Towards Sensitive Terahertz Detection Via Thermoelectric Manipulation
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Liu et al. NPG Asia Materials (2018) 10: 318–327 DOI 10.1038/s41427-018-0032-7 NPG Asia Materials ARTICLE Open Access Towards sensitive terahertz detection via thermoelectric manipulation using graphene transistors Changlong Liu1,2,LeiDu1,2,WeiweiTang1,2,DachengWei3,JinhuaLi4,LinWang1,2,5,GangChen1,2, Xiaoshuang Chen1,2,5 and Wei Lu1,2,5 Abstract Graphene has been highly sought after as a potential candidate for hot-electron terahertz (THz) detection benefiting from its strong photon absorption, fast carrier relaxation, and weak electron-phonon coupling. Nevertheless, to date, graphene-based thermoelectric THz photodetection is hindered by low responsivity owing to relatively low photoelectric efficiency. In this work, we provide a straightforward strategy for enhanced THz detection based on antenna-coupled CVD graphene transistors with the introduction of symmetric paired fingers. This design enables switchable photodetection modes by controlling the interaction between the THz field and free hot carriers in the graphene-channel through different contacting configurations. Hence a novel “bias-field effect” can be activated, which leads to a drastic enhancement in THz detection ability with maximum responsivity of up to 280 V/W at 0.12 THz relative to the antenna area and a Johnson-noise limited minimum noise-equivalent power (NEP) of 100 pW/Hz0.5 at room temperature. The mechanism responsible for the enhancement in the photoelectric gain is attributed to thermophotovoltaic instead of plasma self-mixing effects. Our results offer a promising alternative route toward 1234567890():,; 1234567890():,; scalable, wafer-level production of high-performance graphene detectors. Introduction interact with light10,11, which indicates they are promising Recently, two-dimensional (2D) materials (transition materials with great potential in optoelectronic devices for metal dichalcogenides (TMDs), graphene, black phos- communication, sensing, and imaging, particularly in the phorus, etc.) have ignited intensive interest due to their visible and near-infrared range. However, it is rather dif- novel electronic and photonic properties, which are dis- ficult to extend the spectral response of 2D materials to tinct from those of their bulk counterparts, such as defect- the mid-/far-infrared and terahertz (THz) regime owing – free topological transport1 3, linear dichorism4,5, super- to the relatively large bandgap of these materials. Many conductivity6,7, and plasmonics8,9. It has been proved efforts, such as bandgap engineering and heterogeneous experimentally that few-layer 2D materials can strongly integration with quantum dots or other 2D materials (van der Waals heterostructures), have been dedicated to – resolving this bottleneck12 14. Nevertheless, light absorp- Correspondence: Lin Wang ([email protected])or tion in narrow-gap 2D materials such as TMDs decreases Gang Chen ([email protected]) or Xiaoshuang Chen ([email protected]. significantly when the photon energy is far below the cn) interband threshold, similar to the behavior of other bulk 1State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yutian Road, Shanghai 200083, semiconductors. China On the other hand, gapless graphene, in principal, can 2 University of Chinese Academy of Sciences, No 19 A Yuquan Road, Beijing interact strongly with electromagnetic fields over a much 100049, China Full list of author information is available at the end of the article wider spectral range. The photoresponsivity of graphene These authors contributed equally: Changlong Liu, Lei Du, Weiwei Tang. © The Author(s) 2018 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a linktotheCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. Liu et al. NPG Asia Materials (2018) 10: 318-327 319 Fig. 1 a Schematic of a graphene-based THz detector under bias-field configuration: the graphene-channel is contacted by two log-antenna arms consisting of Cr/Au stacks and two paired fingers in the middle, which results in a four-terminal electrical configuration marked A, B, C, and D in the SEM image in b. The channel features a length L ~6μm and different widths W of 60 and 6 μm in the studied devices; the antenna arm features an outer radius of R ~ 1 mm and a circular tooth span angle of π/4. For more details about the device fabrication process, please see the Materials and Methods section is usually quite low due to the weak light absorption of Materials and methods 2.3% through the interband transition as well as a large Device fabrication dark current15. Nevertheless, by utilizing the Drude The detector was fabricated using chemical vapor response of free electrons in combination with heavy deposition (CVD)-grown single-layer graphene, which chemical doping, one can achieve 40% photon absorption was transferred onto a highly resistive Si/SiO2 (300 nm) in a single graphene layer in the mid-/far-infrared band of substrate for antenna patterning. The purpose of using a – the electromagnetic spectrum16 19. Therefore, graphene highly resistive Si substrate instead of a highly doped one is one of the most preferred materials for THz optoelec- was to eliminate the reflective loss of the incident wave, as tronics. Graphene-based THz detectors have been the THz wavelength is at least two orders of magnitude reported to exhibit suitable performance in the framework longer than the dielectric layer thickness. The graphene- of plasma self-mixing and thermoelectric effects. The channel was contacted symmetrically by a log-periodic plasma self-mixing effect can be achieved via the asym- antenna to maximize the field confinement along the metrical antenna coupling of a THz field between a channel rather than perpendicular to it (see Fig. 1 and 2 source/drain and gate, during which the nonlinear recti- (a), (b)). Thus, particular asymmetrical boundary condi- fication of carrier transport leads to direct detection with tions related to previously reported photothermoelectric – a responsivity of approximately 0.04 ~ 20 V/W19 22. (PTE)18,19and plasma-wave effects21,22 were not con- Moreover, thermoelectric THz detection in graphene can sidered. Oxygen-based plasma etching was performed be achieved by asymmetric contacting, which produces a after the antenna patterning and lift-off processes to form Seebeck coefficient difference and direct photocurrent a graphene-channel (for more details, see Fig. S1, Sup- along the channel due to metal-induced asymmetrical plementary Information). Additional paired contact fin- doping18, and the responsivity can reach as high as 15 V/ gers with different slit widths s were then fabricated in the W. While high-performance graphene-based devices have middle of channel by electron beam lithography, resulting become increasingly critical to satisfying the requirements in a four-terminal configuration24,25. The terminals are of everyday applications, the current implementation of labeled A, B, C, and D in Fig. 1b. Thus, the device direct THz detection is hindered by the lack of sufficient response depended specifically on the different electrical photoelectric gain23. The superior performance of gra- connections among these terminals. The homogeneity of phene can be achieved by engineering the bias-field and the formed graphene-channel was confirmed by Raman electromagnetic size effects to manipulate the hot-carrier scattering spectroscopy, and the current-voltage output distribution. The results of the current study demonstrate characteristics with terminals A and D connected were the important role of carrier-THz field interactions in determined for all the fabricated devices. It could also be detection, which enable switching between the thermo- inferred from the blueshift of the optical phonon-related photovoltaic (PV) and photoconductive (PC) modes, G peak in the Raman spectrum that the Fermi level thereby producing previously unattainable levels of pho- relative to the Dirac point of the transferred sample is toelectric gain. approximately 0.3 eV (see Fig. S7, Supplementary Information). Liu et al. NPG Asia Materials (2018) 10: 318-327 320 Fig. 2 a, b Simulated near-field distribution at 0.12 THz in the center of devices without and with paired fingers, respectively. c, d Respective spectral profiles of the designed devices without and with paired fingers. e Comparison of photocurrent response between the fabricated devices without and with contact fingers measured at 0.12 THz and a power density of 0.6 mW/cm2 Simulations paired contacts or by shortening the channel length/ Because the initial motivation of our work was to antenna gap, both of which aim to enhance the dipole employ the electromagnetic size effect to enhance THz oscillation and capacitive coupling across the gap25. Fig- absorption, we performed a full-vector