經電流輔助退火處理之Bi0.5Sb1.5Te3/Sb多層濺鍍薄膜熱電性質研究

Thermoelectric properties of sputtered Bi-Sb-Te/Sb multilayer films with electric current assisted annealing

張志宇(Chih-Yu Chang)

指導教授 ： 廖建能

國立清華大學/工學院/材料科學工程學系/碩士(2010年)

摘要

Bi2Te3系列化合物在室溫範圍具有極佳之熱電優值，可作為熱電薄膜致冷元件的應用。先前研究發現電流輔助退火處理能夠有效的在短時間內消除Bi-Sb-Te濺鍍薄膜內部晶體缺陷，提升其載子遷移率。然而薄膜在熱處理過程中因電流效應造成的載子濃度下降亦使得Bi-Sb-Te濺鍍薄膜偏離其最佳化熱電性質，因此需要調整Bi-Sb-Te薄膜的成份，使其經電流輔助退火處理後能獲得最佳化熱電性質。
本研究嘗試在不調整濺鍍靶材成份的情況下，利用Bi-Sb-Te與純Sb雙靶交替濺鍍方式，在室溫於聚亞醯胺/矽基板上鍍製Bi0.5Sb1.5Te3/Sb多層複合薄膜。藉由控制Sb鍍層的厚度來調整薄膜的成份組成，經電流輔助退火處理使Sb元素與Bi0.5Sb1.5Te3薄膜反應，補償在熱處理所造成的載子濃度下降。經由電流輔助退火處理後的複合薄膜具有較高的載子濃度與載子遷移率，其熱電性質有明顯的改善。在此我們推論額外Sb的添加能夠抑制因電遷移效應造成晶格中Sb元素被驅動至晶界或表面析出富含Sb相，避免主要電洞載子來源的SbTe錯位缺陷減少，因此Bi0.5Sb1.5Te3薄膜能夠維持較高的載子濃度與載子遷移率，以及因Te揮發在薄膜所形成的微小孔洞。
實驗結果發現在330 °C通電流退火處理5分鐘後的Bi0.5Sb1.5Te3/Sb多層膜具有最佳的熱電性質，其Seebeck係數為187.5 μV/K，電阻率則大幅降低為2.6 mΩcm，熱電功率因子可達到1.36×10-3 W/mK2。相對於原本的Bi0.5Sb1.5Te3濺鍍薄膜，本研究成功利用簡易的Bi-Sb-Te與純Sb交替濺鍍方式調整Bi-Sb-Te薄膜的成份組成，並以後鍍膜的通電退火處理大幅改善薄膜的熱電性質。

關鍵字

熱電薄膜 ； 熱電性質 ； 電遷移

並列摘要

Bimuth telluride-based compound has been considered as promising candidates for thin-film thermoelectric cooling devices due to its superior thermoelectric properties at room temperature regime. Our previous study showed that an electric current assisted annealing can effectively eliminate crystal defects, thus enhance the carrier mobility of sputtered Bi-Sb-Te thin films in short time. However, a reduced carrier density associated with the electrical annealing causes the deviation of optimized composition of Bi-Sb-Te compound. Thus, Bi-Sb-Te thin films need to be adjusted in composition in order to obtain their optimized thermoelectric properties.
In stead of changing the composition of sputter target, we prepared Bi0.5Sb1.5Te3/Sb multilayer films on polyimide/Si substrate at room temperature by consecutively sputtering Bi0.5Sb1.5Te3 and Sb layers alternatively. The overall film composition is modulated by controlling the Sb insertion layer thickness. During the electric current stressing process, the Sb element would diffuse into Bi0.5Sb1.5Te3 layers to compensate the reduced carrier concentration followed by thermal and electrical treatment. The electrically stressed Bi0.5Sb1.5Te3/Sb multilayer film demonstrates high carrier concentration and enhanced Hall mobility, which improves thermoelectric properties. We proposed that the additional Sb supply suppresses electromigration-induced Sb depletion in crystal lattices, which can sustain p-type SbTe antisite defects in Bi0.5Sb1.5Te3 crystal lattice. Thus, Bi0.5Sb1.5Te3 can maintain their high carrier concentration and enhanced Hall mobility after electrical annealing treatment. It not only improves thermoelectric properties of the Bi0.5Sb1.5Te3 films but also decreases the formation of micro voids caused by Te evaporation.
In this study,the Bi0.5Sb1.5Te3/Sb multilayer film with the best thermoelectric properties were obtained for the composite films after electrically stressed at 330 °C for 5 minutes. It has Seebeck coefficient of 187.5 μV/K and the lowest electrical resistivity of 2.6 mΩcm, leading to the highest power factor of 1.36×10-3 W/mK2, As compared to the original Bi0.5Sb1.5Te3 thin films, the Sb-insertion multilayer sputtering approach provides a simple means to modulate the composition and improve thermoelectric properties of Bi–Sb–Te films after electrical annealing treatment.

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