**3. Graphene doping**

technologies. The SiC is not desired for most electronics applications, making it desirable to transfer the graphene from its SiC substrate to a more standard substrate such as Si. The sublimation of graphene from SiC also creates a Si2O3 insulating under layer that could assist with the transfer process. Under high temperatures, a large variety of intercalant species can also be placed between the graphene and SiC layer that can potentially help with the exfoliation or the electrical modification/isolation. [19] Under normal conditions, the graphene SiC interface forms a Schottky contact; however, it has been shown that the oxide can be trans‐ formed to a nitrogen underlayer through a thermal annealing process in a nitride atmosphere

Graphene growth utilizing a carbonaceous source material (such as methane introduced through a CVD process) differs from material to material with the carbon solubility in the metal and the growth conditions determining the deposition mechanism as shown by the phase diagrams in Figure 8. [7] For carbide producing metallic substrates (such as Ni), graphene growth occurs through a precipitation process during cooling from the carbide. [7] The solubility of C in the metal (Ni for example) is higher at higher temperatures, and thus during the furnace cooling phase carbon diffuses out of its Ni host. [7] The process of forming graphene on Ni has the fundamental limitation that single and few layered graphene is obtained over few to tens of micron regions and not homogeneously over the entire substrate. [7] The lack of control over the number of layers is attributed to the difference in out-diffusion of C from the grains and the grain boundaries of Ni creating non-homogeneous growth conditions.

**Figure 8.** The phase diagram for a carbide creating catalyst (Fe) and a non-carbide creating catalyst (Cu) [7].

Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate, where there is registry between the two. In some cases, epitaxial graphene layers are coupled to surfaces

**2.5. Epitaxial growth of graphene on a non-carbide forming catalyst**

modifying the electronic characteristics between the two. [35]

66 Graphene - New Trends and Developments

**2.4. Growth through condensation after carbide formation**

Due to the chemical nature of the graphene with its zero band gap, mobility related to the delocalization of the π-bonding orbitals, and lattice periodicity, the doping of graphene can be achieved either through the breaking of lattice periodicity or the electrostatic confinement of the delocalized pz orbitals. [19] There have been several mechanisms proposed and tested that have been effective in shifting the Fermi energy to either p- or n-type regions of the band structure and the creation of p-n junctions at the interface. [19] It should be noted that by breaking the lattice symmetry, the electronic states and band edges are modified as shown in Figure 2, decreasing the mobility. With chemical functionalization, scattering is introduced into the graphene flakes, also decreasing the mobility.

Since graphene is a self-contained sheet with no real interface layer, it should be noted that process integration with oxide dielectrics as shown in Figure 9 can be difficult due to trapped

**Figure 9.** Image showing trapped charges at graphene oxide interfaces [41].

impurities at the interface in terms of creating floating gates for voltage-controlled variable gate transistors. [41] This effect is relevant for device applications of graphene films. It should also be noted that the introduction of trap centers and doping sites through contamination will degrade the continuity of the 2DEG at the trap or dopant site, causing electron and hole pooling to occur. [16, 17, 42]
