# Fileset

[nanomaterials-14-00383.pdf](https://mdr.nims.go.jp/filesets/2e21275b-0dbd-44c2-be30-7bf9d14146b2/download)

## Creator

E. Abidi, A. Khan, J. A. Delgado-Notario, V. Clericó, J. Calvo-Gallego, [T. Taniguchi](https://orcid.org/0000-0002-1467-3105), [K. Watanabe](https://orcid.org/0000-0003-3701-8119), T. Otsuji, J. E. Velázquez, Y. M. Meziani

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Terahertz Detection by Asymmetric Dual Grating Gate Bilayer Graphene FETs with Integrated Bowtie Antenna](https://mdr.nims.go.jp/datasets/94137e4b-3692-4eca-8641-bb012cc01a81)

## Fulltext

Terahertz Detection by Asymmetric Dual Grating Gate Bilayer Graphene FETs with Integrated Bowtie AntennaCitation: Abidi, E.; Khan, A.;Delgado-Notario, J.A.; Clericó, V.;Calvo-Gallego, J.; Taniguchi, T.;Watanabe, K.; Otsuji, T.; Velázquez,J.E.; Meziani, Y.M. Terahertz Detectionby Asymmetric Dual Grating GateBilayer Graphene FETs withIntegrated Bowtie Antenna.Nanomaterials 2024, 14, 383. https://doi.org/10.3390/nano14040383Academic Editor: Fabrizio PirriReceived: 3 January 2024Revised: 3 February 2024Accepted: 14 February 2024Published: 19 February 2024Copyright: © 2024 by the authors.Licensee MDPI, Basel, Switzerland.This article is an open access articledistributed under the terms andconditions of the Creative CommonsAttribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).nanomaterialsArticleTerahertz Detection by Asymmetric Dual Grating Gate BilayerGraphene FETs with Integrated Bowtie AntennaE. Abidi 1,*, A. Khan 1 , J. A. Delgado-Notario 1 , V. Clericó 1 , J. Calvo-Gallego 1 , T. Taniguchi 2,K. Watanabe 2 , T. Otsuji 3 , J. E. Velázquez 1 and Y. M. Meziani 1,*1 Nanotech Group, Facultad de Ciencias, Universidad de Salamanca, 37008 Salamanca, Spain;atif.khan@usal.es (A.K.); juanandn@usal.es (J.A.D.-N.); vito_clerico@usal.es (V.C.);jaime.calvo@usal.es (J.C.-G.); js@usal.es (J.E.V.)2 National Institute of Material Sciences, 1-1 Namiki, Tsukuba 305-0044, Japan;taniguchi.takashi@nims.go.jp (T.T.); watanabe.kenji.aml@nims.go.jp (K.W.)3 Research Institute of Electrical Communication, Tohoku University, Sendai 980-8577, Japan;otsuji@riec.tohoku.ac.jp* Correspondence: elhadjabidi@usal.es (E.A.); meziani@usal.es (Y.M.M.)Abstract: An asymmetric dual-grating gate bilayer graphene-based field effect transistor (ADGG-GFET) with an integrated bowtie antenna was fabricated and its response as a Terahertz (THz)detector was experimentally investigated. The device was cooled down to 4.5 K, and excited atdifferent frequencies (0.15, 0.3 and 0.6 THz) using a THz solid-state source. The integration of thebowtie antenna allowed to obtain a substantial increase in the photocurrent response (up to 8 nA) ofthe device at the three studied frequencies as compared to similar transistors lacking the integratedantenna (1 nA). The photocurrent increase was observed for all the studied values of the bias voltageapplied to both the top and back gates. Besides the action of the antenna that helps the coupling ofTHz radiation to the transistor channel, the observed enhancement by nearly one order of magnitudeof the photoresponse is also related to the modulation of the hole and electron concentration profilesinside the transistor channel by the bias voltages imposed to the top and back gates. The creation oflocal n and p regions leads to the formation of homojuctions (np, pn or pp+) along the channel thatstrongly affects the overall photoresponse of the detector. Additionally, the bias of both back and topgates could induce an opening of the gap of the bilayer graphene channel that would also contributeto the photocurrent.Keywords: 2D materials; graphene; drag effect; dual grating gate; FETs; plasmons; bowtie antenna1. IntroductionThe terahertz (THz) region, located between 0.1 and 10 THz, has remained for a longtime the only unexplored portion of the electromagnetic (EM) spectrum. The investigationof the THz region has been systematically hindered by the difficulties encountered togenerate and detect THz radiation. The lack of practical THz sources able to provideenough power coined the term “terahertz gap”, that can be extended to include the lack ofTHz detectors. In the present-day the situation of the instrumentation in the THz region isin strong contrast with the more mature available one in the adjacent regions of the EMspectrum, i.e., the infrared (IR) and the microwaves ones.Over the last 20 years, the THz science has made considerable progress fueled by theextraordinarily wide range of potential applications of the THz radiation [1,2]. Terahertzsensing can be applied in many different fields: astronomy [3], spectroscopy (the maximaof the spectral response of many molecules and solids lies in the THz range; as the THzresponse of many substances has been found to be stronger and more distinctive thanthe one found in the microwave, IR and visible spectral ranges, THz spectroscopy can beadvantageously used to define fingerprints of these substances) [4], communications (thatNanomaterials 2024, 14, 383. https://doi.org/10.3390/nano14040383 https://www.mdpi.com/journal/nanomaterialshttps://doi.org/10.3390/nano14040383https://doi.org/10.3390/nano14040383https://creativecommons.org/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.comhttps://orcid.org/0000-0002-0322-5024https://orcid.org/0000-0001-9714-8180https://orcid.org/0000-0001-6646-8309https://orcid.org/0000-0003-4987-4852https://orcid.org/0000-0003-3701-8119https://orcid.org/0000-0002-0887-0479https://orcid.org/0000-0002-6555-9871https://orcid.org/0000-0001-5193-7993https://doi.org/10.3390/nano14040383https://www.mdpi.com/journal/nanomaterialshttps://www.mdpi.com/article/10.3390/nano14040383?type=check_update&version=1Nanomaterials 2024, 14, 383 2 of 12can benefit of a bandwidth