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The use of trimethylgallium (TMGa) as the gallium source during epitaxial growth of GaAs often leads to high levels of carbon incorporation. Using temperature programmed desorption, high-resolution electron energy loss spectroscopy (HREELS), and static secondary ion mass spectroscopy (SSIMS), we have identified a likely carbon incorporation pathway initiated by methyl group dehydrogenation. Methyl group dehydrogenation is evidenced by a small amount of hydrogen evolution around 430 °C. Extended TMGa exposures in this temperature regime yield substantial coverages of methylene (CH2) adsorbate that is detected by HREELS and SSIMS. The CH2 adsorbate undergoes further reaction at higher temperature, yielding acetylene (C2H2), H2, and CH3 radicals as desorption products at ∼550 °C. All of these products can be attributed to a mechanism involving CH2 dehydrogenation, hydrogenation, and recombination reactions. The rate of CH3 dehydrogenation is consistent with carbon doping levels typically obtained by metalorganic molecular beam epitaxy (MOMBE) and related techniques. High temperature exposure to arsine (AsH3) consumes the CH2 adsorbate, apparently by hydrogenating them back to CH3 groups that then desorb. This observation explains why carbon doping is lower during atomic layer epitaxy as compared to MOMBE.

The adsorption of nitromethane (CH3NO2 and CD3NO2) and d 3-methyl nitrite (CD3ONO) on Au(111) was studied by temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). These molecules are only weakly adsorbed on Au(111), and adsorption is completely reversible in both cases. Adsorbed CH3NO2 and CD3ONO in the monolayer each give rise to one thermal desorption peak in TPD with desorption activation energies of 10.5 and 8 kcal/mol, respectively. These desorption energies are close to the values for the heats of adsorption of these molecules, since there is no appreciable activation energy for molecular adsorption. HREELS confirms weak, molecular adsorption in the monolayer for both molecules. Furthermore, nitromethane is bonded on Au(111) in an upright, strongly tilted geometry, suggesting a monodentate coordination to the surface. Methyl nitrite adsorbs on Au(111) with the ONO group in a flat-lying geometry, with evidence for both cis and trans forms. The Au(111) surface does not sufficiently activate nitromethane and methyl nitrite for dissociation or isomerization (CH3NO2 ↔ CH3ONO) to occur under UHV conditions. Thus the activation energies for dissociation and isomerization of nitromethane on Au(111) exceed 10.5 kcal/mol.

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