I. INTRODUCTION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTION <<II. RESULTS AND DISCUSSIO...III. CONCLUSIONIV. METHODSSUPPLEMENTARY MATERIALREFERENCESThe terahertz (THz) wave, which refers to electromagnetic radiation loosely defined as a frequency range from 0.1 to 10 THz,11. C. McDonnell, J. Deng, S. Sideris, T. Ellenbogen, and G. Li, Nat. Commun. 12(1), 30 (2021). https://doi.org/10.1038/s41467-020-20283-0 has raised burgeoning interest in both fundamental science and everyday life due to its application-oriented issues. Benefitting from its special position, the THz wave is endowed with unique advantages, such as the fact that it is penetrable to many objects and overlaps with the rotational and vibrational energy levels of biochemical molecular systems,22. E. T. Papaioannou and R. Beigang, Nanophotonics 10(4), 1243 (2021). https://doi.org/10.1515/nanoph-2020-0563 laying the foundation for applications in biomedicine,33. K. Okada, K. Serita, Q. Cassar, H. Murakami, G. MacGrogan, J.-P. Guillet, P. Mounaix, and M. Tonouchi, J. Phys.: Photonics 2(4), 044008 (2020). https://doi.org/10.1088/2515-7647/abbcda security imaging,44. R. Guo, S. Duan, Z. He, X. Liang, Z. Niu, M. He, Y. Jiang, J. Wu, L. Ji, B. Jin, J. Chen, H. Wang, and P. Wu, Adv. Funct. Mater. 31(52), 2106891 (2021). https://doi.org/10.1002/adfm.202106891 environmental monitoring,55. K. Peng, P. Parkinson, Q. Gao, J. L. Boland, Z. Li, F. Wang, S. Mokkapati, L. Fu, M. B. Johnston, H. H. Tan, and C. Jagadish, Nanotechnology 28(12), 125202 (2017). https://doi.org/10.1088/1361-6528/aa5d80 and communications.66. Y. Ghasempour, R. Shrestha, A. Charous, E. Knightly, and D. M. Mittleman, Nat. Commun. 11(1), 2017 (2020). https://doi.org/10.1038/s41467-020-15761-4 However, efficient exploration of the terahertz technique still lags behind the infrared and microwave ones due to the drawbacks of traditionally light-induced principles that hinder the development of photodetectors with capabilities of high-sensitivity, fast response, as well as on-chip integration.In the last few decades, much effort has been devoted to the innovation and development of efficient THz detection technology.7–97. I. Gayduchenko, S. G. Xu, G. Alymov, M. Moskotin, I. Tretyakov, T. Taniguchi, K. Watanabe, G. Goltsman, A. K. Geim, G. Fedorov, D. Svintsov, and D. A. Bandurin, Nat. Commun. 12(1), 543 (2021). https://doi.org/10.1038/s41467-020-20721-z8. L. L. Hale, C. T. Harris, T. S. Luk, S. J. Addamane, J. L. Reno, I. Brener, and O. Mitrofanov, Opt. Lett. 46(13), 3159 (2021). https://doi.org/10.1364/ol.4277989. N. G. Kalugin, L. Jing, E. S. Morell, G. C. Dyer, L. Wickey, M. Ovezmyradov, A. D. Grine, M. C. Wanke, E. A. Shaner, C. N. Lau, L. E. F. Foa Torres, M. V. Fistul, and K. B. Efetov, 2D Mater. 4(1), 015002 (2016). https://doi.org/10.1088/2053-1583/4/1/015002 Nevertheless, since the frequency of a THz wave is much higher than the cut-off frequency of traditional electronic devices and the photon energy is much lower than thermal-agitation energy at room-temperature,1010. Y. Li, Y. Zhang, T. Li, M. Li, Z. Chen, Q. Li, H. Zhao, Q. Sheng, W. Shi, and J. Yao, Nano Lett. 20(8), 5646 (2020). https://doi.org/10.1021/acs.nanolett.0c00082 it is hard to achieve a photovoltaic (PV) effect by making use of light-induced charge-separation via a built-in field in a reverse-biased semiconductor junction. The emergence of two-dimensional (2D) materials offer a fertile playground for exploring THz optoelectronics at the nanoscale.11–1311. G. Li, N. Amer, H. A. Hafez, S. Huang, D. Turchinovich, V. N. Mochalin, F. A. Hegmann, and L. V. Titova, Nano Lett. 20(1), 636 (2020). https://doi.org/10.1021/acs.nanolett.9b0440412. J. Wang, Z. Xie, and J. T. W. Yeow, Mater. Res. Express. 7(11), 112001 (2020). https://doi.org/10.1088/2053-1591/abc6cc13. Y. Wang, W. Wu, and Z. Zhao, Infrared Phys. Technol. 102, 103024 (2019). https://doi.org/10.1016/j.infrared.2019.103024 Owing to the novel photoelectric properties of 2D materials such as high electron mobility,1414. K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang, Y. Zhang, S. V. Dubonos, I. V. Grigorieva, and A.A. Firsov, Science 306, 666 (2004). https://doi.org/10.1126/science.1102896 intriguing band structure,1515. Y. Xie, F. Liang, S. Chi, D. Wang, K. Zhong, H. Yu, H. Zhang, Y. Chen, and J. Wang, ACS Appl. Mater. Interfaces 12(6), 7351 (2020). https://doi.org/10.1021/acsami.9b21671 and a larger Seebeck coefficient,1616. K. F. Mak, K. L. McGill, J. Park, and P. L. McEuen, Science 344, 1489 (2014). https://doi.org/10.1126/science.1250140 a variety of THz detectors based on 2D materials such as graphene,1717. Z. Huang, H. Chen, Y. Huang, Z. Ge, Y. Zhou, Y. Yang, P. Xiao, J. Liang, T. Zhang, Q. Shi, G. Li, and Y. Chen, Adv. Funct. Mater. 