Abstract

Conventional techniques for the study of surface electronic structure provide information averaged over a macroscopic area of the sample and do not allow direct association of particular electronic features to specific surface sites. The microscopic understanding of surface chemistry, however, requires such an association since chemistry is a localized phenomenon. We will show that STM provides the unique ability to probe surface chemistry at the atomic level. Reactions of Si (111)-(7x7) will be used as prototypes. Using energy-resolved topographs and atom-resolved scanning tunneling spectra (AR-STS) we can follow the spatial distribution of the reactions among the different dangling-bond sites of the 7x7 surface, probe the electronic structure of these sites, and observe the electronic structure changes induced by the reaction. We find that reaction with NH₃ preserves the 7x7 reconstruction but that there is significant selectivity among surface sites. Rest-atom sites are more reactive than ad-atom sites, and center-adatom sites are more reactive than corner-adatom sites. AR-STS reveals rest-atom ↔ adatom interactions at the clean surface which are eliminated by the reaction at rest-atom sites. We will discuss and interpret the observed reactivity differences in terms of silicon hypervalency and reaction-induced lattice strain. Reactions with other gases such as Si₂H₆ proceed in a similar manner. The reaction with evaporated CaF₂ molecules eliminates the 7x7 reconstruction and several different sub-monolayer structures are formed depending on the deposition conditions. We will discuss the topography and bonding in these structures, and the role of steps and other defects in their nucleation. Finally, contrary to the general belief that insulating materials cannot be imaged by the STM we will show that by an appropriate choice of biasing conditions, multilayer insulating CaF₂ films can be imaged by the STM. Mechanisms by which insulators can be imaged will be discussed.

© 1989 Optical Society of America