Local electronic structure of Si semiconductor surface

Local electronic structure around Si-adatoms on Si(111)-7 x 7 surface has b een 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 diff erence of surrounding local structure, the adatoms are classified into four types, i.e., the corner- and center-adatoms in a faulted half (F) cell, an d the corner- and center-adatoms in an unfaulted half (UF) cell. In the STS spectra for each type of adatoms, significant differences are revealed, Th e intensity of the STS spectrum near the HOMO (highest occupied molecular o rbital) 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 electro nic structure indicates the main constitution of the HOMO by the atomic orb itals of adatoms and rest-atoms and also the charge transfer from the adato m to the rest-atom. The charge transfer leads to the intensity difference i n the STS spectra near the HOMO between corner- and center-adatoms, because the corner- and center-adatoms have one and two rest-atom neighbors, respe ctively. The energy gap between the HOMO and LUMO (lowest unoccupied molecu lar orbital) in STS spectra for the corner-adatom in a F cell is larger tha n that in an UF cell. Similar results are obtained for the center-adatom. T he change of the energy gap by the presence of stacking fault is demonstrat ed by the calculation using the cluster models with and without the stackin g fault.