Vacuum-Ultraviolet Photovoltaic Detector - ACS Publications

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Vacuum-Ultraviolet Photovoltaic Detector Wei Zheng, Richeng Lin, Junxue Ran, Zhaojun Zhang, Xu Ji, and Feng Huang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06633 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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Vacuum-Ultraviolet Photovoltaic Detector Wei Zheng,† Richeng Lin,† Junxue Ran,‡ Zhaojun Zhang,† Xu Ji,† Feng Huang †,* †

State key Laboratory of Optoelectronic Materials and Technologies, School of Materials, Sun Yat-Sen University, Guangzhou 510275, PR China ‡ Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China E-mail: [email protected] Abstract. Over the past two decades, solar- and astro-physicists and material scientists have been researching and developing new-generation semiconductor-based vacuum ultraviolet (VUV) detectors with low power consumption and small size for replacing traditional heavy and high-energy-consuming microchannel-detection-systems, to study the formation and evolution of stars. However, the most desirable semiconductor-based VUV photovoltaic detector capable of achieving zero-power-consumption, has not yet been achieved. With high-crystallinity multi-step epitaxial grown AlN as VUV-absorbing layer for photogenerated carriers, and p-type graphene (with unexpected VUV transmittance > 96%) as transparent electrode to collect excited-holes, we constructed a heterojunction device with photovoltaic detection for VUV light. The device exhibits an encouraging VUV photoresponse, high external-quantum-efficiency (EQE), and extremely fast tempera-response (80 ns, 104-106 times faster than that of the currently reported VUV photoconductive-devices). This work has provided an idea for developing zero-power-consumption and integrated VUV photovoltaic detectors with ultra-fast and high-sensitivity VUV detection capability, which not only allows future spacecraft to operate with longer service-time and lower launching-cost, but also ensures an ultra-fast evolution of interstellar objects.

Keywords: vacuum-ultraviolet, photovoltaic detector, AlN, graphene, high-sensitivity, ultrafast

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VUV (10-200 nm) photodetection is extensively employed in the fields of cosmic chemistry and space science, for example, it is used to study the expansion and elements of nebulas, and to monitor the formation and evolution of solar storm.1-3 At present, satellites and spaceexplorers mainly probe VUV by using the combination system of violet-chromatograph and microchannel-plate4,5, which is ponderous with huge size (increase the weight as well as the launching cost) and needs thousands of driving voltage (greatly increase the power consumption). Materials and devices scientists have been dedicating to find new solutions for solar and astro-physicists, and develop low-power-consumption, lightweight, and integrated VUV photodetectors based on ultra-wide band-gap semiconductors, such as AlN and diamonds.6-8 In the field of photoconductive-type VUV photodetectors, a series of state-of-the-art technologies and significant advances have been made over the past 20 years.6-10 In fact, compared to photoconductive-type ones, researchers are more likely to develop photovoltaictype VUV detectors for their zero power-consumption, charge signals linearly proportional to light intensity, and potential to achieve ultra-fast temporal-response and high-sensitivity. However, under current technical conditions, the preparation of semiconductor-based photovoltaic-type VUV detectors is facing an insurmountable bottleneck: a lack of transparent electrode with transmission to VUV light. Here, we have first experimentally confirmed that graphene has an up to 96% transmittance for VUV light, which is a major breakthrough in photovoltaic-type VUV detector fabrication. Furthermore, we have constructed a complex heterojunction device where p-type graphene (without affecting the VUV transmission properties) covers highcrystallinity AlN film. The device exhibits ultra-high sensitive VUV-spectral-selected photoresponse. Under nanowatts 180 nm VUV light irradiation, the device forms a 1.7 V open-circuit-voltage. In the absence of bias, the device produces photocurrent with a high external-quantum-efficiency (EQE) up to 42.6%. Under nanosecond VUV pulse irradiation, its response-time is only 80 ns. This is 10,000 to 1,000,000 times faster than that of the currently reported photoconductive-type VUV devices, which makes it possible to take images of the ultra-fast dynamic process in the celestial activity, for example, the early evolution of the original orbit can be recorded and the chemical-composition of the coronal jet in solar storm can be also probed. RESULTS AND DISCUSSION It is well known that a photovoltaic-detector must have a "sandwich" vertical structure, 2


