Local and gap modes of substitutional 3d transition-metal ions in zinc-blende and wurtzite II-VI semiconductors

The localized vibrational modes of the 3d transition-metal ions (TMI's) substituting for the cations in CdTe, ZnTe, and CdSe appear in infrared absorption with as many lines as there are isotopes of the impurity and with intensities proportional to their natural abundances. The triply degenerate local mode in a zinc-blende host splits into a singlet and a doublet in a wurtzite host, the former appearing only when the electric vector (E) of the incident radiation is parallel to the optic axis ((c) over cap) and the latter when <E perpendicular to(c)over cap>. Replacement of Cd by a TMI in CdSe results in high-frequency local vibrational as well as gap modes lying in the gap between the acoustic- and optic-phonon branches, the latter exhibiting an optical anistropy similar to that of the former. Both the local and the gap modes exhibit a unique nonmonotonic trend across the 3d transition-metal series, with a minimum at Mn.