28(2), 1704363 (2018). https://doi.org/10.1002/adfm.201704363 MXenes,18,1918. S. Li, S. Xu, K. Pan, J. Du, and J. Qiu, Carbon 194, 127 (2022). https://doi.org/10.1016/j.carbon.2022.03.04819. Z. Lin, J. Liu, W. Peng, Y. Zhu, Y. Zhao, K. Jiang, M. Peng, and Y. Tan, ACS Nano 14(2), 2109 (2020). https://doi.org/10.1021/acsnano.9b08832 and TMDCs20–2220. C. Guo, W. Guo, H. Xu, L. Zhang, G. Chen, G. D’Olimpio, C.-N. Kuo, C. S. Lue, L. Wang, A. Politano, X. Chen, and W. Lu, 2D Mater. 7(3), 035026 (2020). https://doi.org/10.1088/2053-1583/ab8ec021. J. Li, W. Ma, L. Jiang, N. Yao, J. Deng, Q. Qiu, Y. Shi, W. Zhou, and Z. Huang, ACS Appl. Mater. Interfaces 14(12), 14331 (2022). https://doi.org/10.1021/acsami.2c0017522. H. Liu, Z. Chen, X. Chen, S. Chu, J. Huang, and R. Peng, J. Mater. Chem. C 4(40), 9399 (2016). https://doi.org/10.1039/c6tc02748b have been reported. However, the performance of 2D material-based THz detectors is aggregated by their weak electromagnetic coupling efficiency,2323. R. Wang, L. Xu, J. Wang, L. Sun, Y. Jiao, Y. Meng, S. Chen, C. Chang, and C. Fan, Nanoscale 13(44), 18467 (2021). https://doi.org/10.1039/d1nr04477j weak absorption,2424. O. Mitrofanov, L. L. Hale, P. P. Vabishchevich, T. S. Luk, S. J. Addamane, J. L. Reno, and I. Brener, APL Photonics 5(10), 101304 (2020). https://doi.org/10.1063/5.0019883 and air-stability,2525. Z. Dong, W. Yu, L. Zhang, H. Mu, L. Xie, J. Li, Y. Zhang, L. Huang, X. He, L. Wang, S. Lin, and K. Zhang, ACS Nano 15(12), 20403 (2021). https://doi.org/10.1021/acsnano.1c08756 which hinders their practical applications.Considerable effort has been invested within the material-science and optoelectronic communities to search for alternative material platforms that enable better device performance and circumvent the above bottlenecks.26–2826. J. Tong, H. Luo, F. Suo, T. Zhang, D. Zhang, and D. H. Zhang, Photonics Res. 10(5), 444354 (2022). https://doi.org/10.1364/prj.44435427. Y. Yang, K. Zhang, L. Zhang, G. Hong, C. Chen, H. Jing, J. Lu, P. Wang, X. Chen, L. Wang, and H. Xu, InfoMat 3(6), 705 (2021). https://doi.org/10.1002/inf2.1219328. S. X. Zhang, J. Zhang, Y. Wu, T. T. Kang, N. Li, X. F. Qiu, and P. P. Chen, Mater. Res. Express. 7(10), 106405 (2020). https://doi.org/10.1088/2053-1591/abc048 The advent of topological semi-metals (TSM) enables alternative platforms for exploring exotic quasi-particles with the topological character of bulk wave functions for THz detection due to their versatile operation of symmetry-breaking.29–3129. Z. K. Liu, B. Zhou, Y. Zhang, Z. J. Wang, H. M. Weng, D. Prabhakaran, S. K. Mo, Z. X. Shen, Z. Fang, X. Dai, Z. Hussain, and Y. L. Chen, Science 343, 864 (2014). https://doi.org/10.1126/science.124508530. T. R. Chang, S. Y. Xu, D. S. Sanchez, W. F. Tsai, S. M. Huang, G. Chang, C. H. Hsu, G. Bian, I. Belopolski, Z. M. Yu, S. A. Yang, T. Neupert, H. T. Jeng, H. Lin, and M. Z. Hasan, Phys. Rev. Lett. 119(2), 026404 (2017). https://doi.org/10.1103/physrevlett.119.02640431. S. Pezzini, M. R. van Delft, L. M. Schoop, B. V. Lotsch, A. Carrington, M. I. Katsnelson, N. E. Hussey, and S. Wiedmann, Nat. Phys. 14, 178 (2018). https://doi.org/10.1038/nphys4306 In the last few years, a few caveats of multiple excitations (e.g., Weyl fermions3232. J. Ma, Q. Gu, Y. Liu, J. Lai, P. Yu, X. Zhuo, Z. Liu, J. H. Chen, J. Feng, and D. Sun, Nat. Mater. 18, 476 (2019). https://doi.org/10.1038/s41563-019-0296-5 and Dirac fermions3333. Z. Dai, M. Manjappa, Y. Yang, T. C. W. Tan, B. Qiang, S. Han, L. J. Wong, F. Xiu, W. Liu, and R. Singh, Adv. Funct. Mater. 