Phonon Engineering at The Nanoscale Makes Silicon a Better Thermoelectric Material

論文發表人：余人侃 (加州理工學院化學系博士班)

 

http://www.mrs.org/s_mrs/index.asp

 

矽一直以來是被認定為效率極低的熱電材料，其熱電係數僅有0.01。 然而，最近的研究發現，奈米矽線的熱電係數可提高至1.0。此熱電係數與商業上常用的碲化鉍熱電材料有著相近的效能。相對於傳統矽材料，奈米矽線的熱電效率高出兩個數量級，此效率上的提升主要來自於因為聲子與材料邊界碰撞頻率增加或聲子維度降低造成之導熱係數降低的影響。

本研究中，我們提出一個全新的奈米矽材料作為熱電材料: 矽奈米網。此結構是利用超磊晶奈米線模組轉移技術製成。其結構包含 34 奈米週期的二維孔洞陣列，每孔洞直徑約為14奈米。該材料實為一種聲子晶格材料。經測量發現，此結構室溫下的導熱係數為 1.7 W/m-K。此數值僅為相同等大小的矽奈米線導熱係數的一半。我們認為此現象係為聲子能階結構因週期孔洞而造成改變，使得聲子群速度降低的結果。因導熱係數的降低，此矽奈米網結構將有著較矽奈米線更高的熱電效率。這也是首次奈米聲子晶格結構在熱電材料上面的應用。

 

Bulk silicon has long been considered a poor thermoelectric (TE) material with the thermoelectric figure-of-merit as low as ZT=0.01. Nevertheless, it has been demonstrated recently that silicon nanowire boasts a ZT=1, comparable to commercially available Bi2Te3 TE devices. The enhancement mainly comes from the nearly two orders of magnitude reduction in thermal conductivity, possibly due to the dimensionality crossover of the phonon modes and/or the increased phonon-boundary scattering.

Here, we study the thermoelectric properties of a novel hole-bar-like silicon thin film (thickness ~ 25 nm). The hole-bar structure consists of a 2-D periodic array of 14 nm wide holes at a pitch of 34 nm, made by Superlattice Nanowire Pattern Transfer (SNAP) technique. This phononic-crystal material exhibits thermal conductivity as low as 2 W/m-K at 300K, which is 3.5 times smaller than that of nanowires with similar dimensions (k~7W/m-K, wire diameter = 22 nm). We attribute this reduction of thermal conductivity to the modification of phonon energy bands by the periodic hole-bar structure. This is the first such demonstration of the TE enhancement mediated by a nanoscale phonon crystal.