considerably higher than the current communication systemsbased on microwaves) [5–8], security (both spectroscopy and imaging to detect concealedobjects can be used) [9–11], metrology [12], etc. The resolution of THz radiation is limitedby light diffraction and its value is similar to the one of the human eye; therefore, THzrays (T-rays) can be used to generate precise images that, additionally, benefit from theunique properties of THz radiation: materials commonly used in packaging for shipping(plastics, cardboard, . . . ) are transparent to T-rays, while opaque in the visible spectrum,allowing the inspection of concealed objects [13]. Additionally, as T-rays can penetrate fewmillimeters deep the human skin, they can be used to examine subcutaneous tissue for invivo diagnosis of skin cancer [14].New emerging 2D materials have shown spectacular properties both for electronicand optoelectronic applications within the terahertz range (detection, emission, modu-lation, etc.). Graphene was the first 2D material to be fabricated and characterized bothoptically and electrically. It exhibits unique properties such as exceptionally high val-ues of carrier mobility (in excess of 100,000 cm2/V·s) of both electrons and holes, highthermal conductivity, fast relaxation of charge carriers, and high transparency (97.3%) inthe visible range of the EM spectrum [15–18]. Bilayer graphene has attracted big interestdue to the extraordinary property of this material that allows the electrical tuning of itsenergy band-gap from µeV to few hundreds of meV [19]. This property makes bilayergraphene very attractive for the development of terahertz detectors. The methods usedfor the synthesis of the 2D materials include mainly the mechanical exfoliation, chemicalvapor deposition (CVD) and the artificial stacking of new individual layers. Creationof heterostructures that combine graphene with atomically thin crystal layers of othermaterials has led to considerable improvements of the quality of the so called van derWaals (vdW) heterostructures; eventually, this has allowed the fabrication of devices withexcellent performance that can be used in future electrical and optoelectronic applica-tions. Hexagonal boron nitride (h-BN) has proven to be the most suitable material tofabricate high-quality graphene-based heterostructures because of the great similarity be-tween the crystal lattice structures of both materials. Due to its large bandgap (∼7 eV)h-BN can be used as a dielectric to build transistors with insulated gates and, addition-ally, it can be effectively used to protect graphene layers from degradation induced byenvironmental contamination.Encapsulation of graphene by hexagonal boron nitride enables the fabricationof devices with very low disorder allowing the achievement of extremely highvalues of carrier mobilities in transistor channels [20]. Accordingly, graphene has beenused to develop high-quality THz sensors [21,22], emitters [23] and modulators [24–26].The spectral response of the collective oscillations of charge carriers, described as plas-mons, in a semiconductor layer or in the channel of a field effect transistor (FET) underEM excitation lies in the THz region. THz response related to the resonant plasmonsof bilayer graphene-based field effect transistors (FETs) at low temperature was alreadyreported [27] and explained in terms of the coupling of the incoming THz beam to the FETchannel through several methods such as antennas [23,28], non-uniform metallization [29],FETs with an asymmetric design of the source and drain contacts [30], the use of a dc currentflowing between the drain and source contacts of a FET to generate asymmetries inside thechannel [31], and the use of asymmetric periodic grating gates [32]. THz detectors based onhigh-electron mobility transistors (HEMTs) with asymmetric dual-grating gates (ADGG)with exceptional high responsivity levels were reported in [33]. The THz response of thesetransistors is attributed to the spatial modulation of plasmons generated underneath thegates’ fingers. A similar phenomenon may be generated in graphene-based transistorswith asymmetric grating gates in which the incident THz beam is converted to propagatingplasmons such that the conservation of momentum is ensured [32,34,35].In this work, an asymmetric dual-grating gate graphene FET (ADGG-GFET) with anintegrated bowtie top antenna was succesfully fabricated and used as a terahertz detector.The channel of the fabricated FET is a sheet of bilayer graphene encapsulated between twoNanomaterials 2024, 14, 383 3 of 12flakes of h-BN, this double junction heterostructure lies on a SiO2/Si wafer in which thedoped substrate was used as the back gate (BG) contact of the transistor. The ADGG-GFETwas excited at low temperature (4.5K) by three tones terahertz radiation: 0.15 THz, 0.3 THzand 0.6 THz. The mapping of the photocurrent as a function of the bias voltage of the topand back gates showed an enhancement of the photocurrent, up to 8 nA, in comparisonwith the photocurrent delivered (close to 1 nA) by a similar transistor devoid of antennaunder the same measurement conditions. A central result of the present work is that theoutput current of this new THz detector based on bilayer graphene is modulated by the dcvoltages applied to the transistor gates. Accordingly, the response level of the detector canbe readily raised through the transistor biasing. This behavior is identified as due to thesetting up of two non-uniform electron and hole concentration profiles along the channel.The formation of these non-uniform profiles give rise to the formation of np, pn or pp+junctions in the channel and these regions will eventually determine the THz response ofthe sensor.2. Materials and MethodsThe bilayer graphene flake