碲化鉍薄膜成長、特性與拓樸性質之實驗研究

Abstract

拓樸絕緣體 (Topological insulator) 因為具有許多獨特的性質因此在近幾年引起大家的關注。拓樸絕緣體的主要特性是它的內部具有絕緣體的性質，但它的表面態卻是呈現金屬的特性，就像是一顆石頭外面包覆著鋁箔，但它是單一種材料。而表面態也被保護著較不會和雜質或缺陷產生散射因此理論上會比一般的三維材料有較好的傳輸特性。通常目前發現有這種特性的物質在以前都用於熱電材料的研究上，例如碲化鉍 (Bi2Te3)、硒化鉍 (Bi2Se3) 和 銻化鉍 (Sb2Te3)。 而在我的論文中我使用物理氣相沉積法 (physical vapor deposition) 成功的長出碲化鉍的奈米結構在矽基板，並且將基板更換成了藍寶石後成功地長出碲化鉍的薄膜。物理氣象沉積法的原理是利用高溫爐加熱使的物質氣化，而氣化的物質將因為氣流的關係會飄向特定的方向，因此我們將基板放在氣流的下游處氣化的物質就會飄過來並且有一部分會附著在基板上，在我們的實驗中粉末和基板的距離大約是15公分。而整個成長的過程皆是放在石英管中並且將兩端封住後用真空泵抽至約10-3 Torr的真空度。 在奈米結構的部分我們將粉末放置的區域加熱到500℃ 並將基板放置的區域加熱至250℃，我們利用了X光繞射儀 (XRD)和能量散佈分析儀 (EDX) 確定我們成長的奈米結構晶格大小和鉍和碲的比例是2 : 3。光學顯微鏡幫助我們知道奈米結構都是幾個微米大小的三角形或是六角形結構。透過原子力顯微鏡 (AFM) 的量測我們知道奈米結構的厚度是隨機的分布，而我們量測到的厚度範圍大約在18~40奈米之間。 在薄膜的部分我們將粉末放置的區域加熱到480℃ 或500℃ 並將基板放置的區域加熱至280℃。我們也試著改面成長的時間觀察是否會對薄膜品質產生影響。我們利用了X光繞射儀，確定我們成長的薄膜晶格大小不會隨成長的時間有劇烈改變，還是碲化鉍的結構，能量散佈分析儀和X射線光電子能譜讓我們確認薄膜鉍和碲的比例是2 : 3而且鉍和碲的電子鍵結正確。而從原子力顯微鏡中我們可以發現到厚度和成長時間有關。 接下來我們將薄膜利用黃光製程製作成我們需要的原件並且量測它的電性，在量測電性上我們使用了物理特性測量系統(PPMS)來幫助我們量測電阻隨、載子濃度、載子遷移率和磁阻隨著溫度的變化。電阻和溫度的變化中我們發現在165 K時碲化鉍會從絕緣體的特性轉變成金屬的特性，而載子濃度會隨著溫度而下降，而載子的遷移率會隨著溫度降低從在300K時的46.7 (cm2/Vs)變成在3 K 時的192.43 (cm2/Vs)。而在磁阻的量測中我們發現在低溫時磁場接近0的附近電阻變化會有較大的變化。我們稱這個現象為弱反侷域化 (weak anti-localization)。這個現象是拓樸絕緣體具有表面態的其中一個條件。為了更進一步的分析Hikami-Larkin-Nagaoka 方程式被我們用來分析磁阻的曲線。分析的結果告訴我們的薄膜上有一個以上的通道有進行電子傳輸的能力且去相位長 (phase coherent length ) 為136 奈米。 關鍵字: 碲化鉍;薄膜成長;物理氣相沉積

Topological insulator (TI) has attracted a lot of attention because of their unique properties. The topological insulator has an insulating bulk state and a gapless metallic surface state. The metallic surface state is topologically protected from scattering by magnetic impurities and defects and has an electric conduction property which is dramatically different from the traditional 3D materials. Bismuth telluride (Bi2Te3) belong to this class of innovative new material and in this thesis we report the successful growth of large area, high quality Bi2Te3 thin film. In this work, physical vapor deposition (PVD) method was used to grow Bi2Te3 thin film on the sapphire (Al2O3) substrate or nanoplate on silicon dioxide substrate. During the growth, the Bi2Te3 powder and the sapphire was placed 15 cm apart in a quartz tube and the temperatures of the source powder and of the substrates were kept at 480 °C or 500 °C and 280 °C, respectively. The structural characteristics of the samples were carefully examined by using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS), the measurement results indicate that the samples have the right chemical composition and are of high crystalline quality. Phi scan measurement shows our film has two domain. The thickness and roughness of the films were analyzed by using atomic force microscopy (AFM). For electric measurements Bi2Te3 Hall bar device was made by using photo-lithography, the physical property measurement system (PPMS) was the employed to measure resistivity, carrier concentration, mobility, and magneto-resistivity of the sample. Electrical measurement showed that Bi2Te3 thin film has insulator behavior when the temperature is above 165 K, metal-insulator transition occurs at 165 K and when the temperature is below at 165 K, the sample becomes metallic. Carrier concentration in this sample is 4.8×1018 (cm-3) at 3 K, the sample’s mobility is 46.7 (cm2/Vs) at 300 K and increases to 192.43 (cm2/Vs) at 3 K. Magneto-resistivity measurement showed that at low temperature a sharp resistance dip appears around B = 0 appeared, a manifestation of the weak anti-localization. WAL was found at low magnetic field in the magneto-resistivity measurement, and showed that the sample has the property expected for a topological insulator. Hikami-Larkin-Nagaoka equation was used to fit the magneto-resistivity curve, and the fitting parameters are α=-0.74, and l_ϕ=136 nm, respectively. Linear non-saturating magneto-resistivity (LMR) at high magnetic field also indicates that the sample has the properties expected for a topological insulator. Key words: Bi2Te3 ; Thin film growth ; PVD