Atomic bonding and thermodynamic properties of pseudo-binary semiconducting alloys

The short-range order of the Cd1-xZnxTe, Hg1-xCdxTe and HgSexTe1-x alloys has been investigated with EXAFS measurements, and a bi-modal distribution of nearest-neighbour bond lengths is found. This behaviour is determined by the minimisation of the bond distortion with respect to the pure compounds, as the authors demonstrate with the use of a thermodynamic model. The same model also allows them to discuss the deviation from the randomness and phase diagrams. The mixing enthalpy hm is determined from the distortion energies of the five tetrahedra which form the basic lattice framework of these solutions. The configurational entropy sm is described by a III-order quasi-chemical approximation using the same tetrahedra as basic clusters. The minimisation of the free energy gm=hm-Tsm determines the equilibrium configuration. The computed distributions of bond lengths in several II-VI and III-V pseudo-binary alloys are in perfect agreement with the experiment and the thermodynamic properties also agree quite well with the existing experimental data. A sharp distinction is made between the behaviour of lattice-matched and lattice-mismatched alloys. The former have a binary-like phase diagram and tend to segregate below the critical temperature. The latter have more structured phase diagrams. In this case the stabilising effect of the elastic strain is outlined and the tendency to form ordered phases is identified.