I. K. Han

Stability of sulfur-treated InP surface studied by photoluminescence and x-ray photoelectron spectroscopy

The degradation behavior of the sulfur-treated InP surface at relatively low temperature has been investigated with x-ray photoelectron and photoluminescence (PL) spectroscopy. The results showed that the treated surfaces were oxidized to In2O3, InPO3, and InPO4 at 250 °C and in a vacuum of 10−3 Torr for 20 min. As the holding time for S-treated InP under a vacuum of 10−3 Torr increased, the PL peak caused by the band edge transition decreased without the formation of oxides. It was therefore suggested that the decrease of the PL intensity for S-treated InP is only related to the generation of phosphorous vacancies at the surface, not to oxide formation. The usefulness of a thin S overlayer on III–V semiconductors was also discussed.

Anticarcinogenic Effects of Products of Heat-Processed Ginsenoside Re, a Major Constituent of Ginseng Berry, on Human Gastric Cancer Cells

Ginsenoside Re is a triol type triterpene glycoside and is abundantly present in ginseng berry. In the present study, we verified that ginsenoside Re can be transformed into less-polar ginsenosides, namely, Rg2, Rg6, and F4, by heat-processing. The products of heat-processed ginsenoside Re inhibited phosphorylation of CDK2 at Thr160 by upregulation of p21 level, resulting in S phase arrest. The products of heat-processed ginsenoside Re also activated caspase-8, caspase-9, and caspase-3, followed by cleavage of PARP, a substrate of caspase-3, in a dose-dependent manner. Concurrently, alteration of mitochondrial factors such as Bcl-2 and Bax was also observed. Moreover, pretreatment with Z-VAD-fmk abrogated caspase-8, -9, and -3 activations by the products of heat-processed ginsenoside Re. We further confirmed that the anticancer effects of the products of heat-processed ginsenoside Re in AGS cells are mainly mediated via generation of less-polar ginsenosides Rg6 and F4.

Beneficial effects of fermented black ginseng and its ginsenoside 20(S)-Rg3 against cisplatin-induced nephrotoxicity in LLC-PK1 cells

Background Nephrotoxicity is a common side effect of medications. Panax ginseng is one of the best-known herbal medicines, and its individual constituents enhance renal function. Identification of its efficacy and mechanisms of action against drug-induced nephrotoxicity, as well as the specific constituents mediating this effect, have recently emerged as an interesting research area focusing on the kidney protective efficacy of P. ginseng. Methods The present study investigated the kidney protective effect of fermented black ginseng (FBG) and its active component ginsenoside 20(S)-Rg3 against cisplatin (chemotherapy drug)-induced damage in pig kidney (LLC-PK1) cells. It focused on assessing the role of mitogen-activated protein kinases as important mechanistic elements in kidney protection. Results The reduced cell viability induced by cisplatin was significantly recovered with FBG extract and ginsenoside 20(S)-Rg3 dose-dependently. The cisplatin-induced elevated protein levels of phosphorylated c-Jun N-terminal kinase (JNK), p53, and cleaved caspase-3 were decreased after cotreatment with FBG extract or ginsenoside 20(S)-Rg3. The elevated percentage of apoptotic LLC-PK1 cells induced by cisplatin treatment was significantly abrogated by cotreatment with FBG and the ginsenoside 20(S)-Rg3. Conclusion FBG and its major ginsenoside 20(S)-Rg3, ameliorated cisplatin-induced nephrotoxicity in LLC-PK1 cells by blocking the JNK–p53–caspase-3 signaling cascade.

Influence of arsenic during indium deposition on the formation of the wetting layers of InAs quantum dots grown by migration enhanced epitaxy

We compared the structural and optical properties of InAs∕GaAs quantum dots grown by migration enhanced epitaxy, with and without arsenic, during indium deposition. The uniformity and size of the quantum dots are enhanced in a sample without arsenic. As a result, narrower and longer wavelength photoluminescence is observed in this sample. Furthermore, the thickness of the wetting layers is reduced by ∼20% in the sample without arsenic, and this result agrees well with the speculation that metallic indium has a smaller driving force for corrugating the InAs wetting layers before they are transformed from two-dimensional to three-dimensional layers. Additionally, the photoluminescence linewidth of the sample without arsenic is insensitive to the cryostat temperature due to two major factors: the reduced thickness of the wetting layers and the enhanced uniformity. In the sample with arsenic, however, the photoluminescence linewidth shows typical anomalies.

