POTASSIUM ADSORPTION AND COADSORPTION WITH OXYGEN ON A SI(111) SURFACE

Adsorption of potassium and its interaction with oxygen on a Si(111) surface have been studied by means of Auger electron spectroscopy (AES), temperature-programmed desorption (TPD), stepped thermal desorption (STD), LEED and work-function (phi) measurements. It has been found that the K coverage, theta(K), attained at 300 K, is 1.15. The coverage dependence of the K desorption energy, E(theta(K)), obtained by the STD method, for a bare and an oxidized Si(111), respectively, shows the stabilization effect of the oxidation. The additional stabilization, observed for an oxidized K layer (as evidenced by the TPD data), is explained by the inclusion of the K adatom in the O-Si bonding (at theta(K) < 0.5), and the formation of dioxygen species (at theta(K) > 0.5). The latter are stable for T < 500 K, and are more likely to coalesce in 3D islands on Si(111). Upon thermal decomposition of dioxygen species, taking place at T > 500 K, active oxygen is transferred to Si. The oxygen bonded to Si and K atoms (at theta(K) < 0.5), has no appreciable contribution to the K-promoted oxidation of the Si(111) surface.