31(17), 2011011 (2021). https://doi.org/10.1002/adfm.202011011) have been reported in three-dimensional solid-state systems, which has prompted enormous interest in exploiting properties of nontrivial topology that are absent in other materials. By breaking either time reversal or inversion symmetry,3434. H. Xu, F. Fei, Z. Chen, X. Bo, Z. Sun, X. Wan, L. Han, L. Wang, K. Zhang, J. Zhang, G. Chen, C. Liu, W. Guo, L. Yang, D. Wei, F. Song, X. Chen, and W. Lu, ACS Nano 15(3), 5138 (2021). https://doi.org/10.1021/acsnano.0c10304 numerous peculiar quantum effects such as nonlinear Hall effect,3535. Q. Ma, S. Y. Xu, H. Shen, D. MacNeill, V. Fatemi, T. R. Chang, A. M. Mier Valdivia, S. Wu, Z. Du, C. H. Hsu, S. Fang, Q. D. Gibson, K. Watanabe, T. Taniguchi, R. J. Cava, E. Kaxiras, H. Z. Lu, H. Lin, L. Fu, N. Gedik et al., Nature 565, 337 (2019). https://doi.org/10.1038/s41586-018-0807-6 nonlinear optical properties,3636. S. Semin, X. Li, Y. Duan, and T. Rasing, Adv. Opt. Mater. 9(23), 2100327 (2021). https://doi.org/10.1002/adom.202100327 and chiral anomalies3737. C.-L. Zhang, S.-Y. Xu, I. Belopolski, Z. Yuan, Z. Lin, B. Tong, G. Bian, N. Alidoust, C.-C. Lee, S.-M. Huang, T.-R. Chang, G. Chang, C.-H. Hsu, H.-T. Jeng, M. Neupane, D. S. Sanchez, H. Zheng, J. Wang, H. Lin, C. Zhang, H.-Z. Lu, S.-Q. Shen, T. Neupert, M. Zahid Hasan, and S. Jia, Nat. Commun. 7, 10735 (2016). https://doi.org/10.1038/ncomms10735 were found, offering ad-hoc properties to target the expected performance and functionalities.HgTe is a semi-metallic material hosting a zinc blende (ZB) crystal structure38,3938. S.-S. Chee, C. Gréboval, D. V. Magalhaes, J. Ramade, A. Chu, J. Qu, P. Rastogi, A. Khalili, T. H. Dang, C. Dabard, Y. Prado, G. Patriarche, J. Chaste, M. Rosticher, S. Bals, C. Delerue, and E. Lhuillier, Nano Lett. 21(10), 4145 (2021). https://doi.org/10.1021/acs.nanolett.0c0434639. J. P. Faurie, S. Sivananthan, M. Boukerche, and J. Reno, Appl. Phys. Lett. 45(12), 1307 (1984). https://doi.org/10.1063/1.95129 and band-inversion with a negative bandgap (−0.3 eV)40,4140. X. C. Zhang, A. Pfeuffer-Jeschke, K. Ortner, V. Hock, H. Buhmann, C. R. Becker, and G. Landwehr, Phys. Rev. B 63, 245305 (2001). https://doi.org/10.1103/physrevb.63.24530541. B. Büttner, C. X. Liu, G. Tkachov, E. G. Novik, C. Brüne, H. Buhmann, E. M. Hankiewicz, P. Recher, B. Trauzettel, S. C. Zhang, and L. W. Molenkamp, Nat. Phys. 7, 418 (2011). https://doi.org/10.1038/nphys1914 so that the selective wavelength dependence of the photoresponse imposed by the gap is absent. Benefiting from the large Seebeck coefficient (about−135 µV/K) and excellent thermoelectric characteristics, a HgTe-based PTE THz detector could possess better response time and high frequency characteristics than electronic detection based on the rectifier effect or nonlinear frequency mixing mechanism in theory. The research on HgTe mainly focuses on quantum wells, which is a typical two-dimension (2D) topological insulator and has shined in the field of infrared detection for decades.4242. J. Ahn and B.-J. Yang, Phys. Rev. Lett. 118(15), 156401 (2017). https://doi.org/10.1103/physrevlett.118.156401 In recent years, HgCdTe has previously been utilized for bow-tie antenna terahertz detectors.43,4443. F. Sizov, V. V. Zabudsky, S. A. Dvoretsky, V. A. Petryiakov, A. G. Golenkov, K. V. Andreyeva et al., Proc. SPIE 9483, (2015). https://doi.org/10.1117/12.217685444. F. T. Zinovia, J. V. Gumenjuk-Sichevska, S. N. Danilov et al., UkrMW (924), 921 (2020). https://doi.org/10.1109/UkrMW49653.2020.9252813 However, since HgCdTe belongs to the ternary metal, this results in a huge obstacle to epitaxial growth. Meanwhile, the expensive CdZnTe substrate used for growth also hindered its further development. HgTe, with its large area epitaxial growth, low cost, and high performance, becomes the best choice. The transformation of dimensions may bring new possibilities. The strain HgTe thin film is a three-dimensional (3D) topological insulator, which could evolve into a semi-metallic state under the regulation of stress and other factors.4545. J. Ruan, S.-K. Jian, H. Yao, H. Zhang, S.-C. Zhang, and D. Xing, Nat. Commun. 7, 11136 (2016). https://doi.org/10.1038/ncomms11136 Traditionally, semimetals have not been considered as candidates for photodetection due to the large dark current that occurs when bias-voltage traverses across the channel. However, the successful demonstration of a graphene-based field-effect-transistor (FET) photodetector offers the feasibility of using semi-metallic materials for high-speed and broadband photodetection down to the far-infrared regime in terms of the photothermoelectric or carrier multiplication effect.4646. H. Qin, J. Sun, S. Liang, X. Li, X. Yang, Z. He, C. Yu, and Z. Feng, Carbon 116, 760 (2017). https://doi.org/10.1016/j.carbon.2017.02.037 A large Seebeck coefficient and the ultra-high electron mobility (4.5 × 104 cm2/V s) in combination with excellent ambient-stability4747. A. Jagtap, N. Goubet, C. Livache, A. Chu, B. Martinez, C. Gréboval, J. Qu, E. Dandeu, L. Becerra, N. Witkowski, S. Ithurria, F. Mathevet, M. G. Silly, B. Dubertret, and E. Lhuillier, J. Phys. Chem. C 122(26), 14979 (2018). https://doi.org/10.1021/acs.jpcc.8b03276 render HgTe an ideal candidate for the development of semimetal-based photodetectors beyond their graphene-based counterparts.

