Observation of Dark Spots and Dark Lines of GaN Microcrystals Grown by Nitridation of Gallium Sulfide

High-spatial-resolution (50 nm) cathodoluminescence images in the ultra-violet region (365 nm) and in the yellow region (570 nm) were taken at room temperature for GaN bulk crystals under a scanning electron microscope equipped with a monochromator and a real-time RGB-separation detector. The crystals were grown by nitridation of Ga2S3 and had a c-facet and side facets with Miller indices of {110x} or {112y}. Dark spots were observed only on the-side facets while dark lines only on a c-facet. Dark lines stretch in the direction of < 1100 >. or < 11200 > and end as a dark spot on side facets. Dark spot density was 10(9) cm(-2). The nonuniformity of yellow luminescence and its irrelevance to the existence of dark lines suggest an extrinsic defect for the origin of yellow luminescence, such as carbon impurity.