本實驗藉由兩步驟化學氣相沉積方式，成功地在攝氏600度合成二硒化鎢薄膜，於八吋預長二氧化矽的矽晶圓上，相較於傳統的化學氣相沉積，此方法可達到大面積二硒化鎢薄膜生成，其製程包含兩步驟:金屬氧化物薄膜沉積以及硒氧離子交換反應，藉由控制氧化物薄膜的厚度，可精確對二硒化鎢的層數進行調變，目前最少層數可達到單層，單層結構時為直接能隙，具備良好的光電轉換效率，能運用於光電子元件領域。
合成二硒化鎢的部分，我們也以材料分析手段，包含拉曼、X射線光電子能譜儀以及電子顯微鏡，對前驅物種類、硒化時間、製程溫度以及製程氣體比例進行參數優化，進而生成高品質二硒化鎢，判斷方式包含以拉曼特徵峰的半高寬及相對強度去決定結晶性、以X射線光電子能譜儀特徵峰的面積比去探討二硒化鎢轉化程度，以及用電子顯微鏡拍攝二硒化鎢薄膜的表面影像，進而以快速傅立葉轉換方式判斷晶粒大小，達到參數優化之目的。
相較於其它過渡金屬硫化物薄膜製備的研究，目前最大的尺寸為長在四吋晶圓大小的二硫化鉬，相較之下，我們所合成的八吋矽晶圓尺寸大小的二硒化鎢，相當具有突破性，此外，N型過渡金屬硫化物中，MoS2已經達到攝氏500度的製程溫度，而P型過渡金屬硫化物，在降低製程溫度的研究上較少，但在此研究我們成功地在攝氏500度製備出高品質二硒化鎢，期待未來能夠朝更低的製程溫度邁進，能夠在軟性電子元件應用有更大的突破。

In this work, we have successfully demonstrated the fabrication of a few WSe2 monolayers on the 8-inch Si/SiO2 wafer by the two-step CVD methods. Compared to conventional CVD ways, the two-step CVD method used in this work, including metal oxide deposition and chalcogenization, can achieve a large-area synthesis with precise thickness control by tuning the thickness of the as-deposited metal oxide. In addition, the thinnest thickness of the WSe2 film obtained by our method approaches to a single monolayer with some mixed bilayer areas and yet displaying photoluminescence due to the transition to direct band gap with the potential for optoelectronic applications.
For the synthesis of WSe2, different parameters were optimized including oxide deposition and quality, substrate temperature, selenization time, forming gas ratio. For the optimization, the films were characterized under several material analysis, including Raman, XPS and TEM. In Raman spectrum, the relative intensity ratio and the full width at half maxima (FWHM) of characteristic peaks served as a criteria to judge the crystallinity of as-grown WSe2. XPS analysis was used to determine the WSe2 transformation rate. Besides, we made a use of fast Fourier transformation(FFT) to judge the grain size of as-grown WSe2 based on the TEM plane-view image. Then, after optimization we demonstrate large area growth with controlled thickness of high quality WSe2 judged from the material characterization.
In the literature review of TMDCs material, the largest size of TMDCs fabrication is at the scale of 4-inch wafer so far. Here we demonstrate the formations of WSe2 with good uniformity in 8-inch wafer size. Besides, only sulfide based TMDCs such MoS2 has been synthesized at temperature closer to 500℃. However, TMDCs based on selenides are usually growth at higher temperatures. We successfully fabricated the high-quality thin layer WSe2 at temperatures as low as 500℃. We also expected to constantly lower the fabrication temperature of TMDCs in the future, especially for some flexible electronic applications.

Contents
Abstract(Chinese) I
Abstract(English) II
Acknowledgement(Chinese) IV
Contents V
Table caption VI
Figure caption VII
Introduction 1
Chapter 1 The properties of TMDCs material 4
1.1 Composition, crystal phases and electronic structure 4
1.2 Optical and vibrational properties 9
1.3 Application of TMDCs material 13
1.3.1 Field effect transistor 13
1.3.2 Optoelectronic devices 15
1.3.3 Other TMDCs devices application 18
Chapter 2 Fabrication processes of TMDCs material 22
2.1 Exfoliation 22
2.2 Chemical vapor deposition 24
2.3 Laser annealing 26
Chapter3 WSe2 fabrication and characterization 28
3.1 Experiment 28
3.2 Parameters optimization 30
3.2.1 Oxide effect 30
3.2.2 Selenization time effect 40
3.2.3 Substrate temperature effect 41
3.2.4 Forming gas ratio effect 42
Chapter 4 Large scale and thickness control 45
4.1 Large scale 45
4.2 Thickness control 46
4.2.1 Approach to WSe2 monolayer 47
Chapter5 Conclusion 48
Chapter6 Future work 49
Chapter7 Reference 51

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