Semimetallic Graphene for IR Sensing

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Surfaces, Interfaces, and Applications

Semimetallic Graphene for IR Sensing Hamza Zad Gul, Wonkil Sakong, Hyunjin Ji, Jorge Torres, Hojoon Yi, Mohan Kumar Ghimire, Jung Hyun Yoon, Min Hee Yun, Ha Ryong Hwang, Young Hee Lee, and Seong Chu Lim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00977 • Publication Date (Web): 02 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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ACS Applied Materials & Interfaces

Semimetallic Graphene for IR Sensing Hamza Zad Gul 1, Wonkil Sakong 1 , Hyunjin Ji 1, Jorge Torres 3, Hojoon Yi 1,2, Mohan Kumar Ghimire 1,2, Jung Hyun Yoon 4, Min Hee Yun 3, Ha Ryong Hwang 4, Young Hee Lee1,2, Seong Chu Lim 1,2,*

1Department 2Center

of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea

for Integrated Nanostructure Physics, Institute for Basic Science, Sungkyunkwan

University, Suwon 16419, Republic of Korea 3Department

of Electrical and Computer Engineering, University of Pittsburgh, Pittsburgh, PA

15261, USA 4WISE

Control Inc., R&D Division, 199, Sanggal-dong, Giheung-gu, Youngin-si, Gyeonggi-do,

17097, Republic of Korea

KEYWORDS: IR detection, Room-temperature operation, Semimetal, Thermoelectric, Layered materials.

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ABSTRACT: Both photothermal and photovoltaic IR detectors employ sensing materials that have an optical band gap. Different from these conventional materials, graphene has a conical band structure that imposes the zero bandgap. In this study, using the semimetallic multilayer graphene (MLG), IR detection at room temperature is realized. The relative high Seebeck coefficient, ranging 40~ 60 V/K, comparing to that of the metal and large optical absorption in mid-IR, ranging the wavelength of 7 to 17 m, benefit the graphene for the detection of IR without an absorber, which is essential for most IR detectors because the band gap of the sensing materials is much larger than the energy of IR and the incident IR can be absorbed directly by sensing material. The developed detector with SiN membrane shows the high responsivity and detectivity, which are 140 V/W and 5 x 108 cm∙Hz1/2/W at 5 Hz, respectively. In addition, the IR sensor shows a response time of 600 sec. In the room temperature operation of IR sensor array without cooling, our sensors detect IR emitted from a human body and track the movement. The availability of large-area graphene in current technology opens new applications for metallic 2D materials and a possibility for scale up.


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Introduction: In contrast to visible light which can be detected by measuring photoelectric current generated from interband transition subject to the band gap of materials, detecting photocurrent in the midIR light ranging from 7 to 14 m at room temperature is scientifically challenging because the generated photocarriers by infrared (IR) source are overwhelmed by the thermalized carriers at room temperature, i. e., the signal is dominated by the thermal noise 1. To circumvent this difficulty, vanadium oxide, showing insulator-to-metal phase-transition material near 330K, was employed to sense IR light 2, which exhibits large resistance change due to the generated heat from an absorber. Polycrystalline Si on SiN membrane coupled to Al electrode exhibited thermoelectric power upon IR irradiation 3. Although both approaches facilitate roomtemperature IR detection, one serious drawback is inhomogeneous optical spectral response due to the limited absorption peak within mid-IR range, let alone high detectivity of the devices. In most applications, the materials that sense the incoming IR are semiconductors, converting the incident IR energy to the electrical current or the heat 4 5. In this regard, graphene is not suitable material for the detection of the IR under the similar detection mechanism because it is semimetal without a bandgap. Nevertheless, graphene exhibits various unique materials properties, which is not easily accessible from conventional materials. For instance, the graphene has no band gap, but the interband transition of graphene is feasible with the light of the energy

E  h > 2EF 6. Even with the interband transition of the semimetallic graphene with the light h  2 EF , no photocurrent is detected because optically excited electrons and holes are

recombined so rapidly 7. Thus, the separation of optically excited carriers, as a means to detect the incident light, was realized by forming metal-graphene junction with work function


