Browsing by Author "Kurth, Matthias"

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    The initial oxidation of magnesium
    (2004) Kurth, Matthias; Mittemeijer, E.J. (Prof. Dr. Ir.)
    Pure Magnesium samples have been oxidised in an UHV chamber under controlled conditions. Pressure range was 10-10 Torr to 10-7 Torr, temperature range was 273 K to 435 K. The samples have then been investigated with XPS, Ellipsometrie and HERDA. Additionally, furnace oxidations at 750 Torr and 673 K have been carried out and investigated with XPS. From the XPS measurements data concerning layer thickness, composition, oxidation state and binding state have been gained. The ellipsometrie measurements yielded additional data concerning layer thickness as well as the size of the band gap of the developing oxide. With the HERDA measurements, the oxygen content within the oxide layer has been determined yielding additional information about composition and layer thickness. The layer thickness as a function of time have then been modelled with a kinetic growth model of Fromhold & Cook. For the refinement of the XPS data concerning layer thickness and composition, the pronounced plasmon excitations that occur in magnesium have been determined with two different procedures which have been developed in the methodical part of this work. The layer thickness and composition values have thus been corrected. Results: Two oxidation stages could be identified: a strong increase for the first few Langmuirs (1L = 1s * 10-6 Torr), followed by a „saturation“ region which was about 1.2 nm to 1.5 nm in magnitude. XPS and ellipsometry results have thereby been in very good agreement. The composition of the developing oxide showed a clear deviation from stoichiometric MgO, mainly caused by an oxygen deficiency; this deficiency has also been confirmed with the HERDA measurements. The Mg/O ratio as a function of layer thickness showed a continous decay starting from very high values for the thinnest layers (>~2.5) down to a saturation value of about 1.4, even for larger layer thicknesses gained with the furnace oxidations. The determination of the size of the band gap which has been gained from the ellipsometry measurements showed a clearly reduced value compared with MgO (~2.5 eV vs. 7.8 eV). The modelling of the kinetic data with the Fromhold & Cook model resulted in good agreement of fit and experimental data. Thereby, fit parameters have been the work function for the interface oxide/oxygen as well as the energy barrier for cation movement (here: Mg cations). For both parameters reasonable values have been gained. Furthermore, a dependence from the oxidation stage corresponding to the changing composition could be realised. In the methodical part of this work mentioned above, the plasmon excitations in Mg have been calculated, for the first time, separately for bulk and surface plasmons. The reconstruction of a Mg spectrum with physically reasonable functions yielded 35 % and 11 %, respectively, relative to the excitation of the main line. With a second method, which is based upon experimental data and which has been further developed here (especially the error estimation procedure), both excitations could have been determined to 17 % and 6 %, respectively.