The surface morphologies of nanopipes were imaged in undoped epilayers, which had been grown onto GaN-based layers by means of metal organic vapour phase epitaxy, using scanning electron microscopy and atomic force microscopy. The nanopipes usually appeared as dodecagonal and hexagonal pyramidal indentations, for larger and smaller sizes, respectively. There were pinholes at their centres. The results indicated that {11•1} facets, as well as {10•1} facets, could play an important role during nanopipe formation in undoped AlGaN epilayers. Energy dispersive X-ray spectroscopy showed that Al atoms precipitated more clearly on the facets of the nanopipes. Yellow cathodoluminescence images were characterized by a bright ring around the outer regions of pyramidal indentations. This was suggested to be a result of competition between the higher concentration of defects, which were responsible for the yellow luminescence, and the thinner epilayer at the facet site.

Nanopipes in Undoped AlGaN Epilayers. J.Kang, S.Tsunekawa, B.Shen, Z.Mai, C.Wang, T.Tsuru, A.Kasuya: Journal of Crystal Growth, 2001, 229[1], 58-62