In this work, a large-area HgTe film with high crystallinity and mobility on a CdTe substrate is successfully grown via the molecular-beam epitaxial (MBE) method, and the light-resonator has been exploited in terms of a bow-tie antenna to achieve strong electromagnetic coupling of THz radiation, which converts the incident electromagnetic waves into a direct current by triggering the photothermoelectric (PTE) effect in the HgTe channel. The excellent broadband responsivity (0.36 A W−1), fast response, high-resolution THz imaging, and blackbody response are all demonstrated in our bow-tie antenna-based HgTe detector. Our results provide a guarantee for sensitive, ultra-fast, and large-area application based on controllable HgTe-film growth.

II. RESULTS AND DISCUSSION

Section:

ChooseTop of pageABSTRACTI. INTRODUCTIONII. RESULTS AND DISCUSSIO... <<III. CONCLUSIONIV. METHODSSUPPLEMENTARY MATERIALREFERENCES

It is well known that there exist a series of difficulties in the growth and device fabrication of Hg-based materials. The adhesion coefficient of Hg on the surface of the crystal material is very low, and resulting in a high vapor pressure at room temperature. Therefore, the growth temperature of the Hg-based material in the MBE system should be controlled precisely. When the growth is completed, the wafer should be cooled below 80 °C in a Hg atmosphere. Otherwise, large quantities of Hg desorb in during cooling, resulting in serious surface degradation. Meanwhile, the selection of the buffer layer and crystal phase is also significant. Both HgTe and CdTe belong to the zinc blende (ZB) structure, and the lattice mismatch is only 0.3%. In theory, CdTe is an ideal lattice matching buffer layer. However, the lattice mismatche between the buffer layer and the substrate is as high as 14.7% with the lattice constants of 6.48 Å for CdTe and 5.65 Å for GaAs. The large lattice mismatch results in the growth of CdTe along different crystalline orientations on the GaAs surface. Improper crystalline orientation matching will produce a large number of stacking faults and twin crystals at the CdTe-GaAs interface, which then extend to the CdTe buffer layer and affect the quality of the HgTe crystal. In the epitaxy of HgTe wafers, our substrate and buffer layer are both [211]-crystal orientation. From the perspective of epitaxial relations, the orientation of CdTe [211]-crystalline is parallel to that of GaAs [211] in the epitaxial plane, which, therefore, greatly reduces the dislocation defects caused by stress release. On the other hand, the bond energy of Hg–Te is only 1/3 that of the Cd–Te bond. HgTe crystal may be damaged in the process of micro/nano fabrication. Compared with traditional dry etching, inductively coupled plasma etching (ICP) is used for device preparation, which is based on the chemical reaction of plasma gas molecules with the material, and less damage is caused to the material. All of the above strategies ensure the success of epitaxial growth and device preparation. In ordinary scientific research, we focus on the exploration and study of the physical mechanisms in individual devices. However, all scientific research will eventually move out of the laboratory and serve practical applications. On the one hand, the market demands large-scale production of large wafers for integrated chips. On the other hand, a single device is only suitable for the exploration of physical mechanisms. In order to further pursue excellent performance, linear array and focal plane array (FPA) detector occupy the majority of practical applications. Therefore, the large size and high quality HgTe wafers we grow lay the foundation for the preparation of FPA devices in practical applications.

