When
a metal surface is exposed to a fluorine-containing molecule, the liberated
F grows a thin metal fluoride film by a redox reaction. Thin metal
fluoride films were grown by reacting XeF2 and SeF6
with polycrystalline iron, vanadium, and copper surfaces at room temperature
[1-3]. X-ray photoelectron spectroscopy was used to ascertain that
films of FeF2, VF3, and CuF2 form on the
respective substrates. The film growth initially follows the Mott-Cabrera
parabolic rate law, indicating that the process is diffusion limited.
At a certain film thickness, however, the growth abruptly stops and a self-limiting
thickness is attained. The sudden stop in film growth is attributed
to the inability of the precursor molecules to dissociate at the surface
when the insulator film becomes too thick for electrons from the substrate
to transport through. The thickness using XeF2 is nearly
double that obtained with SeF6, which suggests that SeF6
must approach the surface more closely then XeF2 in order to
dissociate [1]. Thicker films grow on iron and vanadium than on copper,
which is due to different densities of states at the Fermi level [2].
The growth mechanism is illustrated in the figure. This work suggests the use of molecular precursors to grow thin films via a self-limiting chemical process. In this case, the thickness is determined by the nature of the reactants and substrate, and not by the time of exposure.
The dissociation of SeF6 by iron surfaces also has implications
in the use of zero-valent iron for environmental
remediation [3]. It was found that the reaction is quenched when
the surface is covered by an oxide film of the same thickness as the fluoride
film that stops the reaction. Thus, it was concluded that the oxide
film also inhibits the transoport of electrons needed to dissociate the
precursor.
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