Ultrafast Broadband Second Harmonic Generation and Its Application to the Pump-probe Spectroscopy for the Study of Carrier Dynamics in InGaN Nano-structures

• Main Authors: Hsiang-Chen Wang, 王祥辰
• Other Authors: 楊志忠
• Format: Others
• Language: en_US
• Published: 2006
• Online Access: http://ndltd.ncl.edu.tw/handle/76695227733945645123

博士 === 國立臺灣大學 === 光電工程學研究所 === 94 === In this research, we systematically study ultrafast carrier dynamics in the InGaN compounds by using the degenerate and non-degenerate pump-probe techniques. With the information of ultrafast carrier dynamics, we can understand the nano-structures of InGaN compounds, particularly the connections between the optical characteristics and material nano-structures. Also, we can understand the carrier flow scenarios (in the spectral and spatial domains) that can help us in evaluating the photon emission efficiency. In particular, we can compare the carrier dynamics between the localized states and free-carrier states that can provide us with the clues of the advantages of using clusters as photon emission centers. We use two kinds of sample in our studying, including an InGaN/GaN multiple-quantum-well sample, and an InGaN thin-film sample. In both samples, indium-rich nano-cluster distribute on the backgrounds of large-scale indium composition fluctuations. In the multiple-quantum-well sample, combining with the time-resolved photoluminescence results, we can identify three stages of carrier relaxation. The fast-decay time, ranging from several hundred fs to one ps, corresponds to the process reaching a local quasi-equilibrium condition, in which carriers reach a thermal distribution within one or a few nearby indium-rich clusters. The slow-decay time, ranging from tens to a couple hundred ps, corresponds to the process reaching a global quasi-equilibrium condition, in which carriers reach a thermal distribution among different clusters of various potential minima. In this stage, the mechanism of carrier transport over barriers between clusters dominates the relaxation process. Finally, carrier recombination dominates the relaxation process with the carrier lifetime in the range of a few ns. In the thin-film sample, the observed temperature-, pump-photon-energy-, and pump-intensity-dependent variations of ultrafast carrier dynamics manifest the variation of the space-averaged density of state with energy level in this sample. The carrier dynamics is controlled by the shift of effective bandgap and hence the behavior of band filling, which are determined by the combined effect of bandgap renormalization and phonon effect (bandgap shrinkage with increasing temperature). Two-photon absorption and free-carrier absorption can be observed when the corresponding density of state is low and hence the band-filling effect is weak. The variation of the space-averaged density of state with energy level can be due to the existence of indium-composition-fluctuation nanostructures, which is caused by the spinodal decomposition process, in this sample. To implement the non-degenerate pump-probe experiment, we use two