Seongin Hong

Erratum to: A highly sensitive chemical gas detecting transistor based on highly crystalline CVD-grown MoSe2 films

Layered semiconductors with atomic thicknesses are becoming increasingly important as active elements in high-performance electronic devices owing to their high carrier mobilities, large surface-to-volume ratios, and rapid electrical responses to their surrounding environments. Here, we report the first implementation of a highly sensitive chemical-vapor-deposition-grown multilayer MoSe2 field-effect transistor (FET) in a NO2 gas sensor. This sensor exhibited ultra-high sensitivity (S = ca. 1,907 for NO2 at 300 ppm), real-time response, and rapid on–off switching. The high sensitivity of our MoSe2 gas sensor is attributed to changes in the gap states near the valence band induced by the NO2 gas absorbed in the MoSe2, which leads to a significant increase in hole current in the off-state regime. Device modeling and quantum transport simulations revealed that the variation of gap states with NO2 concentration is the key mechanism in a MoSe2 FET-based NO2 gas sensor. This comprehensive study, which addresses material growth, device fabrication, characterization, and device simulations, not only indicates the utility of MoSe2 FETs for high-performance chemical sensors, but also establishes a fundamental understanding of how surface chemistry influences carrier transport in layered semiconductor devices.

Improving the Stability of High-Performance Multilayer MoS2 Field-Effect Transistors

In this study, we propose a method for improving the stability of multilayer MoS2 field-effect transistors (FETs) by O2 plasma treatment and Al2O3 passivation while sustaining the high performance of bulk MoS2 FET. The MoS2 FETs were exposed to O2 plasma for 30 s before Al2O3 encapsulation to achieve a relatively small hysteresis and high electrical performance. A MoOx layer formed during the plasma treatment was found between MoS2 and the top passivation layer. The MoOx interlayer prevents the generation of excess electron carriers in the channel, owing to Al2O3 passivation, thereby minimizing the shift in the threshold voltage (Vth) and increase of the off-current leakage. However, prolonged exposure of the MoS2 surface to O2 plasma (90 and 120 s) was found to introduce excess oxygen into the MoOx interlayer, leading to more pronounced hysteresis and a high off-current. The stable MoS2 FETs were also subjected to gate-bias stress tests under different conditions. The MoS2 transistors exhibited negligible decline in performance under positive bias stress, positive bias illumination stress, and negative bias stress, but large negative shifts in Vth were observed under negative bias illumination stress, which is attributed to the presence of sulfur vacancies. This simple approach can be applied to other transition metal dichalcogenide materials to understand their FET properties and reliability, and the resulting high-performance hysteresis-free MoS2 transistors are expected to open up new opportunities for the development of sophisticated electronic applications.

Interstitial Mo‐Assisted Photovoltaic Effect in Multilayer MoSe2 Phototransistors

Thin-film transistors (TFTs) based on multilayer molybdenum diselenide (MoSe2 ) synthesized by modified atmospheric pressure chemical vapor deposition (APCVD) exhibit outstanding photoresponsivity (103.1 A W-1 ), while it is generally believed that optical response of multilayer transition metal dichalcogenides (TMDs) is significantly limited due to their indirect bandgap and inefficient photoexcitation process. Here, the fundamental origin of such a high photoresponsivity in the synthesized multilayer MoSe2 TFTs is sought. A unique structural characteristic of the APCVD-grown MoSe2 is observed, in which interstitial Mo atoms exist between basal planes, unlike usual 2H phase TMDs. Density functional theory calculations and photoinduced transfer characteristics reveal that such interstitial Mo atoms form photoreactive electronic states in the bandgap. Models indicate that huge photoamplification is attributed to trapped holes in subgap states, resulting in a significant photovoltaic effect. In this study, the fundamental origin of high responsivity with synthetic MoSe2 phototransistors is identified, suggesting a novel route to high-performance, multifunctional 2D material devices for future wearable sensor applications.

Enhanced photoresponsivity of multilayer MoS2 transistors using high work function MoOx overlayer

Using thin sub-stoichiometric molybdenum trioxide (MoOx, x < 3) overlayer, we demonstrate over 20-folds enhanced photoresponsivity of multilayer MoS2 field-effect transistor. The fabricated device exhibits field-effect mobility (μFE) of up to 41.4 cm2/V s and threshold voltage (VTH) of −9.3 V, which is also modulated by the MoOx overlayer. The MoOx layer (∼25 nm), commonly known for a high work function (∼6.8 eV) material with a band gap of ∼3 eV, is evaporated on top of the MoS2 channel and confirmed by the transmission electron microscope analysis. The electrical and optical modulation effects are associated with interfacial charge transfer and thus an induced built-in electric field at the MoS2/MoOx interface. The results show that high work function MoOx can be a promising heterostructure material in order to enhance the photoresponse characteristics of MoS2-based devices.

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.

Chemical Doping Effects in Multilayer MoS2 and Its Application in Complementary Inverter

Multilayer MoS2 has been gaining interest as a new semiconducting material for flexible displays, memory devices, chemical/biosensors, and photodetectors. However, conventional multilayer MoS2 devices have exhibited limited performances due to the Schottky barrier and defects. Here, we demonstrate poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) doping effects in multilayer MoS2, which results in improved electrical characteristics (∼4.6× higher on-current compared to the baseline and a high current on/off ratio of 106). Synchrotron-based study using X-ray photoelectron spectroscopy and grazing incidence wide-angle X-ray diffraction provides mechanisms that align the edge-on crystallites (97.5%) of the PDPP3T as well as a larger interaction with MoS2 that leads to dipole and charge transfer effects (at annealing temperature of 300 °C), which support the observed enhancement of the electrical characteristics. Furthermore, we demonstrate a complementary metal-oxide-semiconductor inverter that uses a p-type MoSe2 and a PDPP3T-doped MoS2 as charging and discharging channels, respectively.

High-mobility 2D layered semiconducting transistors based on large-area and highly crystalline CVD-grown MoSe 2 for flexible electronics

Layered semiconductor materials of molecular thickness, in particular transition metal dichacholgenides (TMDs), possess desirable characteristics including the existence of bandgap, low-power switching behavior, and high carrier mobility that afford them promising as active components of future high speed and low-power electronics. Reliable large-volume production of 2D layered semiconductors is an essential step for translating their intriguing properties into applications. Although atomically thin flakes of 2D layered TMD such as MoS2 can be peeled from bulk crystals by micromechanical exfoliation, this method is not integrated in large-volume and does not allow systematic control of thickness and size of 2D layered semiconductors. Hence, the growth of large-area MoSe2 is one of the critical challenges to realize its promising potential.potential. Many groups are currently developing various methods to synthesize TMDs materials. However, the growth of highly crystalline TMDC materials over large areas is still a major hurdle to achieve commercially viable integrated circuits. In this work the direct growth of large-area multilayer films is demonstrated by chemical vapour transport (CYT) on SiO2 insulator, with a well-defined crystal structure (2H phase) and large grains reaching several hundred micrometers. As-grown MoSe2 films are characterized by high-resolution scanning/transmission electron microscopy, X-ray diffraction, and high-resolution X-ray photoelectron spectroscopy, which reveal that each grain consists of a single crystal free of structural defects. Multilayer field-effect transistors grown on SiO2 insulator exhibit high mobility up to 121 cm2 V-1 s -1, high on/off current ratio (>104), robust current saturation at large drain voltages, and excellent mechanical flexibility on polyimide substrates with a bending radius as small as 5 mm. All together, the results predict that high mobility materials will be indispensable for various future applications such as high-resolution displays, flexible large-area integrated circuits, 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.