Interaction of Excitons Bound to 3d Transition Metal Ions with Lattice Vibrations in II-VI Semiconductors

Abstract

A large body of experimental information on the interaction of lattice vibration with donor and acceptor excitons (DE and ΑE) bound to isoeIectronic impurities has recently been accumulated. This interaction is very strong. Apart from this, the observed peak position often does not correspond to the phonons from the centre of the Brillouin zone [1]. The analysis of experimental results is based on the fact that DE and ΑE substantially distort ionic oscillations near the impurity, owing to strong localization of the carrier captured by a non-Coulomb potential. This results in a phonon spectum exhibiting normal modes of a cluster that consists of an impurity and some lattice ions surrounding it [2]. Excitons localized on an impurity interact with normal oscillations of a given claster. When an impurity is introduced into the lattice, the motion of the ions that surround the impurity acquires the point symmetry of the impurity site, e.g. Td for crystals with zincblende stucture. Combinations of atomic displacements near the impurity (normal modes) are described by irreducible representations Α1 (breathing mode), E and 2Τ2 of Td group. These are expressed in terms of irreducible representations of phonons of a perfect crystal. The contribution to normal modes originates from phonons from different Brillouin zone points, and the total number of phonons, that is, the projected density of phonon states, is calculated by a computer. Results of such computations for normal modes A1(ZnS), E and Τ2 (in ZnS, ZnSe, ZnTe) are given in [2]. When the difference in force constants and/or ion masses is large, local or quasilocal lattice vibrations may appear.