在1947年Shockley、Bardeen和Brattain在貝爾實驗室製造出第一個具有放大電流的電晶體，爾後，在1956年拿到諾貝爾獎。電晶體是過去近世紀人類歷史中最重要的發明之一，科學研究致力於將其發展為實用的商品。至今，結合許多電晶體的積體電路已帶領人類走入一個科技革命年代。電晶體尺寸隨著莫爾定律已大幅度地下降，因此選用的通道材料成為下一世代元件發展的關鍵。2010年，諾貝爾物理獎頒給撕出單原子層石墨烯的Andre Geim和Kostya Novoselov，而零能隙的石墨烯近年來逐漸被過渡金屬硫族化合物 (TMDs)所取代。這篇論文著墨在製作單層二鎢化硒元件與其電子傳輸特性。為了解電子傳輸機制，利用變溫量測找出蕭特基能障高度並分析金屬與半導體接面間存在的費米能階釘札效應。

第一章介紹半導體的演進史，微縮的部分包含constant-field scaling、generalized scaling和constant-voltage scaling等，在1.3節，藉由傳統三維半導體與金屬能帶結構來討論載子的傳輸行為。第二章介紹過渡金屬硫族化合物的諸多優點、物理性質、晶向、電子結構、能帶與拉曼分析等，說明選擇單層二鎢化硒作為通道材料的原因。

第三章介紹拉曼散射和光致螢光光譜的背景知識，兩者皆為分析材料性質最基礎的方式，觀察電子在基態與虛態或激態的躍遷行為。第四章討論金屬與過渡金屬硫族化合物接面間的現象，因此，接觸金屬的選擇成為載子注入通道材料的重要關鍵之一，其中，載子傳輸機制可分為受元件本身或是其外在因素影響。

一般的場效電晶體藉由施加上閘極電壓來控制閘極絕緣層下方的傳導層，然而這僅可調變閘極下方、整體元件部分的載子，並且，因為通道材料僅數個奈米厚，元件設計改為背閘極式場效電晶體，使得可以調變更多的載子注入通道材料。第五章分別介紹量測系統裝置以及載子在不同溫度下的傳輸行為。第六章則是介紹費米能階釘札的效應。

The Nobel Prize was awarded to Shockley, Bardeen and Brattain in 1956 for their discovery of the first transistor at Bell Lab in 1947. The transistor without a doubt is one of the most important inventions of the past century. Being cited as the example of how scientific research can lead to useful commercial products, the integrated circuit has led humankind into next scientific history. The transistor size has been dramatically shrunk by following Moore’s law; therefore, the choice of conducting material plays an important role in next generation’s transistor. In 2010, the Nobel Prize was awarded to Andre Geim and Kostya Novoselov in the field of Physics for their contribution of revolutionary experiments regarding the two-dimensional material graphene. Beyond the discovery and the astonishing properties of graphene, transition metal dichalcogenides (TMDs) with the formula MX2 have recently been used as alternative materials. This thesis outlines the process of making monolayer WSe2 electronic devices, as well as the study of their electrical transport. Particular emphasis is placed on electrical transport and extraction of the Schottky barrier height via temperature-dependent measurements to understand the mechanism of the Fermi level pinning effect at metal-WSe2 contact.

In chapter 1 we present an introductory overview to the semiconductor engineering, including, but not limited to: constant-field scaling, generalized scaling, and constant-voltage scaling. To have ideas about transport properties, the band structures focusing on contact between conventional bulk semiconductor and metal are discussed in detail by several recommended review papers in Section 1.3. Chapter 2 shows the merits and physical properties of TMDs, composed of a variety of compounds, crystal phases, electronic structure, band diagram and Raman Spectroscopy. This chapter, therefore, indicates why we choose to use the monolayer WSe2 as conducting material for the device.

