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The polarity of the lattice of bulk single GaN crystals and the polarity of homoepitaxial and heteroepitaxial-on-sapphire GaN thin films has been studied using convergent beam electron diffraction. Diffraction patterns obtained at 200 kV for the 〈1–100〉 projection of GaN were matched with calculated patterns. The lattice orientations of two commonly observed bulk single-crystal facets were identified. It is shown that the smooth facets in single crystals correspond to the (0001), Ga-terminated, lattice planes, whereas the rough facets correspond to the (0001̄), N-terminated, planes. It is also shown that metalorganic chemical vapor deposition epitaxy retains the polarity of the substrate, i.e., no inversion boundaries were observed. Heteroepitaxy on sapphire is shown to grow in the (0001), Ga-terminated orientation.

The convergent beam and bend extinction contour techniques of electron microscopy are capable of providing much more information than can be obtained from conventional diffraction patterns and it is the objective of this work to examine the symmetry properties of each of these patterns. The diffraction of fast electrons by a thin parallelsided slab has been studied by group theory and by a graphical construction. We find that the pattern symmetries may be described by thirty-one diffraction groups and that each of these diffraction groups is isomorphic to one of the point groups of diperiodic plane figures and to one of the thirty-one Shubnikov groups of coloured plane figures. A graphical representation of each diffraction group is given, together with tables showing how the diffraction groups are related to the specimen point groups and under certain assumptions to the crystal point groups. These tables assume the symmetric Laue condition and ignore the presence of irreducible lattice translations normal to the slab. By using the tables, crystal point groups can be obtained from convergent beam or bend contour patterns. The method is demonstrated by experiments on several materials, but particularly on germanium and gallium-arsenide specimens since the similarity of these materials exemplifies the sensitivity of the technique.

A combination of transmission electron microscopy imaging and diffraction techniques is used to characterize crystal defects in homoepitaxial GaN thin films. The Burgers vectors of dislocations is established by combining large-angle convergent beam electron diffraction and conventional diffraction contrast techniques. It is shown that dislocations with Burgers vectors c, a, and c+a are present. Evidence is presented that dislocation segments lying in the interfacial plane are dissociated on a fine scale. The significance of the observations for understanding homoepitaxial growth of GaN is discussed.

A 820 micron thick single crystal diamond layer epitaxially grown on a single crystal diamond seed (high pressure, high temperature grown synthetic) by microwave plasma chemical vapor deposition with added nitrogen is characterized by an array of analytical techniques before and after annealing the material at high pressures and temperatures. The most striking result is the conversion of the initially dark colored, highly absorbing CVD layer to clear, transparent material after a 1 hour anneal at 7 GPa and 2200 °C. IR absorption in the region of the CH stretching modes, 2800 to 3107 cm–1 shows a remarkable sharpening and persistence of the observed modes. IR absorption in the one‐phonon region also indicates the presence of significant concentrations of ionized single substitutional nitrogen in the as grown material. EPR indicates a concentration of neutral single substitutional nitrogen at lattice sites of ca. 1 ppm, and this changes by less then 30% when annealed at temperatures up to 2200 °C. EPR also detects 0.1 ppm of the negatively charged nitrogen‐vacancy‐hydrogen complex in the as grown diamond, but this anneals out by 1900 °C, the negatively charged nitrogen‐vacancy complex is below the EPR detection limit in these samples of about 0.1 ppm. Photoluminescence detects the presence of neutral and negatively charged nitrogen‐vacancy complexes in the as grown material, and the formation of new, unassigned bands principally in the 800 to 900 nm region. The total detected nitrogen concentration in the sample is ca. 1.5 ppm. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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