奈米比熱儀的製作及其於物性研究的應用

Abstract

在固態物理學中，比熱的量測可以提供許多資訊，使我們得以研究例如電子及聲子的狀態密度，以及各種相變化等。近年來，為了研究奈米材料迥異於塊材的特性，以及存在於微觀尺度的特殊現象，比熱儀亦朝向縮小尺寸的方向發展。在本實驗中我們利用黃光微影製程設計並成功製作了適用於熱弛法比熱量測的比熱儀。其70微米 x 70微米的樣品搭載平台由厚度約300奈米的氮化矽薄膜為基底，其上鍍有Ni-Cr加熱絲及RuO2溫度計作為樣品加熱與溫度變化監控。此樣品搭載平台由六條金屬線懸吊著，調整其熱傳導可控制放熱的時間常數()在可量測的範圍，一般約介於0.5至5 秒之間。本實驗中樣品搭載平台之背景值比熱在20 時約為 3 x 10-10 J/K，因此可量測樣品的比熱在10-9 J/K範圍，樣品重量約在 10 ng 範圍。我們量測了拓樸絕緣體Bi1.5Sb0.5Te1.7Se1.3 奈米薄片(nanoflakes)在低溫的比熱，並以Debye模型做擬合，所得的Debye溫度與相關參考文獻相符，而根據Dulong-Petit 定律，擬合的結果顯示樣品質量約為11 ng，與以樣品尺寸估計而得的質量相近。本奈米比熱儀不只能以比熱量測研究奈米材料的物性外，也可作為奈米天秤測量奈米材料的質量。本計畫未來將進一步量測電子載子之比熱，探討拓樸絕緣體的表面電子態。

In solid state physics, thermodynamic measurements give a variety of information on fundamental properties and provide direct insight into, e.g. ,phonon and electron density of states and phase transitions. Efforts have been made in calorimetry for reducing size in order to investigate properties of nanoscale structures or novel systems showing finite size effect. In this work, a Si3N4 membrane-based nanocalorimeter designed for thermal relaxation method was fabricated by photolithography. The 70 70 sample holder was supported by 300 -thick Si3N4 membrane, with NiCr heater and RuO2 thermometer deposited on it for sample temperature control and monitoring. The sample holder was suspended by six free-standing wires. Adjustment of the thermal conductance of the wires made the relaxation time constant lie in the measurable range, typically 0.5-5 seconds. The addendum (background heat capacity) was around 300 pJ/K at 20 , so the resolution was good enough for measurement of sample with heat capacity around 1 nJ/K and mass around 10 ng. Low temperature specific heat of topological insulator Bi1.5Sb0.5Te1.7Se1.3 nanoflake was measured and fitted to Debye model. The Dulong-Petit limit gave the mass to be 11 ng, similar to the result estimated by geometry. The nanocalorimater can not only be employed for the study of size dependence of specific heat, but also can be used as a mass balance for nanogram specimen. In the future we will measure specific heat of electrons and investigate the surface states in topological insulators.