固態物理與電聲子動態研究磁化矽奈米線熱電晶片

Solid State Physics and Electron/Phonon Dynamics on Magnetic Silicon Nanowire Thermoelectric Chips

歐祖銘(Tsu-Ming Ou)

指導教授 ： 洪哲文

國立清華大學/工學院/動力機械工程學系/碩士(2012年)

https://doi.org/10.6843/NTHU.2012.00142

摘要

此研究的目的在於探討矽奈米線(silicon nanowire, SiNW)結構，將原無磁性的的矽材料改變結構以附加磁性，探討對於其熱電優質提升的可能性，藉此取代當前普遍採用的熱電材料等如鉍鍗化合物(Bi2Te3)，用以降低材料成本與提高材料的供應優勢及競爭潛力。
影響熱電材料效能的因素包括有電子傳導率(electrical conductivity)、席貝克係數(Seebeck coefficient)及熱傳導係數(thermal conductivity)等，而這些性質皆隨著材料的電子態密度分布(density of state, DOS)與聲子頻散關係(phonon dispersion relation)來改變。本研究將使用第一原理的密度泛函理論(density functional theory, DFT)，並配合周期性邊界條件計算來模擬計算與建立矽奈米線結構，藉由改變其截面積尺寸以及附加磁性與否，來探討其電子態密度分布與能帶結構的改變。此外再以密度泛函微擾理論(density functional perturbation theory, DFPT)來計算取得聲子頻散關係、聲子態密度(phonon density of state)等性質參數。
進行完第一部分的模擬計算後，將獲得的各項參數代入波茲曼傳輸方程式(Boltzmann transport equation)，來推導計算出熱電材料之電子傳導率、熱傳導係數、席貝克係數等，最後便可整理得到用以評斷熱電材料運作效能的熱電優值ZT (figure of merit)。在結果中我們看到材料電子的費米能階在態密度分布因為磁性的附加而向導帶能階偏移，此現象提升了矽奈米線的電子傳導率，而為保留電子自旋量所挖取的缺陷則是降低了熱傳導係數的表現，模擬中的各種結構各尺寸矽奈米線在施加磁性後，多數結果在ZT值上獲得了提升，且與磁性附加的強度成正比。我們推測若是將奈米線尺寸擴大至介於電子與聲子自由徑之間，並配合大量的缺陷與磁性附加將能獲得熱電優質最大的提升效益。

關鍵字

熱電晶片 ； 矽奈米線 ； 聲子 ； 電子傳導率 ； 半導體 ； 鐵磁性 ； 第一原理計算 ； 密度泛涵理論 ； 波茲曼傳輸方程式

並列摘要

This study focuses on investigating the structure of silicon nanowires and the possibility of elevating the Figure of Merit with the addition of magnetic defects. This initiative will provide a chance to have an advantage over the most popular electrical materials, such as 〖Bi〗_2 〖Te〗_3 which has a ZT around 1.0. In addition, with nanowires, the price can be lowered and the supply of materials can be guaranteed not to be in short. Furthermore, the structure of silicon is compatible with the present semiconductor technology, and it will be great potential in marketing.
Some factors which influence the efficiency of thermoelectric materials includes; electrical conductivity, Seebeck coefficient, and thermal conductivity, among others. All of these properties are dependent on the density of states (DOS) and phonon dispersion relations. In addition, these material properties will also vary with different sizes and, inevitably with the magnetic defects, added on purpose. In this research, we use the method of density functional theory (DFT) to build silicon nanowire structures. Then, we try to change the structure and add magnetism effects to find the influence that this conveys in the DOS and band structures. On the other hand, we determine the phonon dispersion relations and the phonon density of states from the density functional perturbation theory (DFPT).
After the previously mentioned simulation results have been obtained, we input these previously calculated properties into the Boltzmann transport equation to obtain key properties of Figure of Merit. Afterwards, we can compare ZT values obtained from the different silicon nanowire structures. In the results, we can see that the Fermi level changed in the DOS with the addition of magnetism. This phenomenon leads to an improvement of electric conductivity, and the defects we designed for the magnetism addition in nanowires reduce the thermal conductivity. In conclusion, we obtain the improvement of ZT in most of the selected magnetism silicon nanowires.

參考文獻

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