Exciton spin thermalization in strained and relaxed Zn1-xMnxSe epilayers

We use polarized time-resolved photoluminescence spectroscopy to study the thermalization of nonequilibrium spin populations of excitons in Zn1-xMnxSe epilayers. A comparison of results from strained and relaxed epilayers shows that the exciton spin-relaxation rate is strongly affected by the degeneracy of the hole bands in these materials. Through rate equation analysis, we find that in relaxed Zn1-xMnxSe epilayers (with degenerate light- and heavy-hole bands) the spin-relaxation rate is rapid, less than 5 ps. Thus, the spin populations are determined solely by the thermal temperature of the excitons within their parabolic bands. In contrast, similar measurements in strained epilayers where the light- and heavy-hole degeneracy is lifted show that the exciton spin-relaxation rate is nearly three orders-of-magnitude slower, with a spin-relaxation time greater than 1 ns. We suggest that the strong decoupling of the exciton spin states from the lattice is caused by exciton localization in these epilayers. [S0163-1829(99)01811-1].