掃描式穿隧電子顯微術與能譜於單層二硒化鉬-二硒化鎢平面異質結內過渡金屬置換之研究

Scanning Tunneling Microscopy and Spectroscopy Investigations into Transition Metal Substitution in Monolayer Lateral MoSe2-WSe2 Heterojunctions

李星燁(Shin-Ye Lee)

指導教授 ： 林敏聰

國立臺灣大學/理學院/物理學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201701776

摘要

自十多年前起，石墨烯成為第一種廣為人知之二維材料。高載子移動率、高熱導率、機械性質……等優異的特性使許多學者致力於石墨烯相關之研究。然而，石墨烯因缺乏能帶隙導致其應用於半導體相關元件相對困難。因此，隨著二維過渡金屬二硫屬化物的發現，合適的能帶隙使其於高開關電流比之場效電晶體材料之應用更具有潛力。另外，兩種不同之單層二維過渡金屬二硫屬化物可穩定存在於同一基板上，形成垂直方向堆疊或單層平面之異質結。藉由選擇不同之材料可將能帶隙調變為P型或N型，並於兩種材料之界面形成P-N結。
本論文主題為探討單層二硒化鉬-二硒化鎢平面異質結之介面幾何性質及電性，此異質結成長以高配向性熱解石墨為基底。藉由掃描式穿隧電子顯微鏡與其能譜之技術研究發現：於異質結介面附近，過渡元素鉬有機會置換二硒化鎢材料結構中的過渡金屬位置形成缺陷。與原始二硒化鎢之能帶隙比較，缺陷造成微小但可觀察之能帶偏移。此外，由表面幾何形態與區域電子狀態密度可得知過渡元素置換產生的缺陷亦具有垂直於異質結界面之空間分佈。藉由比較異質結界面附近置換產生缺陷之擴散與能帶對齊之範圍，得知兩者間沒有明顯對應關係。其中可能的過渡元素之置換原因與其產生之影響將於本論文中詳述。

關鍵字

二硒化鉬 ； 二硒化鎢 ； 二維材料 ； 平面異質結 ； 過渡金屬置換 ； 掃描式穿隧電子顯微鏡 ； 掃描式穿隧電子能譜

並列摘要

Graphene has become the first well-known two-dimensional (2D) material since 2004. According to its exotic features such as high mobility, superior thermal conductivity and mechanical properties, lots of researchers have devoted themselves to studying graphene. However, it is difficult for graphene to become a candidate for semiconductor devices due to its gapless band structure. When 2D transition metal dichalcogenides (TMDs) were explored with suitable band gap and high on/off ratio, they became potential materials for field effect transistor (FET). In addition, two different monolayer TMDs could be grown on the same substrate stably as either vertical or lateral heterojunction. Choice of materials can provide a method of tuning
the band gaps towards either p-type or n-type character. And there could exist a P-N junction at the lateral interface of two materials with this growing process.
In this research, monolayer lateral MoSe2-WSe2 heterojunctions on highly oriented pyrolytic graphite (HOPG) has been investigated to analyze the surface morphology and electronic properties. The techniques of scanning tunneling microscopy and spectroscopy (STM and STS) were employed to survey defects of molybdenum (Mo) substitution on WSe2 transition metal site near the interface. The defects give rise to slight but visible band shift compared with WSe2 band gap. Besides, transition metal substitution diffuses with spatial distribution which could be analyzed by surface morphology and local density of states (LDOS). Comparing diffusion of substitution defects with band alignment near the interface, we do not observe correspondence between them. The possible different reasons for and the effects of transition metal substitution are discussed in this thesis.

並列關鍵字

MoSe2 ； WSe2 ； 2D material ； lateral heterojunction ； transition metal substitution ； scanning tunneling microscopy ； scanning tunneling spectroscopy

參考文獻

[2] Balandin, A. A. et al. Superior thermal conductivity of single-layer graphene. Nano Letters 8, 902-907 (2008).

[3] Hwang, E., Adam, S. & Sarma, S. D. Carrier transport in two-dimensional graphene layers. Physical Review Letters 98, 186806 (2007).

[4] Morozov, S. et al. Giant intrinsic carrier mobilities in graphene and its bilayer. Physical Review Letters 100, 016602 (2008).

[5] Schwierz, F. Graphene transistors. Nature Nanotechnology 5, 487-496 (2010).

[6] Radisavljevic, B., Radenovic, A., Brivio, J., Giacometti, V. & Kis, A. Single layer MoS2 transistors. Nature Nanotechnology 6, 147-150 (2011).