Janghyuk Kim

Wafer-scale synthesis of multi-layer graphene by high-temperature carbon ion implantation

We report on the synthesis of wafer-scale (4 in. in diameter) high-quality multi-layer graphene using high-temperature carbon ion implantation on thin Ni films on a substrate of SiO2/Si. Carbon ions were bombarded at 20 keV and a dose of 1 × 1015 cm−2 onto the surface of the Ni/SiO2/Si substrate at a temperature of 500 °C. This was followed by high-temperature activation annealing (600–900 °C) to form a sp2-bonded honeycomb structure. The effects of post-implantation activation annealing conditions were systematically investigated by micro-Raman spectroscopy and transmission electron microscopy. Carbon ion implantation at elevated temperatures allowed a lower activation annealing temperature for fabricating large-area graphene. Our results indicate that carbon-ion implantation provides a facile and direct route for integrating graphene with Si microelectronics.

Effect of front and back gates on β-Ga2O3 nano-belt field-effect transistors

Field effect transistors (FETs) using SiO2 and Al2O3 as the gate oxides for the back and front sides, respectively, were fabricated on exfoliated two-dimensional (2D) β-Ga2O3 nano-belts transferred to a SiO2/Si substrate. The mechanical exfoliation and transfer process produced nano-belts with smooth surface morphologies and a uniform low defect density interface with the SiO2/Si substrate. The depletion mode nanobelt transistors exhibited better channel modulation with both front and back gates operational compared to either front or back-gating alone. The maximum transconductance was ∼4.4 mS mm−1 with front and back-gating and ∼3.7 mS mm−1 with front-gating only and a maximum drain source current density of 60 mA mm−1 was achieved at a drain-source voltage of 10 V. The FETs had on/off ratios of ∼105 at 25 °C with gate-source current densities of ∼2 × 10−3 mA mm−1 at a gate voltage of −30 V. The device characteristics were stable over more than a month for storage in air ambient and the results show the ...

Quasi-two-dimensional β-gallium oxide solar-blind photodetectors with ultrahigh responsivity

Solar-blind photodetectors have received a great deal of interest owing to their high selectivity for deep ultra-violet light in the presence of visible light. The development of alternative materials and innovative device designs are necessary for such solar-blind photodetectors, as the currently available commercial devices have issues pertaining to chemical and thermal instability, cost, and material handling due to their rigidity. Here, we fabricated solar-blind photodetectors based on exfoliated quasi-two-dimensional β-Ga2O3 flakes with optimal opto-electrical properties (direct bandgap of ∼4.9 eV), chemical and thermal stability, and then systematically characterized their photoresponsive properties. The fabricated device structures were based on back-gated field-effect transistors, allowing us to control the dark currents. These photodetectors exhibit extraordinary photoresponsive properties including the highest responsivity (1.8 × 105 A W−1) among reported semiconductor thin-film solar-blind photodetectors.

Quasi-Two-Dimensional h-BN/β-Ga2O3 Heterostructure Metal–Insulator–Semiconductor Field-Effect Transistor

β-gallium oxide (β-Ga2O3) and hexagonal boron nitride (h-BN) heterostructure-based quasi-two-dimensional metal-insulator-semiconductor field-effect transistors (MISFETs) were demonstrated by integrating mechanical exfoliation of (quasi)-two-dimensional materials with a dry transfer process, wherein nanothin flakes of β-Ga2O3 and h-BN were utilized as the channel and gate dielectric, respectively, of the MISFET. The h-BN dielectric, which has an extraordinarily flat and clean surface, provides a minimal density of charged impurities on the interface between β-Ga2O3 and h-BN, resulting in superior device performances (maximum transconductance, on/off ratio, subthreshold swing, and threshold voltage) compared to those of the conventional back-gated configurations. Also, double-gating of the fabricated device was demonstrated by biasing both top and bottom gates, achieving the modulation of the threshold voltage. This heterostructured wide-band-gap nanodevice shows a new route toward stable and high-power nanoelectronic devices.

Heterostructure WSe2–Ga2O3 junction field-effect transistor for low-dimensional high-power electronics

Layered materials separated from each bulk crystal can be assembled to form a strain-free heterostructure by using the van der Waals interaction. We demonstrated a heterostructure n-channel depletion-mode β-Ga2O3 junction field-effect transistor (JFET) through van der Waals bonding with an exfoliated p-WSe2 flake. Typical diode characteristics with a high rectifying ratio of ∼105 were observed in a p-WSe2/n-Ga2O3 heterostructure diode, where WSe2 and β-Ga2O3 were obtained by mechanically exfoliating each crystal. Layered JFETs exhibited an excellent IDS- VDS output as well as IDS- VGS transfer characteristics with a high on/off ratio (∼108) and low subthreshold swing (133 mV/dec). Saturated output currents were observed with a threshold voltage of -5.1 V and a three-terminal breakdown voltage of +144 V. Electrical performances of the fabricated heterostructure JFET were maintained at elevated temperatures with outstanding air stability. Our WSe2-Ga2O3 heterostructure JFET paves the way to the low-dimensional high-power devices, enabling miniaturization of the integrated power electronic systems.

Broadband low reflectance stepped-cone nanostructures by nanosphere lithography

The authors demonstrated broadband low reflectance through a two-step surface texturing technique that combines nanosphere lithography with dry-etching. Through this, various stepped-cone nanostructures were fabricated on the surface of GaAs to suppress its reflectance, with the shape and height of these nanostructures being precisely controlled by altering the diameter of the etch mask (SiO2 nanospheres) and the etching time. The effects of this stepped-cone nanostructure were analyzed by measuring its reflectance spectra in conjunction with finite-difference time-domain calculations. This found that the average reflectance at wavelengths of 300–2500 nm is reduced from 38.1% to 2.6% due to enhanced light scattering and a gradual change in refractive index. This novel method is therefore considered to represent an easily scalable approach to fabricating broadband antireflective surfaces for solar cell applications.