EVIDENCE FOR A SURFACE METHYLENE SPECIES IN THE DECOMPOSITION OF TRIMETHYLGALLIUM ON GAAS(100)-(4X1) - A HIGH-RESOLUTION ELECTRON-ENERGY-LOSS SPECTROSCOPY STUDY

High resolution electron energy loss spectroscopy (HREELS) has been used to study the adsorption and thermal decomposition of trimethylgallium (TMGa) on GaAs(100). HREEL spectra recorded for adsorption at room temperature show that the dominant surface species is based on methyl groups and is characterised by strong CH3 deformation modes at 1200 and 1450 cm(-1). High resolution data, enhanced by the application of maximum entropy methods and carried out over a wide range of incident electron energies, indicates a second surface species characterised by a mode at 1330 cm(-1). This band dominates the spectra as the temperature is raised to similar to 400 degrees C and there is a corresponding down shift in the C-H stretching frequency. Further increases in temperature result in a significant decrease in the intensity of all the vibrational modes associated with the adsorbate. The results are consistent with TMGa decomposing with increasing temperature by the release of CH3 groups. An additional decomposition route involves the dehydrogenation of the CH3 groups and the formation of a surface methylene species (CH2) which is characterised by the intense CH2 deformation mode at 1330 cm(-1). The observation of this species at higher temperatures supports a recent model explaining the high carbon incorporation levels in the growth of GaAs using TMGa.