拓樸絕緣體硒化鉍之薄膜分析與光激發自旋電子傳輸特性探討

Abstract

拓樸絕緣體為近期新興材料研究中非常熱門的主題之一，在拓撲絕緣體的內部，其電子傳輸情況表現如一般半導體或絕緣體具有一定能隙。但在拓撲絕緣體的邊界或是表面存在一些特殊的量子態而讓電子能在其表面導電。其中三維拓撲絕緣體，如硒化鉍(Bi2Se3)，表現出自旋極化的表面態，其中表面態電子的自旋與它們的動量相鎖定。圓偏振光伏效應（CPGE）是一種光學方法，通過利用圓偏振光在三維拓撲絕緣體中產生自旋光電流來研究其自旋表面態。 在這項研究中，我們使用物相沉積法在藍寶石基板上沉積Bi2Se3薄膜。通過X光繞射儀(XRD)和X光光電子能譜儀(XPS)量測研究材料的晶格結構和化學鍵結情形，其結果指出成長的薄膜具備高指向性的單晶晶格和符合理論值的共價鍵結情形。我們也研究了Bi2Se3正面閘極電晶體中載流子的導電行為，揭示了薄膜在不同閘極偏壓下具備電子與電洞傳輸的雙極性導電特性。我們通過在室溫下分別施加直流偏壓和柵極電壓來研究三維拓撲絕緣體Bi2Se3的圓偏振光伏效應。儘管因為材料中Se的鍵結空缺造成高參雜的電子濃度，但我們發現在室溫下仍然可以明顯地展示Bi2Se3在圓偏振光伏效應和線偏振光伏效應（LPGE）造成的自旋電子流。 我們透過施加不同的平面直流偏壓影響動量空間中電子於表面態的分佈以及激發自旋電子到不同動量方向的導帶，進而導致左旋和右旋圓偏振光激發產生不同大小的自旋電子流。我們還研究了光電流隨柵極電壓的變化，其中費米能級被調控，影響體態的一般載子和自旋電子在表面態中的數量變化。我們的研究表明，在室溫下Bi2Se3的自旋極化光電流可以分別透過平行、垂直電場以及偏振光進行調控。

Three-dimensional topological insulators like Bi2Se3 exhibit spin-polarized surface states which the spin of electron is locked to their momentum[1, 2]. The circular photogalvanic effect (CPGE) is an optical approach to investigate the helical surface states by utilizing circularly polarized light to generate spin photocurrent in three-dimensional topological insulators[3]. In this work, the thermal solid method is used to deposit Bi2Se3 thin films on sapphire substrates. The lattice structure and binding energy of the material are investigated by XRD and XPS measurements which show high-quality single crystal formation and highly coherent covalent bonding. The conduction behavior of carriers in Bi2Se3-based top gate MOSFETs is studied, revealing ambipolar behavior consistent with tuning between electrons and hole carriers within the thin film. The CPGE of the three-dimensional topological insulator Bi2Se3 is investigated by applying DC bias and gate voltage individually at room temperature. It is found that the photocurrent from CPGE and linear photogalvanic effect (LPGE) can be clearly demonstrated for Bi2Se3, even at room temperature, despite the existence of bulk carriers. Different DC biases are applied, and the longitudinal electric field affects the population of electrons in momentum space, resulting in different photocurrents from left and right circularly polarized light excitation. The photocurrent variation with gate voltage is also investigated, where the Fermi level is tuned, leading to changes in the population of bulk state carriers and spin electrons in surface states. Our work showed the helical spin polarized photocurrent of Bi2Se3 can be manipulated by electric field at room temperature.