新穎斜向沉積高方向性排列Bi-Te(Sb,Se)基奈米結構及其高效能薄膜型熱電元件之開發

Abstract

熱電材料具有尺寸小、結構簡單、可靠度高之特點，且可以作為發電或致冷器之用 途，但由於傳統塊材之熱電轉換效率過低，因而使其應用受到極大之限制。近年來由於 奈米科技的發展，透過奈米結構之合成，使得熱電材料轉換效率得以大幅提升，部分熱 電材料系統已達商業應用之要求。根據相關理論計算及重要實驗成果可知，熱電材料之 ZT 值與內部結構之「尺度」、「維度」、「排列方向」及「結晶優選方位」高度相關。因 此，本研究主要是藉由各種新穎及改良的製程技術，來有效控制此些結構參數，合成新 穎熱電薄膜，來達到提昇熱電材料ZT 值之目的。其內容包括兩大主題：利用斜向沉積 技術成長高方向性排列之BiTe (Sb, Se)基奈米結構熱電薄膜，以及高效能薄膜型熱電轉 換元件之開發。 本實驗室已成功利用脈衝雷射沉積技術(Nd:YAG pulsed laser deposition, Nd:YAG PLD)製備一系列垂直排列之Bi-Te 基熱電薄膜，其奈米結構可分為零維奈米晶粒、一 維奈米柱、二維奈米薄片及三維奈米峽谷。另外，再藉由Sb 摻雜可製備出超高導電率 之Bi-Sb-Te 基熱電薄膜，而Bi-Te-Se 基熱電薄膜已進入製備階段。為了更進一步控制 BiTe (Sb, Se)基熱電薄膜之維度、排列方向性、及結晶優選方位，來達到強化其熱電相 關性質之目的，本研究計畫擬於PLD 系統中導入斜向沉積(glancing angle deposition; GLAD)技術。藉由調控鍍源與基板之沈積角度，期待獲得各類不同尺度、維度、間距、 孔隙率、排列方向及結晶優選方位之奈米結構化熱電薄膜材料。最後，更將合成所得 高排列方向性之n 型及p 型熱電薄膜，應用於高效能薄膜型熱電轉換元件之製作。

Thermoelectrics have very attractive features, such as small size, simplicity, and reliability, and have important applications for power generation or cooling. However, the low thermoelectric conversion efficiencies of conventional bulk materials limit their applications. Recently, due to the development of nanoscience and nanotechnology, the thermoelectric conversion efficiencies have been greatly improved by the nano-structured synthesis technique to meet the requirements for commercial use. According to the theoretical calculations and experimental results, the thermoelectric properties extremely depend on the size, dimension, alignment and preferential orientation of structures. In this research plan, we thus expect to control these structure parameters for obtaining unique thermoelectric films with high ZT by various novel and improved approaches. Two main subjects are included: the fabrication of highly oriented and highly ordered nano-structured Sb/Se doped Bi-Te thin films and their applications for high-performance thin-film thermoelectric conversion devices. By using special designed pulsed laser deposition (PLD), we successfully synthesized a series of vertical-aligned Bi-Te thin films respectively consisting of one-dimensional (1-D) nanograins, one-dimensional (1-D) nanorods, two-dimensional (2-D) nanoflakes, and three-dimensional (3-D) nanocanyons. Additionally, the Bi-Sb-Te based thin films were also fabricated and found to exhibit excellent electrical conductivities. The going work is to synthesize Bi-Te-Se based thin films. In order to control the dimensionality, directional growth, and the preferential orientation of these nanostructured Bi-Te films for the further enhancement of the thermoelectric properties, glancing angle deposition (GLAD) will be introduced to the present PLD system. With the adjustment of the deposition angle, the structural factors including size, dimensionality, spacing, porosity, orientation and alignment will be dependently or independently optimized for the highest ZT thermoelectric films. Finally, the prepared highly-ordered n-type and p-type Bi-Te films will be applied for the fabrication of high-performance thin-film thermoelectric devices.