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Abstract
We employ turbostratic stacked chemical vapor deposition (CVD) graphene for a mid-wavelength infrared (MWIR) photodetector using the photogating effect. Turbostratic stacked CVD graphene was fabricated by multiple transfer processes. Graphene field effect transistor-based MWIR photodetectors were developed using an InSb substrate. The effect of the three layers of turbostratic stacked graphene enhanced both the field-effect mobility and MWIR response by approximately three times, compared to that of a conventional single-layer graphene photodetector in vacuum at 77 K. Our results may contribute to the realization of low-cost, mass-producible, high-responsivity graphene-based infrared sensors.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Graphene is composed of two-dimensional atomically thin layers of carbon atoms, and exhibits unique optoelectrical properties such as exceptional broadband photoresponse and significant carrier mobility [1]. Moreover, graphene can be synthesized more cost-effectively compared with typical multicomponent semiconductors by means of chemical vapor deposition (CVD) [2]. Consequently, this material can be used to construct low-cost [2] broadband photodetectors [3] with increased responsiveness [4–6], capable of operating in the terahertz-to-ultraviolet frequency range. An important issue limiting the practical application of graphene is its minimal optical absorbance (2.3% [3]). Previously proposed approaches for increasing its responsivity have employed dissimilar electrodes [5,7], p-n junctions[8,9], bolometers, [10] thermopiles [11], optical cavities [12,13], plasmonic resonance devices [14–16], dual graphene sheets with tunneling morphologies [17], nanoribbons [18], and photogates [19–28]. The use of photogates may be the most viable solution, because this technique yields the largest increase in responsivity, as well as a higher quantum efficiency [24]. In photogating devices, photosensitizers are placed close to the graphene channel, so that they undergo coupling with the incident light [24]. This coupling causes the channel gate voltage to change, which can produce extreme variations in the electrical signal and carrier density of graphene. The carrier mobility in graphene photodetectors that make use of photogating has been shown to correlate well with the responsivity of the device [22]. Consequently, it is essential to increase the carrier mobility to realize the maximum responsivity. The main factors determining mobility are the degree of carrier scattering in the graphene support (a possible source of decreased mobility) and the crystallinity of the graphene specimen. Although exfoliating large surface areas is challenging, graphene carrier mobility can be improved by mechanical exfoliation [1]. Other methods to reduce scattering within the substrate have been proposed, including fabrication of substrates from either hexagonal boron nitride (h-BN) [29] or suspended graphene [30,31]. However, these materials cannot be readily mass-produced. Conventional h-BN fabrication based on mechanical exfoliation cannot yield sufficiently large sections for practical applications. In addition, the fabrication of suspended graphene is complex, and typically results in inhomogeneous specimens. Building on these studies, we previously examined the viability of using turbostratic stacked graphene fabricated by CVD as a channel for transistors [32]. Figs. 1(a) and 1(b) show planar views of the structures of turbostratic stacked and AB-stacked graphene, respectively.
AB-stacked graphene is a stable natural crystal, and graphene multilayers with a highly ordered stacked AB-type structure can be highly conductive, exhibiting parallel conduction. However, similar to bulk graphite, stacked graphene exhibits nonlinear band dispersion, which limits carrier mobility [33,34]. In contrast, turbostratic stacked graphene does not exhibit lattice periodicity, because the layers are misaligned because of translation or rotation. The interlayer interaction of turbostratic graphene is weak because of its random arrangement [35,36]. Thus, multilayered turbostratic graphene may have similar properties to monolayer graphene and may exhibit linear band dispersion [37–39].
It has been demonstrated that transistors fabricated from turbostratic stacked graphene exhibit increased conductivity and greater carrier mobility compared with those incorporating CVD monolayer graphene [32]. CVD graphene is typically polycrystalline with randomly oriented grains [40], and turbostratic stacking structures can be readily generated via multiple transfers of graphene monolayers fabricated using CVD. The gap between graphene layers cannot be controlled in the wet-transferring process, but the interlayer distance is larger than that in AB stacking, where the crystal orientation is aligned. The larger interlayer distance weakens the interaction between the graphene layers [41], and thus, wet transfer is advantageous for fabricating turbostratic stacks. In addition, this technique allows the production of graphene materials with dimensions of several centimeters, which is sufficient to permit commercial-scale fabrication of electronic devices [42]. Therefore, turbostratic stacked graphene is expected to have applications in devices that rely on photogating for improved performance. An enhanced photogating effect at visible wavelengths has been previously demonstrated for photodetectors based on turbostratic stacked graphene [43]. Infrared plasmon enhancement by multilayer graphene nanoribbons has been reported previously [44,45]. However, applications of turbostratic stacked graphene as infrared detectors have not yet been reported. Other two-dimensional materials have been reported as sensors in the mid-infrared region, such as black phosphorus [46], black arsenic phosphorus [47,48], and PdSe2 [49]. However, they are unsuitable for conventional image sensor applications due to stability and large area application issues. In this study, we fabricated turbostratic stacked graphene photodetectors on InSb substrates operating in the mid-wavelength infrared (MWIR) region. These proof-on-concept photodetectors leveraging this photogating effect were fabricated to assess the feasibility of constructing low-cost, highly responsive graphene photodetectors.
