**5. Substrate transfer of doped bilayer graphene**

#### **5.1 Evolution of substrate transfer techniques**

Considerable research has been undertaken to synthesize graphene on metal substrates using the CVD process to produce high-quality large-area graphene films. Sufficiently high quality of graphene films is demonstrated by a single-crystalline structure free of wrinkles, contamination, and cracks [35]. However, various and often critical applications can require that the CVD-grown graphene films be transferred onto other more suitable substrates.

The earliest form of transfer for graphene can be traced back to the synthesis of graphene by "Scotch tape method," whereby graphene flakes were exfoliated and isolated from the graphite substrate onto another target substrate [34]. The development of an efficient process for transfer of graphene from its native substrate onto another foreign nevertheless has since proved relatively challenging, especially for large-area and intact graphene films [36].

While CVD-grown graphene on metal substrates is typically of high quality and purity, the deposited graphene when transferred off onto other types of substrates can easily degrade and suffer from contamination and structural damage [37]. Common sources of contamination during the process of graphene transfer include residues from the source substrate, etchant solutions used to dissolve the source substrates, and unwanted organic contamination due to the adherence of polymer compounds to the graphene following completion of the transfer [38]. These factors can lead to the formation of more charge carrier scattering centers that affect the electrical properties and mechanical stability of that graphene films, generally resulting in undesired doping of the graphene [39].

Additionally, the extreme thinness of graphene (single-layer atomic thickness for monolayer graphene) makes it inherently more vulnerable to altercation and impairment [40]. During cleaning and repeated transfer, mechanical strains can arise in the graphene that potentially can cause irreversible damage [41]. Hence, the need to maintain structural integrity and uniformity of the graphene for various applications can be very essential, for instance for optoelectronic devices or sensors

requiring charge injection between the active functional layer and highly pure and conductive graphene [42].

Furthermore, implementation of an optimized transfer process is critical to boost yield and reproducibility for low cost and scalable production of large-area graphene films [43]. Consequently, significant research is being undertaken to further optimize the process of transferring graphene to attain intact and dislocation- and defect-free graphene films.

The procedure required for preparing graphene for transfer may be characterized by the more relevant process steps involved, which typically include—(a) graphene layer removal from substrate utilizing liquid etchant, bubble transfer, or thermal peel off; (b) use of supportive layers (e.g., polymers such as PMMA and camphor) to prevent cracks, creases, and other structural damage; and (c) cleaning and removal/transfer of the grown layer from the substrate and protective layers [35].

The major graphene transfer methods reported in the literature are as follows:

a.*Bubble-mediated transfer*: In this process, H2 and O2 bubbles are produced due to electrochemical reactions, that is, by the graphene when CVD-grown on a metal substrate such as Cu/Ni acting as an electrode (either anode or cathode). These bubbles when generated apply a pealing-inducing force on the substrate surface, eventually leading to delamination of the graphene from the growth substrate.

Although this method is challenging in certain respects and more limited in that conductive substrates are necessary for the actualization of the electrodeinitiated electrochemical reactions [30], considerable improvements have been discovered and incorporated over time. Goa et al. developed a nondestructive bubble-mediated transfer process that enabled repeated use of the growth (Pt) substrate, whereby the transferred graphene was found to have high carrier mobility along with minimal wrinkles [42]. Another study examined the use of PMMA/graphene/Cu acting as both an anode and cathode to remove a graphene sheet by bubble delamination, resulting in the reportedly high-quality transfer of the graphene films [40].


Through this experimentation toward the achievement of clean, smooth, and reduced-residue transfer, a PMMA-based resist-assisted transfer process was identified and established. In contrast to more conventional PMMA transfer processed known to leave residues during the graphene transfer, the method we have adapted

*Doping and Transfer of High Mobility Graphene Bilayers for Room Temperature Mid-Wave… DOI: http://dx.doi.org/10.5772/intechopen.101851*

and further developed features a more straightforward process that uses different polymeric supportive layers for residue-free and clean transfer of graphene to avoid cleaning and support removal steps. This new method for transferring graphene from Si/SiO2 to HgCdTe substrates combines both wet transfer and nonelectrochemical reaction-based transfer methods.

#### **5.2 Experimental bilayer graphene transfer**

Boron *p*<sup>+</sup> doping of bilayer graphene on Si/SiO2 substrates has been accomplished to provide required electrical performance characteristics for a high mobility graphene channel in MWIR HgCdTe photodetector devices. The final step as discussed in this process involves transferring the sheets of highly *p*-doped bilayer graphene from the original Si/SiO2 onto HgCdTe substrates for incorporation in the MWIR photodetector and FPA devices. This subsequently *p*-doped bilayer graphene on Si/SiO2 is transferred using a PMMA-assisted wet transfer process [44]. **Figure 11** shows schematically this experimental procedure for transferring the graphene onto HgCdTe [18].

Through this relatively straightforward procedure, the successful transfer of doped bilayer graphene sheets deposited on SiO2/Si onto HgCdTe has been demonstrated. The graphene bilayers are preserved, and no morphological changes were observed, following the transfer process with their relocation where the spatial configurations of the bilayers were maintained across macroscopic regions.

#### **5.3 Characterization of graphene transferred onto HgCdTe**

Following the transfer of the *p*-doped graphene onto HgCdTe substrates, the doped bilayer graphene on HgCdTe was measured using optical microscopy and Raman spectroscopy to determine if any significant changes had occurred in its properties through the process. **Figure 12** presents optical microscopy images of

#### **Figure 11.**

*Schematic outline of the experimental process enabling the removable transfer of bilayer graphene from SiO2/Si onto HgCdTe substrates.*

#### **Figure 12.**

*Optical microscopy image of graphene deposited on HgCdTe, where darker areas represent graphene on HgCdTe and lighter area portions the bare HgCdTe substrate.*

#### **Figure 13.**

*Raman spectroscopy analysis of boron-doped graphene transferred onto HgCdTe substrate, compared to spectra of bare HgCdTe substrate, that of the pristine bilayer graphene on Si/SiO2, and the graphene on Si/SiO2 following the boron doping but before transfer.*

the transferred graphene onto HgCdTe. The darker areas represent the part of the HgCdTe covered with graphene (having relatively marginal but practically observable differences in optical absorption), while the lighter areas indicate uncovered portions of the bare HgCdTe substrate.

**Figure 13** shows Raman spectra of the transferred doped graphene on HgCdTe, in comparison to the as-received graphene on Si/SiO2; the graphene following the boron doping; and a bare HgCdTe substrate.

The Raman spectroscopy analysis shows the G-band peak resulting from in-plane vibrations of *sp*<sup>2</sup> -bonded carbon atoms, and the D-band peak due to out-ofplane vibrations attributed to the presence of structural defects. The associated

*Doping and Transfer of High Mobility Graphene Bilayers for Room Temperature Mid-Wave… DOI: http://dx.doi.org/10.5772/intechopen.101851*

D/G ratio relates to the *sp*<sup>3</sup> /*sp*<sup>2</sup> carbon ratio. The 2D-band, the second order of the D-band, is the result of a two-phonon lattice vibrational process.

The ratio of 2D/G intensities provides insight into the properties of the graphene layers. For example, a 2D/G band ratio in this case found in the range of 1–2 indicates a bilayer graphene structure. In addition, the same D-band and 2G-band graphene peaks present in the bilayer graphene prior to doping as well as in graphene samples doped on Si/SiO2 have likewise observed in graphene transferred onto the HgCdTe substrate, thus demonstrating preservation of the structural integrity in the transferred bilayers of doped graphene.
