Single crystal GaN films with a wurtzite structure were grown on the basal plane of sapphire. A high density of threading dislocations parallel to the c-axis crossed the film from the interface to the film surface. They were found to have a predominantly edge character with a Burgers vector. In addition, dislocation hal-loops, elongated along the c-axis of GaN, were also found on the prism planes. These dislocations had a mostly screw character with a [0001] Burgers vector. Substrate surface steps with a height of were found to be accommodated by localized elastic bending of GaN (0001)GaN planes in the vicinity of the film/substrate interface. Observations show that the region of the film, with a thickness of ∼100 nm, adjacent to the interface is highly defective. This region is thought to correspond to the low-temperature GaN “buffer” layer which is initially grown on the sapphire substrate. Based on the experimental observations, a model for the formation of the majority threading dislocations in the film is proposed. The analysis of the results leads us to conclude that the film is under residual biaxial compression.

A thorough investigation of the microstructure of single SCS-6 SiC fibers widely used as reinforcements in metal-matrix and ceramic-matrix composites has been made. Various techniques of electron microscopy (EM) including scanning (SEM), conventional transmission (TEM), high resolution (HREM), parallel electron energy loss spectroscopy (PEELS), and scanning Auger microscopy (SAM) have been used to analyze and characterize the microstructure. The fiber is a complicated composite consisting of many different layers of SiC deposited on a carbon core and different carbonaceous coatings covering the SiC layers. The structural and chemical aspects of each layer are characterized and discussed.

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Twodimensional electron gas properties of AlGaN/GaN heterostructures grown on 6H-SiC and sapphire substrates Appl.

Continuous, ultrathin silicon carbide (Sic) films of less than 10 nm have been grown on Si by rapid thermal chemical vapor deposition carbonization with high propane flow rates at IlOO-1300 "C. X-ray and electron diffraction techniques indicated a monocrystalline structure for these nanometer-scale films. High-resolution transmission electron microscopy reveals that five Sic planes are aligned with four Si planes at the SiC/Si interface. The Fourier transform infrared spectrum of the Sic films exhibits the characteristic SGC absorption peak at around 800 cm-', with a FWHM of 45 cm-'.