電流輔助退火處理對Bi0.5Sb1.5Te3濺鍍薄膜熱電傳輸性質之研究

Effect of electric current stressing on thermal/electrical transport properties of sputtered Bi0.5Sb1.5Te3 thin films

蘇小維(Xiao-Wei Su)

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系列化合物為目前室溫下熱電性質表現最佳的材料系統，且普遍用於商用熱電致冷器模組中。由實驗室之前的研究結果顯示，Bi0.5Sb1.5Te3濺鍍薄膜在經短時間電流輔助退火後，反位缺陷能有效被消除，導致載子遷移率的大幅提升，雖然載子濃度也小幅下降，但電阻率並無上升的情況，導致熱電功率因子可以得到大幅度的改善。

本研究利用在長有二氧化矽薄膜的矽基板上濺鍍Bi0.5Sb1.5Te3薄膜，經單純熱退火及電流輔助退火處理後量測並比較其電性傳輸性質，經電流輔助退火後的Bi0.5Sb1.5Te3薄膜在個別退火溫度下普遍擁有較高的載子遷移率與較低的載子濃度。根據電性傳輸理論計算與變溫霍爾效應的量測，我們確認高溫及電流輔助處理後Bi0.5Sb1.5Te3薄膜的載子傳輸主要由聲子散射機制所主導。接著我們利用3ω熱傳導係數量測法量測經不同退火溫度處理過的Bi0.5Sb1.5Te3薄膜熱傳導係數，並藉由理論計算所得的羅倫茲常數分離出聲子與載子的熱傳導係數。我們發現相較於單純熱退火處理，電流輔助退火後的試片擁有較高的聲子熱傳導係數。根據薄膜聲子熱傳導係數隨退火溫度變化的結果得知Bi0.5Sb1.5Te3薄膜在短時間退火處理後並無明顯晶粒成長的現象，因此推測Bi0.5Sb1.5Te3薄膜內反位缺陷的大量消除為幫助提升電流輔助退火後Bi0.5Sb1.5Te3薄膜聲子熱傳導係數提升的主要原因。

XRD分析結果顯示Bi0.5Sb1.5Te3薄膜經退火處理後，薄膜內部晶粒面向分佈並無嚴重的(00l)方向性，因此在3ω熱傳導係數量測法中所量測到的熱傳導係數可以配合電性量測數據進一步計算出Bi0.5Sb1.5Te3薄膜經不同退火溫度處理後的ZT值。實驗結果發現Bi0.5Sb1.5Te3薄膜經5分鐘330°C電流輔助退火後，其Seebeck係數可以提升至為201.9(μV/K)，電阻率為4.65(mΩ-cm)，熱傳導係數為0.7(W/mK)，其熱電功率因子約為8.76(μW/K2cm)，而ZT值可以達到0.38左右。

關鍵字

熱電薄膜 ； 熱傳導係數 ； 3-omega 法 ； 電遷移 ； Bi0.5Sb1.5Te3

並列摘要

Bismuth telluride-based compound is currently widely used in commercial thermoelectric module due to its excellent thermoelectric performances around room temperature regime. Previous study showed that anti-site defects can be much more effectively eliminated after electric current assisted thermal treatment within a short period of time for sputtered Bi0.5Sb1.5Te3 thin films. Enormous enhancement in carrier mobility compensates the reduction in carrier concentration and results in a reasonable electrical resistivity. Thermoelectric power factor of Bi0.5Sb1.5Te3 thin films has been markedly improved by this post-deposition treatment
In this study, Bi0.5Sb1.5Te3 thin films were sputtered on SiO2/Si substrate. Electrical transport properties were measured and compared for films with thermal annealing and electric current stressing treatment. Bi0.5Sb1.5Te3 films after electric current stressing treatment has much higher carrier mobility and moderately lower carrier concentration than those thermally annealed at the same temperature. From theoretical calculation and Hall measurement at different temperatures, we confirm that the carrier transport properties is dominated by lattice scattering mechanism especially for high-temperature annealed and electric current stressed samples. Then we apply the conventional 3ω thermal conductivities measurement for samples annealed at different temperatures. Using Lorentz number acquired from theoretical calculation, electronic and lattice thermal conductivities were separated from the measured intrinsic thermal conductivities. Compared with thermal annealing samples, the lattice thermal conductivities of electric current stressed Bi0.5Sb1.5Te3 films were found to be higher than that of thermally annealed samples. According to the variation of lattice thermal conductivity with annealing temperature, there is no obvious grain growth for Bi0.5Sb1.5Te3 films with such short thermal treatments. Elimination of a large number of anti-site defects in Bi0.5Sb1.5Te3 films may be the main cause of higher lattice thermal conductivities in electrical stressed samples.
The crystal orientation investigated by XRD analysis indicated no (00l) preferred orientation for processed films. So the ZT values of Bi0.5Sb1.5Te3 films can be further calculated with the value measured by the 3ω thermal conductivities measurement. The experimental results showed that the Bi0.5Sb1.5Te3 films after electrical stressed for 5minutes has a Seebeck coefficient of 201.9(μV/K), a lower electrical resistivity of 4.65(mΩ-cm) and a thermal conductivity of 0.7(W/mK) which brought about a highest thermoelectric power factor of 8.76(μW/K2cm) and a ZT value of 0.38.

並列關鍵字

thermoelectric thin films ； thermal conductivity ； 3-omega method ； electromigration ； Bi0.5Sb1.5Te3

參考文獻

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