Metallization and demetallization of clean and oxygen-covered ultrathin alkali metal films on GaAs(100)

The structure and the electronic valence state occupation of ultrathin K, R b, and Cs films grown on a GaAs(1 0 0)-(4 x 2) surface have been studied by means of metastable He atom scattering (MHAS), He atom scattering (HAS), a nd low-energy electron diffraction (LEED) at temperatures ranging from 150 to 400 K. From the survival probability of the scattered He* atoms, detaile d information on the coverage-dependent filling of the alkali metal valence states and their emptying upon subsequent exposure to oxygen were derived. These data reveal for K and Rb a nearly linear band filling with increasin g coverage starting at about 0.5 ML whereas a more rapid filling is observe d for Cs which is almost completed at about 0.7 ML Subsequent oxygen adsorp tion causes a demetallization of the metallic alkali metal monolayers. In c ase of Cs, a distinct minimum of the He* signal appears at an oxygen exposu re of about 0.8 L, presumably indicating the onset of subsurface, oxidation . (C) 2001 Elsevier Science B.V. All rights reserved.