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Abstract
This study presents high-responsivity graphene-based deep-ultraviolet (DUV) photodetectors using chemical vapor deposition (CVD)-hexagonal boron nitride (h-BN) photogating. To improve the DUV photoresponse, h-BN was used as a photosensitizer in graphene field-effect transistors (GFETs). The h-BN photosensitizers were synthesized using CVD and then transferred onto a SiO2/Si substrate. The behavior of h-BN irradiated with DUV light was investigated using cathodoluminescence and UV–VIS reflectance. Under 260 nm light, it exhibited a clear photoresponse with an ultrahigh responsivity of 19600 AW-1, which was 460% higher than a GFET device without h-BN photosensitizers. A noise equivalent power of 3.09×10−13 W/Hz1/2 was achieved.
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1. Introduction
Ultraviolet (UV) sensing is of interest in various fields, including scientific, commercial, civil, and military applications. In particular, the deep-UV (DUV) range of 190–350 nm has potential in chemical and biological sensing, smoke and fire detection, missile warning systems, combustion monitoring, ozone sensing, and sterilization [1,2]. DUV sensing uses both semiconductors and organic photodetectors. For example, Si-, AlGaN/GaN- [3–8], and SiC-based semiconductors [9–11], and oxide-based wide-bandgap crystal photodetectors [12–17] with responsivities of a few hundred mAW-1 are used for UV sensing with a high UV-radiation hardness. However, although Si is a common material in DUV photodetectors, Si-based detectors absorb both UV and visible light owing to their electronic band structures. Spectral selectivity, and particularly, visible blindness are essential properties for DUV sensing, and optical filters are applied to limit the transparent band to the DUV spectral region. However, these filters also reduce the intensity of the incident DUV light and the responsivity of detectors. Thus, there is a demand for high responsivity and spectral selectivity at DUV wavelengths. Moreover, a wider range of applications can be expected if high-performance DUV photodetectors are realized.
To address this challenge, we propose an ultrahigh-responsivity DUV photodetector using graphene-based photodetectors with a photogating effect. The photogating effect is a promising method for enhancing the responsivity of graphene photodetectors, compared with conventional methods that aim to increase the absorption efficiency, such as hetero-electrodes [18,19], p–n junctions [20–21], optical cavities [22,23], plasmonic metasurfaces [24,25], tunneling currents between parallel graphene sheets [26], nanoribbons [27,28], and carrier multiplication [29,30]. The photogating effect is induced by photosensitizers surrounding a graphene channel that couples incident light. Electrical changes in the photosensitizers produce large modulations in carrier density and electrical signals from the graphene. Therefore, by selecting an appropriate photosensitizer, a photogating effect can be achieved for an arbitrary range of wavelengths. We previously demonstrated enhancements in the responsivity of graphene photodetectors in the visible, middle-wavelength infrared, and long-wavelength infrared regions using Si [31,32], InSb [33–37], and LiNbO3 [38] as photosensitizers. Multilayer hexagonal boron nitride (h-BN) is an insulator with an indirect band gap of ∼5.95 eV, which corresponds to a DUV wavelength of 208 nm, and several devices based on h-BN or graphene/h-BN have been reported for 200–300 nm DUV photodetection [39–42]. However, no reports exist on techniques that drastically enhance the responsivity of these devices.
In this study, the chemical vapor deposition (CVD) of hexagonal boron nitride (h-BN) is used to produce photosensitizers and realize a photogating effect in the DUV spectral region. Multilayer h-BN is produced over a large area using CVD [43,44], which is beneficial for DUV imaging applications. Cathodoluminescence (CL) and the UV–VIS spectra of the h-BN layers are shown, which confirm the behavior of the h-BN photosensitizers in the DUV range. Furthermore, the Raman spectra of the photosensitizers are obtained, and they are used to image graphene field-effect transistors (GFETs). The DUV photoresponses of the devices with and without the h-BN photosensitizers are evaluated under 260 nm DUV light.
