磁性范德华异质结铋-碲化铬的外延生长研究

近年来，单质铋（Bi）由于其显著的自旋轨道耦合效应及独特的二维结构引起了人们的广泛关注。 特别是当二维铋与铁磁材料 Cr₂Te₃ 形成界面时，会引发 Dzyaloshinskii-Moriya 相互作用，从而为磁性斯格明子的产生提供了平台。然而，生长二维铋薄膜的条件相当苛刻，这为在异质结界面精确控制其生长带来了极大的挑战。

本文以 Bi 基范德华异质结 Cr₂Te₃/Bi/Bi₂Te₃ 为研究对象，采用分子束外延技术（MBE），通过还原反应成功克服了生长二维铋的条件限制，最终在Cr₂Te₃/Bi₂Te₃ 异质结的界面处成功合成了具有黑磷结构的双层 Bi 纳米片。通过对生长条件的深入探索和优化，我们构筑了高质量的 Bi 基范德华异质结， 其具体结果如下：

利用 MBE 技术，我们在 Al₂O₃(0001)、 SrTiO₃(100)和 GaAs(111)三种衬底外延生长了 Bi2Te3 薄膜。通过向薄膜表面沉积 Cr 原子的方式还原出铋的纳米片，我们在 Al₂O₃(0001)和 GaAs(111)衬底上构建出 Cr₂Te₃/Bi 的异质结，并证明了衬底与薄膜匹配程度是决定异质外延薄膜质量的关键因素。

借助透射电子显微镜对所有样品进行了结构和形貌表征，结果显示还原方法产生的 Bi 纳米片均为黑磷结构。通过比较三种不同衬底上的薄膜形貌，我们观察到生长在 SrTiO₃衬底上的 Bi₂Te₃薄膜具有双晶畴结构，而在Al₂O₃ 和 GaAs 衬底上的薄膜则没有这种双晶畴结构。此外，我们还发现在GaAs(111)衬底上还原出的 Bi 纳米片在尺寸和层数上均大于在 Al₂O₃(0001)衬底上得到的 Bi 片。 与前人通过退火方式制备的 Bi 片相比，我们的还原方法制备的铋片不仅尺寸更大， 并且更加平整。本文展示了 Bi 基异质结在不同衬底上的制备， 揭示了衬底类型对薄膜生长质量的影响。通过优化生长条件和调整衬底选择， 我们可以有效地控制 Bi 纳米片的尺寸和分布， 为在磁性界面引入重元素提供了一种可行的途径，这进一步推动了在自旋电子学和磁性拓扑材料领域的研究进展。

In recent years, elemental bismuth (Bi) has garnered widespread interest due to  its  pronounced  spin-orbit  coupling  effect  and  unique  two-dimensional structure.  Particularly,  when  two-dimensional  bismuth  interfaces  with  the ferromagnetic material Cr₂Te₃, it triggers the Dzyaloshinskii-Moriya interaction, providing a platform for the generation of magnetic skyrmions. However, the stringent conditions required for growing two-dimensional bismuth thin films pose significant challenges in precisely controlling its growth at heterostructure interfaces.
This  study  focuses  on  the  Bi-based  van  der  Waals  heterostructure
Cr₂Te₃/Bi/Bi₂Te₃. Employing molecular beam epitaxy (MBE) and a reduction reaction technique, we successfully overcame the limitations of growing two -dimensional  bismuth,  synthesizing  black-phosphorus-structured  bilayer  Bi
nanosheets at the Cr₂Te₃/Bi₂Te₃ interface. Through extensive exploration and optimization of growth conditions, we constructed high-quality Bi-based van derWaals heterostructures, with key findings as follows:
Utilizing  MBE,  we epitaxially  grew  Bi₂Te₃  thin  films  on Al₂O₃(0001),
SrTiO₃(100), and GaAs(111) substrates. By depositing Cr atoms on the film
surfaces,  we  reduced  bismuth  nanosheets  and  constructed  Cr₂Te₃/Bi
heterostructures on Al₂O₃(0001) and GaAs(111) substrates, demonstrating that the  compatibility  between  substrate  and  film  is  crucial  for  the  quality  of heteroepitaxial films.
Structural  and  morphological  characterizations  of  all  samples  were
conducted using transmission electron microscopy (TEM). The results revealed that the Bi nanosheets produced through the reduction method all possessed a black  phosphorus  structure.  Comparative  analysis  across  different  substrates showed  that  Bi₂Te₃ films  grown  on  SrTiO₃ substrates  featured  twin  grain boundary structures, which were absent in films grown on Al₂O₃ and GaAs substrates. Additionally, Bi nanosheets reduced on GaAs(111) were larger and more  numerous  in  layers  compared  to  those  on Al₂O₃(0001).  Relative  to previously  annealed  Bi  sheets,  our  reduction  method  produced  larger  and smoother bismuth sheets.
In conclusion, this paper presents the synthesis of Bi-based heterostructures on various substrates, highlighting the influence of substrate type on film growth quality. By optimizing growth conditions and selecting appropriate substrates,
we can effectively control the size and distribution of Bi nanosheets, offering a viable method for incorporating heavy elements into magnetic interfaces. This advancement  propels  research  in  the  fields  of  spintronics  and  magnetic topological materials.

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