Ba atoms adsorbed Si(001) surfaces show 2x3, c(2x6), 2x1 and 2x4 superstructures at submonolayer coverage, successively.@footnote 1,2@ These structures have been examined by LEED I-V curve analysis. The genetic search algorithm proposed by Doll and Van Hove@footnote 3@ has been operated to find a global optimum structure efficiently. After that the automated Tensor LEED analysis is carried out to refine the structure more precisely. Several structure models have been proposed by other researchers.@footnote 4@ In our previous LEED observation using a single domain Si(001)2x1 substrate, the direction of the two-fold periodicity of the 2x4 and 2x3 structures was determined to be parallel to the Si dimer.@footnote 1@ For the 2x3 structure, several models having Ba atoms of 1/6 monolayer(ML) and 1/3 ML, which is consistent with the UPS results showing the existence of dangling bond and dimer bond states, have been considered. The results of STM observations@footnote 5@ have been also considered. At the moment, though the R-factor value is not small enough, the model in which Ba atoms sit on the 3rd layer substrate Si atoms is most suitable. For the 2x1 and 2x4 structures, an amount of Ba atoms in the models are 1/2 and 1 ML, respectively. These results will be shown in the conference.
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@footnote 1@Y.Takeda et.al.,Surf.Sci.402-404(1998)692. @footnote 2@X.Hu et.al.,Surf.Sci.426(1999)69. @footnote 3@R.Doll and M.A.Van Hove, Surf.Sci.355(1996)L393. @footnote 4@W.C.Fan and A.Ignatiev, Surf.Sci.253(1991)297. @footnote 5@K.Ojima, submitted to Surf.Sci.