INTRODUCTION

Section:

ChooseTop of pageABSTRACTINTRODUCTION <<EXPERIMENTAL METHODSRESULTSDISCUSSIONCONCLUSIONSUPPLEMENTARY MATERIALREFERENCESTerahertz (THz) electromagnetic waves have great potential to be utilized for applications in large-capacity, ultrahigh-speed wireless communication technologies in 6G toward 7G.1,21. M. W. Akhtar et al., “The shift to 6G communications: Vision and requirements,” Hum. Cent. Comput. Inf. Sci. 10, 53 (2020). https://doi.org/10.1186/s13673-020-00258-22. M. Z. Chowdhury, Md. Shahjalal, S. Ahmed, and Y. M. Jang, “6G wireless communication systems: Applications, requirements, technologies, challenges, and research directions,” IEEE Open J. Commun. Soc. 1, 957–975 (2019). https://doi.org/10.1109/OJCOMS.2020.3010270 To realize such high-speed communication systems, highly sensitive, fast-response room-temperature detectors operating in the THz and sub-THz ranges are the key elements.3,43. S. S. Dhillon et al., “The 2017 terahertz science and technology roadmap,” J. Phys. D: Appl. Phys. 50, 043001 (2017). https://doi.org/10.1088/1361-6463/50/4/0430014. M. Shur, G. Aizin, T. Otsuji, and V. Ryzhii, “Plasmonic field-effect transistors (TeraFETs) for 6G communications,” Sensors 21, 7907 (2021). https://doi.org/10.3390/s21237907 However, there are many performance limitations for the currently available THz detectors.5,65. A. Rogalski and S. Sizov, “Terahertz detectors and focal plane arrays,” Opto-Electron. Rev. 19, 79–137 (2011). https://doi.org/10.2478/s11772-011-0033-36. T. Otsuji, “Trends in the research of modern terahertz detectors: Plasmon detectors,” IEEE Trans. Terahertz Sci. Technol. 5, 1110–1121 (2015), https://ieeexplore.ieee.org/document/7335433. Two-dimensional (2D) plasmons have attracted increasing attention as a promising mechanism for highly sensitive, fast-response THz detection.6–166. T. Otsuji, “Trends in the research of modern terahertz detectors: Plasmon detectors,” IEEE Trans. Terahertz Sci. Technol. 5, 1110–1121 (2015), https://ieeexplore.ieee.org/document/7335433.7. M. Dyakonov and M. Shur, “Detection, mixing, and frequency multiplication of terahertz radiation by two-dimensional electronic fluid,” IEEE Trans. Electron Devices 43, 380–387 (1996). https://doi.org/10.1109/16.4856508. W. Knap, F. Teppe, Y. Meziani, N. Dyakonova, J. Lusakowski, F. Boeuf, T. Skotnicki, D. Maude, S. Rumyantsev, and M. S. Shur, “Plasma wave detection of sub-terahertz and terahertz radiation by silicon field-effect transistors,” Appl. Phys. Lett. 85, 675–677 (2004). https://doi.org/10.1063/1.17750349. W. Knap, M. Dyakonov, D. Coquillat, F. Teppe, N. Dyakonova, J. Łusakowski, K. Karpierz, M. Sakowicz, G. Valusis, D. Seliuta, I. Kasalynas, A. El Fatimy, Y. M. Meziani, and T. Otsuji, “Field effect transistors for terahertz detection: Physics and first imaging applications,” J. Infrared, Millimeter, Terahertz Waves 30, 1319–1337 (2009). https://doi.org/10.1007/s10762-009-9564-910. S. Boppel et al., “CMOS integrated antenna-coupled field-effect transistors for the detection of radiation from 0.2 to 4.3 THz,” IEEE Trans. Microwave Theory Tech. 60, 3834–3843 (2012). https://doi.org/10.1109/tmtt.2012.222173211. Y. Kurita, G. Ducournau, D. Coquillat, A. Satou, K. Kobayashi, S. Boubanga Tombet, Y. M. Meziani, V. V. Popov, W. Knap, T. Suemitsu, and T. Otsuji, “Ultrahigh sensitive sub-terahertz detection by InP-Based asymmetric dual-grating-gate HEMTs and their broadband characteristics,” Appl. Phys. Lett. 104, 251114 (2014). https://doi.org/10.1063/1.488549912. S. Boubanga-Tombet, F. Teppe, J. Torres, A. El Moutaouakil, D. Coquillat, N. Dyakonova, C. Consejo, P. Arcade, P. Nouvel, H. Marinchio, T. Laurent, C. Palermo, A. Penarier, T. Otsuji, L. Varani, and W. Knap, “Room temperature coherent and voltage tunable terahertz emission from nanometer-sized field effect transistors,” Appl. Phys. Lett. 97, 262108 (2010). https://doi.org/10.1063/1.352946413. L. Vicarelli, M. S. Vitiello, D. Coquillat, A. Lombardo, A. C. Ferrari, W. Knap, M. Polini, V. Pellegrini, and A. Tredicucci, “Graphene field-effect transistors as room-temperature terahertz detectors,” Nat. Mater. 11, 865–871 (2012). https://doi.org/10.1038/nmat341714. D. A. Bandurin, D. Svintsov, I. Gayduchenko, S. G. Xu, A. Principi, M. Moskotin, I. Tretyakov, D. Yagodkin, S. Zhukov, T. Taniguchi, K. Watanabe, I. V. Grigorieva, M. Polini, G. N. Goltsman, A. K. Geim, and G. Fedorov, “Resonant terahertz detection using graphene plasmons,” Nat. Commun. 