拓撲絕緣體碲化銻薄膜之成長及其特性之研究

拓樸絕緣體 (Topological insulator) 重要應用之一為可製成自旋電子元件。因為其擁有的時間反轉對稱性(time reversal symmetry)保護的表面態(surface state)可以抑制背向散射(backscattering)的發生。當拓樸絕緣體的費米能階(fermi level)位於能隙之間時，其樣品內部卻具有絕緣體的特性, 但其樣品表面為金屬態，具有高導電性且能保持其電子自旋特性。因此非常適合用於製成自旋電子元件。在此研究中，我們利用物理汽相沉積法(Physical vapor deposition)成功將拓樸絕緣體碲化銻(Sb2Te3) 薄膜沉積在藍寶石(Sapphire)基板上。和其他成長碲化銻的方法如分子束磊晶法(MBE)和有機金屬化學汽相沉積法(MOCVD)相比，物理汽相沉積法系統簡單，操作容易，可以低廉成本製成高品質的拓樸絕緣體薄膜。原子力顯微鏡(AFM)量測中我們觀察到我們成長的碲化銻薄膜表面具有明顯的三角結構且由五層原子層(quintuple layers)為單位層層堆疊而成。另外我們也發現在同樣的長晶製程下，樣品的厚度會與碲化銻粉末和基板的距離相關。再來由X光繞射儀(XRD) 2-θ掃描模式發現碲化銻的晶格結構具有C軸指向的特性，晶格常數C為30.3埃。由能量色散X射線光譜儀(EDS)發現碲化銻薄膜由接近理想比例的43% 碲(Sb)和57 % 銻(Te)所組成。再來由化學分析電子光譜儀(ESCA)所得到的光譜發現碲化銻薄膜由碲與銻元素組成。我們使用微影製程(lithography)和濕式蝕刻(wet etching)將30 nm 的碲化銻薄膜製做成Hall bar圖案來做所有的電性量測。我們發現碲化銻和鈦(4nm) /金(50nm) 接合下並使用快速高溫熱退火系統(Rapid thermal annealing system)後，材料在室溫到2K之下皆保持歐姆接觸(Ohmic contact)的特性。再來利用鎖向放大器(lock-in amplifier)來量測材料的電性，可以得到在2K時載子的濃度以及電子遷移率分別為1.9x1019 cm-3和184 cm2 /V∙s. 而在磁阻量測中，在低磁下具有反弱局域化現象(Weak anti-localization effect), 證明了碲化銻具有拓樸絕緣體的特性。藉由Hikami-Larkin-Nagaoka(HLN)的模型以及磁阻的量測數據，透過曲線擬和(fitting)的結果，在2K時載子的相位相干長度(Phase coherence length)的值為309 nm.

關鍵字

拓樸絕緣體；物理汽相沉積法；碲化銻

並列摘要

Topological insulator (TI) has great potential in making into spintronic devices because it has a time reversal symmetry protected surface state that forbids backscattering to occur. For TIs, when the Fermi level is within the bulk bandgap, the bulk of the sample is insulating. However, because there is no energy gap in surface states, the surface is metallic and can be used for spin-dependent transport. In this study, we report the successful growth of topological insulator Sb2Te3 thin film on Al2O3 substrate by using a physical vapor deposition (PVD) system. Compared with the other TI thin film growth methods, such as molecular beam epitaxy system (MBE), atomic layer deposition, metal organic chemical vapor deposition (MOCVD), PVD has the advantage that it is a simple and cost effective method to grow high quality TI samples. Atomic force microscopy (AFM) measurements indicate the surface morphology of Sb2Te3 films grown by using PVD have clear triangular structure domains consist of step by step quintuple layers, and the thickness of the thin film is correlated with the distance between Al2O3 substrate and Sb2Te3 source powder. X ray diffraction (XRD) measurement shows Sb2Te3 film grown on Al2O3 substrate is c-axis oriented and the value of lattice constant in the c-direction is 30.3Å. Energy-dispersive spectroscopy (EDS) shows that the percentage composition of Sb and Te atoms in the film are 43% and 57 %, respectively, which is close to ideal 2:3 ratio for Sb2Te3. Electron spectroscopy for chemical analysis (ESCA) measurements indicate that the chemical elements in the thin film consisted of antimony and tellurium. Standard lock-in method to do measurements from 2K to 300K. Temperature dependent resistivity shows metallic behavior but it has metallic insulator transition at 6K. At low temperature range, the resistivity can be fitted with parallel conduction model showing the coexistence of bulk state and surface state. The concentration is in the order of 1019 cm-3 within 2K to 300K and positive slopes in Hall measurements reveals holes dominated behavior. The mobility is 184 cm2/V∙s at 2K. In the magneto-resistivity measurement at 2K, the cusp feature appears at low field shows the evidence of weak anti-localization effect. Fitting data by using HLN model, we found that the dephasing length of carriers of the thin film is 309 nm at 2K.

並列關鍵字

topological insulator ； physical vapor deposition ； Sb2Te3

參考文獻

[1] C. Z. Chang, P. Tang, X. Feng, K. Li, X. C. Ma, W. Duan, K. He and Q. K. Xue, Physical Review Letters, 115(13). (2015)

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[2] Ji. S. Wang, L. Chen, X. K. He, Y. Wang, X. Ma & Q. Xue, Extended Abstracts of the 2015 International Conference on Solid State Devices and Materials. (2015).