working on a variety of problems at the frontiers of engineering, materials science, and physics
CMOS scaling is now running into the nanoscale regime, where quantum nature of electrons comes in. While the limits of silicon are being reached, data in electronic devices keep exponentially expanding. This requires further scaling, lowering power, and improving speed, which are exciting challenges to both academia and industry. Nanoscale materials and their heterostructures are therefore experiencing a burst of activities lately, producing diverse scientific and technological breakthroughs including interesting quantum phenomena. We perform vigorous research in electron transport in nanostructured electron systems. Our major research goals are (1) to understand fundamental electronic properties of emerging low-dimensional materials and their novel heterostructures, and (2) to realize unprecedented high-performance nanoscale device applications based on such basic study. Using advanced transport measurement techniques, we explore how electrons move and interact each other in nanostructured electron systems, and aim to broaden our fundamental understanding of emerging materials and physics. Our biggest motivation is curiosity, but we also have the ambition to bring unprecedented future computing with novel high speed and low power nanoelectronics.

Low-dimensional Electron Systems Lab은 저차원 물질, 반도체 물리학 및 나노 구조 전자 장치에 중점을 둡니다.

Ternary Transistors With Reconfigurable Polarities

Gated PN Junction in Ambipolar MoS2 for Superior Self‐Powered Photodetection

Zero power infrared sensing in 2D/3D-assembled heterogeneous graphene/In/InSe/Au

Gate-tunable resonant tunneling in double bilayer graphene heterostructures

Band Alignment in WSe 2 –Graphene Heterostructures

Chemical potential and quantum Hall ferromagnetism in bilayer graphene

Scaling of Al2O3 dielectric for graphene field-effect transistors

High‐performance near‐infrared photodetectors based on surface‐doped InSe

Giant frictional drag in double bilayer graphene heterostructures

Spin-Polarized to Valley-Polarized Transition in Graphene Bilayers at <?format ?>in High Magnetic Fields

Atomic vacancy control and elemental substitution in a monolayer molybdenum disulfide for high performance optoelectronic device arrays

Bilayer graphene-hexagonal boron nitride heterostructure negative differential resistance interlayer tunnel FET

Quantum Hall effect in Bernal stacked and twisted bilayer graphene grown on Cu by chemical vapor deposition

Magnetotransport properties of quasi-free-standing epitaxial graphene bilayer on SiC: Evidence for Bernal stacking

Substitutional fluorine doping of large-area molybdenum disulfide monolayer films for flexible inverter device arrays

Light‐permeable air filter with self‐polarized nylon‐11 nanofibers for enhanced trapping of particulate matters

HfO2/HfOxNy/HfO2 gate dielectric fabricated by in situ oxidation of plasma-enhanced atomic layer deposition HfN middle layer

Defect-assisted contact property enhancement in a molybdenum disulfide monolayer

Transport gap in dual-gated graphene bilayers using oxides as dielectrics

Wrinkling evolution of a growing bubble: the wonders of petal-like patterns in amorphous silicon membranes

Atomically resolved elucidation of the electrochemical covalent molecular grafting mechanism of single layer graphene

Drain-Induced Multifunctional Ambipolar Electronics Based on Junctionless MoS 2

Kayoung Lee

We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current–voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the resonance width dependence on mobility and rotational alignment.

Using different types of WSe 2 and graphene-based heterostructures, we experimentally determine the offset between the graphene neutrality point and the WSe 2 conduction and valence band edges, as well as the WSe 2 dielectric constant along the c -axis. In a first heterostructure, consisting of WSe 2 -on-graphene, we use the WSe 2 layer as the top dielectric in dual-gated graphene field-effect transistors to determine the WSe 2 capacitance as a function of thickness, and the WSe 2 dielectric constant along the c -axis. In a second heterostructure consisting of graphene-on-WSe 2 , the lateral electron transport shows ambipolar behavior characteristic of graphene combined with a conductivity saturation at sufficiently high positive (negative) gate bias, associated with carrier population of the conduction (valence) band in WSe 2 . By combining the experimental results from both heterostructures, we determine the band …

Bilayer graphene has a distinctive electronic structure influenced by a complex interplay between various degrees of freedom. We probed its chemical potential using double bilayer graphene heterostructures, separated by a hexagonal boron nitride dielectric. The chemical potential has a nonlinear carrier density dependence and bears signatures of electron-electron interactions. The data allowed a direct measurement of the electric field–induced bandgap at zero magnetic field, the orbital Landau level (LL) energies, and the broken-symmetry quantum Hall state gaps at high magnetic fields. We observe spin-to-valley polarized transitions for all half-filled LLs, as well as emerging phases at filling factors ν = 0 and ν = ±2. Furthermore, the data reveal interaction-driven negative compressibility and electron-hole asymmetry in N = 0, 1 LLs.

