Adsorption of tri-methyl aluminum molecules on silicon

The adsorption of tri-methyl aluminum (TMA) on sputter-cleaned Si surfaces was studied using ultra violet and X-ray photoelectron spectroscopy (UPS and XPS). A stable adsorbate layer is formed at temperatures approximately in the range of -80 to -30°C. A lower temperatures, TMA condenses into a multi-molecular-layer film. Both in the multilayer and the adsorbate cases, the UPS spectra indicate that TMA exists in the form of a dimer molecule. In the adsorbate layer, a set of molecular orbitals of the TMA dimer are observed to be stabilized as a result of interaction with the Si surface. A transfer of electronic charge from the Si surface to the TMA molecular adsorbate is observed through chemical shifts in the Si(2p) and Al(2p) XPS core-level spectra. The spectra indicate that the TMA dimer molecule exhibits a tendency to be oriented on the Si surface, with the long molecular axis perpendicular to the surface.