矽鍺奈米線之熱傳導量測

Abstract

在這篇論文中，我們量測了單根矽鍺合金奈米線的熱導率，藉此研究合金散射對於奈米結構的聲子特性所造成的影響。 如何利用結構加工來改變材料的聲子特性而讓材料的熱電轉換效率提高是一個很重要的研究課題。近來，有理論指出合金散射能夠阻絕高頻聲子在材料中的傳播，在微米尺度下造成彈道式的低頻聲子傳播。然而，目前尚未有任何實驗驗證這個特殊的現象。在這篇論文中，我們會用實驗來驗證這個現象。 在論文的第一部分，我會對這個研究主題做一個背景介紹，接著介紹在實驗用到的實驗裝置和技巧，例如：溫度控制系統，氣體注入裝置，還有實驗步驟與數據分析方法。 接著在第二部分，我會介紹研究”矽鍺奈米線中的彈道式聲子”的實驗與其結果。合金散射讓兩種不同結構的矽鍺奈米線中絕大部分的聲子無法傳播，只剩下0.04%的低頻聲子能夠傳熱。從矽鍺奈米線之長度和熱導率的關係圖中我們可以觀察到聲子平均自由徑可能超過8.3μm。另外，矽鍺奈米線直徑的大小幾乎不影響其熱導率。從溫度和熱導率的關係圖中可以推斷聲子間互相散射的效應對熱傳導的影響可以忽略，是合金散射主導了矽鍺奈米線的熱傳導特性。此外很特別的是這些低頻的彈道式聲子不會受到結構形變，區域性結構扭曲，和元素比例的影響。 最後第三部分，根據我們實驗結果，在合金材料中的聲子頻譜是低頻與窄頻帶的，這個特性使得我們有機會利用聲子晶體來製造擁有極低熱導率的矽鍺材料, 使其擁有高熱電效率(ZT>3)。我設計了一個有著蜂窩狀排列孔洞的矽鍺薄膜，此薄膜材料估計可以達到0.05 W/m-K的極低熱導率，並且在室溫下ZT~2.4與在1100K時ZT~8.8。

In this thesis, I describe experimental thermal conductivity measurements on individual SiGe nanowires, which is a model alloy system that the role of alloy scatterings of phonons in nanostructures can be investigated for the first time. Introducing efficient and yet independent methods of engineering phononic properties of a material are important topics for enhancing the energy conversion efficiency of thermoelectric materials. Recently, it has been pointed out theoretically that alloy scatterings of phonons can filter out most high frequency optical phonons and may lead to ballistic phonon phenomena at microscales. However, so far no experimental investigations have been conducted to verify the extraordinary effect. In my thesis, I will show direct evidence that the phonon mean free path of SiGe nanowires exceeds 8.3μm, which is the longest scale ever observed in all thermal conductors at room temperature. In the first part of the thesis, I introduce the background of our research topic, and then I describe various techniques for experimental implementations, including setting up a temperature control system, building a gas injection system in the SEM chamber, designing experimental procedures and analyzing data. In the second part of the thesis, I present the experimental findings of ballistic phonons in SiGe nanowires. Strong alloy scatterings in homo- or hetero-structures of SiGe nanowires effectively filter out most high frequency optical phonons but leave ~0.04% of the excited phonon modes responsible for heat conduction in the nanowire. It results in our observation of a linear length dependence of thermal conductivity with phonon mean free paths exceeding 8.3μm. In addition, thermal conductivities of SiGe nanowires exhibit weak diameter dependence. The absence of umklapp process features in the temperature dependence of thermal conductivity indicates that the phonon-phonon interactions are negligible and the heat transfer of SiGe nanowires is dominated by alloy scatterings. Remarkably, the low frequency ballistic phonons are immune to structural deformation, stacking faults, twin boundaries, local strains, and elemental variations. Based on my result, the nearly monochromatic low frequency phonons in alloyed materials provide a unique opportunity to obtain much reduced thermal conductivities in SiGe via effects of phononic crystals. By introducing complete phononic band gap in a SiGe thin film, the thermal conductivity will likely reach 0.05 W/m-K at room temperature, and correspondingly, increase the figure of merit (ZT) of thermoelectric properties of SiGe. In the third part of my thesis, I design a porous thin film with 2D honeycomb structures. The ultralow thermal conductivity results from the phononic bandgap will likely realize ZT ~ 2.4 at room temperature and display a peak ZT = 8.8 at 1100 K.