Observation of 'ghost' islands and surfactant effect of surface gallium atoms during GaN growth by molecular beam epitaxy

GaN (0001) films grown by molecular beam epitaxy (MBE) were studied using scanning tunneling microscopy (STM). 'Ghost' islands were observed on surfaces grown under excess Ga conditions. These ghost islands were associated to a metastable, intermediate nucleation state of the surface.published_or_final_versio

Initial stage of GaN growth and its implication to defect formation in films

In situ scanning tunneling microscopy (STM) observations of initial growth processes of GaN by molecularbeam epitaxy reveal important differences between growth on vicinal versus flat SiC(0001) substrates. Based on stop-growth STM studies, we explain why there are orders of magnitude reductions in the density of threading screw dislocations in the vicinal films. It is shown that on vicinal surfaces, three-dimensional (3D) islands develop into a characteristic shape. The islands coalesce much sooner than on flat surfaces. Consequently, fewer defects are created at their boundaries.published_or_final_versio

Comparative study on the broadening of exciton luminescence linewidth due to phonon in zinc-blende and wurtzite GaN epilayers

The broadening of exciton luminescence linewidth due to phonon in zinc-blende and wurtzite GaN epilayers was discussed. The coupling parameters between exciton and acoustic and longitudinal optical phonons were obtained for both structures. The analysis showed that the coupling constants of both exciton-acoustic optial phonon coupling and exciton-longitudinal optical phonon coupling for zinc-blende GaN were almost twice as much as the corresponding values of wurtzite GaN.published_or_final_versio

Effect of starvation on growth, histology and ultrastructure of digestive system of juvenile red swamp crayfish (Procambarus clarkii Girard)

The present study was conducted to evaluate the effect of 20 d starvation on growth, survival, histomorphology and ultrastructure changes in the digestive system of juvenile red swamp crayfish (Procambarus clarkii). Juveniles were divided into two groups: a food-deprived group and a control group at 9 day after hatch (DAH). Individuals were sampled at 14, 20, 29 DAH. During the 20 d fasting period, the mean body weight and total body length of crayfish fluctuated around 10.17 mg and 8.12 mm respectively, and the mortality was zero. Histomorphological changes of digestive system were observed in the food-deprived group after 20 days of starvation: the esophagus and stomach walls were thinning, the epithelium atrophied to cuboidal, nuclei were darker and smaller, and nucleolus was difficult to observe; the midgut and hindgut showed wider volume, thinning wall, atrophied epithelial and muscularis and shorter ridges; and hepatopancreas tubule lumens were wider, the lipid droplets in R-cells were smaller and less, and the quantity of typical B-cells decreased. Changes in the ultrastructure of starved crayfish were also observed: the mitochondria of midgut epithelium and R-cells were swollen and vacuolated, and the ridges of which were fractured and reduced. In addition, the electron density of cytoplasmic matrix of R-cells decreased, and the quantity of glycogen granules and lipids also decreased. Changes in the ultrastructure of B-cells were similar to those of R-cells. Though degeneration in histological structure and function of digestive organs were obvious during starvation, juvenile P. clarkii was able to endure a relative long-term starvation

Design and Testing of Cesium Atomic Concentration Detection System Based on TDLAS

In order to better build the Neutral Beam Injector with Negative Ion Source (NNBI), the pre-research on key technologies has been carried out for the Comprehensive Research Facility for Fusion Technology (CRAFT). Cesium seeding into negative-ion sources is a prerequisite to obtain the required negative hydrogen ion. The performance of ion source largely depends on the cesium conditions in the source. It is very necessary to quantitatively measure the amount of cesium in the source during the plasma on and off periods (vacuum stage). This article uses the absorption peak of cesium atoms near 852.1nm to build a cesium atom concentration detection system based on Tunable Diode Laser Absorption Spectroscopy (TDLAS) technology. The test experiment based on the cesium cell is carried out, obtained the variation curve of cesium concentration at different temperatures. The experimental results indicate that: the system detection range is within 5*10E6-2.5*10E7 pieces/cm3 and the system resolution better than 1*10E6 pieces/cm3.Comment: 8 pages,7 figures, the 20th International Symposium on Laser-Aided Plasma Diagnostic

