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We combine two scanning electron microscopy techniques to investigate the influence of dislocations on the light emission from nitride semiconductors. Combining electron channeling contrast imaging and cathodoluminescence imaging enables both the structural and luminescence properties of a sample to be investigated without structural damage to the sample. The electron channeling contrast image is very sensitive to distortions of the crystal lattice, resulting in individual threading dislocations appearing as spots with black–white contrast. Dislocations giving rise to nonradiative recombination are observed as black spots in the cathodoluminescence image. Comparison of the images from exactly the same micron-scale region of a sample demonstrates a one-to-one correlation between the presence of single threading dislocations and resolved dark spots in the cathodoluminescence image. In addition, we have also obtained an atomic force microscopy image from the same region of the sample, which confirms that both pure edge dislocations and those with a screw component (i.e., screw and mixed dislocations) act as nonradiative recombination centers for the Si-doped c-plane GaN thin film investigated.
It has been argued that the initial rarity of an animal species may be a good indicator of subsequent vulnerability. The usefulness of this argument in the conservation of endangered species has been investigated by the authors, who have compared the apparent vulnerability of certain rare animals with their actual status. The two approaches agreed substantially, but some striking differences occurred. Some rare species seem more prone to extinction than is officially recognized, and their status should be reviewed. Other species are not particularly rare, but are threatened for other biological and economic reasons. Knowledge of rarity is a good starting point, but this sould be followed by a detailed examination of other relevant factors to discern genuine risk.