Geonwook Yoo

Thermally activated trap charges responsible for hysteresis in multilayer MoS2 field-effect transistors

Hysteresis, which is induced by both extrinsic and intrinsic causes, is often observed in molybdenum disulphide (MoS2) field-effect transistors (FETs), and several extrinsic hysteresis effects have been reported in unpassivated bottom-gate MoS2 device structures. In this study, interface-trap-induced hysteresis and other electrical properties are examined. We experimentally investigate thermally activated trap charges near a silicon-dioxide (SiO2)-MoS2 interface that gives rise to hysteresis in a multilayer MoS2 FET in a temperature region of 10–300 K. The threshold voltage (VTH) and field-effect mobility (μFE) decrease with the increase in temperature, regardless of the gate-bias sweep direction. The hysteresis that coincides with the trend of subthreshold swing increases sharply above T = 150 K as the released charges from interface traps become dominant over the fixed charges. Based on a temperature-dependent hysteresis analysis, we discussed the activation energy of interface traps and maximum interfa...

Flexible and Wavelength-Selective MoS2 Phototransistors with Monolithically Integrated Transmission Color Filters.

Color-selective or wavelength-tunable capability is a crucial feature for two-dimensional (2-D) semiconducting material-based image sensor applications. Here, we report on flexible and wavelength-selective molybdenum disulfide (MoS2) phototransistors using monolithically integrated transmission Fabry-Perot (F-P) cavity filters. The fabricated multilayer MoS2 phototransistors on a polyarylate substrate exhibit decent electrical characteristics (μFE > 64.4 cm2/Vs, on/off ratio > 106), and the integrated F-P filters, being able to cover whole visible spectrum, successfully modulate the spectral response characteristics of MoS2 phototransistors from ~495 nm (blue) to ~590 nm (amber). Furthermore, power dependence of both responsivity and specific detectivity shows similar trend with other reports, dominated by the photogating effect. When combined with large-area monolayer MoS2 for optical property enhancement and array processing, our results can be further developed into ultra-thin flexible photodetectors for wearables, conformable image sensor, and other optoelectronic applications.

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.

Real-time electrical detection of epidermal skin MoS2 biosensor for point-of-care diagnostics

Various approaches have been proposed for point-of-care diagnostics, and in particular, optical detection is preferred because it is relatively simple and fast. At the same time, field-effect transistor (FET)-based biosensors have attracted great attention because they can provide highly sensitive and label-free detection. In this work, we present highly sensitive, epidermal skin-type point-of-care devices with system-level integration of flexible MoS2 FET biosensors, read-out circuits, and light-emitting diode (LEDs) that enable real-time detection of prostate cancer antigens (PSA). Regardless of the physical forms or mechanical stress conditions, our proposed high-performance MoS2 biosensors can detect a PSA concentration of 1 pg·mL–1 without specific surface treatment for anti-PSA immobilization on the MoS2 surface on which we characterize and confirm physisorption of anti-PSA using Kelvin probe force microscopy (KPFM) and tapping-mode atomic force microscopy (tm-AFM). Furthermore, current modulation induced by the binding process was stably maintained for longer than 2–3 min. The results indicate that flexible MoS2-based FET biosensors have great potential for point-of-care diagnostics for prostate cancer as well as other biomarkers.

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-frequency optoacoustic transmitter based on nanostructured germanium via metal-assisted chemical etching

We propose the use of nanostructured germanium (Ge) fabricated by simple metal-assisted chemical (MAC) etching as a high-frequency optoacoustic ultrasound transmitter. As an acoustic transfer medium, an elastomeric polymer, polydimethylsiloxane (PDMS), is spin-coated on top of Ge nanostructures, which is prepared with three different thicknesses with various MAC etching time in order to compare optoacoustic conversion efficiency. Under pulsed laser excitation, the Ge transmitter generates ultrasound pressure of 7.5 times stronger than that of Cr reference with comparable high frequency spectra (primary: 15 MHz and 6dB roll-off at 27 MHz) to CNT-PDMS composite. Considering its simple fabrication process without substrate limitation, the nanostructured Ge overlaid with PDMS can offer a promising approach for a highly efficient optoacoustic transmitter and toward all-optical high-frequency ultrasound transducers.

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.

All-Optical Ultrasound Transducer Using CNT-PDMS and Etalon Thin-Film Structure

Compared with conventional piezoelectric transductions, an all-optical high-frequency ultrasound (HFUS) transducer is a promising modality for high-resolution ultrasound imaging. We demonstrate an all-optical HFUS transducer by integrating a carbon nanotube-polydimethylsiloxane composite film with an etalon thin-film structure incorporating SiO2/TiO2 distributed Bragg reflectors and an SU-8 resonator. The optical and acoustic characteristics are investigated for two different configurations (forward and backward operation modes). The maximum amplitude of the pulse echo in backward mode is approximately twofold higher than that of the forward mode. This difference is contributed by the increased reflectance and the absorptive loss of the incident pulsed laser in the forward mode. The pulse echo from the transducer exhibits a broad frequency bandwidth of 27 MHz. Furthermore, the scalability of the 2-D all-optical transducer array is also evaluated by characterizing the optical properties of the etalon across an area of 0.1 × 0.2 mm2. Our experimental results show that the proposed transducer is a promising candidate for high-resolution ultrasound imaging systems.

Label-Free and Recalibrated Multilayer MoS2 Biosensor for Point-of-Care Diagnostics

Molybdenum disulfide (MoS2) field-effect transistor (FET)-based biosensors have attracted significant attention as promising candidates for highly sensitive, label-free biomolecule detection devices. In this paper, toward practical applications of biosensors, we demonstrate reliable and quantitative detection of a prostate cancer biomarker using the MoS2 FET biosensor in a nonaqueous environment by reducing nonspecific molecular binding events and realizing uniform chemisorption of anti-PSA onto the MoS2 surface. A systematic and statistical study on the capability of the proposed device is presented, and the biological binding events are directly confirmed and characterized through intensive structural and electrical analysis. Our proposed biosensor can reliably detect various PSA concentrations with a limit of 100 fg/mL. Moreover, rigorous theoretical simulations provide a comprehensive understanding of the operating mechanism of the MoS2 FET biosensors, and further suggests the enhancement of the sensitivity through engineering device design parameters.

Thickness-Dependent Electrical Properties of MoS2 Field-Effect Transistors Fabricated on Sol–Gel Prepared AlOX Layer

Solution-processed high-k oxide layer, which is typically deposited using atomic layer deposition (ALD), has been proposed and recently demonstrated on molybdenum disulfide (MoS2) field-effect transistors (FETs). In this report, we statistically investigate electrical performance of multilayer MoS2 FETs fabricated on sol-gel AlOX gate-dielectric. More than 10 sample devices with different MoS2 thickness are characterized and compared. For electrical parameters extraction, Y -function method is adopted in order to minimize S/D electrode contact-induced variations. In spite of the relatively rougher surface of the sol-gel AlOX film, no significant difference of electrical performance is observed. The sol-gel prepared AlOX can be considered as a promising high-k gate dielectric for high-performance large-area transistion metal dichalcogenides (TMDs) devices fabrication.