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研究生： 劉昆明
Liou, Kuen-Ming
論文名稱： 電流輔助熱退火效應對濺鍍Bi-Sb-Te和Bi-Se-Te熱電薄膜其傳輸性質與缺陷消除的影響
Effect of electric current assisted thermal treatments on transport properties and defect elimination in sputtered Bi−Sb−Te and Bi−Se−Te thermoelectric thin films
指導教授： 廖建能
Liao, Chien-Neng
口試委員:
學位類別： 博士
Doctor
系所名稱： 工學院 - 材料科學工程學系
Materials Science and Engineering
論文出版年： 2010
畢業學年度： 98
語文別： 英文
論文頁數： 78
中文關鍵詞： 熱電 、電遷移 、放電等離子燒結
外文關鍵詞： thermoelectric, electromigration, spark plasma sintering
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    • 近年來環境的危機促進了替代性能源的研究發展，因此能將熱與電能互換的熱電材料重新的吸引人們的注意。熱電薄膜致冷器因具備高冷卻功率密度、短反應時間等優點，故被視為是一種可行的方法能用來解決微電子元件中的熱管理問題。而碲化鉍系化合物是目前所知室溫下最佳的熱電材料，因此是薄膜型致冷器應用上具有潛力的材料。藉由物理氣相沉基法、電化學沉積法來製備的碲化鉍系薄膜，常經由一道後退火製程來消除薄膜內部缺陷進而改善其熱電性質。近年來放電等離子燒結技術(Spark Plasma Sintering, SPS)已被證明能快速的將碲化鉍系粉末緻密化。由放電等離子燒結技術製備出來的碲化鉍系具備緻密、方向性和多晶結構等特性，更重要的是具備了優秀的熱電性質。然而電流在燒結過程中如何與碲化鉍系化合物相互作用卻依舊尚未完全明白。
    在此研究中利用磁控濺鍍法分別製備p型的Bi-Sb-Te與n型的Bi-Se-Te薄膜並接著施加一電流輔助熱退火處理。在熱退火過程中將導入一密度達到~ A/cm2之電流。電流輔助退火試片相較於同溫度下的熱退火試片，具有較低的載子濃度和大幅度提升的載子遷移率。我們提出一個電遷移所引致的Sb與Te元素的優先擴散模型，來解釋所觀察到的Sb與Te富含相的析出，和Seebeck係數與電導的提升。此外因碲化鉍化合物晶體結構所造成的非均向性擴散係數與電導率，將導致在電流的效應下位於(00l)晶粒中的帶電缺陷會被優先的消除。電流輔助退火法是一個更有效的退火方式，能有效的在低溫短時間內消除缺陷，避免在高溫退火製程中會造成性質劣化的晶粒成長與元素揮發等現象發生。

    Recently, environmental crisis spurs the researches on alternative energy and hence thermoelectric materials, which can convert heat into electricity, or alternatively, convert electricity into cooling, attract renewed attention. Due to the advantages of high cooling power density and short response time, thin-film thermoelectric coolers (TECs) have been considered as the plausible solution to the thermal management problem of microelectronics. Bismuth telluride based compounds are known to be the best thermoelectric materials near room temperature regime to date and are the potential candidates for thin-film TECs. A post-deposition thermal treatment is usually used to reduce the structural imperfections and improve thermoelectric properties of bismuth telluride based thin films prepared by physical vapor deposition and electrochemical deposition methods. Recently, spark plasma sintering (SPS) technique, using electric energy as the heat source, was demonstrated to be very effective in the rapid densification of bismuth telluride based powder materials. The SPS-prepared bismuth telluride based compounds have dense, textured and polycrystalline microstructures, and most important of all, excellent thermoelectric properties. Nevertheless, the details of how an electric current interacts with bismuth telluride compounds are still not fully understood.
    In this study, p-type Bi-Sb-Te and n-type Bi-Se-Te nanocrystalline films were prepared by a r.f. magnetron sputtering method followed by a electrical annealing process. An electric current of density ~ A/cm2 was introduced through films during thermal annealing. The electrically stressed p- and n-type films were found to have lower carrier concentration but much higher mobility than those thermally annealed at the same temperatures. A model based on electromigration-induced preferential Sb and Te diffusion is proposed to explain the observed Sb-rich and Te precipitation as well as the enhancement of Seebeck coefficient and electrical conductivity of the electrically stressed thin films. Owing to anisotropic diffusion and electrical transport properties, charged lattice defects in the (00l) grains of bismuth telluride crystal are preferentially eliminated under electric current stressing. The presented current-assisted annealing approach can be an efficient post-deposition treatment in a short cycle time and low temperature that prevents from unfavorable gross grain growth and evaporation of volatile constituents in Bi-Te based nanocrystalline thin films during high-temperature annealing process.

    致謝 I Abstract II 摘要 IV Contents V List of Figures VII List of Tables XII Chapter 1 Introduction & background 1 1.1 Preface 1 1.2 Conversion efficiency and figure-of-merit 2 1.3 Thermoelectric transport theory 5 Chapter 2 Literature review 11 2.1 Preface 11 2.2 Recent remarkable progress in thermoelectrics 14 2.2.1 Quantum confinement effect 14 2.2.2 Localized density of state modulation 16 2.2.3 Large improvement of thermoelectric properties under pressure 19 2.2.4 Nanocrystalline bulk materials 20 2.3 Introduction of bismuth telluride based comompounds 23 2.4 Thin-film-type thermoelectric cooler 25 2.5 Motivation 25 Chapter 3 Experimental design 28 3.1 Thin films preparation 28 3.2 Electrical and thermal annealing 29 3.3 Transport properties characterization 30 3.3.1 Seebeck coefficient measurement 30 3.3.2 Electrical conductivity 31 3.4 Microstructure & phase determination 32 Chapter 4 Results and discussion 33 4.1 Transport properties as a function of annealing temperature 33 4.1.1 For p-type Bi-Sb-Te films 33 4.1.2 For n-type Bi-Se-Te films 35 4.2 Electric current stressing effect 36 4.2.1 Peltier effect caused by applied current 36 4.2.2 The transport properties of p-type Bi-Sb-Te films 37 4.2.3 Surface morphology and microstructure of Bi-Sb-Te films 38 4.2.4 The transport properties of n-type Bi-Se-Te films 42 4.2.5 Surface morphology and microstructure of Bi-Se-Te films 43 4.2.6 Crystalline investigation of both type films 45 4.3 Theoretical calculation for both type films 48 4.4 Hall measurement in variable temperature 51 4.4.1 Temperature-dependent transport properties of Bi-Sb-Te films 52 4.4.2 Temperature-dependent transport properties of Bi-Se-Te films 60 Chapter 5 Conclusions and future prospects 69 5.1 Conclusions 68 5.2 Future prospects 70 Publication list 72 Reference 73

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