was obtained by conventional mechanical exfoliationof bulk graphite using ScotchTM tape first and then by folding the tape several timesto obtain the graphene flakes. Subsequently, the tape containing the flakes was ad-hered to a SiO2/Si substrate, previously cleaned in an oxygen plasma to remove con-taminants, and then annealed for 2 min at 100 ◦C before slowly removing the tape.This last step allowed the fabrication of flakes with bigger area that are needed for ourdesign. The substrate was a 295 nm thick SiO2 layer thermally grown on a 4” highly dopedSi wafer. An optical microscope (Leica DM8000) was used to identify the flakes using thehigh optical transmittance property (∼95% for bilayer). Two relatively thick h-BN layerswere obtained from high-quality ultrapure bulk h-BN crystals following the exfoliationprocess described above without using the oxygen plasma cleaning step as it is not needed.A DektakXT stylus profilometer was used to determine the thickness of both h-BN layers.The h-BN layers were used as the top (∼15 nm thick) and bottom (∼30 nm thick) layersof the h-BN/graphene/h-BN heterostructure. The hot pick-up technique was used forthe encapsulation of the graphene layer between the h-BN flakes [36]. First, the top h-BNwas picked up by a polymer (Polydimethylsiloxane) on a polycarbonate (PC) film andthen transferred on top of the bilayer graphene. The obtained stack PPC/h-BN/graphenewas placed in chloroform solution for 24 h to remove and cleanse it of PC. Finally, thesame process was repeated to pickup the h-BN/graphene stack and placed on the h-BNbottom layer. The vdW heterostructure (a bilayer graphene sheet sandwiched between twolayers of h-BN) has the advantage of protecting graphene from the atmosphere avoidingthe degradation of the transistor as stated above [36]. Both the top and bottom h-BNlayers were also used to electrically insulate the transistor gates, while the bottom one alsoserves to reduce the roughness of the back interface of the graphene layer and to reducethe scattering by remote impurities of the charge carriers inside the transistor channel,thus achieving very high values of the carrier mobility in the transistor channel. Ramanmeasurements were realized on the single flakes (graphene and h-BN) as well as on thefabricated vdW double heterostructure using a micro-Raman spectrometer (LabRAM HREvolution) with a 100× objective using a laser wavelength of 532 nm with a power of1 mW. Taking the G peak as a reference, the Raman spectrum was normalized and used toevaluate the intensities of the 2D and G peaks ratio I2D/IG which was found to be around1.35 (Figure 1a). The asymmetrical shape of the 2D peak and the shoulder found around2650 cm−1 indicate that the graphene sheet is a bilayer one [37]. The measured value of thefull width at half maximum, FWHM, (Figure 1b) was 56 cm−1 which is in good agreementwith previously reported results [37].Nanomaterials 2024, 14, 383 4 of 12Figure 1. (a) Raman spectra of the fabricated vdW heterostructure normalized to the intensity of theG peak. Inset is its optical image. (b) The 2D band peak and its Lorentzian fit.Figure 2 shows the design of the ADGG-GFET with the integrated bowtie antenna.The antenna and both the source (S) and the drain (D) metallic contacts were defined byEBL (Electron Beam Lithography) using a polymethyl methacrylate resin, PMMA (6%in chlorobenzene) as the resist, followed by dry etching using an ICP-RIE (InductivelyCoupled Plasma Reactive Ion Etching) in SF6 atmosphere (40 SCCM, P = 6 mTorr, P = 75 Wat 20 ◦C) and a subsequent electron beam evaporation of Cr/Au (3.5/60 nm thick). Thelarge distance between the target materials and the sample and the very low pressure ofthe ICP-RIE chamber (base pressure 10−10 mbar, process pressure 10−8 mbar) guaranteea very controllable and homogeneous evaporation process (Chromium evaporation rate:0.7 A/s, gold evaporation rate: 1.2 A/s). The pyramidal shape of the quasi one dimensionaledge contacts (an angle of 40◦ between the contact and the horizontal plane) ensuresthat only the S and D metal layers are in contact with the graphene layer which reducesthe contact resistance. Later, a second step of EBL along with another evaporation ofCr/Au (5/45 nm thick) was used to fabricate the two independent grating gate structures(TG1 and TG2 in Figure 2a,b) composed of eight fingers in total while a back-gate contact(BG) was defined on the highly doped Si substrate. The geometrical parameters of theasymmetric gratings of the top gates were: LSD = 13 µm, S1 = 0.79 µm, S2 = 0.75 µm,LG2 = d1 = 0.5 µm and LG1 = d2 = 1 µm (Figure 2a). The top view (Figure 2b) of thedevice shows the bowtie antenna connected to the drain and to the TG2 of the transistor.The arrangement of the two top gates with the antenna was designed using CSTTM, acommercial 3D solver of the Maxwell equations, to maximize the photocurrent signalwithin the sub-THz range. The antenna design was based on the bowtie topology withan angle of 90◦ and a radius of 500 µm. The antenna geometry was designed to obtain abroadband behavior in the terahertz frequency range under study and to ensure that thespectral response of the antenna will not depend on the frequency if scaled as stated byRumsey [38]. The maximum operating frequency is limited by the feed terminals, whileit’s overall size determines the lowest operation frequency of the antenna. The terminals’dimensions (approximately equal to the separation between the vertices of the antennacones) are, in the present case, close to 10 µm and its overall dimension (diameter) is about1 mm, providing a bandwidth of 1:100, i.e., 2 decades approximately, covering the full rangeof frequencies under study. The antenna diameter, 1 mm, leads to a resonant couplingof the incident radiation at the lowest operation frequency (0.15 THz) considered in thiswork. The separation between the antenna feed terminals gives the maximum frequencyof operation (around 1 THz). More details on the theoretical basis of the antenna designcan be found in [39,40]. The inset of Figure 2b highlights the active part of the transistorshowing the asymmetric grating fingers of both top gates of the ADGG-GFET and thevdW heterostructure.Nanomaterials 2024, 14, 383 5 of 12Figure 2. (a) Schematic of the cross-section of the fabricated FET showing the h-BN/graphene/h-BNdouble heterostructure on the SiO2/Si substrate, the two top gates, the source and the drain contacts.