Oxidation von Eisenschichten auf MgO(001)-Substraten - Universität ...

8 Summary
In this work iron oxide was prepared with two different methods. In one method, the oxidation
of iron films on top of MgO(001)-substrates were investigated. First a layer of iron was
deposited on the substrate using molecular beam epitaxy (MBE) at room temperature (RT).
Then the iron film was treated several times for an hour in a thin oxygen atmosphere (p(O
2) = 1× 10 −6 . To investigate effects of the temperature on the oxidation, the iron film were
heated to different temperatures (RT, 373 K, 473 K and 573 K) during oxygen treatment.
After each step the film was investigated. In the second method iron oxide films were
reactively vapour-deposited onto the MgO-substrate in a rare oxygen atmosphere. The effects
of the temperature were investigated by heating the substrate to different temperatures (RT,
373 K, 473 K and 573 K).
The stoichiometric composition of the iron oxide films were identified by X-ray Photoelectron
Spectroscopy (XPS). In addition LEED measurements (Low Energy Electron Diffraction) was
performed to investigate the surface morphology.
The concentration ratios of Fe 0 , Fe 2+ , Fe 3+ and O 2− of the prepared iron oxide films were
determined by the reconstruction of the Fe 2p- and Fe 3p-peaks. In the first method it was
found that the oxidation of the iron films in a rare oxygen atmosphere results in the formation
of iron oxide regardless of the temperature used. It was found that the iron films oxidize
stronger and faster at high temperatures than at low temperatures. While the oxidation of
the iron films stops after a few treatments with oxygen at low temperatures (RT, 373 K),
the iron films oxidized completely at higher temperatures (473 K, 573 K) with the formation
of Fe 3+ ions. The Fe 2p-spectra of the oxidized iron films at 473 K and 573 K both show
the satellite typical for Fe 3+ -ions. The determination of oxygen content after each treatment
showed that the oxygen content after the first treatment almost remains the same regardless
of the temperature. It was found that the number of Fe 2+ -ions in the iron films increased
significantly by annealing without oxygen at 573 K. The typical satellite for Fe 2+ -ions could
be observed in the corresponding Fe 2p-spectra. A growing number of Mg 2+ -ions in the iron
oxide films and the appearance of the Mg 1s signal suggested a diffusion of Mg 2+ ions from the
substrate to the surface at high temperatures. The sharp reflexes in the LEED measurements
showed that the crystallinity of the films was increased by the treatment at high temperature.
While the diffraction pattern of the iron film oxidized at RT showed no reflexes after the last
oxidation step, the iron layer that was oxidized at 373 K had no attributable reflexes. The iron
film, which was oxidized at 473 K, showed a (2 × 1) structure with two domains after the last
treatment step. This structure can not be assigned to any iron oxide phase. The diffraction
pattern of the film oxidized at 573 K showed the strongest reflexes. They formed a (2 × 2)
structure with a ( √ 2 × √ 2)R45 ◦ superstructure. The (2 × 2) structure of the iron oxide
produced at 573 K indicated the formation of maghemite, which can be confirmed by the
high Fe 3+ -content. However, the occurrence of a ( √ 2× √ 2)R45 ◦ superstructure was excluded
for maghemite in the literature [31], so this film can not be pure maghemite. However, the
iron film which was heated to 473 K three times for an hour in oxygen had the structure and
the ion ratio of Fe 2+ / Fe 3+ of magnetite.
The reactive preparation of iron oxide films at different temperatures showed that the ion
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