**6. Graphene Infrared (IR) detectors**

Utilizing a three-terminal top gate design of CVD graphene grown on a SiC substrate, one group

was able to achieve a 350 GHz cutoff frequency, utilizing a channel length of 40 nm as shown in

Figure 24. Image showing the threshold frequency versus gate length for the device

**Figure 24.** Image showing the threshold frequency versus gate length for the device architectures shown on the left,

This group showed that the threshold frequency has a 1/L dependence, where L is the channel length of the graphene FET. This has been modeled and pushed to the limit with an under‐ standing that graphene might be able to break the 1THz limit that InGaAs and SiGe HEMTs can't break. [62] One group theoretically tuned all of the parasitic capacitances that would limit the graphene channel mobility; this includes removing Schottky interactions at the source and drain contacts, removal of any trapped states in the oxide, ignoring any electron/hole pooling effects, and having the gate voltage perfectly coupled to channel potential, allowing for a GFET that operates at 1.5 THz. [62] This GFET is optimized to have zero gain due to the current saturation in the 50 nm channel. [62] By allowing for current saturation in the GFET, a voltage gain can be engineered in the graphene channel; however, this would deteriorate the operating

architectures shown on the left, the epitaxial graphene is on the SiC substrate, and the frequency

shows a 1/L dependence [61].

This group showed that the threshold frequency has a 1/L dependence, where L is the channel

length of the graphene FET. This has been modeled and pushed to the limit with an

the epitaxial graphene is on the SiC substrate, and the frequency shows a 1/L dependence [61].

understanding that graphene might be able to break the 1THz limit that InGaAs and SiGe

HEMTs can't break.<sup>62</sup> One group theoretically tuned all of the parasitic capacitances that would

limit the graphene channel mobility; this includes removing Schottky interactions at the source

and drain contacts, removal of any trapped states in the oxide, ignoring any electron/hole pooling

effects, and having the gate voltage perfectly coupled to channel potential, allowing for a GFET

that operates at 1.5 THz.62 This GFET is optimized to have zero gain due to the current saturation

in the 50 nm channel.62 By allowing for current saturation in the GFET, a voltage gain can be

engineered in the graphene channel; however, this would deteriorate the operating frequency of

**Figure 25.** An image showing the threshold frequency for each possible gain in a GFET for systems with different

One of the interesting applications for graphene is its use in EO devices and lasers. Graphene can absorb wavelengths from the visible to the mid-IR with wavelength modulation enabled

amounts of tuning of parasitic resistance; the blue line has no parasitic resistance [62].

**5. Graphene use in Electro-Optical (EO) devices**

the GFET as shown in Figure 25.<sup>62</sup>

frequency of the GFET as shown in Figure 25. [62]

Figure 24.<sup>61</sup>

78 Graphene - New Trends and Developments

IR detectors can be separated into two separate categories: thermal-based IR detection and photon-based detection. [68] In thermal-based detectors, the incident IR radiation is absorbed, raising the temperature of the material. [68] The raised temperature affects some temperaturedependent property of the material; for pyrometers this is a change in electrical polarization, while for bolometers, this is a change in materials resistance. [68] Another more recent study utilized the photothermoelectric effect in graphene to create a net electric field due to electron diffusion into dissimilar metal contacts. [45] Photon-based detectors utilize band gap-based detection with the arriving photon being absorbed and utilized to promote electron hole pairs to create a photocurrent. [68] The photon-based detectors can be tuned to certain wavelengths by creating a quantum well structure. [68] Photon-based IR absorbers are characterized by having fast absorption response, but usually require cooling due to thermal effects, while thermal-based IR detectors have high responsivity over a large wavelength and can be utilized at room temperature but normally have slow absorption response. [68] This is where utilizing a graphene-based sensing element is attractive due to the high mobility with little temperature sensitivity making it ideal for IR detectors. [2]

Several groups have attempted to integrate graphene into IR detectors. The groups have tried both photon- and the thermal-based absorption methods. [45, 69–74] For photon-based absorption methods, the main focus has been the opening of a band gap through geometric modification. [45, 69] One group utilized bilayer graphene to open a small band gap that is sensitive to thermalization requiring cooling to 5 K for operation. [69]

**Figure 27.** The utilization of graphene nanoribbons to open a small band gap that is enhanced through the use of pand n-type graphene contacts [45]

Another group utilized an array of aligned graphene nanoribbons as shown in Figure 27 to open up a small band gap that has significant difficulties in fabrication and noise properties from the nanoribbon edges. [45] Groups that have tried thermal-based IR detectors seem to have created more novelty, with one group utilizing multiple vertically aligned graphene flakes, while another group utilized a resonant structure of two graphene sheets separated by a dielectric to tune the photon wavelength of absorption as shown in Figure 28. Finally, another group utilized the photothermoelectric effect as shown in Figure 29 to induce an electric current in graphene due to electric gating or dissimilar metal contacts. [45, 70, 71] The bolometer utilizing vertically aligned graphene sheets used distance-based tunneling between sheets for the bolometric effect, which is sensitive to contamination between sheets and alignment of the graphene flakes making reproduction difficult. [71]

**Figure 28.** Phonon resonance-based IR detector [70].

thermal-based IR detectors have high responsivity over a large wavelength and can be utilized at room temperature but normally have slow absorption response. [68] This is where utilizing a graphene-based sensing element is attractive due to the high mobility with little temperature

Several groups have attempted to integrate graphene into IR detectors. The groups have tried both photon- and the thermal-based absorption methods. [45, 69–74] For photon-based absorption methods, the main focus has been the opening of a band gap through geometric modification. [45, 69] One group utilized bilayer graphene to open a small band gap that is

**Figure 27.** The utilization of graphene nanoribbons to open a small band gap that is enhanced through the use of p-

Another group utilized an array of aligned graphene nanoribbons as shown in Figure 27 to open up a small band gap that has significant difficulties in fabrication and noise properties from the nanoribbon edges. [45] Groups that have tried thermal-based IR detectors seem to have created more novelty, with one group utilizing multiple vertically aligned graphene flakes, while another group utilized a resonant structure of two graphene sheets separated by a dielectric to tune the photon wavelength of absorption as shown in Figure 28. Finally, another group utilized the photothermoelectric effect as shown in Figure 29 to induce an electric current in graphene due to electric gating or dissimilar metal contacts. [45, 70, 71] The bolometer

sensitive to thermalization requiring cooling to 5 K for operation. [69]

sensitivity making it ideal for IR detectors. [2]

80 Graphene - New Trends and Developments

and n-type graphene contacts [45]

The resonance-based IR detector shown in Figure 28 utilizes the phonon resonance of two separate graphene sheets separated by a dielectric allowing for the tuning of wavelength detection based upon separation distance, but the fabrication is difficult requiring pristine graphene and no trapped states in the oxide that would both modify the resonant frequency and could possibly contaminate the detector out of detection range. [70]

**Figure 29.** Image of a detector based upon the photothermoelectric effect [45].

The photothermoelectric effect detector shown in Figure 29 is relatively straight forward with contamination only affecting the speed of the detector and the noise only susceptible to trap states of the insulating oxide that the graphene is transferred onto. [45]
