Graphene-Based Terahertz Detectors

Various detection mechanisms are involved in terahertz 2D-material detectors, including the bolometric effect, photothermoelectric (PTE) effect, plasma wave rectification in FET, and so on. Table 6.2 summarizes the performance of representative detectors [69]. The detectivity is estimated from Eq. (2.6), where NEP values are obtained from the references.

THz response can be attributed to the PTE effect as a result of asymmetric thermal distribution. Cai et al. reported a sensitive graphene PTE THz detector with asymmetric electrodes [58]. On account of dissimilar metal contacts, the Fermi energy and Seebeck coefficients are asymmetric across the device channel, which lead to a photovoltage in response to incident radiation. In addition, gate voltage may tune the Fermi energy of graphene, thus influencing the photoresponse. The detector response time is mainly determined by the thermal time constant of graphene, which is about 100 ps.

In addition, Tong et al. have demonstrated a half-edge-contacted graphene PTE detector with asymmetric electrodes (Fig. 6.27) [59]. Its sensitivity was enhanced by a double-patch antenna and an on-chip silicon lens, reaching a maximum responsivity of 4.9 V/W and a typical D* of ~2.2x 10⁶ Jones.

Different modifications of detector design have been proposed to improve sensitivity. One of them is a resonant structure consisting of two graphene sheets, separated by a dielectric layer, to tune the absorbed wavelength, as presented in Table 6.1. The responsivity of that detector exhibits peaks when the frequency of inducing THz radiation reaches the resonant plasma frequencies being tuned by the bias [70]. In order to observe the resonant response of the detector, the frequency of electron collisions as a result of impurities and acoustic phonons must be sufficiently low. Double- graphene-layered (DGL) heterostructures, deploying inter-graphene layer intra-band transitions, show that such structures can be exploited for efficient and tunable THz/IR lasers and photodetectors [71].

The DGL heterostructure consists of a narrow potential barrier like BN and WS₂, sandwiched between two layers of graphene [Fig. 6.28(a)]. The voltage (V,_op._bottom) applied between the source and the drain, combined with the gate voltage, V_g, modulates the Fermi levels and forms a p-i-n structure [Fig. 6.28(b)]. The system is in resonance when the energies of Dirac points of the two layers of graphene are exactly the same. The resonant tunneling between the two layers of graphene, with conservation of energy and momentum, results in the negative differential conductance. A photocurrent is generated due to band-offset (A) between two Dirac points (hco), and the TM-polarized photons with energy hw ~ A can mediate the

FIGURE 6.27 Schematic side-view of the THz PTE detector design.

FIGURE 6.28 Double-graphene-layered (DGL) heterostructure: (a) schematic of the fabricated DGL device, (b) band diagrams of the DGL detector structures with photo-absorption-assisted inter-GL transitions. Wavy arrows indicate the inter-GL radiative V-V and C-C transitions. The inter-GL transitions work for the TM-mode THz photon radiations in the structure (after Ref. [71]).

electron photon-assisted resonant tunneling in two layers of graphene. Yadav et al. reported the first experimental observation of the double-graphene-layered heterostructure THz detector [71].

FET THz detectors conventionally operate at room temperature in the resistive self-missing regime, since the length of the FET channel exceeds the propagation length of plasma waves at room temperature. The plasma waves, excited by THz radiation, could be overdamped, and thus the detectors did not operate in the resonant regime, as was demonstrated by Vicarelli et al. [61]. The first THz FET detector, based on monolayer and bilayer graphene, was exfoliated on Si/Si0₂. In order to satisfy the boundary condition, the ends of the logarithmic antenna were the gate and source, whereas the drain was a metal line leading to a small pad. Figure 6.29(a) shows plasma wave FET with a top-gate antenna-coupled configuration. These detectors demonstrated a noise equivalent power (NEP) ~ 10 ⁹ W/ Hz^1/2 in the range 0.29-0.38 THz. It was reported that, during measurements of a target at room temperature, the bilayer graphene-based FET

FIGURE 6.29 Plasma wave FET THz detector: (a) THz detection configuration in a FET embedding the optical image of the central area of a bilayer graphene- based FET and (b) 0.3 GHz transmission mode image of a leaf (adapted after Ref. [61]).

at gate voltage, V_{=3 V, was mounted on a x-y translation stage, having a spatial resolution of 0.5-pm [61]. The THz image consists of 200x550 scanned points collected by the object raster scanning in the beam focus, with integration time of 20 ms/point [Fig. 6.29(b)]. Further studies carried out by Spirito et al. [63] revealed the influence of the gate configuration in a bilayer graphene FET detector. The device exhibited better performance by employing wide-gate geometries or buried gate configurations [Fig. 6.30(b)], where a responsivity of 1.2 V/W was achieved.

More recently, improvements in THz detection have been described by Liu et al. [68,72]. The detector was built from split-finger gated

FIGURE 6.30 Schematic bilayer graphene FET with (a) top and (b) buried gates (after Ref. [63]) graphene-based FET, coupled with a logarithmic antenna. Its NEP value, determined by the Johnson-Nyquist noise, was less than 0.1 nW/Hz^1/2 at both 0.04 and 0.14 THz. The achieved performances are competitive with those of commercially available detectors, in terms of both sensitivity and NEP.

In comparison with plasmonic TFlz detectors, fabricated by exfoliated graphene [63] or chemical vapor deposition (CVD) deposited on Si/Si0₂ substrates [62], the epitaxial graphene, grown on SiC, is much more promising [72]. The photoresponsivity and NEP of the bilayer graphene (deposited on SiC substrate) FET channel were estimated to be at the levels of 0.25 V/W and 80 nW/Hz^1/2, respectively.