2. Device fabrication
A schematic diagram of the fabrication process of the proposed MWIR photodetector is shown in Fig. 2.
Fig. 2. Schematic diagrams depicting the procedure used to fabricate MWIR turbostratic stacked graphene photodetectors on InSb substrates: (a) growth of TEOS-SiO2 layer on the InSb substrate, (b) transfer of first CVD graphene monolayer, (c) transfer of second CVD graphene monolayer, and (d) formation of Ni/Au electrodes and graphene channel. (e) Schematic diagram of the MWIR turbostratic stacked graphene photodetector with photogating employed in the present work.
In this process, the substrate was first coated with a 310-nm-thick tetraethyl orthosilicate (TEOS)-SiO2 layer using plasma CVD, as shown in Fig. 2(a). InSb was employed as the substrate material because its narrow bandgap corresponds to the MWIR region of the electromagnetic spectrum [22]. A graphene monolayer was subsequently synthesized on Cu via the CVD technique, and then transferred to the TEOS-SiO2 layer using poly(methyl methacrylate) (PMMA) (Fig. 2(b)) [50]. Following this, the specimen was heated at 803 K for 1 h under an Ar/H2 gas mixture flow to ensure that no residual PMMA or undesired substances remained on the surface. A second graphene layer was then fabricated on top of the initial graphene monolayer following the same procedure as described above, and these steps were repeated to construct the turbostratic stacked graphene structure (Fig. 2(c)). This technique is simple, inexpensive, and can be applied to commercial production. Following the formation of the graphene structure on the substrate, four terminal electrodes, each comprising a 15-nm-thick layer of Ni and a 30-nm-thick layer of Au, were added to the graphene surface using electron beam evaporation, followed by a lift-off technique (Fig. 2(d)). Standard photolithography combined with O2 etching was employed to fabricate a graphene channel with a width of 15 μm and length of 7 μm. In this manner, photodetectors incorporating turbostratic stacked graphene with variable graphene layers can be readily produced for experimental trials. Figure 2(e) presents a diagram of the fabricated MWIR turbostratic stacked graphene photodetector. Each of the three devices with different numbers of layers was annealed at 473 K under vacuum, and the Dirac point voltage was adjusted in the vicinity of the back-gate voltage Vbg = 30 V. CVD graphene is naturally doped by atmospheric moisture, oxygen, and SiO2 into a p-type material. In this work, these impurities were removed by annealing to adjust the doping concentration to an appropriate level [51]. This adjustment was required because undoped InSb was used as the substrate material, which meant that photogating to improve the infrared response was possible only at Vbg values greater than 30 V.
3. Results and discussion
3.1 Electrical properties and field-effect mobility
The performance of each device was assessed by measuring Vbg and the source–drain current, Isd. The values presented here were obtained based on four-terminal measurements, which were acquired using a four-point configuration in a Hall bar geometry, independent of the graphene/metal contact resistance values. The measurement data were acquired for each unit at a temperature of 77 K and pressure of 10−4 Pa in a vacuum-cooled chamber. The current data obtained from devices incorporating one, two, or three CVD graphene layers at a source–drain voltage Vsd of 1 V are plotted as functions of the back-gate voltage in Fig. 3(a). As noted in Section 2, the Dirac point voltage was adjusted by vacuum annealing to obtain a Vbg value of 30 V. A significant increase in the current values is evident as the number of graphene layers increases.
Fig. 3. (a) Experimentally determined gating response data obtained from photodetectors incorporating a monolayer of CVD graphene or two or three monolayers of turbostratic stacked graphene. (b) Calculated hole and electron maximum field-effect mobility values as functions of the number of graphene layers. (c) Calculated field-effect mobility as function of Vbg. (d) Temperature dependence of gating response for three-layer turbostratic stacked graphene; the inset shows an expanded view near the charge neutral point.