9, 5392 (2018). https://doi.org/10.1038/s41467-018-07848-w15. J. A. Delgado-Notario, V. Clericò, E. Diez, J. E. Velázquez-Pérez, T. Taniguchi, K. Watanabe, T. Otsuji, and Y. M. Meziani, “Asymmetric dual-grating gates graphene FET for detection of terahertz radiations,” APL Photonics 5, 066102 (2020). https://doi.org/10.1063/5.000724916. J. A. Delgado-Notario, W. Knap, V. Clericò, J. Salvador-Sánchez, J. Calvo-Gallego, T. Taniguchi, K. Watanabe, T. Otsuji, V. V. Popov, D. V. Fateev, E. Diez, J. E. Velázquez-Pérez, and Y. M. Meziani, “Enhanced terahertz detection of multigate graphene nanostructures,” Nanophotonics 11, 519–529 (2022). https://doi.org/10.1515/nanoph-2021-0573 In particular, graphene Dirac plasmons (GDPs)17–1917. A. N. Grigorenko, M. Polini, and K. S. Novoselov, “Graphene plasmonics,” Nat. Photonics 6, 749–758 (2012). https://doi.org/10.1038/nphoton.2012.26218. T. Low and P. Avouris, “Graphene plasmonics for terahertz to mid-infrared applications,” ACS Nano 8, 1086–1101 (2014). https://doi.org/10.1021/nn406627u19. Y. Li, K. Tantiwanichapan, A. K. Swan, and R. Paiella, “Graphene plasmonic devices for terahertz optoelectronics,” Nanophotonics 9, 1901–1920 (2020). https://doi.org/10.1515/nanoph-2020-0211 are believed to be one of the most promising physical principles for breaking through the technological limit for room-temperature, fast, sensitive THz detection capable of 100 Gbit/s class high-data-rate coding of THz- and sub-THz radiation incidence in next-generation 6G- and 7G-class wireless communications systems.2020. I. F. Akyildiz, A. Kak, and S. Nie, “6G and beyond: The future of wireless communications systems,” IEEE Access 8, 133995 (2020). https://doi.org/10.1109/access.2020.3010896 Graphene has also been used for fast photothermoelectric (PTE) THz detection21–2521. X. Xu, N. M. Gabor, J. S. Alden, A. M. van der Zande, and P. L. McEuen, “Photo-thermoelectric effect at a graphene interface junction,” Nano Lett. 10, 562–566 (2010). https://doi.org/10.1021/nl903451y22. T. Mueller, F. Xia, and P. Avouris, “Graphene photodetectors for high-speed optical communications,” Nat. Photonics 4, 297–301 (2010). https://doi.org/10.1038/nphoton.2010.4023. J. Yan et al., “Dual-gated bilayer graphene hot-electron bolometer,” Nat. Nanotechnol. 7, 472–478 (2012). https://doi.org/10.1038/nnano.2012.8824. T. J. Echtermeyer, P. S. Nene, M. Trushin, R. V. Gorbachev, A. L. Eiden, S. Milana, Z. Sun, J. Schliemann, E. Lidorikis, K. S. Novoselov, and A. C. Ferrari, “Photothermoelectric and photoelectric contributions to light detection in metal-graphene-metal photodetectors,” Nano Lett. 14, 3733–3742 (2014). https://doi.org/10.1021/nl500476225. X. Cai, A. B. Sushkov, R. J. Suess, M. M. Jadidi, G. S. Jenkins, L. O. Nyakiti, R. L. Myers-Ward, S. Li, J. Yan, D. K. Gaskill, T. E. Murphy, H. D. Drew, and M. S. Fuhrer, “Sensitive room-temperature terahertz detection via the photothermoelectric effect in graphene,” Nat. Nanotechnol. 9, 814–819 (2014). https://doi.org/10.1038/nnano.2014.182 due to its superior carrier transport and phononic properties.2626. A. A. Balandin, “Phononics of graphene and related materials,” ACS Nano 14, 5170–5178 (2020). https://doi.org/10.1021/acsnano.0c02718 In this paper, we design and fabricate an epitaxial-graphene-channel field-effect transistor (EG-FET) featured by the authors’ original asymmetric dual-grating-gate (ADGG) structure2727. V. V. Popov, D. V. Fateev, T. Otsuji, Y. M. Meziani, D. Coquillat, and W. Knap, “Plasmonic terahertz detection by a double-grating-gate field-effect transistor structure with an asymmetric unit cell,” Appl. Phys. Lett. 99, 243504 (2011). https://doi.org/10.1063/1.3670321 working as a current-driven terahertz detector with applied nonzero drain–source bias voltages and experimentally demonstrate a high responsivity of 0.3 mA/W (equivalently 12 mV/W under the 50 Ω-loaded condition and 84 mV/W under the high (∼1 MΩ) loaded impedance condition) to 0.95 THz radiation incidence at room temperature. The ADGG and drain–source bias dependencies of the measured photoresponse show a clear transition between plasmonic detection and PTE detection while preserving the fast response speed. The experiments also demonstrate fast temporal photoresponses for plasmonic and PTE detection on the order of 10 ps.

