Jungsub Kim

Luminescent properties and reduced dimensional behavior of hydrothermally prepared Y2SiO5:Ce nanophosphors

Hydrothermally prepared nanophosphor Y2SiO5:Ce crystallizes in the P21∕c structure, rather than the B2∕b structure observed in bulk material. Relative to bulk powder, nanophosphors of particle size ∼25–100nm diameter exhibit redshifts of the photoluminescence excitation and emission spectra, reduced self absorption, enhanced light output, and medium-dependent radiative lifetime. Photoluminescence data are consistent with reduced symmetry of the P21∕c structure and are not necessarily related to reduced dimensionality of the nanophosphor. In contrast, medium-dependent lifetime and enhanced light output are attributed to nanoscale behavior. Perturbation of the Ce ion electric field is responsible for the variable lifetime.

Hole emitter of photonic quantum ring

We report on the observation of “hole” whispering gallery lasers from semiconductor microcavities with three dimensional optical confinement, with thresholds potentially reducible to micro- to nanoampere regimes according to a quadratic size-dependent reduction, consistent with a photonic quantum corral effect of naturally formed recombinant quantum rings. If the device size grows over a critical diameter, the quantum ring whispering gallery then begins to disappear. However, cooperative small hole laser arrays, exhibiting photonic crystal structure properties, avoid the criticality and open a possibility of constructing practical high-density electropumped emitter arrays.

Abstraction of CH3 group from the CH3I molecule adsorbed on Cu(100) by gas-phase hydrogen atoms

The reaction of CH 3 I molecules adsorbed on Cu( 100) with a beam of hydrogen atoms has been studied using a mass spectrometer. CH 4 was produced as the only gas-phase product by abstraction of the CH 3 group by a hydrogen atom even at a surface temperature of 100 K, i.e. CH 3 I (ad) + H (g) →CH 4(g) + I (ad) . This is in contrast to the gas-phase reaction, in which HI is believed to be the principal product. As a minor process, concurrent desorption of CH 3 I was also observed. The time variation of the CH 4 signal showed a step-like jump followed by an exponential decay upon opening the beam, from which a reaction cross section σ r = 1.2 x 10 -16 cm2 was estimated at submonolayer coverages of CH 3 I. Why the reaction of adsorbed CH 3 I with the gas-phase H atom is different from that of CH 3 I (g) is discussed in terms of the energetics modified by the surface.

An elegant route to overcome fundamentally-limited light extraction in AlGaN deep-ultraviolet light-emitting diodes: Preferential outcoupling of strong in-plane emission.

While there is an urgent need for semiconductor-based efficient deep ultraviolet (DUV) sources, the efficiency of AlGaN DUV light-emitting diodes (LEDs) remains very low because the extraction of DUV photons is significantly limited by intrinsic material properties of AlGaN. Here, we present an elegant approach based on a DUV LED having multiple mesa stripes whose inclined sidewalls are covered by a MgF2/Al omni-directional mirror to take advantage of the strongly anisotropic transverse-magnetic polarized emission pattern of AlGaN quantum wells. The sidewall-emission-enhanced DUV LED breaks through the fundamental limitations caused by the intrinsic properties of AlGaN, thus shows a remarkable improvement in light extraction as well as operating voltage. Furthermore, an analytic model is developed to understand and precisely estimate the extraction of DUV photons from AlGaN DUV LEDs, and hence to provide promising routes for maximizing the power conversion efficiency.

Complete suppression of large InAs island formation on GaAs by metal organic chemical vapor deposition with periodic AsH3 interruption

Self-assembled InAs quantum dots (QDs) on GaAs substrates were grown by metal organic chemical vapor deposition with periodic AsH3 interruption. In contrast to the conventional InAs QD growth method, AsH3 was interrupted periodically while TMIn was introduced into the reactor continuously. By interrupting AsH3 periodically, the growth surface is modulated between As-stabilized surface and In-stabilized one, resulting in complete suppression of relaxed large island formation and significant improvement in photoluminescence intensity. With further optimization of growth parameters, the authors obtained the emission at 1.32μm and narrow linewidth of 32meV at room temperature.