Figure 1 displays a high-quality single crystal of HgTe films with a diameter of 3 in., which were grown via the MBE method on a CdTe (211)/GaAs substrate (see more details in Sec.  for Materials Preparation). The crystallographic structure of HgTe/CdTe is presented in Fig. 1(a) (the green, yellow, and blue balls denote Hg, Te, and Cd atoms), and the small lattice mismatch between HgTe and CdTe substrate (0.3%) enables the successful growth of single crystal HgTe film on CdTe/GaAs. A scanning electron microscopy (SEM) image of the HgTe/CdTe interface and corresponding energy-dispersive spectroscopy (EDS) elemental mapping images are provided in Figs. 1(b) and 1(c), which reveal a perfect HgTe/CdTe heterointerface accompanied by the uniformity of Hg and Te components across the whole sample. The high-resolution X-ray Diffraction (HRXRD) spectrum of a HgTe film with a thickness of 600 nm is shown in Fig. 1(d). There are two sharp diffraction peaks of (211)-oriented CdTe and HgTe at 70.97° and 71.49°, respectively, corresponding to the 422 crystal plane,4848. P. Ballet, C. Thomas, X. Baudry, C. Bouvier, O. Crauste, T. Meunier, G. Badano, M. Veillerot, J. P. Barnes, P. H. Jouneau, and L. P. Levy, J. Electron. Mater. 43(8), 2955 (2014). https://doi.org/10.1007/s11664-014-3160-z validating the excellent crystalline quality of the as-grown HgTe film. The Raman spectra [see Fig. 1(e)] of HgTe films with the varied thicknesses (240, 400, and 600 nm) show two characteristic Raman peaks at 118 and 137 cm−1, corresponding to the TO and LO phonon mode,4949. J. H. Spencer, J. M. Nesbitt, H. Trewhitt, R. J. Kashtiban, G. Bell, V. G. Ivanov, E. Faulques, J. Sloan, and D. C. Smith, ACS Nano 8, 9044 (2014). https://doi.org/10.1021/nn5023632 and the full width half maximum (FWHM) of the two modes are 12.5 and 8 cm−1, respectively. The microstructure of the HgTe/CdTe interface is revealed by the high-resolution transmission electron microscope (HRTEM) image in Fig. 1(f). No dislocations, stacking or twin faults, at the HgTe/CdTe interface are found, indicating a very smooth transition interface plane on the atomic scale. Correspondingly, selected area electron diffraction (SAED) in Fig. 1(g) further validates the excellent interface (see more details in Sec.  for materials characterization).The electron mobility of HgTe film with a thickness of 600 nm is 2.7 × 104 cm2/V s (300 K) and 4.5 × 104 cm2/V s (77 K) by using the Van der Pauw Hall measurement (see more details in the Hall Test section, supplementary material), where the better crystal quality and electrical properties (such as lower carrier concentration and higher mobility) are revealed compared to the other two thicknesses of wafers and thus performed in THz device preparation. In addition, to achieve efficient conversion of electromagnetic wave into electrical signal, a bow tie antenna structure is engineered numerically by using the finite element software-COMSOL with a particular design for both strong THz-focusing at the channel and readable electrical-output. Figure 2(a) displays schematically the HgTe-based bow-tie antenna THz detector with two arms of antenna connected electrically for measurements (see more details in Sec.  for device fabrication). The optical image of the whole bow tie antenna detector with a 6 µm channel-length and simulated electric-field profile under THz radiation in Fig. 2(b) clearly verifies the efficiency of THz coupling, showing strong polarization-dependence in Fig. 2(c). Meanwhile, the far-field enhancement with varied channel-lengths at different frequencies shown in Fig. 2(d), substantiating evidently stronger light–matter interaction, can be achieved by reducing the antenna-gap.The photoelectronic process of bow-tie antennas-based HgTe THz detector is portrayed in Figs. 2(e) and 2(f) schematically. Due to the extremely low energy of THz waves, it is hard to excite the electrons effectively in traditional semiconductor materials, which could be solved by the inherent negative bandgap characteristics of HgTe topological semi-metals. There exists a strong coupling between THz waves and interband electrons, benefiting from the innate advantages of Dirac fermions.5050. L. Zhang, Z. Chen, K. Zhang, L. Wang, H. Xu, L. Han, W. Guo, Y. Yang, C.-N. Kuo, C. S. Lue, D. Mondal, J. Fuji, I. Vobornik, B. Ghosh, A. Agarwal, H. Xing, X. Chen, A. Politano, and W. Lu, Nat. Commun. 