SHAPE TRANSITION OF GERMANIUM NANOCRYSTALS ON A SILICON(001) SURFACE FROM PYRAMIDS TO DOMES

Chemical vapor deposition of germanium onto the silicon (001) surface at atmospheric pressure and 600 degrees Celsius has previously been sh own to produce distinct families of smaller (up to 6 nanometers high) and larger (all approximately 15 nanometers high) nanocrystals. Under ultrahigh-vacuum conditions, physical vapor deposition at approximatel y the same substrate temperature and growth rate produced a similar bi modal size distribution. In situ scanning tunneling microscopy reveale d that the smaller square-based pyramids transform abruptly during gro wth to significantly larger multifaceted domes, and that few structure s with intermediate size and shape remain. Both nanocrystal shapes hav e size-dependent energy minima that result from the interplay between strain relaxation at the facets and stress concentration at the edges. A thermodynamic model similar to a phase transition accounts for this abrupt morphology change.