Gwangwe Yoo

An Ultrahigh‐Performance Photodetector based on a Perovskite–Transition‐Metal‐Dichalcogenide Hybrid Structure

An ultrahigh performance MoS2 photodetector with high photoresponsivity (1.94 × 10(6) A W(-1) ) and detectivity (1.29 × 10(12) Jones) under 520 nm and 4.63 pW laser exposure is demonstrated. This photodetector is based on a methyl-ammonium lead halide perovskite/MoS2 hybrid structure with (3-aminopropyl)triethoxysilane doping. The performance degradation caused by moisture is also minimized down to 20% by adopting a new encapsulation bilayer of octadecyltrichlorosilane/polymethyl methacrylate.

Wide-range controllable n-doping of molybdenum disulfide (MoS2) through thermal and optical activation.

Despite growing interest in doping two-dimensional (2D) transition metal dichalcogenides (TMDs) for future layered semiconductor devices, controllability is currently limited to only heavy doping (degenerate regime). This causes 2D materials to act as metallic layers, and an ion implantation technique with precise doping controllability is not available for these materials (e.g., MoS2, MoSe2, WS2, WSe2, graphene). Since adjustment of the electrical and optical properties of 2D materials is possible within a light (nondegenerate) doping regime, a wide-range doping capability including nondegenerate and degenerate regimes is a critical aspect of the design and fabrication of 2D TMD-based electronic and optoelectronic devices. Here, we demonstrate a wide-range controllable n-doping method on a 2D TMD material (exfoliated trilayer and bulk MoS2) with the assistance of a phosphorus silicate glass (PSG) insulating layer, which has the broadest doping range among the results reported to date (between 3.6 × 10(10) and 8.3 × 10(12) cm(-2)) and is also applicable to other 2D semiconductors. This is achieved through (1) a three-step process consisting of, first, dopant out-diffusion between 700 and 900 °C, second, thermal activation at 500 °C, and, third, optical activation above 5 μW steps and (2) weight percentage adjustment of P atoms in PSG (2 and 5 wt %). We anticipate our widely controllable n-doping method to be a starting point for the successful integration of future layered semiconductor devices.

A High-Performance WSe2 /h-BN Photodetector using a Triphenylphosphine (PPh3 )-Based n-Doping Technique.

The effects of triphenylphosphine (PPh3 )-based n-doping and hexagonal boron nitride (h-BN) insertion on a tungsten diselenide (WSe2 ) photodetector are systematically studied, and a very high performance WSe2 /h-BN heterostucture-based photodetector is demonstrated with a record photoresponsivity (1.27 × 10(6) A W(-1) ) and temporal photoresponse (rise time: 2.8 ms, decay time: 20.8 ms) under 520 nm wavelength and 5 pW power laser illumination.

Extremely Low Contact Resistance on Graphene through n-Type Doping and Edge Contact Design

The effects of graphene n-doping on a metal-graphene contact are studied in combination with 1D edge contacts, presenting a record contact resistance of 23 Ω μm at room temperature (19 Ω μm at 100 K). This contact scheme is applied to a graphene-perovskite hybrid photodetector, significantly improving its performance (0.6 → 1.8 A W(-1) in photoresponsivity and 3.3 × 10(4) → 5.4 × 10(4) Jones in detectivity).

Controlling Grain Size and Continuous Layer Growth in Two-Dimensional MoS 2 Films for Nanoelectronic Device Application

We report that control over the grain size and lateral growth of monolayer MoS₂ film, yielding a uniform large-area monolayer MoS₂ film, can be achieved by submitting the SiO₂ surfaces of the substrates to oxygen plasma treatment and modulating substrate temperature in chemical vapor deposition (CVD) process. Scanning electron microscopy and atomic force microscopy images and Raman spectra revealed that the MoS₂ lateral growth could be controlled by the surface treatment conditions and process temperatures. Moreover, the obtained monolayer MoS₂ films showed excellent scalable uniformity covering a centimeter-scale SiO₂ /Si substrates, which was confirmed with Raman and photoluminescence mapping studies. Transmission electron microscopy measurements revealed that the MoS₂ film of the monolayer was largely single crystalline in nature. Back-gate field effect transistors based on a CVD-grown uniform monolayer MoS₂ film showed a good current on/off ratio of ~10⁶ and a field effect mobility of 7.23 cm²/V·s. Our new approach to growing MoS₂ films is anticipated to advance studies of MoS₂ or other transition metal dichalcogenide material growth mechanisms and to facilitate the mass production of uniform high-quality MoS₂ films for the commercialization of a variety of applications.

