碳化矽是一個具有良好物理、機械和電子特性的寬能隙材料，使其成為下一個世代中高功率元件的替代材料。在過去大家對於矽基板上磊晶成長碳化矽有很大的興趣，因為其具有低成本以及大面積的優勢，另一方面將矽表面進行碳化能夠有效地解決晶格不匹配的問題並減少晶體缺陷。然而在過去的研究大多使用丙烷對矽表面進行高溫(~1673K)碳化，這導致矽會從矽基板中大量昇華。因此，了解低溫下的碳化機制並改善磊晶層品質非常重要。在過去有些研究指出未飽和碳氫分子因為存在π電子的關係使其比飽和碳氫分子在矽表面具有更高的活性。 在這項研究中，我們結合密度泛函理論以及微觀反應動力學模擬來探討乙炔在矽(100)表面之碳化機制，計算結果顯示end-bridge及di-σ是乙炔在矽表面上的最穩定吸附結構，除此之外，更基於這兩種最穩定的結構探討乙炔在後續的裂解機制。我們發現從end-bridge的結構中存在兩條路徑能在矽表面裂解成carbon dimer，其中乙炔遷移到hollow site後再更進一步脫氫反應是反應能量最小的路徑，而速率決定步驟則是在於第一個氫原子的移除。在di-σ的結構中，我們也發現有兩條路徑能在表面形成carbon dimer，第一條路徑是乙炔直接進行逐步脫氫反應，而另一條則是乙炔經由旋轉後再依序脫氫反應，然而由於他們在表面形成的carbon dimer結構不同，因此在矽表面上他們是互相競爭的反應。
我們的微觀反應動力學結果顯示carbon dimer可以在低溫下以end-bridge的結構生成，並且在660K的溫度產生最大量的carbon dimer。然而在這溫度之後，我們也觀察到從di-σ吸附結構生成的carbon dimer，而這結構將導致多晶碳化矽的產生。隨後由於C-H 裂解以及乙炔旋轉至hollow site形成end-bridge結構之間的競爭反應，促使end-bridge carbon dimer (end-C2H2*)的覆蓋率逐漸下降。我們的結果證實利用乙炔進行矽表面碳化時，在溫度的控制上需要非常小心，除此之外，在660K的溫度附近能生成最多的單晶碳化矽緩衝層。

Silicon carbide (SiC) is a wide gap semiconductor that exhibits promising physical, mechanical, and electronic properties, making it an alternative material for next-generation high-power electronic devices. Previously, the epitaxial growth of SiC on Si substrates has shown a significant interest because of low cost and large surface area. Especially, Si-surface carbonization is an effective method in solving the crystal defect and mismatch problem. Hence, it is important to explore the carbonization mechanism at low temperature in improving the epitaxial film quality. It is reported that unsaturated hydrocarbons have higher reactivity on Si surfaces than saturated hydrocarbons due to the presence of π electrons. Thus, in this study, we explored the carbonization mechanisms of Si(100) surface by acetylene using periodic density functional theory calculations combined with microkinetic simulations. Our calculation indicates that the end-bridge and di-σ configurations are the most stable adsorption configurations for acetylene adsorption on the Si(100) surface. Furthermore, we investigated the subsequent acetylene decomposition mechanisms based on these two most stable adsorption structures. We found that two possible paths for acetylene decomposition to form a carbon dimer on the Si surface in the end-bridge configuration. Among them, the minimum reaction pathway is acetylene diffusion to the hollow site and further step-by-step dehydrogenation, in which the rate-determining step is the removal of the first hydrogen atom. In di-σconfiguration, we also found two pathways to form the carbon dimer on the surface. The first pathway is the step-by-step dehydrogenation of acetylene, and another pathway is the rotation of acetylene and then the step-by-step dehydrogenation. Both of them compete with each other on the surface; however, their final structures are different.
Our microkinetic simulation results suggest that the carbon dimer as an end-bridge structure can be formed at low temperature (580K), and the maximum carbon dimers can be obtained near a temperature of 660K. Whereas, at this maximum temperature, we also observed the carbon dimer formation from the di-σ adsorption configurations, which leads to the polycrystalline SiC formation. Subsequently, the coverage of the end-bridge carbon dimer (end-C2H2*) gradually decreased, as both C-H bond breaking and acetylene rotation to form end bridge configuration reactions compete each other. Our results suggest that if we want to get more single crystalline silicon carbide buffer layer, the temperature should be controlled near 660K and the temperature control careful is essential in the carbonization of the Si (100) surface by acetylene.

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