A flexible graphene terahertz detector

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We present a flexible terahertz (THz) detector based on a graphene field-effect transistor fabricated on a plastic substrate.At room temperature, this detector reveals voltage responsivity above 2 V/W and estimated noise equivalent power (NEP) below 3 nW/ ffiffiffiffiffiffi Hz p at 487 GHz.We have investigated the effects of bending strain on DC characteristics, voltage responsivity, and NEP of the detector, and the results reveal its robust performance.Our findings have shown that graphene is a promising material for the development of THz flexible technology.V C 2017 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4993434]There is increasing interest for terahertz (THz) technology in various applications, such as information and communications, surveillance and security screening, biomedical and material diagnostics, and global environmental monitoring. 1,2Over the past few decades, many breakthroughs in high-power sources, 3 room-temperature detectors, 4,5 and real-time imaging 6,7 have pushed the THz research into future novel applications with significant potential, such as wearable smart electronics and multiple transmit and receive antenna systems (MIMO).However, many of these applications call for flexible, portable, and less expensive solutions compared to existing solid-state technologies.
The advancements in polymer technology have promoted flexible electronics and enabled the fabrication of high-frequency devices on flexible substrates for lightweight, low-cost, and shape-conforming applications.In the radio frequency range, flexible sensing/communication devices have been embedded into clothing or other fabrics. 8,9Thus far, however, only one flexible THz device has been reported: a scanner based on carbon nanotubes. 10 flexible THz detector can be a key element for future niche applications, for instance, bendable THz electronics for high-speed indoor wireless communication and rfenergy harvesting and wearable THz sensors for medical applications.The quest for a material system and the device concept that allow flexible THz electronics is still a great challenge.
As the first real two-dimensional material, graphene possesses extreme thinness and the ability to reversibly undergo high strains and strain rates. 11,12With respect to preparation, graphene can be grown over large areas by chemical vapor deposition (CVD) and subsequently transferred to any substrate.Moreover, graphene, which possesses a high intrinsic carrier mobility, high conductivity, and gapless spectrum, has potential for developing detectors, 13,14 sources, 15 and modulators 16 in the THz range.
Here, we report a flexible THz detector based on an antenna-coupled graphene field-effect transistor (GFET) and demonstrate room-temperature THz detection from 330 GHz to 500 GHz.Additionally, the effect of bending strain on the detector performance is also investigated and analyzed.
The detector was fabricated on a flexible and transparent polyethylene terephthalate (PET) substrate with a dielectric constant of 2.6 (see the supplementary material for detail fabrication processes).Figure 1(a) presents images of the detector with a gate length and channel width of 5.5 lm and 4 lm, respectively.The access area gaps between the gate and source/drain are 0.5 lm.The THz power is received by a split bow-tie antenna, which provides an asymmetric coupling condition between the source and drain. 13The gate capacitance per unit area (C G ) is 6.6 fF/lm 2 , as extracted from the S-parameters.
The detector was characterized using the experimental setup shown in Fig. 1(c) (supplementary material).Since the layered structure of the detector is considerably thinner than that of the substrate, the following simple approximation can be used for the tensile strain in the graphene film: 17 e ¼ t=2r, where t is the thickness of the PET substrate and r is the radius of curvature.The detector was operated in the cold mode, i.e., V ds ¼ 0, to minimize 1/f noise and maximize sensitivity. 18The THz voltage responsivity was calculated as where the factor 2 is due to the peak-to-peak amplitude and the factor ffiffi ffi 2 p originates from the lock-in amplifier rms amplitude.P and DU are the total available THz beam power and the rectified THz voltage of the detector, respectively (see the supplementary material for detail measurements).positive values of V GS .For the analysis, we assume that graphene transport properties are dominated by Coulomb scattering and that the mobility does not depend on the concentration of the charge carriers. 19This assumption allows for extraction of the electron/hole mobility (l e=h ) and residual carrier concentration (n 0 ) by fitting a commonly used semi-empirical model 20 to the measured conductance data in Fig. 2(a).The solid line in Fig. 2(a) represents the fitting results at e ¼ 0%.According to our calculations, the average values of l and n 0 with different strains are 2700 cm 2 /Vs and 1.2 Â 10 12 cm -2 , respectively, and do not reveal a clear dependence on the strain.The latter can be explained by the fact that the deviations of the mobility with strain in the studied range are less than the uncertainty produced by the fitting model. 21s shown in Fig. 2(a), the V Dir of the detector increases with the strain, which is in qualitative agreement with the results presented in Ref. 22

