近年來，陸陸續續有許多二維材料被發現，其中最受矚目的莫過於過渡金屬二硫族化物(TMD)，TMD的導電特性可以由電壓調變且能隙適中，除此之外具有獨特的光學特性、高載子遷移率等，在各方面應用都非常具有潛力。
對半導體材料進行摻雜是調控材料特性的主要方式，二維材料由於其厚度較薄無法使用離子佈值，因此多數是採用化學氣相沉積法成長材料時同時放入摻雜源，然而此摻雜方式為全區性的且無法精準控制濃度，在本論文中我們提出了一種可以達到選區摻雜又同時控制濃度的摻雜方式，以開有窗口的石墨烯作為遮罩保護覆蓋住的二硒化鎢區域，石墨烯惰性且結構穩定，可以控制只有窗口處之二硒化鎢被鉻原子摻雜與置換，且根據掃描穿隧式電子顯微鏡分析原子結構，鉻原子摻雜的濃度隨反應時間呈線性成長，這樣的實驗結果為局部改變二維材料特性提供了更多可能性。
另外，改善接觸電阻對於二維半導體也是一個重要議題，本論文提出了一個新穎的想法嘗試使用氫電漿移除二硒化鎢上層硫族原子層，之後直接鍍上金屬電極，使電極與過渡金屬層接觸形成鍵結，期望能消除穿隧能障，提升電子注入效率，改善接觸電阻。

Over the recent years, many two-dimensional (2D) materials have been explored, and transition-metal dichalcogenide (TMD) semiconductors have become an emerging research topic especially due to their tunable electronic and optical properties. Substitutional doping in TMD can manipulate the electrical, excitonic and optical properties through the variation of d-electron population. So far, the doping strategies in 2D materials are whole-area with the dopants embedded along with the growing process because the 2D thin layer is easily destroyed by ion implantation which is commonly used in 3D material. In this thesis, we demonstrate a post-growth site-selective Cr atoms doping and substituting the W atoms method in a mono-layer WSe2 via patterned graphene mask, which is chemically inert and effectively separated the spatial reaction. Meanwhile, the atomic characterization with scanning transmission electron microscopy shows that the Cr dopants concentration is controllable and increases linearly to the reaction time in the current doping approach.
However, the presence of large contact resistances remains a fundamental challenge in 2D semiconductors. In this thesis, we attempt to do defect engineering at the contact region by using hydrogen plasma treatment to reduce contact resistance. The plasma treatment selectively creates Se vacancies in the top Se atomic layer at the contact regions and the metal electrode directly bonds to transition metal W in WSe2 which eliminates the tunneling barrier and improves the contact resistance. Our findings suggest that defect engineering at the contact regions can be a feasible scheme to lower contact resistance and accomplish high-performance devices.

[1] 李雅明, 半導體的故事：發展與現況. 台灣: 暖暖書屋, 2013.
[2] W. Brinkman, D. Haggan, and W. Troutman, “A history of the invention of the transistor
and where it will lead us,” Solid-State Circuits, IEEE Journal of, vol. SC-32,
pp. 1858–1865, Jan 1998.
[3] Wikipedia contributors, “Moore’s law — Wikipedia, the free encyclopedia,” 2021.
[Online; accessed 6-July-2021].
[4] H. Farkhani, A. Peiravi, J. M. Kargaard, and F. Moradi, “Comparative study of finfets
versus 22nm bulk cmos technologies: Sram design perspective,” in 2014 27th IEEE
International System-on-Chip Conference (SOCC), pp. 449–454, 2014.
[5] “16/12 奈米製程.” https://www.tsmc.com/chinese/dedicatedFoundry/
technology/logic/l_16_12nm.
[6] M. Godara, C. Madhu, and G. Joshi, “Comparison of electrical characteristics of 28
nm bulk mosfet and fdsoi mosfet,” in 2018 IEEE Electron Devices Kolkata Conference
(EDKCON), pp. 413–418, 2018.