EXPERIMENTAL METHODS

Section:

ChooseTop of pageABSTRACTINTRODUCTIONEXPERIMENTAL METHODS <<RESULTSDISCUSSIONCONCLUSIONSUPPLEMENTARY MATERIALREFERENCESA bird’s-eye view of the ADGG-EG-FET structure is schematically shown in Fig. 1. The channel consists of a few layers of epitaxial graphene that were thermally decomposed from a C-face SiC substrate.28–3228. T. Someya, H. Fukidome, H. Watanabe, T. Yamamoto, M. Okada, H. Suzuki, Y. Ogawa, T. Iimori, N. Ishii, T. Kanai, K. Tashima, B. Feng, S. Yamamoto, J. Itatani, F. Komori, K. Okazaki, S. Shin, and I. Matsuda, “Suppression of supercollision carrier cooling in high mobility graphene on SiC(000–1),” Phys. Rev. B 95, 165303 (2017). https://doi.org/10.1103/physrevb.95.16530329. A. J. van Bommel, J. E. Crombeen, and A. van Tooren, “LEED and Auger electron observations of the SiC(0001) surface,” Surf. Sci. 48, 463–472 (1975). https://doi.org/10.1016/0039-6028(75)90419-730. H. Fukidome, Y. Kawai, F. Fromm, M. Kotsugi, H. Handa, T. Ide, T. Ohkouchi, H. Miyashita, Y. Enta, T. Kinoshita, T. Seyller, and M. Suemitsu, “Precise control of epitaxy of graphene by microfabricating SiC substrate,” Appl. Phys. Lett. 101, 041605 (2012). https://doi.org/10.1063/1.474027131. N. Endoh, S. Akiyama, K. Tashima, K. Suwa, T. Kamogawa, R. Kohama, K. Funakubo, S. Konishi, H. Mogi, M. Kawahara, M. Kawai, Y. Kubota, T. Ohkochi, M. Kotsugi, K. Horiba, H. Kumigashira, M. Suemitsu, I. Watanabe, and H. Fukidome, “High-quality few-layer graphene on single-crystalline SiC thin film grown on affordable wafer for device applications,” Nanomater 11, 392 (2021). https://doi.org/10.3390/nano1102039232. H.-C. Kang, H. Karasawa, Y. Miyamoto, H. Handa, H. Fukidome, T. Suemitsu, M. Suemitsu, and T. Otsuji, “Epitaxial graphene top-gate FETs on silicon substrates,” Solid-State Electron. 54, 1071–1075 (2010). https://doi.org/10.1016/j.sse.2010.05.030 After device mesa isolation, which defines an active detector area of 20 × 20 µm2, source and drain ohmic metallic contacts [Ti (10 nm)/Pd (20 nm)/Au (70 nm)] were formed using standard contact lithography, electron-beam evaporation, and lift-off processes. A gate stack was formed with a 40-nm-thick SiN dielectric layer deposited on the graphene-channel layer using plasma-enhanced chemical vapor deposition (PE-CVD).32,3332. H.-C. Kang, H. Karasawa, Y. Miyamoto, H. Handa, H. Fukidome, T. Suemitsu, M. Suemitsu, and T. Otsuji, “Epitaxial graphene top-gate FETs on silicon substrates,” Solid-State Electron. 54, 1071–1075 (2010). https://doi.org/10.1016/j.sse.2010.05.03033. D. Yadav, G. Tamamushi, T. Watanabe, J. Mitsushio, Y. Tobah, K. Sugawara, A. A. Dubinov, A. Satou, M. Ryzhii, V. Ryzhii, and T. Otsuji, “Terahertz light-emitting graphene-channel transistor toward single-mode lasing,” Nanophotonics 7, 741–752 (2018). https://doi.org/10.1515/nanoph-2017-0106 A gate metal electrode [Ti (10 nm)/Pt (20 nm)/Au (70 nm)] was formed in the ADGG structure by using electron-beam lithography, an electron-beam evaporator, and a standard lift-off process.32,3332. H.-C. Kang, H. Karasawa, Y. Miyamoto, H. Handa, H. Fukidome, T. Suemitsu, M. Suemitsu, and T. Otsuji, “Epitaxial graphene top-gate FETs on silicon substrates,” Solid-State Electron. 54, 1071–1075 (2010). https://doi.org/10.1016/j.sse.2010.05.03033. D. Yadav, G. Tamamushi, T. Watanabe, J. Mitsushio, Y. Tobah, K. Sugawara, A. A. Dubinov, A. Satou, M. Ryzhii, V. Ryzhii, and T. Otsuji, “Terahertz light-emitting graphene-channel transistor toward single-mode lasing,” Nanophotonics 7, 741–752 (2018). https://doi.org/10.1515/nanoph-2017-0106 A scanning electron microscope (SEM) image of the fabricated ADGG-EG-FET is depicted in Fig. 2(a). The ADGG electrodes, consisting of two interdigitated grating-shaped metals with grating finger widths of 550 nm (Lg1) and 850 nm (Lg2) were laid out with asymmetric distances of 450 nm (d1) and 750 nm (d2) to the left-side and right-side adjacent fingers, respectively. The source and drain electrodes were formed on top of the graphene channel with planar3434. J. A. Robinson, M. LaBella, M. Zhu, M. Hollander, R. Kasarda, Z. Hughes, K. Trumbull, R. Cavalero, and D. Snyder, “Contacting graphene,” Appl. Phys. Lett. 98, 053103 (2011). https://doi.org/10.1063/1.3549183 and edge3535. L. Wang, I. Meric, P. Y. Huang, Q. Gao, Y. Gao, H. Tran, T. Taniguchi, K. Watanabe, L. M. Campos, D. A. Muller, J. Guo, P. Kim, J. Hone, K. L. Shepard, and C. R. Dean, “One-dimensional electrical contact to a two-dimensional material,” Science 342, 614–617 (2013). https://doi.org/10.1126/science.1244358 ohmic contacts. The crystallinity of the graphene layer