Influence of s,p-d and s-p exchange couplings on exciton splitting in (Zn,Mn)O

摘要

This work presents results of near-band gap magnetooptical studies on(Zn,Mn)O epitaxial layers. We observe excitonic transitions in reflectivity andphotoluminescence, that shift towards higher energies when the Mn concentrationincreases and split nonlinearly under the magnetic field. Excitonic shifts aredetermined by the s,p-d exchange coupling to magnetic ions, by theelectron-hole s-p exchange, and the spin-orbit interactions. A quantitativedescription of the magnetoreflectivity findings indicates that the freeexcitons A and B are associated with the Gamma_7 and Gamma_9 valence bands,respectively, the order reversed as compared to wurtzite GaN. Furthermore, ourresults show that the magnitude of the giant exciton splittings, specific todilute magnetic semiconductors, is unusual: the magnetoreflectivity data isdescribed by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV, whatpoints to small and positive N_0 beta. It is shown that both the increase ofthe gap with x and the small positive value of the exchange energy N_0 betacorroborate recent theory describing the exchange splitting of the valence bandin a non-perturbative way, suitable for the case of a strong p-d hybridization.