown multilayer MoSe2 films exhibit a well-defined crystal structure (2H phase) and large grains reaching several hundred micrometers. Multilayer MoSe2 transistors exhibit high mobility up to 121 cm(2) V(-1) s(-1) and excellent mechanical stability. These results suggest that high mobility materials will be indispensable for various future applications such as high-resolution displays and human-centric soft electronics.

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.

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.

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.

Transistors: High‐Mobility Transistors Based on Large‐Area and Highly Crystalline CVD‐Grown MoSe2 Films on Insulating Substrates (Adv. Mater. 12/2016)

On page 2316, M. A. Alam, S. K. Kim, and co-workers describe a 2D layered semiconductor used to fabricate a mechanically flexible, high-mobility thin-film transistor based on large-area and highly crystalline MoSe2 films grown by chemical vapor deposition (CVD). It is thought that such high-mobility materials will be indispensable for various future applications, such as high-resolution displays and human-centric soft electronics.

Photosensitivity enhancement in hydrogenated amorphous silicon thin-film phototransistors with gate underlap

Conventional ɑ-Si:H phototransistors exhibit poor photosensitivity due to low photo-conversion efficiency. To overcome this intrinsic limit, we introduce gate underlap in ɑ-Si:H phototransistors and demonstrate photosensitivity enhancement. We show that photocurrent can be significantly larger than dark current by 4 orders of magnitude, using 1-μm gate underlap at incident optical power density (Pinc) of 3.2 W/cm2. Our 1-μm gate-underlap phototransistor exhibits higher photosensitivity than the device without gate underlap by 64 times with Pinc = 0.2 W/cm2 and a wavelength of 785 nm. Our gate-underlapped phototransistors also show excellent time-resolved photoswitching behaviors, demonstrating the great potential for highly sensitive photodetectors.

Electrical performance of local bottom-gated MoS2 thin-film transistors

This paper reports the unique electronic properties of the local bottom-gated MoS2 thin-film transistors (TFTs) fabricated on glass substrates. The current–voltage (I–V) characteristics of field effect transistors exhibited the on/off ratio of ∼1×106 and mobility higher than 20 cm2 V−1 s−1. The doping concentration of MoS2 flakes extracted by capacitance–voltage (C–V) measurement is approximately 1016–1017 cm−3. These results demonstrate that the electrical performance of the local bottom-gated TFTs are comparable with the conventional TFTs, providing important technical implications on the feasibility of MoS2 TFTs.