Resonant and nonresonant vibrational and electronic excitations in trimethylaluminum adsorbed on GaAs

The vibrational and electronic excitations of trimethyaluminum (Me3Al) adso rbed on GaAs(100) have been studied using high-resolution electron energy l oss spectroscopy (HREELS). The C-H vibrational modes of the adsorbed Me gro ups are excited by a resonance scattering mechanism involving the formation of a temporary negative ion. Strong vibrational mode enhancement occurs at 4, 8, and 14 eV with several overtone and combination bands observed at th ese energies. Reflectivity measurements of both the clean and adsorbate-cov ered surface are consistent with resonance scattering at these energies. Th e molecular orbitals involved in the resonant process are identifed through calculation of electron affinities and charge distributions for the free M e3Al molecule. HREEL spectra recorded at electron beam energies greater tha n 10 eV reveal new vibrational modes not associated with CH3 groups but cha racteristic of adsorbed CH2, a species also formed after annealing the Me3A l exposed surface to temperatures greater than 350 degrees C. Electron beam induced dissociation of the Me groups occurs with a threshold energy of 10 eV, and the dissociation cross section for CH2 production is very large, v arying from 0.7 x 10(-16) cm(2) at 20 eV to 2.0 x 10(-16) cm2 at 30 eV. The results are consistent with a nonresonant dissociation mechanism, and an e lectron impact ionization mechanism is proposed. The high cross sections re flect the relatively long lifetime of the dissociative excited states, whic h are not quenched efficiently for adsorption on semiconductor surfaces.