Influence of oxygen and argon plasma treatments on the chemical structure and redox state of oxide covered iron

The paper reveals structural changes of the iron surface during the plasma oxidation in an oxygen glow discharge. For this purpose, an in-situ set-up which combines IR-spectroscopy under grazing incidence and quartz crystal micro weighing was used. A Scanning Kelvin Probe was used to measure the influence of the glow discharge on the electronic properties of the iron oxide surface. The glow discharge was initiated by a low power audio frequency voltage source. The results show that an oxygen plasma on iron which is already covered by a natural oxide layer leads to an effective removal of organic contaminations and an increased thickness of the oxide of about 5 to 6 nm. In agreement with the spectroscopic data, the Voltapotential measurements show that the oxide surface is depleted in Fe2+ states. The structure of the additionally grown oxide is most likely to be gamma-Fe2O3. This means that the n-semiconducting properties of the oxide are changed. Within an subsequent argon discharge, the Fe3+ in the oxide surface is again reduced to Fe2+. The paper illustrates the importance of the consideration of the electronic properties while structuring oxide covered metals and the usefulness of the Scanning Kelvin Probe measurement to reveal electronic properties of semiconducting oxide layers. (C) 1999 Elsevier Science B.V. All rights reserved.