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

TL;DR

This study uses a theoretical model to analyze the optical bandedge behavior in diluted magnetic semiconductors, revealing differences between II-VI and III-V types and explaining experimental ferromagnetic-like shifts.

Contribution

It provides a theoretical explanation for the optical bandedge behavior and its dependence on exchange interactions and band offsets in DMSs, especially in GaMnAs.

Findings

Linear relation between exchange splitting and magnetization holds across different x values.

Optical bandedge in low-dilution GaMnAs appears near the host band bottom, not impurity band.

The apparent ferromagnetic shift is due to spin-dependent carrier state shifts, not actual ferromagnetic exchange.

Abstract

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 $A^{\rm II}_{1-x}$Mn$_{x}B^{\rm VI}$-type DMS, the present study reveals that the linear relationship between exchange-spitting $\mit\Delta E_{ex} $ and the averaged magnetization $|x < S_z >|$ widely holds for different values of $x$. The ratio, $\mit\Delta E_{ex}/x < S_z>$, however, depends not only the exchange strength but also the band offset. Furthermore, the present study reveals that in the low dilution of Ga$_{1-x}$Mn$_x$As 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 Ga$_{1-x}$Mn$_x$As.