M.D. Kim

Growth mode and structural characterization of GaSb on Si (001) substrate: A transmission electron microscopy study

Growth mode and structural properties of GaSb layers grown on silicon substrate by molecular beam epitaxy method are investigated by transmission electron microscopy. It is found that the GaSb grows to three-dimensional islands and grains are tilted to reduce a lattice mismatch through twin boundaries when they are directly grown on Si substrate. A low-temperature (LT) AlSb buffer plays a key role in transferring the growth mode from a three-dimensional island to a layer-by-layer structure. When the LT AlSb layer is used as a buffer, 90° misfit dislocations, with the Burgers vector b of 1∕2a⟨110⟩, are observed on the interface.

Microstructural properties and dislocation evolution on a GaN grown on patterned sapphire substrate: A transmission electron microscopy study

The microstructural properties of a GaN layer grown on a patterned sapphire substrate (PSS) were studied in detail using transmission electron microscope techniques to determine dislocation and growth behaviors. Regular and uniform recrystallized GaN islands were observed on the protruding pattern. On a flat sapphire surface, the crystallographic orientation relationship of ⟨1¯21¯0⟩GaN on FS//⟨11¯00⟩sapphire and {11¯01}GaN on FS//{12¯13}sapphire existed between the GaN and the substrate. On the other hand, the orientation relationship of ⟨1¯21¯0⟩GaN layer//⟨1¯21¯0⟩GaN island on IS//⟨11¯00⟩sapphire and {11¯01}GaN layer//{0002}GaN island on IS//{12¯13}sapphire was confirmed among the GaN layer, the recrystallized GaN islands on an inclined sapphire surface and the PSS. The flat surface among the protruding patterns began to fill rapidly with GaN. Then, the GaN gradually overgrew the protruding pattern and coalesced near the summit as the growth time increased. The generation of threading dislocations was ob...

Gain characteristics of InAs∕InGaAsP quantum dot semiconductor optical amplifiers at 1.5μm

The authors have fabricated ridge waveguide quantum dot (QD) semiconductor optical amplifiers (SOAs) on InP substrates that operate in the 1.5μm region. The active layer consists of InAs∕InGaAsP QD layers with a high dot density, but which still have good isolation between dots in the lateral and vertical directions, as confirmed by time-resolved photoluminescence measurements. One of these QD SOAs exhibited a fiber-to-fiber gain of 22.5dB and a chip gain of 37dB at 1.51μm. The spectral gain shape was found to be maintained for variations of the peak gain from 12to22dB, reflecting the zero-dimensional density of states at room temperature.

High-resolution transmission electron microscopy study on the growth modes of GaSb islands grown on a semi-insulating GaAs (001) substrate

The initial growth behaviors of GaSb on a GaAs substrate were studied using a high-resolution electron microscope (HRTEM). Four types of GaSb islands were observed by HRTEM. HRTEM micrographs showed that strain relaxation mechanisms were different in the four types of islands. Although 90° misfit dislocations relieve misfit strain in the islands, additional mechanisms are required to relax the remaining strain. The existence of elastic deformation near the surface related to dislocations and intermediate layers between GaSb and GaAs were demonstrated in island growths. Finally, the generation of planar defects to relieve strain was observed in a specific GaSb growth.

Effects of two-dimensional electron gas on the optical properties of InAs/GaAs quantum dots in modulation-doped heterostructures

The Shubnikov–de Haas data showed that the carrier density of two-dimensional electron gas (2DEG) in the GaAs active region containing InAs quantum dot (QD) arrays embedded between modulation-doped Al0.25Ga0.75As/GaAs heterostructures increased with increasing doping concentration in the modulation layer. The transmission electron microscopy images showed that the sizes of the self-assembled InAs vertically stacked QD arrays inserted in the GaAs did not change significantly with increasing carrier density of the 2DEG. The photoluminescence (PL) spectra showed that the peaks corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole subband of the InAs QDs shifted to the higher energy side with increasing density of the 2DEG and that the full width at half maximum of the PL spectrum increased slightly with increasing density of the 2DEG.

