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Lecture I. Arfaoui


05 september 2003 FWN-building 5113.0202, Nijenborgh 4, 9747 AG, Groningen
Speaker: Dr. I. Arfaoui
Affiliation: EPFL, Suisse
Title: Stoichiometries, structure and electric field modifications of a thin niobium oxide film covering Nb(110)
Date: Fri Sep 5, 2003
Start: 11.00
Location: FWN-building 5113.0202
Host: Petra Rudolf
Telephone: +31 50 363 4736


A (110) surface of a niobium crystal prepared in UHV has been mainly studied by Core Level Spectroscopy (CLS) and Scanning Tunneling Microscopy (STM). Auger Electron Spectroscopy (AES) and Low Energy Diffraction (LEED) have been also used. After UHV annealing, this surface is covered by a thin overlayer of NbOx (with x close to 1) due to surface segregation of O dissolved in the bulk. This niobium oxide layer coexists with other niobium oxides characterized by different stoichiometries and the thickness of this oxide layer corresponds to a single plane of niobium atoms. A huge concentration of interstitial oxygen is found in the nanomete-thick metal skin at NbO/Nb interface. NbOx (with x close to 1) exhibits a complex surface structure, which is stable and reproducible, consisting of a defective arrangement of parallel sticks (length = 3nm, width = 0.6nm). Analysis of all the results reveals that the oxide layer can be described as a side to side arrangement of NbO nanocrystals with a typical size 3.5nm x 1.4 nm. Two epitaxial relationships appear between the oxide overlayer (FCC) and the underlying metal surface (BCC). The origin of this structure is discussed in terms of NbO/Nb misfit. We bring evidence of permanent and irreversible modifications of this oxide layer induced by applying a bias voltage between a STM W-tip and the sample higher than a voltage threshold of 2.5V. This involved voltage and the critical electric peak field which limits accelerating performances of RF cavities are the same.

Laatst gewijzigd:22 oktober 2012 14:30