光电化学刻蚀方法去除SiC衬底外延石墨烯缓冲层及其表征

高温条件下裂解碳化硅（SiC）单晶，在直径5 cm的4H-SiC（0001）面制备出单层石墨烯。利用光电化学刻蚀方法，使KOH刻蚀液与SiC发生反应，降低石墨烯与衬底之间的相互作用力，去掉原位生长过程中SiC衬底与石墨烯之间存在的缓冲层，获得准自由的双层石墨烯。首先通过对比不同的电流密度和光照强度，总结出电流密度为6 mA·cm^-2、紫外灯与样品间距为3 cm时，石墨烯缓冲层的去除效率以及石墨烯质量皆为最佳。采用此优化后工艺处理的样品，拉曼光谱表明原位生长的缓冲层与衬底脱离，表现出准自由石墨烯的特性。X射线光电子能谱（XPS）C1s谱图中代表上层石墨烯与衬底Si悬键结合的S1、S2特征峰消失，即石墨烯缓冲层消失。通过分析刻蚀过程中的电化学曲线，提出了刻蚀过程的化学反应过程中的动态特性。

Photo-electrochemical removal of graphene buffer layer on SiC substrate

Monolayer graphene was fabricated on silicon carbide (SiC) (0001) face by thermal decomposition of SiC single crystal of 5 cm in diameter. The reaction of SiC substrate with aqueous potassium hydroxide (KOH) by photo-electrochemical etching reduced the interaction force between graphene and SiC substrate, and the quasi-free-standing bilayer graphene was obtained by removal of buffer layer between graphene and SiC substrate. Numerous conditions of current densities and illumination intensities were studied. The optimal condition to remove graphene buffer layer, which would synchronously obtain free standing graphene film in the highest quality, was estimated to be in the current intensity of 6 mA·cm^-2 when samples held the distance of 3 cm to UV light source. Among three current densities of 3, 6 and 9 mA·cm^-2, Raman spectra showed that the 6 mA·cm^-2 current density was the most suitable for etching process. The decoupling effect of graphene buffer layer showed a positive correlation to UV light intensity in a certain range. Raman and XPS spectra on graphene film prepared under optimized condition showed broken bonds between SiC substrate and in-situ grown buffer layer and characteristics of free standing graphene film. The removal of graphene buffer layer was shown by disappearance of characteristic S1 and S2 peaks of buffer layer in XPS C1s spectra. The chemical reaction dynamics in etching process was proposed by analyzing electrochemical voltage-time curves.