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The sites for trimethylgallium (TMGa) adsorption on c(2 × 8) GaAs(001) have been identified by hydrogen titration of the surface before and after adsorbing the organometallic molecules. There are four main adsorption sites on GaAs(001): As dimers, Ga dimers, second-layer As atoms, and second-layer Ga atoms. Hydrogen bonds to each of these sites, producing AsH and GaH species with characteristic stretching vibrations in the infrared spectrum. By monitoring the changes in the vibrational spectra of the hydrides upon coadsorption of TMGa, it has been discovered that this molecule dissociatively adsorbs onto an arsenic dimer. One methyl group is transferred to the As atom of the dimer, while at least one other methyl group is transferred to a second-layer Ga atom. At TMGa coverages ≥ 0.9, the adsorbed gallium atoms combine to form dimers. The distribution of the three methyl groups between one As site and two Ga sites is also consistent with the CH stretching vibrations recorded of these species. This model of trimethylgallium adsorption is further supported by the fact that the total number of valence electrons are conserved throughout the reaction. |
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