  We have studied initial growth of Sn atoms on Ge(001) surfaces at room
temperature and 80 K by scanning tunneling microscopy. For Sn deposition onto
the Ge(001) substrate at room temperature, the Sn atoms form two kinds of
one-dimensional structures composed of ad-dimers with different alignment, in
the <310> and the <110> directions, and epitaxial structures. For Sn deposition
onto the substrate at 80 K, the population of the dimer chains aligning in the
<310> direction increases. The diffusion barrier of the Sn adatom on the
substrate kinetically determines the population of the dimer chain. We propose
that the diffusion barrier height depends on surface strain induced by the
adatom. The two kinds of dimer chains appearing on the Ge(001) and Si(001)
surfaces with adatoms of the group-IV elements are systematically interpreted
in terms of the surface stain.