12(1), 1584 (2021). https://doi.org/10.1038/s41467-021-21906-w When the external bias voltage is absent, the electrons are in thermodynamic equilibrium, filling the Dirac cone below the Fermi level. Meanwhile, the electrons are driven into non-equilibrium states under the THz radiation, making it feasible to produce a photocurrent. As the external bias voltage is traversing across the channel, the interface barrier will become asymmetric to facilitate the carrier transport process, and the nonequilibrium carriers will be driven unilaterally to form the bias-dependent photocurrent.In order to better elucidate the underlying mechanism of a bow-tie antenna-based THz detector, the band diagram at the junction region is depicted in Fig. 2(f). Due to the different work functions between electrodes (Au) and HgTe film, the band of HgTe will bend at the metal-material interface. Benefiting from the large Seebeck coefficient and excellent thermoelectric characteristics, the HgTe-based PTE THz detector possesses better response time and high frequency characteristics than electronic detection based on the rectifier effect or nonlinear mixing mechanism. When the electrons/holes are heated up by strong optical-field coupling, depending on the bias conditions, there are two regimes of detector operation that can be distinguished. While the bias voltage is absent, a weak net photocurrent is observable under the THz radiation, which is probably caused by the residual symmetry-breaking artifacts or imperfections. As the external bias voltage is scanned across the channel, the interface barrier of Au–HgTe will become asymmetric, leading to the Seebeck coefficient difference and the non-zero photocurrent production that is changeable by the scanning bias voltage.5151. N. T. Yardimci and M. Jarrahi, Small 14(44), 1802437 (2018). https://doi.org/10.1002/smll.201802437 Alternatively, a large photoresponse at zero bias is achievable by engineering an asymmetric short channel detector with tailored distribution of non-equilibrium carriers.Following the above understandings, we proceed with our experiment by quantifying the performance of the prepared HgTe-based THz photodetector with different channel lengths and symmetry. The Seebeck coefficients of HgTe films with a thickness of 600 nm are measured to be around −135 µV/K at 300 K by the Seebeck coefficient measurement system (see more details in the Seebeck coefficient measurement section, supplementary material). A large Seebeck coefficient is essential to ensure the efficiency of photothermal detection, aside from the strong interband excitation. The schematic magnification of the bow-tie antenna-coupled symmetric long channel detector structure is illustrated in Fig. 3(a), where HgTe serves as the channel connecting the electrodes. The THz wave is coupled to the bow-tie antenna to complete the detection. To validate the underlying mechanism of the bow-tie antenna-based HgTe THz detector, an electromagnetic wave source (Agilent E8257D) at 30 GHz is used to excite the non-equilibrium carriers, and the results are shown in Figs. 3(b) and 3(c). Linear power-dependence of photoresponse at different bias voltages is observed, which is in accordance with the above analysis that the direction of photocurrent is reversible.Furthermore, the fast-pulsed shape of photocurrent is well preserved in Fig. 3(c) at 50 or 100 mV, indicating the excellent performance and good signal-to-noise ratio of the detector. In order to accurately determine the response time, an ultra-fast light-modulator connected with a 100 GHz IMPATT diode is utilized [Fig. 3(d)]. The pulsed-photocurrent at different bias-voltages with the extension of the rising/falling edges is shown in Figs. 3(e) and 3(f), respectively. The rising and recovery time-scales are around 3 and 4 µs, which are defined as the time required to reach 90% of the maximum photocurrent and the time needed to drop to 10% of the maximum photocurrent.5252. Y. Chen, W. Ma, C. Tan, M. Luo, W. Zhou, N. Yao, H. Wang, L. Zhang, T. Xu, T. Tong, Y. Zhou, Y. Xu, C. Yu, C. Shan, H. Peng, F. Yue, P. Wang, Z. Huang, and W. Hu, Adv. Funct. Mater. 31(14), 2170093 (2021). https://doi.org/10.1002/adfm.202170093Actually, the sensitive and stable THz response could be retained across a wide-frequency range from 0.02 to 0.3 THz in our HgTe-based detector (see Figs. S2 and S3 in the supplementary material). The photocurrent responsivity (RI) and noise equivalent power (NEP) vs the bias voltage at 0.03, 0.10, and 0.30 THz are recorded in Figs. 3(g)–3(i). The responsivity, which is derived from the formula RI =Iph/PinSd, is one of the significant norms to evaluate the capacity of a detector. Iph is the detector photocurrent, and Pin and Sd refer to the irradiated power densities of the THz wave and the irradiated device area (140 × 420 µm2),5353. E. Javadi, D. B. But, K. Ikamas, J. Zdanevičius, W. Knap, and A. Lisauskas, Sensors 21(9), 2909 (2021). https://doi.org/10.3390/s21092909 respectively. Under zero bias voltage, the responsivity reaches 554.3, 4.8, and 0.65 mA W−1 at 0.03, 0.1, and 0.3 THz, respectively. It is worth noting that the corresponding