单晶碳化硅在纳米刻划下的变形机理研究

       单晶碳化硅（SiC）是一种重要的宽带隙半导体材料，但由于其硬度高、断裂韧性低，致使其加工难度较大。要制造高性能的碳化硅器件，要求其元件表面和亚表面无加工损伤。已有的理论研究基于分子动力学仿真模拟分析，表明纳米划痕引起的碳化硅亚表面损伤主要是通过非晶态相变引起。而实验研究主要采用纳米压痕的方法，表明碳化硅亚表面的损伤形式包括非晶相、高密度的位错和微裂纹。由于理论研究与实验研究的空间和荷载尺度存在显著差异，其结果不能直接进行比较。因此，碳化硅在纳米尺度下刻划的变形机制目前尚不清楚。
       本文旨在通过在与分子动力学仿真模拟相似的长度和载荷尺度下进行实验研究来阐明碳化硅纳米刻划的变形机理。为此，纳米划痕测试在原子力显微镜（AFM）上进行，采用尖端半径为10 nm和60 nm的金刚石针尖在4H-SiC和6H-SiC的 (0001 ) 和 (000-1) 晶面进行纳米刻划，使用AFM和扫描电子显微镜对纳米沟槽的形貌进行检测，并利用高分辨率透射电镜(HRTEM)对截面亚表面损伤进行分析。此外，本文分别在两种晶型的不同晶面进行力学性能的表征和分析，通过纳米压痕下的变形机理分析，系统对比了两种晶型的物理性能差异。
       研究表明，6H-SiC的碳面硬度最高韧性最差，4H-SiC的硅面硬度最低韧性最好。并且随着正向载荷的增加，纳米硬度和弹性模量逐渐减小并趋于稳定，“pop-in”现象的分析结果表明，第一次“pop-in”出现在压痕深度40-60 nm之间，随后连续出现多次“pop-in”现象。当纳米划痕的载荷小于20 μN时，纳米刻痕深度小于1 nm，非晶态相变是引起亚表面变形的主要原因，这与文献中分子动力学仿真模拟分析的结果保持一致。而且并未产生微裂纹，非晶区域的周围会产生少量的位错。
       该研究通过实验揭示了单晶碳化硅在纳米尺度上的变形和材料去除机制，并为实现低损伤SiC表面提供了可靠的依据。

    Monocrystalline silicon carbide (SiC) is an important wide-bandgap semiconductor material, but the material is difficult to machine due to its high hardness and low fracture toughness. To fabricate high performance of SiC devices, it is required that the surface and subsurface are processed with less machining damage. Theoretical studies based on molecular dynamics simulations have shown that the subsurface damage of SiC is mainly caused by amorphous phase transitions under nano-scratching. On the other hand, the experimental studies based on nanoindentation test showed that subsurface damage of SiC included amorphous phases, high density of dislocations and microcracks. Due to the significant differences in length and loading scales between the theoretical and experimental studies, the results cannot be directly compared. Therefore, the deformation mechanism of silicon carbide due to nano-scratching at the nanoscale is still unclear.
    This study aims to clarify the deformation mechanism of SiC by carrying out experimental investigations at similar length and load scales to those in the theoretical studies based on molecular dynamics. To this end, nano-scratching tests were conducted on an atomic force microscope (AFM). Diamond AFM tips of the radii of 10 nm and 60 nm were used as the nano-scratching tool on the (0001 ) and (000-1) surfaces of 4H-SiC and 6H-SiC single crystals. The nano-grooves were then examined under AFM and SEM, and the machining damages of the subsurface were analyzed by high-resolution transmission electron microscopy (HRTEM). In addition, this paper characterized and analyzed the mechanical properties of different crystalline surfaces for the two types of silicon carbide, respectively, and systematically compared the physical property between the two types after nanoindentation test. 
    It was found that the carbon surface of 6H-SiC has the highest hardness and the lowest toughness, and the silicon surface of 4H-SiC has the lowest hardness and the best toughness. The analysis of the "pop-in" phenomenon shows that the first "pop-in" occurs at the indentation depth of 40-60 nm, and then the "pop-in" phenomenon occurs several times in succession. The results of the "pop-in" phenomenon showed that the first "pop-in" occurred between 40-60 nm, and then there were several "pop-in" phenomena. When the nano-scratching load is less than 20 μN. In such cases, the nano-grooving depth was below 1 nm , and the amorphous phase transition is the main cause of subsurface deformation, which is consistent with the results of molecular dynamics simulations in the literature. Moreover, no microcracks were generated, and a small amount of dislocations are generated around the amorphous regions.
    This study has revealed the deformation and material removal mechanism of monocrystalline SiC at the nanoscale experimentally, and has laid the reliable foundation for achieving minimal damage to the SiC surfaces.

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