Height preference and strain in Ag islands on Si(111)-(7x7)

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.