Chapter 3 gives a brief description of the fundamentals of Raman scattering and photoluminescence emission. Both material analysis measurements are the most fundamental techniques used to identify the material and are based on the concept of electron transitions between ground states and virtual states or excited states as well. Chapter 4 discusses very interesting phenomena at the interface between transition metal dichalcogenides and metal. Thereby, choosing contact metals is one of the foremost issues that correlates with the electron injection efficiency of contacts. To estimate the transport mechanisms of the carriers, we will discuss several scattering effects induced by intrinsic and extrinsic factors of the device. Therefore, to lower down the extrinsic factors, we introduce some treatments applied to enhance the quality of contact.

In conventional metal-oxide-semiconductor field-effect transistors (FETs), a top gate is often used to control the conduction of the inversion layer underneath the gate insulator. This allows the FETs to be partially operated in tuning the Schottky barrier height. In Chapter 5, we applies the back-gated bias as a method for operating our device in real nanoscale. The measurement setup for electrical transport and its electrical properties as a function of temperature are shown at the beginning
and the end of the Chapter 5, respectively. How to mitigate the Fermi level pinning effect at the contact channel is discussed in Chapter 6.

Abstrate........................................................ I
Contents...................................................... III
Chapter1 Introduction........................................... 1
1.1 Development of semiconductor engineering.. . . . . . . . . . 1
1.2 The limitations of MOSFET scaling. . . . . . . . . . . . . . 4
1.2.1 Constant-field scaling . . . . . . . . . . . . . . . . . . 4
1.2.2 Generalized scaling . . . . . . . . . . . . . . . . . . . 5
1.2.3 Constant-voltage scaling . . . . . . . . . . . . . . . . . 5
1.2.4 Physics of difficulties and limitations . .. . . . . . . . 6
1.3 Contact between traditional bulk semiconductor and metal . . 8
1.3.1 Feature of Si and metal contact . . . . . . . . . . . . . 8
1.3.2 Mechanisms of current transport in metal-semiconductor . .10
Chapter2 Transition Metal Dichalcogenides...................... 13
2.1 Introduction . . . . . . .. . . . . . . . . . . . . . . . . 13
2.2 Composition, crystal phases, electronic structure and band structure of TMDs . . . . . . . . . . . . . . . . . . . . . . . 15
2.3 Band diagram of TMDs . . .. . . . . . . . . . . . . . . . . 17
2.4 Raman Spectroscopy of TMDs . .. . . . . . . . . . . . . . . 19
Chapter3 Raman Scattering and Photoluminescence emission....... 23
3.1 Introduction . . . . . . . . . . . . . .. . . . . . . . . . 23
3.2 Fundamental principle of Raman effect . . . . . . . . . . . 24
3.3 Resonance-Enhanced Raman scattering . . . . . . . . . . . . 27
3.4 Physical meaning of Raman spectrum .. . . . . . . . . . . . 28
3.5 Photoluminescence emission . . .. . . . . . . . . . . . . . 30
Chapter4 Interface between Transition Metal Dichalcogenides and the metal ......................................................33
4.1 Basic concept of metal-TMD contact . . . . . . . . . . . . 33
4.1.1 Choosing contact metals . . . . . . . . . . . . . . . . . 33
4.1.2 Electron injection efficiency of contacts . . . . . . . . 34
4.1.3 Transport and scattering mechanisms of the carriers . . . 39
4.2 Fermi level pinning . . . . . . . . . . . . . . . . . . . . 44
4.3 Measurement and treatment to the contact . . . . . . . . . 51
Chapter5 Monolayer WSe2 field effect transistors............... 57
5.1 Experimental techniques . . . . . . . . . . . . . . . . . . 57
5.1.1 Device fabrication . . . . . . . . . . . . . . . . . . . 57
5.1.2 Electrical transport measurements . . . . . . . . . . . . 60
5.2 Electrical transport . . . . . .. . . . . . . . . . . . . . 63
5.2.1 Fermi level pinning . . . . . . . . . . . . . . . . . . . 65
5.2.2 Hysteresis . . . . . . . . . . . . .. . . . . . . . . . . 70
Chapter6 Summary and outlook................................... 73
Reference...................................................... 75