2. Method
2.1 Device fabrication
Figure 1 shows a schematic of the h-BN/graphene photodetector. In this device, 60 nm-thick h-BN, which corresponds to approximately 150 layers, was utilized as the DUV photosensitizer. The h-BN was synthesized by CVD using Fe–Ni alloy foil and then transferred onto a 290 nm-thick SiO2 insulator layer and a 625 µm-thick p-doped Si substrate with a resistivity of 1–20 Ω·cm [43–45]. Source/drain electrodes comprising 10 nm-thick Cr and 20 nm-thick Au layers, respectively, were deposited on the h-BN layers by sputtering. Graphene was synthesized by CVD and subsequently transferred onto the electrodes and h-BN layers. A graphene channel with a wide of 10 µm and length of 10 µm was formed by photolithography and O2 plasma etching. Prior to the electrical measurements, the device was calcined at 423 K for 10 min to remove the moisture and residue adsorbed by the graphene and h-BN layers. A device without h-BN photosensitizers was fabricated in the same manner to confirm the effect of h-BN.
Fig. 1. Schematic of the graphene/h-BN heterostructure of the DUV photodetector. CVD-grown multilayer h-BN and monolayer CVD graphene are transferred onto a SiO2/Si substrate. The graphene channel is connected to Cr/Au source/drain electrodes. Inset: optical microscopy image of the graphene/h-BN heterostructure. Scale bar: 5 µm.
2.2 Optical characterization
The optical properties of the h-BN layers in the DUV region were investigated using CL and UV–VIS measurements. The CL spectrum was measured using a spectrometer (MP-Micro-IRP, Horiba), and a cooled charge-coupled device (CCD) camera (DU420A-OE, Andor Technology) was attached to a field-emission scanning electron microscope (FE-SEM, SU-70, Hitachi) at 70 K. The reflectance spectrum was obtained using a UV–VIS measurement unit (V-770, Jasco) at a specular angle of 5°. The Raman spectra and images were obtained using a 532 nm excitation laser and Raman microscopy (Raman touch, Nanophoton).
2.3 Electric measurement
The electrical characteristics were investigated by assessing the source–drain voltage (Vsd) and back-gate voltage (Vbg). The measurements were performed in a vacuum chamber at 10−4 Pa (GRAIL10-415-4-LV-HT-OP, Nagase Techno-Engineering) and 298 K. The rear of the Si substrate was electrically connected to a vacuum chamber to apply Vbg. The current and applied voltage were measured using a device analyzer (B1500A, Keysight) with an integration time of 60 ms.
2.4 DUV photoresponse evaluation
The DUV photoresponse was also investigated. The change in Isd was measured when the device was irradiated by a 260 nm DUV light-emitting diode (LED, UV-M-0002R3, Nikkiso) under dark conditions. The photocurrent Iph = Ilight − Idark was calculated. The photoresponses were measured without amplification or noise reduction. The device was exposed to the 260 nm DUV LED light with a 2.0 s irradiation cycle and a duty ratio of 60% (1.2 s on; 0.8 s off).
3. Results and discussion
3.1 Optical properties of h-BN photosensitizers
Figure 2(a) shows the CL spectrum of the h-BN layers on the Si substrate. The h-BN layers exhibited DUV luminescence at approximately 220 nm, which corresponds to the band-end emission of h-BN monocrystals [46–49]. Figure 2(b) shows the UV–VIS reflectance at 5°. At 210 nm, the reflectance of the h-BN/Si specimen decreased to 42%, in contrast to 65% for the Si specimen without h-BN layers. The h-BN layers reduced the reflectance by 34%, as shown in Fig. 2(c). Moreover, the diffusion reflectance was negligible and had a value less than 2%. Therefore, the decrease in reflectance can be attributed to the absorption of DUV light by the h-BN layers. These results demonstrate that the h-BN layers possess photoelectric properties in the DUV band, which makes them useful for DUV photosensitizers.
Fig. 2. (a) Normalized CL spectrum of h-BN/Si. Inset: SEM image. Scale bar: 5 µm. (b) Reflectance of Si substrate with (black, solid) and without (red, dotted) h-BN layers. (c) Reflectance ratio of SI with and without h-BN/Si.