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where the photosensitive layer is amid the upper and lower conductive layers, and the upper conductive layer is expected to be transparent to the probe light (as a window layer). Here, it is also necessary for the explored photovoltaic VUV detector with vertical-structure to have a VUV transparent conductive material, so that the upper window layer can cover the photosensitive layer and collect the photogenerated-carriers. Graphene (Gr), due to its quite high mobility at room temperature, is widely used as electrode. Meanwhile, as graphene has a single atomic layer and weak resistance to electromagnetic waves,11 we believe that it may have a high transmittance for VUV light. This assumption is further confirmed by VUV transmission measurement (Figure1). Typically, within the spectral range of 185-200 nm, the transmittance of graphene stands at 96%, almost as high as that of visible light. This result provides a theoretical basis for the realization of VUV photovoltaic device. We constructed a back-to-back p-Gr/AlN/p-GaN heterojunction device as shown in Figure 2a and 2b, by virtue of high VUV transmittance of graphene. This back-to-back structure has an obvious advantage that the thermal diffusion of both positive and negative carriers can be suppressed,12 thus enabling a lower noise-voltage-density, which facilitates the monitoring of extremely weak VUV signals. Figure 2d-f gives the SEM photographs, XRD pattern and Raman spectrum of the device, respectively, all indicating a high crystal quality of its fabricating materials.13 The absorbance layer AlN, as the core part of the device, has a clear symmetrical band edge emission centering at 215 nm (see Figure 2g), implying that the device has a VUV absorption selectivity. It is worth noting that, in order to obtain back-toback photovoltaic devices, here we intentionally performed p-doping on graphene using nitric acid steam (Figure 2c, see detailed description in the experimental method part).14 The I-V output characteristics of the device in dark condition (Figure 3) coincide with the designed back-to-back device structure, which reflect two distinct junctions: p-Gr/AlN junction with low reverse-breakdown-voltage and AlN/p-GaN junction with high reversebreakdown-voltage. The I-V curve can be divided into three characteristic parts according to different linear relations: I, p-Gr/AlN with forward bias, while AlN/p-GaN reverse bias, the device shows as current-off; II, AlN/p-GaN with forward bias, while p-Gr/AlN reverse bias, the device also appears as current-off; III, AlN/p-GaN with forward bias and p-Gr/AlN breakdown, the device is in the state of current-on. It is important to note that the electrode contacts (Au/Ti/p-Gr, In/p-GaN) are both ohmic contacts (see Figure S1-S2) without changing in the device's output characteristics. A series of VUV photodetection tests on the device were conducted. As shown in Figure 3


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4a, under 185 nm monochromatic light illumination, the device produces a 1.7 V electromotive-force difference between its two sides (open-circuit-voltage, see Figure 4b), where the electromotive force of p-Gr-side is higher than that of p-GaN-side. While under 250 nm light, the device does not have any open-circuit-voltage output, same with the situation under dark state. This result, on the one hand, indicates an obvious VUV spectralresponse selectivity (discussed in detail later) of the device, on the other hand, confirms a high crystallinity and high resistance of the AlN absorption layer. This layer neither absorbs the photons (with energy below the bandgap) to produce photogenerated-carriers, nor allows the carriers (even the additional photogenerated-carriers formed in GaN) to diffuse outward. The mechanism of the photovoltaic-effect in the device is shown in Figure 4c. A thick space-charge-region is formed in the AlN at heterojunction, which consists of p-Gr with high carrier-concentration and i-type AlN with low carrier-concentration. When VUV photons penetrate graphene into the AlN absorption layer, most of them will be absorbed in the spacecharge-region in AlN which near the p-Gr side, producing photogenerated electron-hole pairs. They are immediately separated by the built-in electric field (E) in the space-charge-region: the holes drift toward the p-Gr-side while the electrons drift to the opposite direction, and eventually a 1.7 V open-circuit-voltage is formed across the device after overcoming the internal-resistance to become a VUV sensor with photovoltaic-response characteristics. It is worth noting here that there is also a space-charge-region inside the AlN/p-GaN junction, which also includes a built-in electric field (E1). However, the E1 has an adverse effect on the separation of photogenerated-carriers comparing with E in p-Gr/AlN heterojunction. For the current device results, we believe that VUV photons are mainly absorbed in the p-Gr/AlN junction, while are barely absorbed by deeper AlN/p-GaN junction, because the VUV photons have shorter wavelength and can hardly penetrate 100 nm-AlN absorption layer.15 Therefore, for current device structure (with graphene as VUV window), the produced photovoltaic electromotive-force is mainly contributed to the p-Gr/AlN heterojunction. Figure 4d shows the comparative current-voltage (I-V) characteristics of the device illuminated under continuous 185 nm VUV light with a power of 2.87 µW (blue sphere) and under dark condition (black sphere). The results indicate a ~1.7 V open-circuit-voltage coinciding with the results of Figure 4b, and show ultra-high on/off ratios at different voltages. In addition, the device exhibits an ultra-low noise-voltage-density (Figure 4e), for it is almost covered by the background noise of the testing equipment, thus making it reach the expectation by designing the back-to-back structure. 4


