本實驗我們利用光電流顯微術探討二銻化鉬薄膜及二銻化鉬與二錫化硫異質結構之空乏區電場分布情況。藉由機械式剝離法及乾式轉印技術，我們將二銻化鉬與二錫化硫薄膜堆疊在300 nm厚的二氧化矽基板上，並使用標準電子束微影技術製作出鈦金電極(10 nm/90 nm)。光電流顯微術使用633 nm雷射搭配低雜訊電流放大器量測光電流分布，雷射光點約1.5 um。從電壓相關光電流強度分布，我們推得金屬/二銻化鉬接面能障約100 meV。利用電性量測、二次諧波訊號映射、拉曼映射及原子力顯微鏡確認其接面位障變動是因為二銻化鉬晶格結構從2H相位轉變成1T’相位造成的。二銻化鉬堆疊在二錫化硫上形成的異質結構在VDS = 1 V時，其開關比可達103、次臨限擺動約1.5 V/dec.。從變溫量測可得能障約250 meV。光電流結果證明二銻化鉬與二錫化硫異質結構是第二類能帶校準。另外，我們從光電流分佈發現不同堆疊方式其能帶結構會因介面的電子陷獲情況不同而發生改變，導致空乏區內的電場方向改變。最後我們發現光電流顯微術可以檢測一般光學顯微鏡所無法發現的材料表面缺陷。

In this study, we investigate the properties of the depletion region in MoTe2 nano-flake and MoTe2/SnS2 heterostructure devices by scanning photocurrent microscopy (SPCM). We fabricated the multi-layer MoTe2 transistor devices and MoTe2/SnS2 vertical heterostructure transistor devices by the standard mechanically exfoliation method from MoTe2 and SnS2 flakes. The MoTe2 bulk material is obtained from natural mineral, and SnS2 flakes are grown by CVD method. The thickness of the MoTe2 and SnS2 are of the order of 10 nm. The 10-nm-Ti/90-nm-Au metal contacts are defined by e-beam lithography and lift off process. The sample is scanned by a focused 633-nm laser with a 1.5 um diameter on the focused plane and the drain-to-source photocurrent is collected via a low-noise current preamplifier. The DC transport measurements of the MoTe2 transistor shows an ON/OFF ratio about 103 and the field-effect mobility about 9.4 cm2/Vs at VSD = 0.5 V. From the bias dependence of the photocurrent peak obtained from the SPCM line scan data, we extract the contact barrier height of metal/MoTe2 to be about 100 meV. The areal mapping of the photocurrent shows a very large fluctuation near the junction between the metal and MoTe2, indicating that the barrier height is not uniform along the junction. The Schottky barrier fluctuations in metal/MoTe2 junction are identified to be caused by a non-uniform phase transition from a 2H semiconductor phase to a 1T’ metallic phase due to long-term laser beam irradiation. Here we have used I-V transport data, SPCM measurements, second-harmonic-generation (SHG) mapping, and AFM topography to reach the conclusion. The tunnel field effect transistors based on multilayer MoTe2/SnS2 vertical van der Waals heterosturcture devices have been investigated. The device with a MoTe2 layer stacked on a SnS2 layer shows an ON/OFF ratio over 104 and the subthreshold swing is about 1.5 V/dec at VDS = 1 V in ambient air, which is equivalent to about 56 mV/dec. if the dielectric is scaled down to 10 nm equivalent oxide thickness. The energy barrier extracted from the temperature dependence of I-V characteristics at VGS = 30 V is about 250 meV. We use SPCM with a 1 uW 633nm laser to investigate the heterostructure devices. The photocurrent is most significant along the boundary of MoTe2 and MoTe2/SnS2 heterostructure and mainly on the MoTe2 side. This indicates that there exists a depletion region near this boundary. We also conclude that the MoTe2/SnS2 junction should possess a type III band alignment. Besides, we compare the transport and photocurrent results of devices with different stacked sequences, MoTe2/SnS2/SiO2 and SnS2/MoTe2/SiO2. We find that the band alignment between the bare MoTe2 and the heterostructure part is different for these two samples. This could be caused by different charge trapping effect of the layer materials on the substrate. Finally, we report the detection the micro-crack in a multilayer MoTe2/SnS2 heterostructure thin film field effect transistors (MoTe2/SnS2 FETs) by SPCM. The crack with a width less than 50 nm is confirmed by the image of Transmission Electron Microscope (TEM). Near the crack inside the MoTe2/SnS2 heterostrucure, we observed an abnormal photocurrent signal that is due to the fracture in the SnS2 layer.

