Hexagonal Growth Hillocks in GaN Epilayers

Abstract

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.

Influence of growth conditions, inversion domains, and atomic hydrogen on growth of (0001_) GaN by molecular beam epitaxy

“…6. If our assessment is correct, however, the Gaterminated surface can have a significantly higher growth rate than a N-terminated surface, in agreement with the speculations of Middleton et al 24 The growth rate differential is also consistent with recent results on MOCVD GaN reported by Liliental-Weber et al, 25 where ͑0001 គ͒-oriented IDs grow at a slower rate than the ͑0001͒-oriented matrix, leading to oriented ''pits'' that could be described as inverted hillocks. The IDBs were along the ͕101 គ0͖ planes, similar to IDs found in other films grown by MBE 7,15 and MOCVD.…”

Spectroscopic Imaging of InGaN Epilayers

“…Cathodoluminescence (CL) imaging has emerged as an important technique for acquiring very high-resolution images of nitride layers [5,6], with the capability to obtain a vast amount of information from thin semiconductor layers. Panchromatic images can 160 K. P. O'Donnell et al Fig.…”

Study of growth hillocks in GaN:Si films by electron beam induced current imaging

“…CL observations have revealed that the hillocks present specific recombination properties related to their structural features. Middleton et al 1 found enhanced yellow emission in the center of pyramidal hillocks and related it to defects associated with inversion domain boundaries. Herrera Zaldivar et al 2 reported a marked CL contrast at the center and at the border of round shaped hillocks that was explained by an inhomogeneous distribution of point defects and impurities.…”