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The annealing of an AlN buffer layer in a carbon-saturated N 2 -CO gas on a sapphire substrate was investigated. The crystal quality of the buffer layer was significantly improved by annealing at 1650-1700°C. An AlN buffer layer with a thickness of 300 nm was grown by metalorganic vapor phase epitaxy (MOVPE), and was annealed at 1700°C for 1 h. We fabricated a 2-µm-thick AlN layer on the annealed AlN buffer layer by MOVPE. The full widths at half maximum of the (0002)-and (10 12)-plane X-ray rocking curves were 16 and 154 arcsec, respectively, and the threading dislocation density was 4.7 ' 10 8 cm %2 .

InGaN films have been grown on GaN and AlGaN epitaxial layers by metalorganic vapor phase epitaxy. The "composition pulling effect" during the initial InGaN growth stages has been studied as a function of the lattice mismatch between the InGaN and the underlying epitaxial layer. The crystalline quality of the InGaN is good near the InGaN/GaN interface and the composition is close to that of GaN. However, with increasing InGaN film thickness, the crystal quality deteriorates and the indium mole fraction increases. The composition pulling effect becomes stronger with increasing lattice mismatch. It is suggested that indium atoms are excluded from the InGaN lattice during the early growth stages to reduce the deformation energy from the lattice mismatch. TEM observations of the InGaN/GaN structure reveal that the degradation of the crystalline quality of InGaN films grown on GaN is caused by pit formation which arises from edge dislocations propagating through the InGaN film from the underlying GaN.

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The spectral absorption properties of ultrananocrystalline diamond (UNCD)/amorphous carbon composite films grown by pulsed laser deposition with graphite were experimentally investigated. The indirect optical band gaps estimated from the absorption spectrum were 1.0 eV and approximately 5.65 eV, which correspond to those for the amorphous carbon surrounding the UNCDs and for UNCDs themselves, respectively. In addition, the spectrum revealed a direct gap having a value of approximately 2.2 eV. One probable origin can be the presence of grain boundaries between UNCDs and amorphous carbon since they are specific to UNCD/amorphous carbon; further, its appearance is consistent with the theoretical predictions.