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In the study of celestial evolution, ultra-fast temporal-response speed is highly desired for a VUV device, such as in the application of imaging explosion and expansion of stars. Here, we used a 193 nm nanosecond pulse to simulate the ultra-fast changing VUV signal source, and performed a temporal-response test on our photovoltaic devices, as shown in Figure 5a. Figure 5b shows the results of the temporal-dependent photovoltaic-response under illumination of 9 VUV pulses, which reproduces the optical pulse signal better. For a clearer analysis on the response-speed of the device, we stretched the time scale to make the rising edge of the pulse more visible in Figure 5c and Figure 5d. Surprisingly, the device’s response rise-time is only 80 ns, which is 104-106 times faster than that of the currently reported VUV probing devices (see Table 1). Obviously, the ultra-fast temporal-response of the device is mainly attributed to the following reasons: 1. The built-in electric field formed between the graphene and AlN absorption layer rapidly separates the photogenerated carriers; 2 The high crystal quality of the AlN absorption layer reduces the probability of defects trapping carriers; 3. The high mobility p-Gr offers a rapid collection and transmission of the photogenerated carriers (holes).16,17 It should be mentioned that there is a relatively long tailing of the device attenuation time at turn-off VUV light. This phenomenon is common in pulse response test,18 which depends not only on the speed at which the device itself separates and collects the carrier, but also on the internal-impedance and capacitance-effects of test circuit.19 The VUV-selected-photoresponse of the reported VUV devices are further evaluated. Here, relying on the continuous adjustable monochromatic light (from VUV to deep-UV) achieved by the combination of deuterium lamp and monochromator (Figure 6a), we measured the wavelength-dependent photoresponsivity (Rλ) of the device. Rλ denotes the index for the detection capability of a photodetector, which is defined as: Rλ=∆I/PS,20 where ∆I is photocurrent minus dark current, P is incident optical power density, and S is light absorption area. As shown in Figure 6b, the spectral-dependent Rλ of the device shows a rejection ratio of 195 nm VUV light versus 260 nm UV light is about three orders of magnitude. Ultraviolet photovoltaic detectors based on ZnO, GaN, ZnS, SnO2, and Ga2O3 materials with excellent performance have been reported in many papers, among which, the shortest cutoff wavelength obtained in Ga2O3 so far is 260 nm,21 which still remains unqualified for VUV photovoltaic detection. Our recent p-Gr/AlN/p-GaN photodetector is suitable for VUV photovoltaic detection for it has a response wavelength of λ < 200 nm and a cutoff wavelength of 205 nm. This is consistent with the spectral-absorption characteristics of pure AlN, benefiting from the high crystallinity of the epitaxial AlN absorption layer in 5


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device. The EQE of the device at 195 nm (EQE=hcR/eλ, where h is the Planck constant, c is the speed of light, e is the electron charge, and λ is the incident light wavelength) is 42.6%, see Table 1, which is 13.7 times higher than that of conventional AlN-based photoconductive devices. The stability of a photodetector is an important parameter for its practical application. In order to verify the stability of our device, we made a long-term stability test on its photoreaction. The device shows no significant change in photogenerated-voltage and photocurrent after a two-months storage in the air (see Figure S3), indicating a high environmental-stability.