Figure 3(b) shows a plot of the maximum electron and hole field-effect mobility values, μFE, for the devices as functions of the number of graphene monolayers. Figure 3(c) shows a plot of μFE as a function of Vbg for each of the specimens containing one-, two-, and three graphene layers. These data were calculated using the standard formula [22]:
Here, L and W are the length (7 μm) and width (15 μm) of the graphene channel, respectively, COX is the capacitance of the insulator (calculated as εiε0/t, in which εi = 3.9 is the relative permittivity of TEOS-SiO2, ε0 is the vacuum permittivity, and t is the thickness of the insulating layer (310 nm)). It has been shown previously that the mobility of monolayer graphene exceeds that of multilayer graphene with orderly stacking fabricated via mechanical exfoliation [34,52]. In the present work, the μFE values increase significantly with increase in the number of monolayers in the device. The mobility obtained for three layers of turbostratic stacked graphene was found to be approximately three times that observed for a single monolayer. The pronounced increase in mobility is attributed to the lower degree of carrier scattering in this material, in addition to its linear band dispersion [32]. In a previous study, Coulomb scattering resulting from charge impurities (including inhomogeneous SiO2 surface charge) was found to significantly affect the characteristics of graphene [53–56]. In the present study, the underlying graphene monolayer reduced carrier scattering by TEOS-SiO2, leading to an increase in μFE. In this study, two-layer graphene showed only a slight increase in mobility compared to single-layer graphene, and three-layer graphene increased the mobility significantly compared to single or two-layer graphene. It has been reported previously that two-layer graphene exhibits a significant increase in mobility compared to single-layer graphene [43]. In previous studies, we have used thermally oxidized SiO2 as the underlying film of graphene [43]. In this study, TEOS-SiO2 is used as the underlying film, which has worse surface roughness and larger carrier scattering than thermally oxidized SiO2. Therefore, in the case of thermally oxidized SiO2, two-layer graphene was enough to suppress carrier scattering, but in the case of TEOS-SiO2, three-layer graphene was required to yield the same scattering suppression effect.
To study this phenomenon in detail, we investigated the temperature dependence of the properties of the three-layer turbostratic stacked graphene. Figure 3(d) shows the temperature dependence of the gating response for the three-layer turbostratic stacked graphene. The source–drain current changes in both the high electric field region and near the charge neutral point. The behavior in the high electric field region is similar to that of monolayer graphene [57], where the mobility is increased by cooling. Furthermore, near the charge neutral point, the behavior is similar to that of an AB-stacked bilayer graphene[57], and the conductivity decreases with cooling. It is known that the temperature change near the charge neutral point is not large for monolayer graphene [32,57], because of the Coulomb scattering in the substrate. In the three-layer turbostratic stacked graphene prepared in the present study, the temperature change was observed to occur near the charge neutral point, as in the case of AB-stacked graphene layers. This is a result of the suppression of the effect of Coulomb scattering in the substrate and the dominance of phonon scattering and thermal excitation of graphene, which are more affected by temperature variation. This result is important for infrared sensors, where thermal noise has a significant impact, and suppressing scattering by cooling can significantly increase the carrier mobility.
3.2 Infrared response
The infrared response of each device was assessed by monitoring Isd over time. A heated filament with an associated bandpass filter (3–5 μm transmittance) and an intensity of 13.2 mW/cm2 was employed as the light source. Light was irradiated to the devices from above with a duty ratio of 0.4 and a frequency of 0.5 Hz. The photocurrent values, Ip, were determined without amplification, by subtracting the Isd values obtained under dark conditions from those measured during exposure to the light source. The photoresponse characteristics of the device follow the same principles as in field effect transistors, and Ip can be obtained from the product of the conductance change, ΔG, and Vsd, as in [22]
Figure 4(a) shows the gated photocurrent as a function of the back-gate voltage. The photocurrent was significantly affected by the back-gate voltage and was clearly increased, when the value of Vbg was in the >30 V region, as the number of graphene layers increased. The effect of the back-gate voltage on the photocurrent in the pink region of this plot, i.e., at Vbg > 30, where the photogating effect occurs, was found to be correlated with the field-effect mobility in the same back-gate voltage region, as shown in Fig. 3(c). These results provide direct evidence that the photoresponses of the devices are enhanced by the photogating effect. The resulting Ip data are plotted against time for the three devices in Fig. 4(b). These data were acquired using a Vsd of 1 V and a Vbg value selected to obtain the maximum Ip. The devices containing one, two, and three graphene layers were found to have Ip values of 0.71, 0.85, and 1.94 μA at Vbg values of 40, 35, and 50 V, respectively, indicating that the incorporation of three turbostratic stacked graphene monolayers led to an approximately three-fold increase in the current. This result is attributed to the enhanced photogating effect due to the increased field-effect mobility of graphene. The underlying graphene in contact with the substrate suppresses the effects of carrier scattering from the substrate without diminishing the photogating effect. This is because the modulation of the gate voltage due to the photogating effect far exceeds the variation due to carrier scattering. The response time of these devices is at least lower than the measurement interval of 60 ms. The actual response time is determined by the photoresponse of the InSb photosensitizer and is expected to be a few nanoseconds.