Chemical and Free Radical-scavenging Activity Changes of Ginsenoside Re by Maillard Reaction and Its Possible Use as a Renoprotective Agent

Reactive oxygen species play critical role in kidney damage. Free radical-scavenging activities of Panax ginseng are known to be increased by heat-processing. The structural change of ginsenoside and the generation of Maillard reaction products (MRPs) are closely related to the increased free radical-scavenging activities. In the present study, we have demonstrated the Maillard reaction model experiment using ginsenoside Re and glycine mixture to identify the renoprotective effect of MRPs from ginseng or ginsenosides. Ginsenoside Re was transformed into less-polar ginsenosides, namely Rg2, Rg6 and F4 by heat-processing. The free radical-scavenging activity of ginsenoside Re-glycine mixture was increased in a temperature-dependant manner by heatprocessing. The improved free radical-scavenging activity by heat-processing was mediated by the generation of antioxidant MRPs which led to the protection of LLC-PK1 renal epithelial cells from oxidative stress. Although the free radical scavenging activities of less-polar ginsenosides were weak, they could protect LLC-PK1 cells from oxidative stress. Therefore, MRPs and less-polar ginsenosides contributed to the combined renoprotective effects against oxidative renal damage.

Growth and characterization of silicon-nitride films by plasma-enhanced chemical vapor deposition

Thin films of silicon nitride were deposited on Si wafers by plasma-enhanced chemical vapor deposition (PECVD). For deposition we designed and made hot wall capacitively coupled PECVD equipment which has a radial flow reactor. Using an RF generator of frequency 13.56 MHz and SiH4 (5% SiH4 in N2) + NH3 and N2 as reactive gases and the carrier gas, respectively, we systematically varied the substrate temperature (240–360°C), the partial pressure of reactive gases (0.35 <PSiH3PSiH4<1.32, 5% of S iH4 in N2) and the RF power (20–160 W), as deposition parameters. The characteristics of the films such as composition, deposition rate, refractive index and hydrogen content were investigated by AES, ellipsometry, FTIR spectrometry, nanospec and spectroscopic ellipsometry. As a result of these measurements, well-known characteristics were observed as a function of the substrate temperature and the partial pressure of the reactive gases. However, in our investigation of the RF power dependence of the refractive index of the film, we found that the refractive index increases and then decreases as we increase the RF power. To explain this, we considered the RF power-dependent heating effect in the glow discharge process and the amount of NH radicals which increases with the RF power.

Parametric study on optical properties of digital-alloy In(Ga1−zAlz)As/InP grown by molecular-beam epitaxy

Optical properties of digital-alloy InGaAlAs grown by molecular-beam epitaxy were parametrically investigated by 10-K-photoluminescence (PL) study on (In0.53Ga0.47As)n/(In0.52Al0.48As)n short-period superlattices (SPSs) in the range of n=1–5 monolayers. Two different peaks are resolved in PL spectra, and the higher energy peak (H) results from an excitonic transition while the lower energy peak (L) is related to a phonon-assisted transition. The H peak energies decrease monotonously as n increases, and it is in good agreement with the band-gap calculation with transfer matrix methods. It is found that two monolayer-period length (n=2) is the optimum one, where the PL intensity is largest and the ratio of L peak to H peak intensity is lowest. The various compositions (z) of digital-alloy In(Ga1−zAlz)As are prepared and their optical properties are investigated for z=0.2, 0.4, 0.6, and 0.8. The linewidths of 9-K-PL spectra for various z values are within the range of 10–15 meV, which are comparable to the b...

Enhanced disordering of GaAs/AIGaAs multiple quantum well by rapid thermal annealing using plasma enhanced chemical vapour deposited SiN capping layer grown at high RF power condition

