||Joshua Robinson, Kathleen Trumbull, Michael LaBella, Randall Cavalero, Matthew Hollander, Michael Zhu, Maxwell Wetherington, Mark Fanton, David Snyder
We investigate carrier transport and graphene structural properties as a function of silicon carbide (SiC) wafer orientation and growth ambient. Monolayer epitaxial graphene is found to exhibit Raman 2D/G ratios greater than 0.85, while multilayer graphene 2D/G ratios average 0.50. Additionally, increased terrace edges are observed as wafer orientation diverges from the (0001) crystallographic direction, which leads to increased carrier density and decreased mobility. We report that while those angles closest to the (0001) direction produce superior transport properties (room temperature mobility (µe) up to 2400cm2/Vs), the presence of uncontrolled terrace formation can result in significant scatter in the carrier mobility. As the SiC surface normal diverges from (0001), the terrace edge formation becomes uniform and parallel, leading to a significant improvement in Hall transport uniformity across the wafer. Raman spectroscopy maps of the same Hall cross provide evidence that the G peak intensity, 2D-peak intensity, and 2D/G peak ratio are all affected by the presence of the SiC terrace edge. The G-peak intensity doubles at the terrace edge, while the 2D peak is significantly less affected. As a result, we find that the 2D/G peak ratio is reduced from >1 at the terrace center to 0.4 – 0.5 at the terrace edge. This alone does not constitute layer thickness variation; however, according to Lorentzian fitting of the 2D peak from individual Raman spectra the graphene on the terrace center and edge is monolayer and bilayer graphene, respectively. Hall transport measurements indicate that room temperature carrier density and mobility are affected by the presence of step edges. Hall crosses exhibiting high mobility also exhibit zero or one narrow step edge (Figure 1a), while highly doped samples exhibit multiple (≥ 2) step edges. When miscut is 0.02 degree or greater, the carrier transport properties become significantly more uniform, with deviation around the mean averaging < 6%. However, in addition to improved uniformity, the average carrier density and mobility is changed by as much as +30% and -40%, respectively, compared to the 0.02 degree sample. Finally, Ar growth ambient is shown to result in carrier mobilities three times that of graphene grown in He, indicating that graphene transport properties may be further improved via growth in heavy molecular weight gaseous environments. This is due to the reduced probability for Si evaporation as the molecular weight of the gas ambient increases.