3.2 Raman imaging of graphene/h-BN DUV photodetectors
Figures 3(a) and 3(b) show the Raman spectra for the graphene/h-BN channel and high-purity h-BN crystals, respectively; these were obtained using an excitation laser with a wavelength of 512 nm. The spectrum for the graphene/h-BN channel shows characteristics typical of the monolayer graphene, including a G peak at 1587 cm-1 and a 2D peak at 2692 cm-1, which correspond to the bond stretching and second order breathing modes, respectively, of sp2 carbon atoms. Both spectra show peaks at 1365 cm-1, which corresponds to the in-plane vibrational mode of h-BN (E2g peak) [50]. The full width at half maximum (FWHM) of the E2g peak for the graphene/h-BN channel was 11.2 cm-1. By contrast, the FWHM of the E2g peak for the high-purity h-BN crystals was 9.0 cm-1 [46], which indicates that the h-BN layers maintained high crystallinity in our devices. Figure 3(c) shows a Raman mapping image of the h-BN layers at 1365 cm-1, and Fig. 3(d) shows the intensity ratio of the 2D and G peaks of graphene. These figures indicate that a monolayer-graphene/h-BN heterostructure was fabricated successfully.
Fig. 3. Raman spectra of (a) graphene/h-BN channel and (b) high-purity h-BN crystals. Inset in (a): light microscopy image of the graphene/h-BN channel. The Raman spectra were acquired from the region indicated by (a) the white dotted line and (b) the black broken line. Scale bar: 5 µm. (c) Raman image at 1365 cm-1 and (d) intensity ratio of 2700 cm-1 2D and 1580 cm-1 G peaks of graphene. Scale bar: 5 µm.
3.3 Gating response
Figure 4 summarizes the correlation between the source and drain current Isd and the field-effect (FE) mobility μFE calculated from the transconductance of the device. The Dirac point of graphene was obtained at a Vbg of −1.4 V with the lowest Isd. The maximum electron and hole μFE values were 812 and 444 cm2V-1s-1 at −5.8 and 10.9 V, respectively.
Fig. 4. Gating response of the device under dark conditions. The solid black line denotes the source–drain current at a source–drain voltage of 0.1 V, and the dotted red line denotes the FE mobility of the device. The numbers above zero on the FE mobility axis indicate electron mobility, and the numbers below zero indicate hole mobility.
3.4 DUV photoresponse
Figure 5 compares the DUV photoresponses of the devices with and without h-BN photosensitizers. The device with h-BN photosensitizers exhibited clear photoresponses and generated an Iph of 57.1 ± 7.7 nA, whereas the device without h-BN photosensitizers generated an Iph of 10.6 ± 2.1 nA. The modulation of Ibg correlates well with the irradiation by DUV light. The device with h-BN photosensitizers produced a DUV photoresponse that was 460% larger than that of the device without them. By contrast, the μFE value of the device with h-BN photosensitizers was low (approximately 591 cm2V-1s-1), whereas the μFE value of the device without them reached 8694 cm2V-1s-1. In addition, the photo noise of the device with h-BN photosensitizers was larger than that of the device without them. This was attributed to the roughness of the h-BN layers and the adhesion between graphene and h-BN. Further improvement of the properties of the h-BN layer and the CVD process is expected to enhance the μFE value and DUV photoresponse.
Fig. 5. DUV photoresponse of devices (a) with and (b) without h-BN photosensitizers under pulsed DUV light from an LED with a wavelength of 260 nm. The solid black line denotes the photocurrent, and the dotted red line denotes the gate current. Pulse periods: 1.2 s on; 0.8 s off.
The photogating effect was attributed to the gate voltage modulation of the GFETs. Figure 6 shows the schematic of the photogating effect occurring in the device. Because the proposed device follows the electrical characteristics of metal–insulator (oxide)-semiconductor FETs, the photoresponse is dependent on the GFET performance parameters [33]. The contribution of the h-BN photosensitizer toward the improvement in the DUV photoresponse was quantitatively evaluated by calculating the modulation of the gate voltage $\Delta {{V}_{\textrm{bg}}}$ using the following equation:
Fig. 6. Schematic of the photogating effect in the device under DUV light irradiation: (a) photocarrier behavior and ΔVbg in the device and (b) mechanism of photocurrent generation via the photogating effect.