Shallow optically active structural defect in wurtzite GaN epilayers grown on stepped 4H-SiC substrates

Shallow optically active structural defect in wurtzite GaN epilayers grown on stepped 4H-SiC substrates was investigated. The GaN epilayers grown with plasma-assisted molecular-beam epitaxy were optically characterized by photoluminescence and excitation spectra. Results showed that the localized states which were induced by the structural defect located about 100 meV above the maximum valence band of GaN.published_or_final_versio

The Quantitative Significance of the Transsulfuration Enzymes for H2S Production in Murine Tissues

The enzymes of the transsulfuration pathway, cystathionine --synthase (CBS) and cystathionine --lyase (CSE), are important for the endogenous production of hydrogen sulfide (H2S), a gaseous signaling molecule. The relative contributions of CBS and CSE to H2S generation in different tissues are not known. In this study, we report quantification of CBS and CSE in murine liver and kidney and their contribution to H2S generation in these tissues and in brain at saturating substrate concentrations. We show that CBS protein levels are significantly lower than those of CSE; 60-fold and 20-fold in liver and kidney, respectively. Each enzyme is more abundant in liver compared with kidney, twofold and sixfold for CBS and CSE, respectively. At high substrate concentrations (20-mM each cysteine and homocysteine), the capacity for liver H2S production is approximately equal for CBS and CSE, whereas in kidney and brain, CBS constitutes the major source of H2S, accounting for -80% and -95%, respectively, of the total output. At physiologically relevant concentrations of substrate, and adjusting for the differences in CBS versus CSE levels, we estimate that CBS accounts for only 3% of H2S production by the transsulfuration pathway enzymes in liver. Antioxid. Redox Signal. 15, 363-372.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/90506/1/ars-2E2010-2E3781.pd

Direct observation of a Ga adlayer on a GaN(0001) surface by LEED Patterson inversion

A low-energy electron diffraction (LEED) Patterson function (PF) with multiple incident angles is used to obtain three-dimensional interatomic information of hexagonal GaN(0001) grown on a 6H-SiC(0001)-√3 x √3 surface. A Ga-Ga atomic pair between the Ga adlayer and the terminating Ga layer is observed in the LEED PF. This provides direct experimental evidence to support the structural model proposed by first-principles calculations. The LEED PF also shows that the GaN film has a hexagonal structure and the surface has single-bilayer steps.published_or_final_versio

Step bunching of vicinal GaN(0001) surfaces during molecular beam epitaxy

Step bunching of vicinal GaN(0001) surface during epitaxial growth is observed by scanning tunneling microscopy. Large step stiffness and repulsive step-step interaction are suggested based on step morphology observations. The size of the bunch changes with time, depending on the direction in which the substrate is heated by a direct current. This observation provides evidence for the electromigration effect causing the step bunching, and from the field dependence we infer that adatoms, which are likely N, have effective positive charges. ©2000 The American Physical Society.published_or_final_versio

Scaling of three-dimensional InN islands grown on GaN(0001) by molecular-beam epitaxy

The scaling property of three-dimensional InN islands nucleated on GaN(0001) surface during molecular-beam epitaxy (MBE) is investigated. Due to the large lattice mismatch between InN and GaN (∼10%), the islands formed from the Stranski-Krastanow growth mode are dislocated. Despite the variations in (residual) strain and the shape, both the island size and pair separation distributions show the scaling behavior. Further, the size distribution resembles that for submonolayer homoepitaxy with the critical island size i = 1, suggesting that detachment of atoms is not significant. The above results also indicate strain is insignificant in determining the nucleation and growth of dislocated islands during heteroepitaxy by MBE.published_or_final_versio