臺灣博碩士論文加值系統

本實驗採用機械剝離法、熱蒸鍍等製程製備二硫化鉬場效電晶體，於高真空環境下進行熱退火350℃、4小時，利用熱退火來提升金屬電極與二硫化鉬接觸並降低接觸電阻。並於同一樣品製作兩點量測MoS2場效電晶體元件及穿隧結元件。在變溫環境下量測MoS2元件的電流對電壓關係並討論其電性傳輸機制，同時也用穿隧結量測穿隧電流，以求得MoS2元件的能態密度。
首先於室溫300 K下固定同一載子濃度分別求出兩個樣品的電阻率，並繪製電阻率與溫度的關係比較圖，發現電阻率隨著溫度下降而逐漸上升，呈現半導體行為。且可用二維變程跳躍傳輸理論解釋其電性傳輸機制。接著利用穿隧結結構探討不同二硫化鉬能帶結構的差異，將穿隧電流與電壓的關係轉換成微分電導對電壓的關係，發現電阻率大的元件其能隙值較大。另外亦發現隨外加正閘極偏壓的增加，其能隙位置會漸漸往負外加偏壓地方位移，由能帶圖來看費米能階往導電帶靠近，使樣品呈現 n 型半導體行為。
另外在不同溫度下其能態密度亦不相同，微分電導值會隨溫度降低而略減，能隙會略微增加。藉由穿隧結結構和兩點量測的變溫實驗，將MoS2的能態密度、電阻率及電子傳輸機制作連結，得知電阻率大的樣品其能隙寬度較寬，及電阻率與擬合參數T_0 、E_a 呈正相關。

In this experiment, a molybdenum disulfide field-effect transistor (FET) was fabricated by mechanical exfoliation, electron-beam lithography, and thermal evaporation. The as fabricated devices were then annealed at 350°C for 4 hours in a high vacuum environment. The thermal annealing may improve the contact between the metal electrodes and the MoS2 flakes so as to decrease the contact resistance. Here we made a two-probe MoS2 FET together with hBN-MoS2 tunneling junction on the same MoS2 flake. We carried out current-voltage measurements at variable temperatures we discussed the conduction mechanism. At the same time, the tunneling current was measured and the density of states of the MoS2 were calculated.
Fixing at the the same carrier concentration of two devices, we compared their resistivities and their temperature behaviors. It is found that the resistivity increases with decreasing temperature, showing a semiconductor behavior. The data can be well described by the two-dimensional Mott’s variable range hopping. We further implemented the tunneling junction structure to explore the change of the band structure. The density of states of MoS2 were calculated using differential conductance from the data of tunneling current. It is found that the device with a larger room-temperature resistivity exhibits a larger energy gap. With increasing gate voltage, the position of the energy gap will shift to negative bias voltage. It indicated that the Fermi level is moved to be more close to the conduction band, showing an n-type semiconductor behavior.
In addition, the density of states changed at different temperatures. The differential conductance (dI/dV) decreased slightly and the energy gap increased at lower temperature. Using the tunneling junction and the FET structure on the same MoS2 flake, the density of states, the resistivities, and the electron conduction mechanism of MoS2 were explored and their relationship was discussed. It is observed that the device of a large room-temperature resistivity shows a wider energy gap, a larger disorder parameter of T_0 (from Mott’s VRH model), and a higher exponential component of E_a (from thermal activation model).

摘要 i
Abstract ii
誌謝 iv
目錄 v
表目錄 viii
圖目錄 ix
第一章 緒論 1
1.1簡介 1
1.2 研究動機 2
第二章 文獻回顧 3
2.1二硫化鉬的結構與基本特性 3
2.2二硫化鉬的電性傳輸 5
2.3二硫化鉬的能態密度 7
2.3.1利用STM量測在不同閘極偏壓下二硫化鉬的能態密度的變化 7
2.3.2利用理論計算及STM量測探討二硫化鉬氧化製程的能態密度 10
2.3.3製作穿隧結的結構以探討還原氧化石墨烯的能態密度 12
第三章 實驗理論 14
3.1場效電晶體 (Field-Effect Transistor, FET) 14
3.1.1 N-MOSFET之基本構造與操作原理 14
3.1.2 N-MOSFET之電流對電壓關係 15
3.1.3 溫度與散射機制 18
3.2蕭特基位障與熱離子散射 19
3.3熱活化傳輸 (Thermally Activated Transport) 22
3.4變程跳躍傳輸 (Variable Range Hopping , VRH) 24
3.5量子穿隧效應 (Tunneling Effect) 26
3.5.1 穿隧效應 26
3.5.2 電流-電壓特徵曲線之物理意義 27
第四章 元件製程與實驗步驟 30
4.1實驗儀器介紹 30
4.1.1超音波震洗機 30
4.1.2原子力顯微鏡 (Atomic Force Microscope , AFM) 30
4.1.3撕貼機 31
4.1.4電子束微影系統 (Electron-Beam Lithography , EBL) 32
4.1.5熱蒸鍍 (Thermal Evaporation) 33
4.1.6高溫加熱爐退火 (Annealing) 34
4.1.7電性量測系統 (Probe Station) 35
4.2穿隧結元件製程 36
4.2.1矽基板清洗 36
4.2.2撕貼穿隧結材料 (二硫化鉬及氮化硼) 37
4.2.3旋轉塗佈光阻 37
4.2.4樣品定位 (設計樣品電極圖檔) 38
4.2.5 電子束微影 (曝寫欲得到之電極圖形) 38
4.2.6 顯影 (清除曝寫區域的光阻) 38
4.2.7 蒸鍍 (鍍上金屬薄膜) 39
4.2.8 舉離 (形成電極) 40
4.2.9 退火 (提高二硫化鉬的導電性與活性) 40
4.3電性量測方法 41
4.3.1室溫環境電性量測 41
4.3.2變溫環境電性量測 42
第五章 結果與討論 43
5.1二硫化鉬元件基本特性 43
5.1.1二硫化鉬元件基本結構與層數分析 43
5.1.2二硫化鉬元件基本電性分析 45
5.2 二硫化鉬元件在變溫系統下的電性傳輸機制 47
5.3二硫化鉬之能態密度 52
5.3.1二硫化鉬隨電阻率不同其能態密度差異 53
5.3.2 二硫化鉬隨閘極偏壓改變下之能態密度變化 55
第六章 結論 58
參考文獻 60