FIG. 2. Detector Characterization. (a)
The conductance of the detector as a function of V GS at V DS ¼ 0.5 mV with different strains of 0%, 0.875%, and 1.25%.The solid line is the fitting results without strain.(b) The measured R v as a function of frequency at V GS ¼ 1:4 V without strain.The black square markers are the experimental data, and the dashed line is a fit to the data.(c) The measured R v as a function of V GS at 487 GHz with different strains of 0%, 0.875%, and 1.25%.The solid line is the modelled results without strain.(d) The estimated NEP as a function of V GS at 487 GHz with different strains of 0%, 0.875%, and 1.25%.(e) The measured THz voltage response of the detector as a function of the available THz power at 487 GHz with V GS ¼ 1:4 V and without strain.
at the graphene-dielectric interface as the substrate is bending. 23In particular, it was shown that the stress is responsible for the change in the trap activation energy level. 24igure 2(b) shows the voltage responsivity versus frequency in the entire available frequency range measured at V GS ¼ 1.4 V and without bending (the data with strains are shown in the supplementary material).As shown, the voltage responsivity generally increases with the frequency up to approximately 2 V/W.According to our experiments, the deviations around the average value are very reproducible; hence, they can be associated with the standing waves in the optical setup. 25][27][28] Figure 2(c) shows the measured voltage responsivity as a function of the gate voltage at 487 GHz with different strains.As shown in this figure, the maximum value of the responsivity decreases with the strain.An empirical nonlinear equivalent circuit model predicts a second-order nonlinear response in the GFET-based THz detectors when an oscillating THz field is applied between the gate and source such that 28 where r is the channel conductivity.It can be shown that the both overdamped plasma model and thermoelectric model arrive at a similar V GS dependence. 14,29The second derivative of the channel conductivity in the form of q el e=h provides the maximum of the responsivity as As shown in Eq. ( 3), a decrease in the dielectric constants due to the out-of-plane compressive strain caused by bending should result in a decrease in the responsivity.Additionally, in accordance with the model of screening of the charged impurity potential, one can expect an increase in n 0 with a decrease in the dielectric constant because of the reduced screening effect.Indeed, the model provides the following relationship: 30 where C 0 is the voltage fluctuation, r s ¼ e 2 =ðk h F Þ is the effective fine structure constant, F is the Fermi velocity, and k is the dielectric constant.Therefore, the bending strain should result in an increase in n 0 due to the lower dielectric constant and hence a decrease in the voltage response.Thus, the reduction in the voltage response with bending strain observed in our experiments [see Fig. 2(c)] is consistent with the empirical nonlinear model.The solid line in Fig. 2(c) represents the simulation results at e ¼ 0% based on the model.As the gate voltage increases, the measured responsivity varies from negative to positive, which is consistent with the modelled responsivity.7]31 The noise equivalent power (NEP) of the detector with zero bias is dominated by thermal Johnson-Nyquist noise and can be calculated as where k B is the Boltzmann constant and T ¼ 300 K is the room temperature.Figure 2 Figure 2(e) shows that the measured THz voltage response of the detector at the given V GS ¼ 1.4 V is linear with the incident THz power.The available output power from the rf-source was not able to drive the detector into saturation.
In summary, we have presented a flexible THz detector based on an antenna-coupled GFET for the development of novel THz devices.The THz responses of the detector with different strains and different frequencies were measured.The detector without bending offers THz voltage responsivity above 2 V/W and an NEP below 3 nW/ ffiffiffiffiffiffi Hz p at 487 GHz for room-temperature operation.Based on the bending tests, the responsivity only has a small reduction with increasing strain, which demonstrates the robustness of the detector.Furthermore, this work provides an important route towards high-performance and low-cost THz flexible technology.
See supplementary material for device fabrication and the experimental setup.

Figure 2 (
a) shows the conductance of the detector versus the gate-source voltage (V GS ) with different bending strains of e ¼ 0%, 0.875%, and 1.25%.The conductance is calculated as G ¼ I DS /V DS , where I DS and V DS are the drain current and the drain-source voltage, respectively, from the transfer characteristics measured at V DS ¼ 0.5 mV.As shown in this figure, the Dirac voltages (V Dir ) of the detector are located at positive values of V GS , indicating that the graphene channel is p-type.The asymmetric conductance curves are due to the additional resistance produced by the p-n junctions between the n-type gated channel and the p-type ungated regions at a) Author to whom correspondence should be addressed: xinxiny@chalmers.se.0003-6951/2017/111(2)/021102/4 V C Author(s) 2017.111, 021102-1 FIG. 1. Flexible GFET-THz detector and experimental setup.(a) Optical microscopy images of the detector.(b) Schematic of the bent detector.(c) Experimental setup for THz characterization.The figure is only for reference, not in the actual size.(d) Photographs of the detector under test with a bending radius of r ¼ 7 mm, which corresponds to a strain of e ¼ 1:25%.The inset shows the corresponding side view.
(d)shows the estimated NEP of the detector as a function of V GS with different strains.