Scanning tunneling microscopy of insulators: CaF₂ epitaxy on Si (111)

Abstract

Scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) have been used to study the epitaxy of CaF2 on Si (111). Energy-resolved images of the submonolayer structures produced at the initial stages of CaF2 deposition were obtained. We found that in these structures and also at the 1×1 interface, bonding involves the Ca atom in a reduced, Ca+-like state. Using STS we were able to measure the CaSi bonding-antibonding level splitting at the interface. The distribution of charged defects was also imaged by the STM. More important, we found that we can image strongly insulating CaF2 multilayers by tunneling into their conduction band.