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Isolated modes and percolation in the lattice dynamics of (Be, Zn)Se

The mixed II–VI semiconductor Zn1 −  x Be x Se possesses non-trivial vibration properties, because its two constituent compounds, ZnSe and BeSe, show very different degrees of covalency and hence high elastic contrast. An anomalous Be–Se vibration line has been observed, mostly at intermediate Be content, in the Raman spectra of thin (Zn,Be)Se films. In order to explain the microscopic origin and detailed composition of these lines, a first-principles calculation of the vibration frequencies in a mixed crystal has been performed, with the frozen-phonon technique and supercell setup within the density functional theory, using the SIESTA method, which uses norm-conserving pseudopotentials and strictly localized numerical basis functions. The calculations confirmed an earlier assumption that the anomalous Be–Se line appears due to the formation of continuous chains of a more rigid Be-rich pseudo-continuous phase formed within the more soft Zn-rich host region on crossing the Be–Se bond percolation threshold (x ∼ 0.19). Different local deformations in percolated and non-percolated regions affect the interatomic elastic interactions and split the corresponding vibration lines. Besides confirming the percolation model qualitatively, the calculation provides details concerning the vibration patterns in different phonon modes.