Yellow luminescence from precipitates in GaN epilayers

The room-temperature photoluminescence spectra of GaN epilayers with different surface morphologies grown by metalorganic vapor phase epitaxy exhibited a near-band-gap emission peak and a broad emission band in the yellow region. The intensity distributions of the yellow luminescence observed by cathodoluminescence (CL) were characterized by a number of dark and bright tortuous regions in sample A with a specular surface and by many bright hexagrams in sample B with a hexagonal hillock surface. Instead of the CL bright hexagrams, a number of CL bright line segments parallel to <112_0> directions were visible in sample B after the epilayer surface hillocks were polished. GaN epilayer images by high-resolution transmission electron microscopy exhibited a number of circular or hexagonal defects in sample A and many longer defects arranged in <112_0> directions in sample B. The compositions of the defects in these images were analyzed by energy-dispersive X-ray spectrometry and found to be composed of carbon, oxygen, and gallium elements. The results suggest that the C- and O-related defects are responsible for the yellow luminescence from the precipitates in imperfect coalescence.