Ga clusters on a GaAs (0 0 1)(2 x 4)-As surface were successfully observed
by a system in which scanning tunneling microscopy (STM) and molecular beam
epitaxy (MBE) were equipped with not separated each chambers but one incor
porate unit. It is found that the boundary between crystalline adatoms and
magic clusters was found as a Ga trimer (N = 3) cluster. The dangling bonds
of a Ga dimer (N = 2) are completely empty, satisfying electron counting h
euristics, resulting no clear observation by the filled state imaging, and
showing the crystalline adatoms. However, the Ga trimer is clearly observed
. Moreover, the Ga trimer and heptamer (N = 7) have 9 and 21 electrons in m
olecular orbital states, which are closed to the magic numbers of spherical
jellium, 8 and 20 respectively. It seems that this excess one electron bec
omes the support between the cluster and the substrate. (C) 2000 Elsevier S
cience B.V. All rights reserved.