[1] K. S. Novoselov, et al. “Electric Field Effect in Atomically Thin Carbon Films,” Science Vol. 306, 666-669 (2004).
[2] K. I. Bolotin, et al. “Ultrahigh electron mobility in suspended graphene,” Solid State Communications Vol. 146, 351-335 (2008).
[3] A. A. Balandin, et al. “Superior Thermal Conductivity of Single-Layer Graphene,” Nano Lett, Vol. 8, 902-907 (2008).
[4] Zuoli He, Wenxiu Que, “Molybdenum disulfide nanomaterials: Structures, properties, synthesis and recent progress on hydrogen evolution reaction”, Applied Materials Today, Vol. 3, 23-56 (2016).
[5] Subhamoy Ghatak, Atindra Nath Pal, and Arindam Ghosh, “Nature of Electronic States in Atomically Thin MoS2 Field-Effect Transistors” ACS Nano, 7707-7712 (2011).
[6] Q. H. Wang, et al. “Electronics and optoelectronics of two-dimensional transition metal dichalcogenides” Nature Nanotechnology, Vol.7, 699-712 (2012).
[7] Hai Li, et al. “Rapid and Reliable Thickness Identification of Two-Dimensional Nanosheets Using Optical Microscopy”, ACS Nano, 10344–10353 (2013).
[8] Deep Jariwala, et al. “Band-like transport in high mobility unencapsulated single-layer MoS2 transistors” Appl. Phys. Lett, Vol.102, 173107 (2013).
[9] B. Radisavljevic, A.Kis “Mobility Engineering and A Metal–Insulator Transition in Monolayer MoS2” Nature Materials, Vol.12, 815-820 (2013).
[10] C.P Lu, et al. “Bandgap and doping effects in MoS2 measured by Scanning Tunneling Microscopy and Spectroscopy” Nano Lett, 4628 (2014).
[11] Hyeokshin Kwon, et al. “Characterization of Edge Contact: Atomically Resolved Semiconductor–Metal Lateral Boundary in MoS2” Adv. Mater, Vol.29, 1702931 (2017).
[12] Anna V. Krivosheeva, et al. “Theoretical Study of defect impact on two-dimensional MoS2”, Journal of Semiconductors, Vol. 36 (2015).
[13] Santosh KC, et al. “Surface oxidation energetics and kinetics on MoS2 monolayer”, Journal of Applied Physics 117, 135301 (2015).
[14] 謝秉軒，「利用掃描探針顯微鏡分析二硫化鉬氧化前後之表面形貌與電性結構」，國立交通大學電子物理研究所，碩士論文，民國105年.
[15] Sheng-Tsung Wang, and Wen-Bin Jian, et al. “Direct probing of density of states of reduced graphene oxides in a wide voltage range by tunneling junction”, Appl. Phys. Lett. Vol.101, 183110 (2012).
[16] D.A.Neamen, “Semiconductor physics and devices: basic principles,” (2012).
[17] S.M.Sze, “Semiconductor devices: physics and technology,” (2008).
[18] P.M Chaikin, T.C Lubensky, T.A Witten, “Principles of condensed matter physics,” (1995).
[19] P.A.Lee and T.V. Ramakrishnan, “Disordered electronic systems,” Rev. Mod. Phys. Vol.57,287-337 (1985).
[20] P.W.Anderson, “Absence of Diffusion in Certain Random Lattices ,” Phys. Rev. Vol.109,1492 (1958)
[21] Tal Schwartz, et al. “Transport and Anderson localization in disordered two-dimensional photonic lattices,” Nature, Vol.446,52-55 (2007)
[22] Stephen Gasiorowicz, “Quantum physics” 10. rev. (2007)
[23] C. Julian Chen, “Introduction to Scanning Tunneling Microscopy” (2008).
[24] A Zabet-Khosousi, A.A Dhirani, “Charge Transport in Nanoparticle Assemblies”, Chem. Rev. Vol.108, 4084-4124, (2008).