Exploring Local Doping in CVD-Grown Graphene on Sapphire Through Correlative Kelvin Probe Force Microscopy and Raman Spectroscopy Techniques

Graphene, transition metal dichalcogenides and synthetic 2D materials offer a vast potential for novel (opto-) electronic devices, membranes and energy storage among others. In particular, industrial application of 2D materials in high-performance nanoelectronics require homogeneous distributions of electrical properties on the nanoscale. Advanced atomic force microscopy (AFM) techniques like Kelvin probe force microscopy (KPFM) and conductive AFM (cAFM) resolve local surface potential and conductivity with a nanometer resolution and directly relate this information to the materials surface structure [1, 2]. Here, I present a correlative KPFM and µRaman study on wafer-scale monolayer graphene on an insulating sapphire substrate prepared via chemical vapor deposition. The use of high temperature H₂-annealed sapphire instead of pristine sapphire improves the resulting graphene film by reducing the overall density of strain-induced wrinkles and increasing the crystal quality [3]. We found the annealing-induced sapphire steps introduce a heterogeneous surface potential distribution on the graphene monolayers caused by spatially variant p-doping concentrations. The decreased p-doping concentration at the position of underlying step edges suggest that a change in substrate-film interaction leads to the highly heterogenous surface potential distribution.<br/>[1] U. Zerweck et al., Phys. Rev. B 71 (2005) 125424<br/>[2] J. Ludwig et al., Nanotechnology 30 (2019), 285705<br/>[3] N. Mishra et al., Small 15 (2019), 1904906