**3.1. Electrostatic doping**

Electrostatic doping as shown in Figure 10 can be controlled through a variety of methods; some use floating gates with oxide buffer layers and others use direct gate contacts to locally modify the Fermi level allowing for the voltage-controlled operation of the graphene device. [43] Most electrostatic gating is accomplished through a horizontal device architecture to preserve the mobility of graphene for ultrafast devices. With both direct and indirect contact, electrostatic gating can be accomplished by utilizing metals with two different work functions; polymers with different end groups as shown in Figure 11; and finally, layered materials with different opposing band gaps with the higher band gap being the acceptor and the lower being the donor as shown in Figure 12. [19] Metals with dissimilar work functions are normally integrated into a horizontal device with many different combinations to choose from. [44, 45] The amount of gap opening is defined by the difference between the two metal work functions and the induced electric field decreasing down the length of the sheet making the contact placement critical. [44, 45]

**Figure 10.** Diagram showing charge injection and Fermi modification of a graphene Schottky contact [43].

Review of Graphene Technology and Its Applications for Electronic Devices http://dx.doi.org/10.5772/61316 69

**Figure 11.** Diagram showing the doping of graphene utilizing different polymer end groups [18].

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

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

Electrostatic doping as shown in Figure 10 can be controlled through a variety of methods; some use floating gates with oxide buffer layers and others use direct gate contacts to locally modify the Fermi level allowing for the voltage-controlled operation of the graphene device. [43] Most electrostatic gating is accomplished through a horizontal device architecture to preserve the mobility of graphene for ultrafast devices. With both direct and indirect contact, electrostatic gating can be accomplished by utilizing metals with two different work functions; polymers with different end groups as shown in Figure 11; and finally, layered materials with different opposing band gaps with the higher band gap being the acceptor and the lower being the donor as shown in Figure 12. [19] Metals with dissimilar work functions are normally integrated into a horizontal device with many different combinations to choose from. [44, 45] The amount of gap opening is defined by the difference between the two metal work functions and the induced electric field decreasing down the length of the sheet making the contact

**Figure 10.** Diagram showing charge injection and Fermi modification of a graphene Schottky contact [43].

to occur. [16, 17, 42]

**3.1. Electrostatic doping**

68 Graphene - New Trends and Developments

placement critical. [44, 45]

For polymers, the use of different functional groups can electrostatically dope a horizon‐ tal graphene sheet with an isolated amine group (isolated nitrogen atom as in nitric acid) n-doping the graphene sheet while fluorine is well known as a good electron acceptor so a polymer containing an isolated fluorine end group p-dopes the polymer as shown in Figure 11. [18] For the polymer electrostatic doping technique, similar atomic dopants are utilized for the chemical doping regime with atoms lower than group V providing n-type doping and elements higher than group V creating p-type dopants (this will create environmental sensitivity within an exposed graphene sheet due to the oxygen and hydroxide adatoms p-doping the graphene). [18, 46]

Finally, for electrostatic doping, the utilization of other 2D materials as shown in Figure 12 can be used by vertical device integration with either a homojunction-based device or a hetero‐ junction-based device. For a homojunction-based device, graphene is utilized in a double layer with electrostatic doping coming from a layer above one graphene sheet with a lower band gap (such as tungsten diselenide WSe2) and one below the other graphene sheet with a higher band gap (such as molybdenum disulfide MoS2) creating an electric field between the two 2D materials with different band gaps and electrostatically doping the graphene as shown in Figure 12. [48] Since the electrostatic potential outside a sheet with a band gap will only induce a shift in the Fermi energy for graphene, a heterojunction can be formed between the junction of the 2D materials with a tuning of the upper and lower contacts required. [48] The use of 2D stacked devices is interesting but it should be remembered that many of these layers have not been shown to be readily deposited on top of the other, requiring transfer techniques that can induce defects, transfer contaminants, and have alignment issues between the lattices creating different properties across the lattice due to misalignment as shown in Figure 13. [47, 49]

**Figure 12.** Diagram showing the stacking of multiple Van der Waals materials in order to create unique and tunable electrical properties [47]

**Figure 13.** Image showing that the misalignment of 2D materials can electrically isolate the two sheets by separating the Dirac cones [50].

It has been shown that a twist angle between two graphene sheets above 2° electrically isolates the two graphene sheets from one another except at certain twist angles as shown in Figure 13. [49] Most 2D materials have also been shown to have intrinsic doping due to vacancies and edge defects that create more problems for device integration. [49] It should be noted that the mobilities in graphene on boron nitride (BN) substrates have been measured up to 140,000 cm2 /Vs, which is very close to completely suspended graphene grown from a SiC step edge, showing the validity of using 2D heterostructures for device integration and isolation. [32, 42]
