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Letter

Vol. 43, No. 8 / 15 April 2018 / Optics Letters

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Graphene-based broadband terahertz detector integrated with a square-spiral antenna WANLONG GUO,1,2,3 LIN WANG,1,* XIAOSHUANG CHEN,1,4 CHANGLONG LIU,1 WEIWEI TANG,1 CHENG GUO,1 JIN WANG,1 AND WEI LU1 1

State Key Laboratory of Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, 500 Yutian Road, Shanghai 200083, China 2 University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China 3 School of Physical Science and Technology, ShanghaiTech University, Shanghai 201210, China 4 e-mail: [email protected] *Corresponding author: [email protected] Received 19 December 2017; revised 4 March 2018; accepted 6 March 2018; posted 7 March 2018 (Doc. ID 315510); published 4 April 2018

Raising interest in terahertz radiation (loosely defined as the 0.1∼10 THz frequency range) for the applicationoriented issues in everyday life requires progressive development of fast, sensitive, and portable photodetectors. In this Letter, a broadband graphene-based terahertz detector with good integrability and sensitivity at room temperature is proposed. It is based on the chemical vapor depositedgrown graphene integrated with a square-spiral metal antenna which, on one hand, improves the efficiency for electromagnetic coupling and, on the other hand, facilitates the hot-electron photo-thermoelectric process for photodetection. Sensitivity over 28 V/W at room temperature and noise-equivalent power of less than 0.35 nW∕Hz0.5 are demonstrated in reference to the incident power. The presented results appealingly open an alternative way to realize chip-level graphene-based terahertz optoelectronics with good scalability and expected performance for targeted terahertz applications. © 2018 Optical Society of America OCIS

codes:

(300.6340)

Spectroscopy,

infrared;

(040.0040)

Detectors; (160.4236) Nanomaterials; (230.0250) Optoelectronics. https://doi.org/10.1364/OL.43.001647

Graphene, as an expressively charming material, has attracted great interest and been widely studied due to many intriguing properties such as the high carrier mobility, gapless spectrum, and massless Dirac fermions [1]. The advantages of this material enable its application in the areas of ultrafast and ultrasensitive detection of electromagnetic radiation in the visible, infrared, and even terahertz frequency ranges [2–6]. Owing to its increased intraband absorption and weak electron-phonon coupling, graphene is well suited for hot-electron photothermoelectric detection when extending to the terahertz band [7]. Furthermore, the little electronic heat capacity of graphene results in a larger change of temperature within the same absorbed energy case, contributing to a large photocurrent. Therefore, graphene is undisputedly one of the most promising 0146-9592/18/081647-04 Journal © 2018 Optical Society of America

materials for terahertz photodetectors from the viewpoint of high responsivity and large modulation frequency [6,8,9]. The critical process of a photodetector relies on the conversion of an absorbed optical signal into an electrical signal. To date, photodetection mechanisms in a typical graphene photodetector depend on the specific architecture and can be categorized into three main sorts: the photo-thermoelectric effect, the bolometric effect, and the plasma-wave-assisted mechanism [10–13]. In a plasma-wave-assisted terahertz detection process, the nonlinearity of the transfer characteristic rectifies the ac signal. The oscillating terahertz field is coupled asymmetrically between the source and the gate, leading to the longitudinal electric field along the channel following the so-called selfmixing theory first proposed by Dyakonov and Shur [14,15]. In a photo-thermoelectric process, terahertz radiation is absorbed by means of the intraband transition and then converts into the electronic heat in graphene [16,17]. Afterwards, the photocurrent gives rise following the Seebeck effect, which is facilitated by either the temperature gradient along the channel or the Seebeck coefficient difference enabled by the non-uniform carrier distribution [18]. Therefore, the photothermoelectric effect is remarkable in close to the interface at graphene (2D material)/metal contact, and its photo-response is proportional to the incident terahertz power [19]. In the bolometric case, it is associated with the change of transport conductance produced by the lattice heating [20]; instead of direct photocurrent generation, it requires externally applied bias [21,22] and can operate on material with high temperature coefficient of resistance. Here we introduce the design of thermoelectric photodetector in a convenient way by integrating only a square-spiral antenna under terahertz illumination [23]. Without using dissimilar metallic contacts, the detector can be finished in one step by placing the monolayer graphene stripe in departure from the center between two gold electrodes. Here the spiral antenna serves as both the coupler and the electrode, and the graphene channel is extended from the center to the third contact finger (see the inset of Fig. 1), leading to the