Carrier capture before entering into a semiconductor quantum dot as a dominant pathway for the reduction of emission efficiency

To study the carrier reduction pathway for quantum dots (QDs), we have measured carrier lifetimes and photoluminescence spectra both at 10 K and at higher temperatures. We found that the carriers captured in QDs are robust and are not lost to nearby defects, even at elevated temperature, and that the lower emission efficiency of QDs with defects compared to the corresponding defect-free QDs is due to the capture of carriers to defects before entering into the QDs.

Temperature and excitation power dependence of photoluminescence from high quality GaSb grown on AlSb and GaSb buffer layers

High quality GaSb layers were grown on semi-insulating (001) GaAs substrates by molecular-beam epitaxy, using AlSb and GaSb buffer layers. We observed strong photoluminescence even for temperatures higher than 100 K. The photoluminescence intensity was significantly increased when AlSb/GaSb superlattices were grown on the GaSb layer. With increasing the excitation power, the ratio of the acceptor-bound exciton with respect to the donor-acceptor pair transition is increased due to the saturation of the donor-acceptor pair transitions. We also observed an abnormal increase in the intensity with increasing temperatures up to 100 K. This unusual behavior is attributed to the influence of deep centers.

Strong photoluminescence at 1.53 μm from GaSb/AlGaSb multiple quantum wells grown on Si substrate

Strong photoluminescence at 1.53 μm was obtained from a GaSb/Al0.4Ga0.6Sb multiple-quantum-well sample grown on Si substrate, indicating greatly reduced defects by InSb quantum-dot layers that terminate dislocations. The carrier lifetime of 1.4 ns, comparable to typical InP-based quantum wells, and its independence on excitation power indicates the low defect density. Due to the wide well width and tensile strain, photoluminescence was dominated by the light hole-electron transition at low temperature. However, the heavy hole-electron transition was dominant at room temperature due to the proximity of energy levels and higher density of states for the heavy hole transition.

Controlled Aggregation and Increased Stability of β-Glucuronidase by Cellulose Binding Domain Fusion

Cellulose-binding domains (CBDs) are protein domains with cellulose-binding activity, and some act as leaders in the localization of cellulosomal scaffoldin proteins to the hydrophobic surface of crystalline cellulose. In this study, we found that a CBD fusion enhanced and improved soluble β-glucuronidase (GusA) enzyme properties through the formation of an artificially oligomeric state. First, a soluble CBD fused to the C-terminus of GusA (GusA-CBD) was obtained and characterized. Interestingly, the soluble GusA-CBD showed maximum activity at higher temperatures (65°C) and more acidic pH values (pH 6.0) than free GusA did (60°C and pH 7.5). Moreover, the GusA-CBD enzyme showed higher thermal and pH stabilities than the free GusA enzyme did. Additionally, GusA-CBD showed higher enzymatic activity in the presence of methanol than free GusA did. Evaluation of the protease accessibility of both enzymes revealed that GusA-CBD retained 100% of its activity after 1 h incubation in 0.5 mg/ml protease K, while free GusA completely lost its activity. Simple fusion of CBD as a single domain may be useful for tunable enzyme states to improve enzyme stability in industrial applications.

Microstructural characteristics of GaN/AlN thin films grown on a Si (110) substrate by molecular beam epitaxy: Transmission electron microscopy study

1 Korea Research Institute of Standards and Science, 267 Gajeong-Ro, Yuseong-Gu, Daejeon 34113, Republic of Korea 2 IV Works Co., Ansan, Kyungki-do 425-833, Republic of Korea 3 Department of Physics, Chungnam National University, 99 Daehak-Ro, Yuseong-Gu, Daejeon 34134, Republic of Korea 4 Division of Electrical and Computer Engineering, Hanyang University, Ansan city, Kyunggi-do 15588, Republic of Korea