  FeTe is known to become a superconductor when doped with oxygen. Using layer
by layer growth of single crystal films by molecular beam epitaxy (MBE), we
have studied how oxygen incorporates. If oxygen is supplied during growth of a
layer, it substitutes for tellurium inhomogeneously in oxygen domains that are
not associated with superconductivity. When oxygen is supplied after growth, it
diffuses homogeneously into the crystalline film and incorporates
interstitially. Only the interstitial oxygen causes superconductivity to
emerge. This suggests that the superconductivity observed in this material is
spatially uniform and not filamentary.
