臺灣博碩士論文加值系統

由於半導體產業不斷的藉由縮小電晶體尺度來提升元件效能，然而日漸縮小的元件在製程技術與元件操作上勢必面臨瓶頸，因此科學家已經開始尋求替代材料並且引進新的元件設計概念。其中僅有原子層厚度的二維層狀半導體材料在近年來備受矚目。過渡金屬二硫族化物(transition metal dichalcogenides)，如二硫化鉬(MoS2)、二硫化鎢(WS2)、二硒化鉬(MoSe2)、二硒化鎢(WSe2)等，其單層晶體結構與石墨烯相似，即是屬於二維層狀半導體材料，圖1-1為此種二為材料的結構，MX2(M為圖中藍綠色原子，代表過渡金屬；而X為黃色原子，代表硫族化物) [15]，以二硫化鉬(MoS2)為例，層與層之間約為0.7 nm。由於單層過渡金屬二硫族化物具有直接能隙[1][7-8]、製作成場效電晶體後具有極佳的電流開關比(on/off current ratio)，而且這種僅有原子層厚度的材料也具有非常特殊的光學特性，因此極具潛力成為下一世代電子與光電產業的重要材料。與石墨烯的研究脈絡類似，早期多半仰賴膠帶法剝離單晶塊材，來取得單層過渡金屬二硫族化物。近年來已經可以利用化學氣相沉積法製備二維材料，可以成長大面積（晶圓尺寸）的單層材料，對於未來開發二維材料在電子元件與邏輯電路的應用是極為重要。二維層狀材料的另一特色是可以自由堆疊不同材料[2]，形成新型的層狀垂直異質結構(vertical heterostructures)[3-5]，其電學、光學及傳輸特性可以因選擇的材料及堆疊形式而改變，成為量身訂作的人造材料，是目前物理及材料界研究的重要領域之一[13-14]。使用太赫茲系統去量測過渡金屬二硫族化物，能夠以非接觸性的方式取得他的介電係數毅可研究載子動力學，而因為此二維材料厚度為奈米尺度，又利用穿透式太赫茲系統量測，圖1-2與圖1-3中，太赫茲訊號穿過樣品與基板的訊號大小差1-2%，樣品穿透率太大，較難取得樣品的資訊，而實驗中系統使用之量測樣品的波長太長，所以我們量測較厚的二維材料-過渡金屬二硫族化物[6]，並且希望利用孔洞做出高頻濾波器的效果並改善量測系統，使得分析更加準確。
而鈣鈦礦(Perovskites)太陽能電池的快速發展，使得它成為光伏材料的明日之星，各界也對此物質有著重大的興趣。經過最近幾年的研究，鈣鈦礦的效率及技術都有著卓越的發展。近年來光電轉換效率更可以高達22 %了。鈣鈦礦是一種礦物，最早是在烏拉爾山脈被發現，並以俄羅斯人Lev Perovski為命名。而真正的鈣鈦礦物是由鈣、鈦、氧形成的〖CaTiO〗_3，但本文所通稱的鈣鈦礦通常是以〖ABX〗_3為晶體結構的化合物。鈣鈦礦晶格結構如圖1-3，A是帶正電荷的大原子或陽離子，通常位於立方體的中心，B也是陽離子居多，位於立方體的四個角，X通常是帶負電荷的陰離子，位於立方體的面上，以我們實驗中的MAPbI_3來說明的話，A、B、X分別對應的就是MA(CH_3 NH_3)、Pb、I。在之前的文獻中，研究CH_3 NH_3 PbI_3鈣鈦礦的聲子模態在太赫茲的範圍(0.3~2.1 THz)，而這些聲子模態主要是跟Pb-I鍵結的振動有關。造成分析時複數導電率會有在1、2 THz的地方有波峰出現，但其餘地方震盪過多導致研究與分析較難進行，結果可以看出波峰的位置變得較為明顯，震盪減少許多。
太赫茲光譜是一種非接觸和非破壞性光學方法，可用於取得遠紅外光譜區域中各種材料的介電特性。對於金屬和金屬材料，介電特性由Drude模型描述，其複數導電率只對應於頻率。對於由重原子組成的材料，振動模式發生在低頻區域（<100 cm-1），其落入太赫茲區域（2 THz~70 cm-1）。同時，這些物理參數是透過模擬擬合從透射/反射THz測量的信號獲得的，並且將通常不必要的振盪與頻率對應到的介電特性重疊。為了改善材料特性的太赫茲分析，我們在樣品前面加入了一個直徑適當(約4 mm）的孔徑，用作空間濾波器。當孔徑與束腰匹配時，我們觀察到系統造成的額外振蕩減少。我們應用這種改進的分析設置來測量過渡金屬二硫化物（TMDCs）和鈣鈦礦的介電性質。儘管由於機械剝離法導致厚度限制和樣品的小面積覆蓋，我們可以使用具有前孔的THz系統測量相對厚的TMDC的光電導率。對於具有1和2 THz特徵振動模式的鈣鈦礦，前孔徑可以使得諧振峰值更平滑。