Figure 7(a) shows the relationship between Iph and light intensity between 27.0 and 1.5 µW/cm2. As the light intensity increased, Iph decreased nonlinearly from 477 to 166 nA. The noise equivalent power, NEP, can be calculated using the following equation:
where Δf is the noise bandwidth, and SNR denotes the signal-to-noise ratio. The NEP reached a minimum of 3.09 × 10−13 W/Hz1/2, which is comparable to that of UV-enhanced avalanche photodetectors or amplified photodetectors. This high sensitivity to low-power light is a distinguishing and beneficial feature of graphene-based photodetectors using the photogating effect [52]. The response time was estimated to be 200–300 ms. The response time of the proposed device was affected by the direct carrier transfer from the h-BN photosensitizer to the graphene channel and the accumulation of generated photocarriers in the h-BN photosensitizers. The main mechanism of the DUV photoresponse is dependent on the FE modulation by the photocarrier generation in the h-BN layers, and the response time should be restricted by the carrier mobility in the photosensitizers. Improvements in the CVD process and the h-BN layers are expected to result in faster response times.Fig. 7. (a) DUV photoresponse characteristics of the proposed device under different light intensities. (b) Photo current and responsivity of devices with (solid black line) and without (dotted red line) h-BN layers.
Figure 7(b) shows the Iph and responsivity dependence of the devices with and without h-BN photosensitizers as a function of Vsd. The photoresponse of the device with h-BN photosensitizers increased linearly with Vsd, and the DUV responsivity reached a maximum of 19600 AW-1, which corresponds to a quantum efficiency of 9.3 × 106%. These values are higher than those of other graphene-based UV photodetectors, such as 8.0 × 103%@325 nm for a graphene/h-BN/GaAs sandwich diode [39], 3.6 × 104%@385 nm for a graphene/ZnO Schottky barrier diode [53], and 2.9 × 103%@210 nm for h-BN nanosheet DUV photodetectors [42]. These results also indicate that a sufficiently strong FE modulation is obtained when h-BN is used as a photosensitizer in the DUV region.
4. Conclusion
In conclusion, we demonstrated a graphene/h-BN heterostructure photodetector that incorporates a photogating effect to enhance the DUV photoresponse of graphene. Specifically, h-BN layers were utilized as the DUV photosensitizers in GFETs. The CL and reflectance properties indicate that the h-BN layers exhibit DUV photoelectric properties. The proposed device exhibited a clear DUV photoresponse under 260 nm light. The observed variations in the gate current suggest that the h-BN layers modulate the back-gate voltage applied to the graphene channel and produce a photogating effect. The DUV photoresponse of the proposed device was enhanced by 460%, compared with that of a GFET without h-BN photosensitizers. Further, the modulation of the back-gate voltage increased by more than 20 times when h-BN photosensitizers were introduced. The proposed device exhibited a minimum NEP and maximum responsivity of 3.09 × 10−13 W/Hz1/2 and 19600 A/W, respectively. The substrate in the graphene/h-BN heterojunction need not be limited to Si substrates; more flexible DUV devices can be produced using polymer substrates. The results of this study will help realize high-performance DUV sensors and image sensors for a wide range of applications.
Funding
Acquisition, Technology & Logistics Agency (JPJ004596); Japan Society for the Promotion of Science (21H05232, 21H05233).
Acknowledgments
The part of this work performed by Mitsubishi Electric Corp. was supported by Innovative Science and Technology Initiative for Security Grant Number JPJ004596, ATLA, Japan. HA acknowledges the JSPS KAKENHI Grant-in-Aid for Transformative Research Areas (A) “Science of 2.5 dimensional materials” (21H05232, 21H05233).
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are all presented within this article, and no data were generated or analyzed in the presented research.
