Surface reaction mechanisms as a function of surface composition, adsorbate coverage, and adsorbate acidity: I. Trimethylgallium on metal-treated silicon. II. Hexafluoropentanedione, methanol, and acetone on copper

The heterogeneous reaction of trimethylgallium was determined as a function of adsorbate coverage, surface composition, and surface temperature. As the average of trimethylgallium on Si(100) is increased, the reaction mechanism evolves from intramolecular decomposition, forming methane at very low coverage, to intermolecular interactions at half monolayer coverage, which form ethylene and facilitate methyl-gallium bond scission, to the desorption of gallium-alkyl species and methyl species at multilayer coverage. Desorption is favored at higher coverage, since reaction sites on Si(100) are blocked. There is no significant difference between the reaction mechanism of a saturation exposure of trimethylgallium on a sample held at 100 K and one held at 280 K, besides the increased adsorption of trimethylgallium into the multilayer adsorption state on the former. Surfaces with 0.5 ML of aluminum or gallium metal on Si(100) form the same surface reconstructions, but exhibit dramatically different reactivities towards trimethylgallium. On the gallium-treated surface desorption is favored, whereas on the aluminum-treated surface trimethylgallium decomposes by 250 K. Methyl groups from the trimethylgallium are transferred to the surface aluminum atoms and trimethylaluminum desorbs. Gallium atoms are left on the surface. The reactivities of the group III organometallics on group III metal-rich surfaces depend on the electronegativity differences between the coordinated metal atom in the adsorbate and the metal on the surface. Reactions favor the formation or maintenance of the strongest alkyl-metal bonds. The reactivities of acetone and hexafluoropentanedione on Cu(110), and the reactivity of methanol on Cu(210), were determined with and without oxygen coadsorption. Acetone, a weakly acidic molecule, was unreactive on Cu(110). Hexafluoropentanedione, a strongly acidic molecule, decomposed on Cu(110). H...