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Fig. 1. Schematic of the graphene-based terahertz detector integrated with a square-spiral antenna. The source is the left electrode connected to the right one, and the middle electrode is the drain electrode. The inset shows a zoomed image of the terahertz detector in the center of the square-spiral antenna.

asymmetrical construction. A significant thermoelectric effect can be observed across the device under terahertz excitation, and exhibiting a quasi-linear relationship under the applied bias. The devices demonstrated room-temperature responsivity over 28 V/W. During the fabrication, the monolayer graphene which is grown by a CVD method is transferred onto a highly intrinsic silicon (the resistivity ρ > 20,000 Ω · cm) wafer coated by a 300 nm SiO2 . The number of layers is confirmed by a combination of optical microscopy and Raman spectroscopy. The latter is also used to characterize the sample quality by measuring the D to G intensity ratio and the doping level. The square-spiral antenna is patterned via an ultraviolet lithography technique and finished after Cr/Au metallization to make an electrical connection with a graphene stripe which is defined by oxygen plasma etching. To realize the functionality of hotelectron thermoelectric terahertz detection, spatial asymmetry is endued by dislocating the graphene from the center of the antenna. The top view of the photodetector is shown in Fig. 1, from where it can be seen that the graphene channel is contacted by three metallic fingers causing the structural asymmetry. The graphene channel has length L of 9 μm, and the width W is 4 μm. The antennas are designed to concentrate the local terahertz field in the center upon the incident terahertz radiation. It is demonstrated that the terahertz radiation is focused at the center of the antenna and decreases gradually along axis x, generating a temperature gradient to one side due to the asymmetrical construction of the graphene and Au contact [8]. The Drude free carrier absorption of terahertz radiation in the graphene channel is strongest at the center of the antenna, where the terahertz field is also strongest, following the simulated results of Fig. 2. The absorption becomes much weaker at the part of the channel that is not located at the center, which facilitates the hot carrier diffusion along the graphene channel. Each antenna block is 100 μm × 100 μm in size so that more terahertz photons at different frequencies can be funneled into the center, as displayed in Fig. 2. Owing to the symmetrical broken, a temperature gradient, rather than the Seebeck coefficient difference is given along the graphene channel when terahertz radiation is coupled with the graphene channel, which produces thermoelectric photoR voltaic response V  − S · ∇T xdx  −S · ΔT , as shown in Fig. 3(a) [18].

Letter

Fig. 2. (a)–(d) show the incident power onto the active channel simulated by the conventional FDTD method at 26.9, 100, 140, and 300 GHz, respectively.

The I–V characteristics of our device is measured by using a Keithley 4200 parameter analyzer with the bias voltage swept from −0.5 to 0.5 V, and the resistance is determined to be 2 kHz for the studied device. After that, in order to calibrate the broadband nature of the designed device, it is wire-bonded and illuminated by a linearly polarized microwave source (Agilent E8257D) with a frequency tunable from 20 to 40 GHz, a VDI tripler with a frequency tunable from 80 to 120 GHz, and Gunns oscillators operate at 140 and 300 GHz, respectively. The electromagnetic radiation is modulated from 100 Hz to 40 kHz by using a chopper, and the photocurrent is recorded using a current pre-amplifier (SR570) connected to a lock-in amplifier (SR830). The device is exposed to the terahertz beam with a power density tuned from 0.5 mW∕cm2 up to 2 mW∕cm2 . The received power at every electromagnetic frequency is calibrated using a Golay cell, and the photocurrent spectra are normalized to this power to calculate the responsivity demonstrated in Fig. 3(b). All the experiments are performed in ambient environment at room temperature, and it is observed that a short-circuit thermoelectric photocurrent (I ph ) is successfully produced in response to the terahertz radiation. Figure 4(b) displays the polarization dependence of the photocurrent as observed by rotating the direction of terahertz electric polarization. Such polarization dependence indicates the anisotropic response in the (x) direction versus (y) direction. The origin of such anisotropy is attributed to the different absorption coefficients when the terahertz field is parallel to the arms of the antenna (x) and perpendicular (y) to the arms. It shows a factor of two changes for the photocurrent response when the polarization is rotated between these two directions. In Fig. 4(c), it can be seen that the modulation frequency dependence of the

