反射異向性光譜是一種非破壞性的光學量測技術，可量測半導體表面特性，實驗所得到的光譜圖在臨界點的位置會有某些特殊變化，其強度大小與形狀都與樣品的載子濃度以及表面結構有關，如果將入射光的能量固定在樣品材料的臨界點位置，則可用RAS監控分子束磊晶（MBE）的生長情況，近年亦常用於金屬表面的量測。
烏采（Wurtzite）結構氧化鋅是我們量測的樣品，此樣品結構的m-plane在垂直c軸跟平行c軸兩個方向具有不對稱性，且對於兩種偏振光有不同的躍遷機制，因此可利用線性偏振光入射樣品表面，即便探測光的貫穿深度（penetration depth）雖有一百至一千奈米，不論從表面或樣品內部反射均會得到反射差異訊號，經分析後可了解樣品特性。
以往RAS利用光彈調變器（photo-elastic modulator, PEM）調變反射光，因PEM中熔凝石英的x、y方向折射率不同，故光通過後x、y方向會有相位延遲，橢圓偏振光由此產生，為了改善偏振方向切換有過渡期的缺點，我們以扭轉向列型液晶加偏壓搭配偏振片使用，用以調控x、y偏振方向的切換，而PEM只能固定50 kHz或100 kHz頻率上的限制也可迎刃而解，並且液晶相較PEM也便宜許多，使得實驗進行愈加便利。

Semiconductors of （1-100） face ZnO with Wurtzite structure aren’t symmetric in two axes which are perpendicular or parallel to c-axis. This provides an optical technique to measure properties of ZnO sample. Reflectance anisotropy spectroscopy (RAS) is an important optical technique to determine surface properties of semiconductors. Although optical penetration depths are often in the range of several hundred nanometers, we can still observe difference signals. That’s because our ZnO sample is a bulk. A photo-elastic modulator (PEM) was often used to change polarization-state of incident light to measure reflectance difference of polarization between (E⊥c) and (E∥c) these two directions. There are some features around critical points and their amplitudes or/and shapes are dependent on carrier concentration of the sample or/and structure of the surface. If photon-energy of the incident light is fixed at critical point energy, it can be used to in-situ monitor the growth of GaAs by molecular beam-epitaxy.
However, the PEM can be operated only at a fixed frequency (50 kHz or 100 kHz) and if the state of polarization is set to change from x to y linear polarization, there are elliptical polarizations lying between them. In this work, a method using TN liquid crystals (LCs) to change the direction of polarization of the incident light was used. A linear-polarized light will be rotated by 90 degrees by passing through TN LCs. Nevertheless, the polarized direction of the incident light will not be changed if there are enough voltages applied on LCs because they are aligned perpendicular to front and back electrodes. Using a synthesized signal generator, the direction of the incident light can be changed between x and y directions and there are almost no elliptical polarizations lying between them.

論文審定書 i
摘要 ii
Abstract iii
第一章 前言 1
第二章 液晶簡介 2
2.1 液晶分子排列 2
2.2 液晶特性 4
2.3 TN液晶配向 8
第三章 反射異向性光譜 9
3.1反射異向性光譜簡述 9
3.2電磁波反射 9
3.3 Epioptics 12
第四章 樣品特性 15
4.1水熱法簡介 15
4.2氧化鋅(ZnO)樣品簡介 15
4.3氧化鋅(ZnO)結構與能帶關係 17
第五章 實驗設計 18
5.1 實驗配置比較 18
5.2 液晶調控 19
5.3 實驗結果與分析 26
第六章 結論 28
參考文獻 29
附錄 31

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