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Weyl semimetal Td-phase WTe2, a novel topological matter, possesses a strong spin-orbit coupling and non-trivial topological band structure, and thus becomes a very promising superior spin current source material. By constructing the WTe2/Ti heterostructures, the issue that the ferromagnetic layer with perpendicular magnetic anisotropy cannot be directly prepared on WTe2 layer can be well addressed, and meet the requirements for high-performance spin-orbit torque devices. To be compatible with the semiconductor technology, the device integration usually involves a high temperature process. Therefore, the thermal stability of WTe2/Ti is critical for practical device fabrication and performance. However, the thermal stability of WTe2/Ti interface has not been very clear yet. In this work, the micro-Raman scattering technique is used to systematically study the WTe2/Ti interface annealed at different temperatures. It is found that the thermal stability of the interface between WTe2 and Ti is related to the thickness of WTe2 flake; appropriate increase of the WTe2 thickness can lead to the improvement of thermal stability in WTe2/Ti heterostructures. In addition, high temperature annealing can cause a significant interfacial reaction. After annealed at 473 K for 30 min, the interface between WTe2 (12 nm) and Ti changes dramatically, leading to the formation of Ti-Te interface layer. This observation is highly consistent with the observations by high-resolution transmission electron microscopy and the elemental analysis results as well. This study will provide useful information for further exploring the influence of the WTe2/Ti interface on the spin-orbit torque effect, and greatly invigorate the research area of energy efficient spintronic devices based on WTe2 and other novel topological materials.
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Keywords:
- Weyl semimetal WTe2 /
- WTe2/Ti heterostructure /
- thermal stability /
- Raman scattering
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图 1 (a) 外尔半金属Td-WTe2单晶的晶体结构示意图; (b)机械剥离的WTe2纳米片的光学显微镜照片; (c) 机械剥离的WTe2纳米片的室温拉曼光谱图, 测量区域为图(b)中红色虚线框所示
Figure 1. (a) Schematic diagram of the crystal structure of Weyl semimetal Td-WTe2 single crystal; (b) optical image of mechanically exfoliated WTe2 flake; (c) room temperature Raman spectra of mechanically exfoliated WTe2 flake, the measurement area is indicated by the red dashed box in panel (b).
图 2 (a) WTe2/Ti异质结的光学显微镜照片, 图中①—⑤代表具有不同WTe2厚度的异质结区域; (b) AFM扫描图, 测量区域为图(a)中红色虚线框所示; (c)从AFM扫描图中沿着红色虚线的异质结台阶高度图
Figure 2. (a) Optical image of WTe2/Ti heterostructures with different WTe2 thickness denoted by ①–⑤; (b) AFM image of WTe2/Ti heterostructure, the scanned area is denoted by the red dashed box in panel (a); (c) the height of one WTe2/Ti step along the red dashed line in panel (b).
图 3 室温下WTe2 (12—32 nm)/Ti异质结的(a) 非偏振拉曼光谱图, (b) 垂直偏振拉曼光谱图, (c) 平行偏振拉曼光谱图. 图中数字代表不同的WTe2厚度, “WTe2”代表机械剥离的WTe2单晶对照样品, 其厚度大于100 nm
Figure 3. (a) Unpolarized Raman spectra, (b) vertically polarized Raman spectra, and (c) parallel polarized Raman spectra of WTe2 (12–32 nm)/Ti heterostructures at room temperature. The numbers in all figures represent WTe2 thickness, “WTe2” denotes the mechanically exfoliated WTe2 single crystal with thickness larger than 100 nm.
图 4 不同温度退火的WTe2 (12—32 nm)/Ti异质结的室温拉曼光谱 (a) WTe2 (12 nm)/Ti异质结分别在制备态和323—523 K退火后的非偏振拉曼光谱图; (b) WTe2 (12 nm)/Ti异质结在473 K退火后界面反应生成Ti-Te化合物的非偏振拉曼光谱放大图; (c) WTe2 (32 nm)/Ti异质结分别在制备态和 323—523 K退火后的非偏振拉曼光谱图; (d) WTe2 (12, 18, 19, 20, 32 nm)/Ti异质结退火后界面生成Ti-Te的拉曼峰峰强随着退火温度的变化曲线
Figure 4. Room temperature Raman spectra of WTe2 (12–32 nm)/Ti heterostructures annealed at different temperatures: (a) Unpolarized Raman spectra of WTe2 (12 nm)/Ti heterostructure at as-grown state and annealed at 323-523 K, respectively; (b) enlarged unpolarized Raman spectra of Ti-Te interfacial reaction layer in WTe2 (12 nm)/Ti heterostructure annealed at 473 K; (c) unpolarized Raman spectra of WTe2 (32 nm)/Ti heterostructure at as-grown state and annealed at 323–523 K, respectively; (d) Raman intensity of the Ti-Te interfacial reaction layer in WTe2 (12, 18, 19, 20, 32 nm)/Ti heterostructures as a function of the annealing temperature.
图 5 (a) WTe2/Ti (30 nm) 异质结的高分辨TEM图片, 样品在473 K退火30 min; (b) 放大的WTe2/Ti界面高分辨TEM图片; (c) EDS元素分析图像; (d) 沿着图 (c) 中箭头方向的EDS线扫结果
Figure 5. (a) High-resolution TEM image of WTe2/Ti (30 nm) heterostructure annealed at 473 K for 30 min; (b) enlarged high-resolution TEM image of WTe2/Ti interface; (c) EDS mapping image; (d) EDS line scanning along the arrow direction in panel (c).
