| Summary: | 碩士 === 國立臺北科技大學 === 光電工程系研究所 === 97 === In thesis, we used femtosecond-laser as the light source to investigated the ultrafast carrier dynamics of ZnO thin films at room temperature by using time-resolved measurement technology. As the photo-excited energy is above band-gap of ZnO, the effect of band-filling and bandgap renormalization will occur simultaneously and compete each other that can be seen in time resolved photo-reflectance change while the carrier is excited from the valence band to the conduction bnad. By decomposition the trace with three components, we obtain the hot carrier cooling about 1.2 ps followed by BGR recovery about 4.8 ps, and a relative long time scale about 100 ps which is spontaneous emission. As the photo-excited energy is located near exciton states, we observed relative short decay time about 1 ps due to exciton thermalization, and a slow recovery time of 50 ps due to free exciton spontaneous emission (SPE). As the photo-excited energy is located in shallow band-tail states, it showed the behavior of phonon-assisted-tunneling and defect absorption. In deep band-tail states, the positive transmission change can be seen by the effect of BF due to absorption saturation of defect level.
|
|---|
