Molten-alkali etching has been widely used to reveal dislocations in 4H silicon carbide (4H-SiC), which has promoted the identification and statistics of dislocation density in 4H-SiC single crystals. However, the etching mechanism of 4H-SiC is limited misunderstood. In this letter, we reveal the anisotropic etching mechanism of the Si face and C face of 4H-SiC by combining molten-KOH etching, X-ray photoelectron spectroscopy (XPS) and first-principles investigations. The activation energies for the molten-KOH etching of the C face and Si face of 4H-SiC are calculated to be 25.09 and 35.75 kcal/mol, respectively. The molten-KOH etching rate of the C face is higher than the Si face. Combining XPS analysis and first-principles calculations, we find that the molten-KOH etching of 4H-SiC is proceeded by the cycling of the oxidation of 4H-SiC by the dissolved oxygen and the removal of oxides by molten KOH. The faster etching rate of the C face is caused by the fact that the oxides on the C face are unstable, and easier to be removed with molten alkali, rather than the C face being easier to be oxidized.

NECP-SARAX是西安交通大学核工程计算物理实验室开发的先进反应堆中子学分析系统，近些年基于CEFR、PHENIX、SUPERPHENIX、JOYO MK-I、ZPR和ZPPR等反应堆开展了程序的验证与确认工作，计算结果表明：NECP-SARAX在快堆堆芯物理特性分析具有良好的性能。同时，利用上述反应堆中的燃料组件和控制棒组件，初步展现了截面计算程序TULIP在快谱问题计算分析上的精确性，但是仍然缺乏系统性地验证与确认。为了确认TULIP程序对于不同类型快谱问题的适用性，从国际临界安全基准评估项目中选取了147组临界实验装置进行计算分析，发现对于带有厚反射层的实验装置，TULIP程序计算k_eff和蒙特卡罗程序计算值的偏差超过10^-2。以HMF021-002实验装置构造均匀两核素问题展开研究，中等质量核素在非共振区表现出类似共振波动状的散射截面，对于使用大量结构材料的快谱系统，其非共振区类似共振波动截面的自屏效应变得不可忽略。针对此现象，对TULIP程序的共振计算策略和非共振区计算方法进行优化，采用超细群（Ultra Fine Group，UFG）的共振计算方法，对中等质量核素高装载量情况下非共振区类似共振波动状截面的自屏效应进行处理，改进程序后相应实验装置计算偏差降低到3×10^-3内，数值结果表明，改进后的TULIP程序对于快谱系统具有良好的计算分析能力。

Discrimination of dislocations is critical to the statistics of dislocation densities in 4H silicon carbide (4H-SiC), which are routinely used to evaluate the quality of 4H-SiC single crystals and homoepitaxial layers. In this work, we show that the inclination angles of the etch pits of molten-alkali etched 4H-SiC can be adopted to discriminate threading screw dislocations (TSDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) in 4H-SiC. In n-type 4H-SiC, the inclination angles of the etch pits of TSDs, TEDs and BPDs in molten-alkali etched 4H-SiC are in the ranges of 27°?35°, 8°?15° and 2°?4°, respectively. In semi-insulating 4H-SiC, the inclination angles of the etch pits of TSDs and TEDs are in the ranges of 31°?34° and 21°?24°, respectively. The inclination angles of dislocation-related etch pits are independent of the etching duration, which facilitates the discrimination and statistic of dislocations in 4H-SiC. More significantly, the inclination angle of a threading mixed dislocations (TMDs) is found to consist of characteristic angles of both TEDs and TSDs. This enables to distinguish TMDs from TSDs in 4H-SiC.Discrimination of dislocations is critical to the statistics of dislocation densities in 4H silicon carbide (4H-SiC), which are routinely used to evaluate the quality of 4H-SiC single crystals and homoepitaxial layers. In this work, we show that the inclination angles of the etch pits of molten-alkali etched 4H-SiC can be adopted to discriminate threading screw dislocations (TSDs), threading edge dislocations (TEDs) and basal plane dislocations (BPDs) in 4H-SiC. In n-type 4H-SiC, the inclination angles of the etch pits of TSDs, TEDs and BPDs in molten-alkali etched 4H-SiC are in the ranges of 27°?35°, 8°?15° and 2°?4°, respectively. In semi-insulating 4H-SiC, the inclination angles of the etch pits of TSDs and TEDs are in the ranges of 31°?34° and 21°?24°, respectively. The inclination angles of dislocation-related etch pits are independent of the etching duration, which facilitates the discrimination and statistic of dislocations in 4H-SiC. More significantly, the inclination angle of a threading mixed dislocations (TMDs) is found to consist of characteristic angles of both TEDs and TSDs. This enables to distinguish TMDs from TSDs in 4H-SiC.