本實驗使用分子束磊晶(Molecular Beam Epitaxy, MBE)系統成長拓樸絕緣體MnSb2Te4於藍寶石(sapphire)(0001)基板上，透過調控Mn、Sb、Te三者的分壓比以得到晶向和表面結構穩定的MnSb2Te4薄膜。
因為MnSb2Te4薄膜本身表面容易氧化，因此會在MnSb2Te4薄膜表面成長2 nm厚的碲(Te)層作為保護(capping)層來保護表面，以便我們拿出分子束磊晶腔體後轉移到掃描穿隧電子顯微鏡腔體進行MnSb2Te4 表面結構的探測。
本實驗使用掃描穿隧電子顯微鏡(Scanning Tunneling Microscopy, STM)技術，此技術能呈現原子等級高解析度的表面形貌。
我們利用內部的燈絲通上電流以加熱燈絲，目的是要烘烤掉上述的碲保護層，使我們能更清楚的觀察 MnSb2Te4 薄膜的表面形貌。因為樣品本身脆弱容易被烘烤掉，因此我們會分數次去烘烤掉 MnSb2Te4 薄膜表面上的 Te 層，每次烘烤完時都會使用 STM 測量其表面形貌且觀察每次的表面形貌狀況。
在進行前三次的烘烤後，首先我們在大範圍下(100 nm×100 nm)觀察表面是否有明顯的乾淨表面平台與階梯結構，我們發現表面都有不規則的顆粒結構、表面粗糙度都偏大且階梯結構也不明顯，這表示碲保護層仍未完全清乾淨。
再進行到第四次烘烤後，因薄膜表面的碲保護層已減少許多，這時我們可觀察平坦的階梯表面。在小範圍下(10.5 nm×10.5 nm)可觀察到原子尺度的表面形貌，並確認表面Te層的結構與原子間的距離。
最後，我們使用掃描穿隧電子能譜(Scanning Tunneling Spectroscopy, STS)技術來觀察MnSb2Te4表面局域態密度(Local Density Of State, LDOS)結構且發現其狄拉克點(Dirac point)的位置。

The most important feature of a Topological Insulator (TI) is that it conducts electricity on the surface but does not conduct electricity inside. There is a surface state between the conduction band and valence band of the topological insulator, enabling electrons to move between the two band to form a Dirac cone.
MnSb2Te4 is a magnetic topological insulator thin film. In this study, we grew MnSb2Te4 thin films on the Al2O3 sapphire c-plane (0001) substrate with Molecular Beam Epitaxy (MBE)system in an ultrahigh vacuum (UHV) chamber highly pure Mn (99.99 %), Sb (99.9999 %) and Te (99.9999 %) were co-evaporated from Knudsen effusion cells. The ratio of Mn, Sb and Te is 1 : 2 : 4. By controlling with the flux ratio to get a single-phase structure and stable surface structure of MnSb2Te4 thin films.
Because the MnSb2Te4 thin films is easy to oxidize, we grew a 2 nm thick Te capping layer with above-mentioned MBE system to prevent the situation.
First, we measured the MnSb2Te4 and Sb2Te3 crystal structure with X-ray Diffraction (XRD).
Next, we put the above thin films sample into the Scanning Tunneling Microscopy (STM) instrument, and applied current to the internal tungsten filament to heat the filament. The purpose was to bake off the Te capping layer, so that we could observe the surface morphology of the MnSb2Te4 thin films more clearly.
Because the sample itself is fragile and easy to be baked off, we would bake off the Te capping layer for several times.
Next, we would use large-scale mode (100 nm × 100 nm) in STM to measure the surface morphology after each baking and compare the difference in surface and Root Mean Square roughness value Rq.
Whether MnSb2Te4 was an integer Septuple Layers(SLs) structure was very important in this experiment.If it was not, which represented a residual Te capping layer that still existed on the surface of the films, then we would perform the baking step again.
After the fourth baking, we would begin to use the small-scale mode (10.5 nm × 10.5 nm or 6.0 nm × 6.0 nm) in STM to confirm the structural shape of the MnSb2Te4 itself surface Te layer and calculated the Te interatomic distance.After measuring the surface morphology in STM, we would use some STM software to do the analysis.
Finally, the Local Density Of States (LDOS) structure on the surface of MnSb2Te4 had been observed utilizing Scanning Tunneling Spectroscopy (STS) technique. The size of the energy gap of the surface states and Dirac point position of MnSb2Te4 could be confirmed from STS dI/dV-V curve results.