responsivity could be further improved to 25.6, 0.48, and 0.038 A W−1 when the voltage is fixed at 100 mV. The NEP, which is defined as the minimum incident power required when the signal-to-noise ratio reaches unity within the 1 Hz bandwidth, has been extracted from the ratio νn/RI.5454. L. Viti, D. G. Purdie, A. Lombardo, A. C. Ferrari, and M. S. Vitiello, Nano Lett. 20(5), 3169 (2020). https://doi.org/10.1021/acs.nanolett.9b05207νn is the root-mean-square of noise rooted from thermal Johnson–Nyquist noise (νth) related to the non-zero resistance detector and shot noise due to bias current (νb), in the form of νn = (νth + νb)1/2 = (4kBT/r + 2qId)1/2, in which kB is the Boltzmann constant, T is room temperature, r is detector resistance (r = 107 Ω is obtained from the measurement), q is the elementary charge, and Id is a direct current. The system-specific flicker noise (ν1/f) is dominated at low-frequency (less than 150 Hz), and it has been ignored. When the bias voltage is zero, the value of NEP is large since the configuration of antennas is geometrically symmetric and the photocurrent depends weakly on the excitation of the THz wave. Nevertheless, when the bias voltage increases, the non-equilibrium carriers are transported under the action of an external direct current electric field, and the required threshold power of the THz wave decreases rapidly. Therefore, a smaller NEP value is obtained. Under 0.03, 0.10, and 0.30 THz-irradiation, the measured NEPs are 0.008, 2.73, and 28.6 nW/Hz1/2 at 0 mV bias and 0.8, 44.8, and 560 pW/Hz1/2 at 100 mV bias voltage, respectively. Benefitting from the nature of the negative bandgap and oxidation resistance in HgTe, the symmetric bow-tie antenna-assisted HgTe-based THz detector exhibits excellent detection stability, as could also be inferred from the photoresponse at 3.3 THz from the QCL laser after two months [see Fig. S2(b) in the supplementary material].It is worth mentioning that, in terms of the optical coupling with the magnetic dipole (MD) mode, the symmetric cubic resonator across the 6 µm HgTe channel in the aforementioned detector supports all magnetic dipole modes. Unfortunately, the MD mode Mz does not couple to a plane wave with the k-vector normal to the xy-plane; only the Mx or My mode could be directly excited by this plane wave. Therefore, perfect absorption using only MD modes in symmetric cubic resonators is impossible.5555. T. Siday, P. P. Vabishchevich, L. Hale, C. T. Harris, T. S. Luk, J. L. Reno, I. Brener, and O. Mitrofanov, Nano Lett. 19(5), 2888 (2019). https://doi.org/10.1021/acs.nanolett.8b05118 In order to further improve the sensitivity of the HgTe-based THz detector, short channel bow-tie antennas formed by incorporating an asymmetric cubic resonator and a 2 µm gap-channel are also implemented.Figure 4(a) illustrates the position-dependence of the Seebeck coefficient for scanning a THz spot. The fact that the S(x) achieves its maximum with opposite signs at two junctions, combined with a value close to zero with the THz spot focused at the middle of the channel. The simulated localized electric-field enhancement (for the situation of a THz spot focused at the middle of the channel) is shown in Fig. 4(b) (see simulation method in supplementary material).5656. X. Zuo, Z. Li, W. W. Wong, Y. Yu, X. Li, J. He, L. Fu, H. H. Tan, C. Jagadish, and X. Yuan, Appl. Phys. Lett. 120(7), 071109 (2022). https://doi.org/10.1063/5.0066507 Here, we simultaneously excite the MD modes, in particular Mx and Mz, and thus realize perfect absorption in the HgTe channel through breaking the resonator cubic symmetry, i.e., a bow-tie antenna-based asymmetric cubic resonator. Compared with the previous symmetric long channel bow-tie antenna, the electromagnetic coupling gain in the HgTe channel increases by ∼11%, and the far-field enhancement reaches 140 at 0.03 THz. Figure 4(c) shows the physical process of hot electron transport excited by a potential gradient ∇V(x) with asymmetric THz-irradiation, in which a maximum value of S(x) could be completed. Corresponding profiles of electron temperature T(x), Seebeck coefficient S(x), and potential gradient ∇V(x) along the channel are all shown in Fig. 4(d). Hence, the detector equipped with an asymmetric short channel cubic resonator leads to a high temperature T(x) along the channel. The incident photons will heat the carriers in HgTe via strong electron–electron interactions. The hot carriers and the lattice finally reach thermal equilibrium via the scattering process between carriers and phonons. By taking advantage of the large Seebeck coefficient in HgTe, a potential gradient ∇V(x) = –S × ∇T(x) opens up the feasibility of unbiased operation, obliterating the excess thermal-noise and power-consumption.Figures 