We use photoluminescence spectroscopy to investigate the optical properties of two different II-VI semiconductor nanostructure systems: ZnMnSe/ZnSe multiple quantum wells and self-assembled CdSe quantum dots. The behavior of excitons in ZnMnSe quantum wells is examined using polarized magneto-photoluminescence, while (temperature-dependent) time-resolved photoluminescence is used to study the dynamics of excitons confined to quantum dot structures.
When Zn0.86Mn0.14Se/ZnSe multiple quantum wells are placed in an external magnetic fields, the spin-down holes become confined to the ZnMnSe “barriers”, while the electrons remain in the ZnSe “wells”. This spatial separation of electrons and holes results in weak electron-hole overlap, which should result in a large increase of the radiative lifetime of the spin-down exciton. As a result, we see the formation of exciton magnetic polarons (EMP) which lower their energy by spontaneously aligning the magnetic impurities in the exciton Bohr radius. We find that the EMP polarization approaches 100% in fields as small as 200 mT in these spatially indirect EMP, which is consistent with their extremely long recombination lifetime (~10 ns). This contrasts with previous measurements on other magnetic quantum well systems for which the polarization of short-lived spatially direct EMP never saturates.
Time-resolved photoluminescence measurements of excitons confined to CdSe self-assembled quantum dots (SAQDs) reveal the existence of two different exciton decay times: a short 450 ps lifetime and a much longer (> 4 ns) lifetime. While the emission resulting from the short lifetime excitons persists to room temperatures, the long lifetime component decreases in intensity with increasing temperature and is nearly completely gone by 60 K. Time-resolved spectra further reveal that the long lifetime component arises from spectrally sharp features (~200 μeV), while the rapid decay results from an underlying broad emission (~60 meV). Further analysis reveals that both states exhibit an activated behavior with activation energies that differ by over an order of magnitude for the two components. These interesting differences may reflect the complex nature of the electron and hole wave functions confined to these CdSe structures.
|School:||University of Cincinnati|
|School Location:||United States -- Ohio|
|Source:||DAI-B 79/10(E), Dissertation Abstracts International|
|Keywords:||Cdse quantum dots, Diluted magnetic semiconductors, Exciton magnetic polaron (emp), Nanostructure, Time-resolved photoluminescence|
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