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

TL;DR

This study investigates how s,p-d and s-p exchange interactions influence exciton splitting in (Zn,Mn)O, revealing unusual giant exciton splittings and supporting a non-perturbative exchange theory for dilute magnetic semiconductors.

Contribution

It provides a detailed magnetooptical analysis of exciton behavior in (Zn,Mn)O, highlighting the role of exchange couplings and validating a non-perturbative exchange model.

Findings

Excitonic transitions shift with Mn concentration and split nonlinearly under magnetic field.

The free excitons are associated with Gamma_7 and Gamma_9 valence bands, reversing the order seen in GaN.

The giant exciton splittings are described by an effective exchange energy N_0(beta-alpha)=+0.2+/-0.1 eV.

Abstract

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