[1] J. Bardeen, W. H. Brattain, Phys. Rev., 1948, 74, 230-231.
[2] The first bipolar junction transistor, http://microblog.routed.net/2006/12/12/shockleys-and-pearsons-bipolar-junction-transistor/.
[3] The first silicon integrated circuit chip, http://www.computerhistory.org/timeline/?category=cmpnt.
[4] F. Braun, Annalen der Physik und Chemie, 1874, 153, 556-563.
[5] D. Sarkar, X. Xie, W. Liu, W. Cao, J. Kang, Y. Gong, S. Kraemer, P. M. Ajayan, K. Banerjee, Nature, 2015, 526, 91-95.
[6] J. Bardeen, Phys. Rev. Lett., 1947, 71, 717-727.
[7] V. Heine, Phys. Rev. Lett., 1965, 138, A1689-A1696.
[8] J. Tersoff, Springer, 1984, 52, 155-156.
[9] H. Hasegawa, T. Sawada, ELSEVIER, 1983, 103, 119-140.
[10] G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh, S. K. Banerjee, L. Colombo, Nat. Nanotechnol., 2014, 9, 768-779.
[11] K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, A. A. Firsov, Science, 2004, 306, 666-669.
[12] M. Y. Han, B. Ozyilmaz, Y. Zhang, P. Kim, Phys. Rev. Lett., 2007, 98, 206805.
[13] D. A. Areshkin, C. T. White, Nano Lett., 2007, 7, 3253-3259.
[14] D. C. Elias, R. V. Gorbachev, A. S. Mayorov, S. V. Morozov, A. A. Zhukov, P. Blake, L. A. Ponomarenko, I. V. Grigorieva, K. S. Novoselov, F. Guinea, A. K. Geim, Nature Phys., 2011, 7, 701-704.
[15] J. A. Wilson, A. D. Yoffe, Adv. Phys., 1969, 18, 193-335.
[16] A. D. Yoffe, Annu. Rev. Mater. Sci., 1993, 3, 147-170.
[17] A. D. Yoffe, Adv. Phys., 1993, 42, 173-266.
[18] T. Björkman, A. Gulans, A. V. Krasheninnikov, R. M. Nieminen, J. Phys.: Condens. Matter, 2012, 24, 424218.
[19] M. Luisier, M. Lundstrom, D. A. Antoniadis, J. Bokor, IEEE Int. Electron Devices Meet., 2011, 251-254.
[20] C. Manish, S. S. Hyeon, G. Eda, L. J. Li, K. P. Loh, H. Zhang, Nature Chem., 2013, 5, 263-275.
[21] S. Lebegue, T. Bjorkman, M. Klintenberg, R. M. Nieminen, O. Eriksson, Phys. Rev. X, 2013, 3, 031002.
[22] J. Kang, W. Liu, D. Sarkar, D. Jena, K. Banerjee, PHYSICAL REVIEW X, 2014, 4, 031005.
[23] M. Chhowalla, H. S. Shin, G. Eda, L. J. Li, K. P. Loh, H. Zhang, Nat. Chem., 2013, 5, 263-275.
[24] O. V. Yazyev, E. Kioupakis, J. E. Moore, S. G. Louie, Phys. Rev. B, 2012, 85, 161101.
[25] W. Zhao, Z. Ghorannevis, K. K. Amara, J. R. Pang, M. Toh, X. Zhang, C. Kloc, P. H. Tane, G. Eda, Nanoscale, 2013, 5, 9677.
