Chel-Jong Choi

Epitaxial growth of ZnO nanowall networks on GaN/sapphire substrates

Heteroepitaxy of vertically well-aligned ZnO nanowall networks with a honeycomblike pattern on GaN∕c-Al2O3 substrates by the help of a Au catalyst was realized. The ZnO nanowall networks with wall thicknesses of 80–140nm and an average height of about 2μm were grown on a self-formed ZnO thin film during the growth on the GaN∕c-Al2O3 substrates. It was found that both single-crystalline ZnO nanowalls and catalytic Au have an epitaxial relation to the GaN thin film in synchrotron x-ray scattering experiments. Hydrogen-sensing properties of the ZnO nanowall networks have also been investigated.

Improved heat dissipation in gallium nitride light-emitting diodes with embedded graphene oxide pattern

The future of solid-state lighting relies on how the performance parameters will be improved further for developing high-brightness light-emitting diodes. Eventually, heat removal is becoming a crucial issue because the requirement of high brightness necessitates high-operating current densities that would trigger more joule heating. Here we demonstrate that the embedded graphene oxide in a gallium nitride light-emitting diode alleviates the self-heating issues by virtue of its heat-spreading ability and reducing the thermal boundary resistance. The fabrication process involves the generation of scalable graphene oxide microscale patterns on a sapphire substrate, followed by its thermal reduction and epitaxial lateral overgrowth of gallium nitride in a metal-organic chemical vapour deposition system under one-step process. The device with embedded graphene oxide outperforms its conventional counterpart by emitting bright light with relatively low-junction temperature and thermal resistance. This facile strategy may enable integration of large-scale graphene into practical devices for effective heat removal.

Wafer-Scale, Homogeneous MoS2 Layers on Plastic Substrates for Flexible Visible-Light Photodetectors

An appropriate solution is suggested for synthesizing wafer-scale, continuous, and stoichiometric MoS2 layers with spatial homogeneity at the low temperature of 450 °C. It is also demonstrated that the MoS2 -based visible-light photodetector arrays are both fabricated on 4 inch SiO2 /Si wafer and polyimide films, revealing 100% active devices with a narrow photocurrent distribution and excellent mechanical durability.

CdS quantum dots grown by in situ chemical bath deposition for quantum dot-sensitized solar cells

CdS quantum dots (QDs) are grown on mesoporous TiO2 films by 3 to 13 repeated cycles of in situ chemical bath deposition (CBD). The overall energy conversion efficiency of CdS quantum dot-sensitized solar cells (QD-SSCs) increases as the number of cycles increases to the eight — peaking at 2.13%. This is attributable to efficient light harvesting and charge-collection resulting from enhanced light absorption and faster charge transport. However a further increase of CBD cycles to thirteen reduces QD-SSCs performance, despite better light absorption in the long wavelengths. This is attributed to decreased charge-injection efficiency, which is due to reduced driving forces of carrier injection, increased trap-mediated recombination in the QDs, and hindered ion transport.

Enhancement in Light Emission Efficiency of a Silicon Nanocrystal Light‐Emitting Diode by Multiple‐Luminescent Structures

A lot of research has been devoted towards the Si-based microphotonics due to the applications in silicon-based optoelectronic devices. [ 1–4 ] The Si-based light sources could reduce the fabrication cost because the compatibility with a conventional Si technology is better than any other materials such as conventional GaAsand GaN-based materials. Bulk silicon has poor luminescence effi ciency due to the indirect nature of its band gap and is thus highly ineffi cient for the light sources. However, if the size of Si nanocrystals (nc-Si) is smaller than the free exciton Bohr radius of bulk Si ( ∼ 4.6 nm), the light emission effi ciency could be much enhanced due to an increase in overlapping of electron-hole wave functions, that is, a quantum confi nement effect. [ 5 ] Because of this, nc-Si light emitting diodes (LEDs) have been investigated as promising light sources for the next generation of photonic applications. [ 6 , 7 ]