was confirmed by Raman spectroscopy, as shown in Fig. 2(b). The G and G′ bands at 1590 and 2700 cm−1 were clearly identified in the Raman spectra; in contrast, the defect-oriented D band at 1350 cm−1 was as weak as the background noise floor, indicating the favorable high crystallinity of the graphene layer.3636. A. C. Ferrari, J. C. Meyer, V. Scardaci, C. Casiraghi, M. Lazzeri, F. Mauri, S. Piscanec, D. Jiang, K. S. Novoselov, S. Roth, and A. K. Geim, “Raman spectrum of graphene and graphene layers,” Phys. Rev. Lett. 97, 187401 (2006). https://doi.org/10.1103/physrevlett.97.187401 The ratio between the intensities of the G and G′ peaks was ∼3.5, indicating that the samples are few-layer graphene within the three layers.3737. R. Saito, A. Jorio, A. G. Souza Filho, G. Dresselhaus, M. S. Dresselhaus, and M. A. Pimenta, “Probing phonon dispersion relations of graphite by double resonance Raman scattering,” Phys. Rev. Lett. 88, 027401 (2002). https://doi.org/10.1103/PhysRevLett.88.027401 The surface morphology of the graphene layer was characterized by atomic force microscopy (AFM), as shown in Fig. 2(c). Crystal domains with a size of several micrometers were identified in the AFM image.3838. M. L. Bolen, S. E. Harrison, L. B. Biedermann, and M. A. Capano, “Graphene formation mechanisms on 4H-SiC (0001),” Phys. Rev. B 80, 115433 (2009). https://doi.org/10.1103/physrevb.80.115433When THz radiation is incident on the surface of the device, the ADGG electrodes work as a broadband antenna that can efficiently convert the incident THz photons into GDPs.27,3927. V. V. Popov, D. V. Fateev, T. Otsuji, Y. M. Meziani, D. Coquillat, and W. Knap, “Plasmonic terahertz detection by a double-grating-gate field-effect transistor structure with an asymmetric unit cell,” Appl. Phys. Lett. 99, 243504 (2011). https://doi.org/10.1063/1.367032139. T. Otsuji, M. Hanabe, T. Nishimura, and E. Sano, “A grating-bicoupled plasma-wave photomixer with resonant-cavity enhanced structure,” Opt. Express 14, 4815–4825 (2006). https://doi.org/10.1364/oe.14.004815 When one grating gate of the ADGG electrodes, Gate 1 (G1), is electrically biased at a high voltage, whereas another gate, Gate 2 (G2), is biased at the Dirac voltage to deplete the carriers, the channel underneath the high-biased G1 gate finger becomes a plasmonic cavity, working as a plasmonic detector producing a rectified direct current (DC) photocurrent due to the hydrodynamic nonlinearity of the GDPs.15,16,2715. J. A. Delgado-Notario, V. Clericò, E. Diez, J. E. Velázquez-Pérez, T. Taniguchi, K. Watanabe, T. Otsuji, and Y. M. Meziani, “Asymmetric dual-grating gates graphene FET for detection of terahertz radiations,” APL Photonics 5, 066102 (2020). https://doi.org/10.1063/5.000724916. J. A. Delgado-Notario, W. Knap, V. Clericò, J. Salvador-Sánchez, J. Calvo-Gallego, T. Taniguchi, K. Watanabe, T. Otsuji, V. V. Popov, D. V. Fateev, E. Diez, J. E. Velázquez-Pérez, and Y. M. Meziani, “Enhanced terahertz detection of multigate graphene nanostructures,” Nanophotonics 11, 519–529 (2022). https://doi.org/10.1515/nanoph-2021-057327. V. V. Popov, D. V. Fateev, T. Otsuji, Y. M. Meziani, D. Coquillat, and W. Knap, “Plasmonic terahertz detection by a double-grating-gate field-effect transistor structure with an asymmetric unit cell,” Appl. Phys. Lett. 99, 243504 (2011). https://doi.org/10.1063/1.3670321 The depleted channel underneath the low-biased G2 gate finger becomes a highly resistive load, working as a transducer to produce a photovoltage from the photocurrent.11,1611. Y. Kurita, G. Ducournau, D. Coquillat, A. Satou, K. Kobayashi, S. Boubanga Tombet, Y. M. Meziani, V. V. Popov, W. Knap, T. Suemitsu, and T. Otsuji, “Ultrahigh sensitive sub-terahertz detection by InP-Based asymmetric dual-grating-gate HEMTs and their broadband characteristics,” Appl. Phys. Lett. 104, 251114 (2014). https://doi.org/10.1063/1.488549916. J. A. Delgado-Notario, W. Knap, V. Clericò, J. Salvador-Sánchez, J. Calvo-Gallego, T. Taniguchi, K. Watanabe, T. Otsuji, V. V. Popov, D. V. Fateev, E. Diez, J. E. Velázquez-Pérez, and Y. M. Meziani, “Enhanced terahertz detection of multigate graphene nanostructures,” Nanophotonics 11, 519–529 (2022). https://doi.org/10.1515/nanoph-2021-0573 Due to the periodic arrangement of such a unit pair of the plasmonic cavity and the resistive load in the “ADGG” structure, the photovoltage generated in each unit pair accumulates in a cascading manner, resulting in highly sensitive THz detection. It is noted that the difference between d1 (=450 nm) and d2 (=750 nm) or the asymmetricity between them (d1/d2 ≠ 1) is the key to unbalancing