循環拉伸應力與電流輔助熱退火對可撓式基板上碲化鉍系薄膜微結構與熱電性質影響之研究

Effect of cyclic stress and current-assisted annealing on microstructure and thermoelectric properties of Bi-Sb-Te films on flexible substrates

沈孜璨(Shen, Tzu Tsan)

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(2015年)

摘要

碲化鉍系統為室溫範圍下具優異表現之熱電材料，若將此材料系統以薄膜形式與軟性基板結合，將可得質輕、低熱導、高Seebeck係數之可撓式熱電薄膜元件，在自體發電—穿戴式電子元件這方面之應用具有極大潛力。然而在元件製作與操作過程中，導入熱電薄膜之應力將影響元件之效能及可靠性，因此探討應力對可撓式基板上碲化鉍系薄膜微結構與熱電性質之影響遂顯相形重要。本研究計畫將p型碲化鉍薄膜濺鍍於軟性聚亞醯胺基板上，分別經高溫熱退火或電流輔助熱退火後，再將薄膜施以循環拉伸應力，並觀察上述兩種熱處理下薄膜電阻值隨拉伸數量之改變，進而探討拉伸應力對薄膜之熱電性質與微結構影響。研究結果顯示碲化鉍熱電薄膜其載子濃度與Seebeck係數並不受室溫下所施加之機械應力影響，但在經相同拉伸次數作用後，碲化鉍熱電薄膜隨不同熱處理竟產生相異電阻上升程度—當拉伸次數達三十五次時，高溫熱退火薄膜電阻上升了30 %，電流輔助熱退火薄膜僅上升了7 %。為探討此現象成因，本研究相繼觀察不同熱處理下熱電薄膜微結構隨拉伸次數之變化，發現熱電薄膜受拉伸作用後皆會產生與拉伸方向垂直之裂紋，且應力移除後之裂紋密合程度會導致熱電薄膜隨循環拉伸應力產生不同之電阻上升幅度：因碲化鉍熱電薄膜經電流輔助熱退火後，內部晶粒受電流燒結影響使晶粒間鍵結較強，因此受相同循環拉伸次數作用後產生晶粒脫落之程度較少，導致裂紋在應力移除後之密合程度較佳，因此能觀察到電流輔助熱退火薄膜上留下較窄與較淺之裂紋形貌，並達到有效減緩熱電薄膜受循環應力影響時產生之電阻成長幅度。此外，量測結果中亦顯示經相同拉伸次數作用後，電流輔助熱退火薄膜所產生之載子遷移率下降程度小於高溫熱退火薄膜，因此可進一步證明碲化鉍熱電薄膜經電流輔助熱退火後能有效抑制其受應力影響所造成之電阻上升情況。

關鍵字

碲化鉍 ； 拉伸應力 ； 裂紋

並列摘要

Bismuth telluride has been considered as a promising candidate for thermoelectric devices due to its superior thermoelectric properties at room temperature. By developing thermoelectric devices using thin films and flexible substrates, we can fabricate light, wearable and high performance thermoelectric devices which may contribute to the development of self-powered wearable mobile electronics. However, during fabrication and operation of such devices, a different degree of stress will be inevitably introduced, therefore, the understanding of mechanical and electrical properties of thermoelectric thin films on flexible substrates is crucial for device performance and reliability considerations. In this study, p type Bi-Sb-Te thin films were deposited on polyimide substrates by RF magnetron sputtering. After deposition, thin films were subject to conventional thermal annealing or current-assisted annealing and the electrical resistivity of Bi-Sb-Te thin films was in-situ measured during cyclic tensile stressing. The results show that the mechanical stress induced at room temperature has no significant effect on Seebeck coefficient and carrier concentration of Bi-Sb-Te thin films, however, current-assisted annealing is beneficial for suppressing the stress-induced increase in electrical resistivity of Bi-Sb-Te films compared with conventional thermal annealing. To investigate this phenomenon, this study aimed at the observation of microstructure of Bi-Sb-Te thin films and found the stress-induced resistivity change at room temperature is associated with the degree of microcrack-close-up. Both crack width and crack depth of Bi-Sb-Te films with current-assisted annealing is smaller than those with conventional thermal annealing after stress is released. The current-assisted annealing is able to mitigate the degradation of carrier mobility induced by cyclic stressing for the Bi-Sb-Te films.

並列關鍵字

Bi2Te3 ； tensile stress ； crack

參考文獻

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