The bonding of alkali metals to semiconductor surfaces: a direct and inverse photoemission study

Abstract

The bonding character (ionic versus covalent) of alkali metals on semiconductor surfaces is controversial. This concerns the amount of charge transfer and the question where a possible metallization takes place (alkali overlayer or semiconductor substrate). To give an answer we have employed direct and inverse photoemission spectroscopy to Na, K, and Cs on Si(111)2×1 and 7 × 7 and on GaAs(110), and K on Si(100)2 × 1. We find that there is no iso-electronic behavior, e.g., monolayer coverages may result in a metallic surface {Si(111)2 × 1K} or leave the surface semiconducting {Si(111)1 × 1Na, Si(111)√3 × √3R30°Cs} with specific surface atom re-arrangements. The reason may lie in the increasing competition between the alkali-semiconductor and alkali-alkali interaction as a function of coverage, both being dependent on the atomic radii of the alkali-metal atoms. Our results will be compared to theoretical calculations as well as to the findings of other experimental techniques. © 1992.