Free and bound exciton in II-VI semimagnetic compounds

Abstract

The extremely large exchange interaction typical of semimagnetic semiconductors strongly affects the exciton energies. In hexagonal Cd 1- x Mn x Se crystals, the existence of a uniaxial crystal field leads to a strong dependence of exciton splittings not only on the value, but also on the direction of the magnetic field with respect to the crystal axis. In ordinary wurtzite type crystals such as CdSe, the transverse component g h⊥ of the g-factor at the top of the valence band (A-band) is equal to zero and these states are thus not split in the transverse magnetic field ( H⊥ C 6). By contrast, a large exchange interaction in the Cd 1- x Mn x Se crystals results in a non-zero g-factor and spin splitting of the top valence states in the H⊥ C 6 magnetic field is observed. This splitting arises as a result of a different valence subbands mixing in the exchange field and its proportional to the cube of the exchange field ∝ G 3 h . The subban mixing leads also to new selection rules for optical exciton transitions. The valence and conduction band spin splittings result in ionization of the neutral donor (acceptor) bound excitons in semimagnetic semiconductors. The influence of spin-polaron effects on the energies and stability of the bound excitons is discussed.