The microstructure of iron oxide thin films

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Abstract
The microstructural dependence of iron oxide thin films on the film preparation conditions has been systematically evaluated. The main process parameters in this study were substrate temperature, which varied from room temperature to 325 °C, and oxygen flow rate, which was from 3.0 to 9.5 sccm. For the oxygen flow rate below 6.0 sccm during deposition, the crystalline grains are columnar as shown by cross-section TEM. The columnar structure varied with the deposition temperature. For lower deposition temperature, the columnar diameter is nearly constant across the entire thickness (1000 Å) of the film; increasing the temperature tends to increase the diameter of columnar grains along the thickness of the film toward the top surface. It was also noted that the microvoids between columns were present only in the room-temperature deposited films. These voids are believed due to the shadowing effect, and resulted from the low surface atomic mobility at low deposition temperature. For higher oxygen flow rate (>8.5 sccm) the columnar structure no longer existed; instead, large α-Fe2O3 grains are present in a rather diffuse amorphouslike matrix which is believed to consist of numerous very small microcrystalline Fe3O4 particles as evidenced by the electron diffraction patterns. The postoxidization treatment was found to have very little effect on both grain size and the crystalline texture of the as-deposited thin films. However, an additional surface oxide layer of about 70 Å was seen after the oxidation process. This oxide layer was found to be polycrystalline in nature and was speculated to have resulted from the iron atoms which diffused to the surface and were subsequently oxidized to form this layer.