Barium induced 2 x 3 and c(2 x 6) structures on Si(l 0 0), which appear at
the Ba coverage below 1/3 ML, have been studied by scanning tunneling micro
scopy (STM). It is revealed that the 2 x 3 structure has only one adsorptio
n phase, being independent of Ba coverage. Based on high resolution filled-
state STM images where four protrusions are resolved in a unit cell, a mode
l of 2 x 3 structure is proposed where the building unit consists of a Ba d
imer and two Si dimers. It is also found that the c(2 x 6) structure always
coexists with the 2 x 3 structure and has the same building block. The c(2
x 6) is realized when adjacent rows of building blocks are out of phase. (
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