Improved performance of AlGaN-based deep ultraviolet light-emitting diodes with nano-patterned AlN/sapphire substrates

We demonstrated AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) with periodic air-voids-incorporated nanoscale patterns enabled by nanosphere lithography and epitaxial lateral overgrowth (ELO) on a 4-in. sapphire substrate. The nanoscale ELO improved the crystal quality of overgrown epitaxial layers at a relatively low growth temperature of 1050 °C and at small coalescence thickness less than 2 μm. The light output power of the DUV LED was enhanced significantly by 67% at an injection current of 20 mA. We attribute such a remarkable enhancement to the formation of embedded periodic air voids which cause simultaneous improvements in the crystal quality of epitaxial layers by ELO and light extraction efficiency enabled by breaking the predominant in-plane guided propagation of DUV photons.

Variation of the external quantum efficiency with temperature and current density in red, blue, and deep ultraviolet light-emitting diodes

The temperature-dependent external quantum efficiencies (EQEs) were investigated for a 620 nm AlGaInP red light-emitting diodes (LEDs), a 450 nm GaInN blue LED, and a 285 nm AlGaN deep-ultraviolet (DUV) LED. We observed distinct differences in the variation of the EQE with temperature and current density for the three types of LEDs. Whereas the EQE of the AlGaInP red LED increases as temperature decreases below room temperature, the EQEs of GaInN blue and AlGaN DUV LEDs decrease for the same change in temperature in a low-current density regime. The free carrier concentration, as determined from the dopant ionization energy, shows a strong material-system-specific dependence, leading to different degrees of asymmetry in carrier concentration for the three types of LEDs. We attribute the EQE variation of the red, blue, and DUV LEDs to the different degrees of asymmetry in carrier concentration, which can be exacerbated at cryogenic temperatures. As for the EQE variation with temperature in a high-current d...

Repeated oral administration of capsaicin increases anxiety-like behaviours with prolonged stress-response in rats

This study was conducted to examine the psycho-emotional effects of repeated oral exposure to capsaicin, the principal active component of chili peppers. Each rat received 1 mL of 0.02% capsaicin into its oral cavity daily, and was subjected to behavioural tests following 10 daily administrations of capsaicin. Stereotypy counts and rostral grooming were significantly increased, and caudal grooming decreased, in capsaicin-treated rats during the ambulatory activity test. In elevated plus maze test, not only the time spent in open arms but also the percent arm entry into open arms was reduced in capsaicin-treated rats compared with control rats. In forced swim test, although swimming duration was decreased, struggling increased in the capsaicin group, immobility duration did not differ between the groups. Repeated oral capsaicin did not affect the basal levels of plasma corticosterone; however, the stress-induced elevation of plasma corticosterone was prolonged in capsaicin treated rats. Oral capsaicin exposure significantly increased c-Fos expression not only in the nucleus tractus of solitarius but also in the paraventricular nucleus. Results suggest that repeated oral exposure to capsaicin increases anxiety-like behaviours in rats, and dysfunction of the hypothalamic-pituitary-adrenal axis may play a role in its pathophysiology.

Study of Multi-stacked InAs Quantum Dot Infrared Photodetectors Grown by Metal Organic Chemical Vapor Deposition

We grew multi-stacked InAs/ DWELL (dot-in-a-well) structure by metal organic chemical vapor deposition and investigated optical properties by photoluminescence and I-V characteristics by dark current measurement. When stacking InAs quantum dots (QDs) with same growth parameter, the size and density of QDs were changed, resulting in the bimodal emission peak. By decreasing the flow rate of TMIn, we achieved the uniform multi-stacked QD structure which had the single emission peak and high PL intensity. As the growth temperature of n-type GaAs top contact layer (TCL) is above , the PL intensity severely decreased and dark current level increased. At bias of 0.5 V, the activation energy for temperature dependence of dark current decreased from 106 meV to 48 meV with increasing the growth temperature of n-type GaAs TCL from 580 to . This suggest that the thermal escape of bounded electrons and non-radiative transition become dominant due to the thermal inter-diffusion at the interface between InAs QDs and well layer.

Enhanced light extraction efficiency of micro-ring array AlGaN deep ultraviolet light-emitting diodes

An effective approach to overcome inherently poor light extraction efficiency of AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) is presented. We demonstrated the 5 × 5 array micro-ring DUV LED having an inclined sidewall at the outer perimeter and a p-GaN-removed inner circle of the micro-ring, together with MgF2/Al omnidirectional reflectors. The micro-ring array DUV LED shows remarkably higher light output power by 70% than the reference, consistent with the calculated result, as well as comparable turn-on and operational voltages, which are attributed to the effective extraction of strong transverse-magnetic polarized anisotropic emission and the reduction of the absorption loss by the p-GaN contact layer, simultaneously.