CONCLUSIONS In summary, in order to meet the urgent need of VUV photodetection in astro-physics, we constructed a back-to-back p-Gr/AlN/p-GaN photovoltaic device with graphene as its VUV window. The device exhibits photodetection selectivity for VUV light, which is a kind of semiconductor-based VUV photovoltaic detector that has been long-awaited by solar and astro-physicists and device experts. The reported device possesses 80 ns ultra-fast temporalresponse and high EQE, which are attributed to the high mobility and high hole-collectionefficiency of the graphene window. Our results provide a reference for developing highperformance, zero-power-consumption, and integrated VUV photovoltaic-detectors.

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METHODS Thin films growth: Epitaxial growth of GaN and AlN was conducted by using 48 pieces of MOCVD, HICRO-I, MTM Semiconductor Equipment Co., Ltd. First, a 25 nm GaN buffer was grown on a c-plane sapphire at 530 °C; then a 2.5 µm unintentionally doped GaN was grown at 1050 °C and under 200 torr pressure; followed by a 200 nm p-GaN (carrier concentration of 3×1017/cm³, mobility of 10 cm³/V•s) growing at 950 °C. And then anneal it at 725 °C in a nitrogen atmosphere for 10 mins; finally a secondary epitaxial growth of 100 nm AlN was carried out at 1050 °C and under 50 torr pressure. Device preparation: After the epitaxial growth of AlN film, we constructed a 3×5 mm graphene on the AlN surface as a transparent conductive window via wet transfer method. Then, 20 nm Ti and 50 nm Au were successively deposited at one end of the graphene by thermal evaporation. Finally, a gold wire (25 µm diameter) was connected to the gold electrode using liquid Ga droplets. meanwhile, we constructed ohmic contact with thermally fused In as the electrode at the p-GaN side. Material characterization: The VUV to near-infrared transmission spectrum in Figure 1 was measured by a deep-UV spectrophotometer of Shimadzu UV-2600. The SEM image, X-ray patterns, Raman spectrum, and VUV Photoluminescence were collected by a field emission scanning electron microscope of ZEISS AURIGA, Bruker D8 Advance X-ray diffractometer, Renishaw inVia reflex microRaman spectroscopy (with 514 nm pump laser), and Exacted ArF excimer lasers, EX5/250 Mini Excimer Laser, GAM LASER, respectively. The spectrum acquisition relied on QE65PRO Scientific-Grade Spectrometer with H70 grating (200-289 nm). The noise-voltage-density was tested using the dynamic signal analyzer of Stanford SR785. Device photoelectric measurements: In the photovoltaic-effect test (Figure 4), the 185 nm monochromatic light is from the spectral line of a quartz-packaged low-pressure mercury lamp through a prism; and the 250 nm light from a deep-UV LED. In the temporal-dependent photoresponse test in Figure 5, the 193 nm pulsed light source is an ArF excimer laser, EX5/250 Mini Excimer Laser (GAM LASER), and the high-speed voltage signal acquisition device is a 6G oscilloscope, KEYSIGHT DSOS604A. The VUV spectral-response test system shown in Figure 6 used Shimadzu UV-2600 as continuous adjustable light source, with VXUV20A photodetector (PTO DIODE CORP) as light-power calibration, and KEITHLEY 2636b as SourceMeter.

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ASSOCIATED CONTENT Supporting information The Supporting Information is available free of charge on the ACS Publications website at DOI:************* Ohmic contact between p-Gr and Ti/Au electrodes as well as between p-GaN and In electrodes, wavelength-dependent photoresponsivity after two months.

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (No. 61604178, 91333207, U1505252 and 61427901), the Guangdong Natural Science Foundation (No. 2014A030310014).