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[1] Slonczewski J C 1996 J. Magn. Magn. Mater. 159 L1
Google Scholar
[2] Berger L 1996 Phys. Rev. B 54 9353
Google Scholar
[3] Miron I M, Garello K, Gaudin G, Zermatten P J, Costache M V, Auffret S, Bandiera S, Rodmacq B, Schuhl A, Gambardella P 2011 Nature 476 189
Google Scholar
[4] Bhatti S, Sbiaa R, Hirohata A, Ohno H, Fukami S, Piramanayagam S N 2017 Mater. Today 20 530
Google Scholar
[5] Lee K S, Lee S W, Min B C, Lee K J 2013 Appl. Phys. Lett. 102 112410
Google Scholar
[6] Fukami S, Anekawa T, Zhang C, Ohno H 2016 Nat. Nanotechnol. 11 621
Google Scholar
[7] Liu L Q, Pai C F, Li Y, Tseng H W, Ralph D C, Buhrman R A 2012 Science 336 555
Google Scholar
[8] Liu L Q, Lee O J, Gudmundsen T J, Ralph D C, Buhrman R A 2012 Phys. Rev. Lett. 109 096602
Google Scholar
[9] Wang Y, Deorani P, Qiu X P, Kwon J H, Yang H 2014 Appl. Phys. Lett. 105 152412
Google Scholar
[10] Liu L Q, Moriyama T, Ralph D C, Buhrman R A 2011 Phys. Rev. Lett. 106 036601
Google Scholar
[11] Pai C F, Liu L Q, Li Y, Tseng H W, Ralph D C, Buhrman R A 2012 Appl. Phys. Lett. 101 122404
Google Scholar
[12] Wang Y, Deorani P, Banerjee K, Koirala N, Brahlek M, Oh S, Yang H 2015 Phys. Rev. Lett. 114 257202
Google Scholar
[13] Wang Y, Zhu D P, Wu Y, Yang Y M, Yu J W, Ramaswamy R, Mishra R, Shi S Y, Elyasi M, Teo K L, Wu Y H, Yang H 2017 Nat. Commun. 8 1364
Google Scholar
[14] Wang Y, Zhu D P, Yang Y M, Lee K, Mishra R, Go G, Oh S H, Kim D H, Cai K M, Liu E, Pollard S D, Shi S Y, Lee J, Teo K L, Wu Y H, Lee K J, Yang H 2019 Science 366 1125
Google Scholar
[15] Han X F, Wan C H, Yu G Q 2021 Appl. Phys. Lett. 118 180401
Google Scholar
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Google Scholar
He C L, Xu H J, Tang J, Wang X, Wei J W, Shen S P, Chen Q Q, Shao Q M, Yu G Q, Zhang G Y, Wang S G 2021 Acta Phys. Sin. 70 127501
Google Scholar
[17] 强晓斌, 卢海舟 2021 物理学报 70 027201
Google Scholar
Qiang X B, Lu H Z 2021 Acta Phys. Sin. 70 027201
Google Scholar
[18] MacNeill D, Stiehl G M, Guimaraes M H D, Buhrman R A, Park J, Ralph D C 2017 Nat. Phys. 13 300
Google Scholar
[19] MacNeill D, Stiehl G M, Guimaraes M H D, Reynolds N D, Buhrman R A, Ralph D C 2017 Phys. Rev. B 96 054450
Google Scholar
[20] Shi S Y, Liang S H, Zhu Z F, Cai K M, Pollard S D, Wang Y, Wang J Y, Wang Q S, He P, Yu J W, Eda G, Liang G C, Yang H 2019 Nat. Nanotechnol. 14 945
Google Scholar
[21] Li P F, Kang Y, Zhao Y B, Qin J H, Song W G 2018 ISA Trans. 80 1
Google Scholar
[22] Shi S Y, Li J, Hsu C H, Lee K, Wang Y, Yang L, Wang J Y, Wang Q S, Wu H, Zhang W F, Eda G, Liang G C, Chang H X, Yang H 2021 Adv. Quantum Technol. 4 2100038
Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
[26] Lee H Y, Kim S, Park J Y, Oh Y W, Park S Y, Ham W, Kotani Y, Nakamura T, Suzuki M, Ono T, Lee K J, Park B G 2019 APL Mater. 7 031110
Google Scholar
[27] Zhang L Y, Liu N, Li W B, Luo L M, Wang Y 2022 Solid State Commun. 342 114620
Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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Google Scholar
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[34] Kim M, Han S, Kim J H, Lee J U, Lee Z, Cheong H 2016 2 D Mater. 3 034004
[35] Cordes H, Schmidfetzer R 1995 J. Mater. Sci. -Mater. Electron. 6 118
[36] Cooley K A, Mohney S E 2019 J. Vac. Sci. Technol. A 37 061510
Google Scholar
[37] Yu C C, Wu H J, Agne M T, Witting I T, Deng P Y, Snyder G J, Chu J P 2019 APL Mater. 7 013001
Google Scholar
[38] Hangyo M, Nakashima S I, Mitsuishi A 1983 Ferroelectrics 52 151
Google Scholar
[39] Rajaji V, Dutta U, Sreeparvathy P C, Sarma S C, Sorb Y A, Joseph B, Sahoo S, Peter S C, Kanchana V, Narayana C 2018 Phys. Rev. B 97 085107
Google Scholar
[40] Ding H, Xu B 2012 J. Chem. Phys. 137 224509
Google Scholar
[41] Khan J, Nolen C M, Teweldebrhan D, Wickramaratne D, Lake R K, Balandin A A 2012 Appl. Phys. Lett. 100 043109
Google Scholar
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