Wang Shutang, Huang Qiyu, Guo Rensong, Xu Jian, Lin He, Cao Jiefeng
School of Electronic Information and Electrical Engineering, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China.
Nanotechnology. 2020 May 15;31(20):205702. doi: 10.1088/1361-6528/ab704a. Epub 2020 Jan 27.
Electrochemical etching of silicon carbide (SiC) material has received increasing attention in recent years, due to its simple procedure, low cost and significance in the exploration of novel optoelectronic devices. In this paper, 4H-SiC substrates were electrochemically etched at a constant current density of 392.98 mA cm in an electrolyte made up of hydrofluoric acid and deionized water. The layering of a SiC porous layer and periodic fluctuation of the voltage were witnessed for the first time, with the layering phenomenon corresponding well to the voltage period. However, no such phenomenon was observed when the SiC substrates were anodic etched under the same conditions with magnet stirring. As a result, the periodic variation of voltage was hypothesized to be the cause of regular layering during constant current electrochemical etching. Electrochemical etching in potentiostatic mode was thus performed at different voltages. We found that the diameter of the SiC nanopores increased while the thickness of the sidewall decreased with the increasing voltage. Based on the experimental findings, a model of mass transport was proposed. The mass transport process leads to periodic changes in resistance, hence the periodic change in voltage. This successfully explained the reason for the layering. Furthermore, SiC substrates were also electrochemically etched at high and low current densities, finding the existence of a threshold current density for the occurrence of the layering. Energy dispersive x-ray spectroscopy analysis showed that the composition of the SiC porous layer remained unchanged compared to the pure SiC wafer, implying that the peeling-off of the SiC porous layer obtained by electrochemical etching can be directly adopted for use on devices requiring a SiC porous structure.
近年来，碳化硅（SiC）材料的电化学蚀刻因其工艺简单、成本低以及在新型光电器件探索中的重要意义而受到越来越多的关注。在本文中，4H-SiC衬底在由氢氟酸和去离子水组成的电解液中以392.98 mA/cm²的恒定电流密度进行电化学蚀刻。首次观察到SiC多孔层的分层现象以及电压的周期性波动，分层现象与电压周期对应良好。然而，当在相同条件下进行磁搅拌阳极蚀刻SiC衬底时，未观察到这种现象。因此，推测电压的周期性变化是恒流电化学蚀刻过程中规则分层的原因。于是在不同电压下进行恒电位模式的电化学蚀刻。我们发现，随着电压升高，SiC纳米孔的直径增大而侧壁厚度减小。基于实验结果，提出了一个质量传输模型。质量传输过程导致电阻的周期性变化，从而引起电压的周期性变化。这成功解释了分层的原因。此外，还在高电流密度和低电流密度下对SiC衬底进行电化学蚀刻，发现分层现象的发生存在一个临界电流密度。能量色散X射线光谱分析表明，与纯SiC晶片相比，SiC多孔层的成分保持不变，这意味着通过电化学蚀刻获得的SiC多孔层的剥离可直接用于需要SiC多孔结构的器件。