the boundary conditions at the left side and right side of the plasmonic cavity so that the plasmonic displacement current flowing from source/drain to drain/source becomes unbalanced, resulting in rectified DC photovoltaic output at the drain terminal. The length of the high-biased gate (Lg1 = 550 nm in the above context) determines the plasmonic resonant mode frequency, whereas the length of the low-biased gate (Lg2 = 850 nm in the above context) determines the load resistance value.First, we measured the electrical DC and voltage characteristics of the as-fabricated EG-FET using a semiconductor parametric analyzer. The DC drain–source current vs the gate bias Vg1 scanning from/to 0 V to/from −20 V was measured under the condition of Vg2 = 0 V and Vds = 0.1 V. As shown in Fig. 3, clear ambipolar characteristics near the Dirac voltage (VDirac, defined as the charge neutrality voltage point) were observed under a negative gate voltage. DC measurements in the p-type high-current operation region with negative gate voltages larger than VDirac could not be conducted due to the gate breakdown limitation. Such a wide shift in VDirac is considered to be due to the relatively high unintentional n-type doping of the graphene channel that occurred during the SiN insulator deposition process using PE-CVD.3232. H.-C. Kang, H. Karasawa, Y. Miyamoto, H. Handa, H. Fukidome, T. Suemitsu, M. Suemitsu, and T. Otsuji, “Epitaxial graphene top-gate FETs on silicon substrates,” Solid-State Electron. 54, 1071–1075 (2010). https://doi.org/10.1016/j.sse.2010.05.030 Furthermore, the measured hysteresis was weak; the difference in current between the forward and backward bias applications was merely several μAs. It was confirmed that the heterointerfaces of the graphene-SiN-gate stack suppressed undesired defect- and/or strain-induced trap centers, resulting in almost no hysteresis.Next, we measured the temporal response of the photovoltage output from the drain electrode in response to pulsed quasi continuous-wave (CW) radiation incidence centered at 0.95 THz at room temperature. The THz detection measurement was conducted and implemented with an injection-seeded THz parametric generator (is-TPG)4040. S. Hayashi, K. Nawata, T. Taira, J. Shikata, K. Kawase, and H. Minamide, “Ultrabright continuously tunable terahertz-wave generation at room temperature,” Sci. Rep. 4, 5045 (2014). https://doi.org/10.1038/srep05045 as the THz radiation incident source (Fig. 4). The is-TPG generated pulsed-CW THz radiation with an envelope pulse width of 155 ps and a repetition rate of 200 Hz.4141. Y. Takida, K. Nawata, T. Notake, T. Otsuji, and H. Minamide, “Optical up-conversion-based cross-correlation for characterization of sub-nanosecond terahertz-wave pulses,” Opt. Express 30, 11217–11224 (2022). https://doi.org/10.1364/oe.452310 The envelope pulse width of 155 ps was identified by using an optically upconverted cross correlation method with a subnanosecond Nd:YAG infrared pump pulse whose wavelength was centered at 1064 nm, as described in Ref. 4141. Y. Takida, K. Nawata, T. Notake, T. Otsuji, and H. Minamide, “Optical up-conversion-based cross-correlation for characterization of sub-nanosecond terahertz-wave pulses,” Opt. Express 30, 11217–11224 (2022). https://doi.org/10.1364/oe.452310. The THz waves that were output from the is-TPG traveling in free space were focused by a Tsurupica lens with a focal distance of 100 mm and directed via an indium-tin oxide (ITO) mirror to the sample surface placed at the focal point. The radiation incidence energy was 137 nJ/envelope (peak power of ∼911 W). A set of radio frequency (RF) probes were contacted to the ADGG-EG-FET electrode pads to apply the bias voltages (the drain-to-source bias Vds and two ADGG biases Vg1 and Vg2). To observe the temporal photoresponse waveform without distortions caused by the multireflection between the device output and the far end of the measurement equipment, we used a 50 Ω-impedance measurement setup consisting of a 50 Ω-input-impedance 22 dB-gain wideband preamplifier, a 50 Ω-input-impedance 33 GHz-bandwidth digitizing oscilloscope, and a 1m-long 50Ω-coaxial-cabled transmission line to connect the device output terminal and the oscilloscope. Compared to the high-impedance measurement setup that is frequently utilized for static DC-voltage photoresponse measurements, the measured photovoltage under the 50Ω-loaded condition becomes small by a factor of the voltage divider ratio between the internal channel resistance Rch (∼300 Ω) and the load resistance RL (=50 Ω) given by RL/(Rch + RL) ∼ 0.14 in this experiment.

