https://scholars.lib.ntu.edu.tw/handle/123456789/120296
標題: | 超快寬頻倍頻技術及其應用於氮化銦鎵奈米結構內載子動態之研究 Ultrafast Broadband Second Harmonic Generation and Its Application to the Pump-probe Spectroscopy for the Study of Carrier Dynamics in InGaN Nano-structures |
作者: | 王祥辰 Wang, Hsiang-Chen |
關鍵字: | 超快載子動態;氮化銦鎵;銦聚集;非簡併激發-探測;ultrsfast carrier dynamics;InGaN;In cluster;Non-degenerate pump-probe | 公開日期: | 2006 | 摘要: | 在本論文中,我們有系統的研究氮化銦鎵奈米結構材料中的超快載子動力現象,所應用的研究方法包含簡併與非簡併的激發-探測技術,透過這些方法我們可以瞭解試片的光學特性與材料特性的關聯性,進一步幫助我們瞭解載子在半導體奈米結構中流動的過程與機制,使我們更清楚的理解發光效率與材料特性的關聯性,進而改善並提升材料的發光效率。特別是我們的試片具有侷限聚集能階與自由載子能階,透過這些研究,我們可以瞭解如何利用銦聚集作為提升氮化銦鎵奈米結構材料的發光效率。 我們共探討兩種氮化銦鎵試片:一為多層量子井結構;另一為薄膜結構。在量子井的實驗中,我們結合了時間解析光致螢光的結果,可以將載子鬆弛的過程分成三個階段,最快的衰減時間大概在幾百個飛秒到一個皮秒左右,為區域近似平衡階段,在這段時間內載子在一個或數個銦聚集內達到熱平衡。衰減時間範圍從十幾個皮秒到幾百個皮秒是屬於第二階段的衰減,在這階段裡載子在各個最小位能的聚集內達到熱平衡,載子鬆弛的機制在於載子越過位能障於聚集間移動。最後的衰減階段為電子與電洞的結合放光,其載子的生命期為幾個奈秒。在薄膜試片的實驗結果裡,利用變化溫度、變化激發的能量與強度的激發-探測實驗,可以觀察出在不同能隙上之載子密度的相對變化。薄膜試片的載子鬆弛機制控制於有效能隙的變化與能帶填滿的行為,而主要的貢獻來自於能階正規化和聲子現象的作用(包括隨著溫度上升的能階縮小現象),當載子的密度很低,能帶填滿的貢獻較低時,我們可以觀察到雙光子吸收與自由載子吸收。這種奈米結構上隨著不同的能帶,空間平均之載子密度變化來自於銦成分濃度的不均勻分布,最主要是隱晶分解(spinodal decomposition)的現象所致。 在非簡併的實驗裡,我們利用了兩組BBO晶體及七個飛秒的超快雷射,發展出可任意變化探測與激發波長的非簡併架構,我們使用氮化銦鎵薄膜試片來測試這種技術,而此試片已證實有奈米銦聚集。利用這個技術,可以觀察出載子從自由載子能階被捕獲至侷限聚集能階所需的時間,當激發與探測的能量都落在自由載子能階上時,其訊號上升的時間約為300飛秒,當激發的能量落在自由載子能帶上且激發與探測的能量都落在侷限聚集能階上時,其上升的時間約為590-715飛秒。 最後,我門開發出一種應用超寬頻倍頻技術於非簡併的激發與探測技術裡,試片同樣是氮化銦鎵薄膜,為了瞭解不同波長之間的時間差(色散),我們利用不同的激發波長在不同平移台位置上找到干涉的方法求得色散關係,透過這種非簡併超寬頻倍頻的探測技術,可以幫助我們進一步觀察瞭解超快載子動力現象。 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 |
URI: | http://ntur.lib.ntu.edu.tw//handle/246246/50868 | 其他識別: | en-US |
顯示於: | 光電工程學研究所 |
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ntu-95-D90941002-1.pdf | 23.31 kB | Adobe PDF | 檢視/開啟 |
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