We investigate the scaling of Al 2 O 3 dielectric on graphene by atomic layer deposition (ALD) using ultra-thin, oxidized Ti and Al films as nucleation layers. We show that the nucleation layer significantly impacts the dielectric constant (k) and morphology of the ALD Al 2 O 3, yielding k= 5.5 and k= 12.7 for Al and Ti nucleation layers, respectively. Transmission electron microscopy shows that Al 2 O 3 grown using the Ti interface is partially crystalline, while Al 2 O 3 grown on Al is amorphous. Using a spatially uniform 0.6 nm-thick Ti nucleation layer, we demonstrate graphene field-effect transistors with top dielectric stacks as thin as 2.6 nm.

2D InSe is one of the semimetal chalcogenides that has been recently given attention thanks to its excellent electrical properties, such as high mobility near 1000 cm 2 V −1 s −1 and moderate band gap of ≈1.26 eV suitable for IR detection. Here, high‐performance visible to near‐infrared (470–980 nm wavelength (λ)) photodetectors using surface‐doped InSe as a channel and few‐layer graphenes (FLG) as electrodes are reported, where the InSe top region is relatively p ‐doped using AuCl 3 . The surface‐doped InSe photodetectors show outstanding performance, achieving a photoresponsivity ( R ) of ≈19 300 A W −1 and a detectivity ( D *) of ≈3 × 10 13 Jones at λ = 470 nm, and R of ≈7870 A W −1 and D * of ≈1.5 × 10 13 Jones at λ = 980 nm, superior to previously reported 2D material‐based IR photodetectors operating without an applied gate bias. Surface doping using AuCl 3 renders a band bending at the …

We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2–5 nm thick hexagonal boron nitride dielectric. At temperatures ( ) lower than , we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing and becomes comparable to the layer resistivity at the lowest . At low the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficient suggests a thermoelectric origin of this anomalous drag.

We investigate the transverse electric field ( ) dependence of the quantum Hall state (QHS) in dual-gated graphene bilayers in high magnetic fields. The longitudinal resistivity measured at shows an insulating behavior which is strongest in the vicinity of , as well as at large fields. At a fixed perpendicular magnetic field ( ), the QHS undergoes a transition as a function of the applied , marked by a minimum, temperature-independent . This observation is explained by a transition from a spin-polarized QHS at small fields to a valley- (layer-)polarized QHS at large fields. The field value at which the transition occurs follows a linear dependence on .

Defect engineering of 2D transition metal dichalcogenides (TMDCs) is essential to modulate their optoelectrical functionalities, but there are only a few reports on defect‐engineered TMDC device arrays. Herein, the atomic vacancy control and elemental substitution in a chemical vapor deposition (CVD)‐grown molybdenum disulfide (MoS 2 ) monolayer via mild photon irradiation under controlled atmospheres are reported. Raman spectroscopy, photoluminescence, X‐ray, and ultraviolet photoelectron spectroscopy comprehensively demonstrate that the well‐controlled photoactivation delicately modulates the sulfur‐to‐molybdenum ratio as well as the work function of a MoS 2 monolayer. Furthermore, the atomic‐resolution scanning transmission electron microscopy directly confirms that small portions (2–4 at% corresponding to the defect density of 4.6 × 10 12 to 9.2 × 10 13 cm −2 ) of sulfur vacancies and oxygen …

We present the room temperature operation of a vertical tunneling field-effect transistor using a stacked double bilayer graphene (BLG) and hexagonal boron nitride heterostructure. The device shows two tunneling resonances with negative differential resistance (NDR). An analysis of the electrostatic potential drop across the heterostructure indicates the resonances are associated with the relative alignment of the lower or upper bands of the two BLG. Using the NDR characteristic of the device, one-transistor latch or SRAM operation is demonstrated. The device characteristics are largely insensitive to temperature from 1.5 to 300 K.

We examine the quantum Hall effect in bilayer graphene grown on Cu substrates by chemical vapor deposition. Spatially resolved Raman spectroscopy suggests a mixture of Bernal (A-B) stacked and rotationally faulted (twisted) domains. Magnetotransport measurements performed on bilayer domains with a wide 2D band reveal quantum Hall states (QHSs) at filling factors , 8, 12, consistent with a Bernal stacked bilayer, while magnetotransport measurements in bilayer domains defined by a narrow 2D band show a superposition of QHSs of two independent monolayers. The analysis of the Shubnikov–de Haas oscillations measured in twisted graphene bilayers provides the carrier density in each layer as a function of the gate bias and the interlayer capacitance.