[7] J. BARNUM, “The challenges of process control on finfets
and fd-soi,” May 2018. https://semiengineering.com/
the-challenges-of-process-control-on-finfets-and-fd-soi/.
[8] G. Fiori, F. Bonaccorso, G. Iannaccone, T. Palacios, D. Neumaier, A. Seabaugh,
S. Banerjee, and L. Colombo, “Electronics based on two-dimensional materials,”
Nature Nanotechnology, vol. 9, pp. 768–779, 10 2014.
[9] L. W. T. N. et al, Applications of Printed 2D Materials. In: Printing of Graphene
and Related 2D Materials. Springer, Cham, 2019. https://doi.org/10.1007/
978-3-319-91572-2_6.
[10] H. Liu, A. T. Neal, and P. D. Ye, “Channel length scaling of mos2 mosfets,” ACS
Nano, vol. 6, no. 10, pp. 8563–8569, 2012.
[11] K. Xu, D. Chen, F. Yang, Z. Wang, L. Yin, F. Wang, R. Cheng, K. Liu, J. Xiong,
Q. Liu, and J. He, “Sub-10 nm nanopattern architecture for 2d material field-effect
transistors,” Nano Letters, vol. 17, no. 2, pp. 1065–1070, 2017.
[12] H. Liu, M. Si, Y. Deng, A. Neal, Y. Du, S. Najmaei, P. Ajayan, J. Lou, and P. Ye,
“Switching mechanism in single-layer molybdenum disulfide transistors: An insight
into current flow across schottky barriers,” ACS nano, vol. 8, 12 2013.
[13] L. Wang, I. Meric, P. Huang, Q. Gao, Y. Gao, H. Tran, T. Taniguchi, K. Watanabe,
L. Campos, D. Muller, J. Guo, P. Kim, J. Hone, K. Shepard, and C. Dean, “Onedimensional
electrical contact to a two-dimensional material,” Science, vol. 342,
pp. 614–7, 11 2013.
[14] Y. Xu, C. Cheng, S. Du, J. Yang, B. Yu, W. Yin, E. Li, S. Dong, P. Ye, and X. Duan,
“Contacts between two- and three-dimensional materials: Ohmic, schottky and p-n
heterojunctions,” ACS Nano, vol. 10, 04 2016.
[15] M. Chhowalla, H. Shin, G. Eda, L. Li, K. Loh, and H. Zhang, “The chemistry of twodimensional
layered transition metal dichalcogenide nanosheets,” Nature chemistry,
vol. 5, pp. 263–75, 04 2013.
[16] Q. H. Wang, K. Kalantar-zadeh, A. Kis, J. Coleman, and M. Strano, “Electronics
and optoelectronics of two-dimensional transition metal dichalcogenides,” Nature
nanotechnology, vol. 7, pp. 699–712, 11 2012.
[17] Y. Xiao, M. Zhou, J. Liu, J. Xu, and L. Fu, “Phase engineering of two-dimensional
transition metal dichalcogenides,” Science China Materials, vol. 62, 02 2019.
[18] D. Voiry, A. Mohite, and M. Chhowalla, “Cheminform abstract: Phase engineering
of transition metal dichalcogenides,” Chemical Society reviews, vol. 44, 04 2015.
[19] C. Gong, H. Zhang, W. Wang, L. Colombo, R. Wallace, and K. Cho, “Band alignment
of two-dimensional transition metal dichalcogenides: Application in tunnel
field effect transistors,” Applied Physics Letters, vol. 103, 08 2013.
[20] H. Yuan, H. Wang, and Y. Cui, “Two-dimensional layered chalcogenides: From
rational synthesis to property control via orbital occupation and electron filling,”
Accounts of chemical research, vol. 48, 01 2015.
