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The effects of indium segregation and hydrogen on the optical and structural properties of InGaN/GaN multiple quantum wells, grown by metalorganic chemical vapor deposition were investigated. Photoluminescence and high-resolution transmission electron microscopy analysis showed that two types of indium-rich regions can be formed in the InGaN well layers. Self-assembled quantum dot-like indium-rich regions were found in the well layer grown at a normal growth temperature. These regions behaved as luminescent centers, showing a maximum indium content at the center of indium-rich region. However, randomly-distributed indium-segregated regions, which formed near the upper interface of the InGaN well layers during the subsequent high-temperature annealing process led to the degradation of the optical properties by generating defects such as misfit dislocations. The use of hydrogen during the growth interruption was found to be very effective in suppressing the formation of indium-segregated regions in the InGaN well layers.

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We report on the reduction of threading dislocations in GaN overlayers grown by organometallic vapor phase epitaxy on micro-porous TiN networks. These networks were obtained by in situ annealing of thin Ti layers deposited in a metalization chamber, on the ͑0001͒ face of GaN templates. Observations by transmission electron microscopy indicate dislocation reduction by factors of up to 10 in GaN layers grown on TiN networks compared with the control GaN. X-ray diffraction shows that GaN grown on the TiN network has a smaller ͑102͒ plane peak width ͑4.6 arcmin͒ than the control GaN ͑7.8 arcmin͒. In low temperature photoluminescence spectra, a narrow excitonic full-width-at-half-maximum of 2.4 meV was obtained, as compared to 3.0 meV for the control GaN, confirming the improved crystalline quality of the overgrown GaN layers.