References
1. M. Razeghi and A. Rogalski, “Semiconductor ultraviolet detectors,” J. Appl. Phys. 79(10), 7433–7473 (1996). [CrossRef]
2. E. Monroy, F. Omnes, and F. Calle, “Wide-bandgap semiconductor ultraviolet photodetectors,” Semicond. Sci. Technol. 18(4), R33–R51 (2003). [CrossRef]
3. O. Katz, V. Garber, B. Meyler, G. Bahir, and J. Salzman, “Gain mechanism in GaN Schottky ultraviolet detectors,” Appl. Phys. Lett. 79(10), 1417–1419 (2001). [CrossRef]
4. J. Li, Z. Y. Fan, R. Dahal, M. L. Nakarmi, J. Y. Lin, and H. X. Jiang, “200 nm deep ultraviolet photodetectors based on AlN,” Appl. Phys. Lett. 89(21), 213510 (2006). [CrossRef]
5. C. Huang, H. C. Zhang, and H. D. Sun, “Ultraviolet optoelectronic devices based on AlGaN-SiC platform: Towards monolithic photonics integration system,” Nano Energy 77, 105149 (2020). [CrossRef]
6. K. Jiang, X. Sun, Y. Chen, S. Zhang, J. Ben, Y. Chen, Z.-H. Zhang, Y. Jia, Z. Shi, and D. Li, “Three-dimensional metal–semiconductor–metal bipolar ultraviolet phototransistor based on GaN p-i-n epilayer,” Appl. Phys. Lett. 119(16), 161105 (2021). [CrossRef]
7. H. Zhang, F. Liang, K. Song, C. Xing, D. Wang, H. Yu, C. Huang, Y. Sun, L. Yang, X. Zhao, H. Sun, and S. Long, “Demonstration of AlGaN/GaN-based ultraviolet phototransistor with a record high responsivity over 3.6 × 107 A/W,” Appl. Phys. Lett. 118(24), 242105 (2021). [CrossRef]
8. R. N. Tan, Q. Cai, J. Wang, D. F. Pan, Z. Y. Li, and D. J. Chen, “Highly solar-blind ultraviolet selective metal-semiconductor-metal photodetector based on back-illuminated AlGaN heterostructure with integrated photonic crystal filter,” Appl. Phys. Lett. 118(14), 142105 (2021). [CrossRef]
9. X. P. Chen, H. L. Zhu, J. F. Cai, and Z. Y. Wu, “High-performance 4H-SiC-based ultraviolet p-i-n photodetector,” J. Appl. Phys. 102(2), 024505 (2007). [CrossRef]
10. G. Lioliou, M. C. Mazzillo, A. Sciuto, and A. M. Barnett, “Electrical and ultraviolet characterization of 4H-SiC Schottky photodiodes,” Opt. Express 23(17), 21657–21670 (2015). [CrossRef]
11. A. Aldalbahi, E. Li, M. Rivera, R. Velazquez, T. Altalhi, X. Peng, and P. X. Feng, “A new approach for fabrications of SiC based photodetectors,” Sci. Rep. 6(1), 23457 (2016). [CrossRef]
12. X. H. Xie, Z. Z. Zhang, C. X. Shan, H. Y. Chen, and D. Z. Shen, “Dual-color ultraviolet photodetector based on mixed-phase-MgZnO/i-MgO/p-Si double heterojunction,” Appl. Phys. Lett. 101(8), 081104 (2012). [CrossRef]
13. S. H. Tsai, S. Basu, C. Y. Huang, L. C. Hsu, Y. G. Lin, and R. H. Horng, “Deep-ultraviolet photodetectors based on epitaxial ZnGa2O4 thin films,” Sci. Rep. 8(1), 14056 (2018). [CrossRef]
14. B. R. Tak, M. M. Yang, Y. H. Lai, Y. H. Chu, M. Alexe, and R. Singh, “Photovoltaic and flexible deep ultraviolet wavelength detector based on novel beta-Ga2O3/muscovite heteroepitaxy,” Sci. Rep. 10(1), 16098 (2020). [CrossRef]
15. S. Kim and J. Kim, “Highly selective ozone-treated β-Ga2O3 solar-blind deep-UV photodetectors,” Appl. Phys. Lett. 117(26), 261101 (2020). [CrossRef]