The multiple quantum well (MQW) structure is a good candidate to monolithically integrate lasers and waveguides for realizing photonic integrated circuits. For such an application, selective disordering techniques are required, because these can make it possible to define a waveguide section laterally by inducing local changes in absorption and refractive indices of a MQW structure, which is needed to fabricate waveguide devices including laser. There are several techniques to selectively disorder the I I I -V MQW structures. These are impurity induced disordering [1,2], ion implantation disordering [3, 4], and impurity-free disordering [5-12] followed by thermal treatment. Impurity induced disordering and ion implantation disordering can disorder MQW structure perfectly, but these techniques introduce many defects and high doping concentrations which may deteriorate the performances of waveguide devices because of losses from scatterings by defects and free carrier absorption in the waveguide. Impurity-free techniques employ dielectrics, such as SiO2 or SiN, a capped annealing technique, which employs rapid thermal annealing (RTA) [5-10] or sealed ampoule annealing with an As overpressure [11, 12]. Impurity-free techniques do not disorder MQW structure completely, but disorder it enough to fabricate waveguides and lasers [9-12]. For impurity-free disordering, there are various film growth techniques, such as chemical vapour deposition (CVD) [11], e-beam evaporation [6, 8, 9], sputtering [5], and plasma enhanced chemical vapour deposition (PECVD) [7, 10, 12], which can deposit SiN and/or Si te films on a MQW structure. The behaviour of disordering is affected by the film quality used in the impurity-free disordering technique. Therefore, if an optimum film growth condition can be found by varying process conditions, then selective disordering can be achieved with the same material without surface degradation which should be accompanied in the disordering process without an encapsulation. The characteristics of film deposited by the PECVD technique can be varied by varying process conditions, such as substrate temperature, ratio of reactant gas, and RF power [13]. As a first step to finding a process-dependent impurity-free disordering, we varied the RF power. We carried out impurity-free disordering using a PECVD SiN cap layer with RTA. We found that this technique could disorder the GaAs/A1GaAs MQW structure and the disordering was enhanced by using SiN film grown at high RF power. We used a GaAs/A1GaAs MQW laser structure which was grown by a metal organic chemical vapour deposition (MOCVD) technique on Si-doped n ÷ GaAs substrate. The structure has the following layers from the top of the substrate: 0.5/xm of n (1018 cm -3) GaAs buffer, 1/zm of n (1017 cm -3) A10.4~Ga0.s3As, 0.1/xm of undoped A10.24Ga0.76As, four 7 nm undoped GaAs quantum wells with 10 nm A10.24Ga0.76As barrier, 0.1/xm of undoped A10.24Ga0.76As, 1/zm of p (1017 cm -3) A10.47Ga0.53As, and 0.2/xm of p+ (10 ~8 cm -3) GaAs. In the substrate design, separate confinement design is chosen to enhance the optical confinement in the quantum wells. SiN films were deposited by a PECVD technique for cap layers. We used dilute silane (5% Sill4 in N:) and high purity NH 3 (99.999%) and N2 (99.9999%) as a reactant gas. During plasma deposition, the ratio of partial pressure (PNH3/Psi~,) was kept at 1, total pressure was kept as 120 Pa by adding N2 gas and the substrate temperature was 300 °C. SiN films were grown with various RF powers (0 W, 60 W and 90 W). The thicknesses and refractive indices, for each RF power condition, were determined by an ellipsometer (Gaertner, Ll17) and a surface profiler (Tencor, alpha step 200). Disordering of MQW samples was accomplished by RTA at 850 °C for 35 s in Ar atmosphere with a heating rate of 65 °C/s. Disordering of MQW

Formation of self-assembled large droplet-epitaxial GaAs islands for the application to reduced reflection

The structural and reflectance properties of large gallium (Ga) droplets and GaAs islands grown by droplet epitaxy (DE) were presented. The reflectance results of self-assembled large GaAs islands by DE suggest the possibility of a novel method for antireflective coating. The diameter, height, density, and aspect ratio of large Ga droplets were investigated up to the scale of optical size. After GaAs island growth, the reflectance of s-polarization at 70° on in-situ measurement was reduced up to approximately 2%–20% in the wavelength range of 350–900 nm. For large GaAs islands, reduction of reflectance for s-, p-polarization at 20°–80° and reduction of reflectance at normal incidence was presented. This result shows that a layer of self-assembled large GaAs islands by DE can be a good candidate for an antireflector for high-quality optoelectronic devices.

THERMAL STABILITY OF SULFUR-TREATED INP INVESTIGATED BY PHOTOLUMINESCENCE

The effect of sulfur (S) treatments on InP is investigated by low‐temperature photoluminescence (PL) measurements. For both n‐ and p‐InP, the PL intensity is observed to increase about four times in magnitude if the scattering by the S overlayer is relatively small. Some PL bands are observed to disappear after S treatments and then reappear if the S‐treated surface is heat treated at 220 °C in a vacuum of 10−3 Torr. By observing their dependence on the excitation power density, the doping level of the samples, and measurement temperature, these PL bands are ascribed to the optical transitions via surface states. Our results thus indicate that the S‐treated InP surface may not be stable at a subsequent processing temperature of about 250 °C.