Application of Nano-photonics Technology to the Development of Light-emitting Diode

為應用表面電漿子與量子井耦合以提升發光二極體效率，我們首先實現在氮化銦鎵/氮化鎵量子井結構上，使用熱退火技術來製作不同尺寸的銀奈米結構，經由表面電漿子與量子井的耦合，光致螢光頻譜的峰值位置和強度會隨著不同尺寸的銀奈米結構而變化。經由將銀薄膜轉變成奈米島狀型的結構，我們不但可以增強光致螢光的強度，也能調整表面電漿子的色散曲線，使得量子井的發光頻譜具有明顯的紅移現象。此外，我們探討在氮化銦鎵/氮化鎵的量子井與表面電漿子的耦合效應中，表面電漿子經由p-型或n-型氮化鎵層的歐姆接點之耗損。研究顯示當歐姆接點形成於摻雜的半導體層之上，光致螢光強度明顯減弱，相反地，如於摻雜的半導體層和歐姆接點接之間成長一絕緣層，光致螢光強度會顯著地提升。應用表面電漿子與量子井的耦合效應於氮化銦鎵/氮化鎵的量子井發光二極體上，我們於藍光之單層氮化銦鎵/氮化鎵的量子井發光二極體的p-型氮化鎵層上製作能產生表面電漿子的結構，並且研究表面電漿子與量子井的耦合效應。我們製作一個具有較強的表面電漿子與量子井的耦合並有較低的表面電漿子能量耗損的發光二極體，證實此發光二極體相對於較低的表面電漿子與量子井的耦合或較強的表面電漿子能量耗損的發光二極體，能增強電致螢光強度。同時，在綠光之單層氮化銦鎵/氮化鎵的量子井發光二極體的p-型氮化鎵層上製作銀奈米結構，此奈米結構可產生侷限性表面電漿子，透過量子井與侷限性表面電漿子的耦合，我們證實綠光之單層氮化銦鎵/氮化鎵的量子井發光二極體的發光效率顯著的增強。經由控制銀薄膜的厚度和熱退火條件，可以產生適當的銀奈米結構，此奈米結構產生的侷限性表面電漿子的能量與發光二極體的能隙一致。當發光二極體操作於20毫安培時，我們觀察到電致螢光的峰值強度增強至2.5倍，而整體的發光強度增強至2.2倍。外，我們製作藍紅光之多色彩發光元件，係將發紅光的硒化鎘/硫化鋅奈米晶體塗佈於發藍光的氮化銦鎵/氮化鎵的多層量子井發光二極體上。為改善紅光和藍光的強度對比，製作不同孔徑大小的微米級孔洞於發光二極體上，以增加多層量子井的主動層和硒化鎘/硫化鋅奈米晶體直接接觸的面積。同時，我們也製作白光之發光元件，係將硒化鎘/硫化鋅奈米晶體塗佈於發藍綠雙波長的氮化銦鎵/氮化鎵量子井發光二極體上，藉由吸收/再放光之機制，部份的藍光和綠光轉換成紅光，剩餘的藍光、綠光與紅光組合成白光。同時，將金奈米粒子摻入硒化鎘/硫化鋅奈米晶體內，使其產生侷限性表面電漿子，此侷限性表面電漿子可吸收綠光，而且有效率地轉換能量至硒化鎘/硫化鋅奈米晶體，增強奈米晶體的吸收效果，進而增強紅光發光強度。經由侷限性表面電漿子耦合之機制，從藍綠光轉換成紅光的轉換效率增加30%，而量子轉換效率可以達到52.8%。In this dissertation, we first demonstrate the variations of the photoluminescence (PL) spectral peak position and intensity through the surface plasmon (SP) coupling with an InGaN/GaN quantum-well (QW) by forming Ag nanostructures of different scale sizes on the QW structure with thermal annealing. By transferring an Ag thin film into a nano-island structure, we can not only enhance the PL intensity, but also adjust the SP dispersion relation and hence red-shift the effective QW emission wavelength. Also, the leakage of SP through the Ohmic contact of either p-type or n-type GaN layer in the coupling process between SP and an InGaN/GaN QW is studied. It is shown that the PL intensity is significantly reduced when an Ohmic contact is formed, in contrast to the case of significant PL enhancement when an insulating thin layer is applied between the doped semiconductor and metal. or practical application of the QW-SP coupling, we study the coupling effects between the QW and SP generated nearby on the p-type side in a blue InGaN/GaN single-QW LED. The QW-SP coupling leads to the enhancement of the electroluminescence (EL) intensity in the LED sample designed for QW-SP coupling and reduced SP energy leakage, when compared to an LED sample of weak QW-SP coupling or significant SP energy loss. Meanwhile, the output enhancement of a green InGaN/GaN QW LED through the coupling of QW with localized surface plasmons (LSPs), which are generated on Ag nanostructures on the top of the device, is also demonstrated. The suitable Ag nanostructures for generating LSPs of resonance energies around the LED wavelength are formed by controlling the Ag deposition thickness and the post-thermal-annealing condition. With a 20 mA current injected onto the LED, enhancements of up to 150 % in electroluminescence peak intensity and of 120 % in integrated intensity are observed. esides, blue-red polychromatic light-emitting devices are fabricated by attaching red-emitting CdSe/ZnS nano-crystals (NCs) on a blue-emitting InGaN/GaN multiple-quantum-well (MQW) structure. To improve the red/blue intensity contrast, holes of different diameters are fabricated for increasing the direct contact area between the MQW active regions and CdSe/ZnS NCs. In addition, we demonstrate the implementation of a white-light device by spin-coating CdSe/ZnS NCs on the top of a blue/green two-color InGaN/GaN QW LED for converting blue and green emissions into red light through the absorption/reemission process. Meanwhile, Au nano-particles (NPs) are mixed with CdSe/ZnS NCs for generating LSP modes to couple with the CdSe/ZnS NCs. The LSP modes can absorb green emission and effectively transfer the energy into the CdSe/ZnS NCs through the coupling process for enhancing red emission. With the LSP coupling process, the conversion efficiency from the blue/green range into red light can be increased by around 30 %. The conversion quantum efficiency can reach 52.8 %.Contents文摘要…………………………………………………ibstract…………………………………………………ivontents…………………………………………………viihapter 1ntroduction.1 Applications of Wide Bandgap Nitride Semiconductors…………………………………………1.2 Surface Plasmons…………………………………4.3 CdSe/ZnS Nano-Crystals…………………………12.4 Research Motivations……………………………15.5 Thesis Organization……………...……………20eferences………………………………………………21hapter 2oupling between Surface Plasmon and Quantum Well in an InGaN/GaN Quantum-well through Metal/semiconductor Interface Light-emitting Diode .1 Annealing-induced Metal Nano-structures for Surface Plasmon Coupling…………………………………………………40 .2 Surface Plasmon Leakage Effect…………………………53.3 Surface Plasmon-Coupled Blue Light-emitting Diode…64.4 Surface Plasmon-Coupled Green Light-emitting Diode…73eferences……………………………………………………………82hapter 3se of CdSe/ZnS Nanocrystals for Color Conversion in an InGaN/GaN Quantum-well Light-emitting Diode.1Control of Color Contrast of Blue/red Lights…………128.2White-light Device Based on Surface Plasmon-enhanced CdSe/ZnS Nanocrystal Wavelength Conversion on a Blue/green Two-color Light-emitting Diode…………………………………135eferences……………………………………………………………147hapter 4onclusions…………………………………………………………171 ublication List…………………………………………………17