Electrical phase transitions on the alkali-metal-adsorbed Si(001) surfaces

The adsorption of alkali metals (AM's) on the Si(001) surface has been studied using high-resolution electron-energy-loss spectroscopy. We find that the metallization at low AM coverages manifests itself with a unique surface plasmon excitation. Which turns out to be common to the AM (Li,Na,Cs)/Si(001) surfaces. With increasing AM coverage, these AM-induced metallic surfaces become semiconducting again revealing the characteristic metal-semiconductor transitions (MST's) at the onset of the first ordered structures: (4 X 1) and (2 X 3) for Na and Cs adsorption, respectively. We invoke a mechanism leading to a band insulator to understand such a MST accompanying a disorder-order structural transition. In addition, the distinctly different electrical properties of the AM saturated surfaces, semiconducting for Na and metallic for Cs, are attributed to the availability of some excess adatoms beyond 1 monolayer at room temperature.