[21] W. S. Yun, S. Han, S. C. Hong, I. G. Kim, and J. Lee, “Thickness and strain effects on
electronic structures of transition metal dichalcogenides: 2h-m x 2 semiconductors
(m= mo, w; x= s, se, te),” Physical Review B, vol. 85, no. 3, p. 033305, 2012.
[22] M. Xu, T. Liang, M. Shi, and H. Chen, “Graphene-like two-dimensional materials,”
Chemical reviews, vol. 113, 01 2013.
[23] K. Novoselov, D. Jiang, F. Schedin, T. Booth, V. Khotkevich, S. Morozov, and
A. Geim, “Two-dimensional atomic crystals,” Proceedings of the National Academy
of Sciences of the United States of America, vol. 102, pp. 10451–3, 08 2005.
[24] “以石墨烯為例，談談化學氣相沉積法.” https://m.sohu.com/a/458729373_
777213?_trans_=010004_pcwzy.
[25] Y.-C. Lin, W. Zhang, J.-K. Huang, K.-K. Liu, Y.-H. Lee, C.-T. Liang, C. W. Chu, and
L. Li, “Wafer-scale mos2 thin layers prepared by moo3 sulfurization,” Nanoscale,
vol. 4, pp. 6637–41, 09 2012.
[26] Y.-H. Lee, X.-Q. Zhang, W. Zhang, M.-T. Chang, C.-T. Lin, K.-D. Chang, Y.-C. Yu,
J. Wang, C.-S. Chang, L. Li, and T.-W. Lin, “Synthesis of large-area mos2 atomic
layers with chemical vapor deposition,” Advanced materials (Deerfield Beach, Fla.),
vol. 24, pp. 2320–5, 05 2012.
[27] J. You, M. Hossain, and Z. Luo, “Synthesis of 2d transition metal dichalcogenides
by chemical vapor deposition with controlled layer number and morphology,” Nano
Convergence, vol. 5, 12 2018.
[28] X. Zhang, X.-F. Qiao, W. Shi, J. Wu, D.-S. Jiang, and P.-H. Tan, “Phonon and
raman scattering of two-dimensional transition metal dichalcogenides from monolayer,
multilayer to bulk material,” Chemical Society reviews, vol. 44, p. 2757, 04
2015.
[29] X. Luo, Y. Zhao, J. Zhang, M. Toh, C. Kloc, Q. Xiong, and S. Quek, “Effects of
lower symmetry and dimensionality on raman spectra in 2d wse2,” Physical Review
B, vol. 88, 01 2014.
[30] N. Huo, Y. Yang, and J. Li, “Optoelectronics based on 2d tmds and heterostructures,”
Journal of Semiconductors, vol. 38, p. 031002, 03 2017.
[31] W. Liu and S. Bai, “Thermoelectric interface materials: A perspective to the challenge
of thermoelectric power generation module,” Journal of Materiomics, vol. 5,
04 2019.
[32] W. Mönch, “Metal-semiconductor contacts: electronic properties,” Surface science,
vol. 299, pp. 928–944, 1994.
[33] J. Bardeen, “Surface states and rectification at a metal semi-conductor contact,”
Phys. Rev., vol. 71, pp. 717–727, May 1947.
[34] A. Zur, T. McGill, and D. Smith, “Fermi-level position at a semiconductor-metal
interface,” Phys. Rev. B, vol. 28, 08 1983.
[35] H. Hasegawa and T. Sawada, “On the electrical properties of compound semiconductor
interfaces in metal/insulator/ semiconductor structures and the possible origin
of interface states,” Thin Solid Films, vol. 103, pp. 119–140, 1983.
[36] Y. Liu, J. Guo, E. Zhu, L. Liao, S.-J. Lee, M. Ding, I. Shakir, V. Gambin, Y. Huang,
and X. Duan, “Approaching the schottky–mott limit in van der waals metal–semiconductor
junctions,” Nature, vol. 557, 05 2018.
