Lattice dynamics of the mixed semiconductors (Be,Zn)Se from first-principles calculations

Vibration properties of ${\mathrm{Zn}}_{1\ensuremath{-}x}{\mathrm{Be}}_{x}\mathrm{Se}$, a mixed II-VI semiconductor characterized by a high contrast in elastic properties of its pure constituents ZnSe and BeSe, are simulated by first-principles calculations of electronic structure, lattice relaxation, and frozen phonons. The calculations within the local density approximation have been done with the SIESTA method, using norm-conserving pseudopotentials and localized basis functions; the benchmark calculations for pure end systems were, moreover, done also by the all-electron WIEN2k code. An immediate motivation for the study was to analyze, at the microscopic level, the appearance of anomalous phonon modes detected early in Raman spectra in the intermediate region (20%\char21{}80%) of ZnBe concentration. This was discussed early on the basis of a percolation phenomenon\char22{}i.e., the result of the formation of wall-to-wall BeSe chains throughout the crystal. The presence of such chains was explicitly allowed in our simulation and indeed brought about a softening and splitting off of particular modes, in accordance with experimental observation, due to a relative elongation of BeSe bonds along the chain as compared to those involving isolated Be atoms. The variation of force constants with interatomic distances shows common trends in relative independence of the short-range order.