Reaction of I2 with the (001) surfaces of GaAs, InAs, and InSb. I. Chemical interaction with the substrate

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.

Coupling vs surface-etching reactions of alkyl halides on GaAs(100). 2. CH2I2 reactions

Surface reactions of CH2I2 on gallium-rich GaAs(100)-(4 x 1), studied by temperature programmed desorption and X-ray photoelectron spectroscopy (XPS), show CH2I2 adsorbs dissociatively at liquid nitrogen temperatures to form surface chemisorbed CH2(ads) and I(ads) species. Controlled hydrogenation of a fraction of the CH2(ads) species in the chemisorbed layer by the background hydrogen radicals results in a surface layer comprising both CH3(ads) and CH2(ads) species. This hydrogenation step initiates a plethora of further surface reactions involving these two species and I(ads). Thermal activation leads to three sequential methylene insertions (CH2(ads)) into the CH3-surface bond to form three higher alkyl (ethyl (C2), propyl (C3), and butyl (C4)) species, which undergo beta-hydride elimination to evolve the respective higher alkene (ethene, propene, and butene). In competition with beta-hydride elimination, reductive elimination of the ethyl and propyl species with I(ads) occurs to liberate the respective alkyl iodide. Beta-hydride elimination in the alkyls, in the temperature range 420-520 K, is the more dominant pathway, and it is also the rate-limiting step for further chain propagation. The evolution of the alkyl iodides represents the only pathway for the removal of surface iodines in this study and is different from previous investigations where gallium and arsenic iodide etch products (GaI(x), AsI(x) (x = 1-3)) formed instead. The desorption of methane and methyl iodide, formed from surface CH3(ads) species at high temperatures by the reaction between surface methylenes and hydrogens eliminated from the surface C2-C4 alkyls, terminates the chain propagation. We discuss the reaction mechanisms by which the observed reaction products form and postulate reasons for the reaction pathways adopted by the surface species.

Surface-reconstruction-switched adsorbate photofragmentation dynamics

Energy-resolved angular distributions of neutral fragments ejected during photoinduced electron transfer reaction of CH3Br on GaAs(100) exhibit three distinct methyl-radical ejection channels. These undergo marked changes when the termination is switched from the Ga-rich c(8 x 2) to the As-rich c(2 x 8). Our observations are consistent with a strong adsorption-site dependence of the dynamics.

FUNDAMENTAL STUDIES OF HALOGEN REACTIONS WITH III-V SEMICONDUCTOR SURFACES

▪ Abstract  It is technologically important to understand how halogens react with semiconductor surfaces because halogen compounds are commonly used to etch semiconductor wafers in the microelectronics industry. Halogens are also model adsorbates for studying chemisorption on covalently bonded materials, such as semiconductors, owing to the simple nature of the bonds that they form. The growing use of III-V materials in the manufacture of optoelectronic devices has prompted investigations of the reactions of molecular halogens (XeF2, Cl2, Br2, and I2) with III-V semiconductor surfaces (GaAs, GaSb, InP, InAs, and InSb). This review examines the more fundamental of these investigations, which involve model systems in ultra-high vacuum, focusing on the chemistry of the halogen surface reactions and the physical and electronic structure of the reacted surfaces.