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.
Kemp
Neil T
NT
School of Chemistry, The University of New South Wales, Sydney 2052, Australia.
Paris
Nathan J
NJ
Giveen
Deborah
D
Singh
Nagindar K
NK