RESULTS

Section:

ChooseTop of pageABSTRACTINTRODUCTIONEXPERIMENTAL METHODSRESULTS <<DISCUSSIONCONCLUSIONSUPPLEMENTARY MATERIALREFERENCESFirst, we conducted experiments at a sufficiently doped voltage of +15 V for Vg1 and at the Dirac voltage (=VDirac, the charge neutrality point) for Vg2 to ensure that the channel regions underneath G1 (G2) were sufficiently doped (depleted); thus, in theory, only the plasmonic rectification should be responsible for THz detection. In addition, Vds was biased at 1 V to drive the current in the channel. As shown under the condition Vg2 = −15.5 V in Fig. 5, we confirmed a clear photovoltaic response at room temperature, indicating that the ADGG-EG-FET works properly as a plasmonic THz detector. The observed tail-free pulse width full-width at half-maximum (FWHM) value of 199 ps was slightly wider than the envelope width of the is-TPG radiation incidence, which was characterized to be 155 ps. The discrepancy between them might be due to several systematic factors caused by the different routes of pulse-width characterization. To the best of the authors’ knowledge, this is the first experimental demonstration of a fast temporal photoresponse of a graphene-channel FET THz detector at room temperature.Second, we measured the photoresponse by increasing Vg2 from −15.5 V (=VDirac) to +15 V (=)Vg1) under fixed Vg1 (=+15.5 V) and Vds (=+1.0 V) conditions. As shown in Fig. 5, the measured temporal photovoltage output increased with increasing Vg2 and corresponding electron densities underneath G2 while preserving the high-speed response. The peak values of the photovoltage vs Vg2 are plotted in Fig. 6(a). With increasing Vg2, the photoresponse increased and started to saturate at ∼−5 V. We claim that the observed increase in the photoresponsivity is due to a new type of unipolar PTE effect assisted by electrostatic carrier drift/diffusion, which will be discussed in the section titled Discussion.The gate bias conditions for G1 and G2, which correspond to the plasmonic rectification and the PTE effect, are shown in Figs. 6(b) and 6(c), respectively. To confirm the behavior of photoelectrons due to the PTE process, we fixed Vg1 = Vg2 = 0 V, which ensured that the graphene-channel area was entirely sufficiently doped, and then increased Vds from 0 to +1.5 V. The temporal photoresponse waveforms under typical nonzero Vds bias voltage conditions were measured, as plotted in Fig. 7(a). The temporal photoresponse preserved its waveform with an FWHM value of 199 ps, independent of the applied nonzero Vds bias voltages.The peak values of the temporal photoresponse under the 50-Ω-loaded condition shown in Fig. 7(a) are plotted in Fig. 7(b) as a function of Vds. The output photovoltage increased linearly with increasing Vds. More importantly, when Vds = 0 V, preventing the PTE rectification operation, no photoresponse was observed. This is clear evidence that the photoresponse observed in Figs. 7(a) and 7(b) under the fully doped conditions resulted from the PTE rectification effect.

DISCUSSION

Section:

ChooseTop of pageABSTRACTINTRODUCTIONEXPERIMENTAL METHODSRESULTSDISCUSSION <<CONCLUSIONSUPPLEMENTARY MATERIALREFERENCESIn addition to the mechanism of THz detection by the GDPs, the experimental results suggest that the ADGG-EG-FET is also able to work as a current-driven PTE THz detector21–2521. X. Xu, N. M. Gabor, J. S. Alden, A. M. van der Zande, and P. L. McEuen, “Photo-thermoelectric effect at a graphene interface junction,” Nano Lett. 10, 562–566 (2010). https://doi.org/10.1021/nl903451y22. T. Mueller, F. Xia, and P. Avouris, “Graphene photodetectors for high-speed optical communications,” Nat. Photonics 4, 297–301 (2010). https://doi.org/10.1038/nphoton.2010.4023. J. Yan et al., “Dual-gated bilayer graphene hot-electron bolometer,” Nat. Nanotechnol. 7, 472–478 (2012). https://doi.org/10.1038/nnano.2012.8824. T. J. Echtermeyer, P. S. Nene, M. Trushin, R. V. Gorbachev, A. L. Eiden, S. Milana, Z. Sun, J. Schliemann, E. Lidorikis, K. S. Novoselov, and A. C. Ferrari, “Photothermoelectric and photoelectric contributions to light detection in metal-graphene-metal photodetectors,” Nano Lett. 14, 3733–3742 (2014). https://doi.org/10.1021/nl500476225. X. Cai, A. B. Sushkov, R. J. Suess, M. M. Jadidi, G. S. Jenkins, L. O. Nyakiti, R. L. Myers-Ward, S. Li, J. Yan, D. K. Gaskill, T. E. Murphy, H. D. Drew, and M. S. Fuhrer, “Sensitive room-temperature terahertz detection via the photothermoelectric effect in graphene,” Nat. Nanotechnol. 9, 814–819 (2014). https://doi.org/10.1038/nnano.2014.182 due to the photo-Seebeck effect. When Vg2 is biased near the Dirac point, the carrier density in the channel is not sufficient to drive the PTE effect; plasmon rectification dominates the detection, which is shown in the blue part of Fig. 6(a). With increasing Vg2, the density of electrons under Vg2 increases toward the highly doped level under Vg1. This makes generating a photovoltaic response through plasmonic rectification difficult. Therefore, one needs to consider different mechanisms to interpret the observed result of increasing photovoltage with increasing Vg2, as shown in the orange part of Fig. 6(a).

