Molecular modeling of covalently attached alkyl monolayers an the hydrogen-terminated Si(111) surface

The structure of octadecyl monolayers on the H-terminated Si(111) surface i s investigated by molecular modeling simulations, using substitution percen tages from 33.3% to 100% of the Si-H moieties by Sialkyl groups. In all cal culations, two-dimensionally repeating boxes were used to mimic the modifie d surface. Calculations without this repeating box approach were shown to b e unsuccessful. The results on the repeating boxes showed that only with a substitution percentage of similar to 50% is there a good correlation betwe en the structure of the monolayers as obtained from molecular modeling and the available experimental data. A variety of substitution patterns with a substitution percentage of 50% on the Si(111) surface were investigated, wh ich showed that a zigzag-type pattern is most suitable to describe the stru cture of the layers. From the results of the investigations, an important c onclusion for future experimental work is drawn. It is shown that the exper imentally determined substitution percentage of 50-55% of the Si-H for Si-a lkyl groups is close to the maximum value that can be reached on the H-term inated Si(111) surface.