Optical studies of bonding in coevaporated amorphous silicon-tin alloys

Homogeneous amorphous ${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Sn}}_{\mathrm{x}}$ alloys have been prepared by coevaporation on cold substrates over a wide composition range, 0≤x≤0.50. Their complex dielectric constant ε̃ has been accurately determined between 0.5 and 3 eV by combining different methods on films with different thicknesses (from 30 to 300–500 nm). The derivation of two characteristic alloy parameters (the optical gap and the average gap) from these data is discussed in detail. While the optical gap decreases linearly with increasing x, the average-gap variation shows a change in slope for x≃0.30, suggesting a modification of the alloy average bonding. A careful analysis of the overall evolution upon alloying of the ${\epsilon }_1$ and ${\epsilon }_2$ spectra supports the conclusion that these a-${\mathrm{Si}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Sn}}_{\mathrm{x}}$ alloys are chemically ordered.