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mismatch8-9 10 11-12, graphene p-n junction13 , pyroelectric detector14 and two graphene layers apart by a thin insulator, segregating electron and hole carriers into different layers15. These approaches are still limited in optical range and pushing the detection limit to mid-IR range inevitably faces the thermal noise issue. Therefore, photoelectric approaches to detect a wide range of mid-IR with uniform responsivity and high detectivity without cooling are fundamentally limited. RESULT AND DISCUSSION: Utilization of thermoelectric effect could be an alternative approach. Different from VOx, the graphene has no band gap. The semi-metallicity benefits the graphene since the graphene can absorb a wide range of IR directly without IR absorber16 17 that is essential for VOx-based bolometer. Because of large bandgap of VOx, the direction absorption of IR is not possible. In addition to wide absorption spectrum, under a temperature gradient, the graphene can generate a thermoelectric voltage, V = S·ΔT, where S is the Seebeck coefficient of graphene and ΔT is the temperature difference. The absorption of long wavelength and the large absorption rate combined with thermoelectric properties of graphene are expected to benefit graphene IR detector (GIRD) with simple device structure without IR absorbing layer, which is not accessible from VOx and other types of IR absorbing materials 1 2 3. In addition to this, the simple device structure and the scalability of IR sensor using the graphene is expected to be a landmark in the commercialization of graphene. In the present study, we realized GIRDs on a 4-inch wafer using multilayer graphene (MLG) available from chemical vapor deposition (CVD). Dual roles of not only absorbing, but also detecting materials of incident IR are served by MLG. Such a simple structure is not available


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from the conventional IR detectors, which employ IR absorbing layer for improved detection. Without it, our device shows a responsivity and detectivity of 140 V/W and 5 x 108 cm. Hz1/2 /W, respectively at 5 Hz. The response time is approximately 600 sec. In this study, we demonstrate real applications that includes the motion tracking of human body (See Supplementary Videos 1-3) and the detection of CO2 gas at the concentration as low as 10 ppm. For the application of MLG for IR sensing, the material properties of CVD-grown MLG used in this study are found in the literature18. We transferred MLG on Si (500 m)/SiO2 (300 nm) substrate using wet process, patterned using oxygen plasma, metallized the device (See Supplementary Information 1). The device is configured with 2 four-terminal electrodes and a heater electrode for the characterization of thermoelectric properties of MLG. Using the device, the temperature coefficient of resistance (TCR, ) and Seebeck coefficient S of MLG are measured as a function of temperature, as shown in Fig. 1(a) and 1(b). Sheet resistance of MLG decreases with increasing the temperature, showing a semiconducting behavior in Fig. 1(a). Thus, the TCR of MLG is negative with the range of MLG = -3 x 10-4/K. Such a low MLG is due to weak electron-optical phonon scattering at room temperature 19. Such a low TCR of graphene is advantageous for application to thermoelectric devices because the output voltage is dominated by thermoelectric carriers rather than by the change in electrical conductivity. Since MLG is placed on SiO2 layer that contains charged puddles, the transport property of MLG is subject to the change. The effects of ionic impurities and surface optical phonons of the SiO2 substrate on the transport of carriers in the MLG are screened by the first few bottom layers. Therefore, the inner layers are expected to exhibit the intrinsic properties of the band overlap between the valence band and the conduction band. For fitting R-T curve in Fig 1 (a), 𝑛 ~ 𝜅𝐵𝑇


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∆

ln (1 + 𝑒 20 21.

2𝜅𝐵𝑇

) has been used, where Δ is the band overlap energy and kB is Boltzmann constant

The approximation holds the validity since the carrier mobility of graphene  is known to

be remain almost constant with the temperature range22. Hence, MLG is expected to exhibit a decrease in resistance as the temperature increases, as shown in Fig. 1(a). The band overlap energy of the MLG is estimated to be roughly 40 meV and is also subject to change to an external electric field23

24.