16. X. Y. Sun, X. H. Chen, J. G. Hao, Z. P. Wang, Y. Xu, H. H. Gong, Y. J. Zhang, X. X. Yu, C. D. Zhang, F. F. Ren, S. L. Gu, R. Zhang, and J. D. Ye, “A self-powered solar-blind photodetector based on polyaniline/α-Ga2O3 p–n heterojunction,” Appl. Phys. Lett. 119(14), 141601 (2021). [CrossRef]
17. D. Y. Han, K. W. Liu, X. Chen, B. H. Li, T. Y. Zhai, L. Liu, and D. Z. Shen, “Performance enhancement of a self-powered solar-blind UV photodetector based on ZnGa2O4/Si heterojunction via interface pyroelectric effect,” Appl. Phys. Lett. 118(25), 251101 (2021). [CrossRef]
18. T. Mueller, F. N. A. Xia, and P. Avouris, “Graphene photodetectors for high-speed optical communications,” Nat. Photonics 4(5), 297–301 (2010). [CrossRef]
19. X. Cai, A. B. Sushkov, R. J. Suess, M. M. Jadidi, G. S. Jenkins, L. O. Nyakiti, R. L. Myers-Ward, S. Li, J. Yan, D. K. Gaskill, T. E. Murphy, H. D. Drew, and M. S. Fuhrer, “Sensitive room-temperature terahertz detection via the photothermoelectric effect in graphene,” Nat. Nanotechnol. 9(10), 814–819 (2014). [CrossRef]
20. N. Liu, H. Tian, G. Schwartz, J. B. Tok, T. L. Ren, and Z. Bao, “Large-area, transparent, and flexible infrared photodetector fabricated using P-N junctions formed by N-doping chemical vapor deposition grown graphene,” Nano Lett. 14(7), 3702–3708 (2014). [CrossRef]
21. M. Shimatani, S. Ogawa, D. Fujisawa, S. Okuda, Y. Kanai, T. Ono, and K. Matsumoto, “Photocurrent enhancement of graphene phototransistors using p–n junction formed by conventional photolithography process,” Jpn. J. Appl. Phys. 55(11), 110307 (2016). [CrossRef]
22. M. Furchi, A. Urich, A. Pospischil, G. Lilley, K. Unterrainer, H. Detz, P. Klang, A. M. Andrews, W. Schrenk, G. Strasser, and T. Mueller, “Microcavity-integrated graphene photodetector,” Nano Lett. 12(6), 2773–2777 (2012). [CrossRef]
23. M. Engel, M. Steiner, A. Lombardo, A. C. Ferrari, H. V. Löhneysen, P. Avouris, and R. Krupke, “Light–matter interaction in a microcavity-controlled graphene transistor,” Nat. Commun. 3(1), 906 (2012). [CrossRef]
24. Y. Yao, R. Shankar, P. Rauter, Y. Song, J. Kong, M. Loncar, and F. Capasso, “High-responsivity mid-infrared graphene detectors with antenna-enhanced photocarrier generation and collection,” Nano Lett. 14(7), 3749–3754 (2014). [CrossRef]
25. S. Ogawa, M. Shimatani, S. Fukushima, S. Okuda, and K. Matsumoto, “Graphene on metal-insulator-metal-based plasmonic metamaterials at infrared wavelengths,” Opt. Express 26(5), 5665 (2018). [CrossRef]
26. C. H. Liu, Y. C. Chang, T. B. Norris, and Z. Zhong, “Graphene photodetectors with ultra-broadband and high responsivity at room temperature,” Nat. Nanotechnol. 9(4), 273–278 (2014). [CrossRef]
27. M. Freitag, T. Low, L. Martin-Moreno, W. Zhu, F. Guinea, and P. Avouris, “Substrate-sensitive mid-infrared photoresponse in graphene,” ACS Nano 8(8), 8350–8356 (2014). [CrossRef]
28. X. Yu, Z. Dong, Y. Liu, T. Liu, J. Tao, Y. Zeng, J. K. W. Yang, and Q. J. Wang, “A high performance, visible to mid-infrared photodetector based on graphene nanoribbons passivated with HfO2,” Nanoscale 8(1), 327–332 (2016). [CrossRef]
