Coupling versus surface-etching reactions of alkyl halides on GaAs(100): I. CF3CH2I reactions

We report the rich surface chemistry exhibited by the reactions of 1,1,1-trifluoroethyl iodide (CF3CH2I) adsorbed onto gallium-rich GaAs(100)-(4 x 1), studied by temperature-programmed desorption (TPD) and low-energy electron diffraction (LEED) studies and X-ray photoelectron spectroscopy (XPS). CF3CH2I adsorbs molecularly at 150 K but dissociates, below room temperature, to form a chemisorbed monolayer of CF3CH2 and I species. Recombinative desorption of molecular CF3CH2I competes with the further reactions of the CF3CH2 and I chemisorbed species. The CF3CH2 species can either undergo beta-fluoride elimination to yield gaseous CF2=CH2 or it can undergo self-coupling to form the corresponding higher alkane, CF3CH2CH2CF3. A second coupling product, CF3CH2CH=CF2, is also evolved, and it is postulated that migratory insertion of the liberated CF2=CH2 into the surface-carbon bond of the chemisorbed CF3CH2 is responsible for its formation. The iodines, formed by C-I scission in the chemisorbed CF3CH2I, and the fluorines, derived from beta-fluoride elimination in CF3CH2, react with the surface gallium dimers, and Ga-As back-bonds to generate five etch products (GaF, AsF, GaI, AsI, and As2) that desorb in the temperature range of 420 to >600 K. XPS data reveal that the surface stoichiometry remains constant throughout the entire annealing temperature range because of the desorption of both gallium- and arsenic-containing etch products, which occur sequentially. In this article, plausible mechanisms by which all products form and the binding sites of these reactions in the (4 x 1) reconstruction are discussed. Factors that control the rate constants of etch product versus hydrocarbon product formation and in particular how they impact on the respective desorption temperatures will be discussed.