Local vibrational modes of substitutional ${\mathrm{Mg}}^{2+}$, ${\mathrm{Ca}}^{2+}$, and ${\mathrm{S}}^{2\mathrm{-}}$ in zinc-blende and wurtzite II-VI semiconductors

Localized vibrations of substitutional ${\mathrm{Mg}}^{2+}$, ${\mathrm{Ca}}^{2+}$, and ${\mathrm{S}}^{2\mathrm{-}}$ in CdTe, ZnTe, and CdSe are observed as sharp absorption lines in the infrared and display as many lines as there are isotopes of the impurity, their intensities being proportional to their natural abundances. The lowering of the site symmetry from ${\mathit{T}}_{\mathit{d}}$ in the zinc-blende CdTe and ZnTe to ${\mathit{C}}_{3\mathit{v}}$ in the wurtzite CdSe causes the triply degenerate (${\mathrm{\Gamma }}_5$) mode in the former to split into a singlet (${\mathrm{\Gamma }}_1$) and a doublet (${\mathrm{\Gamma }}_3$) in the latter, the singlet and the doublet appearing only for the electric vector along the optic axis and normal to it, respectively. The features of the local mode are interpreted in terms of an anharmonic oscillator consistent with the site symmetry of the impurity. The evolution of the ${\mathrm{Mg}}^{2+}$ local mode from that in CdTe to that in ZnTe, as observed in a series of Mg doped ${\mathrm{Cd}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Zn}}_{\mathit{x}}$Te samples, provides insights into the variation of the local mode frequency or, equivalently, the Mg-Te force constant in the context of virtual crystal approximation. Their linewidths, however, can be correlated with the x(1-x) disorder in the ternary. Ultrahigh resolution measurements of the ${\mathrm{Mg}}^{2+}$ local mode in ZnTe and CdSe reveal fine structures originating in the isotopic disorder of the nearest neighbor Te and Se, respectively.