分子束磊晶成長之拓樸絕緣體：透過磁鄰近效應及磁性參雜磁化高品質碲化鉍銻薄膜

Abstract

最近，透過引入磁序來破壞時間反演對稱性，進而在拓撲表面態打開一個交換間隙的拓撲絕緣體中已觀察到量子反常霍爾效應。當費米能級被調入表面帶隙之中，便可以觀察到在沒有外加磁場的情況下達到量化的霍爾電阻。拓撲絕緣體與磁性絕緣體之異質結構上的磁鄰近效應被證明是一種在顯著高溫下實現量子異常霍爾效應的方式。本論文中，首先透過分子束磊晶技術及二階段長晶(two-step growth) ，以非晶的碲化鉍銻((Bi,Sb)2Te3)薄膜作為緩衝層的方式，於銪鐵石榴石(Eu3Fe5O12, EuIG)上成長c軸取向外延(c-axis oriented epitaxial)之拓撲絕緣體薄膜碲化鉍銻。在成長碲化鉍銻薄膜前會對銪鐵石榴石進行不同溫度的高溫退火處理，以優化銪鐵石榴石的起始表面狀態。我們展示了有序的碲化鉍銻表面及碲化鉍銻與銪鐵石榴石之間的銳利介面，並且在這個異質結構上量測到巨大的反常霍爾電阻。隨著退火溫度的提升，反常霍爾電阻的大小顯著增加，這可能是由於更乾淨且富有鐵的介面使碲化鉍銻與銪鐵石榴石之間的交換耦合更強。此異質結構中反常霍爾效應的可調性展現了其在為室溫下的自旋電子學應用的巨大潛力。除了磁鄰近效應外，具有強鐵磁有序(ferromagnetic order)的磁性參雜亦是達成量子異常霍爾效應的方式。本論文中，不同參雜量的鉻參雜碲化鉍銻被成長在藍寶石基板上。我們校正了樣品中的鉻含量及展示了樣品的結晶性。樣品透過反射式高能電子繞射振盪揭示了其二維層狀成長模式，並透過拉賽福溝道效應(Rutherford backscattering spectrometry (RBS) channeling)證實了其良好的晶體結構。此外，隨鉻參雜量變化的反常霍爾電阻及居禮溫度展示出和其他報導中的相似的趨勢。可重複成長的具有可控制鉻參雜量的碲化鉍銻薄膜展現了在同時磁化上下表面態之拓撲絕緣體的研究中作為上層可能性。

Recently, quantum anomalous Hall effect (QAHE) has been observed in the time-reversal symmetry (TRS) broken topological insulators (TIs) by introducing magnetic order, which resulting in the opening of an exchange gap at the surface state. When the Fermi level is tuned within the surface band gap, the Hall resistance can reach a quantized value without external magnetic fields. The magnetic proximity effect (MPE) of topological insulator (TI)/magnetic insulator (MI) heterostructures has been demonstrated to be a viable route for realizing QAHE at significantly higher temperatures. First, in this work, c-axis oriented epitaxial TI films (Bi,Sb)2Te3 were grown on the ferrimagnetic insulator europium iron garnet (Eu3Fe5O12, EuIG) films by molecular beam epitaxy (MBE) and using a two-step growth method, where an amorphous (Bi,Sb)2Te3 was grown as a buffer layer. A high-temperature annealing process with different annealing temperatures (Ta) was implemented before the (Bi,Sb)2Te3 growth to improve the starting EuIG surface conditions. The well-ordered (Bi,Sb)2Te3 surface and the sharp interface between (Bi,Sb)2Te3 and EuIG were demonstrated, and an enormous anomalous Hall resistance (RAHE) was observed in this heterostructure at room temperature. The RAHE was enhanced significantly in (Bi,Sb)2Te3/EuIG with increased Ta, which could be attributed to the stronger exchange coupling between (Bi,Sb)2Te3 and EuIG due to the clean and Fe-rich EuIG surface. The tunability of AHE strength in (Bi,Sb)2Te3/EuIG provides great potential for spintronic applications at room temperature. Apart from MPE, the magnetic doping with strong ferromagnetic order is another approach to attain QAHE. In this work, the Cr-doped (Bi,Sb)2Te3 thin films were grown on α-Al2O3(0001) substrates with various Cr doping level. We calibrated the Cr concentration (Cr(y)) and characterized the crystallinities of our samples. The 2D layer-by-layer growth mode was revealed by the reflection high-energy electron diffraction (RHEED) oscillation, and the excellent crystalline structure of our sample was indicated by the Rutherford backscattering spectrometry (RBS) channeling mode. Additionally, the enhanced RAHE and Curie temperature (TC) with increased Cr(y) were demonstrated, displaying a similar trend reported by other groups. The reproducible growth of the Cr-doped (Bi,Sb)2Te3 demonstrates the possibility to be utilized as the top layer in the studies of TI with magnetized top and bottom surface state.