# 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-Ambient Interaction  
The surface of EG is sensitive to a range of gases present in the ambient air [7; 8; 9; 10; 11; 12]. Early on, researchers realized that adsorbed atoms or molecules on a graphene device could modify the channel conductivity. [PERSON] et al. [108] utilized this property to create a detector with sensitivity to very low concentrations of NH\({}_{3}\), NO\({}_{2}\), H\({}_{2}\)O\({}_{2}\), and CO\({}_{2}\). Yet, graphene is relatively sensitive to all adsorbed molecules, and, hence, a requirement on building any sensor requires the ability to discriminate the target molecule from the background. An additional implication is that the surface of a graphene sample must be prepared to mitigate the effects of adsorbed molecules on transport properties. For example, heating the sample to temperatures \(\sim\)400 \({}^{\circ}\)C in ultra-high vacuum (UHV) results in a clean, usable surface [109]. In a similar vein, [PERSON] et al. prepared a reproducible graphene surface to study the effects of adsorbed gases on work function and electrical properties. This led to several important findings. First, there are a range of molecules in the ambient (O\({}_{2}\), H\({}_{2}\)O, and NO\({}_{2}\)), which act as p-type dopants and reduces the n-type sheet charge density of graphene. Yet, there are other unidentified atmospheric contaminants, which also act as p-type dopants. It was suggested that some candidates are N\({}_{2}\)O\({}_{4}\), CO\({}_{2}\), and various hydrocarbons. Lastly, it was shown that, for more than one graphene layer, the effect on conductivity was less, and it was proposed this was due to charge screening of the outermost layer. First-principles modeling indicates that the physical mechanism of the doping compensation (for H\({}_{2}\)O, NH\({}_{3}\), CO, NO\({}_{2}\), and NO) and is due to charge transfer [110] from graphene to the molecule when the lowest unoccupied molecular orbital is below the Dirac point. The polarity of the Si-face SiC gives rise to high hydrophilicity, which is relatively unchanged with the addition of a graphene monolayer [111]. Graphene's response to water and changes in the environment are strongly thickness dependent with a monolayer on Si-face being most sensitive to water adsorption than the bilayer or tri-layer graphene [5; 112; 113]. Yet, the understanding of the exact H\({}_{2}\)O doping mechanism is not as well-understood since chemical processes may be involved. [PERSON] et al. noted that, upon ambient exposure at 300 K, both the MEG and MLG on SiC(000\(\overline{1}\)) typically exhibit p-type conduction, \(4.5\times 10^{14}\) cm\({}^{-2}\) (Figure 12). The p-type doping is due to the ambient adsorbed film containing water vapour (H\({}_{2}\)O), O\({}_{2}\), and NO\({}_{2}\) on the exposed surface [5; 114]. The Dirac energy of the graphene lies near the redox potential of dissolved oxygen, which causes the surface charge transfer from graphene into the film (water/oxygen redox couple) and results in compensation of the n-type sheet density [2; 5; 46; 114; 115]. In addition, the conductivity state of vacuum annealed EG on SiC(000\(\overline{1}\)) is n-type (Figure 10b) [112; 114]. This n-type conductivity was related to the work function considerations and \(\mathbf{E_{F}}\) pinning at the state related to dangling bonds, as discussed in Section 4.1.3 [5]. Yet, we now know that the pseudo charge effect induced by substrate hexagonality dominates the graphene charge state. [PERSON] et al. exposed monolayer EG on SiC(0001) to different inert gases and performed in-situ, contactless majority free charge carrier determination using terahertz-frequency optical Hall effect measurements [2]. The transport properties of graphene are also sensitive to other adsorbed gas molecules N\({}_{2}\)O\({}_{4}\), CO\({}_{2}\), and hydrocarbons [114; 116; 117; 118; 2; 5]. The mechanism of ambient doping is described as ambient-exposed graphene forms a thin film of water containing dissolved CO\({}_{2}\) that reacts with water forming H\({}^{+}\) ions.