Chemical etching of boron-rich layer and its impact on high efficiency n-type silicon solar cells

This paper reports on an effective chemical etching treatment to remove a boron-rich layer which has a significant negative impact on n-type silicon (Si) solar cells with boron emitter. A nitric acid-grown oxide/silicon nitride stack passivation on the boron-rich layer-etched boron emitter markedly decreases the emitter saturation current density J0e from 430 to 100 fA/cm2. This led to 1.6% increase in absolute cell efficiency including 22 mV increase in open-circuit voltage Voc and 1.9 mA/cm2 increase in short-circuit current density Jsc. This resulted in screen-printed large area (239 cm2) n-type Si solar cells with efficiency of 19.0%.

Analysis of Transconductance

This paper experimentally investigates the unique behavior of transconductance (gm) in the Schottky-barrier metal-oxide-semiconductor field-effect transistors (SB-MOSFETs) with various silicide materials. When the Schottky-barrier height (SBH) or a scaling parameter is not properly optimized, a peculiar shape of gm is observed. Thus, gm can be used as a novel metric that exhibits the transition of the carrier injection mechanisms from a thermionic emission (TE) to thermally assisted tunneling (TU) in the SB-MOSFETs. When the local maximum point of gm is observed, it can be expected that an incomplete transition occurs between TE and TU in SB-MOSFETs. When a dopant-segregation (DS) technique is implemented in the SB-MOSFETs, however, the carrier injection efficiency from the source to the channel is significantly improved, although the SBH is not minimized. As a consequence, the peculiar shape of the gm disappears, i.e., a complete transition from TE to TU can be enabled by the DS technique.

(g_{m})

We report an enhancement in light emission efficiency form Si nanocrystal (NC) light-emitting diodes (LEDs) via surface plasmons (SPs) by employing Au nanoparticles (NPs). Photoluminescence intensity of Si NCs with Au NPs was enhanced by 2 factors of magnitude due to the strong coupling of Si NCs and SP resonance modes of Au NPs. The electrical characteristics of Si NC LED were significantly improved, which was attributed to an increase in an electron injection into the Si NCs due to the formation of inhomogeneous Schottky barrier at the SiC-indium tin oxide interface. Moreover, light output power from the Si NC LED was enhanced by 50% due to both SP coupling and improved electrical properties. The results presented here can provide a very promising way to significantly enhance the performance of Si NC LED.

in Schottky-Barrier MOSFETs

CdS quantum dots (QDs) of 6.8–6.9 nm were assembled in situ on conventional TiO2 nanotube arrays (Type I) and nanoporous-layer-covered nanotube arrays (Type II). The QD-sensitized solar cell with the Type II nanotubes exhibited significantly enhanced overall energy conversion efficiency, despite having less assembled QDs. This was due to the Type II nanotube arrays having fewer defects and suppressed recombination rate (or back electron transport) from surface traps in the TiO2 to electron traps in the QDs, resulting in significantly improved electron lifetime.

Enhancement in light emission efficiency of Si nanocrystal light-emitting diodes by a surface plasmon coupling

The effect of the number of InGaN/GaN quantum well (QW) pairs on the interfacial structural and optical properties of InGaN/GaN multiple quantum wells (MQWs), as grown by low-pressure metalorganic vapor-phase epitaxy was examined. As the number of QW pairs increased, In-rich InGaN precipitates were more readily detected in the InGaN/GaN MQWs by cross-sectional transmission electron microscope. The intensity of the photoluminescence (PL) peak was decreased and the PL peak was red-shifted with an increase in the number of QW pairs. X-ray diffraction measurements revealed that the interfacial structure between InGaN and GaN were also deteriorated with the increasing number of QW pairs. These results can be attributed to the relaxation of an accumulated strain through the dislocations induced by an increase in the total thickness of the MQWs with an increase in the number of QW pairs. These results suggest that the defects such as dislocations facilitate the formation of In-rich phases in the InGaN layers in the MQWs.