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 f ind that the metallization at low AM coverages manifests itself with a unique surface plasmon excitation. Which turns out to be common to th e AM (Li,Na,Cs)/Si(001) surfaces. With increasing AM coverage, these A M-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 adsor ption, respectively. We invoke a mechanism leading to a band insulator to understand such a MST accompanying a disorder-order structural tra nsition. In addition, the distinctly different electrical properties o f the AM saturated surfaces, semiconducting for Na and metallic for Cs , are attributed to the availability of some excess adatoms beyond 1 m onolayer at room temperature.