碲化鉍((Bi2Te3)是熱電材料轉換效率較高的元件，其優質係數ZT值約為1。希望藉由奈米的量子效應提升它的熱電性質，我們製作一系列低維度的奈米線和薄膜來進行研究。本實驗使用的碲化鉍奈米線乃利用薄膜樣品與基板的熱膨脹係數不同，經由熱處理在碲化鉍的薄膜上長出奈米線。由掃描式電子式顯微鏡和穿隧式電子顯微鏡可以觀察到菱形晶胞(Rhombohedral unit cell)結構的碲化鉍奈米線沿著(110)方向生長，直徑約150-330 nm長度約20-30 μm。將碲化鉍奈米線轉移到矽晶片上，運用半導體製程中的熱蒸鍍(Evaporator)以及電子束曝光系統(E-Beam writer)製作電極、熱電偶和加熱器來量測席貝克(Seebeck) 係數、電傳導率和熱傳導率。最後成功的製作與量測出p型(107 μV/k) 和n型(-52.8 μV/k) 的奈米線，雖然其席貝克係數小於塊材，但奈米線的熱傳導率低於塊材兩倍以上，研究發現最好的碲化鉍奈米線的熱電優值(ZT value) 可達1.18略大於塊材。 碲化鉍薄膜是以分子束磊晶 (Molecular Beam epitaxy)成長，分子束磊晶是在高真空下以物理的方式將高純度的材料4N (99.99%)將原子傳遞至基板上進行沉積反應形成，鍍率可低於0.1 nm/秒以下，因此可以製備出高品質的薄膜樣品，製造出各種不同比例的Bi-Te的薄膜。藉由X光繞射儀可以得知薄膜是菱形晶胞結構並且延著(0,0,l)的平面所成長。並用熱電偶成功的量測出薄膜的席貝克係數在室溫下座落於80-80 μV/k，電阻率5-30 μΩ-m，計算出功率因子(power factor)最高可達2000 μW/mK^2，與塊材相比低於一半，但是薄膜的熱傳導率同樣也低於塊材兩倍以上。最後得到最佳的碲化鉍薄膜的熱電優值(ZT value) 可達到1.01等同於塊材。 / Bismuth telluride (Bi2Te3) is the thermoelectric material used for high-efficiency energy conversion. The figure of merit ZT of bulk is around 1. To study the promising positive effects on the thermoelectric properties, low dimensional nanowires and thin films of Bi2Te3 were prepared and measurements were performed. Here the method applied to nanowires growth on Bi2Te3 thin films is the mismatch of thermal expansion between substrate and thin films. By annealing at 300-350℃ for a week, the nanowires were grown on the thin films. Rhombohedral structure of Bi2Te3 nanowires with diameter ~150-330 nm and length ~20-30 μm grew along (110) direction was confirmed by Transmission Electron Microscopy (TEM) and Selected Area Electron Diffraction Pattern (SAED). To measure the Seebeck coefficient, electrical conductivity and thermal conductivity, Bi2Te3 nanowires were moved to silicon chips. Electrodes, thermometers and heaters were fabricated through thermal evaporation and E-Beam lithography processes. We successfully grew p-type(107 μV/k) and n-type(-52.8 μV/k) nanowires. Although Seebeck coefficient of nanowires is smaller than that of bulks, its thermal conductivity is less than half of that of bulks. The best ZT value of nanowires we obtained was 1.18, which was slightly larger than that of the bulks. Molecular beam epitaxy (MBE) is a technique to grow Bi2Te3 thin films under extremely high vacuum, which is undergoing a physical vapor deposition to atomically grow thin films layer by layer. Due to the deposition rate is lower than 0.1 nm/s, we can deposit the high-quality thin films and adjust the ratio between bismuth and telluride. Rhombohedral structure of thin films grew along (110) plane was confirmed by X-Ray Diffraction (XRD). The Seebeck coefficient (80-80 μV/k) and electrical resistivity (5-30 μΩ-m) in room temperature are obtained by the thermocouples. The highest power factor can reach to 2000 μW/mK^2. While the power factor of thin films is about half of bulk ‘s value, the thermal conductivity of thin films is also half of that of bulks. The best ZT value of thin films obtained was nearly as same as that of bulks, 1.01.

諸多的研究顯示，和塊材相比，低維度的材料其物理性質會有所不同，為了探究熱電材料在低維度下對其熱電性質所造成的效應，我們合成了BixSb2-xTe3-y奈米線並量測其熱電性質。本實驗藉由熱處理薄膜製備奈米線的方法合成單晶BixSb2-x Te3-y奈米線。我們先利用脈衝雷射沉積系統將BixSb2-x Te3鍍在矽基板上形成薄膜，再將薄膜以350 ℃至490 ℃熱處理5到21天，奈米線即為了平衡因薄膜與矽基板彼此熱膨脹係數不同所造成的應力而自薄膜上長出，其直徑為幾十奈米至幾百奈米不等，長度則為幾微米至幾十微米。為了瞭解奈米線之構成與量測其熱電性質，我們結合微影製程及操縱技術，將單根奈米線架空於附有電極、加熱元件及溫度感測元件之量測平台上，由於奈米線已被架空，我們便能透過選區繞射分析奈米線其結晶性，並使用能量散射分析儀得知奈米線之成分，利用四點量測可得知奈米線的電阻率ρ，以加熱元件在奈米線兩端產生溫差，並量測因西貝克效應 (Seebeck effect) 所造成之電壓差即能得到西貝克係數 S (Seebeck coefficient)，三倍頻技術要求所量測的樣品必須要架空於基板上，運用三倍頻技術 (3ω method) 可量測奈米線之熱導率κ及比熱。結合微影製程、操縱技術以及量測系統，我們成功得到單根奈米線的三個熱電係數ρ、S以及κ，並了解低維度對熱電性質所造成的影響。 / Compare with the bulk materials, many researches had revealed that physical properties were different in low dimensional materials. To study the low-dimensional effects on thermoelectric properties of thermoelectric materials, BixSb2-xTe3-y nanowires were synthesized and studied for their thermoelectric properties. Single-crystallized BixSb2-xTe3-y nanowires were synthesized by on-film formation of nanowires. First, BixSb2-xTe3 thin films were deposited on SiO2/Si substrates by using the pulsed laser deposition system. BixSb2-xTe3-y nanowires grew from the films by annealing the films at 350~490 ℃ for 5~21 days through the stress release of the thermal expansion mismatch between the film and the substrate. A series of BixSb2-xTe3-y nanowires were prepared with the diameter from few tens of nanometers to few hundreds of nanometers and the length from few micrometers to few tens of micrometers. In order to analyze the components and measure the thermoelectric properties of the nanowires, the technique of combining microfabrication and manipulation for suspending a single BixSb2-xTe3-y nanowire on a measurement platform with electrodes, heater and thermometers was developed. As long as the wire is suspended, the crystallization of the nanowire is able to be analyzed by the selected area electron diffraction (SAED). The composition of the nanowire can be analyzed by the STEM-EDX. Resistivity ρ is measured by the four-point probe method. In order to get the Seebeck coefficient S, temperature difference were generated by the heater and thermoelectric voltage generated by Seebeck effect were measured. The 3ω method which demands that the wire should be suspended was applied to measure the thermal conductivity κ and specific heat c. By using the developed technique and the measurement system, three thermoelectric parameter ρ, S, κ of a single nanowire were successfully measured and the low-dimensional effect on thermoelectric properties were examined.