Electronic and chemical behavior of oxygen in BaO films on Ir(100)

Abstract

A new surface analysis technique involving low energy electron reflection (LEER) has been used to separate the electron affinity and position of the conduction band edge for BaO single crystal films epitaxially grown on Ir(100) substrates. AES and LEED were used in the study for characterizing chemical and structural effects. Active films have a work function of 1.3 eV with an electron affinity of 0.6 eV. Thermal activation was principally the result of the generation of O-vacancy donors by O diffusion into the substrate and not by evaporation of O from the coating. Donor generation could be retarded by the formation of an oxide diffusion barrier at the substrate interface or by pre-saturating the substrate surface region with oxygen. Electron bombardment desorption caused changes in surface stoichiometry which increased the worl function by increasing the electron affinity. Poisoning by oxygen took place by neutralization of O- vacancy donors until near intrinsic conditions occurred. Additional oxygen accumulates at the surface increasing the electron affinity. Since the Fermi level does not go below the mid-gap position, the formation of Ba-vacancies does not seem likely. Reactivation is possible but the initial donor density cannot be achieved due to O saturation of the substrate.