The fit in Fig. 1(a) indicates that the band overlap energy of our CVD-

grown MLG is approximately 59 meV. The Seebeck coefficient S both of SLG and MLG is also measured as shown in Fig. 1b (Supplementary Information 1). A positive Seebeck coefficient for both graphene implies hole as a majority carrier, which is owing to the doping by the adsorption of oxygen gases and moistures. The Seebeck coefficient is supposed to show a linear dependence on the temperature by Mott relation25. A non-linear behavior of S on the temperature is attributed to the scattering of carriers by grain boundaries, charged impurities, defects, and SiO2 substrate26 27. Furthermore, S of our graphene is approximately 20 - 30 μV/K at 300 K in Fig. 1b. In addition to measuring S from each MLG and SLG, we investigate the correlation of S to the resistance of MLG. The sheet resistance of MLG ranges about 90 Ω/

. In general, the

thermoelectric IR device has a long channel connected in series, resulting in a high device resistance. In this regard, in order to reduce the overall device resistance, we stack MLG. Figure 1(c) demonstrates the correlation of R vs. S as a function of the number of the MLG transfer. The R of MLG is 450 Ω at first transfer of MLG and reduces to 83 Ω at fifth one. Opposing to the R, S remains intact regardless of the transfer time. According to Mott’s relation, S is observed to be high in electrically resistive materials 28 , but the high electrical resistance contributes to the


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increase of thermal noises in devices 29. Since the same MLG is overlayered by itself, the electrical conductivity did not change, nor the S. Only R drops with increasing thickness. Therefore, increasing the electrical conductivity of MLG without scarifying S will enhance the device performance. In order to harness photothermoelectric (PTE) effect for IR detection, a device with MLG and Au electrodes are fabricated on double sided SiN/SiO2/Si/SiO2/SiN substrate. The device is thermally divided into Si and SiN membrane area by etching underlying Si substrate marked by a white dashed line, shown in the inset of Fig. 2(a). A pair of serpentine MLG and Au electrodes is placed on each Si substrate and SiN membrane. The temperature variations of each electrode are measured under constant and pulsed IR inputs. For the estimation of the temperature rise, the temperature-resistance (T-R) curve was measured in CCR near room temperature and the alteration of electrical resistance of MLG and Au electrode was probed. For both MLG and Au, a global illumination is applied on the device (See Supplementary Information 2 for the effect of contact area). Under a constant light input, MLG electrode on SiN membrane shows a higher temperature increase than the one on Si substrate in Fig. 2(a). The higher temperature rise inside SiN membrane is responsible for lower thermal conduction of SiN than Si substrate. Similar result is also observed from Au electrode in SiN membrane, as found in Fig. 2(b). On Si substrate, both MLG and Au electrode exhibits lower temperature increase. The thickness of Au, MLG, and SiN are 70nm, 4 nm, and 1 m, respectively. Much higher thermal mass of SiN is expected to rule the temperature change of Au and MLG. However, the significant difference in the temperature between MLG and Au electrode inside SiN is observed in Figs. 2(a) and 2(b). The results imply that, rather than the thermal contact to SiN substrate, the thermal conductivity


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and IR absorption properties of MLG and Au electrode are expected to attribute more to the temperature difference between MLG and Au electrode inside SiN membrane. The frequency dependence of the temperature changes ∆T of both Au and MLG has been further studied on SiN membrane. In a case of MLG, the temperature of MLG on SiN membrane is raised by 2.4 K and it remains constant through the frequency range, whereas Au electrode experiences the temperature increase ranging from 0.3 K to 1.5 K depending on the frequency of the pulsed incident IR. The larger increase of the temperature of MLG is largely due to the higher absorption rate of IR, whereas most IR is reflected on the surface of Au (See Supplementary Information 3 for IR absorption). As we increase the frequency of IR pulses, almost no variation is observed in ∆T of MLG. However, the dependence of ∆T on the frequency is quite different in a case of Au electrode. A power law dependence of ∆T on the frequency is well manifested for Au on Si and SiN substrate (more detail provided in supplementary information 4) In our device, the substance like SiN, MLG, and Au are very anisotropic such that the lateral heat conduction dominates the thermal properties of the device. In this regard, the heat diffusivity D

