Inyeal Lee

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.

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.

Room temperature operation of InGaAs∕InGaAsP∕InP quantum dot lasers

The growth conditions for InGaAs∕InGaAsP∕InP quantum dots (QDs) have been optimized and QDs of high luminescence efficiency and the room temperature operation of QD lasers emitting at ∼1.5μm have been demonstrated. Lattice-matched InGaAsP (λg=1.0–1.1μm) was used as a barrier layer for the InGaAs QDs and the emission wavelength was controlled by the QD composition. High-density InGaAs QDs with an areal density as high as 1.13×1011cm−2 have been grown. The integrated and peak intensity of the photoluminescence (PL) spectra at room temperature are as high as 25% and 10% of those at 10K, respectively. The room temperature PL peak intensity is about 50% that of a high-quality InGaAs∕InP quantum well. Room temperature, pulsed operation at ∼1.5μm has been achieved from broad area lasers with a 1mm cavity length. Threshold current density per QD stack of ∼430A∕cm2 is measured for the five-, seven-, and ten-stack QD lasers.

Silicon nitride films prepared by high-density plasma chemical vapor deposition for solar cell applications

Abstract Silicon nitride films were deposited by means of high-density inductively coupled plasma chemical vapor deposition in a planar coil reactor. The process gases used were pure nitrogen and a mixture of silane and helium. Buried contact solar cells, passivated by the silicon nitride layer, show efficiency above 17%. Strong H-atom release from the growing SiN film and Si–N bond healing are responsible for the improved electrical and passivation properties of the SiN film. This paper presents the optimal refractive index of SiN for a single layer antireflection (SLAR) coating in solar cell applications.

The effects of a double layer anti-reflection coating for a buried contact solar cell application

Abstract Theoretical and experimental investigations were performed on a double layer anti-reflection (DLAR) coating of MgF 2 /CeO 2 . We investigated CeO 2 films as an AR layer because they have a proper refractive index of 2.46 and demonstrate the same lattice constant as the Si substrate. An optimized DLAR coating showed a reflectance value as low as 1.87% over wavelengths ranging from 0.4 to 1.1 μm. Buried contact solar cells (BCSC) were investigated using the following structure: MgF 2 /CeO 2 /Ag/Cu/Ni grid/n + emitter/p-type Si base/P + /Al. The BCSC efficiency under an illumination of 50 mW/cm 2 measured as high as 19.9% when employing DLAR coatings. MgF 2 /CeO 2 DLAR coatings on the BCSC cell contributed to an increase of the fill factor (from 71% to 75%) and the J sc (from 19.8 mA/cm 2 to 22.6 mA/cm 2 ).

Construction and characterization of Cu2+, Ni2+, Zn2+, and Co2+ modified-DNA crystals

We studied the physical characteristics of modified-DNA (M-DNA) double crossover crystals fabricated via substrate-assisted growth with various concentrations of four different divalent metallic ions, Cu(2+), Ni(2+), Zn(2+), and Co(2+). Atomic force microscopy (AFM) was used to test the stability of the M-DNA crystals with different metal ion concentrations. The AFM images show that M-DNA crystals formed without deformation at up to the critical concentrations of 6 mM of [Cu(2+)], 1.5 mM of [Ni(2+)], 1 mM of [Zn(2+)], and 1 mM of [Co(2+)]. Above these critical concentrations, the M-DNA crystals exhibited deformed, amorphous structures. Raman spectroscopy was then used to identify the preference of the metal ion coordinate sites. The intensities of the Raman bands gradually decreased as the concentration of the metal ions increased, and when the metal ion concentrations increased beyond the critical values, the Raman band of the amorphous M-DNA was significantly suppressed. The metal ions had a preferential binding order in the DNA molecules with G-C and A-T base pairs followed by the phosphate backbone. A two-probe station was used to measure the electrical current-voltage properties of the crystals which indicated that the maximum currents of the M-DNA complexes could be achieved at around the critical concentration of each ion. We expect that the functionalized ion-doped M-DNA crystals will allow for efficient devices and sensors to be fabricated in the near future.

Alternating Current Dielectrophoresis Optimization of Pt-Decorated Graphene Oxide Nanostructures for Proficient Hydrogen Gas Sensor

Alternating current dielectrophoresis (DEP) is an excellent technique to assemble nanoscale materials. For efficient DEP, the optimization of the key parameters like peak-to-peak voltage, applied frequency, and processing time is required for good device. In this work, we have assembled graphene oxide (GO) nanostructures mixed with platinum (Pt) nanoparticles between the micro gap electrodes for a proficient hydrogen gas sensors. The Pt-decorated GO nanostructures were well located between a pair of prepatterned Ti/Au electrodes by controlling the DEP technique with the optimized parameters and subsequently thermally reduced before sensing. The device fabricated using the DEP technique with the optimized parameters showed relatively high sensitivity (∼10%) to 200 ppm hydrogen gas at room temperature. The results indicates that the device could be used in several industry applications, such as gas storage and leak detection.

Electrical characterization of multilayer HfSe2 field-effect transistors on SiO2 substrate

We fabricated and characterized two-dimensional field-effect transistors (FETs) based on hafnium diselenide (HfSe2) crystalline nanoflakes. The HfSe2 FET exhibits an n-type semiconductor behavior with a high on/off current ratio exceeding 7.5 × 106. In the temperature range of 120 K–280 K, the thermally activated transport is observed at high carrier concentrations, while at low concentrations and low temperatures hopping conduction dominates the transport mechanism. We also observed the metal insulator transition at carrier density of ∼1.8 × 1012 cm−2. This initial report on the physical and electrical characterization of two dimensional HfSe2 material demonstrates the feasibility of this semiconducting material for electronic devices.

Non-degenerate n-type doping by hydrazine treatment in metal work function engineered WSe₂ field-effect transistor.

We report a facile and highly effective n-doping method using hydrazine solution to realize enhanced electron conduction in a WSe2 field-effect transistor (FET) with three different metal contacts of varying work functions-namely, Ti, Co, and Pt. Before hydrazine treatment, the Ti- and Co-contacted WSe2 FETs show weak ambipolar behaviour with electron dominant transport, whereas in the Pt-contacted WSe2 FETs, the p-type unipolar behaviour was observed with the transport dominated by holes. In the hydrazine treatment, a p-type WSe2 FET (Pt contacted) was converted to n-type with enhanced electron conduction, whereas highly n-doped properties were achieved for both Ti- and Co-contacted WSe2 FETs with on-current increasing by three orders of magnitude for Ti. All n-doped WSe2 FETs exhibited enhanced hysteresis in their transfer characteristics, which opens up the possibility of developing memories using transition metal dichalcogenides.

Photoluminescence and lasing characteristics of InGaAs∕InGaAsP∕InP quantum dots

The InGaAs quantum dots (QDs) were grown with InGaAsP(λg=1.0–1.1μm) barrier, and the emission wavelength was controlled by the composition of InGaAs QD material in the range between 1.35 and 1.65μm. It is observed that the lateral size increases and the height of the QDs decreases with the increase in relative concentration of trimethylgallium to trimethylindium supplied during InGaAs QD growth. It is seen that the higher concentration of group III alkyl supply per unit time leads to higher QD areal density, indicating that the higher concentration causes more QDs to nucleate. By optimizing the growth conditions, the QDs emitting at around 1.55μm were grown with an areal density as high as 8×1010cm−2. The lasing action between the first excited subband states at the wavelength of 1.488μm has been observed from the ridge waveguide lasers with five QD stacks up to 260K. The threshold current density of 3.3kA∕cm2 at 200K and a characteristic temperature of 118K were measured.