Abstract (Chinese). . . . . . . . . . . . . .i
Abstract. . . . . . . . . . . . . . . . . . .ii
List of Tables. . . . . . . . . . . . . . . .vi
List of Figures. . . . . . . . . . . . . . . vi
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
1.1 MoTe2 and SnS2 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Scanning photocurrent microscopy . . . . . . . . . . . .5
2. Samples preparation . . . . . . . . . . . . . . . . . . . . . . . 11
2.1 Exfoliation and thickness identification . . . . . . . .11
2.2 Device fabrication . . . . . . . . . . . . . . . . . . . . . .12
3. Instrumentation and measurement methods . . . .18
3.1 Instrumentation of the I-V measurements . .18
3.2 Instrumentation of SPCM . . . . . . . . . . . . . .18
3.3 Calibration and setup . . . . . . . . . . . . . . . . . 20
4. Results and discussion . . . . . . . . . . . . . . . . . . . .24
4.1 MoTe2 FET . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.1.1 Electrical properties . . . . . . . . . . . . . . .25
4.1.2 Schottky barrier variation . . . . . . . . . . 29
4.2 MoTe2/SnS2 Heterojunctions . . . . . . . . . . . .34
4.2.1 Temperature dependent transport . . . . 34
4.2.2 Band diagram and SPCM data . . . . . . 37
4.2.3 Surface charge trapping effect . . . . . . .41
4.2.4 Detection of invisible crack by using SPCM . . . . .45
5. Conclusions . . . . . . . . . . . . . . . . . . . . . 85
Bibliography. . . . . . . . . . . . . . . . . . . . . . . 87
Appendix A: Design and construction a confocal microscope system. . . . .94

[1] A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, Rev. Mod. Phys. 81, 109 (2009).
[2] S. D. Sarma, S. Adam, E. H. Hwang, and E. Rossi, Rev. Mod. Phys. 83, 407 (2011).
[3] Q. H. Wang, K. Kalantar-Zadeh, A. Kis, J. N. Coleman, and M. S. Strano, Nat. Nanotech. 7, 699 (2012).
[4] M. Chhowalla, H. S. Shin, G. Eda, L. J. Li, K. P. Loh, and H. Zhang, Nat. Chem. 5, 263 (2013).
[5] A. K. Geim and I. V. Grigorieva, Nature 499, 419 (2013).
[6] R. Ganatra and Q. Zhang, ACS Nano 8, 4074 (2014).
[7] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, Nat. Nanotech. 6, 147 (2011).
[8] K. F. Mak, C. Lee, J. Hone, J. Shan, and T. F. Heinz, Phys. Rev. Lett. 105, 136805 (2010).
[9] A. Splendiani, L. Sun, Y. Zhang, T. Li, J. Kim, C. Y. Chim, G. Galli, and F. Wang, Nano Lett. 10, 1271 (2010).
[10] G. Eda, H. Yamaguchi, D. Voiry, T. Fujita, M. Chen, and M. Chhowalla, Nano Lett. 11, 5111 (2011).
[11] O. Lopez-Sanchez, D. Lembke, M. Kayci, A. Radenovic, and A. Kis. Nat. Nanotech. 8, 497 (2013).
[12] Y. Guo, X. Wei, J. Shu, B. Liu, J. Yin, C. Guan, Y. Han, S. Gao, and Q. Chen, Appl. Phys. Lett. 106, 103109 (2015).
[13] S. Rathi, I. Lee, D. Lim, J. Wang, Y. Ochiai, N. Aoki, K. Watanabe, T. Taniguchi, G. H. Lee, Y. J. Yu, P. Kim, and G. H. Kim, Nano Lett. 15, 5017 (2015).
[14] R. Cheng, F. Wang, L. Yin, K. Xu, T. A. Shifa, Y. Wen, X. Zhan, J. Li, C. Jiang, Z. Wang, and J. He. Appl. Phys. Lett. 110, 173507 (2017).
[15] M. H. Doan, Y. Jin, S. Adhikari, S. Lee, J. Zhao, S. C. Lim, and Y. H. Lee, ACS Nano 11, 3832 (2017).
[16] A. Nourbakhsh, A. Zubair, M. S. Dresselhaus, and T. Palacios, Nano Lett. 15, 5791 (2015).
