以分子動力學模擬研究氮化鋁薄膜之熱傳性質

Abstract

本論文使用平衡分子動力學(EMD)方法來研究纖鋅礦氮化鋁塊材以及薄膜之熱傳導係數，亦使用非平衡分子動力學(NEMD)方法來研究厚度不對稱氮化鋁薄膜結構之熱整流性質，並與熱擴散方程式之結果進行比較。模擬使用Tersoff勢能函數來描述原子之間的作用力。 透過EMD模擬得到的室溫氮化鋁塊材熱傳導係數會大於實驗及第一原理結果，且在溫度介於100 K到500 K間時，熱傳導係數會隨著溫度上升而下降，非等向性落在1.9%至 6.8%之間，可視為等向性材料。而透過EMD模擬得到的氮化鋁薄膜平面及垂直平面熱傳導係數在溫度介於100 K到500 K間時，也都會隨著溫度上升而下降。隨著薄膜厚度增加，平面熱傳導係數會略微上升，而垂直平面熱傳導係數則會明顯的增加。 在透過EMD模擬得到的薄膜熱傳導係數的溫度相依性後，可將其用於求解熱擴散方程式，以初步探討厚度不對稱氮化鋁薄膜結構的熱整流現象。結果顯示，由於二維熱擴散方程式有考慮到截面積變化所造成的擴散熱阻，因此熱傳量略小於一維之結果，且熱偏好從面積小流向面積大的地方。相較於熱擴散方程式，由於NEMD模擬包含了尺寸效應以及微觀介面熱阻等現象，因此熱傳量小了一個數量級，熱整流方向也與二維熱擴散方程式之結果相反。

In this study, we use equilibrium molecular dynamics (EMD) simulation to study the thermal conductivity of bulk and thin film wurtzite aluminum nitride (AlN). In addition, we also use nonequilibrium molecular dynamics (NEMD) simulation to investigate the thermal rectification phenomenon of thickness-asymmetric AlN thin film structure and compare the result with those from the heat diffusion equation. The thermal conductivity of bulk AlN at room temperature obtained through EMD simulations is slightly higher than experimental and first-principles results. When the temperature is between 100 K and 500 K, the thermal conductivity decreases as the temperature rises, with anisotropy ranging from 1.9% to 6.8%, indicating that it can be considered as an isotropic material. For AlN thin films, the in-plane and out-plane thermal conductivity also decrease as the temperature increases between 100 K and 500 K. As the film thickness increases, the in-plane thermal conductivity slightly increases, while the out-plane thermal conductivity significantly increases. After obtaining the temperature-dependent thermal conductivity of the thin film through EMD simulations, it can be used to solve the heat diffusion equation to explore the thermal properties of thickness-asymmetric AlN thin film structure. The results show that, due to the interface thermal resistance caused by the change in cross-sectional area considered in the 2D heat diffusion equation, the heat is slightly smaller than the 1D result, and heat tends to flow from the smaller to the larger area. Compared to the heat diffusion equation, the NEMD simulations, which consider size effects and microscopic effect, result in a much smaller heat with the thermal rectification direction opposite to that of the 2D heat diffusion equation.