29. B. Y. Zhang, T. Liu, B. Meng, X. Li, G. Liang, X. Hu, and Q. J. Wang, “Broadband high photoresponse from pure monolayer graphene photodetector,” Nat. Commun. 4(1), 1811 (2013). [CrossRef]
30. F. Wendler, A. Knorr, and E. Malic, “Carrier multiplication in graphene under Landau quantization,” Nat. Commun. 5(1), 3703 (2014). [CrossRef]
31. M. Shimatani, S. Ogawa, D. Fujisawa, S. Okuda, Y. Kanai, T. Ono, and K. Matsumoto, “Giant Dirac point shift of graphene phototransistors by doped silicon substrate current,” AIP Adv. 6(3), 035113 (2016). [CrossRef]
32. M. Shimatani, N. Yamada, S. Fukushima, S. Okuda, S. Ogawa, T. Ikuta, and K. Maehashi, “High-responsivity turbostratic stacked graphene photodetectors using enhanced photogating,” Appl. Phys. Express 12(12), 122010 (2019). [CrossRef]
33. S. Fukushima, M. Shimatani, S. Okuda, S. Ogawa, Y. Kanai, T. Ono, and K. Matsumoto, “High responsivity middle-wavelength infrared graphene photodetectors using photo-gating,” Appl. Phys. Lett. 113(6), 061102 (2018). [CrossRef]
34. S. Fukushima, M. Shimatani, S. Okuda, S. Ogawa, Y. Kanai, T. Ono, K. Inoue, and K. Matsumoto, “Low dark current and high-responsivity graphene mid-infrared photodetectors using amplification of injected photo-carriers by photo-gating,” Opt. Lett. 44(10), 2598–2601 (2019). [CrossRef]
35. S. Fukushima, M. Shimatani, S. Okuda, and S. Ogawa, “Carrier density modulation and photocarrier transportation of graphene/InSb heterojunction middle-wavelength infrared photodetectors,” Opt. Eng. 59(09), 097101 (2020). [CrossRef]
36. M. Shimatani, S. Fukushima, S. Okuda, and S. Ogawa, “High-performance graphene/InSb heterojunction photodetectors for high-resolution mid-infrared image sensors,” Appl. Phys. Lett. 117(17), 173102 (2020). [CrossRef]
37. S. Fukushima, M. Shimatani, S. Okuda, S. Ogawa, Y. Kanai, T. Ono, K. Inoue, and K. Matsumoto, “Photogating for small high-responsivity graphene middle-wavelength infrared photodetectors,” Opt. Eng. 59(03), 037101 (2020). [CrossRef]
38. M. Shimatani, S. Ogawa, S. Fukushima, S. Okuda, Y. Kanai, T. Ono, and K. Matsumoto, “Enhanced photogating via pyroelectric effect induced by insulator layer for high-responsivity long-wavelength infrared graphene-based photodetectors operating at room temperature,” Appl. Phys. Express 12(2), 025001 (2019). [CrossRef]
39. X. Li, S. Lin, X. Lin, Z. Xu, P. Wang, S. Zhang, H. Zhong, W. Xu, Z. Wu, and W. Fang, “Graphene/h-BN/GaAs sandwich diode as solar cell and photodetector,” Opt. Express 24(1), 134–145 (2016). [CrossRef]
40. A. F. Zhou, A. Aldalbahi, and P. Feng, “Vertical metal-semiconductor-metal deep UV photodetectors based on hexagonal boron nitride nanosheets prepared by laser plasma deposition,” Opt. Mater. Express 6(10), 3286–3292 (2016). [CrossRef]
41. J. E. Thompson, D. Smalley, and M. Ishigami, “Solar-blind ultraviolet photodetectors based on vertical graphene-hexagonal boron nitride heterostructures,” MRS Adv. 5(37-38), 1993–2002 (2020). [CrossRef]
