Jun-Long Li

EVOLUTION OF MOSAIC STRUCTURE IN SI0.7GE0.3 EPILAYERS GROWN ON SI(001) SUBSTRATES

In this article, we report a study of mosaic structures in partially relaxed Si0.7Ge0.3 epilayers grown on Si(001) substrates by x-ray double- and triple-axis diffractometry. The samples have different layer thicknesses and hence different degrees of strain relaxation. Our results show that, at early stages of strain relaxation, the films contain mosaic regions laterally separated by perfect regions. This is because the mosaic structure caused by a misfit dislocation is effectively localized in a lateral range of the layer thickness. Therefore, far from the dislocations, the film is virtually a perfect crystal. With the increase in the degree of strain relaxation, and consequently in the dislocation density, the mosaic regions of the layer expand while the perfect regions shrink and finally vanish completely. Moreover, our results indicate that the conventional method of estimating dislocation density from the x-ray rocking curve width fails in our case.

Relaxed Ge0.9Si0.1 alloy layers with low threading dislocation densities grown on low-temperature Si buffers

Relaxed GexSi1−x epilayers with high Ge fractions but low threading dislocation densities have been successfully grown on Si (001) substrate by employing a stepped-up strategy and a set of low-temperature GeySi1−y buffers. We show that even if the Ge fraction rises up to 90%, the threading dislocation density can be kept lower than 5×106 cm−2 in the top layers, while the total thickness of the structure is no more than 1.7 μm.

Two-step method to grow InAs epilayer on GaAs substrate using a new prelayer

The strain relaxed InAs grown on GaAs(0 0 1) under In-rich condition is investigated by X-ray double-crystal diffraction. It is found that the strain is fully relaxed within 10 nm thick InAs layer grown under this condition, which is much faster than the growth of a similar layer of InAs under As-condition. A new growth method to obtain a high-quality InAs layer is suggested

Strain relaxation of GeSi alloy with low dislocation density grown on low-temperature Si buffers

We have developed a low-temperature (LT) growth technique. Even with Ge fraction x upto 90%, the total thickness of fully relaxed GexSi1-x buffers can he reduced to 1.7 mu m with dislocation density lower than 5 x 10(6) cm(-2). The surface roughness is no more than 6 nm. The strain relaxation is quite inhomogeneous From the beginning. Stacking faults generate and form the mismatch dislocations in the interface of GeSi/LT-Si

Interfacial structure of molecular beam epitaxial grown cubic-GaN films on GaAs(001) probed by x-ray gazing-angle specular reflection

We report on a study of interfacial structure of GaN films grown on GaAs(001) substrates by plasma-assisted molecular beam epitaxy using x-ray grazing-angle specular reflection. We show that interfacial layers with electron densities differing from those of GaN and GaAs were formed upon deposition of GaN. It is also found that the interfacial structure of our systems depends strongly on the course of the initial layer deposition. The phase purity of the GaN films was examined by x-ray reciprocal space mapping. A simple kinetic growth model suggested by our results has been presented.

Thermal stability of cubic GaN film grown by molecular-beam epitaxy on GaAs(001)

The thermal stability of cubic-phase GaN (c-GaN) film grown by molecular-beam epitaxy was investigated by Raman scattering spectroscopy and X-ray scattering. The results of Raman scattering shows that, after annealing at 1000degreesC, the intensity of transverse (TO) and longitudinal (LO) optical peaks from cubic phase obviously decreases while the intensity of TOb peak from the boundary effect slightly decreases, but the transformation of the hexagonal phase (alpha-GaN) can not be detected due to a little of alpha-GaN inclusion. X-ray reflectivity measurements indicate that there is a high-electron-density layer between the substrate and the GaN film, and it becomes uniform and much thinner after high-temperature annealing, counting for the Raman results of the intensity change of the TOb peak. The results of high-angle X-ray diffraction and X-ray reciprocal space mapping revealed that the relative content of alpha-GaN obviously increases after annealing at 1000degreesC, and (10 (1) over bar1) is the most stable diffraction lattice of the alpha-GaN hexagonal phase

A practical green chemistry approach to synthesize fused bicyclic 4H-pyranes via an amine catalysed 1,4-addition and cyclization cascade

A newly developed synthetic approach to densely functionalized 4H-pyrane derivatives via an amine catalysed cascade reaction is presented. This protocol is relatively environmentally benign because it proceeds smoothly in water or ethanol at ambient temperature with low catalyst loading; more importantly, the products are easily purified, and some show promising antibacterial activity.

Direct Sulfide-Catalyzed Enantioselective Cyclopropanations of Electron-Deficient Dienes and Bromides

A catalytic highly regioselective, diastereoselective, and enantioselective cyclopropanation of electron-deficient dienes and bromides via direct sulfide organocatalysis is reported. A variety of vinylcyclopropanes featuring a quaternary chiral center were synthesized in up to 99% yield and up to 98:2 enantiomeric ratio (er). These products could be facilely transformed to various interesting molecules with great structural diversity.

Facile synthesis of novel spiroheterocycles via diastereoselective aziridination of cyclic enones

A collection of novel spiroheterocycles with strained aziridine moieties have been facilely synthesized by using easily accessible starting materials under mild reaction conditions. This procedure is robust, scalable and highly diastereoselective, and also features broad substrate scope. In addition, some of the products show promising in vitro biological activity against a spectrum of pathogens, which might be considered as a clue for the discovery of new antimicrobial agents.

Protective Effect of 2,4',5'-Trihydroxyl-5,2'-dibromo diphenylmethanone, a New Halophenol, against Hydrogen Peroxide-Induced EA.hy926 Cells Injury.

Vascular endothelial cells produce reactive oxygen species (ROS) during the process of energy metabolism in aerobic respiration. A growing body of evidence indicates that excessive ROS is implicated in the pathogenesis of cardiovascular diseases including atherosclerosis. The newly synthesized halophenol, 2,4′,5′-trihydroxyl-5,2′-dibromo diphenylmethanone (TDD), exhibits antioxidative and cytoprotective activities in vitro. In this study, the protective effect of TDD against hydrogen peroxide (H2O2)-induced oxidative injury of EA.hy926 cells was investigated. Cell viability was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-dephenyltetrazolium bromide (MTT) assay, while the effect of TDD on the transcription profile of EA.hy926 cells subjected to H2O2-induced oxidative injury was evaluated by microarray analysis. Several signaling pathways, including apoptosis, were significantly associated with TDD. Flow cytometric analysis was used to evaluate anti-apoptotic effect of TDD. Subsequently, RT-PCR and Western blot were used to detect the expressions of the apoptosis-associated protein, Bcl-2 and Bax. Meanwhile the expression of cleaved caspase-3, an executioner of apoptosis, was also detected by Western blot. The results showed that pretreatment of EA.hy926 cells with TDD prevented the decrease of cell viability induced by H2O2, and attenuated H2O2-induced elevation of Bax and cleaved caspase-3 while increased Bcl-2 expressions. In summary, TDD inhibited H2O2-induced oxidative injury of EA.hy926 cells through negative regulation of apoptosis. These findings suggest that TDD is a potential candidate for therapeutic intervention in oxidative stress-associated cardiovascular diseases.