 C p

of each material is a crucial parameter.  is thermal conductivity and  is density and

Cp specific heat capacity. Because of the low thermal conductivity and large thermal mass, the frequency response in Fig. 2(c) and 2(d) are not resulting from SiN membrane. The high thermal conductivity and low thermal mass of MLG can give rise to the fast heating and cooling, whereas, in a case of Au, rather slower cooling is expected due to low thermal conductivity and high heat capacity. (heat diffusivity DAu = 60 x 10-6 m2/s, 30 DSiN = 8.65 x 10-6 m2/s 31 and DMLG = 6.5 x 10-4 m2/s 32) Therefore, the thermal response of SiN at high frequency is not possible.


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Thus, the temperature difference we observed from Fig. 2(c) and (d) is probably caused by the heat conduction of MLG and Au along the lateral direction. Under alternating IR inputs, if the thermal constant

τ=(L2  ρ  CP )/κ , where L is the length of the SiN membrane, is much shorter

than the pulse period T, the temperature would not change as the function of frequency increases, because the incident heat by IR is quickly dissipated by the thermal conduction during the offtime. In such a case, the temperature is constant regardless of the frequency. However, if the  is comparable or longer than pulse period, the complete cooling is not possible, but the heat accumulates during the off-time, leading to the temperature rise of the electrode. The  for serpentine MLG, serpentine Au and SiN (as shown in insert figure 2(a)) is calculated, through length and heat diffusivity, to be 9.76 msec, 100 msec and 509 msec respectively. We implant the thermoelectric behavior of MLG into SiN device for the detection of IR. Since the thermal voltage from a single MLG channel is weak, in order to amplify the thermoelectric voltage, we have fabricated a device as shown in Fig. 3. It shows that MLG channels are connected in series in electrical, but in parallel in thermal. As we mentioned in Fig. 1(c), for increasing the IR absorption and reduce the device resistance, the CVD-grown MLG was then transferred three times onto double-sided Si (400 m)/SiO2 (0.5 m)/SiN (1 m) for the fabrication of optical detectors (Figs. 3a and 3b). The graphene was then patterned using oxygen plasma (Fig. 3c), followed by the deposition of Cr (10 nm)/Au (60 nm) for the electrode (Fig. 3d). A free-standing low-stress SiN membrane (200-300 MPa) was adapted to implement a substantial temperature gradient in the device (Fig. 3e). The backside SiN layer was removed by the standard reactive ion etching (RIE) process using CF4 plasma, and a buried SiO2 layer was then removed using a standard 10:1 buffered oxide etch (BOE) to expose the bulk Si substrate. Deep trench RIE was used to remove the backside bulk Si with a thickness of 500 µm. A free-


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standing SiN membrane was finally obtained after etching of the frontside SiO2 by BOE (Fig. 3e). Figures 3f and 3g present optical images of the SiN before and after backside Si etching, respectively (Supplementary Information 5 for the device dimensions). In Fig. 3g, the long and narrow graphene channels are placed across from the free-standing SiN membrane (hot zone) to the Si substrate (cold zone) and connected in series through Au electrodes, as schematically shown in Fig. 3h. The fabrication on 4” wafer produces 630 devices shown in Fig. 3i. The result supports that the process with MLG is feasible for the further scale-up, which is not accessible from the other graphene photodetectors [27]. GIRD Application: The IR detectors in Fig. 3(i) are characterized and tested for the real applications. In Figure 4(a) and 4(b), we test the response time of our device. In general, the thermal response of the typical IR detector is affected by the thermal properties, heat capacity and thermal conductivity, of the IR absorbers. Since, in our device, we do not have the IR absorber, MLG, Au, and SiN are exposed to IR and give different thermal behavior depending on their thermal properties. Based on the device structure in Fig. 3(a), the estimated heat diffusion time of MLG, Au, and SiN are 753sec, 8.4msec and 29msec respectively for our device. In order to see, the thermal response from MLG, IR pulses with the frequency higher than 1.3 kHz should be applied to the sensor. However, the maximum frequency of IR output from the controller is 1 KHz. Due to this reason, a single IR pulse with on-time of 2msec is provided as an input and the development of output voltage is time-stamped as shown in Fig. 4(a). A timedomain plot of the output voltage of IR sensor demonstrates that nearly 1 msec after the thermal input, the voltage output of the sensor reaches about 2.25 mV and saturates. For reaching 80% of the saturation output voltage marked by an arrow in the figure, GIRD takes approximately 600 sec, which is quite similar to the heat diffusion time of MLG, 753sec. Such a fast response is


