HOST-ISOTOPE FINE-STRUCTURE OF LOCAL AND GAP MODES OF SUBSTITUTIONAL IMPURITIES IN ZINCBLENDE AND WURTZITE II-VI SEMICONDUCTORS

The frequency of a local vibrational mode (LVM) of a light impurity su btituting for either the group II or the group VI host atom in a II-VI semiconductor (e.g., Mg2+ replacing Zn2+ 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 w ith the isotopes of the nearest neighbor (NN) atoms of the impurity, T e in the above case. Depending on the number of isotopes of host atoms , a variety of isotopic configurations in the surrounding ''cage'' of NN's can occur. The striking similarity of this host-isotopic fine str ucture for the same type of NN's (e.g., Te in ZnTe and CdTe), irrespec tive of the impurity (X) and its isotopes, justifies the LVM to be ass ociated with the high-frequency, infrared active, triply degererate (G amma(5)) mode of an XY(4) molecule, the four Y's being the NN's. A num erical calculation of the normal modes of such an XY(4) molecule, usin g stretching and bending force constants, reproduces the entire host-i sotope fine structure of an LVM with remarkable fidelity when all the isotopic NN combinations are appropriately taken into account. When th e 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-frequ ency Gamma(5) mode. When the substitutional impurity replaces the ligh ter 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 Ga mma(5). If only a gap mode is to be expected as for Se2- in ZnS, both computer calculation as well as experimental observation display strik ing isotopic shifts associated with both the impurity and the host ato ms. The model also satisfactorily describes the anisotropic effects fo r an LVM in a wurtzite host (e.g., Mg2+ in CdSe).