本論文探討以化學氣相傳輸法(CVT)成長的高品質六方晶系(2H)二硫化鎢(WS2) 層狀半導體之電傳輸及表面電子結構特性。經由機械剝離法將WS2晶體製作成具有原始表面(non-fresh surface)和新撕表面歐姆接觸電極之奈米元件。發現奈米結構電導率比塊材高出約三個數量級，且電導率會隨厚度遞減而上升。由transfer length method (TLM)模型分析，可計算元件接觸電阻，發現電子傳輸行為並非遵循傳統的三維模式，而是二維的傳輸機制。透過變溫電導率量測觀察到，具有原始表面的奈米結構擁有比塊材低的活化能。上述結果皆指出WS2奈米結構存在表面主導之電傳輸特性。藉由掃描穿隧顯微鏡(STM)和角解析光電子能譜(ARPES)量測，證實了原始表面和新撕表面存在兩種不同的電子結構特性。量測結果顯示原始表面具有極高的電洞濃度，相反的新撕表面較偏向本質特性，不具有高的電洞濃度。此結果證實WS2存在表面電洞累積。最後利用場效電晶體(FET)量測得到出電洞遷移率，新撕表面遷移率為4.5 cm2V-1s-1。

We study the electronic transport properties and surface electronic structure of two-hexagonal (2H) tungsten disulfide (WS2) layered semiconductors grown by chemical vapor transport (CVT). The two-terminal devices of WS2 nanosheets with fresh and non-fresh surfaces were fabricated by the mechanical exfoliation and focused-ion beam (FIB) methods. It was found that the conductivity of the WS2 nanostructure is about three orders of magnitude higher than the bulk, and the conductivity value increases as the thickness decreases. Using the transfer length method (TLM) model, we observed that the WS2 nanostructures follow a two-dimensional (2D) transport behavior rather than the conventional 3D mode. The carrier activation energy of the nanostructures is lower than that of the bulk counterparts. These results imply a surface-dominant electrical characteristics in WS2 nanostructures. The scanning tunneling microscopy (STM) and angle-resolved photoemission spectroscopy (ARPES) measurements indicates that the nonfresh surface of WS2 possesses high hole concentration and on the contrary the fresh surface shows nearly intrinsic nature. The results confirm the presence of surface hole accumulation (SHA) in WS2. Presence of SHA provides an explanation for the physical origin of the 2D electronic transport and anomalously high conductivity in WS2 nanostructures. In addition, the carrier mobility of the WS2 nanosheets was measured by the field-effect transistor (FET) approach. The hole mobility was estimated to be 4.5cm2V-1s-1

中文摘要 I
Abstract II
目錄 III
圖目錄 V
表目錄 VIII
第一章 緒論 1
1.1二維層狀材料 2
1.2過渡金屬硫屬化合物 2
第二章 樣品介紹 4
2.1 二硫化鎢晶體成長 4
2.2 二硫化鎢晶體結構與基本特性 6
第三章 實驗方法 8
3.1 二硫化鎢層狀材料之形貌、結構特性及檢測介紹 8
3.1.1 X光繞射實驗(X-ray Diffraction, XRD) 8
3.1.2 拉曼光譜儀 (Raman Spectrometer) 11
3.2 二硫化鎢層狀半導體元件製作 24
3.3 元件之暗電導量測 28
第四章 結果與討論 34
4.1二硫化鎢晶體形貌與結構分 34
4.2二硫化鎢層狀半導體元件 38
4.3二硫化鎢奈米結構暗電導 40
4.4奈米結構電導率量測 49
4.5 電導率厚度相依特性之人為因素排除 52
4.5.1 材料與白金電極間接觸電阻影響排除 52
4.6變溫電導率與活化能量測 57
4.7 由STM觀察表面電洞聚集效應 61
4.8由ARPES證實表面電洞聚集來源 70
4.9 載子遷移率與表面電洞濃度 73
第五章 結論 76
參考文獻 78

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