臺灣博碩士論文加值系統

本篇論文在研究石墨烯-鈮超導接面的電性分析並著重在探討當改變石墨烯的費米能量時其安德烈夫反射行為，石墨烯因其特殊的能帶結構，被稱為零能隙的半導體，而超導體會在其費米面上開一個超導能隙，因石墨烯的載子為無質量的相對論性狄拉克費米子，無質量的狄拉克費米子與庫柏電子對之間的交互作用令人感到興趣，本論文中使用鈮薄膜沉積在石英基板上其超導臨界溫度大約9 K，石墨烯則是使用化學氣相沉積法(CVD)所成長後轉移到鈮圖形之上接觸後形成兩個串聯接面，在此之前我們特別處理鈮的介面來確保乾淨的介面可以被達成，其中為了調控石墨烯的費米能量，我們使用液態離子凝膠覆蓋在石墨烯之上當作石墨烯的閘極，實驗中首先在室溫量測閘極偏壓對石墨烯電阻關係，決定好所調控的費米能量相對於電荷中性點(CNP)後降溫到臨界溫度以下，我們發現當費米面被調控為遠離電荷中性點時，歸一化微分電導對外加偏壓關係表現出抑制其下沉結構的行為，且其下沉結構特徵接近超導能隙，也就說當費米能量鄰近電荷中性點時其歸一化微分電導的下沉行為較為明顯，在數據分析上指出當鄰近電荷中性點時因為其安德烈夫臨界角較小所以安德烈夫反射行為被抑制，我們的實驗結果提供了第一個綜合的研究石墨烯-鈮超導接面的電學特性和拓展了一個新的辦法來更深入探討調控安德烈夫反射行為在於狄拉克費米子之上。

We investigate the electric properties through graphene-Nb superconductor junction and aim to study Andreev reflection (AR) as a function of Fermi-level EF of graphene. The Nb thin film is deposited on quartz with superconducting temperature Tc close to 9 K. The graphene is prepared by chemical vapor deposition (CVD) and transfer on top of Nb-pattern, in which two series coupled junctions are defined. In particular, we carefully treat Nb surface so to ensure clean interface is achieved. To control EF of graphene, we employ ion-gel on graphene as a gate electrode. We first characterize the resistance of graphene with the gate voltage Vg, determine EF respected to the charge neutral point (CNP) and then cool down the sample below Tc,. We find as EF is tuned away CNP, the differential conductivity dI/dV v.s. V shows a suppressed dip structures, which width is comparable with the expected superconducting gap of Nb. Distinctly, as EF is in the vicinity of CNP, the downward feature of dI/dV becomes more pronounced. Data analysis suggest the retro-AR is enhanced near CNP due to smaller the critical angle of Andreev reflection. Our results provide the first comprehensive studies of electric properties of graphene-supercondutor interface and open a new route to further explore the tunneling processes of AR for Dirac fermions.

摘要 i
Abstract ii
目錄 iii
第一章 導論
1-1超導現象(Superconductivity) 1
1-2 安德烈夫反射(Andreev reflection) 3
1-3布蘭德-廷肯-克拉普威理論(Blonder- Tinkham- Klapwijk
theory) 4
1-4 石墨烯(Graphene)簡介 6
第二章 文獻回顧與實驗動機
2-1 (理論回顧)石墨烯鏡像安德烈夫反射 10
2-2 (理論回顧)石墨烯-絕緣體-超導體接面穿隧電導理論 14
2-3 (實驗回顧)臨近狄拉克點安德烈夫反射在石墨烯-二硒化鈮
接面 16
2-4(實驗回顧)鏡像帶內安德烈夫反射在石墨烯-二硒化鈮凡德瓦
力接面 22
2-5實驗動機 26
第三章 S-G元件設計製備與特性
3-1 S-G元件設計 27
3-2 S-G元件製程 29
3-3實驗電性量測架設 32
3-3超導體鈮和石墨烯基本特性檢測 36
3-4 鈮介面檢測 39
第四章 實驗數據與討論
4-1 實驗樣品特徵總結 42
4-2 S-G接面實驗數據 43
4-3 S-G接面數據歸納與討論 53
4-4 S-G接面缺點討論與改善 56
第五章 結論 57
參考文獻 58
附件 60

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