本論文主要是以分子動力學(molecular dynamics)模擬並探討單晶矽奈米結構受應變作用後之電性性質與結構變化，模擬研究的方向針對不同維度的單晶矽材料做研究，其中包括奈米線(nanowires)與奈米塊(nanocubes)。原子間相互作用力採用Tersoff勢能與Morse勢能函數來計算，並依循牛頓運動方程式，採用Gear五階預測修正運算法來分析各個步階的分子位置、速度等相對物理量。關於矽結構的相變化，此次採用配位數法則來連續監控矽單晶奈米結構的相變行為，此方式能確保相變化的過程清楚呈現，以便追蹤金屬相矽(Si-II)產生的過程和蔓延方向，進而預測並探討矽單晶奈米結構的導電率(Conductivity)，使矽材料的電性與相變關係得以詮釋。同時在奈米塊的最底層和奈米線兩側則設定為固定層，並在上下兩側施加一虛擬電源，藉此追蹤單晶矽材料受應力作用後，材料的電性變化。本研究也將以往模擬較難過濾出的bc8&R8相給過濾出來，此相的配位數和Si-I相一樣，因此不容易過濾。奈米線拉伸模擬發現：應變作用過程中，矽奈米結構內部會先產生差排(dislocation)再產生相變化。因為矽奈米結構受拉伸應變作用時，首先會藉由差排原子滑移來釋放拉伸應力，緊接著結構會重組滑移的差排原子，使其轉變成有意義的相變結構，此過程即為相變過程。於是差排原子先產生，緊接著才由相變化從後遞補。當應變施加於(011)和(111)方向矽奈米線時，必須超越材料的降伏應力使結構產生破壞後才能改變其材料電性。但是針對(001)方向來說，無需破壞整個結構即可改變電性。於是藉由應力改變材料電性的研究中，(001)方向的矽奈米線較較好的研究題材。同時發現，適度的應力作用確實能轉變矽奈米線的電性，即使將負載移除，殘留的應力仍然會產生作用並導致相變化，使材料維持在導體階段。藉由壓痕模擬發現：模型的尺度大小與半圓球探針的半徑和壓深有關，探針壓深至少要大於探針半徑的70%才可對結構造成塑性變型，此時卸載後的區域為永久殘留的塑性變型區，有利於觀察整個結構的相變化。

Molecular dynamics (MD) simulation is adopted to examine the deformation and phase transformation of mono-crystalline Si. In this study, the inter-atomic interaction of Si atoms is modeled by Tersoff potential, while the interaction between Si atoms and diamond indenter atoms is modeled by Morse potential. The techniques of coordination number (CN) and centro-symmetry parameter (CSP) are used to monitor and elucidate the detailed mechanisms of the phase transformation throughout the loading process in which the evolution of structural phase change and the dislocation pattern can be identified. Therefore, the relationship between phase transformation and dislocation pattern can be established and illustrated. In addition, the electrical resistance and conductivity of mono-crystalline Si was evaluated by using the concept of virtual electric source during loading and unloading similar to in situ electrical measurements. Simulation results show that before the failure of the material, the dislocations are introduced first and then the phase transformation such that the total energy of the system tends to approach a minimum level. Moreover, the electrical resistance of (001)- rather than (011)- and (111)-oriented SiNW’s is changed before failure. As the stress level of the (001) SiNW reaches 24 GPa, a significant amount of metallic Si-II and amorphous phase is produced from the semiconducting Si-I phase and leads a pronounced decrease of electrical resistance. In this article, the phases of BC8 and R8, which have the same coordinate number as the phase Si-I and were difficult to distinguish from each other in previous studies, are successfully identifiedand extracted from the deformed region during unloading. As the effect of the indenter-radius size on thestructural phase transformation of mono-crystalline Si for three different crystallographically oriented surfaces is investigated. It is also found that as the onset of the plastic deformation tends to take place only as the ratio of the indentation depth to the tip radius is larger than 0.7. Under this condition the structural phase transformation can be easily observed in the residual deformed region after unloading.

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