Hexagonal growth hillocks in GaN epilayers

We describe a study of the hexagonal growth hillocks commonly present in gallium nitride films. The MOVPE-grown epilayers of the present work exhibit a predominantly smooth morphology but small groups of hexagonal hillocks were found to populate the surface, particularly at the sample edges. Scanning electron (SE) micrographs were taken of several groups of hillocks. At the maximum beam energy of 25 keV, two types of hexagonal hillock are visible. Hillocks in the first group are terminated by an apex (ie. they are pyramidal in form), while the other, flat-topped, hillocks terminate on (0001)-facets. As one lowers the electron beam energy, thereby reducing beam penetration, some of the flat-topped hillocks disappear from the image. From this we tentatively deduce that these hillocks are buried. The result of further investigations, using an atomic force microscope, are consistent with the presence of sub-surface features. The relationship between the luminescence and morphological properties of a pyramidal hillock is studied via cathodoluminescence imaging. The band-edge emission originates from the full hexagonal structure, except for the central region, where only the defect-related yellow luminescence is apparent. We suggest this might be explained by defects associated with inversion domain boundaries at the hillock centre.