Transition density of states for cleaved and excited-oxygen exposed surfaces of Ge(111) derived from the M2,3M4,5V Auger transition

The M_2,3M_4,5V Auger transition obtained from vacuum-cleaved Ge(111) surfaces has been debroadened to remove inelastic scattering, instrumental effects and core level lifetime broadening. The resultant spectrum was further separated into its component - M₂M_4,5V, M₃M_4,5V - transitions to obtain the transition density of states function (TDOS). For the 15.3 eV wide valence band, four definite peaks were found at -4.3, -8.0, -11.9 and -13.7 eV and the main TDOS peak at - 4.3 eV is at variance with the accepted bulk value of approximately - 2.7 eV (analogous to previous silicon results). Detailed AES measurements for excited-oxygen exposed cleaved germanium surfaces are also reported. For oxygen exposures in the range 0.1 to 10³ L a continuous shift was observed in the negative excursion of the M_2,3M_4,5V [N'(E)] transition and the derived TDOS curves showed that a number of oxygen-induced peaks appeared at well-defined energies, -7.7, -8.4, -11.3, -13.5, -14.9, -17.8 and -19.7 eV, below the vacuum level. Three of these peaks, -11.3, -14.9 and -19.7 eV, are consistent with GeO₂ formation. For the range of exposures studied, the valence-band spectroscopic data reported have been interpreted as showing the simultaneous existence of both atomic and molecular chemisorption states and their coexistence with GeO₂ formation during the early oxidation stage.