Surface segregation in pseudobinary alloys

Alloys of the form $A_x$ $B_{1\mathrm{-}x}$C almost always have a different surface concentration from the bulk in order to maintain a constant chemical potential for each layer in the alloy. We have calculated the degree of surface segregation for the pseudobinary alloys ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Cd}}_{\mathrm{x}}$Te and ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Zn}}_{\mathrm{x}}$Te. The enthalpy responsible for segregation is the difference in the energies for moving an A or B atom from the bulk alloy to the surface. There are two major contributions to this energy process: (1) a bond-breaking mechanism, whereby the element with the lowest surface energy segregates to the top, and (2) strain release, where the dilute element in the compound segregates to the surface to alleviate the strain energy due to mismatch of the AC and BC bond lengths. In our segregation model, the free energy of each layer is calculated in the regular and quasichemical approximations. By equating the chemical potentials of each successive layer to the bulk, the composition of each layer is obtained. Our results indicate that there is strong surface enrichment of Hg in ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Cd}}_{\mathrm{x}}$Te while there is less surface segregation of Hg in ${\mathrm{Hg}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Zn}}_{\mathrm{x}}$Te at the low x values appropriate for infrared application. Mercury segregation to the surface will lower the band gap or may turn the surface into a semimetal, thereby affecting the passivation of the surface.