We study the adsorption of Al on the Si(100) surface by ab initio calc
ulations for a complete range of coverages from an isolated Al adatom
up to full surface coverage. Analysis of the energetics of adsorption
and the mobility of Al adatoms reveals that the long, one-dimensional
strings of Al dimers observed experimentally result from a sequence of
reaction steps which can be classified as a surface polymerization. T
he calculated local density of states (LDOS) of the adsorbed Al row is
compared with scanning tunneling microscopy images. The LDOS characte
ristic of the reaction sites which mediate the polymerization should b
e observable by scanning tunneling spectroscopy.