42. S. Veeralingam, L. Durai, P. Yadav, and S. Badhulika, “Record-high responsivity and detectivity of a flexible deep-ultraviolet photodetector based on solid state-assisted synthesized hBN nanosheets,” ACS Appl. Electron. Mater. 3(3), 1162–1169 (2021). [CrossRef]
43. Y. Uchida, S. Nakandakari, K. Kawahara, S. Yamasaki, M. Mitsuhara, and H. Ago, “Controlled growth of large-area uniform multilayer hexagonal boron nitride as an effective 2D substrate,” ACS Nano 12(6), 6236–6244 (2018). [CrossRef]
44. Y. Uchida, K. Kawahara, S. Fukamachi, and H. Ago, “Chemical vapor deposition growth of uniform multilayer hexagonal boron nitride driven by structural transformation of a metal thin film,” ACS Appl. Electron. Mater. 2(10), 3270–3278 (2020). [CrossRef]
45. B. Hu, H. Ago, Y. Ito, K. Kawahara, M. Tsuji, E. Magome, K. Sumitani, N. Mizuta, K.-i. Ikeda, and S. Mizuno, “Epitaxial growth of large-area single-layer graphene over Cu(111)/sapphire by atmospheric pressure CVD,” Carbon 50(1), 57–65 (2012). [CrossRef]
46. Y. Kubota, K. Watanabe, O. Tsuda, and T. Taniguchi, “Deep ultraviolet light-emitting hexagonal boron nitride synthesized at atmospheric pressure,” Science 317(5840), 932–934 (2007). [CrossRef]
47. P. Jaffrennou, J. Barjon, J. S. Lauret, B. Attal-Tretout, F. Ducastelle, and A. Loiseau, “Origin of the excitonic recombinations in hexagonal boron nitride by spatially resolved cathodoluminescence spectroscopy,” J. Appl. Phys. 102(11), 116102 (2007). [CrossRef]
48. O. Tsuda, K. Watanabe, and T. Taniguchi, “Crystallization of hexagonal boron nitride exhibiting excitonic luminescence in the deep ultraviolet region at room temperature via thermal chemical vapor phase deposition,” Diamond Relat. Mater. 19(1), 83–90 (2010). [CrossRef]
49. K. Watanabe, T. Taniguchi, K. Miya, Y. Sato, K. Nakamura, T. Niiyama, and M. Taniguchi, “Hexagonal boron nitride as a new ultraviolet luminescent material and its application-Fluorescence properties of hBN single-crystal powder,” Diamond Relat. Mater. 20(5-6), 849–852 (2011). [CrossRef]
50. Y. Miyata, E. Maeda, K. Kamon, R. Kitaura, Y. Sasaki, S. Suzuki, and H. Shinohara, “Fabrication and characterization of graphene/hexagonal boron nitride hybrid sheets,” Appl. Phys. Express. 5(8), 085102 (2012). [CrossRef]
51. H. Jiang, C. Nie, J. Fu, L. Tang, J. Shen, F. Sun, J. Sun, M. Zhu, S. Feng, Y. Liu, H. Shi, and X. Wei, “Ultrasensitive and fast photoresponse in graphene/silicon-on-insulator hybrid structure by manipulating the photogating effect,” Nanophotonics 9(11), 3663–3672 (2020). [CrossRef]
52. S. Ogawa, M. Shimatani, S. Fukushima, S. Okuda, Y. Kanai, T. Ono, and K. Matsumoto, “Broadband photoresponse of graphene photodetector from visible to long-wavelength infrared wavelengths,” Opt. Eng. 58(02), 057106 (2019). [CrossRef]
53. B. Nie, J. G. Hu, L. B. Luo, C. Xie, L. H. Zeng, P. Lv, F. Z. Li, J. S. Jie, M. Feng, C. Y. Wu, Y. Q. Yu, and S. H. Yu, “Monolayer graphene film on ZnO nanorod array for high-performance Schottky junction ultraviolet photodetectors,” Small 9(17), 2872–2879 (2013). [CrossRef]