[26] H. Terrones, E. D. Corro, S. Feng, J. M. Poumirol, D. Rhodes, D. Smirnov, N. R. Pradhan, Z. Lin, M. A. T. Nguyen, A. L. Elı´as, T. E. Mallouk, L. Balicas, M. A. Pimenta, M. Terrones, Scientific Reports, 2014, 4, 1-9.
[27] W. Zhao, Z. Ghorannevis, L. Chu, M. Toh, C. Kloc, P. H. Tan, G. Eda, ACS Nano, 2013, 7, 791-797.
[28] K. Xu, Z. Wang, X. Du, M. Safdar, C. Jiang, J. He, Nanotechnology, 2013, 24, 1-7.
[29] A. M. Jones, H. Yu, N. J. Ghimire, S. Wu, G. Aivazian, J. S. Ross, B. Zhao, J. Yan, D. G. Mandrus, D. Xiao, W. Yao, X. Xu, Nat Nanotechnol., 2013, 8, 634-638.
[30] K. F. Mak, K. He, C. Lee, G. H. Lee, J. Hone, T. F. Heinz, J. Shan, Nat. Mater., 2013, 12, 207-211.
[31] S. Tongay, J. Suh, C. Ataca, W. Fan, A. Luce, J. S. Kang, J. Liu, C. Ko, R. Raghunathanan, J. Zhou, F. Ogletree, J. Li, J. C. Grossman, and J. Wu, Sci. Rep., 2013, 3, 2657.
[32] T. Korn, S. Heydrich, M. Hirmer, J. Schmutzler, C. Schuller, Appl. Phys. Lett., 2011, 99, 102109.
[33] Z. Wang, H. Yin, C. Jiang, M. Safdar, J. He, Appl. Phys. Lett., 2012, 101, 253109.
[34] K. Xu, D. Chen, F. Yang, Z. Wang, L. Yin, F. Wang, R. Cheng, K. Liu, J. Xiong, Q. Liu, J. He, Nano Lett., 2017, 17, 1065-1070.
[35] H. Y. Park, S. R. Dugasani, D. H. Kang, J. Jeon, S. K. Jang, S. Lee, Y. Roh, S. H. Park, J. H. Park, ACS Nano, 2014, 8, 11603-11613.
[36] K. Kaasbjerg, K. S. Thygesen, K. W. Jacobsen, Phys. Rev. B, 2012, 85, 115317.
[37] Q. H. Wang, K. K. Zadeh, A. Kis, J. N. Coleman, M. S. Strano, Nat. Nanotechnol., 2012, 7, 699-712.
[38] Y. Q. Wu, Y. M. Lin, A. A. Bol, K. A. Jenkins, F. N. Xia, D. B. Farmer, Y. Zhu, P. Avouris, Nature, 2011, 472, 74-78.
[39] D. V. Tsu, G. Lucovsky, M. J. Mantini, Phys. Rev. B, 1986, 33, 7069.
[40] T. P. Ma, IEEE Trans. Electron Devices, 1998, 45, 680.
[41] G. N. Parsons, J. H. Souk, J. J. Batey, Appl. Phys., 1991, 70, 1553.
[42] H. Nienhaus, T. U. Kampen, W. Monch, Surf. Sci., 1995, 324, 328-332.
[43] M. V. Fischetti, D. A. Neumayer, E. J. Cartier, Appl. Phys., 2001, 90, 4587.
[44]J. Brivio, D. T. Alexander, A. Kis, Nano Lett., 2011, 11, 5148-5153.
[45] P. Miró, M. Ghorbani-Asl, T. Heine, Adv. Mater., 2013, 25, 5473-5475.
[46] V. Heine, Phys. Rev., 1965, 138, A1689.
[47] S. G. Louie, M. L. Cohen, Phys. Rev. Lett., 1975, 35, 866-869.
[48] H. Hasegawa, T. Sawada, Thin Solid Films, 1983, 103, 119-140.
[49] D. Liu, Y. Guo, L. Fang, J. Robertson, App. Phys. Lett., 2013, 103, 183113.