Fig. 4. (a) Back-gate voltage as function of Ip for monolayer graphene and two-layer and three-layer turbostratic stacked graphene photodetectors. Vsd for all measurements was 1 V. The area of the plot shaded pink indicates the voltage range in which the photogating effect occurs. (b) MWIR responses of photodetectors incorporating a monolayer of graphene or two or three turbostratic stacked graphene layers.
Subsequently, the effect of Vsd and light intensity on the photoresponse of the three-layer turbostratic stacked graphene photodetector was assessed. Throughout all these tests, Vbg was maintained at 50 V. The responsivity, R, can be calculated using the equation
where P is the incident light intensity and A is the irradiated surface area calculated using the graphene channel area. Note that the light outside the channel was not considered. Specifically, the carrier diffusion length of InSb should be considered. However, it was not calculated in this study because the carrier lifetime of the InSb substrate has not yet been unambiguously determined. Figure 5(a) shows Ip and the responsivity as functions of the light intensity for the sample containing three-layer turbostratic stacked graphene, where Ip increases with the intensity of the incident light. The change in the slope of the photocurrent and responsivity at approximately 1 nW light intensity is worth noting. This result indicates that low-power incident light of less than 1 nW is highly responsive. This suggests that turbostratic stacked graphene infrared sensors based on photogating can be expected to have high dynamic ranges, similar to those of turbostratic stacked graphene photodetectors in the visible range [43]. Fig. 5(b) shows the dependence of Ip and the responsivity on Vsd for the sample with three-layer turbostratic stacked graphene. These data demonstrate that Ip is proportional to Vsd in the low Vsd region of Vsd ≤ 0.5 V, and that the responsivity of the device can be effectively tuned. Although Ip increases more slowly in the high Vsd region of Vsd > 0.5 V, the results show that the responsivity can be tuned using the bias voltage, which is advantageous for image sensor applications.Fig. 5. Ip and responsivity at Vbg = 50 V for three-layer turbostratic stacked graphene photodetector as function of (a) light intensity and (b) Vsd.
These characteristic infrared responses are consistent in trend with photoresponses obtained by harnessing the photogating effect [22,24,43]. Therefore, we conclude that turbostratic stacked graphene effectively generates a photogating effect in the MWIR region, which can be used to effectively increase MWIR photodetection responsivity. In addition, since infrared sensors are used in a cooled state, turbostratic stacked graphene—in which phonon scattering dominates because of the suppression of the effect of Coulomb scattering in the substrate—can be expected to be highly effective as a detector owing to increased mobility and noise reduction via cooling.
4. Conclusions
MWIR photodetectors based on turbostratic stacked CVD graphene employing the photogating effect were fabricated and characterized to assess the possibility of realizing high-responsivity devices. The performance in the MWIR region of the spectrum, resulting from the photogating effect, was found to increase with the field-effect mobility of graphene. A device composed of three layers of turbostratic stacked graphene exhibited an infrared response in the range of 3–5 μm and field-effect mobility approximately three times that of a monolayer graphene photodetector. Thus, if we select the appropriate photosensitizer, the same photogating effect can be obtained in the visible and infrared wavelength region. The increased responsivity observed in the present work can be attributed to the higher carrier mobility associated with linear band dispersion in turbostratic stacked graphene, together with the inhibition of the carrier scattering in the TEOS-SiO2 substrate. Although multilayer graphene has interface interactions and defects, turbostratic graphene can produce higher carrier mobility, which can lead to higher responsivity. Moreover, large-area turbostratic stacked graphene fabricated using CVD can be produced to realize improved responsivity when incorporated into photodetectors, and mass production based on the fabrication method we present is feasible. Furthermore, this material is expected to be applied to achieve the photogating effect in other regions of the electromagnetic spectrum, such as the long-wavelength infrared region, and for the development of graphene/InSb heterojunction photodetectors. Exploiting the effects of other 2D materials such as MoS2 may yield more flexible control of graphene plasmons [58,59]. Turbostratic graphene with 2D materials may contribute to such advanced functional photon control. Our results will aid in the future development of inexpensive, highly responsive, graphene-based infrared sensors that can be produced on a commercial scale.
Funding
Acquisition, Technology & Logistics Agency (JPJ004596).
Acknowledgments
Portions of this work were presented at the SPIE Defense + Security conferences in 2021 [60]. This work was supported by Innovative Science and Technology Initiative for Security Grant number JPJ004596, ATLA, Japan.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are all presented within this article.
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