Joonggyu Kim

A Van Der Waals Homojunction: Ideal p–n Diode Behavior in MoSe2

A MoSe2 p-n diode with a van der Waals homojunction is demonstrated by stacking undoped (n-type) and Nb-doped (p-type) semiconducting MoSe2 synthesized by chemical vapor transport for Nb substitutional doping. The p-n diode reveals an ideality factor of ≈1.0 and a high external quantum efficiency (≈52%), which increases in response to light intensity due to the negligible recombination rate at the clean homojunction interface.

Wafer-Scale Single-Crystalline AB-Stacked Bilayer Graphene.

Single-crystalline artificial AB-stacked bilayer graphene is formed by aligned transfer of two single-crystalline monolayers on a wafer-scale. The obtained bilayer has a well-defined interface and is electronically equivalent to exfoliated or direct-grown AB-stacked bilayers.

Large-Scale Graphene on Hexagonal-BN Hall Elements: Prediction of Sensor Performance without Magnetic Field.

A graphene Hall element (GHE) is an optimal system for a magnetic sensor because of its perfect two-dimensional (2-D) structure, high carrier mobility, and widely tunable carrier concentration. Even though several proof-of-concept devices have been proposed, manufacturing them by mechanical exfoliation of 2-D material or electron-beam lithography is of limited feasibility. Here, we demonstrate a high quality GHE array having a graphene on hexagonal-BN (h-BN) heterostructure, fabricated by photolithography and large-area 2-D materials grown by chemical vapor deposition techniques. A superior performance of GHE was achieved with the help of a bottom h-BN layer, and showed a maximum current-normalized sensitivity of 1986 V/AT, a minimum magnetic resolution of 0.5 mG/Hz(0.5) at f = 300 Hz, and an effective dynamic range larger than 74 dB. Furthermore, on the basis of a thorough understanding of the shift of charge neutrality point depending on various parameters, an analytical model that predicts the magnetic sensor operation of a GHE from its transconductance data without magnetic field is proposed, simplifying the evaluation of each GHE design. These results demonstrate the feasibility of this highly performing graphene device using large-scale manufacturing-friendly fabrication methods.

Nanofabrication of a sub-wavelength size aperture using anisotropic inductively coupled plasma processing

We successfully fabricated a nano-size silicon oxide aperture using inductively coupled plasma (ICP) anisotropic etching as a potential near-field optical probe application. Several other anisotropic semiconductor processes were also utilized for sub-wavelength size aperture fabrication. Initially, a 2 μm size dot array was photolithographically patterned on an Si(100) wafer. After the formation of a hollow pyramid by anisotropic KOH etching, stress-dependent oxide growth was performed at 1000°C to give an oxide etch-mask for dry etching. Reactive ion etching by 100 W, 9 mTorr, 40 sccm Cl2 feed gas using the ICP system with a negatively biased substrate was performed in order to fabricate the nano-size aperture. After etching at the bias voltages of 500 and 540 V, the diameter of the aperture was measured to be ~ 120 and ~ 200 nm, respectively. Finally, the oxide aperture with a sub-wavelength size diameter was obtained after Si bulk micromachining.

Quantum Hall conductance of graphene combined with charge-trap memory operation.

The combination of quantum Hall conductance and charge-trap memory operation was qualitatively examined using a graphene field-effect transistor. The characteristics of two terminal quantum Hall conductance appeared clearly on the background of a huge conductance hysteresis during a gate-voltage sweep for a device using monolayer graphene as a channel,hexagonal boron-nitride flakes as a tunneling dielectric and defective silicon oxide as the charge storage node. Even though there was a giant shift of the charge neutrality point, the deviation of quantized resistance value at the state of filling factor 2 was less than 1.6% from half of the von Klitzing constant. At high Landau level indices, the behaviors of quantum conductance oscillation between the increasing and the decreasing electron densities were identical in spite ofa huge memory window exceeding 100 V. Our results indicate that the two physical phenomena, two-terminal quantum Hall conductance and charge-trap memory operation, can be integrated into one device without affecting each other.