A Study of Two-Dimensional Exciton Localization and Mobility Edge by Ultrafast Time-Resolved Spectroscopy

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In this project, we have studied dynamics of exciton localization and correlation between localization and exciton transfer perpendicular to the superlatticelayers by means of the picosecond time-resolve spectroscopy. We have used GaAs/AlAs short-period superlattices with a an enlarged well (SQW) which is intentionally introduced.In order to perform time-resolved spectroscopy, we have installed a time-correlated single photon counting apparatus including a constant fraction discriminator and a fast timing amplifier. We could obtain 50ps time-resolution using this photon counting system and a mode-locke Nd:YAG laer.We have performed time-resolved photoluminescence experiments in GaAs/AlAs short-period superlattices(SPS). We have found, for the first time, the tunneing-assisted recombination of electrons and holes in different wells of SPS with thin barriers. It is also found that the direct correlation between the carrier flow from SPS and the carrier sink into SQW.Perpendicular transport is strongly dependent on the barrier thickness. Varying the barrier thickness from 0.9nm to 1.8nm, we have observed the increase in the tunneling transfer and then the formation of the miniband state for the thiner barrier.In coclusion, picosecond time-resolved spectroscopy of GaAs/AlAs short-period superlattices gives us some important informations on the quantum confinement of carriers and excitons, i.e. the localization(two-dimensionality) and the perpendicular transport(three-dimensionality) due to the formation of Bloch states by the coupling between wells.