Hoonbae Kim

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.

A chlorinated barium titanate-filled polymer composite with a high dielectric constant and its application to electroluminescent devices

We have studied the effect of chlorination on the dielectric performance of a barium titanate (BTO)–polymer film. Functionalization of BTO powder particles with chlorine (Cl) atoms and/or Cl-containing functional groups was achieved by a simple treatment using chlorinated solvents. The chlorinated BTO (Cl-BTO) particles were incorporated into a cyanoethyl-based polymer and the mixture was spin-coated to produce a composite film. The dielectric constant of the composite film with Cl-BTO was as high as 208 at a frequency of 10 Hz, showing a 2.5-fold increase in dielectric constant compared to composites composed of the neat BTO and the cyanoethyl-based polymer at a frequency of 10 Hz. The observed dramatic increase in dielectric constant would be caused by the interfacial polarization due to the p-type doping effect resulting from the presence of strong electronegative Cl atoms. Given a relatively high dielectric constant, composite films with Cl-BTO were further exploited as a dielectric layer in an inorganic electroluminescence (EL) device. The luminance of the EL device with Cl-BTO was 4090 cd m−2 at a frequency of 1 kHz, showing a 2.03-fold increase in luminance compared to that with the untreated BTO. The improved performance of the EL device is attributed to a high dielectric constant of the composite films that allow for efficient charge carrier tunneling into the phosphor and therefore enhanced luminance and efficiency.

Enhanced electroluminescence performances by controlling the position of carbon nanotubes

Alternating current driven powder electroluminescence (EL) devices with single-walled carbon nanotubes (SWCNTs) were fabricated to utilize the field enhancement by controlling the position of SWCNTs. The SWCNT conditions, which could lead to increased EL performance, were optimized by examining the characteristics of EL devices after controlling the length of the SWCNTs, where the SWCNT layer was positioned at the interface between the dielectric and emitting layers. The EL device with a short SWCNT layer for a crushing time of 20 min exhibited the highest EL performance of the samples examined. In addition, SWCNTs with the optimized SWCNT condition were inserted at various interfaces to determine the effective position of the SWCNT layer in the EL device. The highest EL performance, such as a 51% and 65% increase in luminance and efficiency, respectively, was achieved by inserting SWCNTs at the interface between the dielectric and emitting layers together with SWCNTs between the emitting layer and bottom...

Low-Temperature Process for Atomic Layer Chemical Vapor Deposition of an Al2O3 Passivation Layer for Organic Photovoltaic Cells.

Flexible organic photovoltaic (OPV) cells have drawn extensive attention due to their light weight, cost efficiency, portability, and so on. However, OPV cells degrade quickly due to organic damage by water vapor or oxygen penetration when the devices are driven in the atmosphere without a passivation layer. In order to prevent damage due to water vapor or oxygen permeation into the devices, passivation layers have been introduced through methods such as sputtering, plasma enhanced chemical vapor deposition, and atomic layer chemical vapor deposition (ALCVD). In this work, the structural and chemical properties of Al2O3 films, deposited via ALCVD at relatively low temperatures of 109 degrees C, 200 degrees C, and 300 degrees C, are analyzed. In our experiment, trimethylaluminum (TMA) and H2O were used as precursors for Al2O3 film deposition via ALCVD. All of the Al2O3 films showed very smooth, featureless surfaces without notable defects. However, we found that the plastic flexible substrate of an OPV device passivated with 300 degrees C deposition temperature was partially bended and melted, indicating that passivation layers for OPV cells on plastic flexible substrates need to be formed at temperatures lower than 300 degrees C. The OPV cells on plastic flexible substrates were passivated by the Al2O3 film deposited at the temperature of 109 degrees C. Thereafter, the photovoltaic properties of passivated OPV cells were investigated as a function of exposure time under the atmosphere.

Optical characteristics of In(Ga)As quantum dots on (100) InP substrate for 1.5 μm laser diodes

We report on round quantum dots grown on InP (100) substrate, which emit around 1.55 μm. At 10 K the full width at half maximum is as small as 28 meV, attesting a rather uniform size distribution. The carrier lifetimes are almost the same across the whole photoluminescence band, indicating the good isolation of quantum dots. Continuous-wave lasings are observed at room temperature from laser diodes made of these quantum dots.