We investigate the magnetotransport properties of quasi-free-standing epitaxial graphene bilayer on SiC, grown by atmospheric pressure graphitization in Ar, followed by H 2 intercalation. At the charge neutrality point, the longitudinal resistance shows an insulating behavior, which follows a temperature dependence consistent with variable range hopping transport in a gapped state. In a perpendicular magnetic field, we observe quantum Hall states (QHSs) both at filling factors (ν) multiples of four (ν = 4, 8, 12), as well as broken valley symmetry QHSs at ν = 0 and ν = 6. These results unambiguously show that the quasi-free-standing graphene bilayer grown on the Si-face of SiC exhibits Bernal stacking.

Reliable and controllable doping of transition metal dichalcogenides (TMDCs) is a mandatory requirement for practical large-scale electronic applications. However, most of the literature on the doping methodologies of TMDCs has focused on n -type doping and multilayer TMDC rather than a monolayer one enabling large-scale growth. Herein, we report substitutional fluorine doping of a chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS 2 ) monolayer film using a delicate SF 6 plasma treatment. Our SF 6 -treated MoS 2 monolayer shows a p -type doping effect with fluorine substitution. The doping concentration is controlled by the plasma treatment time (2–4.9 atom %) while maintaining the structural integrity of the MoS 2 monolayer. Such reliable and tunable substitutional doping is attributed to preventing direct ion bombardment to the MoS 2 monolayer by our gentle plasma treatment system. Finally …

Recently, there is a growing demand of high‐performance, lightweight, and light‐permeable air filters due to severe air pollution. Nanofiber air filters have been actively investigated to filter out ultrafine particulate matters (PMs) in air. Among various polymer nanofibers, Nylon‐6 nanofibers have been widely adopted to fabricate air filters owing to their mechanical strength and chemical stability. However, the filtering efficiency of the Nylon‐6 air filter is relatively low when it has high light transmittance. Thus, a relatively thick nanofiber layer is required to ensure the filtering efficiency, leading to a high pressure drop and low light transmittance. Herein, the fabrication and performance of an air filter made of ferroelectric Nylon‐11 nanofibers is introduced. The air filters fabricated with ferroelectric Nylon‐11 demonstrate much improved PM filtering efficiency due to their spontaneous polarization. Specifically, for inorganic …

By using plasma as a reactant with tetrakis (dimethylamino) hafnium precursor during plasma-enhanced atomic layer deposition, we deposited the layer between layers. The 5 nm thick (HfONO) trilayer gate oxide shows reduced capacitance equivalent oxide thickness (≅ 1.25 nm) than that (≅ 1.40 nm) of the film with the same thickness due to the contribution of nitrogen incorporation to the high dielectric constant. The HfONO film utilizing plasma shows lower values of interface trap density, trapped positive charge density, and gate leakage currents than the layer with the same thickness while maintaining comparable hysteresis. The results can be attributed to the presence of N–H bonds, which can reduce localized states below the conduction band and prevent the conduction-band lowering, and decrement of N–N and N–O bonds, which contribute to trap density, confirmed by the combination of X-ray …

Contact engineering for two-dimensional (2D) transition metal dichalcogenides (TMDCs) is crucial for realizing high-performance 2D TMDC devices, and most studies on contact properties of 2D TMDCs have mainly focused on Fermi level unpinning. Here, we investigated electrical and photoelectrical properties of chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS 2 ) monolayer devices depending on metal contacts, Ti/Pt, Ti/Au, Ti, and Ag, and particularly demonstrated the essential role of defects in MoS 2 in contact properties. Remarkably, MoS 2 devices with Ag contacts show a field-effect mobility of 12.2 cm 2 V –1 s –1 , an on/off current ratio of 7 × 10 7 , and a photoresponsivity of 1020 A W –1 , which are outstanding compared to similar devices with other metal contacts. These improvements are attributed to a reduced Schottky barrier height, thanks to the small work function of Ag and Ag–MoS 2 …

Graphene bilayers in Bernal stacking exhibit a transverse electric (E) field-dependent band gap, which can be used to increase the channel resistivity and enable higher on/off ratio devices. We provide a systematic investigation of transport characteristics in dual-gated graphene bilayer devices as a function of density and E field and at temperatures from room temperature down to 0.3 K. The sample conductivity shows finite threshold voltages along the electron and hole branches, which increase as the E field increases, similar to a gapped semiconductor. We extract the transport gap as a function of E field and discuss the impact of disorder. In addition, we show that beyond the threshold, the bilayer conductivity shows a highly linear dependence on density, which is largely insensitive to the applied E field and the temperature.