(b) Photograph of the FET with the 90◦ bowtie antenna, along with a zoomed in photo of the transistoractive area showing the asymmetric gratings fingers of both top gates.The device was placed inside a variable temperature pulse-tube cryostat with apolyethylene window highly transparent to THz waves. The channel resistance (RSD)of the FET was measured as follows: the internal oscillator of the lock-in amplifier(a Standford Research lock-in model SR860) was used to apply a quasi-dc voltage(1 V, 13 Hz) through a 100 MΩ resistor in series with the channel of the ADGG-GFETensuring the flow of a quasi-dc current with an intensity of 10 nA. The voltage drop in thechannel (between source and drain) was then measured as a function of the gates biases(top and back gates). This electrical characterization of the device versus both gates allowedthe extraction of different parameters like the CNP (Charge Neutrality Point), the contactresistances, and the carrier mobility’s.The ADGG-GFET was then excited, at 4.5 K, by continous-wave terahertz radiation atthree tones 0.15, 0.3 and 0.6 THz. We used a TeraSchottky solid-state source with multiplierstages to reach 0.15, 0.3, and 0.6 THz with an emitted power around 6, 3 and 0.8 mW,respectively. This power was measured close to the output of the source by a calibratedpyroelectric detector (THZ 20 from Pyrosensor) with an aperture of 20 mm. The terahertzbeam was collimated and focused on the ADGG-GFET using first a 90º off-axis parabolicmirror (with reflected focal distance of 152.4 mm and ∼95% reflectivity) followed by aplano-convex, aspherical TPX (Polymethylpentene) THz lens with a diameter of 50.8 mm,a focal length of 100 mm and transparency around 95% (https://www.tydexoptics.com/pdf/THz_Materials.pdf, accessed on 3 January 2024). The losses of the terahertz powerwere mainly related to the water vapor absorption in air during the beam propagation. Theterahertz beam was electrically modulated at a frequency of 298 Hz that was used as thereference signal for the lock-in amplifier. A low-noise current preamplifier (a StandfordResearch current preamplifier model SR570) with a sensitivity of 1 µA/V, was placedbetween the input signal of the lock-in and the drain used to measure the generatedphotocurrent by the ADGG-GFET. To simplify the analysis of the effect of both top andback gates on the photocurrent, TG1 was kept grounded in all measurements. Accordingly,to simplify the notation, hereafter we will refer to top gate TG2 as TG bearing in mind thatTG1 was grounded.https://www.tydexoptics.com/pdf/THz_Materials.pdfhttps://www.tydexoptics.com/pdf/THz_Materials.pdfNanomaterials 2024, 14, 383 6 of 123. Results3.1. DC Measurements ResultsFigure 3a shows the channel resistance (RSD) of the ADGG-GFET versus the backgate voltage (VBG) while TG was biased at 0 V. The CNP was observed at VBG ∼ −2 V,which indicates that the graphene channel is unintentionally n-doped. RSD exhibited twoshoulders located approximately at VBG ∼ −4 and +1 V. They are attributed to the influ-ence of the top grating gates that induced asymmetric doping profiles along the channel.A similar behavior has been already reported in [34,41]. The charge carrier mobilitywas extracted from measurements using the model presented by Gammelgaard et al.in [42]. It was found to be around 23,000 cm2/V·s and 17,000 cm2/V·s for electrons andholes, respectively.Figure 3. Source-to-drain resistance RSD at 4.5 K (a) with respect to VBG where both G1 and G2 weregrounded and (b), its mapping with respect to VTG and VBG.Figure 3b shows the channel resistance map as a function of both TG and BG biasvoltages. While the back gate electrode controls the carrier distribution across the wholechannel (including the regions beneath the top gates), the TG competes for control overthe electric potential inside the channel and, eventually, modulates the carrier densityin the channel’s portions directly below its fingers. For VTG < 0 V, the RSD versusVBG plot exhibits the same behavior observed in Figure 3a with a maximum of RSDat VBG = −2 V (CNP indicated with dashed line). When a positive bias was appliedto the top gate, a new maximum (with a value of 3 kΩ) is set up for negative voltages of theback gate bias (marked as region 1 in Figure 3b). The shift of the Dirac point entails that,when a negative VBG is applied, specific changes in the concentration levels of electronsor holes in the graphene sheet occur [35,43]. This results in the creation of n and p typeregions along the channel that are induced by the applied bias on both gates. Due tothe simultaneous presence of both n and p regions, interband tunneling may take placebetween them and, as a consequence, asymmetric bell-shaped RSD plots are observed [44].3.2. THz Measurements ResultsThe ADGG-GFET was cooled down to 4.5 K and excited with three-tones terahertzradiation. The photocurrent induced by the excitation beam was measured at the draincontact using the lock-in technique as mentioned above. Figure 4a shows photocurrentversus time when the THz beam was turned on and off periodically. The signal wasobtained under