Transition-metal dichalcogenides (TMDC), as a new category of two dimensional(2D) materials, draw intense research interest in the post-graphene era due to their exceptional optoelectronic properties as 2D semiconductor counterpart of graphene[1-5]. There are many opportunities for two-dimensional layers of thin TMDCs in nanoelectronics and optoelectronics. TMDCs such as MoS2, MoSe2, WS2 and WSe2 have sizable bandgaps that change from indirect to direct in single layers, allowing applications such as transistors, photodetectors and electroluminescent devices. 　　
THz spectroscopy is a noncontact and non-destructive optical method which can be used to charcterize the dielectric function of a wide range of materials in the far-infrared spectral region. For metals and metallic materials, the dielectric properties are described by the Drude model, whose complex conductivity is monotonically depending on the frequency. For materials composed of heavy atoms, the vibrational modes occur at low frequency regions (<100 cm-1), which fall in the THz regime (2 THz~70 cm-1). Meanwhile, those physical parameters are obtained through model fitting to the measured transmitted/reflected THz signals and often unnecessary oscillation components overlap with the dielectric spectra. In order to improve the THz analysis of material properties, we introduce an aperture with a proper diameter (~ 4 mm) in front of samples, which works as a spatial filter. When the aperture diameter is matching with the beam waist, we observed the reduction of extra oscillation. We applied this spatial filter to measure the dielectric properties of transition-metal dichalcogenides (TMDCs) and perovskites. Despite of thickness limit and the small area coverage of TMDC samples due to sample preparation, we could measure the photoconductivity of TMDC layers using the THz system with the front aperture. For perovskites with the characteristic vibrational modes at 1 and 2 THz, the front aperture smoothes the resonance peaks since it is used to be spatial filter.

Abstract III
摘要 V
致謝 IX
目錄 X
圖目錄 XIII
表目錄 XV
第一章 緒論 1
1.1 研究動機 1
1.2 論文主軸 4
第二章 實驗原理 5
2.1 太赫茲時域頻譜分析系統(THz-TDS system) 5
2.2 太赫茲波的輻射與探測 8
2.3 分析方法 9
2.3.1 厚片樣品 10
2.3.2 薄膜樣品 12
2.4 擬合模型 14
2.4.1德汝德模型(Drude Model) 14
2.4.2德汝德-史密斯模型(Drude-Smith Model) 16
2.4.3 德汝德-勞倫茲模型(Drude-Lorentz model) 17
第三章 實驗方法 19
3.1機械剝離法 19
3.2樣品表面拍攝 20
第四章 利用穿透是太赫茲時域頻譜分析系統量測過渡金屬二硫族化物
22
4.1過渡金屬二硫族化物 22
4.2過渡金屬二硫族化物的拉曼光譜 22
4.2.1 二硫化鉬(MoS2) 22
4.2.2 二硫化鎢(WS2) 25
4.3原子力顯微鏡(AFM)下的過渡金屬二硫族化物 27
4.3.1 二硫化鉬(MoS2) 27
4.3.2 二硫化鎢(WS2) 28
4.4 等效介質理論(Effective Medium Theory，EMT) 29
4.5 利用THz-TDS分析過渡金屬二硫族化物 31
4.6穿透是太赫茲系統中的震盪 38
第五章 孔洞對於系統量測之影響 40
5.1 孔洞(Front aperture) 40
5.2 先前的實驗結果以及此次量測的比對 41
5.3 在系統中放置孔洞 43
5.4孔洞的基本原理與效果 45
5.5 鈣鈦礦之量測結果 55
第六章 結論與未來展望 58
6.1 結論 58
6.2 未來展望 59
參考文獻 60

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