  Growth and strain behavior of thin Ag films on Si substrate have been
investigated by scanning tunneling microscopy, cross-sectional transmission
electron microscopy and high resolution x-ray diffraction studies. Ag islands
formed on Si at room temperature growth show strongly preferred heights and
flat top. At low coverage, islands containing two atomic layers of Ag are
overwhelmingly formed. At higher coverages island height distribution shows
strong peaks at relative heights corresponding to an even number of Ag atomic
layers. This appears to be a quantum size effect. Hexagonal disc-like islands
with flat top are formed upon annealing. The annealed film shows two
closely-spaced Ag(111) diffraction peaks - one weak and broad and the other
narrow and more intense. The intense peak corresponds to a shorter Ag(111)
planar spacing compared to the bulk value. This can be explained in terms of
changes in the Ag lattice during the heating-cooling cycle due to thermal
expansion coefficient mismatch between Ag and Si.