[17] R. K. Ghosh and S. Mahapatra, IEEE J. Electron Devi. 1, 175 (2013).
[18] J. Kang, S. Tongay, J. Zhou, J. B. Li, and J. Q. Wu, Appl. Phys. Lett. 102, 012111 (2013).
[19] Y. F. Liang, S. T. Huang, R. Soklaski, and L. Yang, Appl. Phys. Lett. 103, 042106 (2013).
[20] C. Gong, H. J. Zhang, W. H. Wang, L. Colombo, R. M. Wallace, and K. J. Cho, Appl. Phys. Lett. 103, 053513 (2013).
[21] Y. Guo and J. Robertson, Appl. Phys. Lett. 108, 233104 (2016).
[22] J. Xu, J. Jia, S. Lai, J. Ju, and S. Lee, Appl. Phys. Lett. 110, 033103 (2017).
[23] Y. W. Lan, C. M. Torres, S. H. Tsai, X. Zhu, Y. Shi, M. Y. Li, L. J. Li, W. K. Yeh, and K. L. Wang, Small 2, 5676 (2016).
[24] T. Roy, M. Tosun, M. Hettick, G. H. Ahn, C. Hu, and A. Javey, Appl. Phys. Lett. 108, 083111 (2016).
[25] Á. Szabó, S. J. Koester, and M. Luisier, IEEE Electron Dev. Lett. 36, 514 (2015).
[26] Y. F. Lin , Y. Xu, S. T. Wang, S. L. Li, M. Yamamoto, A. Aparecido-Ferreira, W. Li, H. Sun, S. Nakaharai, W. B. Jian, K. Ueno, and K. Tsukagoshi, Adv. Mater. 26, 3263 (2014).
[27] S. Fathipour, N. Ma, W. S. Hwang, V. Protasenko, S. Vishwanath, H. G. Xing, H. Xu, D. Jena, J. Appenzeller, and A. Seabaugh, Appl. Phys. Lett. 105, 192101(2014).
[28] A. Conan, A. Bonnet, M. Zoaeter, and D. Ramoul, Phys. Status Solidi B, 124, 403 (1984).
[29] S. Nakaharai, M. Yamamoto, K. Ueno, Y. F. Lin, S. L. Li, and K. Tsukagoshi, ACS Nano 9, 5976 (2015).
[30] H. Li, Z. Yin, Q. He, H. Li, X. Huang, G. Lu, D. W. H. Fam, A. I. Y. Tok, Q. Zhang, and H. Zhang, Small 8, 63 (2012).
[31] D. S. L. Abergel, V. Apalkov, J. Berashevich, K. Ziegler, and T. Chakraborty, Adv. Phys. 59, 261 (2010).
[32] Y. F. Lin, Y. Xu, C. Y. Lin, Y. W. Suen, M. Yamamoto, S. Nakaharai, K. Ueno, and K. Tsukagoshi, Adv. Mater. 26, 3263 (2015).
[33] I. G. Lezama, A. Arora, A. Ubaldini, C. Barreteau, E. Giannini, M. Potemski, and A. F. Morpurgo, Nano Lett. 15, 2336 (2015).
[34] D. H. Keum, S. Cho, J. H. Kim, D. H. Choe, H. J. Sung, M. Kan, H. Kang, J. Y. Hwang, S. W. Kim, H. Yang, K. J. Chang, and Y. H. Lee, Nat. Phys. 11, 482 (2015).
[35] L. Yang, W. Zhang, J. Li, S. Cheng, Z. Xie, and H. Chang, ACS Nano, 11, 1964 (2017).
[36] Y. C. Lin, D. O. Dumcenco, Y. S. Huang, and K. Suenaga, Nat. Nanotechnol. 9, 391 (2014).
[37] S. Song, D. H. Keum, S. Cho, D. Perello, Y. Kim, and Y. H. Lee, Nano Lett. 16, 188 (2016).
[38] S. Cho, S. Kim, J. H. Kim, J. Zhao, J. Seok, D. H. Keum, J. Baik, D. H. Choe, K. J. Chang, K. Suenaga, S. W. Kim, Y. H. Lee, and H. Yang, Science 349, 625 (2015).
[39] R. H. Williams, R. B. Murray, D. W. Govan, J. M. Thomas, and E. L. Evans, J. Phys. C 6, 3631 (1973).
[40] L. A. Burton, D. Colombara, R. D. Abellon, F. C. Grozema, L. M. Peter, T. J. Savenije, G. Dennler, and A. Walsh, Chem. Mater. 25, 4908 (2013).