A large-area amorphous silicon (a-Si) membrane on an oxidized wafer (SiO2/Si) is detached from the substrate by dissolving the sandwiched oxide layer using hydrofluoric acid solution (HF/H2O). Due to the atomic disorder in the structure of a-Si, the rate of molecular diffusion inside the a-Si film and etching of the oxide layer is not uniform over the film; thus, certain over-susceptible spots for etchant infiltration starts to locally detach. Using an in situ optical microscope, initially the detached region is observed to be buckled as a circular dome-shaped protuberance, which then forms wrinkles around the rim of the a-Si bubble. Around the rim, the deformation pattern was strongly dependant on the thickness h of the film, where the number of facets in the engendered petal-like patterns decreased with the increasing h. The tension-induced wrinkling in a-Si membranes was analytically and semi-quantitatively examined …

Engineering graphene at the atomic level via chemical doping, substrate interactions or lateral confinement opens up avenues for precise tuning of its electronic and magnetic properties. Chemical doping by covalent modification routes using electrochemical tools offers rich opportunities that are yet to be fully explored. The key to controlling graphene's physicochemical properties requires a detailed atomistic understanding of the geometry and mechanism of the covalent attachment process. By employing diaryliodonium salts instead of the commonly used diazonium salts, precise molecular grafting onto epitaxial graphene is achieved. Using atomically resolved imaging via scanning tunneling microscopy it is shown that for single layer, high quality, low defect graphene, the functionalization process is controlled by kinetics rather than thermodynamics in accord with Marcus–Gerisher theory. The predominance of …

Applying a drain bias to a strongly gate-coupled semiconductor influences the carrier density of the channel. However, practical applications of this drain-bias-induced effect in the advancement of switching electronics have remained elusive due to the limited capabilities of its current modulation known to date. Here, we show strategies to largely control the current by utilizing drain-bias-induced carrier type switching in an ambipolar molybdenum disulfide (MoS 2 ) field-effect transistor with Pt bottom contacts. Our CMOS-compatible device architecture, incorporating a partially gate-coupled p–n junction, achieves multifunctionality. The ambipolar MoS 2 device operates as an ambipolar transistor (on/off ratios exceeding 10 7 for both NMOS and PMOS), a rectifier (rectification ratio of ∼3 × 10 6 ), a reversible negative breakdown diode with an adjustable breakdown voltage (on/off ratio exceeding 10 9 with a maximum …

Low- or self-powered infrared sensors can be used in a broad range of applications, including networking mobile edge devices and image recognition for autonomous driving technology. Here, we show state-of-the-art self-powered near-infrared (NIR) sensors using graphene/In/InSe/Au as a photoactive region. The self-powered NIR sensors show outstanding performance, achieving a photoresponsivity of ∼8.5 A W−1 and a detectivity of ∼1012 Jones at 850 nm light. Multiple self-powered InSe photodetectors with different device structures and contacts were systematically investigated. In particular, the asymmetrically assembled graphene/In/InSe/Au vertical heterostructure offers a high built-in field, which gives rise to efficient electron–hole pair separation and transit time that is shorter than the photocarrier lifetime. The built-in potential across the InSe was estimated using the Schottky barrier height at each metal …

Low-dimensional Electron Systems Lab

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Ternary Transistors With Reconfigurable Polarities

Gated PN Junction in Ambipolar MoS2 for Superior Self‐Powered Photodetection

Zero power infrared sensing in 2D/3D-assembled heterogeneous graphene/In/InSe/Au

연구실 보유 특허

대학별 입시끈

우리 연구실 모집

교수님 소개

이가영

연구실 키워드

알럼나이가 간 회사

연구실 지원 방법

대우 조건

연도별 논문 인용수

저널 & 학회 논문 출판 수

연구실 논문 목록

Ternary Transistors With Reconfigurable Polarities

Gated PN Junction in Ambipolar MoS2 for Superior Self‐Powered Photodetection

Zero power infrared sensing in 2D/3D-assembled heterogeneous graphene/In/InSe/Au

연구실 보유 특허

최근 3년간 연구실적 높은 연구실

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