Microscopic Views of Atomic and Molecular Oxygen Bonding with epi Ge(001)-2×1 Studied by High-Resolution Synchrotron Radiation Photoemission

Embryo stage of oxidation of an epi Ge(001)-2×1 by atomic oxygen and molecular O₂ is studied via synchrotron radiation photoemission. The topmost surface buckled with the up- and down-dimer atoms and the first subsurface layer behave distinctly from the bulk by exhibiting surface core-level shifts in the Ge 3d core-level spectrum. The O₂ molecules become dissociated upon reaching the epi Ge(001)-2×1 surface. One of the O atom removes off the up-dimer atom, and the other bonds with the underneath Ge atom in the subsurface layer. Atomic oxygen adsorbed on the epi Ge(001)-2×1 preferentially in between the up-dimer atoms and the underneath subsurface atoms without affecting the down-dimer atoms. The electronic environment of the O-affiliated Ge up-dimer atoms becomes similar to that of the down-dimer atoms. Both exhibit an enrichment in charge, where the subsurface of the Ge layer is maintained in a charge-deficient state. The dipole moment originally generated in the buckled reconstruction no longer exists, thereby resulting in a decrease in the ionization potential. The down-dimer Ge atoms and the back-bonded subsurface atoms remain inert to atomic O and molecular O₂, a possible cause of low reliability in Ge-related metal-oxide-semiconductor (MOS) devices.