Studies of Ultrafast Carrier Dynamics in ZnO thin film and Nanostructures
Date Issued
2006
Date
2006
Author(s)
Sun, Shih-Ze
DOI
en-US
Abstract
ZnO-related material has got more and more attentions because of its excellent material properties in device application. In recent years, more and more nanostructure ZnO have been synthesized. However, ultrafast carrier dynamics in nanostructure ZnO even free carrier dynamics in bulk ZnO have been less investigated. To develop ZnO-based devices, it is essential to know the physical mechanisms
For this reason, in the thesis, exciton and above-band-gap free carrier dynamics in ZnO nanorods and ZnO thin films have been investigated at room temperature with a femtosecond transient transmission measurement. Following the photoexcitation of above-band-gap free carriers, an extremely fast external thermalization time on the order of 200 fs can be observed. Under high excitation, hot phonon effects were found to delay the carrier cooling process. While the photoexcitation energy was tuned to match the bound exciton transition in ZnO nanorods, stable exciton formation can be uncovered while no evident exciton ionization process can be found unless the photoexcited exciton density exceeded the Mott density. Whereas, while the photoexcitation energy was tuned to match the free exciton transition in ZnO thin films, a weak exciton ionization process with an exciton ionization time of 800 fs can still be found even photoexcited exciton density below the Mott density.
In addition to the investigation carrier dynamics in ZnO, we have also utilized femtosecond reflectivity pump-probe technique to measure sound velocity in ZnO thin films. However, the measured sound speed on the order of 4000 m/s appear much slower than the previous reported value on the order of 6000 m/s. Some possible reasons were explore according to the analysis of experimental results. On the other hand, some interesting phenomena observed prompted us to discuss extensively.
Subjects
氧化鋅
奈米柱
載子動力學
激子解離
熱化
熱聲子效應
znic oxide
nanorods
carrier dynamics
exciton ionization
thermalization
hot phonon effect
Type
thesis