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only possible from MLG in our device. The response time of our device is further tested with much shorter pulses. In the inset of Fig. 4(a), 1 KHz pulses with duty cycle 0.1% is introduced. Thus, the pulse on-time is 10 sec. With such an extremely short on-time, the device is unable to keep up the thermal cycle of heating and cooling process of incident IR. During the irradiation of 1 sec, the output voltage, in an order of a few v range, is still distinguishable from the background noise level. In a test of low frequency range shown in Fig. 4(b), the responsivity of our IR sensor is dominated by the heat diffusivity of SiN, whose thermal time constant is approximately 29 msec. The responsivity of graphene detector reduces as the frequency increase. When the frequency reaches about 100 Hz, the responsivity Rresp drops from 140 V/W to 20 V/W

and the detectivity

D* 

Rresp  A NEP

1/2 reduces from 5.0 to 2.3 × 108 cm  Hz / W . A is the device area

and NEP is the noise equivalent power. It is learned that in Fig. 2 (c) and (d), the thermal response of both MLG and Au electrode is much faster than GIRD in Fig. 4(b). Although we observed the time constant of our device as 753sec due to the MLG, using the equation R = 𝑉 |𝑇𝐶𝑅| 2

3

∑𝑖 = 1

𝐴𝑖 (1 + (2𝜋𝑓𝜏𝑖)2

33,

where i indices MLG, Au, and SiN, V is voltage, f is frequency, A is

a free factor for fitting and  is a response time, we perform a fitting as shown in the inset Fig. 4(b). Owing to limitation of our IR source, the frequency-dependent responsivity is characterized up to 100 Hz in fig 4(b). Therefore, a fitting in high frequency region is not available. Our fit for the responsivity is in a close agreement with the observation. The delayed thermal response of the detector is attributed to the diffusion time constant of SiN membrane. The faster response of our detector shorter than 1 msec is expected to appear with the IR pulses at higher than 1 kHz. Our GIRD is tested in different environments. In particular, our sensor detects the motion of the human body (Supplementary video 1, 2, and 3). In particular, in the supplementary video 3, the


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presence of the human body at 10 m away from the sensor is well detected and even more, the movement of the body is traced and recognized using 5 × 5 GIRD array. The application of an array of GIRD indicates that the security measures can be boosted up by recognizing the intruder using small array of thermoelectric IR sensor that are much more inexpensive comparing to VOx bolometers (Supplementary Information 6). In addition to the motion detection, we applied GIRD for gas sensing. CO2 gases are known as greenhouse gas causing global warming. In this reason, its concentration in atmosphere is under constant monitoring. However, they are chemically inert so that detection of CO2 based on the chemical reaction resulting in charge transfer is not accessible at room temperature unless the catalytic reaction is incorporated with. The antisymmetric stretching mode of C=O has a resonant absorption band between 2000-2400 cm-1, which is corresponding to the wavelength of 4.3 m 34-35.