REFERENCES (1) Baker, D. N. How to Cope with Space Weather. Science 2002, 297, 1486-1487. (2) Baker, D. N.; Kanekal, S. G.; Li, X.; Monk, S. P.; Goldstein, J.; Burch, J. L. An Extreme Distortion of the Van Allen Belt Arising From the `Hallowe'en' Solar Storm in 2003. Nature 2004, 432, 878-881. (3) Guerrero, M. A.; De Marco, O. Analysis of Far-UV Data of Central Stars of Planetary Nebulae: Occurrence and Variability of Stellar Winds. Astron. Astrophys. 2013, 553, A126. (4) Venot, O.; Fray, N.; Bénilan, Y.; Gazeau, M.-C.; Hébrard, E.; Larcher, G.; Schwell, M.; Dobrijevic, M.; Selsis, F. High-Temperature Measurements of VUV-Absorption Cross Sections of CO2 and Their Application to Exoplanets. Astron. Astrophys. 2013, 551, A131. (5) Moos, H. W.; Cash, W. C.; Cowie, L. L.; Davidsen, A. F.; Dupree, A. K.; Feldman, P. D.; Friedman, S. D.; Green, J. C.; Green, R. F.; Gry, C.; Hutchings, J. B.; Jenkins, E. B.; Linsky, J. L.; Malina, R. F.; Michalitsianos, A. G.; Savage, B. D.; Shull, J. M.; Siegmund, O. H. W.; Snow, T. P.; Sonneborn, G., et al. Overview of The Far Ultraviolet Spectroscopic Explorer Mission. Astrophys. J. 2000, 538, L1-L6. (6) Zheng, W.; Huang, F.; Zheng, R.; Wu, H. Low-Dimensional Structure Vacuum-Ultraviolet-Sensitive (λ < 200 nm) Photodetector with Fast-Response Speed Based on High-Quality AlN Micro/Nanowire. Adv. Mater. 2015, 27, 3921-3927. (7) BenMoussa, A.; Soltani, A.; Schühle, U.; Haenen, K.; Chong, Y. M.; Zhang, W. J.; Dahal, R.; Lin, J. Y.; Jiang, H. X.; Barkad, H. A.; BenMoussa, B.; Bolsee, D.; Hermans, C.; Kroth, U.; Laubis, C.; Mortet, V.; De Jaeger, J. C.; Giordanengo, B.; Richter, M.; Scholze, F.; Hochedez, J. F. Recent Developments of WideBandgap Semiconductor Based UV Sensors. Diamond Relat. Mater. 2009, 18, 860-864. (8) Li, J.; Fan, Z. Y.; Dahal, R.; Nakarmi, M. L.; Lin, J. Y.; Jiang, H. X. 200 nm Deep Ultraviolet Photodetectors Based on AlN. Appl. Phys. Lett. 2006, 89, 213510. (9) BenMoussa, A.; Soltani, A.; Gerbedoen, J. C.; Saito, T.; Averin, S.; Gissot, S.; Giordanengo, B.; Berger, G.; Kroth, U.; De Jaeger, J. C.; Gottwald, A. Developments, Characterization and Proton Irradiation Damage Tests of AlN Detectors for VUV Solar Observations. Nucl. Instrum. Methods Phys. Res., Sect. B 2013, 312, 48-53. (10) Balducci, A.; Marinelli, M.; Milani, E.; Morgada, M. E.; Tucciarone, A.; Verona-Rinati, G.; Angelone, M.; Pillon, M. Extreme Ultraviolet Single-Crystal Diamond Detectors by Chemical Vapor Deposition. Appl. Phys. Lett. 2005, 86, 193509. (11) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J.-H.; Kim, P.; Choi, J.-Y.; Hong, B. H. Large-Scale Pattern Growth of Graphene Films For Stretchable Transparent Electrodes. Nature 2009, 457, 706-710. (12) Chui, C. O.; Okyay, A. K.; Saraswat, K. C. Effective Dark Current Suppression With Asymmetric MSM Photodetectors in Group IV Semiconductors. IEEE Photonics Technol. Lett. 2003, 15, 1585-1587. 8