M-DNA/Transition Metal Dichalcogenide Hybrid Structure-based Bio-FET sensor with Ultra-high Sensitivity

Here, we report a high performance biosensor based on (i) a Cu2+-DNA/MoS2 hybrid structure and (ii) a field effect transistor, which we refer to as a bio-FET, presenting a high sensitivity of 1.7 × 103 A/A. This high sensitivity was achieved by using a DNA nanostructure with copper ions (Cu2+) that induced a positive polarity in the DNA (receptor). This strategy improved the detecting ability for doxorubicin-like molecules (target) that have a negative polarity. Very short distance between the biomolecules and the sensor surface was obtained without using a dielectric layer, contributing to the high sensitivity. We first investigated the effect of doxorubicin on DNA/MoS2 and Cu2+-DNA/MoS2 nanostructures using Raman spectroscopy and Kelvin force probe microscopy. Then, we analyzed the sensing mechanism and performance in DNA/MoS2- and Cu2+-DNA/MoS2-based bio-FETs by electrical measurements (ID-VG at various VD) for various concentrations of doxorubicin. Finally, successful operation of the Cu2+-DNA/MoS2 bio-FET was demonstrated for six cycles (each cycle consisted of four steps: 2 preparation steps, a sensing step, and an erasing step) with different doxorubicin concentrations. The bio-FET showed excellent reusability, which has not been achieved previously in 2D biosensors.

Effect of Hydrogen Annealing on Contact Resistance Reduction of Metal–Interlayer–n-Germanium Source/Drain Structure

The effect of post-deposition H2 annealing (PDHA) on the reduction of a contact resistance by the metal-interlayer-semiconductor (M-I-S) source/drain (S/D) structure of the germanium (Ge) n-channel field-effect transistor (FET) is demonstrated in this letter. The M-I-S structure reduces the contact resistance of the metal/n-type Ge (n-Ge) contact by alleviating the Fermi-level pinning (FLP). In addition, the PDHA induces interlayer doping and interface controlling effects that result in a reduction of the tunneling resistance and the series resistance regarding the interlayer and an alleviation of the FLP, respectively. A specific contact resistivity (pc) of 3.4×10-4Ω·cm2 was achieved on a moderately doped n-Ge substrate (1×1017 cm-3), whereby 5900× reduction was exhibited from the Ti/n-Ge structure, and a 10× reduction was achieved from the Ti/Ar plasma-treated TiO2-x/n-Ge structure. The PDHA technique is, therefore, presented as a promising S/D contact technique for the development of the Ge n-channel FET, as it can further lower the contact resistance of the M-I-S structure.

Theoretical and Experimental Investigation of Graphene/High-

Here, we theoretically and experimentally investigate the impact of a high-κ layer inserted between graphene and p-Si in a graphene/Si junction. We have achieved 86-fold and 222-fold reductions in a specific contact resistivity (ρ_c) by inserting 1-nm-thick Al₂O₃ and 2-nm-thick TiO₂ in the graphene-semiconductor junction, respectively, corresponding to lowering the effective barrier height by 0.24 and 0.12 eV. Furthermore, we propose a graphene-induced gap state model that simultaneously considers the graphenes modulation by a gate bias and the effect of the high-κ insertion.

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The effects of graphene n-doping on metal-graphene (M-G) contacts in combination with 1D edge contacts is discussed by J.-H. Park and co-workers, as described on page 864, presenting a record contact resistance of 23 Ω μm at room temperature (19 Ω μm at 100 K). This is lower than the value required for the latest Si CMOS technology. This contact scheme is applied to graphene-perovskite hybrid photo-detectors, significantly improvement of its performance (0.6 → 1.8 A W(-1) in photoresponsivity and 3.3 × 10(4) → 5.4 × 10(4) Jones in detectivity).

/p-Si Junctions

We investigated the effect of an electric field-based post exposure bake (EF-PEB) process on photoacid diffusion and pattern formation. To investigate the control of photoacid diffusion experimentally, the EF-PEB processes was performed at various temperatures. Cross sectional images of various EF-PEB processed samples were obtained by scanning electron microscopy (SEM) after ion beam milling. In addition, we conducted a numerical analysis of photoacid distribution and diffusion with following Ficks second law and compared the experimental results with our theoretical model. The drift distance was theoretically predicted by multiplying drift velocity and EF-PEB time, and the experimental values were obtained by finding the difference in pattern depths of PEB/EFPEB samples. Finally, an EF-PEB temperature of 85 °C was confirmed as the optimum condition to maximize photoacid drift distance using the electric field.