Therefore, the existence and concentration of CO2 in air can be probed by measuring the

intensity change of incident IR from constant IR source. For this experiment, the detection of CO2 using the GIRD is tested by measuring the difference in output voltage of the IR sensor in between N2 and CO2 at atmospheric pressures. Figure 4c presents the voltage output from GIRD in different ambient gas. The voltage in N2 environment reaches about 142 volt, but when the N2 is replaced with 200 ppm CO2 in N2 gas, it drops to 73 volt. From our measurement, it is estimated that the CO2 gas absorbs approximately 50% of the incident laser hence reducing the incident IR onto the sample by half. The detection limit of CO2 concentration is examined as shown in Figure 4(d). At the concentration > 600 ppm, the sensitivity of our CO2 sensor saturates. At this concentration, the output voltage drops below 1 volt. However, as the CO2 concentration reduces below 600 ppm, the output voltage from the sensor goes down with the concentration. When the CO2 concentration reaches to 10 ppm, a few percentages of output


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voltage decreases comparing the ambient condition. So, our sensor can be suitable to monitor the air quality indoor. Further improvement can be made for the lower detection limit by introducing more MLG channels and thinner SiN membrane. Low Frequency Noise: We also studied the noise property of IR detector. To characterize the low frequency noise (LNF) of the device (Supplementary Information 7), we used a homemade system including battery-powered DC voltage source and DAQ-4431 from National Instruments. The further descriptions on the system are available in elsewhere36. We modulated applied voltage VD from 0V to 15V in LF noise system, which gives rise to the current flow in the device ranging from ~10 nA to ~20 A. The resistance of IR graphene detector is around 700k. Figure 5(a) shows the drain current power spectral density (SID) vs. 1/f noise, indicating that 1/f behavior is well followed in our device. Our fit of the plots in Fig. 5(a) to SID~1/f between 5 Hz and 50 Hz reveals that  ranges from 0.8 to 1.3, as shown in Fig. 5(b). Thus, the thermally excited carriers in the IR detector experience the local carrier number and mobility fluctuations3738.

The perturbations in the flow of the carrier through the graphene and metal electrodes are

owing to the defect sites, grain boundaries, chemical ionic charges at the interface between graphene and SiN substrate, and dangle bonds at the graphene channel edge sties that traps and releases the carriers. The further analysis of the noise behavior of the device is expected to focus the control of the interface traps through chemical treatment that can suppress the trap site density39, the growth of the high crystalline MLG, passivation of channel edges, all of which should lead to enhance the signal to noise ratio (SNR) in the frequency range lower than 60 Hz. In summary, up to date, using the graphene, a various of photodetectors has been demonstrated. Monolayer graphene has exhibited excellent sensitivity, but most of application remains in the


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lab scale. Therefore, the feasibility for scale-up and real device application has been put into question. Different from the other graphene photo detector, our approach can be commercialized. What is important for our detector is that the MLG is a semimetal without band gap and it plays as a sensing material and the IR absorber. Thus, the device structure is much simpler than the established one. In order to enhance the performance, the control of the material properties of MLG using N doping that can display n-type behavior, whose combination with normal MLG is expected to increase the thermoelectric response at given temperature gradient in SiN membrane device. Experimental Section IR response of graphene detector: The IR response of the device was characterized by mounting the detector inside a cryo-chamber. Then, we pumped the chamber down to the pressure of 4× 10-6 Torr. The characterization was conducted at 300K. As IR sources, IR LED with the wavelength of 4.5m (QF4550CM1) and 9.6m (QD9550CM1) from Thor Labs were utilized and fixed on a positioner. The ITC4005 Laser Diode/Temperature controller modulates the electrical input to IR source. The controller also allows the control of duty cycle of the current pulse from 0.002% to 99.99%. Hence, the laser diode was run with the current pulse at 5Hz with the duty cycle of 1 %, corresponding to the pulse on-time of 2 msec. The emitted IR from LEDs was sent through Ge window of 5 cm in a diameter that was attached on the outside chamber wall. The gap between the IR source and the sample inside CCR was approximately 11 cm. The output voltage from the detector was fed into a low noise voltage preamplifier (Ametek N5113) that magnified the input voltage, followed by filtering and registering the signal using a SR830 lock-in amplifier from Stanford Research System Inc.