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(13) Zheng, W.; Zheng, R.; Huang, F.; Wu, H.; Li, F. Raman Tensor of AlN Bulk Single Crystal. Photonics Res. 2015, 3, 38-43. (14) Yan, H.; Li, X.; Chandra, B.; Tulevski, G.; Wu, Y.; Freitag, M.; Zhu, W.; Avouris, P.; Xia, F. Tunable Infrared Plasmonic Devices Using Graphene/Insulator Stacks. Nat. Nanotechnol. 2012, 7, 330-334. (15) Ozgit, C.; Donmez, I.; Alevli, M.; Biyikli, N. Self-Limiting Low-Temperature Growth of Crystalline AlN Thin Films by Plasma-Enhanced Atomic Layer Deposition. Thin Solid Films 2012, 520, 2750-2755. (16) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.; Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.; Hone, J. Boron Nitride Substrates for High-Quality Graphene Electronics. Nat. Nanotechnol. 2010, 5, 722-726. (17) Schwierz, F. Graphene Transistors. Nat. Nanotechnol. 2010, 5, 487-496. (18) Koppens, F. H. L.; Mueller, T.; Avouris, P.; Ferrari, A. C.; Vitiello, M. S.; Polini, M. Photodetectors Based on Graphene, Other Two-Dimensional Materials and Hybrid Systems. Nat. Nanotechnol. 2014, 9, 780-793. (19) Hawkins, R. T., II; Jones, M. D.; Pepper, S. H.; Goll, J. H.; Ravel, M. K. Vector Characterization Of Photodetectors, Photoreceivers, and Optical Pulse Sources by Time-Domain Pulse Response Measurements. IEEE Trans. Instrum. Meas. 1992, 41, 467-75. (20) Peng, L.; Hu, L.; Fang, X. Low-Dimensional Nanostructure Ultraviolet Photodetectors. Adv. Mater. 2013, 25, 5321-5328. (21) Guo, D.; Liu, H.; Li, P.; Wu, Z.; Wang, S.; Cui, C.; Li, C.; Tang, W. Zero-Power-Consumption Solar-Blind Photodetector Based on β-Ga2O3/NSTO Heterojunction. ACS Appl. Mater. Interfaces 2017, 9, 1619-1628. (22) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J.-S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I.; Kim, Y.-J.; Kim, K. S.; Ozyilmaz, B.; Ahn, J.-H.; Hong, B. H.; Iijima, S. Roll-to-Roll Production of 30Inch Graphene Films for Transparent Electrodes. Nat. Nanotechnol. 2010, 5, 574-578. (23) Mak, K. F.; Shan, J.; Heinz, T. F. Seeing Many-Body Effects in Single- and Few-Layer Graphene: Observation of Two-Dimensional Saddle-Point Excitons. Phys. Rev. Lett. 2011, 106, 046401. (24) BenMoussa, A.; Hochedez, J. F.; Dahal, R.; Li, J.; Lin, J. Y.; Jiang, H. X.; Soltani, A.; Jaeger, J.-C. D.; Kroth, U.; Richter, M. Characterization of AlN Metal-Semiconductor-Metal Diodes in the Spectral Range of 44–360 nm: Photoemission Assessments. Appl. Phys. Lett. 2008, 92, 022108.

