Gil-Ho Kim

Large-Scale Synthesis of High-Quality Hexagonal Boron Nitride Nanosheets for Large-Area Graphene Electronics

Hexagonal boron nitride (h-BN) has received a great deal of attention as a substrate material for high-performance graphene electronics because it has an atomically smooth surface, lattice constant similar to that of graphene, large optical phonon modes, and a large electrical band gap. Herein, we report the large-scale synthesis of high-quality h-BN nanosheets in a chemical vapor deposition (CVD) process by controlling the surface morphologies of the copper (Cu) catalysts. It was found that morphology control of the Cu foil is much critical for the formation of the pure h-BN nanosheets as well as the improvement of their crystallinity. For the first time, we demonstrate the performance enhancement of CVD-based graphene devices with large-scale h-BN nanosheets. The mobility of the graphene device on the h-BN nanosheets was increased 3 times compared to that without the h-BN nanosheets. The on-off ratio of the drain current is 2 times higher than that of the graphene device without h-BN. This work suggests that high-quality h-BN nanosheets based on CVD are very promising for high-performance large-area graphene electronics.

Modification of InAs quantum dot structure by the growth of the capping layer

InAs quantum dots inserted at the middle of a GaAs quantum well structure have been investigated by transmission electron microscopy and scanning transmission electron microscopy. We find that the growth condition of the overlayer on the InAs dots can lead to drastic changes in the structure of the dots. We attribute the changes to a combination of factors such as preferential growth of the overlayer above the wetting layers because of the strained surfaces and to the thermal instability of the InAs dots at elevated temperature. The result suggests that controlled sublimation, through suitable manipulation of the overlayer growth conditions, can be an effective tool to improve the structure of the self-organized quantum dots and can help tailor their physical properties to any specific requirements of the device applications

Tunable Electrical and Optical Characteristics in Monolayer Graphene and Few-Layer MoS2 Heterostructure Devices

Lateral and vertical two-dimensional heterostructure devices, in particular graphene-MoS2, have attracted profound interest as they offer additional functionalities over normal two-dimensional devices. Here, we have carried out electrical and optical characterization of graphene-MoS2 heterostructure. The few-layer MoS2 devices with metal electrode at one end and monolayer graphene electrode at the other end show nonlinearity in drain current with drain voltage sweep due to asymmetrical Schottky barrier height at the contacts and can be modulated with an external gate field. The doping effect of MoS2 on graphene was observed as double Dirac points in the transfer characteristics of the graphene field-effect transistor (FET) with a few-layer MoS2 overlapping the middle part of the channel, whereas the underlapping of graphene have negligible effect on MoS2 FET characteristics, which showed typical n-type behavior. The heterostructure also exhibits a strongest optical response for 520 nm wavelength, which decreases with higher wavelengths. Another distinct feature observed in the heterostructure is the peak in the photocurrent around zero gate voltage. This peak is distinguished from conventional MoS2 FETs, which show a continuous increase in photocurrent with back-gate voltage. These results offer significant insight and further enhance the understanding of the graphene-MoS2 heterostructure.

Assembly of gold nanoparticles of different diameters between nanogap electrodes

Gold nanoparticles (NPs) of different diameters i.e., 5, 10, and 20 nm, were assembled between 20 nm gap electrodes using ac dielectrophoresis (DEP) process. DEP parameters, such as frequency, trapping time, and voltage of value 1 MHz, 1 s, and 2–3 V, respectively, led to the pearl-chain assembly corresponding to each type of NPs between 20 nm gap electrodes. Mutual DEP could be attributed to the NPs chaining in low field regions and subsequently the DEP force directs these chains to the trapping region. Such controlled assembly of individual NPs may find application in fabricating devices for molecular electronics.

Transport properties of two-dimensional electron gases containing InAs self-assembled dots

We present a study of the transport properties of two-dimensional electron gases formed in GaAs/AlGaAs heterostructures in which InAs self-assembled quantum dots have been inserted in the center of a GaAs quantum well. We observed that, while maintaining a constant carrier density, the mobility increased as the InAs dot density was reduced. The ratio of the transport to the quantum lifetime was measured to be approximately five with the dominant scattering mechanism attributed to short-range scattering from the inserted InAs dots.

