Proceedings of nanoGe Fall Meeting19 (NFM19)
DOI: https://doi.org/10.29363/nanoge.nfm.2019.332
Publication date: 18th July 2019
In recent years, quantum dots made of III-V semiconductors have attracted increasing interest as a non-restricted alternative to the more prevailing CdSe-based QDs. In this presentation, we discuss the properties of the bright exciton in InP QDs using a combination of ensemble and single dot studies.
Using cryogenic photoluminescence microscopy, we first show that the emission features of single InP QDs exhibit a transition from single exciton to trion and to biexciton emission with increasing excitation power, not unlike CdSe-based QDs. Following this confirmation that emission involves exciton, we analyze the bright exciton emission in more detail using fluorescence line narrowing spectroscopy in magnetic fields and four-wave mixing spectroscopy. Most importantly, we find that the bright exciton line splits into 3 different sub-levels with increasing magnetic field strength, whereas the exciton dephasing rate increases strongly with increasing temperatures, even at temperatures of 4.2 K.
While a 3-fold degenerate bright exciton is expected for spherically symmetrical nanocrystals of zinc blende semiconductors, it was found that a slight shape anisotropy sufficed to fully split this isotropic exciton in the case of CdSe QDs. We argue that the particular light to heavy hole mass ratio in InP QDs may render the bright exciton in such QDs insensitive to shape anisotropy, which makes that the bright exciton in InP QDs is effectively isotropic. Not only does this interpretation account for the fluorescence line narrowing spectroscopy, it also explains why thermalization across different exciton levels still determines the bright exciton dephasing at cryogenic temperatures.
The observation of nearly isotropic excitons in InP QDs is of fundamental and practical interest, making such QDs ideal model system to study the size-dependence of exchange interaction or providing single photon emitters with orientation-independent properties.
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