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

在这个项目中，我们利用皮秒时间分辨光谱研究了激子局域化的动力学以及激子局域化与垂直于超晶格层的激子转移之间的关系。为了实现时间分辨光谱，我们安装了一台包括恒定分数鉴别器和快速定时放大器的时间相关单光子计数装置。利用该光子计数系统和锁模Nd：YAGe激光器，我们获得了50ps的时间分辨率。我们在GaAsAlAs短周期超晶格中进行了时间分辨光致发光实验。我们首次在不同势垒的SPS中发现了电子和空穴在调谐辅助下的复合。研究还发现，SPS的载流子流量与进入量子阱的载流子汇量直接相关，垂直输运强烈依赖于势垒厚度。将势垒厚度从0.9 nm改变到1.8 nm，我们观察到较薄势垒的隧道传输增加，进而形成了微带态。结论：通过对GaAsAlAs短周期超晶格的皮秒时间分辨光谱，我们得到了一些关于载流子和激子的量子限制的重要信息，即由于势垒之间的耦合而形成的布洛赫态的局域性(二维)和垂直输运(三维)。

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.