9.2.1.3 Electron structure effects

This chapter presents electronic structure effects on scanning tunneling microscopy (STM). On surfaces of semiconductors and semimetals, numerous experiments have clearly demonstrated that tunneling images are strongly affected by, and often dominated by, local electronic structure effects. In order to understand how electronic structure effects are observed in STM, consider first the energy level diagram. When there is no bias applied between a sample and a tip, their Fermi levels are aligned, and the tunneling barrier has a trapezoidal shape determined by the work functions of sample and tip and the separation Z between the sample and the tip. At negative sample bias, electrons originating in occupied states of the sample with energy between E_F-eV and E_F contribute to the tunneling current. At positive sample bias, electrons in the tip between E_F-eV and E_F tunnel into empty states of the sample. Thus, the sign and magnitude of the sample-tip bias determines which electronic states of the sample and the tip can contribute to the net tunneling current. Ordered overlayers of silver (Ag) and aluminum (Al) on Si(111) show dramatic changes with the applied bias, revealing the locations of the occupied and unoccupied electronic states of the surface.