excitation at 0.3 THz (blue) and at 0.6 THz (red) for VBG = −6 V andVTG = 0 V. A photocurrent with a value close to 1.5 nA was measured with a good signal-to-noise ratio (SNR). The SNR is given by: SNR = 10 × log[PSignal/PNoise] where Psignaland PNoise are the signal and noise power obtained under illumination and dark conditions,Nanomaterials 2024, 14, 383 7 of 12respectively. We defined PSignal = RSD× < ion >2 and PNoise = RSD× < io f f >2. RSDwas assumed to be the same at both conditions (dark and illumination). From the aboveequation, the new SNR formula is given as:SNR = 20 × log[< ion >< io f f >]< ion > and < io f f > are the root mean square of the measured photocurrent when thebeam was switched on and off respectively. They were obtained as < i >= 1N√∑Ni=0 i2iwhere N is the number of samples taken from the measurements (Figure 4a) for eachcondition and frequency. The obtained SNR values were around 36 dB and 37 dB at0.3 and 0.6 THz, respectively which shows a good level of the photocurrent signal. Thegeneration of the photocurrent is the device response to THz radiation related to theplasmonic nonlinearities in the channel of the FET [32,33,45] and, ultimately, it shows theefficient coupling of the sub-THz wave to the FET channel carried by the ADGG structureand the bowtie antenna [46].Figure 4. (a) Photocurrent vs. time under on/off excitation at 0.3 THz (blue) and 0.6 THz (red) at4.5 K, while having both top gates grounded. The BG bias was fixed at the maximum intensity of thephotocurrent (VBG = −6 V for both frequencies). The rectangles were introduced to guide the eye.(b–d) Photocurrent maps versus top and back gate under excitation at (b) 0.15 THz, (c) 0.3 THz and(d) 0.6 THz.As the multi-gate topology of the transistor consists of three different gates, an analysisof the influence of each independent gate on the THz response of the transistor is necessary.As mentioned before, in this work only the influence of two of the three gates (TG andBG) has been considered. Figure 4b–d shows the photocurrent measured under excitationat three tones (0.15, 0.3 and 0.6 THz) as a function of VTG and VBG. Under excitation at0.15 THz (Figure 4b), the photocurrent shows a maximum of the device response close to±4 nA along the region 1. The sign changes at the new CNP induced by the TG. Underillumination at 0.3 THz (Figure 4c), a similar behavior was observed with a maximum of thephotocurrent of ±8 nA along the region 1. Here, the maximum of the photocurrent remainsNanomaterials 2024, 14, 383 8 of 12constant for VBG lower than −6 V (region 2). In both cases (Figure 4b,c), a relative maxi-mum of the photocurrent was also observed around the CNP imposed by the BG voltage(VBG ∼ −2 V), but with an intensity lower than the obtained along the region 1. It canbe observed that the magnitude of the photocurrent increased when biases with oppositepolarities were applied to TG and BG (along the region 1). The maximum photocurrentlevels induced by the 0.15 and 0.3 THz radiation are significantly higher than the onesreported for a similar structure [34] lacking the bowtie antenna. This justifies the use of anantenna to improve the coupling between the THz beam and the channel of the device thatgives rise to a strong response of the transistor. In contrast to the behavior observed underexcitation at 0.15 and 0.3 THz, the behavior changes radically under excitation at 0.6 THz(Figure 4d) as no significant effect of the TG biasing on the device response was found. Themaximum value of the signal was close to ∼1.5 nA at VBG ∼ −6 V.4. DiscussionThe maximum photocurrent under excitation at 0.15 & 0.3 THz was obtained aroundregion 1 when the BG voltage was kept below (above) −2 V and a positive (negative)bias voltage was applied to TG (Figure 4b,c). Depending on the biasing conditions ofboth gates (BG and TG), regions with different carrier’s type (n or p regions) are createdalong the channel. The interplay of TG & BG biases determines the electric potentialprofile in the channel and, consistently, the set-up of regions n and p with local differentcarrier concentrations and carrier types (electrons and holes) along the channel. These are:(i) channel regions covered by the TG fingers and whose electric potential is controlledby both TG and BG biases, (ii) the intergate regions (regions between adjacent top gatesfingers) in which the electrical potential in the channel portion underneath is essentiallycontrolled by VBG. It has been demonstrated [45,47] that the biasing conditions give riseto the so called plasmonic drag effect that shapes the photocurrent response. Figure 5gives a qualitative description of the internal carrier distributions within the channel thatexplains the observed improvement of the photocurrent signal. The BG voltage belowthe CNP (∼−2 V) tends to impose holes (p-type region) as majority carriers in the wholechannel, while positive voltages on TG induce the formation of n-type regions underneaththe fingers of the top gate electrode as shown in Figure 5a. As a result, pn regions arecreated along the channel. If the bias is inverted (above the CNP for the BG and negativevalues on the TG), np regions will be created. A similar behavior occurs in the case ofregion 2 (Figure 4) where, in this case, a p region is imposed in the channel by the BGand the bias on the TG induced a heavier p doping (p+) in the regions beneath the TG(Figure 5b). The plasmonic drag effect leads to the generation of the photocurrent asobserved in our experiments. In a bilayer graphene channel, an opening of the gap couldbe induced for the same biasing conditions as previously reported [48–50] that would alsolead to a behavior very similar to the one observed in the present study.The photocurrent obtained under excitation at 0.6 THz (Figure 4d) exhibits a behaviordistinctively different from the one obtained under illumination at 0.15 & 0.3 THz as nodependence of the photocurrent upon the TG bias was observed. A preliminary interpre-tation of results points towards a control of the behavior of the device photoresponse bythe plasmons generated in the intergate regions of the channel that are essentially con-trolled by the back gate electrode. This initial interpretation still needs to be confirmed byfurther measurements and theoretically analyzed to fully understand the origins of thedevice response.Nanomaterials 2024, 14, 383 9 of 12Figure 5. Schematic description of the charge carriers distribution in the channel for (a) np regionsand (b) pp+ junctions. Since no bias was applied on TG1, it is assumed that it doesn’t affect thecharge carriers.5. ConclusionsA bilayer graphene based ADGG-GFET (asymmetric dual-grating gate Graphenefield-effect transistor) with an integrated bowtie antenna was fabricated and character-ized as a terahertz detector at three frequencies (0.15, 0.3 and 0.6 THz). Both h-BN andbilayer graphene sheets were obtained by mechanical exfoliation. A van der Waals doublejunction heterostructure was obtained encapsulating the bilayer graphene sheet betweentwo h-BN layers. The heterostructure was first characterized by Raman spectroscopy (thevalue of the I2D/IG intensity ratio obtained was close to 1.35) and subsequently used tofabricate the ADGG-GFET. The integration of the bowtie antenna as well as a proper com-bination of the bias voltages applied to the back and top gates improve the photocurrentresponse by a factor of 8 in comparison with a similar transistor lacking the bowtie antenna.This enhancement by nearly one order of magnitude of the photoresponse is also related tothe modulation of the hole and electron concentration profiles inside the transistor channelby the bias voltages imposed to the top and back gates. The creation of local n and p regionsleads to the formation of pn homojuctions along the channel that strongly affects the overallphotoresponse of the detector. Additionally, the bias of both back and top gates couldinduce an opening of the gap of the bilayer graphene channel that would also contributeto the photocurrent. The present work opens the way for the development of novel THzdevices based on the new 2D materials.Author Contributions: Conceptualization, Y.M.M., J.E.V. and J.A.D.-N.; methodology, Y.M.M. andJ.A.D.-N.; investigation, J.C.-G.; data curation, E.A., J.C.-G. and A.K.; writing—original draft prepa-ration, Y.M.M.; writing—review and editing, all authors; supervision, Y.M.M. and J.E.V.; projectadministration, Y.M.M., J.E.V. and J.C.-G.; funding acquisition, Y.M.M., J.E.V. and J.C.-G.; Materials,T.T. and K.W. All authors have read and agreed to the published version of the manuscript.Funding: This research was funded by the Spanish Agencia Estatal de Investigación under GrantsNumbers PID2021-126483OB-I00 and PID2022-136869NB-C33, the Consejería de Educación, Juntade Castilla y León under Grant Number SA121P20, the Universidad de Salamanca Research Pro-gram under Grant Number PIC2-2021-02 and the Conselleria de Innovación, Universidades, Cien-cia y Sociedad Digital of the Generalitat Valenciana under the Grant Number MFA/2022/056.This work was performed in the Cooperative Research Project of the Research Institute of Elec-Nanomaterials 2024, 14, 383 10 of 12trical Communication, Tohoku University. This work was funded by the JSPS KAKENHI under grantnumber 21H04546, Japan.Data Availability Statement: The research data of this works are available upon request to authors.Acknowledgments: The authors acknowledge the support from the National Institute of Mate-rial Science for providing high quality h-BN crystals. EL HADJ ABIDI acknowledges the grant002/PG/Espagne/2020-2021 of Ministère de L’Enseignement Supérieur et de la Recherche Sci-entifique, République Algérienne Démocratique et Populaire. Authors also acknowledge USAL-NANOLAB for Clean room and Raman facilities. J.A.D.-N. thanks the support from the Universidadde Salamanca for the María Zambrano postdoctoral grant funded by the Next Generation EU Fundingfor the Requalification of the Spanish University System 2021–23, Spanish Ministry of Universities.Conflicts of Interest: The authors declare no conflict of interest.AbbreviationsThe following abbreviations are used in this manuscript:ADGG Asymmetric Dual Grating GatesBG Back GateCNP Charge Neutrality PointD drainEBL Electron Beam LithographyEM ElectromagneticFET Field Effect TransistorFWHM full width at half maximumS SourceSNR Signal to Noise RatioTG Top GateTHz TerahertzvdW van der WaalsReferences1. Leitenstorfer, A.; Moskalenko, A.S.; Kampfrath, T.; Kono, J.; Castro-Camus, E.; Peng, K.; Qureshi, N.; Turchinovich, D.; Tanaka, K.;Markelz, A.G.; et al. The 2023 terahertz science and technology roadmap. J. Phys. D Appl. Phys. 2023, 56, 223001. [CrossRef]2. Markelz, A.G.; Mittleman, D.M. Perspective on Terahertz Applications in Bioscience and Biotechnology. ACS Photonics 2022,9, 1117–1126. [CrossRef]3. Rainsford, T.J.; Mickan, S.P.; Abbott, D. T-ray sensing applications: review of global developments. Smart Struct. Devices Syst.2005, 5649, 826–838.4. Fu, X.; Liu, Y.; Chen, Q.; Fu, Y.; Cui, T.J. Applications of Terahertz Spectroscopy in the Detection and Recognition of Substances.Front. Phys. 2022, 10, 869537. [CrossRef]5. Chen, Z.; Ma, X.; Zhang, B.; Zhang, Y.; Niu, Z.; Kuang, N.; Chen, W.; Li, L.; Li, S. A survey on terahertz communications. ChinaCommun. 2019, 16, 1–35. [CrossRef]6. Song, H.J.; Nagatsuma, T. Present and Future of Terahertz Communications. IEEE Trans. Terahertz Sci. Technol. 2011, 1, 256–263.