Electronic characteristics of thin films

In very thin films, new effects may take place, because the finite thickness is responsible for further modifications of the Madelung potential. This shows up, for example, when one considers unsupported MgO films, with thicknesses n ranging from 1 to 6 atomic planes and several orientations ((100), (110) and (211)). As a first gross approximation, the atoms may be assumed to remain at their bulk positions. The application of the self-consistent tight-binding method yields the gap width A and the ionic charges Qs borne by the surface atoms, as a function of their coordination number Zg. The results are as follows  

The gap width J presents large variations as a function of n, especially along the (211) orientation. However, when the film thickness exceeds one repeat unit in the direction perpendicular to the surface, the electronic structure on surface atoms tends towards that of a semi-infinite system. The variations of A are driven by the values of the Madelung potential. Along (211), this latter varies non-monotonically with n, in a way which may be understood by counting how many atoms belong to the first and second coordination shells of the surface atoms. 

The surface charges turn out to be very close to the bulk charges. The rough balance between electrostatic and covalent effects invoked for semi-infinite planar surfaces takes place in most cases, except for the smallest thicknesses in the less-dense orientations, when the coordination number is smaller than or equal to 2. 

The values of A are fixed by the positions of the renormalized atomic levels of the surface atoms, which partly determine the transport properties of these films, but also their chemical reactivity. Fig. 3.5 shows the large variations of the lowest cation ec and highest anion 6a renormalized atomic energies, as a function of the film thickness n, for different orientations. 

These considerations suggest that very thin oxide films may present specific electronic properties, which might find interesting applications, for example in the field of micro-electronics or opto-electronics, and an enhanced reactivity similar to that of oxide powders. This point is more and more recognized in the literature. Thin oxide films may be grown on metallic substrates by deposition of metallic atoms and subsequent oxidation. By this method, many crystalline films have been obtained, for example BaO on W(llO) (Shih et al, 1988 Mueller et al, 1990) - a 

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