When the drain terminal is DC-biased, the electric potential forms a slope along the channel, resulting in asymmetric thermodiffusion of photogenerated hot electrons under THz radiation incidence due to the photo-Seebeck effect along the channel with the help of field-induced electrostatic drift/diffusion; the hot electrons photoexcited by THz electromagnetic radiation diffuse and become biased in the direction of the potential slope. Such a PTE detection mechanism can well support the photoresponse vs Vg2 tendency in our observation when all gate electrodes are zero biased. In this regard, this mechanism might be said to be electrostatic-drift/diffusion-assisted PTE detection. When the drain electrode is zero biased, in contrast, thermodiffusion occurs isotropically, and there is no specific fraction of the diffusion direction going either to the source or to the drain, resulting in no photoresponse. It is worth mentioning that one merit of PTE detection is the possibility of conducting zero gate-bias operations. If the THz radiation spot size is sufficiently small with respect to the channel area to realize local THz irradiation, photothermodiffusion is dominant in either direction toward the source or drain electrode, depending on which electrode is closer to the spot. In this case, a nonzero photoresponse can be obtained. Our experiments fall into the former case; the spot covers the entire channel region, so no photoresponse can be obtained under zero drain-bias conditions.

We also consider whether the plasmonic rectification mechanism could be responsible for the zero gate-bias detection in the EG-FET with the Dirac voltage deeply shifted to the negative region. Since the spatial carrier density distribution along the channel under zero gate-bias conditions is rather monotonic and less periodically modulated, the photoresponse is saturated at Vg2 ∼ 0 V and beyond [see Fig. 6(a)]. Thus, plasmonic detection cannot be dominated. As a consequence, as long as the drain is nonzero biased, the photovoltaic response is associated with the PTE rectification mechanism. This is not similar to the standard PTE rectification in bipolar p–n junction diode structures21–2521. X. Xu, N. M. Gabor, J. S. Alden, A. M. van der Zande, and P. L. McEuen, “Photo-thermoelectric effect at a graphene interface junction,” Nano Lett. 10, 562–566 (2010). https://doi.org/10.1021/nl903451y22. T. Mueller, F. Xia, and P. Avouris, “Graphene photodetectors for high-speed optical communications,” Nat. Photonics 4, 297–301 (2010). https://doi.org/10.1038/nphoton.2010.4023. J. Yan et al., “Dual-gated bilayer graphene hot-electron bolometer,” Nat. Nanotechnol. 7, 472–478 (2012). https://doi.org/10.1038/nnano.2012.8824. T. J. Echtermeyer, P. S. Nene, M. Trushin, R. V. Gorbachev, A. L. Eiden, S. Milana, Z. Sun, J. Schliemann, E. Lidorikis, K. S. Novoselov, and A. C. Ferrari, “Photothermoelectric and photoelectric contributions to light detection in metal-graphene-metal photodetectors,” Nano Lett. 14, 3733–3742 (2014). https://doi.org/10.1021/nl500476225. X. Cai, A. B. Sushkov, R. J. Suess, M. M. Jadidi, G. S. Jenkins, L. O. Nyakiti, R. L. Myers-Ward, S. Li, J. Yan, D. K. Gaskill, T. E. Murphy, H. D. Drew, and M. S. Fuhrer, “Sensitive room-temperature terahertz detection via the photothermoelectric effect in graphene,” Nat. Nanotechnol. 9, 814–819 (2014). https://doi.org/10.1038/nnano.2014.182 with metals of the anode and cathode electrodes with different work functions in which both electrons and holes contribute to the rectification function. Our current-driven ADGG-EG-FET contributes only unipolar carriers of hot electrons or hot holes excited by THz radiation incidence. Therefore, the observed THz radiation rectification mechanism is regarded as a new type of unipolar PTE detection assisted by field-induced electrostatic drift/diffusion. The results shown in Fig. 6 suggest that plasmonic rectification and/or PTE rectification take place in the ADGG-EG-FET under THz radiation incidence depending on the ADGG bias conditions, and that these two effects coexist under a wide range of Vg1 and Vg2 conditions. The theoretical estimation of the contributions of these two mechanisms to photocurrent generation revealed that they are on the same order of magnitude (see Sec. II of the supplementary material) and supported the experimental results.In general, there can be additional contributions from ratchet mechanisms42–4642. A. V. Nalitov, L. E. Golub, and E. L. Ivchenko, “Ratchet effects in two-dimensional systems with a lateral periodic potential,” Phys. Rev. B 86, 115301 (2012). https://doi.org/10.1103/physrevb.86.11530143. V. V. Popov, D. V. Fateev, E. L. Ivchenko, and S. D. Ganichev, “Noncentrosymmetric plasmon modes and giant terahertz photocurrent in a two-dimensional plasmonic