[41] Y. Huang, E. Sutter, J. T. Sadowski, M. Cotlet, O. L.A. Monti, D. A. Racke, M. R. Neupane, D. Wickramaratne, R. K. Lake, B. A. Parkinson, and P. Sutter, ACS Nano 8, 10734 (2014).
[42] J. M. Gonzalez and I. I. Oleynik, Phys. Rev. B 94, 125443 (2016).
[43] K. R. Reddy, N. K. Reddy, and R. Miles, Sol. Energy Mater. Sol. Cells, 90, 3041 (2006).
[44] B. Luo, Y. Fang, B. Wang, J. Zhou, H. Song, and L, Zhi, Energy Environ. Sci. 5, 5226 (2012).
[45] W. Shi, L. Huo, H. Wang, H. Zhang, J. Yang, and P. Wei, Nanotechnology 17, 2918 (2006).
[46] H. Zhong, G. Yang, H. Song, Q. Liao, H. Cui, P. Shen, and C. Wang, J. Phys. Chem. C 116, 9319 (2012).
[47] Y. Sun, H. Cheng, S. Gao, Z. Sun, Q. Liu, F. Lei, T. Yao, J. He, S. Wei, and Y. Xie. Angew, Chem., Int. Ed., 51, 8727 (2012).
[48] C. Bacaksiz, S. Cahangirov, A. Rubio, R. T. Senger, F. M. Peeters, and H. Sahin. Phys. Rev. B 93, 125403 (2016).
[49] S. Datta and B. Das, Appl. Phys. Lett. 56, 665 (1990).
[50] I. Zutic, J. Fabian, and S. D. Sarma, Rev. of Mod. Phys. 76, 323 (2004).
[51] Y. Ahn, J. Dunning, and J. Park, Nano Lett. 5, 1367 (2005).
[52] Y. Yang, J. Li, H. Wu, E. Oh, and D. Yu, Nano Lett. 12, 5890 (2012).
[53] Y.H. Ahn, A.W. Tsen, B. Kim, Y.W. Park, and J. Park, Nano Lett. 7, 3320 (2007).
[54] M. Burghard and A. Mews, ACS Nano 6, 5752 (2012).
[55] W. J. Yu, Q. A. Vu, H. Oh, H. G. Nam, H. Zhou, S. Cha, J. Y. Kim, A. Carvalho, M. Jeong, H. Choi, A. H. C. Neto, Y. H. Lee, and X. Duan, Nat. Commun. 7, 13278 (2013).
[56] D. Jariwala, S. L. Howell, K. S. Chen, J. Kang, V. K. Sangwan, S. A. Filippone, R. Turrisi, T. J. Marks, L. J. Lauhon, and M. C. Hersam, Nano Lett. 16, 497 (2016).
[57] T. Wilson, Appl. Phys. 22, 119 (1980).
[58] M. H. Hecht, Phys. Rev. B 41, 7918 (1990).
[59] D. V. Lang and C. H. Henry, Solid-State Electron 21, 1519 (1978).
[60] J. Park, Y. H. Ahn, and C. Ruiz-Vargas, Naon Lett. 9, 1742 (2009).
[61] C. C. Wu, D. Jariwala, V. K. Sangwan, T. J. Marks, M. C. Hersam, and L. J. Lauhon, J. Phys. Chem. Lett. 4, 2508 (2013).
[62] M. Buscema, M. Barkelid, V. Zwiller, H. S. J. van der Zant, G. A. Steele, and A. Castellanos-Gomez, Nano Lett. 13, 358 (2013).
[63] Y. Yi, C. Wu, H. Liu, J. Zeng, H. He, and J. Wang, Nanoscale 7, 15711 (2015).
[64] H. Yamaguchi, J. C. Blancon, R. Kappera, S. Lei, S. Najmaei, B. D. Mangum, G. Gupta, P. M. Ajayan, J. Lou, M. Chhowalla, J. J. Crochet, and A. D. Mohite, ACS Nano 9, 840 (2015).
[65] 2D semiconductors, http://www.2dsemiconductors.com/
[66] H. Li, J. Wu, X. Huang, G. Lu, J. Yang, X. Lu, Q. Xiong, and H. Zhang, ACS Nano 7, 10344 (2013).
[67] M. Yamamoto, S. T. Wang, M. Ni, Y. F. Lin, S. L. Li, S. Aikawa, W. B. Jian, K. Ueno, K. Wakabayashi, and K. Tsukagoshi, ACS Nano 8, 3895 (2014).
