A lowenergy electron diffraction data acquisition system for very low electron doses based upon a slow scan charge coupled device camera Rev.The adsorption and decomposition of ethylbromide on the Si͑100͒ and Si͑111͒ surfaces is investigated. Ethylbromide adsorbs molecularly on Si at surface temperatures below 110 K. Warming the ethylbromide-covered surface above 200 K results in the cleavage of the carbonbromine bond with the formation of surface ethyl groups and coadsorbed bromine atoms. Further heating of the surface leads to the decomposition of surface ethyl groups which desorb as ethylene leaving hydrogen at the Si surface. The hydrogen atoms recombine at higher surface temperatures ͑near 800 K͒ and desorb as molecular hydrogen. Isotopic labeling of the hydrogen within the ethyl group provides greater insight into the mechanism for ethylene formation. A branching reaction is observed between and ␣-hydride elimination within the ethyl group to form ethylene. The ratio of the amount of ethylene produced by -hydride to ␣-hydride elimination can be varied by the position of the deuterium label within the ethyl group. The reaction rate for ethylene production is measured and how this rate is influenced by the isotopic substitution is discussed.
The adsorption and decomposition of germane GeH4 and digermane Ge2H6 on Si(100) have been investigated. Exposure of GeH4 to Si(100) at 110 K results in dissociative chemisorption to GeH3 and H. The adsorption of GeH4 is via a precursor state. Heating to higher temperatures results in the decomposition of GeH3. H2 desorption from GeH4 decomposition exhibits two desorption states. One state is dominated by Si and the second state is influenced by Ge on the surface. The preadsorption of Ge strongly alters the desorption of H2, shifting most of the desorption into the lower temperature state most strongly influenced by Ge. The H2 thermal desorption from the decomposition of Ge2H6 is examined and is qualitatively similar to that of GeH4.
The adsorption and decomposition of digermane, Ge2H6, on the Si(100)-(2×1) surface has been investigated with the intent of elucidating the surface processes leading to the deposition of epitaxial Ge thin films from gaseous Ge-containing sources. Exposure of Ge2H6 to Si(100) at 110 K results in molecular adsorption. Upon heating to 150 K, Ge–Ge bond scission occurs and a surface covered with germyl groups is observed. Warming the surface to higher temperatures decomposes the germyl surface groups. The final reaction products are H2 and atomic Ge. The hydrogen thermal desorption that results from the decomposition of Ge2H6 contains features from a distribution of surface sites. Some of the desorption sites are strongly influenced by Ge, lowering the desorption temperature of H2 by over 150 K when compared to the clean silicon surface.
The adsorption and thermal decomposition of tetramethylgermane on Si( 100) have been studied by ultraviolet photoelectron spectroscopy, static secondary ion mass spectrometry, temperature-programmed desorption, and Auger electron spectroscopy. Tetramethylgermane adsorbs molecularly on Si( 100) at 1 10 K. In temperature-programmed desorption experiments, most of the tetramethylgermane reversibly desorbs at 141 f 4 K. The remaining tetramethylgermane decomposes at higher surface temperatures by breaking of a C-Ge bond resulting in the formation of a methyl group and Ge(CH3)3, the latter evolving into the gas phase. Further heating causes decomposition of the methyl group. The hydrogen atoms released by methyl group decomposition form surface Si monohydrides, which later combine to desorb molecular H2 The methyl groups decompose with a pseudo-first-order preexponential of (1 f 5) X lo8 s-I and an activation energy of 29 f 1 kcal mol-].
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