使用垂直矽奈米線降低熱傳導之理論分析與實驗結果

Abstract

我們研究矽奈米線在矽薄片上的等效熱傳導係數，做為使用矽奈米線來製作熱電元件的第一步研究。應用奈米結構降低熱傳導係數是目前的趨勢，好的熱電材料需要有良好的Seebeck coefficient、導電率與低的熱傳導係數，當材料尺寸與聲子平均自由路徑相近時，會產生尺寸效應，造成熱傳導值的降低，使熱電優值提升。本文第一部分將簡介熱電材料原理與應用。第二部分介紹使用無電鍍自組裝溼式蝕刻來製作奈米線的方法與結果。第三部分介紹Hot disk熱特性量測分析儀的原理與量測結果。第四部分使用聲子輻射方程式來模擬實驗的結果。本文使用無電鍍蝕刻，在硝酸銀與氫氟酸的混合溶液下，銀粒子還原析出在矽晶片，並產生二氧化矽於銀粒子底部，隨後被氫氟酸蝕刻，將矽晶片蝕刻成矽奈米線，長度從數百奈米至上百微米。接著我們使用Hot disk 2500，利用瞬態平面熱源法量測奈米結構的等效熱傳導係數，發現等效熱傳導係數隨著奈米線的長度增加而降低。可能的原因在於空氣的影響與奈米線本身熱傳導值的降低，我們使用聲子輻射方程式來模擬實際的結構，並討論當材料發生尺寸效應時所能達到的熱傳導降低與僅受空氣影響的熱阻串聯模型的比較。

Since silicon is widely used in the integrated circuit (IC) industry, the ability to tailor the thermoelectric properties of bulk silicon using its nanostructures can enable a variety of exciting applications, such as efficient thermo-photovoltaic devices, and monolithically- integrated electronic and optoelectronic device cooling. In late 2007, scientists have reported that a single silicon nanowire (Si NW) with a diameter less than 52nm exhibits a low thermal conductivity ~1.6 Wm-1k-1 and ZT ~1 at room temperature, suggesting the potential of using Si NWs for efficient thermoelectric energy conversion. In order to realize a practical thermoelectric device based on Si NWs, it is essential to fabricate large-area and highly-oriented Si NW arrays on silicon substrates. In this paper, we demonstrate the preparation of vertically-aligned Si NWs with diameters of tens of nanometers, heights ranging from tens of micrometers to over 100 μm, and most importantly, an area over 5x5 cm2. Characterizations using a hot-disk slab-module system show the thermal conductivity reduced for the fabricated Si NW samples, compared to that of bulk silicon. Two-dimensional microscale heat-transfer analyses of Si NWs based on the equation of phonon radiative transfer are presented. The high-aspect-ratio and vertically-aligned Si NW arrays were fabricated using a silver-induced wet deposition and wet chemical etching method. The surface morphology and the etching length depend on the concentration of both AgNO3 and HF solution, the etching temperature, and etching time. The concentration of AgNO3 affects the structure of the nanowire arrays because the porosity is determined by the density of deposited Ag particles. A Hot Disk 2500 slab-module system was then employed to measure the thermal conductivity of the fabricated NWs. During the measurement, the sensor was sandwiched between two samples, while the other side of samples was insulated by a material with a low thermal conductivity in order to reduce the heat losses to the surroundings. The thermal conductivities obtained from Si NW samples were lower than bare Si substrates for all six individual experiments. Then we use equation of phonon radiative transfer to simulate our structure. Compare to thermal resistance model, the equation of phonon radiative transfer consider size effect when the boundary is comparable to phonon mean free path. In summary, we have successfully fabricated large-area, high-aspect-ratio Si NW arrays, and measured their thermal properties using a Hot Disk system. The Si NW samples exhibit a reduced thermal conductivity, compared to that of bulk Si, showing great potential for next-generation thermoelectric devices.