利用射頻磁控濺鍍法製備P型鍺銻碲薄膜之熱電性質研究

熱電元件由於能直接將熱能與電能相互轉換，長久以來被視為有開發潛力的能量轉換元件。儘管如此，過低的轉換效率仍是阻礙其普遍化的原因之一。當前研究的重點包括如何提升其熱電優值 (ZT)。鍺銻碲合金長期以來被應用在相變記憶體材料當中，近年來的研究顯示該材料擁有合適的熱電特性，適合被使用在熱電發電/致冷的應用中。然而該材料在薄膜的熱電特性上還未被廣泛研究。在本論文中，我們利用射頻磁控濺鍍法沉積鍺銻碲薄膜材料於矽基板上，並利用退火處理增加薄膜的結晶性。藉由調整不同的濺鍍壓力及退火溫度，達到增進薄膜的熱電性質的目的。材料的分析包含晶體結構，成份分析，以及熱電特性。由霍爾量測的結果顯示出鍺銻碲薄膜材料為P型高摻雜半導體，其載子濃度約為1021/cm3。X光晶體繞射顯示出該材料退火後為多晶材料，類似氯化鈉的面心立方結構。隨著濺鍍壓力的降低，X光繞射圖譜中的(200)峰值強度明顯增強許多，表示(200)方向的晶粒分佈有增加的趨勢。此種現象結合晶粒邊界位能障的觀念，極有可能解釋在低壓的製程下，載子擁有較高的遷移率。另一方面，隨者退火溫度的增加，薄膜擁有較佳的結晶性，同時也觀察到當退火溫度高於攝氏350度時，碲元素有缺陷的情形發生。在退火攝氏350度以上的樣品中，席貝克係數擁有顯著的增加，主要原因是由於較低的載子濃度。載子濃度的降低與碲元素的缺陷有明顯的關聯，本論文試著以鍺元素空缺的角度去解釋該現象。在較低的濺鍍壓力(0.1mTorr)與較高的退火溫度(450oC)的情況下，鍺銻碲薄膜材料在量測溫度為623K時擁有最佳的功率因子(21uW/cmK2)。帶入塊材的晶格熱傳導係數值，此材料的熱電優值(ZT)可達到0.6，相較於塊材的ZT值有兩倍以上的增加。

關鍵字

熱電；薄膜；鍺銻碲；射頻磁控濺鍍；功率因子

並列摘要

Thermoelectric devices have long been regarded as potential energy converter owing to their ability to convert waste heat directly into useful electricity. Nevertheless, the low efficiency made it difficult to become widely used. Current research is focused on finding thermoelectric materials with high ZT value. Pseudo-binary GeTe-Sb2Te3 compounds have long been used for phase-change memory (PCM) applications. It has been shown that GeTe-rich phases exhibit interesting thermoelectric properties, which can be applied to thermoelectric generator/cooler (TEG/TEC). However, the research on thin film case has not been widely explored. In this work, Ge-Sb-Te thin films were deposited on Si substrate using radio-frequency (RF) magnetron sputtering at room temperature with stoichiometric Ge19Sb2Te22 compound as the sputtering target. The as-deposited amorphous thin films were annealed in Ar atmosphere for 30 minutes to increase the crystallinity. Various deposition working pressures and post-annealing temperatures were used to improve thermoelectric properties. The films were analyzed in terms of their crystalline structure, composition, Seebeck coefficient, electrical properties, and thermal properties. The Hall measurement results indicated that the GST films were heavily doped p-type semiconductors with carrier concentration around 1021/cm3. The p-type behavior was confirmed by the positive Seebeck coefficient. X-ray diffraction (XRD) revealed that the annealed films were polycrystalline in nature, which crystallized in NaCl-type fcc structure that resembled the pure GeTe. With the decreasing working pressure, preferred orientation of (200) grain was observed from XRD patterns, which could possibly explain the higher mobility obtained at lower working pressure. On the other hand, the crystalline quality was found enhanced with increasing annealing temperatures. More severe Te-deficient was also observed from energy dispersion spectroscopy (EDS) at high annealing temperatures (>=350oC). As the annealing temperature increased above 350oC, the Seebeck coefficient was significantly improved due to the lower carrier concentration, which was assumed to be caused by the Te-deficient. Optimized thermoelectric power factor (21uW/cmK2) measured at 623K was obtained for thin film deposited at lowest working pressure (0.1mTorr) and annealed at highest temperature (450oC). The estimated ZT value could achieve to 0.6 using kL of the corresponding bulk materials.

並列關鍵字

thermoelectric ； thin film ； germanium antimony telluride ； RF magnetron sputtering ； power factor

參考文獻

1. Alam, H.; Ramakrishna, S., A review on the enhancement of figure of merit from bulk to nano-thermoelectric materials. Nano Energy 2013, 2 (2), 190-212.

2. DiSalvo; F. J., Thermoelectric Cooling and Power Generation. Science 1999, 285 (5428), 703-706.

5. Kordas, K.; Toth, G.; Moilanen, P.; Kumpumaki, M.; Vahakangas, J.; Uusimaki, A.; Vajtai, R.; Ajayan, Chip cooling with integrated carbon nanotube microfin architectures. Applied Physics Letters 2007, 90 (12), 123105.

6. L.T.Yeh, Review of Heat Transfer Technologies in Electronic Equipment. JournalofElectronicPackaging 1996, 117, 333–339.

8. Bell, Cooling, Heating, Generating Power,and Recovering Waste Heat with Thermoelectric Systems. Science 2008, 321, 1457.

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利用射頻磁控濺鍍法製備P型鍺銻碲薄膜之熱電性質研究

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