[CrossRef]7. Kleine-Ostmann, T.; Nagatsuma, T. A Review on Terahertz Communications Research. J. Infrared Millim. Terahertz Waves 2011,32, 143–171. [CrossRef]8. Federici, J.; Moeller, L.; Su, K. 6 - Terahertz wireless communications. In Handbook of Terahertz Technology for Imaging, Sensingand Communications; Saeedkia, D., Ed.; Woodhead Publishing Series in Electronic and Optical Materials; Woodhead Publishing:Cambridge, UK , 2013; pp. 156–214. [CrossRef]9. Tzydynzhapov, G.; Gusikhin, P.; Muravev, V.; Dremin, A.; Nefyodov, Y.; Kukushkin, I. New Real-Time Sub-Terahertz SecurityBody Scanner. J. Infrared Millim. Terahertz Waves 2020, 41, 632–641. [CrossRef]10. Mittleman, D.M. Twenty years of terahertz imaging (Invited). Opt. Express 2018, 26, 9417–9431. [CrossRef]11. Kemp, M.C.; Taday, P.; Cole, B.E.; Cluff, J.; Fitzgerald, A.J.; Tribe, W.R. Security applications of terahertz technology. InProceedings of the Terahertz for Military and Security Applications. SPIE, Orlando, FL, USA, 21–25 April 2003; Volume 5070,pp. 44–52.12. Popovic, Z.; Grossman, E.N. THz Metrology and Instrumentation. IEEE Trans. Terahertz Sci. Technol. 2011, 1, 133–144. [CrossRef]13. Li, X.; Li, J.; Li, Y.; Ozcan, A.; Jarrahi, M. High-throughput terahertz imaging: Progress and challenges. Light Sci. Appl. 2023,12, 233. [CrossRef]http://doi.org/10.1088/1361-6463/acbe4chttp://dx.doi.org/10.1021/acsphotonics.2c00228http://dx.doi.org/10.3389/fphy.2022.869537http://dx.doi.org/10.23919/JCC.2019.09.001http://dx.doi.org/10.1109/TTHZ.2011.2159552http://dx.doi.org/10.1007/s10762-010-9758-1http://dx.doi.org/10.1533/9780857096494.1.156http://dx.doi.org/10.1007/s10762-020-00683-5http://dx.doi.org/10.1364/OE.26.009417http://dx.doi.org/10.1109/TTHZ.2011.2159553http://dx.doi.org/10.1038/s41377-023-01278-0Nanomaterials 2024, 14, 383 11 of 1214. Woodward, R.M.; Cole, B.E.; Wallace, V.P.; Pye, R.J.; Arnone, D.D.; Linfield, E.H.; Pepper, M. Terahertz pulse imaging in reflectiongeometry of human skin cancer and skin tissue. Phys. Med. Biol. 2002, 47, 3853. [CrossRef] [PubMed]15. Castro Neto, A.H.; Guinea, F.; Peres, N.M.R.; Novoselov, K.S.; Geim, A.K. The electronic properties of graphene. Rev. Mod. Phys.2009, 81, 109–162. [CrossRef]16. Falkovsky, L.A. Optical properties of graphene. J. Phys. Conf. Ser. 2008, 129, 012004. [CrossRef]17. Wallace, P.R. The Band Theory of Graphite. Phys. Rev. 1947, 71, 622–634. [CrossRef]18. Novoselov, K.S.; Geim, A.K.; Morozov, S.V.; Jiang, D.; Zhang, Y.; Dubonos, S.V.; Grigorieva, I.V.; Firsov, A.A. Electric Field Effectin Atomically Thin Carbon Films. Science 2004, 306, 666–669. [CrossRef] [PubMed]19. Kanayama, K.; Nagashio, K. Gap state analysis in electric-field-induced band gap for bilayer graphene. Sci. Rep. 2015, 5, 15789.[CrossRef]20. Yankowitz, M.; Ma, Q.; Jarillo-Herrero, P.; LeRoy, B.J. van der Waals heterostructures combining graphene and hexagonal boronnitride. Nat. Rev. Phys. 2019, 1, 112–125. [CrossRef]21. Yang, X.; Vorobiev, A.; Generalov, A.; Andersson, M.A.; Stake, J. A flexible graphene terahertz detector. Appl. Phys. Lett. 2017,111, 021102. [CrossRef]22. Bianco, F.; Perenzoni, D.; Convertino, D.; De Bonis, S.L.; Spirito, D.; Perenzoni, M.; Coletti, C.; Vitiello, M.S.; Tredicucci, A.Terahertz detection by epitaxial-graphene field-effect-transistors on silicon carbide. Appl. Phys. Lett. 2015, 107, 131104. [CrossRef]23. Jiang, Z.; Wang, Y.; Chen, L.; Yu, Y.; Yuan, S.; Deng, W.; Wang, R.; Wang, Z.; Yan, Q.; Wu, X.; et al. Antenna-integratedsilicon–plasmonic graphene sub-terahertz emitter. APL Photonics 2021, 6, 066102. [CrossRef]24. Chen, X.; Tian, Z.; Li, Q.; Li, S.; Zhang, X.; Ouyang, C.; Gu, J.; Han, J.; Zhang, W. Recent progress in graphene terahertz modulators.Chin. Phys. B 2020, 29, 077803. [CrossRef]25. Kakenov, N.; Ergoktas, M.S.; Balci, O.; Kocabas, C. Graphene based terahertz phase modulators. 2D Mater. 2018, 5, 035018.[CrossRef]26. Jessop, D.S.; Kindness, S.J.; Xiao, L.; Braeuninger-Weimer, P.; Lin, H.; Ren, Y.; Ren, C.X.; Hofmann, S.; Zeitler, J.A.; Beere, H.E.;et al. Graphene based plasmonic terahertz amplitude modulator operating above 100 MHz. Appl. Phys. Lett. 2016, 108, 171101.[CrossRef]27. Bandurin, D.A.; Svintsov, D.; Gayduchenko, I.; Xu, S.G.; Principi, A.; Moskotin, M.; Tretyakov, I.; Yagodkin, D.; Zhukov, S.;Taniguchi, T.; et al. Resonant terahertz detection using graphene plasmons. Nat. Commun. 2018, 9, 5392. [CrossRef] [PubMed]28. Zak, A.; Andersson, M.A.; Bauer, M.; Matukas, J.; Lisauskas, A.; Roskos, H.G.; Stake, J. Antenna-Integrated 0.6 THz FET DirectDetectors Based on CVD Graphene. Nano Lett. 2014, 14, 5834–5838. [CrossRef] [PubMed]29. Fedorov, G.; Gayduchenko, I.; Titova, N.; Gazaliev, A.; Moskotin, M.; Kaurova, N.; Voronov, B.; Goltsman, G. Carbon NanotubeBased Schottky Diodes as Uncooled Terahertz Radiation Detectors. Phys. Status Solidi B 2018, 255, 1700227. [CrossRef]30. Ryu, M.; Kim, S.H.; Kim, K. Enhanced Photoresponse of Plasmonic Terahertz Wave Detector Based on Silicon Field EffectTransistors with Asymmetric Source and Drain Structures. J. Semicond. Technol. Sci. 2013, 13, 576–580. [CrossRef]31. Lisauskas, A.; Boppel, S.; Matukas, J.; Palenskis, V.; Minkevičius, L.; Valušis, G.; Haring-Bolívar, P.; Roskos, H.G. Terahertzresponsivity and low-frequency noise in biased silicon field-effect transistors. Appl. Phys. Lett. 2013, 102, 153505. [CrossRef]32. Popov, V.V.; Fateev, D.V.; Otsuji, T.; Meziani, Y.M.; Coquillat, D.; Knap, W. Plasmonic terahertz detection by a double-grating-gatefield-effect transistor structure with an asymmetric unit cell. Appl. Phys. Lett. 2011, 99, 243504. [CrossRef]33. Kurita, Y.; Ducournau, G.; Coquillat, D.; Satou, A.; Kobayashi, K.; Boubanga Tombet, S.; Meziani, Y.M.; Popov, V.V.; Knap, W.;Suemitsu, T.; et al. Ultrahigh sensitive sub-terahertz detection by InP-based asymmetric dual-grating-gate high-electron-mobilitytransistors and their broadband characteristics. Appl. Phys. Lett. 2014, 104, 251114. [CrossRef]34. Notario, J.A.D.; Clericò, V.; Diez, E.; Velázquez-Pérez, J.E.; Taniguchi, T.; Watanabe, K.; Otsuji, T.; Meziani, Y.M. Asymmetricdual-grating gates graphene FET for detection of terahertz radiations. APL Photonics 2020, 5, 66102. [CrossRef]35. Delgado-Notario, J.A.; Knap, W.; Clericò, V.; Salvador-Sánchez, J.; Calvo-Gallego, J.; Taniguchi, T.; Watanabe, K.; Otsuji, T.; Popov,V.V.; Fateev, D.V.; et al. Enhanced terahertz detection of multigate graphene nanostructures. Nanophotonics 2022, 11, 519–529.