液態反射鏡與碳化矽基材在高靈敏度感測元件及高功率發光二極體應用之研究

Abstract

在本論文中，首先第一部分將針對利用化學還原法合成出液態面鏡（liquid mirror），改善並提升其反射率。第二部分利用此液態面鏡做為一個很好的基材，有效的大幅增加表面增強拉曼散射效果。第三部分探討利用碳化矽基板取代藍寶石基材之發光二極體與傳統藍寶石基板發光二極體兩者出光行為差異與最佳化光取出效率，並評估碳化矽基發光二極體的優缺點。 在第一部分論文中，對於目前應用於天文望遠鏡中的液態面鏡提升其短波段反射率。首先利用硝酸銀與檸檬酸鈉還原出銀粒子懸浮液，再利用配位基（ligand）製備出液態面鏡，並搭配不同合成過程添加還原劑的含量與反應溫度下可以得到不同尺寸大小的銀粒子。利用不同大小的銀粒子混合之液態面鏡可改善其反射率值。相較於目前文獻中液態面鏡在可見光波段400~1000nm平均反射率值最高約為69%、較短波段400~550nm平均反射率約為56%，由本研究發現利用大小顆銀粒子混合，小顆銀粒子有效填補到大顆銀粒子間隙而提升液態面鏡表面平整度，有效再提升可見光區域較短波段400~550nm的平均反射率值由56%提升至68%，而整體在400~1000nm平均反射率也提升至77%。同時可由掃描式電子顯微鏡與原子力顯微鏡驗證此液態面鏡表面形貌。第二部份論文中，因此液態面鏡粒子之間彼此緊密堆積聚集造成電場效應增加，有很強的表面拉曼散射增強效果。利用待測分子4-ATP可偵測極限濃度（limit of detection；LOD）最低至1.01x10-17M而待測分子增強因子（Analytical EF；AEF）可達1013。 在論文的第三部份中，主要研究利用碳化矽取代藍寶石為基材之發光二極體（Light emitting diode；LED）光學行為差異與光取出效率之研究。首先針對覆晶型LED的出光行為利用嚴格耦合波分析法（Rigorous Coupled-Wave Analysis；RCWA）模擬做光學模擬的分析，可得到在碳化矽表面製作週期性結構可提升出光表現，與有效的增加臨界角外的穿透強度有關。再利用時域有限差分法（Finite-Difference Time Domain method；FDTD）分析碳化矽基發光二極體與藍寶石基發光二極體兩者間的出光行為差異。因為基材折射率與氮化鎵折射率間的差異，造成兩種不同出光行為：對於藍寶石基發光二極體，大部分的光容易侷限於氮化鎵層且可透過表面週期性結構來破壞波導模態增加光取出效率。反之對於碳化矽基發光二極體而言，光容易侷限於碳化矽層，造成氮化鎵層內部沒有波導模態，無法利用氮化鎵表面製作周期性結構提升光取出效率，因此一次直接出光相對重要。同時也實際利用市售的碳化矽基材量測其光學性質，並評估碳化矽基發光二極體的可行性與優缺點。未來將繼續針提升碳化矽基發光二極體之光取出效率研究並可嘗試實際製作出元件結構應用。

In this thesis, we first fabricated and improved the reflectance of liquid mirror by using chemical reduction method. Second, we used this highly reflectance liquid mirror with surface enhancement Raman scattering (SERS) property to be a highly sensitive Raman sensor. Third, we investigated the different light performance behavior between silicon carbide based light emitting diodes (LEDs) and sapphire based LEDs.and evaluated the optical properties of the SiC based LEDs. In the first part of this thesis, we have improved the reflectance of liquid mirror in the short wavelength regime for astronomical telescope application. We used silver nitrate and sodium citrate to prepare silver suspension, and then used ligand to form a metal-liquid like film at the liquid-liquid interface. By controlling the amount of the reducing agent and the reaction temperature, the particle-size of a liquid mirror can be changed. Comparing to the highest reflectance in previous literature, here we mixed the nanoparticles with large and small sizes to effectively increase the reflectance of liquid mirror in the visible region, The improvement of the reflectance of liquid mirror can be resulted from the surface roughness of the liquid mirror reduced by mixing nanoparticle in different sizes. The results were also verified by measuring SEM and AFM images. In the second part of this thesis, due to the closely packing of these silver nanoparticles caused the huge electric field enhancement; liquid mirror has a very good SERS property. We demonstrated that liquid mirror can be used to detect the trace amount of 4-aminothiophenol in Raman spectrometer, and the lowest detective limit can be down to 1.01x10-17M and the analytical enhancement factor can be up to 1013. In the third part of this thesis, a silicon carbide substrate can be used to replace a sapphire substrate to enhance the LED performance due to its special mechanical and thermal properties. However, the difference optical properties between silicon carbide and sapphire affect the light extraction performance of LEDs. By using rigorous coupled-wave analysis (RCWA) method, we simulated the optical properties of periodic structures fabricated on the silicon carbide surface of flip-chip SiC LEDs. These periodic structures increase the light extraction behavior effectively compared to the flat silicon carbide due to increase the diffracted transmittance intensity. However, the commercial silicon carbide substrate possesses about 30% absorption in the visible region, hence light may be absorbed by silicon carbide material during processing through silicon carbide/air interface and reduced the light extraction efficiency of flip-chip SiC LEDs. Moreover, by using the finite-difference time domain (FDTD) method, we simulated the light behavior between two different substrates based LEDs. From our FDTD results, the light was mainly confined in the gallium nitride (GaN) layer to form waveguide mode for sapphire based LEDs, and this waveguide mode can be extracted by fabricating periodic structures on the GaN surface to enhance the extraction performance.