An atomic view of Fermi level pinning of Ge(100) by O2

An experimental atomic-level study of the structural and electronic properties of the oxidation of the Ge(100) surface was performed using scanning tunneling microscopy (STM) and spectroscopy (STS). Room-temperature O2-dosed Ge(100) surfaces at sub-monolayer coverages (with and without post-oxidation annealing) were imaged via STM in order to identify the bonding geometries of the oxidation reaction products, and STS spectra were taken for the characterization of the surface electronic structures resulting from those structures. DFT modeling, including STM simulations, was performed for the various potential adsorbate structures indicated by STM imaging in order to elucidate the most likely bonding geometries. Long, low-temperature post-oxidation anneals (325 deg C) were used to eliminate some metastable oxidation reaction products and to drive the coalescence of the stable products. The O2-reacted Ge(100) surfaces, both the disordered pre-annealed and the ordered post-annealed (325 deg C), were found to exhibit Fermi level pinning near the valence band. However, proper Fermi level position was restored upon desorption of the GeO at 500 deg C, indicating that the presence of germanium suboxide at the Ge(100) surface is a source of Fermi level pinning for annealed surfaces. The pinning observed on the room-temperature as-oxidized surface is most likely also due to the suboxide coverage; it is likely that additional components to the pinning states also arise from the displaced Ge ad-species.