Percolation scheme for the very basic understanding of the Raman spectra of semiconductor mixed crystals

A1-xBxC semiconductor mixed crystals with zincblende structure are benchmark systems for the experimental study of the chemical disorder due to alloying, which relates to the percolation site theory. The C-invariant and (A,B)-substituting sublattices intercalate through a tetrahedral bonding, resulting in two bond species (A-C and B-C) arranged with maximal (cubic) symmetry. This forms the most “simple” three-dimensional (3D) disordered system of chemical bonds one can imagine, comparable to an ideal object, which can be experimentally tested and confronted with models ran at the ultimate atom scale. The simplicity gives grounds for hope to solve certain critical issues behind alloying. One refers to (i) the nature of the A⇿B atom substitution, as to whether this is ideally random, or not. Another one, emphasized in this talk, is (ii) to elucidate how lattice-supported complex media engage their pressure-induced structural transition at the local scale.
In order to address experimentally such issues, one not only needs a suitable system as described above but also a local probe, such as the bond force constant, as conveniently measured at the laboratory scale by Raman scattering.  Precisely, over the past decade and half, we proposed a unified understanding of the phonon mode behavior of the common zincblende-type semiconductor mixed crystals within our so-called percolation scheme. This formalizes a multi-mode Raman pattern per bond, reflecting a sensitivity of bond vibrations to their local environment. Such sensitivity is presently exploited to tackle the raised issue (ii).