- 學位論文
矽鍺材料介面熱阻之分子動力學及波包模擬研究
An investigation of the Si-Ge thermal boundary resistance in use of MD and wave packet methods
摘要
本論文利用非平衡分子動力學數值工具來研究矽/鍺薄膜完美及非完美介面在500K下之熱阻。在此研究中，原子間的作用力採用Stillinger-Weber勢能函數；介面特徵包括厚度及其內鍺原子所佔比例。研究結果顯示，介面熱阻隨介面內鍺原子比例的變化趨勢因介面厚薄可有完全不同的情形發生。厚介面其熱阻值呈現一單峰曲線，在鍺原子比例為0.5時為最大。此結果說明了在厚介面中，質量差異散射為主宰因素。薄介面其熱阻值卻是在0.5的峰值兩側另有兩個比完美介面熱阻值還低的局部最小值。為了更進一步瞭解，本研究乃進行波包測試：將波包打入材料內，藉此量測介面在0K及500K下的聲子穿透率。研究結果顯示溫度對穿透率隨介面內鍺原子比例變化的影響並不大。兩溫度的結果皆顯示無論厚或薄介面，無論波長，穿透率皆是隨鍺原子比例先降後升，由此證明質量差異散射確實為主宰因素。穿透率的結果符合厚介面的熱阻結果，可是卻與薄介面的結果不符。造成此現象的原因極有可能是我們所打入的波包只有一個震動模式及入射波的角度一直是垂直於介面。此外，在非平衡分子動力學模擬中，我們所用的模擬長度不夠大以及採用的絕熱邊界條件也可能造成熱阻值量測上的誤差。
並列摘要
This thesis employs the non-equilibrium molecular dynamics (NEMD) to investigate the thermal transport phenomena across both perfect and imperfect interfaces formed by two dielectric thin films (Si and Ge) at 500K. The conditions of imperfect interfaces, the interface thickness and compositions, were manipulated and their effects on the thermal boundary resistance (TBR) were explored. Changing the Ge atomic composition creates different levels of mass disorders. The Stillinger-Weber (SW) potentials were used to describe the interaction between and among atoms of Si and Ge. The adiabatic boundary condition was applied in the main heat transfer direction at both ends of the simulation domain. The simulation results show that the TBRs of thick and thin interfaces possess remarkably different variation trends. The thick interface reveals a single bump in its variation curve having the maximum occurring at a Ge atomic fraction of 0.5. This implies that the mass difference scattering effect overtakes other scattering effects. The thin interface nonetheless has two additional dips on both sides of the peak, both smaller than the TBR of the perfect interface. For a further understanding, the wave-packet experiment was employed to measure the acoustic transmissivities at 0K as well as 500K. Two alloy interfaces were inserted in the simulation domain and the periodic boundary conditions were employed in all three directions. The so measured longitudinal acoustic transmissivities at 0K decrease first and increase later with an increasing Ge atomic fraction for both thick and thin interfaces and for all wave numbers, implying the dominance of the mass difference scattering effect. The TA transmissitivity has a similar trend except that the dip is smaller in the thin interface case. The 500K experiments show reduced transmissivities compared to those measured at 0K but the variation trend remains unchanged. The measured transmissivities support the TBR results of the alloy interface of thickness 10UC. Nonetheless, they are contradictive to the TBRs associated with the interfaces of thickness 2UC. This discrepancy may be caused by the limitations of the wave packet experiment which (1) treats the phonon mode in isolation and (2) has the incident wave always normal to the interface. Additionally, the adiabatic boundary condition and the insufficiently long simulation domain in the NEMD simulations may contribute to the discrepancy as well.
參考文獻
延伸閱讀
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