Very Low Pressure Oxidation of Si and Ge Surfaces

ABSTRACTAlthough the oxidation of Si(111) 7×7, Ge(111) 2×8 surfaces are relatively well understood very little work has been performed at sub monolayer coverage. We have utillsed a custom-built high resolution Kelvin probe to follow the changes in work function, and changes in band-bending and surface state occupancy via Surface Photovoltage Spectroscopy (SPV). The Kelvin probe Is an Ideal tool for such analysis due to Its extremely high surface sensitivity, equivalent to 4×1010 molecules/cm2, or 0.01% of the available sites.We observe a very rapid initial adsorption phase, which has not been found by other surface probes, e.g., AES, EELS, etc, corresponding to the formation of an elementary dipole layer. During this phase the Increase In the effective electron affinity with oxygen uptake achieves a maximum of 1V at 0.3 monolayer (ML), on SI(111) at 300 K. We report, for the first time, SPV spectra of SI(111), at 100 K, indicating quenching of a band of surface states centered around the fermi-level, quenching Is completed at the max-Imum of the work function change.The second adsorption phase, involving oxygen penetration through the surface layer and incorporation Into the lattice, commences at 0.25 ML for Si(111) at 300 K. a much earlier stage than previously reported. Further. we find evidence of a metastable SiO2 precursor at coverages below 1 ML. The dipole layer formation (and breakup) is shown to be very sensitive to both temperature and amount of surface Carbon contamination. The second phase also Involves a substantial reduction In band-bending to nearly flat bands at 2 ML, accounting for 20% of the work function change In the case of SI and almost 90% for Ge.