臺灣博碩士論文加值系統

自2004年成功地在實驗中從石墨分離出石墨烯後，石墨烯開始廣泛的在各領域中被研究。然而，據我們所知只有少數研究關注單層石墨烯和雙層石墨烯交界面的量子傳輸變化。在這理，我們研究了交替之單雙層石墨烯的電子能帶結構和傳輸特性，在實驗中，我們觀察到額外的狄拉克點(Dirac point)和額外的朗道能階(Landau levels)，並且發現兩組朗道能階在磁場B=3.8T時產生交點。為了更加清楚地量測量子霍爾效應的演化情況，我們成功地開發了封裝技術，使封裝的雙層石墨烯裝置(Devices)之遷移率(mobility)可以提高到100,000 cm2 V-1 s-1。這項技術不僅提供了樣品新的設計可能性，亦使量子現象能夠被更深入的研究。除此之外，交替之單雙層石墨烯在磁場B=8T時觀察到不等差的填充因子v=2, 6, 10, 12, 16, 18...，但是由於遷移率(mobility)不夠理想，無法觀察到後續的填充因子變化，亦無法確切對兩組朗道能階(Landau levels)發生交叉點處進行分析，因此如何將交替的單雙層石墨烯異質結構與封裝技術結合起來以達到提升遷移率(mobility)的目的，將是我們未來的主要工作。

Few-layer graphene has been extensively studied since 2004 when it was experimentally demonstrated for the first time[1]. The mono- and bilayer graphene and lead to exhibit distinct electronic properties such as massless and massive Dirac fermion, and lead to different applications and research directions. However, the quantum transport in monolayer and bilayer graphene heterojunctions had only been investigated in few researches[2, 14, 18]. Here, we investigate the transport behaviors of alternating monolayer-bilayer graphene devices. In the experiment, we may observe an additional Dirac point and additional Landau levels. Furthermore, in order to clearly observe the evolution of quantum Hall effect, we have successfully developed the encapsulated technology. Based on this technology, the mobility of encapsulated bilayer graphene devices can be raised to 100,000 cm2 V-1 s-1. Indeed, such encapsulated processes provide new design possibilities for the devices, which allows us to deeply study of quantum phenomena. Additionally, the filling factor v=2, 6, 10, 12, 16, 18... are detected in the alternating monolayer-bilayer graphene at applied magnetic field B=8T, but the evolution of Landau levels near the applied magnetic field B=3.8T can’t be analyzed. At the applied magnetic field B=3.8T, two groups of Landau levels generate a crossing point. Therefore, how to combine the alternating monolayer-bilayer graphene heterostructures and encapsulated technology to improve the mobility is our future work.

Abstrate　　i

1 Introduction　　1

2 Theoretical background .......... 3
2.1 Graphene properties .......... 3
2.1.1 The crystal structure of graphene .......... 3
2.1.2 Electronic properties of graphene .......... 4
2.2 Quantum Hall effect .......... 8
2.2.1 Landau levels .......... 8
2.2.2 Density of states in a Landau levels .......... 10
2.2.3 Quantum Hall effect .......... 10
2.2.4 Quantum Hall effect in graphene .......... 11

3 Fabrication　　16
3.1 Mechanical exfoliation .......... 16
3.2 Hot pick-up transfer technique .......... 19
3.2.1 Actual process of pick-up, transfer and Meltdown .......... 19
3.3 E-beam lithography and contact evaporation .......... 21

4 Experimental result and discussion　　22
4.1 Quantum transport in the monolayer-bilayer graphene heterostructure ... 22
4.1.1 Introduction .......... 22
4.1.2 The alternating monolayer-bilayer graphene heterostructure .......... 24
4.1.3 Measurement result .......... 28
4.2 High mobility encapsulated graphene device .......... 33
4.2.1 Encapsulated bilayer graphene heterostructure device .......... 33
4.2.2 Measurement result .......... 36

5 Conclusion　　40
5.1 Future work .......... 41

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