Ultraviolet photoemission studies of the oxidation of thin Bi films using synchrotron radiation

Three aspects of photoelectron spectroscopy are used to examine the oxidation process in thin Bi films with cleaved layered crystals of $\mathrm{V}B$-$\mathrm{VI}B$ compounds used as substrates. Valence-band emission, $5d$ core emissions, and the conduction-band density of states are all examined separately. The changes in the electronic structure shows that oxidation occurs in three stages as oxygen exposure increases: (i) a precursor stage of weak chemisorption; (ii) a sudden onset of strong rapid oxidation and growth of a two-dimensional oxides; and (iii) a slower growth of the oxide layer. Bi $5d$ core-level shifts, the opening of a band gap at the Fermi level, and spin-orbit splitting in the conduction band in the oxide are all discussed.