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This work investigates dislocation etch pits in epitaxial lateral overgrowth (ELO) GaN by wet chemical etching. A mixture of H2SO4 and H3PO4 was used as a dislocation etchant, and SEM and AFM were employed to observe the surface topography. For the as-grown sample, SEM images present the flat, smooth surface without any pits or hillocks. After the chemical etching, hexagonal shaped etch pits were observed at the edge of ELO GaN. AFM observation of etched ELO GaN displayed high densities of etch pits clustered in the “window” region and the coalescent line of two growing fronts. In contrast, the overgrowth region was nearly free of etch pits. Moreover, we observed that different sizes of etch pits dominated in “window” region and coalescent region. This implied different types dislocations dominated in these regions.
The metal-ceramic interface in an XDTM Al/TiCp metal matrix composite has been characterized in as-extruded, recrystallized, and high temperature heat-treated conditions. In both the as-extruded and recrystallized composite, the interface is atomically abrupt. Localized orientation relationships exist between Al and Tic that lead to some degree of coherency at the interface. Recrystallization produces semicoherent interfaces by formation of subgrains in the Al adjacent to the Tie particles. Few interfaces show cracking, even after extensive deformation. Lack of cracking together with the direct contact down to atomic level, observed between the two phases are evidence for excellent bonding between the carbide particles and the aluminum matrix. Heat treating samples at 913 k for 24 hours produces reaction products like Al3Ti and Al4C3. These reactions are explained on the basis of thermodynamic data.
The morphology and crystal structure of diamond crystallites grown by radio frequency (RF) and microwave (MW) plasma enhanced chemical vapor deposition (PECVD) under comparable conditions are examined by electron microscopy and found to be a strong function of the plasma. A novel TEM sample preparation method is developed to examine the internal structure of diamond as well as interfacial structure between diamond and substrate by direct deposition of diamond crystallites on prethinned metal grids. The diamond crystallites grown by RF PECVD are highly defective and composed of nanocrystalline grains of random orientation while those grown by MW PECVD are wellfaceted and process near perfect long range order in crystal structure.
Al-Ti multilayered films were deposited by magnetron sputtering of Al and Ti targets on to Si (100) or NaCl substrates. The bi-layer thickness was 16 nm with Ti constituting 12% of the total thickness. The films were subsequently annealed in vacuum at 400°C for periods between 2 and 24 h. In the course of the annealing, interdiffusion and chemical reaction between Al and Ti layers led to the precipitation of Al3Ti particles. Plan view and cross-section TEM examination of as-deposited and annealed films were performed to study the microstructural evolution, and to estimate the Al grain and AI3Ti particle size distributions. Cross-section TEM and X-ray diffraction showed a well-defined layered structure in the as-deposited films. The microstructure was found to be metastable in the first 6 h of annealing, with Al-Ti multilayers being gradually replaced by an AI-AI3Ti composite structure. The Al3Ti particles were uniformly distributed throughout the film. X-ray and electron diffraction analyses showed that Al3Ti possessed the ordered DO22 structure. The hardness of the Al-Ti films in as-deposited and annealed conditions was determined using a nanoindenter and the data have been correlated with the microstructural changes with annealing.