# Electronic and Transport Properties of Epitaxial Graphene on SiC and 3C-SiC/Si: A Review
## 4 Electronic and Transport Properties: Effect of Interactions with the Surroundings
### Effect of Epitaxial Graphene-Contact Metal Interaction  
Since electronic transport measurements involve making metal contacts, understanding the influence of metal contacts on the transport of EG is also a concern [3]. The work function difference between graphene and a metal induces doping in graphene since the Fermi levels must equilibrate [3]. Since the \(E_{\text{F}}\) of freestanding graphene meets the \(E_{\text{D}}\), the adsorption of metal can significantly alter its electronic properties [3], as the DOS of graphene at Dirac point is much lower than that of the metal. A tiny amount of charge transfer shifts the \(E_{\text{F}}\) significantly [33; 34; 25] as it is reported that 0.01 electrons per carbon atom would shift the \(E_{\text{F}}\) by 470 meV. The charge transfer creates a dipole layer at the graphene-metal interface with \(\Delta\)V potential. The value of \(\Delta\)V depends on the strength of the metal-graphene interaction. Even a weak interaction can cause a large shift in the \(E_{\text{F}}\) away from the \(E_{\text{D}}\)[3; 119]. [PERSON] et al. [119] used DFT to characterize the adsorption of graphene on various metals and found graphene interacts more strongly to Ni, Co, Ti, and Pd (chemisorption). The chemisorption involves hybridization between graphene p\({}_{z}\) and metal d-states that cause an opening of a band gap in the graphene. In the case of chemisorption, the band structure is disturbed, and the doping is estimated from the difference of work function between the graphene covered with metal and metal-free graphene. In the case of Ni, Co, Ti, and Pd, the graphene is n-type doped [119]. Physisorption, e.g., interaction with metals such as Ag, Al, Cu, Au, and Pt, involves only weak binding to graphene. Therefore, the electronic structure is preserved. In the case of physisorption, the electrons transfer from the  
Figure 12: P-type doping of graphene upon the ambient exposure at 300 K. The p-type doping correspond to depletion of \(4.5\times 10^{14}\) electrons/cm\({}^{-2}\). Adapted with permission from Reference [5]. Copyright (2012) AIP Publishing. graphene to the metal, which causes the \(\mathbf{E_{\mathrm{F}}}\) to move closer to the \(\mathbf{E_{\mathrm{D}}}\)--and the hole doping is estimated from the change. At an equilibrium distance from the metal, the graphene is reported to be p-doped on Au and Pt and n-doped on Al, Ag, and Cu. [PERSON] et al. also reported that the graphene is doped n-type when the metal work function \(<\)5.4 eV, while it is p-type when the work function is \(>\)5.4 eV [119]. Using femtosecond laser-patterned microstructures (resist free, defect-free method), [PERSON] et al. [120] studied the graphene-metal interactions on EG synthesized on 6H-SiC(0001) via thermal decomposition. They found that the nickel does not form bonds with EG on SiC in contrast to the Ni-C formation in chemical vapour deposition (CVD) grown graphene on metals due to the absence of defect sites [119]. However, it was shown that the contact resistance (R\({}_{\mathrm{C}}\)) of Ni to EG with resistance residue under the contact is an order of magnitude greater than the intrinsic quantum-limited R\({}_{\mathrm{C}}\), which proposes the requirement for both end-contacts as well as a residue-free graphene-metal interface (which can form resistive NiCO\({}_{3}\)) as requirements to obtain quantum-limited contact resistance [120]. [PERSON] et al. [121] calculated the work functions of a number of high purity noble metals such as Cr, Cu, Rh, Ni, and Pt, and rare earth metals such as Pr, Eu, Er, Yb, and Y, in contact with MLG, using Kelvin Probe Force Microscopy. The metals were deposited under manufacturing-like conditions and Au (and its published work function) was used as the standard. The work functions were compared to published values for the other metals measured under careful, i.e., UHV, conditions. For the noble metals, a comparison showed Cr, Cu, and Rh had higher work functions (\(\sim\)0.5 eV) than expected and Ni and Pt were lower than expected (also \(\sim\)0.5 eV).