調變複晶矽鍺之鍺含量與材料維度對於熱傳導率以及電傳導率之影響評估研究; Ge-content and geometrical effects on thermal conductivity and electrical conductivity of poly-Si1-xGex nanopillars

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Title:	調變複晶矽鍺之鍺含量與材料維度對於熱傳導率以及電傳導率之影響評估研究;Ge-content and geometrical effects on thermal conductivity and electrical conductivity of poly-Si1-xGex nanopillars
Authors:	曾瀚陞;Tseng,Han-sheng
Contributors:	電機工程學系
Keywords:	矽鍺合金;矽鍺奈米柱;熱傳導率;電傳導率
Date:	2013-08-22
Issue Date:	2013-10-08 15:24:42 (UTC+8)
Publisher:	國立中央大學
Abstract:	本論文利用低壓化學氣相沉積(LPCVD)方式沉積不同鍺含量之複晶矽鍺薄膜，再藉由電子束微影與乾蝕刻的方式定義製作出約80-120 nm柱寬的奈米柱(線)陣列。並藉由植入不同摻雜源，來觀察n型與p型複晶矽鍺奈米柱的溫度相依之熱傳導率與電傳導率。本文藉由三倍頻量測法以及四點探針法對於300-340 nm高的複晶矽鍺柱陣列分別進行熱傳導率以及電傳導率之量測分析。實驗觀察得知複晶矽鍺奈米柱之熱傳導率會隨著溫度升高而呈現1/T下降之趨勢，其溫度相依之行為模式與鍺塊材之熱傳導率類似。有趣的是，隨著鍺含量之增加可進一步藉由合金散射降低奈米柱之熱傳導係數。奈米柱所呈現之熱傳導率值介於1-2 W/m．K，遠較塊材鍺之熱傳導率(60W/m．K)下降50倍以上。足以佐證可以利用低維度以及合金的設計與製作來大幅降低複晶矽鍺奈米柱的熱傳導率。藉由溫度相依之功率因素(n)之分析也証明此低熱導率是分別由聲子散射、合金散射及晶粒邊界散射等機制在奈米尺度下交互作用所致。可喜的是，聲子散射及晶粒邊界散射並未影響電荷在奈米柱(線)的傳輸能力。藉由帶入矽塊材的席貝克係數及先前所量測到的電導率可估算出矽鍺奈米柱之ZT值。P型矽鍺奈米柱因具有較低的熱傳導率以及較高的電導率，因此擁有接超過1的高ZT值。本文成功地運用鍺含量與維度設計來製作出複晶矽鍺奈米柱(線)，並獲得低熱傳導率、高電導率以及高ZT值。此奈米柱陣列未來可運用於奈米熱電致冷器元件之熱點主動層，提供在晶片上積體電路之局部熱點直接冷卻之應用。 This thesis experimentally realized Si1-xGex nanopillar array and investigated effects of Ge content and geometrical sizes of poly-Si1-xGex nanopillar on the electrical and thermal conductivities in temperature of 100－400K. The fabrication of poly-Si1-xGex nanopillar started with the deposition of a tri-layer of Si3N4/poly-Si1-xGex (x=0, 0.12, 0.24 and 0.36)/Si3N4 thin film over the Si substrate using low pressure chemical vapor deposition (LPCVD). The dense array of nanopillar with diameter of 80-120 nm was generated using the combination of e-beam lithography and SF6/C4F8 etching, followed by implanting phosphorous and boron dopants. To understand effects of Ge content and dopants on the carrier and phonon transport within poly-Si1-xGex nanopillar array, temperature-dependent characterisitics of the thermal conductivity and electrical conductivity of poly-Si1-xGex nanopillar array were conducted using the three omega method and four-point-probe technique. measured effective thermal conductivity of poly-Si1-xGex nanopillar array declines inversely with temperature (k*(T) ? 1/T ) just like in bulk Ge. An interesting finding is that the thermal conductivity of poly-Si1-xGex nanopillars appears to have an inverse dependence on the Ge content as a consequence of the enhanced alloy scattering. The thermal conductivity of nanopillars is around 1-2 W/m?K, which is much below the thermal conductivity of bulk Ge (60 W/m?K) with reduction factor of 50. We demonstrated the low-dimensional nanostructure and design of Ge content in SiGe alloy, leading to a large reduction in the thermal conductivity for poly-Si1-xGex nanopillars. Analysis of temperature-dependent thermal conductivity shows that the interactions between phonon scattering, alloy scattering and boundary scattering drag the phonon transport within the nanopillars when the pillar size is less than 100nm, primarily because of the geometrical size is less than the phonon wavelength. Fortunately the reduced dimensionality does not affect the electron transport in the nanopillars, consequently, the estimated ZT possibly achieves to unity at T = 400 K, primarily a result of a much higher electrical conductivity and a fairly lower thermal conductivity. We envisaged the reported the enhanced electrical conductivity and reduced thermal conductivity conducive for the further fabrication of poly-Si1-xGex nanopillars thermoelectric nanocoolers.
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