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Figure Captions Figure 1. The transmittance of graphene on electromagnetic waves in VUV reaches nearinfrared band. It can be seen that graphene has a high transmittance for VUV light, similar to the other three lights (uv, visible and infrared). The lowest absorption near 270 nm is derived from the saddle-point exciton absorption.22,23 Figure 2. Device structure and electrical characteristics. a. Cross-section structure of the pGr/AlN/p-GaN heterojunction device. b. Optical image of the device. c. p-doping process via graphene electrode to obtain back-to-back photovoltaic devices. d. XRD pattern of the device, and (002) peak of AlN observed at 36 degrees with FWHM of 320 arcseconds. e. Raman scattering pattern of the device, the characteristic signals of components AlN (inset) and GaN and Gr are all observed. f. SEM image of the device’s cross-section, where AlN is about 100 nm, p-GaN is 300 nm, and un-doped GaN is 500 nm. g. Photoluminescence spectrum of AlN film, with luminescent center at 214 nm. Figure 3. I-V output characteristics of the device in dark state, where the inset illustrates the equivalent circuit of the device under different bias voltages. Here, for the sake of simplicity, the device's internal resistance and junction capacitance are omitted in the equivalent circuit. The I-V curve can be divided into three characteristic parts according to different linear relations: I, p-Gr/AlN with forward bias and AlN/p-GaN with reverse bias, the device shows a current-off state; II, AlN/p-GaN with forward bias and p-Gr/AlN with breakdown voltage, this device also shows a power failure; III, AlN/p-GaN with forward bias and p-Gr/AlN breakdown, the device is in the current-on state. Here, the electrode contacts (Au/Ti/p-Gr, In/p-GaN) are ohmic contacts (see Figure S1-S2), leaving the device's output characteristics unchanged. Figure 4. VUV photovoltaic testing and mechanisms. a. Photovoltaic response of the device under irradiation of 185 nm monochromatic light, and the device produces a 1.7 V electromotive-force difference between its two sides. b. Voltage output characteristics of the device under different lighting conditions. When put under 250 nm light, the device does not have any open-circuit-voltage output, remaining the same when put under dark state. c. The mechanism of photovoltaic effect of the device under VUV illumination. A thick spacecharge-region is formed in a heterojunction composed of high-carrier-concentration p-Gr and low-carrier-concentration i-type AlN. When VUV photons penetrate the graphene into the AlN absorbing layer, most of them are absorbed in the space-charge-region of AlN near the p-Gr side to produce photogenerated electron-hole pairs. Then, the electrons and holes are separated by the built-in electric field (E): the holes move toward the p-Gr side, while the electrons drift to the opposite direction, the open-circuit voltage of 1.7 V is finally formed. d. I-V characteristics of the device under dark condition and illumination of 185 nm monochromatic light (2.87 µw). e. Noise-voltage-density of the device. It can be seen that an ultra-low noise-voltage-density is almost covered by the background noise of the testing equipment. Figure 5. Temporal-dependent photovoltaic-response measurements. a. Schematic diagram of the photovoltaic-response under VUV nanosecond pulse, where the device is connected in parallel with a resistance (0.1 MΩ) and an oscilloscope. b. Voltage output signal of the device generated under nine consecutive equal interval pulses. Applying a 193 nm nanosecond pulse to simulate the ultra-fast changing VUV signal source, we performed a temporal-response test as shown in Figure 5a. Figure 5b shows the results of the temporal-dependent 10


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ACS Nano

photovoltaic-response under illumination of 9 VUV pulses, which reproduces the optical pulse signal better. c. Relationship between time (ms) and normalized ∆V. we stretched the time scale to make the rising edge of the pulse more visible d. Relationship between time (µs) and normalized ∆V. For a clearer analysis on the response-speed of the device, we amplifies the typical pulse. It can be seen that the device’s response rise-time is 80 ns. Figure 6. Spectral-dependent photoresponse measurements. a. Schematic diagram of the spectral-dependent photoresponse measurement on the device. b. Photoresponse of the device at different wavelengths. Here, Rλ is wavelength-dependent photoresponsivity (Rλ), denoting the index for the detection capability of a photodetector, which is defined as: Rλ=∆I/PS.20

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Table 1. Comparison of index parameters with previously reported semiconductorbased VUV detectors. Materials

VUV light Bias

Photoresponsi vity

EQE

Rejection ratio of VUV/UV-C

Rise time

Decay time

Reference

AlN

190 nm

30 V

0.0045 A/W

3.1 %

~ 104

~8s

~3s

24,7

Diamond

58.4 nm

-

-

-

~ 104

~4s

-

10

p-Gr/AlN

195 nm

0V

0.067 A/W

42. 6%

~ 103

8×10-8 s

4×10-4 s

This work

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Figure 1. The transmittance of graphene on electromagnetic waves in VUV reaches near-infrared band. It can be seen that graphene has a high transmittance for VUV light, similar to the other three lights (uv, visible and infrared). The lowest absorption near 270 nm is derived from the saddle-point exciton absorption.22,23 73x53mm (300 x 300 DPI)