Fig. 3. (a) Graphene photo-thermoelectric detector device structure and principle of operation. The left electrode is the source, and the right one is the drain. (b) Detection of terahertz radiation using a home-built experimental setup.

Vol. 43, No. 8 / 15 April 2018 / Optics Letters

Letter

Fig. 4. Room-temperature characterization. (a) Current–voltage characteristic of the device. (b) Bias voltage dependence of a detector with quadrate helical antenna. (c) Photo-thermoelectric response versus amplitude modulation frequency of electromagnetic wave. (d) Power dependence of the device, which exhibits good linear behavior.

photocurrent is almost flat at the range from 100 Hz to 30 kHz and drops abruptly beyond 30 kHz, as limited by the bandwidth of readout electronics. Figure 4(d) shows the photocurrent response versus the incident power, which exhibits perfect linear relationship, corroborating again the power dependent property of the photo-thermoelectric effect. Figure 5(a) depicts the spectral response of our photodetector at varying illumination frequencies from 80 to 120 GHz under the power intensity of 0.5 mW∕cm2 . The highest photo response appears at around 100 GHz being which is consistent with the simulation result. The smaller spectral response around 100 GHz is not consistent with the experiment, probably resulting from the reflection and diffraction due to the non-uniform plane wave terahertz radiation. As shown in Fig. 5(d), the voltage responsivity can reach as high as 28 V/W, when the electromagnetic frequency is in resonance with the structure, while it is still moderate over 8 V/W, even though the incident frequency is a departure from resonance. The responsivity here is defined as the photocurrent I ph divided by the incident radiation power P in, R I  I ph ∕P in ; or equivalently defined as photovoltage V pv divided by Pin, R V  I ph R∕P in  V pv ∕P in . In this detector, R V is extracted pffiffiffi via: from photo-induced voltage ΔU  2 2 × π4 × LIA G Sa ΔU R V  P in , P in  P total × S b , where the amplification factor of the transimpedance amplifier G  106 V∕A, S b is the radiation beam cross-sectional area, S b  2 cm2 . S a is the detector active area, S a  0.02 mm2 , and LIA is the lock-in signal.

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Fig. 5. (a) Spectral photocurrent response of the device from 80 to 120 GHz at the radiation power density of 1 mW∕cm2 ; the red line shows the result of FDTD simulation for the field intensity at the center of antenna. (b), (c) Temporal responses of a device chopped by an electronic modulator at 1 kHz under varying power density of the terahertz radiation: P 1  0.75 mW∕cm2 , P 2  1 mW∕cm2 , P 3  1.25 mW∕cm2 , and P 4  1.5 mW∕cm2 . (d) Electrical bias dependence of the responsivity at incident frequencies of 100, 140, and 300 GHz, respectively.

