Surface valence states and stoichiometry of non-superconducting and superconducting FeTe films

We report the surface electronic structure and stoichiometry of FeTe films following the incorporation of oxygen by three different methods: air exposure, dry oxygen exposure and low temperature oxygen annealing. X-ray photoemission experiments show that oxygen incorporation changes the initial valence state of Fe from 0 to mainly 3+. We also observe that the Te changes valence from initially 0 to mixed 0 and 4+. The rate of valence changes is seen to depend on the method of incorporation. In addition, it is observed that the surface of the FeTe films is left in a Te deficient state following any type of exposure to oxygen.