雷射干涉重力波天文台(Laser Interferometer Gravitational-Wave Observatory,LIGO)，主要目的為偵測來自宇宙中天體運行的重力波訊號，然而重力波訊號非常的微弱，容易受到雜訊干擾，因此降低系統雜訊為重點目標。重力波偵測儀以麥克森干涉儀為架構，當中40Hz~400Hz為系統總雜訊最小的區間，此區間主要的雜訊來自於，高反射鏡堆疊材料之熱雜訊(Coating Brownian noise)。相關論文中指出材料熱雜訊與機械損耗為正比，因此本實驗室致力於研究高品質光學薄膜，並要求薄膜同時兼顧良好的光學性質與機械損耗。
本研究的重心為找到降低氮化矽、氮氧化矽光學吸收的方法，為了明白氮化矽、氮氧化矽光學吸收的成因，筆者從非晶材料吸收光譜開始探討光學吸收，並將吸收光譜分成三個區段，光子能量由高到低排序，分別為Band-to-band absorption、Urbach absorption、Defect absorption，解釋每個區段在能帶圖(Band diagram)中，相應吸收光子的機制。同時了解材料中的懸鍵(Dangling bond)，會在能隙(Energy gap)中產生缺陷能態(Defect State)，進而造成低能量的光學吸收。
本論文透過整理實驗數據，歸納出光學吸收經驗公式，藉由經驗公式得到光學吸收、矽懸鍵與能隙，三個參數的趨勢關係，從中得知若薄膜矽懸鍵濃度大於〖10〗^18 〖cm〗^(-3)，減少矽懸鍵濃度即為降低光學吸收最好的方法，反之矽懸鍵濃度小於〖10〗^18 〖cm〗^(-3)，降低光學吸收的方法則是設法提高能隙大小。另外，經驗公式的模擬結果透露，降低光學吸收的方法，除了減少矽懸鍵濃度、拉大能隙之外，降低Urbach absorption也能達到降低光學吸收的目標。

The purpose of Laser Interferometer Gravitational-Wave Observatory(LIGO) is to detect the gravitational waves signals from celestial bodies in the universe and this system is based on Michelson interferometer. However, the gravitational waves signals are very weak and are easily disturbed by noise, so reducing system noise is the key target for our study. One of the dominant noise is thermal noise, which comes from the stacked material of the high reflector. Some papers pointed out that thermal noise of materials is proportional to mechanical loss. Therefore, our laboratory is dedicated to researching high-quality optical films with low mechanical loss and low optical absorption.
The focus of this study is on reducing the optical absorption of silicon nitride and silicon oxynitride. In order to understand the mechanism of the optical absorption, the author first discusses the three principal regions of optical absorption in an amorphous semiconductor, dividing it into three principal regions when arranged in photon energy from high to low, which are Band-to-band absorption, Urbach absorption, and Defect absorption respectively and explains the mechanism how each region in Band diagram absorbs the photon. Meanwhile, understanding that the dangling bond in the material will generate defect states in the energy gap, which will cause low-energy Defect absorption.
In this thesis, the empirical equation of optical absorption is obtained by analyzed experimental data. The empirical equation is used to comprehend the relationship among three parameters – optical absorption, silicon dangling bonds and energy gap, so as to know that if the concentration of silicon dangling bonds is higher than 〖10〗^18 〖cm〗^(-3), decreasing the concentration of silicon dangling bonds is the best way to reduce optical absorption. On the contrary, if the concentration of silicon dangling bonds is less than 〖10〗^18 〖cm〗^(-3), widening the energy gap is the best way to reduce optical absorption. In addition, the simulation results of the empirical equation revealed that apart from decreasing the concentration of silicon dangling bonds and widening the energy gap, lowering the Urbach absorption can also achieve the purpose of reducing optical absorption.

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