Shape Transition of Germanium Nanocrystals on a Silicon (001) Surface from Pyramids to Domes
Gilberto Medeiros-Ribeiro,
Alexander M. Bratkovski,
Theodore I. Kamins,
Douglas
A. A. Ohlberg,
R. Stanley Williams
*
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.
Hewlett-Packard Laboratories, 3500 Deer Creek Road, Mail Stop 26U,
Palo Alto, CA 94304-1392, USA.
*
To whom correspondence should be addressed. E-mail:
stan-williams{at}hpl.hp.com
