以物理汽相沉積法成長拓樸絕緣體Bi(2-x)SbxTe(3-y)Sey薄膜及其特性之研究

拓樸絕緣體是一種新穎材料，因其具有相當獨特之性質而在近幾年引起不少關注。與一般所見之導體或絕緣體不同，拓樸絕緣體內部的能帶結構是一有能距之能帶結構，但在其表面能帶結構卻是一無能距之Dirac cone 能帶結構，兩種迥異的性質卻同時存在於單一種材料上。該表面態受時間反演對稱保護；處於表面態的載子其自旋方向永遠與動量方向維持90°，故而可以大幅抑制傳輸過程中載子的背向散射機率，故呈現高度金屬性。二元硫族化物，如硒化鉍(Bi2Se3)、碲化鉍(Bi2Te3)以及碲化銻(Sb2Te3)已被證實為三維拓樸絕緣體且已被廣泛研究。然而部分研究已指出，由於製備樣品時產生的大量缺陷導致傳輸特性是被內部塊材的載子所主導要想研究這些物質表面態的特性是有一定困難度的。近來有研究指出，要為研究拓樸絕緣體表面之性質，三元甚至四元的碲鉍礦可能是較合適的選擇，這類的材料通常會有較低的載子濃度因此內部塊材較容易呈現出絕緣態的特性。目前絕大部分成長四元碲鉍礦的方法是用布里奇曼法(Bridgeman method)來製備單晶塊材，接著以剝離法(exfoliation)撕起一小塊加以量測。非常少數的研究著重於此四元碲鉍礦薄膜的製備。在此篇論文中，我們發展了以硒化鉍以及碲化銻作為來源粉末，使用物理汽象沉積法 (PVD) 在藍寶石基板上成長四元碲鉍礦的薄膜的方法。藉由原子力顯微鏡(AFM)以及X光繞射儀(XRD)，我們發現用此一方法成長的薄膜具有層狀結構和良好的結晶性。X光能譜儀(XPS)以及電子維探儀(EPMA)來量測表面成份(比)，分析結果明確指出存有鉍、銻、碲和硒四種元素且成份比(鉍：銻：碲：硒)為：1.47:0.68:0.74:2.11。電性以及磁阻特性是藉由物理特性量測系統(PPMS)量測；由霍爾效應可得載子為電子(n-type)，載子濃度在絕對溫度2 K時是2.3 x 1018 cm-3，同時，載子遷移率是277 cm2/Vs；弱反局部化效應 (weak anti-localization effect) 在低溫低磁場下也可明顯觀察到，並可藉由Hikami-Larkin-Nagaoka (HLN)方程式加以分析。分析結果發現我們的薄膜電子在低溫時相位相干長度(phase coherence length)為149奈米，且樣品上下表面皆有表面態載子傳輸，證實了我們的薄膜具有拓樸絕緣體的性質。

關鍵字

拓樸絕緣體；四元；薄膜；物理汽相沉積法

並列摘要

Topological insulators (TIs) are a new kind of novel materials that have attracted much attentions recently. One of the most unusual properties is that TIs have a full band gap in the bulk and a gapless linearly dispersed Dirac-cone states in the surface. The surface states of the TIs are protected by time-reversal symmetry, the carriers in the surface states have their spin locked to their momentum, and as a result the probabilities of backscattering of conducting carrier is strongly suppressed. The surface state is thus a highly conductive metallic state. Binary chalcogenides Bi2Se3, Bi2Te3 and Sb2Te3 are model TIs and have been widely studied. However, it is found that it is difficult to investigate the properties of the surface states in these materials because large number of defects in the material were generated during the samples growth and make thee bulk material highly conducting. It is thus difficult to isolate the surface conduction properties from electric transport measurements. On the other hand, it is shown recently that Bi2-xSbxTe3-ySey (BSTS) is a good candidate for studying three-dimensional (3D) TI because it has relatively low carrier concentration in the bulk and is less conducting, and is probably a better candidate for the study of the surface state of the TI. BSTS thin film or flake is usually obtained by exfoliation of bulk BSTS single crystal grown by modified Bridgeman method. Very few studies have been made on the growth of BSTS thin film. In this study, with Bi2Se3 powder and Sb2Te3 powder used as source powders, a physical vapor deposition (PVD) system was employed to grow BSTS thin films on c-plane sapphire substrates. Atomic force microscope (AFM) and X-ray diffraction (XRD) measurements show that the as-grown thin films have layer-by-layer structure and have high crystalline quality. X-ray photoelectron spectroscopy (XPS) and electron probe micron analysis (EPMA) indicate that the elemental distribution of our sample is uniform and the atomic ratio of Bi: Sb: Te: Se is found to be about 1.47: 0.68: 0.74: 2.11. Electric and magnetoresistance (MR) properties were measured by physical properties measurement system (PPMS). The measurement of resistivity at different temperature indeed manifests an insulating behavior. From Hall measurement, the carrier density and the mobility in our thin film at T = 2 K are 2.3×1018 cm-3, 277 cm2/Vs, respectively and the carrier type is found to be n-type. Weak anti-localization effect (WAL) was observed from the MR measurement at low temperature and can be fitted well by using Hikami-Larkin-Nagaoka (HLN) equation. From the fitting it is found that both sides of the surface are conducting and the phase coherence length lψ= 149 nm at T = 2 K.