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外尔半金属Td-WTe2是一种新型的拓扑量子材料, 具有很强的自旋轨道耦合作用和独特的拓扑能带结构, 被认为是一种非常有潜力的自旋电子材料. 通过构造WTe2/Ti异质结构, 能够解决原本在WTe2上无法直接制备出具有垂直磁各向异性铁磁层的难题. 与现有半导体工艺相兼容, 器件集成需要经受高温处理过程, 因此WTe2/Ti的热稳定性对于实际器件制备和应用至关重要. 然而, WTe2/Ti界面的热稳定性目前仍然不清楚. 本文利用显微拉曼散射技术系统研究了不同温度退火后的WTe2/Ti异质结的热稳定性, 发现WTe2和Ti的界面热稳定性与WTe2纳米片的厚度相关, WTe2纳米片厚度适当增加, WTe2/Ti异质结更加稳定. 此外, 高温退火会导致更加强烈的界面反应, 在473 K退火30 min后, WTe2 (12 nm)与Ti发生明显界面反应, 生成Ti-Te化合物, 该现象与高分辨透射电子显微镜测量和元素分析结果高度一致. 研究结果为进一步探究WTe2/Ti界面对于自旋轨道转矩效应的影响提供有用信息, 激发基于WTe2等拓扑材料的低功耗自旋器件研究.
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关键词:
- 外尔半金属WTe2 /
- WTe2/Ti异质结 /
- 热稳定性 /
- 拉曼散射
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.-
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
Fig. 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
Fig. 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
Fig. 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
Fig. 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
Fig. 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
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Google Scholar
[6] Fukami S, Anekawa T, Zhang C, Ohno H 2016 Nat. Nanotechnol. 11 621
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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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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