Dae-Yeong Lee

Lateral MoS2 p-n junction formed by chemical doping for use in high-performance optoelectronics.

This paper demonstrates a technique to form a lateral homogeneous 2D MoS2 p-n junction by partially stacking 2D h-BN as a mask to p-dope MoS2. The fabricated lateral MoS2 p-n junction with asymmetric electrodes of Pd and Cr/Au displayed a highly efficient photoresponse (maximum external quantum efficiency of ∼7000%, specific detectivity of ∼5 × 10(10) Jones, and light switching ratio of ∼10(3)) and ideal rectifying behavior. The enhanced photoresponse and generation of open-circuit voltage (VOC) and short-circuit current (ISC) were understood to originate from the formation of a p-n junction after chemical doping. Due to the high photoresponse at low VD and VG attributed to its built-in potential, our MoS2 p-n diode made progress toward the realization of low-power operating photodevices. Thus, this study suggests an effective way to form a lateral p-n junction by the h-BN hard masking technique and to improve the photoresponse of MoS2 by the chemical doping process.

Ultimate thin vertical p–n junction composed of two-dimensional layered molybdenum disulfide

Semiconducting two-dimensional crystals are currently receiving significant attention because of their great potential to be an ultrathin body for efficient electrostatic modulation, which enables to overcome the limitations of silicon technology. Here we report that, as a key building block for two-dimensional semiconductor devices, vertical p–n junctions are fabricated in ultrathin MoS2 by introducing AuCl3 and benzyl viologen dopants. Unlike usual unipolar MoS2, the MoS2 p–n junctions show ambipolar carrier transport, current rectification via modulation of potential barrier in films thicker than 8 nm and reversed current rectification via tunnelling in films thinner than 8 nm. The ultimate thinness of the vertical p–n homogeneous junctions in MoS2 is experimentally found to be 3 nm, and the chemical doping depth is found to be 1.5 nm. The ultrathin MoS2 p–n junctions present a significant potential of the two-dimensional crystals for flexible, transparent, high-efficiency electronic and optoelectronic applications.

Si-Compatible Cleaning Process for Graphene Using Low-Density Inductively Coupled Plasma

We report a novel cleaning technique for few-layer graphene (FLG) by using inductively coupled plasma (ICP) of Ar with an extremely low plasma density of 3.5 × 10(8) cm(-3). It is known that conventional capacitively coupled plasma (CCP) treatments destroy the planar symmetry of FLG, giving rise to the generation of defects. However, ICP treatment with extremely low plasma density is able to remove polymer resist residues from FLG within 3 min at a room temperature of 300 K while retaining the carbon sp(2)-bonding of FLG. It is found that the carrier mobility and charge neutrality point of FLG are restored to their pristine defect-free state after the ICP treatment. Considering the application of graphene to silicon-based electronic devices, such a cleaning method can replace thermal vacuum annealing, electrical current annealing, and wet-chemical treatment due to its advantages of being a low-temperature, large-area, high-throughput, and Si-compatible process.

Metal-Semiconductor Barrier Modulation for High Photoresponse in Transition Metal Dichalcogenide Field Effect Transistors

A gate-controlled metal-semiconductor barrier modulation and its effect on carrier transport were investigated in two-dimensional (2D) transition metal dichalcogenide (TMDC) field effect transistors (FETs). A strong photoresponse was observed in both unipolar MoS2 and ambipolar WSe2 FETs (i) at the high drain voltage due to a high electric field along the channel for separating photo-excited charge carriers and (ii) at the certain gate voltage due to the optimized barriers for the collection of photo-excited charge carriers at metal contacts. The effective barrier height between Ti/Au and TMDCs was estimated by a low temperature measurement. An ohmic contact behavior and drain-induced barrier lowering (DIBL) were clearly observed in MoS2 FET. In contrast, a Schottky-to-ohmic contact transition was observed in WSe2 FET as the gate voltage increases, due to the change of majority carrier transport from holes to electrons. The gate-dependent barrier modulation effectively controls the carrier transport, demonstrating its great potential in 2D TMDCs for electronic and optoelectronic applications.

Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (Rc). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS2 and MoTe2 by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS2 and MoTe2, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS2. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that Rc is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.

P‐Type Polar Transition of Chemically Doped Multilayer MoS2 Transistor

A high-performance multilayer MoS2 p-type field-effect transistor is realized via controllable chemical doping, which shows an excellent on/off ratio of 10(9) and a maximum hole mobility of 132 cm(2) V(-1) s(-1) at 133 K. The developed technique will enable 2D materials to be used for future high-efficiency and low-power semiconductor device applications.

High-performance photocurrent generation from two-dimensional WS2 field-effect transistors

The generation of a photocurrent from two-dimensional tungsten disulfide (WS2) field-effect transistors is examined here, and its dependence on the photon energy is characterized. We found from the WS2 devices that a significant enhancement in the ratio of illuminated current against dark current (Iillum/Idark) of ∼102–103 is attained, even with the application of electric fields of ED = 0.02 and EG = −22 mV/nm, which are much smaller than that of the bulk MoS2 phototransistor. Most importantly, we demonstrate that our multilayer WS2 shows an extremely high external quantum efficiency of ∼7000%, even with the smallest electrical field applied. We also found that photons with an energy near the direct band gap of the bulk WS2, in the range of 1.9–2.34 eV, give rise to a photoresponsivity of ∼0.27 A/W, which exceeds the photoresponsivity of the bulk MoS2 phototransistor. The superior photosensing properties of WS2 demonstrated in this work are expected to be utilized in the development of future high perfor...

Enhancement of light absorption using high-k dielectric in localized surface plasmon resonance for silicon-based thin film solar cells

The application of high-dielectric-constant (k) materials, e.g., Si3N4, ZrO2, and HfO2, to localized surface plasmon resonance (LSPR) excited by a Au nanoparticle structure has been investigated and simulated for the enhancement of light absorption in Si-based thin film solar cells by using Mie theory and three-dimensional finite-difference time-domain computational simulations. As compared to a conventional SiO2 dielectric spacing layer, the high-k dielectrics have significant advantages, such as (i) a polarizability over two times higher, (ii) an extinction cross-section 4.1 times larger, (iii) a 5.6% higher transmission coefficient, (iv) a maximal 39.9% and average 25.0% increase in the transmission of the electromagnetic field, (v) an absorption of the transmitted electromagnetic field that is a maximum of 2.8 times and an average of 1.4 times greater, and (vi) increased absorption efficiency and extended cover range. Experimental results show that the average absorptance in the visible spectrum using...

Gate-controlled Schottky barrier modulation for superior photoresponse of MoS 2 field effect transistor

An ultrahigh photocurrent (PC) signal which was about thousand times higher compared to the corresponding dark current was achieved in a two-dimensional (2D) multi-layer MoS2 field effect transistor (FET), owing to a gate-controlled MoS2/Ti/Au Schottky barrier (SB) modulation. The SBs can be enlarged for suppressing the electron drift along the channel in dark environment, and be reduced for the collection of photo-excited charge carriers in illuminating environment, providing the great potential for 2D electronic and optoelectronic applications.

Effects of Plasma Treatment on Contact Resistance and Sheet Resistance of Graphene FET

We investigated the effect of capacitively coupled Ar plasma treatment on contact resistance (R c ) and channel sheet resistance (R sh ) of graphene field effect transistors (FETs), by varying their channel length in the wide range from 200 nm to 50 μm which formed the transfer length method (TLM) patterns. When the Ar plasma treatment was performed on the long channel (10 ~ 50 μm) graphene FETs for 20 s, R c decreased from 2.4 to 1.15 kΩ·μm. It is understood that this improvement in R c is attributed to the formation of sp³ bonds and dangling bonds by the plasma. However, when the channel length of the FETs decreased down to 200nm, the drain current (I d ) decreased upon the plasma treatment because of the significant increase of channel R sh which was attributed to the atomic structural disorder induced by the plasma across the transfer length at the edge of the channel region. This study suggests a practical guideline to reduce R c using various plasma treatments for the R c sensitive graphene and other 2D material devices, where R c is traded off with Rsh.