Loading effects due to the finite impedance of the preamplifier input are neglected. We assume that the entire power incident on the antenna is fully coupled to the graphene detector, meaning that the present R V up to 28 V/W is a lower limit. To estimate the sensitivity of our detector, the noise equivalent power is another important figure of merit which is usually characterizing the minimum detectable power of a photodetector when the signal-to-noise ratio is unity in a 1 Hz bandwidth. The electrical noise is dominated by the Johnson–Nyquist type pffiffiffiffiffiffiffiffiffiffiffiffiffiffi given by noise−equivalent power NEP  4k B TR ∕R V [24], where k B is the Boltzmann constant, T  300 K, R is the resistance of the device, and R V is responsivity. The NEP of this detector reaches a minimum level of 0.35 nW∕Hz0.5 [21]. The response time of this photodetector is characterized by measuring the time between 10% and 90% of the generated signal under modulated excitation intensity, either on the rising or the falling edge [22]. Usually the response time of the thermal detector is on the order of microseconds due to the slow relaxation time, but the discussed detector here demonstrated a short response time up to 9 μs, as exhibited in Fig. 5(b). In graphene, the electron-electron interaction is very fast and the electron-phonon interaction is slow, so electron temperature gradient forms in the active region before relaxation through crystal lattice, giving rise to a rapid terahertz response [16,25]. In this scenario, hot electrons result from

Table 1. Comparison of the Performance and Efficiency of this Work and the Earlier Works About Graphene Based Terahertz Detectors Materials Device Architecture Monolayer Graphene Square-Spiral Antenna Bilayer Graphene FET Graphene FET on Flexible Substrate Monolayer Graphene FET Monolayer Graphene Asymmetrical Electrodes Monolayer Graphene Split Gating

Spectral Range (Terahertz)

NEP (nW∕Hz0.5 )

Responsivity (V/W)

References

0.08 ∼ 0.12, 0.14, 0.3 0.29 ∼ 0.38 0.487 0.3 1.9 0.4

0.35 2 3 30 1.7 0.13

28 1.2 2 0.15 4.9 74

This Letter [6] [26] [15] [8] [21]

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ultrafast heating of carriers through strong electron-electron interactions and can maintain at a higher temperature than the lattice till relaxation by lattice phonons. In addition, thermal equilibration of hot electrons in graphene can sustain a timescale of up to nanoseconds. Therefore, the operation speed of the device is ultrafast compared with photovoltaic photodetectors. The photodetector with a short response time is an important parameter to define the merit of the photodetector, which is usually desired to allow for high-speed devices. Further, Fig. 5(d) shows the electrical bias dependence of responsivity at 100, 140, and 300 GHz, demonstrating that the detector allows for a multi-waveband response. The performance of the graphene Terahertz detector is also comparable or even superior to other works (see Table 1). In conclusion, we have achieved a fast photodetector response at a terahertz regime based on monolayer graphene, which is obtained by the CVD method and taking into consideration the contact effect. The device response is in excess of the former devices in which the photo-thermal effect dominates. A minimum NEP of 0.35 nW∕Hz0.5 and the responsivity over 28 V/W referenced to the incident power are achieved by integration with a square-spiral antenna, and the response time up to the standard value of 9 μs. On account of the roomtemperature operation, high sensitivity, and stability of the device, it can be predicted that this detector will be a suitable component of the future terahertz detector array for fast imaging. Funding. State Key Program for Basic Research of China (2017YFA0205801, 2017YFA0305500, 2013CB632705); National Natural Science Foundation of China (NSFC) (11334008, 61290301, 61521005, 61405230, 61675222); Youth Innovation Promotion Association (CAS); Aviation Science Fund (20162490001). REFERENCES 1. F. H. L. Koppens, T. Mueller, P. Avouris, A. C. Ferrari, M. S. Vitiello, and M. Polini, Nat. Nanotechnol. 9, 780 (2014). 2. P. Avouris and F. N. Xia, MRS Bull. 37, 1225 (2012). 3. H. Zhao, Q. S. Guo, F. N. Xia, and H. Wang, Nanophotonics 4, 128 (2015). 4. B. C. Deng, Q. S. Guo, C. Li, H. Z. Wang, X. Ling, D. B. Farmer, S. J. Han, J. Kong, and F. N. Xia, ACS Nano 10, 11172 (2016).

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