[1] L. Chen et al., "Electronic structure and magnetism of MnSb2Te4," J. Mater. Sci., vol. 55, pp. 14292-14300, 2020.
[2] S. C. Huan et al., "Magnetism-induced ideal Weyl state in bulk van der Waals crystal MnSb2Te4," (in English), Appl. Phys. Lett., Article vol. 118, no. 19, p. 5, May 2021, Art no. 192105, doi: 10.1063/5.0047438.
[3] W. Ko et al., "Realizing gapped surface states in the magnetic topological insulator Mn Bi 2− x Sb x Te 4," Phys. Rev. B, vol. 102, no. 11, p. 115402, 2020.
[4] S. Wimmer et al., "Mn-Rich MnSb2Te4: A Topological Insulator with Magnetic Gap Closing at High Curie Temperatures of 45-50 K," (in English), Adv. Mater., Article vol. 33, no. 42, p. 11, Oct 2021, Art no. 2102935, doi: 10.1002/adma.202102935.
[5] Z. H. Zang et al., "Layer-Number-Dependent Antiferromagnetic and Ferromagnetic Behavior in MnSb2Te4," (in English), Phys. Rev. Lett., Article vol. 128, no. 1, p. 6, Jan 2022, Art no. 017201, doi: 10.1103/PhysRevLett.128.017201.
[6] H. S. Zhang, W. J. Yang, Y. Y. Wang, and X. H. Xu, "Tunable topological states in layered magnetic materials of MnSb2Te4, MnBi2Se4, and MnSb2Se4," (in English), Phys. Rev. B, Article vol. 103, no. 9, p. 9, Mar 2021, Art no. 094433, doi: 10.1103/PhysRevB.103.094433.
[7] "拓撲絕緣體," Wikipedia, May 2 2015.
[8] "實驗二十五-掃描式穿隧顯微鏡," 國立交通大學 物理實驗手冊 掃描穿隧式顯微鏡 (STM), p. 9.

[9] C.-Y. W. 簡紋濱、吳至原 Wen-Bin Jian, "拆解掃描穿隧顯微鏡(Disassembling Scanning Tunneling Microscope)," 科儀新知第二十七卷第二期, p. 6, October 94.
[10] 葉勝玄, "穿隧接點微分電導之物理簡介," 物理雙月刊（廿八卷五期）, p. 6, October 2006.
[11] 范戊靖, "開發節能型研究用之掃描式穿隧電流顯微鏡(Design of green and research type scanning tunneling microscope)," 學術論文, p. 53, 2011.
[12] "如何用穿隧效應，洞察量子天地？中研院自行架設掃描穿隧能譜," 中央研究院 研之有物, June 2017.
[13] 莊維倫, "半導體量子點陣列中電性傳輸與電子結構對尺寸之相關性(Size dependence in charge transport and electronic structure of semiconductor quantum-dot arrays)," 學術論文, p. 64, July 2010.
[14] 王雅馨, "鎖相放大器研究及其去除光雜訊之應用(A Study on Lock-in Amplifier and Its Application on Removing Optical Noise)," 學術論文, p. 96, July 2015.
[15] 蘇維彬W.-B. Su, "低溫掃描穿隧顯微儀及其應用
(Low Temperature Scanning Tunneling Microscope and Related Applications)," 科儀新知第二十四卷第一期, p. 8.
[16] "Scanning tunneling microscopy (STM)," Zeljkovic Lab Nanoscale Synthesis and Characterization of Quantum Materials, p. 2, 2015.
[17] "何謂表面粗糙度," KEYENCE.
[18] J.-T. CHANG, "MnBi2Te4 薄膜缺陷結構之研究(The study of defect structures in MnBi2Te4 grown thin films)," 學術論文, p. 146, September 2021.
[19] M. W. Y. Liu, and L. Li, "Spiral Growth without Dislocations: Molecular Beam Epitaxy of the Topological Insulator Bi2Se3 on Epitaxial Graphene/SiC(0001)," Phys. Rev. Lett., vol. 108, p. 5, March 14 2012., doi:10.1103/PhysRevLett.108.115501.
[20] Y.-S. Yang, "磁性拓樸絕緣體Mn(Sb1-xBix)2Te4之能帶結構與磁電性分析(Band Structure and Magnetoelectric Analysis of Magnetic Topological Insulator Mn(Sb1-xBix)2Te4)," 學術論文, p. 101, 2022.