Ja Hoon Koo

Synthesis of Few-Layered Graphene Nanoballs with Copper Cores Using Solid Carbon Source

We report the fabrication of graphene-encapsulated nanoballs with copper nanoparticle (Cu NP) cores whose size range from 40 nm to 1 μm using a solid carbon source of poly(methyl methacrylate) (PMMA). The Cu NPs were prone to agglomerate during the annealing process at high temperatures of 800 to 900 °C when gas carbon source such as methane was used for the growth of graphene. On the contrary, the morphologies of the Cu NPs were unchanged during the growth of graphene at the same temperature range when PMMA coating was used. The solid source of PMMA was first converted to amorphous carbon layers through a pyrolysis process at the temperature regime of 400 °C, which prevented the Cu NPs from agglomeration, and they were converted to few-layered graphene (FLG) at the elevated temperatures. Raman and transmission electron microscope analyses confirmed the synthesis of FLG with thickness of approximately 3 nm directly on the surface of the Cu NPs. X-ray diffraction and X-ray photoelectron spectroscopy analyses, along with electrical resistance measurement according to temperature changes showed that the FLG-encapsulated Cu NPs were highly resistant to oxidation even after exposure to severe oxidation conditions.

Flatband voltage control in p-metal gate metal-oxide-semiconductor field effect transistor by insertion of TiO2 layer

Titanium oxide (TiO2) layer was used to control the flatband voltage (VFB) of p-type metal-oxide-semiconductor field effect transistors. TiO2 was deposited by plasma enhanced atomic layer deposition (PE-ALD) on hafnium oxide (HfO2) gate dielectrics. Comparative studies between TiO2 and Al2O3 as capping layer have shown that improved device properties with lower capacitance equivalent thickness (CET), interface state density (Dit), and flatband voltage (VFB) shift were achieved by PE-ALD TiO2 capping layer.

Formation of Vertically Aligned Cobalt Silicide Nanowire Arrays Through a Solid-State Reaction

We report for the first time synthesis of high-density arrays of vertically well-aligned cobalt monosilicide (CoSi) nanowires (NWs) in a large area via a solid-state reaction. The vertical arrays of 1-μm-long Si NWs were first grown on a p-type (1 0 0) Si substrate by the aqueous electroless etching (AEE) method, and a 40-nm-thick Co layer was conformally deposited using a thermal atomic layer deposition system as revealed by SEM and transmission electron microscope analyses. The rapid thermal annealing process was carried out at various temperatures ranging from 700 to 1000 °C; the X-ray diffraction analysis confirmed that the polycrystalline CoSi NW arrays were formed at temperatures above 900 °C. The required high driving force for this silicide formation can be attributed to the significant amounts of oxygen-related contaminants at the defect sites of the highly rough surfaces of AEE-grown Si NWs. To demonstrate practical applications, field emitters and Schottky diodes were fabricated using the vertically aligned CoSi NW arrays. The field emission measurements showed a turn-on field of 10.9 V/μm and a field enhancement factor of 328, indicating the feasibility of vertically aligned CoSi NW arrays as promising field emitters. For the Schottky diodes, the measured Schottky barrier height was 0.52 eV and the estimated ideality factor obtained from the I-V characteristic curves was 2.28.

Single-Crystalline Silicon-Based Heterojunction Photodiode Arrays on Flexible Plastic Substrates

A silicon-based photodiode array was fabricated on a flexible polyethylene terephthalate substrate using a transfer printing technique. A heterojunction structure composed of a 15-nm-thick highly doped hydrogenated amorphous-silicon (n+ a-Si:H) layer and a 3-μm-thick p-type single-crystal silicon (p c-Si) membrane layer was adopted as the active layer of the flexible photodiode. The highly ordered photodiode array formed on the flexible substrate exhibited superior stability in electrical properties under bent conditions with no mechanical deformation. The variation of the spectral quantum efficiency (QE) under short-wavelength light illumination (λ ≤ 580 nm) was in excellent agreement with that of a heterojunction photodiode composed of a-Si:H and a bulk c-Si substrate. Relatively low QE values were observed under longer wavelength (λ ≥ 600 nm) illumination due to the finite thickness of the active layer. The C-V measurement results of the fabricated photodiode array were in accordance with the abrupt junction model. A closer inspection of the junction area of the device using high-resolution cross-sectional transmission micrograph exhibited an interface depth of 2 ± 0.5 nm, which is unavoidable in plasma-enhanced a-Si:H deposition processes.

Induced Transition of CdSe Nanoparticle Superstructures by Controlling the Internal Flow of Colloidal Solution

The self-assembly behaviors of flow-enhanced CdSe nanoparticle (NP) colloidal systems were investigated, which were systemically prepared by adding ethylene glycol (EG) or acetic acid (AA) to NP suspensions with deionized water (DI water) base. The additive solvents, which had higher boiling points and lower surface tensions than those of the DI water, modified the internal flow of the NP colloidal system, consequently affecting the morphologies of the generated NP superstructures after the full evaporation of their droplets. In flow-enhanced systems, NPs were formed into highly elongated dendrites that stretched from the center region to the edges along the direction of convective flow inside the droplet, while NPs in random drift system were easily aggregated to form cluster-shaped thick dendritic structures. When the volume fraction of EG was increased, the dominant superstructures were changed from dendrites to clusters, which can be mainly attributed to the changes in the dielectric properties of the NP droplets as evaporation proceeded because of the large discrepancy in the vapor pressures of EG and DI water. The balance between the interparticle potentials of electrostatic repulsion and van der Waals attraction was continuously altered, resulting in the formation of clusters with increasing EG ratio. Contrastively, the transition of superstructures could not be observed in the case of colloidal system prepared by mixing DI water and AA, which can be ascribed to the similar vapor pressures of the two solvents; the dielectric properties of the solution mixture was barely changed throughout the steady evaporation process, which resulted in the formation of uniformly distributed highly elongated dendrites. Polarization-dependent imaging experiments and photoluminescence measurements revealed that the stretched dendrites formed under the flow-enhanced conditions showed higher crystallinity than that of the clusters.