Donghan Lee

Growth of Si-doped InAs quantum dots and annealing effects on size distribution

We investigated the Si-doped InAs quantum dots (QDs) grown by molecular beam epitaxy and the annealing effects on the QD size distribution through photoluminescence (PL) spectroscopy. A double-peak feature in PL was observed from as-grown InAs QDs with Si-doping, and excitation intensity dependence of PL indicated that the double-peak feature is related to the ground-state emission from InAs QDs with bimodal size distribution. The PL spectrum from Si-doped InAs QDs subjected to annealing treatment at 800°C in nitrogen ambient showed three additional PL peaks and blue-shift of the double-peak feature observed from as-grown sample. The excitation-intensity-dependent PL and consideration of thermal stability of carriers through temperature-dependent PL measurement demonstrated that three additional peaks come from the InAs QDs with three new branches of QD occurring during annealing process.

Optical and microstructural studies of atomically flat ultrathin In-rich InGaN∕GaN multiple quantum wells

Optical and microstructural properties of atomically flat ultrathin In-rich (UTIR) InGaN∕GaN multiple quantum well were investigated by means of photoluminescence (PL), time-resolved PL (TRPL), and cathodoluminescence (CL) experiments. The sample exhibits efficient trapping of the photoexcited carriers into quantum wells (QWs) and the effect of internal electric field in the QWs was found negligible by excitation power-dependent PL and TRPL. These phenomena were attributed to the nature of UTIR InGaN QWs, indicating the potential of this system for application in optoelectronic devices. Variation of TRPL lifetime across the PL band and spatially resolved monochromatic CL mapping images strongly suggest that there is micrometer-scale inhomogeneity in effective band gap in UTIR InGaN∕GaN QWs, which is originated from two types of localized areas.

A study on doping density in InAs/GaAs quantum dot infrared photodetector

We study the influence of doping density and the resulting optimum operation voltage on the performance of quantum dot infrared photodetectors (QDIPs). The optimum operation voltage, where detectivity becomes maximum, becomes smaller as the doping density increases. This is because the optimum dark current levels are similar regardless of the doping density. We confirmed experimentally that the optimum dark current level is ~5 mA (current density: ~A/cm2) for our samples. It is found that the higher doping density improves the performance in the range used in this experiment (5×1016–5×1017/cm3). The response to a normal incident light is confirmed and the possibility of high-temperature operation of QDIP is shown.

Coherent Lattice Vibrations in Mono- and Few-Layer WSe2.

We report the observation of coherent lattice vibrations in mono- and few-layer WSe2 in the time domain, which were obtained by performing time-resolved transmission measurements. Upon the excitation of ultrashort pulses with the energy resonant to that of A excitons, coherent oscillations of the A1g optical phonon and longitudinal acoustic phonon at the M point of the Brillouin zone (LA(M)) were impulsively generated in monolayer WSe2. In multilayer WSe2 flakes, the interlayer breathing mode (B1) is found to be sensitive to the number of layers, demonstrating its usefulness in characterizing layered transition metal dichalcogenide materials. On the basis of temperature-dependent measurements, we find that the A1g optical phonon mode decays into two acoustic phonons through the anharmonic decay process.

Optical Characteristics of InAs/InGaAsP/InP Self-Assembled Quantum Dots Emitting at 1.4–1.6 µm

InAs/InGaAsP quantum dots (QD) grown on InP substrates show strong photoluminescence (PL) signals, with peaks from 1.4 to 1.6 µm at room temperature. Time-resolved PL measurements reveal that carrier lifetimes are the same across the entire PL band at low temperature, as well as at room temperature. This is strong evidence that the PL originates from the inhomogeneously broadened states of well-isolated QDs even at room temperature. These good characteristics, with the availability of additional InGaAsP confinement layers of different band gaps, make the present QD system a good candidate for low threshold lasers at 1.5 µm.

Serological update and molecular characterization of Dirofilaria immitis in dogs, South Korea.

Eighty-one dogs in the Chungnam province were tested for heartworm (Dirofilaria immitis) infection by ELISA (SNAP test, IDEXX Laboratories, Maine, USA). Seventeen (20.9%) of the 81 samples were found to be positive and further analyzed by 16S rRNA sequencing. In this study, all dogs tested lived outdoors. Using the chi(2) test and Fishers exact test, no significant differences in the prevalence of dirofilariasis were observed among different gender and age groups, although the prevalence of this disease among dogs 2-4yrs of age remains highest. Sequence analysis revealed that the species prevalent in Chungnam province were genetically distinct from the type strain of D. immitis based on the nucleotide deletion found at position nt 276 (cytosine) and nucleotide substitution at position 428 (G to A) of the partial 16S rRNA sequence of the type strain. Furthermore, phylogenetic analysis suggests at least 2 groups of D. immitis circulating in the Chungnam area between the year 2007 and 2008.

Polarization anisotropy of transient carrier and phonon dynamics in carbon nanotubes

We report on polarization-dependent transient carrier dynamics and coherent phonon oscillations in single-walled carbon nanotubes by determining the relation between the nanotube axis and the incident light polarization. Due to the anisotropic shape of nanotubes, optical absorption strongly depends on the polarization direction. We observed three decay components when the excitation wavelength was resonant with the E22 transition energy and observed two-decay components under off-resonance conditions. The transient absorption and coherent phonon amplitudes were measured as a function of the angle between the pump and probe polarizations and were analyzed based on the absorption anisotropy of carbon nanotubes.

Effects of Beryllium Doping into InGaAlAs Metamorphic Buffer on High-Electron-Mobility Transistor Structure

The effects of beryllium (Be) doping into InGaAlAs metamorphic buffer (M-buffer) on the surface morphology, residual strain, and transport property of metamorphic high-electron-mobility transistors (MHEMTs) were investigated. The results of atomic force microscopy, photoluminescence, and Hall measurements indicated improved surface morphology and transport property of MHEMTs due to Be doping in the M-buffer layer, particularly at a high M-buffer growth temperature (450°C). However, the electron mobility degraded at high Be doping concentrations (~1017/cm3) due to anomalous out-diffusion of Be into the channel of the MHEMT.

Carrier dynamics of low-temperature-grown In0.53Ga0.47As on GaAs using an InGaAlAs metamorphic buffer

Structural and carrier dynamic properties of low-temperature-grown In0.53Ga0.47As (LT-InGaAs) on GaAs using an InGaAlAs metamorphic buffer were studied. Investigation of dependence of the structural and carrier dynamic properties of the LT-InGaAs on annealing temperature showed that they were closely related. The use of the metamorphic buffer was effective in reducing the carrier lifetime in the LT-InGaAs. A carrier lifetime as short as 2.14ps, comparable to that of Be-doped LT-InGaAs was achieved.

Carrier lifetime of low-temperature-grown In/sub 0.53/Ga/sub 0.47/As on GaAs using an InGaAlAs metamorphic buffer

Dependence of carrier lifetime of low-temperature-grown In/sub 0.53/Ga/sub 0.47/As (LT-InGaAs) on GaAs using an InGaAlAs metamorphic buffer (M-buffer) on post-growth rapid thermal annealing was investigated. Utilization of residual dislocation in the LT-InGaAs on the M-buffer was effective in reducing the carrier lifetime, producing the carrier lifetime of 2.14 ps that is comparable to that of the Be-doped LT-InGaAs.