4(e)–4(i) summarize the detector performance of a bow-tie antenna-based HgTe THz detector with an asymmetric 2 µm channel length. The comparison of photocurrent between the symmetric long channel and asymmetric short channel detectors in a single time-period of 0.1 THz with a 100 mV bias voltage is also displayed in Fig. 4(e), substantiating the improved performance and speed. To evaluate the detector performance at higher frequency operation, the transient photocurrent is also presented at 0.3 THz in Fig. 4(f), where the pulsed shape is well preserved with a better signal-to-noise ratio than that in Fig. 3(f).In addition, the responsivity and NEP vs the bias voltage at 0.3 THz are summarized in Figs. 4(g) and 4(h), respectively. In Fig. 4(g), the responsivity shows a linear growth by varying bias voltage, which could be regarded as strong evidence that non-equilibrium carriers are driven by the external bias for THz detection. Notably, the asymmetric nanostructure near the HgTe channel manipulates the localized intensity-distribution of THz waves, whereas it is immune to the intrinsic symmetry of bow-tie antennas, as can be inferred from the far-field characters in a 2 µm channel [see Fig. S4(b) in the supplementary material]. Therefore, the detector shows excellent prospects for improving the ability to detect polarization. Figure 4(i) depicts both the responsivity and the NEP of the detector at different frequencies with a bias voltage of 100 mV for a symmetric long channel detector and an asymmetric short channel detector, respectively. Here, the photodetector acquires a higher responsivity (0.36 A W−1) and a lower NEP (88.5 pW Hz−1/2) at 0.3 THz. Compared with the previous detector, the sensitivity of the asymmetric short channel detector at 0.1 and 0.3 THz is improved by an order of magnitude.The stronger zero-bias photocurrent mediated by the unilateral hot-carrier flow validates the success of the implementation of the asymmetric structure that strengthens the efficiency of charge-separation with a higher signal-to-noise ratio and lower power-consumption [see Fig. S5(b) in the supplementary material]. Even though it is only a preliminary attempt at HgTe-based THz detectors and far from an optimal detector design method. However, the obtained RI and NEP have shown comparable properties to other existing materials. To prove the superiority of the HgTe-based photodetector, the room-temperature responsivity, NEP, and bandwidth of the detector are compared with those of the commercial photodetector and the two-dimensional based photodetectors in Table I. The 3 dB bandwidth corresponds to the frequency when the normalized responsivity decreases to 0.707 of its original value, and a value of 30 kHz is obtained for the HgTe photodetector as shown in Fig. S5(c) (see more details in the supplementary material). The responsivity and bandwidth of the HgTe photodetector are comparable to those of the most advanced commercial photodetectors, and the NEP is lower than that of photodetectors based on 2D materials and commercial photodetectors in the 0.3–3.3 THz region. In addition, it is worth mentioning that the photoresponse is also valid at higher frequency (e.g., 3.3 THz from a QCL laser) despite the size of the antenna being incommensurate with the incident photons, which is leveraged on the semi-metallic nature and ultrahigh carrier mobility of HgTe film.

TABLE I. Comparison of NEP and THz imaging capability of various photodetectors.

Material typeRI (A/W)NEP (pW Hz−1/2)Bandwidth (kHz)Response timeNormalization methodRT imagingReferencesGraphene0.15 at 0.33 THz163 at 0.33 THz⋯⋯Device areaOK4646. H. Qin, J. Sun, S. Liang, X. Li, X. Yang, Z. He, C. Yu, and Z. Feng, Carbon 116, 760 (2017). https://doi.org/10.1016/j.carbon.2017.02.037InSb97 at 0.03 THz0.1 at 0.03 THz2415 µsDevice area⋯2626. J. Tong, H. Luo, F. Suo, T. Zhang, D. Zhang, and D. H. Zhang, Photonics Res. 10(5), 444354 (2022). https://doi.org/10.1364/prj.444354BP0.05 at 0.29 THz7000 at 0.29 THz⋯⋯λ2/4⋯5757. L. Viti, J. Hu, D. Coquillat, A. Politano, W. Knap, and M. S. Vitiello, Sci. Rep. 6, 20474 (2016). https://doi.org/10.1038/srep20474PdSe20.005 at 0.3 THz900 at 0.3 THz227.5 µsλ2/4πOK2525. Z. Dong, W. Yu, L. Zhang, H. Mu, L. Xie, J. Li, Y. Zhang, L. Huang, X. He, L. Wang, S. Lin, and K. Zhang, ACS Nano 15(12), 20403 (2021). https://doi.org/10.1021/acsnano.1c087560.02 at 0.12 THz142 at 0.12 THzSilicon0.11 at 0.3 THz260 at 0.3 THz⋯⋯Device area⋯5858. I. V. Minin, O. V. Minin, J. Salvador-Sánchez, J. A. Delgado-Notario, J. Calvo-Gallego, M. Ferrando-Bataller, K. Fobelets, J. E. Velázquez-Pérez, and Y. M. Meziani, Opt. Lett. 46(13), 3061 (2021). https://doi.org/10.1364/ol.431175HgTe0.04 at 3.3 THz1400 at 3.3 THz302.5 µsDevice areaOKThis work0.36 at 0.3 THz88.5 at 0.3 THz