[68] Stanford Research Systems Inc. Model SR570 Low-Noise Current Preamplifer. 1290-D Reamwood Avenue, Sunnyvale, CA 94089, U.S.A., 1995.
[69] Stanford Research Systems Inc. Model SR560 Low-Noise Voltage Preamplifer. 1290-D Reamwood Avenue, Sunnyvale, CA 94089, U.S.A., 1995.
[70] Stanford Research Systems Inc. Model SR830 Lock-In Amplifier. 1290-D Reamwood Avenue, Sunnyvale, CA 94089, U.S.A., 1995.
[71] N. R. Pradhan, D. Rhodes, S. Feng, Y. Xin, S. Memaran, B. H. Moon, H. Terrones, M. Terrones, and L. Balicas, ACS Nano 8, 5911 (2014).
[72] H. Liu, N. Han, and J. Zhao, RSC Adv. 5, 17572 (2015).
[73] L. Yin, X. Zhan, K. Xu, F. Wang, Z. Wang, Y. Huang, Q. Wang, C. Jiang, and J. He, Appl. Phys. Lett. 108, 043503 (2016).
[74] C.C. Wu, D. Jariwala, V. K. Sangwan, T. J. Marks, M. C. Hersam, and L. J. Lauhon, J. Phys. Chem. Lett. 4, 2508 (2013).
[75] N. Ubrig, S. Jo, H. Berger, A. F. Morpurgo, and A. B. Kuzmenko, Appl. Phys. Lett. 104, 171112 (2014).
[76] S. M. Sze and K. K. Ng, Physics of Semiconductor Devices, 3rd Edition, Wiley, New York, 2007.
[77] S. Kim, J. H. Kim, D, Kim, G, Hwang, J. Baik, H. Yang, and S. Cho, 2D Mater. 4, 024004 (2017).
[78] M. A. Camacho-Lopez, L. Escobar-Alarcon, M. Picquart, R. Arroyo, G. Cordoba, and E. Haro-Poniatowski, Optical Mater. 33, 480 (2011).
[79] J. Susoma, L. Karvonen, A. Saynatjoki, S. Mehravar, R. A. Norwood, N. Peyghambarian, K. Kieu, H. Lipsanen, and J. Riikonen, Appl. Phys. Lett. 108, 073103 (2016).
[80] C. Janisch, Y. Wang, D. Ma, N. Mehta, A. L. Elias, N. Perea-Lopez, M. Terrones, V. Crespi, and Z. Liu, Sci. Rep. 4, 5530 (2014).
[81] Y. Miyauchi, R. Morishita, M. Tanaka, S. Ohno, G. Mizutani, and T. Suzuki, Jpn. J. Appl. Phys. 55, 085801 (2016).
[82] R. Beams, L. G. Cançado, S. Krylyuk, I. Kalish, B. Kalanyan, A. K. Singh, K. Choudhary, A. Bruma, P. M. Vora, F. Tavazza, A. V. Davydov, and S. J. Stranick, ACS Nano 10, 9626 (2016).
[83] A. Castellanos-Gomez, R. Roldán, E. Cappelluti, M. Buscema, F. Guinea, H. S. J. van der Zant, and G. A. Steele, Nano Lett. 13, 5361 (2013).
[84] A. Allain, J. Kang, K. Banerjee, and A. Kis, Nature Mater. 14, 1195 (2015).
[85] J. K. Kim, K. Cho, T. Y. Kim, J. Pak, J. Jang, Y. Song, Y. Kim, B. Y. Choi, S. Chung, W. K. Hong, and T. Lee, Sci Rep. 6, 36775 (2016).
[86] J. Kang, D. Jariwala, C. R. Ryder, S. A. Wells, Y. Choi, E. Hwang, J. H. Cho, T. J. Marks, and M. C. Hersam, Nano Lett. 16, 2580 (2016).
[87] B. Akdim, R. Pachter, and S. Mou, Nanotechnology 27, 185701 (2016).
[88] X. Yan, C. Liu, C. Li, W. Bao, S. Ding, D. W. Zhang, and P. Zhou, Small 13, 1701478 (2017).
[89] H. Ji, M. K. Joo, Y. Yun, J. H. Park, G. Lee, B. H. Moon, H. Yi, D. Suh, and S. C. Lim, ACS Appl. Mater. Interfaces 8, 19092 (2016).