以第一原理計算小半徑二矽化鎳奈米線的結構與電性性質

First Principle Calculation: Structural and Electronic Properties of Small Diameter NiSi2 NWs

鄭舒婷(Su-Ting Cheng)

指導教授 ： 蔡哲正

國立清華大學/工學院/材料科學工程學系/碩士(2009年)

摘要

奈米線的各種性質在尺度小於數十奈米時，會有強烈的尺寸效應，也就是說，其各種性質會和奈米線半徑(假設奈米線長度相對於半徑為無限長)有關係存在。此一效應源自於在奈米結構中，表面對體積的比例(surface to volume ratio:SVR)急遽的上升，使得表面會影響整體結構的性質。
我們的模擬是針對NiSi2的奈米線，使用第一原理計算預測其可能穩定的結構，楊氏係數，以及相關電性行為。在我們的結果中顯示，[111]軸向奈米線相對於[100]奈米線，在直徑小於1奈米時具有相對較好的穩定性，這可供成長超細奈米線參考。在機械行為方面，NiSi2奈米線在我們所計算的範圍(直徑0.8-1.3nm)僅具有原本塊材12.5%的楊氏系數，這樣的改變，我們相信來自於奈米線表面應力，此表面應力隨著奈米線尺寸上升會跟著慢慢下降，而表面應力和奈米線側面的方向有密切的關係，奈米線各種側面對機械行為的影響是值得繼續探討的問題。
在電性方面也深受各種奈米線側面的影響，NiSi2在製程上的應用主要用在歐姆接面(ohmic contact), 而幾乎所有我們進行計算的NiSi2奈米線都具有半金屬(semi-metal)的性質，且它們的能帶結構卻有很大的差異。[100]和[110]為典型半金屬能帶，而[111]則較接近半導體的能帶型態，這樣的差異我們認為主要來自於側面(facet)的不同。而側面對電性造成的影響，是來自於表面低鍵結度(low coordination number)的原子，從投影態密度(PDOS)中可發現，表面低鍵結度的原子可貢獻許多額外的態密度，造成與塊材迥異的能帶結構。

關鍵字

第一原理計算 ； 二矽化鎳 ； 奈米線 ； 楊氏系數

並列摘要

Physical properties at the nanoscale are known to be different from those in macro-dimensions. It was indicated that both structural and electronic properties are dependent on size effect or surface effect according to the high surface-to-volume ratio (SVR), which will exaggerate the surface status.
Our research examined the small diameter NiSi2 NWs with first principle calculations via CAmbridge Serial Total Energy Package (CASTEP). Numerical convergence has been checked with different k-point sets and cut-off energy from 300 to 380 eV. We have grown self-assembled NiSi2 NWs with triangular cross section on (100) Si substrate and these kinds of wires have been observed having very low resistivity. On the other hand, free-standing NiSi2 NWs have been fabricated by silicidation of Si NWs produced by etching Si substrates. Hence, we attempted to simulate the properties of low index axis NiSi2 NWs.
(1) Structural Properties
All the structures were carried out with total energy minimization to reach the most stable state and tensile and compressive strain were applied to analyze the Young’s modulus. Both types of NWs were found to have shorter bond length for edge (surface) bonding compared to bulk NiSi2 but longer bond length for inner bonding. The Young’s modulus was found to be much softer than that of the bulk. The difference of the bond length can be considered as “surface stress” which accounts for the softening.
(2) Electronic Properties
As to know the electronic properties in such low dimensions, we examined the band structure and density of states (DOS). The band structure was computed along the direction from  (0,0,0) to Z (0,0,0.5). All the NWs simulated are calculated to be semimetal. Ni and Si atoms both contributed to the low resistivity because half of the DOS near the Fermi level comes from Si and half from Ni. Further analyzing the Partial Density of States (PDOS), the surface Si atoms with low coordination number make greater contribution to the total Si DOS.

並列關鍵字

First principle calculation ； NiSi2 ； Young's modulus ； nanowire

參考文獻

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