Local Electronic Structure of Si Semiconductor Surface

Abstract

Local electronic structure around Si-adatoms on Si(111)-7 x 7 surface has been investigated using the methods of STM (scanning tunneling microscopy), STS (scanning tunneling spectroscopy) and the molecular orbital calculation for the cluster of local structure around each adatom. In view of the difference of surrounding local structure, the adatoms are classified into four types, i.e., the corner- and center-adatoms in a faulted half (F) cell, and the comer- and center-adatoms in an unfaulted half (UF) cell. In the STS spectra for each type of adatoms, significant differences are revealed. The intensity of the STS spectrum near the HOMO (highest occupied molecular orbital) level for a coner- and center-adatoms in the F cell is larger than for the respective adatoms in the UF cell. The calculation of local electronic structure indicates the main constitution of the HOMO by the atomic orbitals of adatoms and rest-atoms and also the charge transfer from the adatom to the rest-atom. The charge transfer leads to the intensity difference in the STS spectra near the HOMO between corner- and center-adatoms, because the corner- and center-adatoms have one and two rest-atom neighbors, respectively. The energy gap between the HOMO and LUMO (lowest unoccupied molecular orbital) in STS spectra for the corner-adatom in a F cell is larger than that in an UF cell. Similar results are obtained for the center-adatom. The change of the energy gap by the presence of stacking fault is demonstrated by the calculation using the cluster models with and without the stacking fault.