Exciton spin relaxation in znmnse-based diulute magnetic semiconductor heterostructures

by Hodges, Alex Randall.

Abstract (Summary)
We use time-resolved magneto-photoluminescence spectroscopy to study spin relaxation of excitons in a series of strained and unstrained ZnMnSe-based heterostructures. In an unstrained Zn0.98Mn0.02Se epilayer, we find rapid spin relaxation, with a spin relaxation time of less than five picoseconds. We attribute this rapid spin relaxation to the complicated band structure: the various exciton spin bands intersect each other numerous times. The excitons can freely scatter between the spin bands, resulting in rapid spin relaxation. In contrast, we find extremely slow spin relaxation in a strained Zn0.98Mn0.02Se/Zn0.98Fe0.01Se multiple quantum well, with a spin relaxation time of greater than one nanosecond. Once excitons cool to the bottom of the band, very little spin relaxation occurs, and an extremely non-thermal exciton spin distribution persists throughout the lifetime of the exciton. In addition, we show that the dominant spin relaxation mechanism in this structure is LO-phonon emission during the momentum relaxation process, which occurs within 1 ps of the exciting laser pulse. We find similar results in two additional strained structures. For a strained Zn0.98Mn0.02Se epilayer and a strained Zn0.98Mn0.02Se/ZnSe multiple quantum well, we also see very slow spin relaxation, with spin relaxation times of greater than 1 ns. We conclude that this effect is due to the removal of the light hole - heavy hole valence band degeneracy by the lattice strain. This eliminates the band-mixing effects that lead to rapid spin relaxation in unstrained ZnMnSe-based heterostructures, thus resulting in extremely slow spin relaxation. We also find that the addition of a small fraction of cadmium to a strained quantum well strongly increases the spin relaxation rate. In a strained Zn0.933Cd0.036Mn0.031Se/ZnSe quantum well, we see rapid spin relaxation. The spin relaxation time is less than 5 ps, similar to that of the unstrained Zn0.98Mn0.02Se epilayer. However, as in the other strained structures, the excitons spins never fully thermalize with the lattice. The spin relaxation, while initially rapid, is incomplete. In memory of my father, Richard Gordon Hodges 1944 – 1998.
Bibliographical Information:


School:University of Cincinnati

School Location:USA - Ohio

Source Type:Master's Thesis



Date of Publication:

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