Host-isotope fine structure of local and gap modes of substitutional impurities in zinc-blende and wurtzite II-VI semiconductors.

Abstract

The frequency of a local vibrational mode (LVM) of a light impurity substituting for either the group II or the group VI host atom in a II-VI semiconductor (e.g., ${\mathrm{Mg}}^{2+}$ replacing ${\mathrm{Zn}}^{2+}$ in ZnTe) exhibits two types of isotope shifts: in one, the shift is caused by the specific isotope of the impurity, whereas in the other a shift occurs in association with the isotopes of the nearest neighbor (NN) atoms of the impurity, Te in the above case. Depending on the number of isotopes of the host atoms, a variety of isotopic configurations in the surrounding ``cage'' of NN's can occur. The striking similarity of this host-isotope fine structure for the same type of NN's (e.g., Te in ZnTe and CdTe), irrespective of the impurity (X) and its isotopes, justifies the LVM to be associated with the high-frequency, infrared active, triply degererate (${\mathrm{\ensuremath{\Gamma}}}_{5}$) mode of an ${\mathit{XY}}_{4}$ molecule, the four Y's being the NN's. A numerical calculation of the normal modes of such an ${\mathit{XY}}_{4}$ molecule, using stretching and bending force constants, reproduces the entire host-isotope fine structure of an LVM with remarkable fidelity when all the isotopic NN combinations are appropriately taken into account. When the impurity has a mass such that in addition to a local mode a gap mode results, the numerical calculations correctly predict the absence of observable isotopic shifts in the gap mode ascribed to the lower-frequency ${\mathrm{\ensuremath{\Gamma}}}_{5}$ mode. When the substitutional impurity replaces the lighter host atom (while being heavier), the local mode is absent and only a gap mode can occur and can be associated with the high-frequency ${\mathrm{\ensuremath{\Gamma}}}_{5}$. If only a gap mode is to be expected as for ${\mathrm{Se}}^{2\mathrm{\ensuremath{-}}}$ in ZnS, both computer calculation as well as experimental observation display striking isotopic shifts associated with both the impurity and the host atoms. The model also satisfactorily describes the anisotropic effects for an LVM in a wurtzite host (e.g., ${\mathrm{Mg}}^{2+}$ in CdSe). \textcopyright{} 1996 The American Physical Society.