[50] J. Robertson, J. Vac. Sci. Technol. B, 2000, 18, 1785.
[51] J. Tersoff, Phys. Rev. Lett., 1984, 52, 465.
[52]W. M€onch, Phys. Rev. Lett., 1987, 58, 1260.
[53]W. M€onch, Surf. Sci., 1994, 300, 928-944.
[54] C. Tejedor, F. Flores, and E. Louis, J. Phys. C, 1977, 10, 2163.
[55]I. Popov, G. Seifert, and D. Tomanek, Phys. Rev. Lett., 2012, 108, 156802.
[56]Y. Liu, P. Stradins, and S. H. Wei, Sci. Adv., 2016, 2, e1600069.
[57]C. Kim, I. Moon, D. Lee, M. S. Choi, F. Ahmed, S. Nam, Y. Cho, H. J. Shin, S. Park, W. J. Yoo, ACS Nano, 2017, 11, 1588-1596.
[58]J. Robertson, J. Vac. Sci. Technol. B, 2000, 18, 1785−1791.
[59]C. Gong, L. Colombo, R. M. Wallace, K. Cho, Nano Lett., 2014, 14, 1714−1720.
[60] T. C. Leung, C. L. Kao, W. S. Su, Y. J. Feng, C. T. Chan, Phys. Rev. B, 2003, 68, 195408.
[61]Chan, K. T.; Neaton, J. B.; Cohen, M. L., Phys. Rev. B, 2008, 77, 235430.
[62] H. Yang, J. Heo, S. Park, H. J. Song, D. H. Seo, K. E. Byun, P. Kim, I. Yoo, H. J. Chung, K. Kim, Science, 2012, 336, 1140−1143.
[63] W. Zhou, X. Zou, S. Najmaei, Z. Liu, Y. Shi, J. Kong, J. Lou, P. M. Ajayan, B. I. Yakobson, J. C. Idrobo, Nano Lett., 2013, 13, 2615−2622.
[64] D. Liu, Y. Guo, L. Fang, J. Robertson, Appl. Phys. Lett., 2013, 103, 183113.
[65] Y. Guo, D. Liu, J. Robertson, Appl. Phys. Lett., 2015, 106, 173106.
[66]H. H. Berger, Solid-State Electronics., 1972, 15, 145-158.
[67]H. Liu, M. Si, Y. Deng, A. T. Neal, Y. Du, S. Najmaei, P. M. Ajayan, J. Lou, P. D. Ye, ACS Nano, 2014, 8, 1031-1038.
[68] Y. Guo, Y. Han, J. Li, A. Xiang, X. Wei, S. Gao, Q. Chen, ACS Nano, 2014, 8, 7771-7779.
[69] W. Li, Y. Liang, D. Yu, L. Peng, K. P. Pernstich, T. Shen, A. R. H. Walker, G. Cheng, C. A. Hacker, C. A. Richter, Q. Li, D. J. Gundlach, X. Liang, Appl. Phys. Lett., 2013, 102, 183110.
[70] Y. Huang, E. Sutter, N. N. Shi, J. Zheng, T. Yang, D. Englund, H. J. Gao, P. Sutter, ACS Nano, 2015, 9, 10612-10620.
[71] J. A. Robinson, M. LaBella, M. Zhu, M. Hollander, R. Kasarda, Z. Hughes, K. Trumbull, R. Cavalero, D. Snyder, Appl. Phys. Lett., 2011, 98, 053103.
[72] Y. Dan, Y. Lu, N. J. Kybert, Z. Luo, A. T. Charlie Johnson, Nano Lett., 2009, 9, 1472.
[73] T. Lohmann, K. von Klitzing, J. H. Smet, Nano Lett., 2009, 9, 1973.
[74] X. Luo, Y. Zhao, J. Zhang, M. Toh, C. Kloc, Q. Xiong, S. Y. Quek, PHYSICAL REVIEW B, 2013, 88, 195313.
[75] L. T. Zhuravlev, Colloids Surf. A, 2000, 173, 1-38.