crystal,” Phys. Rev. B 91, 235436 (2015). https://doi.org/10.1103/physrevb.91.23543644. P. Olbrich, J. Kamann, M. Konig, J. Munzert, L. Tutsch, J. Eroms, D. Weiss, M. H. Liu, L. E. Golub, E. L. Ivchenko, V. V. Popov, D. V. Fateev, K. V. Mashinsky, F. Fromm, T. Seyller, and S. D. Ganichev, “Terahertz ratchet effects in graphene with a lateral superlattice,” Phys. Rev. B 93, 075422 (2016). https://doi.org/10.1103/physrevb.93.07542245. S. Hubmann, V. V. Bel’kov, L. E. Golub, V. Y. Kachorovskii, M. Drienovsky, J. Eroms, D. Weiss, and S. D. Ganichev, “Giant ratchet magneto-photocurrent in graphene lateral superlattices,” Phys. Rev. Res. 2, 033186 (2020). https://doi.org/10.1103/physrevresearch.2.03318646. E. Mönch, S. O. Potashin, K. Lindner, I. Yahniuk, L. E. Golub, V. Y. Kachorovskii, V. V. Bel’kov, R. Huber, K. Watanabe, T. Taniguchi, J. Eroms, D. Weiss, and S. D. Ganichev, “Ratchet effect in spatially modulated bilayer graphene: Signature of hydrodynamic transport,” Phys. Rev. B 105, 045404 (2022). https://doi.org/10.1103/PhysRevB.105.045404 in graphene with spatially modulated carrier density other than the plasmonic ratchet mechanism4747. V. V. Popov, “Terahertz rectification by periodic two-dimensional electron plasma,” Appl. Phys. Lett. 102, 253504 (2013). https://doi.org/10.1063/1.4811706 included in our model. The reported ratchet mechanisms work even under the zero drain-bias condition without dc channel current flow, whereas no photoresponse was observed with our ADGG-EG-FET detector in the zero drain-bias condition, as shown in Fig. 7(b). As those prior works on ratchet mechanisms42–4642. A. V. Nalitov, L. E. Golub, and E. L. Ivchenko, “Ratchet effects in two-dimensional systems with a lateral periodic potential,” Phys. Rev. B 86, 115301 (2012). https://doi.org/10.1103/physrevb.86.11530143. V. V. Popov, D. V. Fateev, E. L. Ivchenko, and S. D. Ganichev, “Noncentrosymmetric plasmon modes and giant terahertz photocurrent in a two-dimensional plasmonic crystal,” Phys. Rev. B 91, 235436 (2015). https://doi.org/10.1103/physrevb.91.23543644. P. Olbrich, J. Kamann, M. Konig, J. Munzert, L. Tutsch, J. Eroms, D. Weiss, M. H. Liu, L. E. Golub, E. L. Ivchenko, V. V. Popov, D. V. Fateev, K. V. Mashinsky, F. Fromm, T. Seyller, and S. D. Ganichev, “Terahertz ratchet effects in graphene with a lateral superlattice,” Phys. Rev. B 93, 075422 (2016). https://doi.org/10.1103/physrevb.93.07542245. S. Hubmann, V. V. Bel’kov, L. E. Golub, V. Y. Kachorovskii, M. Drienovsky, J. Eroms, D. Weiss, and S. D. Ganichev, “Giant ratchet magneto-photocurrent in graphene lateral superlattices,” Phys. Rev. Res. 2, 033186 (2020). https://doi.org/10.1103/physrevresearch.2.03318646. E. Mönch, S. O. Potashin, K. Lindner, I. Yahniuk, L. E. Golub, V. Y. Kachorovskii, V. V. Bel’kov, R. Huber, K. Watanabe, T. Taniguchi, J. Eroms, D. Weiss, and S. D. Ganichev, “Ratchet effect in spatially modulated bilayer graphene: Signature of hydrodynamic transport,” Phys. Rev. B 105, 045404 (2022). https://doi.org/10.1103/PhysRevB.105.045404 clearly suggested that the geometrically asymmetric dual-grating-gate layout breaks the symmetry of the ratchet photocurrent flow, there might be contributions from ratchet mechanisms in our case. However, since we measured the detection photoresponse at the sensitivity level of plasmonic detection under periodic charge density modulations as the reference and observed an increase in the photoresponse with increasing Vg2, as the charge density increased from the charge neutrality point in the initially depleted region up to the entirely fully doped condition, the level of detection responsivity due to the ratchet mechanisms was considered to be too small to be identified in our measured range of the scale. Further investigation of the ratchet mechanisms in our detector will be performed in future work.Next, we investigate the response speed of our ADGG-EG-FET by fitting the response peaks using Gaussian functions, as shown in Fig. 8. For each pulse of the applied gate bias (Vg2) from −15.5 to 15 V, which corresponds to the transition from the plasmonic detection region to the plasmonic/PTE hybridized detection and PTE detection regions, the FWHM increases from ∼190–200 ps, as shown in Fig. 8(a). The plasmonic detection mechanism is very fast, on the order of 1 ps.4848. S. Rudin, G. Rupper, and M. Shur, “Ultimate response time of high electron mobility transistors,” J. Appl. Phys. 117, 174502 (2015). https://doi.org/10.1063/1.4919706 However, as discussed in Sec. II of the supplementary material, the response time of the PTE detection mechanism in our ADGG-EG-FET with a very long channel length is intrinsically limited by the intraband optical-phonon emission, which has an energy relaxation time on the order of 10 ps.4949. A. Satou, V. Ryzhii, Y. Kurita, and T. Otsuji, “