Manipulation and trapping of semiconducting ZnO nanoparticles into nanogap electrodes by dielectrophoresis technique

The assembly of ZnO nanoparticles into nanogap electrodes using ac dielectrophoresis (DEP) process is reported. DEP parameters such as frequency, voltage, and time were optimized to assemble minimum number of nanoparticles into nanogap electrodes. Frequency variation study revealed that positive DEP is active at frequencies less than 500 kHz; whereas negative DEP starts dominating at 1 MHz. The fabricated device exhibited a nonlinear I-V characteristic and under ultraviolet (UV) illumination a remarkable change in conductivity of one order of magnitude was observed. The results show the potential for realizing future optoelectronic devices such as miniaturized UV sensor.

Assembly of thermally reduced graphene oxide nanostructures by alternating current dielectrophoresis as hydrogen-gas sensors

Chemo-resistive hydrogen-gas sensors based on thermally reduced graphene oxide (rGO) have been fabricated on a micro-hotplate by positive ac dielectrophoresis (DEP). The optimized DEP parameters for manipulating rGO nanostructures into Au electrodes for hydrogen sensing are: applied frequency = 1 MHz, peak-to-peak voltage = 5 V, and DEP time = 30 s. The device exhibits good sensitivity (∼6%) with fast response time (∼11 s) and recovery time (∼36 s) for 200 ppm hydrogen gas at room temperature. This result indicates that the DEP process has great potential for assembling rGO for hydrogen-gas sensor in many industrial and scientific applications.

Mechanism of ultraviolet photoconductivity in zinc oxide nanoneedles

Ultraviolet photoconductivity in zinc oxide (ZnO) nanoneedles grown on the surface of a multilayer structure comprised of ZnO film (50 nm)/Zn layer (20 nm)/ZnO film (2 μm) fabricated on a stainless steel substrate using an unbalanced magnetron sputtering technique is reported. It was observed that the multilayered structure with ZnO nanoneedles exhibited enhanced ultraviolet photoconductivity in comparison to the ZnO films that were without nanoneedles. The enhancement in the photoconductivity is attributed to the increase in the quantum yield of the photogenerated charge carriers due to the presence of nanoneedles. A successive slow photoresponse transient following after a fast rise is due to the establishment of equilibrium between the charge carriers in the conduction band and the trapping centers created due to the shallow defects in the ZnO film. The observed photoresponse is critically analyzed on the basis of trapping levels created by the oxygen species during the high pressure deposition of the ZnO multilayer. Results show the promise of ZnO nanostructures in ultraviolet detection applications.

Transport in a gated Al0.18Ga0.82N/GaN electron system

We have investigated the low-temperature transport properties of front-gated Al0.18Ga0.82N/GaN heterostructures. At zero gate voltage, the Hall mobility increases with decreasing temperature (20 K⩽T⩽190 K) due to a reduction in phonon scattering. For T⩽20 K, the mobility decreases with decreasing temperature. This is due to weak localization in a weakly disordered two-dimensional system. By changing the applied gate voltage, we can vary the carrier density n from 3.11×1012 to 6.95×1012 cm−2 in our system. The carrier density shows a linear dependence on the applied gate voltage, consistent with a simple parallel-plate capacitor model. The average distance between the GaN electron system and the AlGaN/GaN interface is estimated to be 240 A. At high carrier densities (n>4.65×1012 cm−2), the measured mobility (μ) is found to be a decreasing function of carrier density as μ∼n−0.31. Loss of mobility with increasing carrier density is dominated by interface roughness scattering. At low carrier densities (n 4.65×1012 cm−2), the measured mobility (μ) is found to be a decreasing function of carrier density as μ∼n−0.31. Loss of mobility with increasing carrier density is dominated by interface roughness scattering. At low carrier densities (n<4.24...

Electrically detected and microwave-modulated Shubnikov–de Haas oscillations in an Al0.4Ga0.6N/GaN heterostructure

We report the drastic enhancement pattern of Shubnikov–de Haas (SdH) oscillations observed in an AlGaN/GaN heterostructure by microwave modulation. The dependence of the SdH pattern on microwave power and temperature is investigated. The underlying mechanism is attributed to the effect of carrier heating. This technique helps study the transport properties of two-dimensional electrons in many wide-band-gap heterostructures, in which moderate mobilities and heavier electron effective mass (rapidly damping SdH amplitudes) are frequently encountered. In addition, this method has the advantage of keeping the carrier concentration fixed and not requiring expensive high-energy laser facilities compared with carrier-modulated SdH measurements.