At the surface, the three dimensional symmetry of a solid is broken. Electr
ons and atoms near the surface may rearrange to lower the free energy of th
e system. Scattering by defects and confinement by boundaries of electrons
may produce long-range charge density oscillations. Adatoms interact with e
ach other via mutual perturbation of the surface, known as indirect electro
nic and elastic interactions. These interactions are very weak and are also
oscillatory. For some systems, formation of adsorption layer superstructur
es can be directly correlated to adatom-adatom interactions. When the tempe
rature is raised, adatoms and admolecules can start to diffuse, interact, o
r react. They may aggregate into clusters and islands, and grown into a thi
n film. The stability of clusters may exhibit magic numbers in size and thi
ckness. When the temperature is changed, island shape transitions may occur
. The growth of islands and ultra-thin films can also be influenced by elec
tronic effects as well as by the addition of a surfactant layer. All these
growth behaviors in epitaxy can be understood from the mechanisms and energ
etics of elementary surface atomic processes, and atom and electron dynamic
s. They, in turn, can be studied in details using atomic resolution microsc
opy. (C) 2001 Elsevier Science Ltd. All rights reserved.