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CO2 Detection: For detection of CO2, we used, as an IR source, a diode laser with center wavelength of 4.55 m (Model number QF4550CM1), which is powered by ITC4005QCL controller. For the comparison, the chamber was pumped using mechanical pump to a few millitorr. Then, 99.999 purity N2 gases are purged into the chamber upto atmospheric pressure. Then, IR was shined on the detector and the output voltage was measured. Then, after the chamber was evacuated to millitorr, the chamber was filled again with CO2 gases at various concentration diluted in N2 gas. The beam path of the incident IR is approximately 10cm. The sensitivity of the detector to CO2 gas was estimated by comparing the output voltage in different ambient between N2 and CO2 gases. Temperature evaluation of SiN membrane: For a quantitative estimation of temperature increase of MLG and Au on Si and SiN substrate under IR illumination, firstly temperature coefficient of resistance (TCR) of MLG and Au on different substrate was measured. To evaluate TCR of them, the device was loaded into the same cryo-chamber. In order to measure the variation of the resistance upon temperature change, a current source (Keithley 6221) and nanovoltmeter (Keithley 2182A) was used for a four-terminal method. To MLG and Au, DC current was applied and the voltage variations were measured from 300 to 302K with a step size of 0.2 K. After registering the resistance-temperature correlation, at room temperature, we applied DC current on MLG and Au and shined them with IR with the wavelength of 9.6 um that was chopped. The periodic fluctuation of voltage from MLG and Au, due to temperature change by alternating IR source, is converted to resistance, which is further utilized to convert to estimate the temperature of the sample.


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FIGURES

Figure 1. (a) Resistance vs. temperature (R-T) and TCR plots of MLG and a curve fit to R-T plot with layer-to-layer interaction energy Δ=59 meV. (b) Thermoelectric power of SLG and MLG measured from 250 to 350 K. (c) Variation of thermopower (V/K) and resistance (R) of MLG depending on the number of transfer times.


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Figure 2. (a) Temperature variation ΔT of MLG on SiN and Si substrate as a function of time. (b) Temperature variation ΔT of Au electrode on SiN and Si substrate as a function of time. (c) Temperature variation ΔT of MLG on SiN and Si substrate as a function of frequency. (d) Temperature variation ΔT of Au electrode on SiN and Si substrate as a function of frequency.


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Figure 3. (a) Double-sided Si (400 m)/SiO2 (0.5 m)/ SiN (1 m) substrate. (b) Transfer of MLG. (c) Patterning of graphene channels. (d) Deposition of Au electrodes. (e) Si backside etching. Optical image of an optical detector (f) before and (g) after Si backside etching. (h) A schematic of Au-graphene arrays on SiN membrane. (i) 4-inch-wafer-scale fabrication of the IR detector


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Figure 4. (a) Response curve of IR detector taken with a single IR pulse of 2 msec. (Inset figure) The response curve of the sensor taken with IR pulses at 1 kHz with the on-time of 100 sec. (b) Responsivity and Detectivity of the detector as a function of frequency. The inset shows a fit to the responsivity. (c) The detection of CO2 at 200 ppm taken at 5 Hz. (d) Sensitivity curve of our IR detector as a function of CO2 concentration.


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Figure 5. (a) The drain current power spectral density vs. Frequency plots measured at various drain current. (b) The variations of  obtained from a power law fit of SID~1/f between 5 Hz and 50 Hz.


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AUTHOR INFORMATION Supplementary Information The supplementary information details the fabrication of graphene field effect transistor, Effect of contact resistance on photovoltage, IR absorption spectroscopy, Photothermal mechanism, SiN membrane structure, Comparison of different IR detectors and Low frequency measurements.

Corresponding Author *Corresponding author e-mail: [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the Institute for Basic Science (IBS-R011-D1).


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Table Of Content: 160 120

Output (volt)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dry air CO2 Frequency: 5Hz

80 40 0 0

250

500

750

1000

1250

Time (Sec)

The output voltage of Graphene IR detector when ambient gas is replaced with CO2 contained in N2 gas


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Semimetallic Graphene for IR Sensing

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