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Figure 2. Device structure and electrical characteristics. a. Cross-section structure of the p-Gr/AlN/p-GaN heterojunction device. b. Optical image of the device. c. p-doping process via graphene electrode to obtain back-to-back photovoltaic devices. d. XRD pattern of the device, and (002) peak of AlN observed at 36 degrees with FWHM of 320 arcseconds. e. Raman scattering pattern of the device, the characteristic signals of components AlN (inset) and GaN and Gr are all observed. f. SEM image of the device’s cross-section, where AlN is about 100 nm, p-GaN is 300 nm, and un-doped GaN is 500 nm. g. Photoluminescence spectrum of AlN film, with luminescent center at 214 nm. 131x108mm (300 x 300 DPI)


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Figure 3. I-V output characteristics of the device in dark state, where the inset illustrates the equivalent circuit of the device under different bias voltages. Here, for the sake of simplicity, the device's internal resistance and junction capacitance are omitted in the equivalent circuit. The I-V curve can be divided into three characteristic parts according to different linear relations: I, p-Gr/AlN with forward bias and AlN/p-GaN with reverse bias, the device shows a current-off state; II, AlN/p-GaN with forward bias and p-Gr/AlN with breakdown voltage, this device also shows a power failure; III, AlN/p-GaN with forward bias and p-Gr/AlN breakdown, the device is in the current-on state. Here, the electrode contacts (Au/Ti/p-Gr, In/p-GaN) are ohmic contacts (see Figure S1-S2), leaving the device's output characteristics unchanged. 65x47mm (300 x 300 DPI)


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Figure 4. VUV photovoltaic testing and mechanisms. a. Photovoltaic response of the device under irradiation of 185 nm monochromatic light, and the device produces a 1.7 V electromotive-force difference between its two sides. b. Voltage output characteristics of the device under different lighting conditions. When put under 250 nm light, the device does not have any open-circuit-voltage output, remaining the same when put under dark state. c. The mechanism of photovoltaic effect of the device under VUV illumination. A thick space-charge-region is formed in a heterojunction composed of high-carrier-concentration p-Gr and lowcarrier-concentration i-type AlN. When VUV photons penetrate the graphene into the AlN absorbing layer, most of them are absorbed in the space-charge-region of AlN near the p-Gr side to produce photogenerated electron-hole pairs. Then, the electrons and holes are separated by the built-in electric field (E): the holes move toward the p-Gr side, while the electrons drift to the opposite direction, the open-circuit voltage of 1.7 V is finally formed. d. I-V characteristics of the device under dark condition and illumination of 185 nm monochromatic light (2.87 µw). e. Noise-voltage-density of the device. It can be seen that an ultra-low noise-voltage-density is almost covered by the background noise of the testing equipment. 93x55mm (300 x 300 DPI)


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Figure 5. Temporal-dependent photovoltaic-response measurements. a. Schematic diagram of the photovoltaic-response under VUV nanosecond pulse, where the device is connected in parallel with a resistance (0.1 MΩ) and an oscilloscope. b. Voltage output signal of the device generated under nine consecutive equal interval pulses. Applying a 193 nm nanosecond pulse to simulate the ultra-fast changing VUV signal source, we performed a temporal-response test as shown in Figure 5a. Figure 5b shows the results of the temporal-dependent photovoltaic-response under illumination of 9 VUV pulses, which reproduces the optical pulse signal better. c. Relationship between time (ms) and normalized ∆V. we stretched the time scale to make the rising edge of the pulse more visible d. Relationship between time (µs) and normalized ∆V. For a clearer analysis on the response-speed of the device, we amplifies the typical pulse. It can be seen that the device’s response rise-time is 80 ns. 112x90mm (300 x 300 DPI)


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Figure 6. Spectral-dependent photoresponse measurements. a. Schematic diagram of the spectraldependent photoresponse measurement on the device. b. Photoresponse of the device at different wavelengths. Here, Rλ is wavelength-dependent photoresponsivity (Rλ), denoting the index for the detection capability of a photodetector, which is defined as: Rλ=∆I/PS.20 60x25mm (300 x 300 DPI)


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TOC Figure 38x18mm (300 x 300 DPI)


Vacuum-Ultraviolet Photovoltaic Detector - ACS Publications

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