**4.2 Boron doping of bilayer graphene**

Dopants used for chemically doping graphene include, but are not limited to, S, N, B, P, I, Se, O, and I [20, 21]. Among these dopant elements, the two most common and notable are nitrogen (N) and boron (B), for inducing *n*-type conductivity *p*+ -type conductivity, respectively, in graphene [22].

As the MWIR photodetector device under consideration requires *p*-doped graphene for optimal performance, the boron doping method is here in view. Boron is one of the most natural choices for doping among the atomic elements, having valence atoms that differ in number by only a single atom compared to the number of its carbon ones [29]. Atoms of boron also have a similar atomic size (0.088 nm atomic radius) to those of carbon (atomic radius of 0.077 nm), a factor that further facilitates *p*-type conduction in graphene [30].

When a dopant atom such as in this instance boron is bonded within a carbon framework, a defect is introduced into the neighboring site. Since boron only contains three valence electrons compared to four in carbon atoms, this can cause uneven charge distribution resulting in charge transfer between nearby carbon atoms, further expounding their electrochemical behavior [31]. The incorporation of a relatively small number of boron atoms thereby effectively lowers the Fermi level and formation of an acceptor level in the doped graphene. The introduction of boron likewise contributes to improved stabilization of the extremities of the graphene material and similarly aids in mitigating the termination of its layers, thereby promoting layer-by-layer growth of larger portions or sections of graphene [32].

#### **4.3 Graphene spin-on doping process**

We have developed and implemented a distinct spin-on dopant (SOD) process to produce highly boron-doped bilayer graphene. The SOD process involves spincoating a dopant solution onto a source substrate and annealing the latter in conjunction with a target substrate in a tube furnace [31]. The bilayer graphene sheets doped using this process were deposited on SiO2/Si substrates (300 nm SiO2, *p*-type doped) by CVD acquired from Graphenea, Inc.

In the high-temperature environment and inert gas (e.g., argon) atmosphere, diffusion of the dopant (boron) from the source substrate into the target sample (bilayer graphene) occurs when the B atoms replace the C atoms to form *p*-doped graphene. The main advantages of this technique are its low cost and simplistic setup, combined with the capability to provide uniform and consistent doping profiles [33]. **Figure 9** depicts a schematic representation for substitutional doping of boron in the honeycomb lattice of graphene by the SOD process.

For the SOD procedure schematically illustrated in **Figure 9**, we start with the CVD-deposited graphene on Si/SiO2 substrates. The spin-on diffusant used is Filmtronics B-155 (4% boron conc.). This boron source is spin-coated onto a Si wafer at 2300 rpm for 30 s using a CEE vacuum coater tool [34].

This boron-solution-coated source wafer is then placed in a custom-designed silica boat approximately 10 mm apart from and facing a target graphene sample. These are each inserted into a tube furnace that is pumped down to 10 Torr vacuum pressure and annealed in the presence of flowing Ar gas.

*21st Century Nanostructured Materials – Physics, Chemistry, Classification, and Emerging…*

#### **Figure 9.**

*Schematic of processing steps for the fabrication of boron-doped GFs through a spin-on dopant (SOD) method [32].*


**Table 1.**

*Annealing and spin coating parameters for the SOD process.*

Experimental parameters for the boron doping of graphene are given in **Table 1**. The goal of this process is to achieve required high doping levels while maintaining graphene surface features and quality for graphene-enhanced HgCdTe MWIR photodetectors.

#### **4.4 Graphene boron-doping concentration analysis**

To determine the structural properties, chemical bonding states, and doping concentration changes in the doped bilayer graphene, Raman spectroscopy and time-of-flight (ToF) secondary-ion mass spectroscopy (SIMS) techniques were performed as along with X-ray photoelectron spectroscopy (XPS).

Doping concentration vs. depth profile results for different boron-doped graphene bilayers using SIMS are presented in **Figure 10**. This graphene sample on Si/ SiO2 was doped with the SOD process using 15 min. Annealing duration. The SIMS analysis indicates boron doping levels of ~1.8 × 1020 cm−3 of boron in the graphene bilayers further confirmed by XPS.

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

**Figure 10.** *SIMS atomic dopant concentrations vs. depth profiles for two different p-doped graphene samples on Si/SiO2 substrates.*
