Optical bandedge of diluted magnetic semiconductors: difference between II-VI and III-V-based DMSs

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Applying the dynamical coherent potential approximation to a simple model, we have theoretically studied the behavior of the optical bandedge in diluted magnetic semiconductors (DMSs). For A II₁₋ₓMnₓB VI-type DMS, the present study reveals that the linear relationship between exchange-spitting ΔEex and the averaged magnetization |x < Sz >| widely holds for different values of x. The ratio, ΔEex/x < Sz>, however, depends not only the exchange strength but also the band offset. Furthermore, the present study reveals that in the low dilution of Ga₁₋ₓMnₓAs the optical bandedge exists not at the bandedge of the impurity band but near the bottom of host band. The optical bandedge behaves as if the exchange interaction is ferromagnetic although the antiferromagnetic exchange interaction actually operates at Mn site. We conclude that the spin-dependent shift of the carrier states between the impurity band and host band accompanying with the change of magnetization causes the apparently ferromagnetic behavior of the optical bandedge which was reported in the magnetoreflection measurement of Ga₁₋ₓMnₓAs.