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 shown 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 approximately the same substrate temperature and growth rate produced a similar bimodal size distribution. In situ scanning tunneling microscopy revealed that the smaller square-based pyramids transform abruptly during growth to significantly larger multifaceted domes, and that few structures with intermediate size and shape remain. Both nanocrystal shapes have 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.
Medeiros-Ribeiro
G
Hewlett-Packard Laboratories, 3500 Deer Creek Road, Mail Stop 26U, Palo Alto, CA 94304-1392, USA.
Bratkovski
AM
Kamins
TI
Ohlberg
DAA
Williams
RS