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  • 學位論文

表面電漿子與氮化銦鎵/氮化鎵量子井耦合研究

Study on Surface Plasmon Coupling with InGaN/GaN Quantum Wells

指導教授 : 楊志忠

摘要


在本論文中,我們首先研究在有富銦奈米顆粒結構的氮化銦鎵/氮化鎵量子井中,載子侷限效應對螢光頻譜衰減時間的影響。我們利用時間解析光激螢光頻譜和蒙地卡羅模擬載子的躍遷及結合放光來研究在氮化銦鎵中的載子動態。在實驗中,我們使用兩片不同矽摻雜條件的樣品做為兩種不同奈米顆粒密度的情形。在高奈米顆粒密度的樣品中,我們發現其螢光衰減時間在高能量處有一上升的趨勢;而在低奈米顆粒密度的樣品中,則無此一現象。此差異和模擬的結果相吻合,有助於我們了解此一上升趨勢來自於在自由能階中的載子侷限效應。   接著我們研究表面電漿子和氮化銦鎵/氮化鎵量子井耦合的溫變特性。載子要耦合成表面電漿子須具備足夠的能量和動量。在低溫時,因載子被侷限在位能低處,此時量子井和表面電漿子的耦合較弱。隨著溫度上升,有越來越多的載子可以脫離位能低處,耦合強度也越來越強。當溫度接近室溫時,此時因為大部分的載子均已脫離位能低處,耦合強度呈一飽和狀態。此三段溫度範圍可利用螢光強度增強的比例和時間解析螢光頻譜區分出來;其範圍和量子井中載子隨溫變的侷限過程相同。   我們亦觀察到利用在量子井上方的氮化矽和銀膜產生的表面電漿電磁耦子和侷域表面電漿子來提升螢光激發光譜。在對應到侷域表面電漿子的波段,入射光可先被侷域表面電漿子吸收,然後將能量藉由消散波傳給量子井。當載子鬆弛至螢光能階時,表面電漿電磁耦子和載子的耦合則可增強發光。   最後,我們藉由在氮化鎵和銀膜中加入一層低折射率的二氧化矽來增強和表面電漿子耦合的量子井發光強度。此二氧化矽層可降低表面電漿子的損耗、增長消散波的長度,其代價為降低表面電漿子的能階密度。綜合這些效應後,我們仍可進一步透過表面電漿波來提升量子井的發光。對發光二極體而言,延伸消波的長度可使得p型導電層的厚度不再受限。更重要的是降低表面電漿子的損耗,可使得整體的發光效率提升。

並列摘要


In this dissertation, we first demonstrate the carrier trapping effects on photoluminescence (PL) decay time in InGaN/GaN quantum wells (QWs) with nano-cluster structures. Carrier dynamics in InGaN/GaN QWs with compositional fluctuations is studied with time-resolved PL experiment and Monte Carlo simulations of exciton hopping and recombination. In particular, the effects of indium-rich nano-clusters in such a QW structure on the photon-energy dependent PL decay time are investigated. In our experiments, two InGaN/GaN QW samples of different silicon doping conditions are used for demonstrating the two cases of different nano-cluster densities. An increasing trend of PL decay time on the high-energy side of the PL spectrum is observed in the sample of high nano-cluster density. Such a trend is not observed in another sample with few clusters. This difference is consistent with the simulation results which can help us in identifying the origin of the increasing trend as the exciton trapping by the local potential minima in the spectral range of the free-carrier states.   Then, we study the temperature-dependent behavior of the surface plasmon (SP) coupling with an InGaN/GaN QW. The SP coupling efficiency relies on the availability of carriers with sufficient momentum for transferring the energy and momentum into the SP modes. At low temperatures, the carriers are trapped by the potential minima in the QW and the SP coupling is weak. As temperature increases, more and more carriers escape from the potential minima leading to the stronger and stronger SP coupling. When the temperature is close to the room condition, the SP coupling strength saturates because most carriers have escaped from the potential minima. The three temperature ranges of different SP coupling behaviors can be clearly identified from the data of PL enhancement ratio and PL intensity decay rate, and are consistent with those of temperature dependent PL feature variation, which clearly indicates the carrier localization process.   Next, we report the observation of the enhancement of photoluminescence excitation through the couplings of an InGaN/GaN QW with localized surface plasmons (LSPs) and surface plasmon polaritons (SPPs), which are generated on an Ag nanostructure deposited on the SiN-coated QW epitaxial sample. At the wavelengths corresponding to the LSP modes, the excitation light is first absorbed by the LSPs. The LSP energy is then transferred into the QW through the absorption of the LSP evanescent fields by the carriers. After the carriers are relaxed down to the PL emission levels, the coupling of the carriers with the SPPs enhances light emission. The coupling of the SPPs with the relaxed carriers becomes stronger as temperature increases because of the increased carrier momentum.   Finally, the improved emission enhancement in SPP coupling with an InGaN/GaN QW by inserting a SiO2 layer of lower refractive index between the deposited Ag and GaN layers is experimentally and numerically demonstrated. The inserted SiO2 layer leads to reduced SPP dissipation rate, increased evanescent field intensity beyond a certain depth in GaN, and decreased SPP density of state. The combination of these factors can result in further emission enhancement of QW through SPP coupling. For light-emitting diode application, the elongated evanescent field coverage can release the constraint of thin p-type GaN for effective SPP coupling. More importantly, the reduced SPP dissipation can result in more effective emission in such an SPP-QW coupling mechanism.

並列關鍵字

InGaN Surface plasmon

參考文獻


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