[37] J. Kang, W. Liu, D. Sarkar, D. Jena, and K. Banerjee, “Computational study of metal
contacts to monolayer transition-metal dichalcogenide semiconductors,” Phys. Rev.
X, vol. 4, p. 031005, 07 2014.
[38] C. Kim, I. Moon, D. Lee, M. S. Choi, F. Ahmed, S. Nam, Y. Cho, H.-J. Shin, S. Park,
and W. Yoo, “Fermi level pinning at electrical metal contacts of monolayer molybdenum
dichalcogenides,” ACS Nano, vol. 11, 01 2017.
[39] C. , L. Colombo, R. Wallace, and K. Cho, “The unusual mechanism of partial fermi
level pinning at metal-mos2 interfaces,” Nano letters, vol. 14, 03 2014.
[40] K. Sotthewes, R. Bremen, E. Dollekamp, T. Boulogne, K. Nowakowski, D. Kas,
H. Zandvliet, and P. Bampoulis, “Universal fermi level pinning in transition metal
dichalcogenides,” The Journal of Physical Chemistry C, vol. 123, 02 2019.
[41] A. Dankert, L. Langouche, M. V. Kamalakar, and S. P. Dash, “High-performance
molybdenum disulfide field-effect transistors with spin tunnel contacts,” ACS Nano,
vol. 8, no. 1, pp. 476–482, 2014. PMID: 24377305.
[42] M. Farmanbar and G. Brocks, “Ohmic contacts to 2d semiconductors through van
der waals bonding,” Advanced Electronic Materials, vol. 2, 01 2016.
[43] K. Liu, P. Luo, W. Han, S. Yang, S. Zhou, H. Li, and T. Zhai, “Approaching ohmic
contact to two-dimensional semiconductors,” Science Bulletin, vol. 64, 06 2019.
[44] Y. Chai, R. Ionescu, S. Su, R. Lake, M. Ozkan, and C. Ozkan, “Making onedimensional
electrical contacts to molybdenum disulfide-based heterostructures
through plasma etching,” physica status solidi (a), vol. 213, pp. n/a–n/a, 05 2016.
[45] Z. Cheng, Y. Yu, S. Singh, K. Price, S. G. Noyce, Y.-C. Lin, L. Cao, and A. D.
Franklin, “Immunity to contact scaling in mos2 transistors using in situ edge contacts,”
Nano Letters, vol. 19, no. 8, pp. 5077–5085, 2019.
[46] C.-H. Chu, H.-C. Lin, C.-H. Yeh, Z.-Y. Liang, M.-Y. Chou, and P.-W. Chiu, “Endbonded
metal contacts on wse2 field-effect transistors,” ACS Nano, vol. 13, no. 7,
pp. 8146–8154, 2019. PMID: 31244047.
[47] R. Kappera, D. Voiry, S. E. Yalcin, B. Branch, G. Gupta, A. Mohite, and
M. Chhowalla, “Phase-engineered low-resistance contacts for ultrathin mos2 transistors,”
Nature materials, vol. 13, 08 2014.
[48] A. Allain, J. Kang, K. Banerjee, and A. Kis, “Electrical contacts to two-dimensional
semiconductors,” Nature Materials, vol. 14, pp. 1195–1205, 11 2015.
[49] J. Zhang, S. Jia, I. Kholmanov, L. Dong, D. Er, W. Chen, H. Guo, Z. Jin, V. B.
Shenoy, L. Shi, et al., “Janus monolayer transition-metal dichalcogenides,” ACS
nano, vol. 11, no. 8, pp. 8192–8198, 2017.
[50] D. B. Trivedi, G. Turgut, Y. Qin, M. Y. Sayyad, D. Hajra, M. Howell, L. Liu, S. Yang,
N. H. Patoary, H. Li, et al., “Room-temperature synthesis of 2d janus crystals and
their heterostructures,” Advanced Materials, vol. 32, no. 50, p. 2006320, 2020.