[CrossRef]36. Pizzocchero, F.; Gammelgaard, L.; Jessen, B.S.; Caridad, J.e.M.; Wang, L.; Hone, J.; Booth, T.J. The hot pick-up technique for batchassembly of van der Waals heterostructures. Nat. Commun. 2016, 7, 11894. [CrossRef]37. Hao, Y.; Wang, Y.; Wang, L.; Ni, Z.; Wang, Z.; Wang, R.; Koo, C.K.; Shen, Z.; Thong, J.T.L. Probing Layer Number and StackingOrder of Few-Layer Graphene by Raman Spectroscopy. Small 2010, 6, 195–200. [CrossRef]38. Rumsey, V. Frequency independent antennas. In Proceedings of the 1958 IRE International Convention Record, New York, NY,USA, 21–25 March 1966; Volume 5, pp. 114–118.39. Balanis, C.A. Antenna Theory: Analysis and Design; John Wiley & Sons: Hoboken, NJ, USA, 2016.40. Brown, G.; Woodward, O., Jr. Experimentally Determined Radiation Characteristics of Conical and Triangular Antennas. RCARev. 1952, 13, 425–452.41. Velasco, J., Jr.; Lee, Y.; Jing, L.; Liu, G.; Bao, W.; Lau, C.N. Quantum transport in double-gated graphene devices. Solid StateCommun. 2012, 152, 1301–1305. [CrossRef]42. Gammelgaard, L.; Caridad, J.M.; Cagliani, A.; Mackenzie, D.M.A.; Petersen, D.H.; Booth, T.J.; Bøggild, P. Graphene transportproperties upon exposure to PMMA processing and heat treatments. 2D Mater. 2014, 1, 035005-12. [CrossRef]http://dx.doi.org/10.1088/0031-9155/47/21/325http://www.ncbi.nlm.nih.gov/pubmed/12452577http://dx.doi.org/10.1103/RevModPhys.81.109http://dx.doi.org/10.1088/1742-6596/129/1/012004http://dx.doi.org/10.1103/PhysRev.71.622http://dx.doi.org/10.1126/science.1102896http://www.ncbi.nlm.nih.gov/pubmed/15499015http://dx.doi.org/10.1038/srep15789http://dx.doi.org/10.1038/s42254-018-0016-0http://dx.doi.org/10.1063/1.4993434http://dx.doi.org/10.1063/1.4932091http://dx.doi.org/10.1063/5.0047070http://dx.doi.org/10.1088/1674-1056/ab9433http://dx.doi.org/10.1088/2053-1583/aabfaahttp://dx.doi.org/10.1063/1.4947596http://dx.doi.org/10.1038/s41467-018-07848-whttp://www.ncbi.nlm.nih.gov/pubmed/30568184http://dx.doi.org/10.1021/nl5027309http://www.ncbi.nlm.nih.gov/pubmed/25203787http://dx.doi.org/10.1002/pssb.201700227http://dx.doi.org/10.5573/JSTS.2013.13.6.576http://dx.doi.org/10.1063/1.4802208http://dx.doi.org/10.1063/1.3670321http://dx.doi.org/10.1063/1.4885499http://dx.doi.org/10.1063/5.0007249http://dx.doi.org/10.1515/nanoph-2021-0573http://dx.doi.org/10.1038/ncomms11894http://dx.doi.org/10.1002/smll.200901173http://dx.doi.org/10.1016/j.ssc.2012.04.024http://dx.doi.org/10.1088/2053-1583/1/3/035005Nanomaterials 2024, 14, 383 12 of 1243. Kim, S.; Nah, J.; Jo, I.; Shahrjerdi, D.; Colombo, L.; Yao, Z.; Tutuc, E.; Banerjee, S.K. Realization of a high mobility dual-gatedgraphene field-effect transistor with Al2O3 dielectric. Appl. Phys. Lett. 2009, 94, 062107-4. [CrossRef]44. Satou, A.; Tamamushi, G.; Sugawara, K.; Mitsushio, J.; Ryzhii, V.; Otsuji, T. A Fitting Model for Asymmetric I–V Characteristicsof Graphene FETs for Extraction of Intrinsic Mobilities. IEEE Trans. Electron Devices 2016, 63, 3300–3306. [CrossRef]45. Fateev, D.; Mashinsky, K.; Polischuk, O.; Popov, V. Excitation of Propagating Plasmons in a Periodic Graphene Structure byIncident Terahertz Waves. Phys. Rev. Appl. 2019, 11, 064002. [CrossRef]46. Delgado-Notario, J.A.; Velazquez-Perez, J.E.; Meziani, Y.M.; Fobelets, K. Sub-THz Imaging Using Non-Resonant HEMT Detectors.Sensors 2018, 18, 543. [CrossRef]47. Olbrich, P.; Kamann, J.; König, M.; Munzert, J.; Tutsch, L.; Eroms, J.; Weiss, D.; Liu, M.H.; Golub, L.E.; Ivchenko, E.L.; et al.Terahertz ratchet effects in graphene with a lateral superlattice. Phys. Rev. B 2016, 93, 075422. [CrossRef]48. Zhang, Y.; Tang, T.T.; Girit, C.; Hao, Z.; Martin, M.C.; Zettl, A.; Crommie, M.F.; Shen, Y.R.; Wang, F. Direct observation of a widelytunable bandgap in bilayer graphene. Nature 2009, 459, 820–823. [CrossRef] [PubMed]49. Titova, E.; Mylnikov, D.; Kashchenko, M.; Safonov, I.; Zhukov, S.; Dzhikirba, K.; Novoselov, K.S.; Bandurin, D.A.; Alymov, G.;Svintsov, D. Ultralow-noise Terahertz Detection by p–n Junctions in Gapped Bilayer Graphene. ACS Nano 2023, 17, 8223–8232.[CrossRef] [PubMed]50. Mylnikov, D.A.; Titova, E.I.; Kashchenko, M.A.; Safonov, I.V.; Zhukov, S.S.; Semkin, V.A.; Novoselov, K.S.; Bandurin, D.A.;Svintsov, D.A. Terahertz Photoconductivity in Bilayer Graphene Transistors: Evidence for Tunneling at Gate-Induced Junctions.Nano Lett. 2023, 23, 220–226. [CrossRef] [PubMed]Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individualauthor(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury topeople or property resulting from any ideas, methods, instructions or products referred to in the content.http://dx.doi.org/10.1063/1.3077021http://dx.doi.org/10.1109/TED.2016.2578325http://dx.doi.org/10.1103/PhysRevApplied.11.064002http://dx.doi.org/10.3390/s18020543http://dx.doi.org/10.1103/PhysRevB.93.075422http://dx.doi.org/10.1038/nature08105http://www.ncbi.nlm.nih.gov/pubmed/19516337http://dx.doi.org/10.1021/acsnano.2c12285http://www.ncbi.nlm.nih.gov/pubmed/37094175http://dx.doi.org/10.1021/acs.nanolett.2c04119http://www.ncbi.nlm.nih.gov/pubmed/36546884 Introduction Materials and Methods Results DC Measurements Results THz Measurements Results Discussion Conclusions References