Î²-Ga2O3 Heterojunction Deep

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Functional Inorganic Materials and Devices

High-performance Graphene/#-Ga2O3 Heterojunction Deep-Ultraviolet Photodetector with Hot-electron Excited Carrier Multiplication Richeng Lin, Wei Zheng, Dan Zhang, Zhaojun Zhang, Qixian Liao, Lu Yang, and Feng Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05336 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018

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ACS Applied Materials & Interfaces

High-performance Graphene/β-Ga2O3 Heterojunction DeepUltraviolet Photodetector with Hot-electron Excited Carrier Multiplication Richeng Lin, Wei Zheng, Dan Zhang, Zhaojun Zhang, Qixian Liao, Lu Yang, Feng Huang* State key Laboratory of Optoelectronic Materials and Technologies, School of Materials, Sun Yat-sen University, Guangzhou 510275, P. R. China E-mail: [email protected]

ABSTRACT: Solar blind ultraviolet (SBUV) detection has important applications in wireless secure communication and early warning, etc. However, the desired key device for SBUV detection and high-sensitivity and low-noise “sandwich” photodetector with large-detective-area is difficult to be fabricated, since it’s usually hard for traditional wide-bandgap semiconductors to boast both high conductivity and high-SBUV-transparency. Here, we proposed to use graphene as the transparent conductive layer to form Graphene/β-Ga2O3 heterojunction. With the help of largearea graphene and hot carrier multiplication, a SBUV photodetector with large-detective-area, low-dark-current and high-sensitivity was successfully assembled. Its photoresponsivity is 1~3 orders of magnitude higher than that of the conventional SBUV photodetectors, and its response speed can rival the best device ever reported. KEYWORDS: Solar-blind ultraviolet, large-area detection, graphene/β-Ga2O3 heterojunction,

transparent conductive layer, hot carrier multiplication.

INTRODUCTION Solar-blind ultraviolet (SBUV) detection, owing to its “blank background noise”, has important applications in wireless secure communication. Due to multiple reflections and long-range transmission, such communication adopting Rayleigh scattering of SBUV only involves extremely weak optical signals, thus it is a prerequisite for the photodetectors to have large-detective-area and high-sensitivity.1 As is well known, wide bandgap semiconductor (WBS) based SBUV photodetector has compact size, low operating-voltage, and spectral selectivity, all these make it a more promising candidate in communication than photomultiplier tube.2,3,4 In order to detect the weak optical signals mentioned above, SBUV photodetector must be designed into "sandwich" structure with a large-detective-area, and the photosensitive layer (PSL) be placed between the transparent conductive layer (TCL) and the back

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conductive layer (BCL).5,6 Usually, researchers believe that PSL film is the key component of a photodetector, however, SBUV-TCL film actually should be given more attention in developing large-detective-area SBUV photodetectors. Generally, two strategies are adopted to achieve the above described SBUV-TCL films. Nano-scale metal thin film with optical transmittance is the first typical strategy, however, such film involves high electrical resistance.7 As a result, different working voltages exist between the central and edge regions of the photodetector, which is unfavorable for large-area detection. The second strategy is to render conductive WBS film as SBUV-TCL.8,9,10 Unfortunately, the typical nature of semiconductor is: the wider the bandgap of the semiconductor, the harder the realization of high conductivity. Obviously, these two strategies face bottlenecks in device fabrication and physics of materials, respectively. Graphene, a new photoelectronic material composed of monolayer carbon atoms, has quite high roomtemperature carrier mobility owing to its small electronic mass in Dirac point.11,12 Such single-atomic-layer material has adequate transmittance to wide-frequency light, and simultaneously possesses high conductivity with atomic-scale thickness. We believe that, with the development of large-area fabrication technology, graphene will replace the above mentioned two films and be used as a promising SBUV-TCL film.13,14 In this work, we assembled a typical “sandwich" structure SBUV photodetector with a 4 mm x 6 mm graphene. The large-area graphene epitaxial grown on copper foil was transferred to β-Ga2O3/p-GaN heterojunction substrate via wet-chemical method. Using rectifying effect of p-GaN/β-Ga2O3 diode, the photodetector obtained an extremely low dark current density of 1.25 x 10-8 A/cm2, and settled the problem of large background current in conventional large-area photodetector. The photodetector behaved a responsivity of 12.8 A/W (at -6 V), which is 1-3 orders of magnitude higher than that of the conventional WBS-based SBUV photodetectors.15,16,17,18 Such high responsivity was due to its unique gain mechanism. And the hot carrier multiplication in graphene generates additional photocurrent under SBUV illumination. In addition, our photodetector possesses ultrafast response speeds of tr=1.5 ms and td=2 ms, which is much faster than that of the previously reported devices.3 Our fabricated photodetector exhibits great potential in large-detective-area SBUV detection for communication, and our designing strategy also provides a reference for manufacturing large-active-area, high-sensitivity, and high-speed


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photodetector. EXPERIMENTAL METHOD Material preparation: Large-area single-layer graphene grown on copper foil is commercially available. High crystallinity β-Ga2O3 was grown by a self-designed MOCVD assisted with ‘inert’ N2O as oxygen source. The p-GaN substrate with a carrier concentration of 1016 cm-3 was purchased from MTM Semiconductor Equipment Co. Ltd. PMMA and ammonium persulfate were from Sinopharm Chemical Reagent Co. Ltd. Photodetector fabrication: The as-grown graphene was cut to targeted size. 5% of PMMA chlorobenzene solution was spin-coated on graphene at 4000 rmp and 60 s. Then the sample was baked at 70 oC for 10 mins. After cooling to room temperature, the sample was put to 0.2 M ammonium persulfate solution for one night. After completely dissolving the Cu substrate, the PMMA/graphene composite films were transferred to deionized water to remove metal residue. This process was repeated three times, followed by transferring the films to the targeted β-Ga2O3/p-GaN substrates. After natural drying, the sample was baked at 70 oC for 10 min. Then PMMA layer was removed with acetone, ethyl alcohol and deionized water. Then, this processes were also repeated three times to ensure a complete removal of PMMA. Eventually, large-area graphene was transferred and TCL was successfully obtained. It’s worth noting that the interface structure between graphene/β-Ga2O3 is of vital importance in improving the performance of our devices. Firstly, graphene/β-Ga2O3 forms a schottky junction whose built-in electric field is advantageous for the separation of carrier. Secondly, due to the nano-scale thick graphene has limited light absorption, β-Ga2O3was used as the main light absorption layer in the device. Therefore, high quality graphene/β-Ga2O3 interface structure is a prerequisite for high performance of the device. Then 10 nm Ti and 70 nm Au electrodes were deposited by thermal evaporation using a physical mask. The In electrode is obtained by electric welding. Characterizations and measurements: Crystalline structure of the epitaxial-grown β-Ga2O3 film was identified using an X-ray diNractometer (Panalytical X’Pert Pro) with Cu-Kα radiation (λ = 1.5406 Å), and Raman spectra at room temperature was measured by Jobin Yvon LabRAM HR Raman spectrometer with a 514 nm laser as excitation. SEM images were collected using scanning electron microscopy instrument of ZEISS AURIGA, Oxford INCAPent aFET-x3 and Hitachi S-4800.



Absorption spectrum was obtained via a Shimadzu UV-2600 UV-Vis spectrometer. All the electrical properties were measured in atmospheric environment using a prober, a SourceMeter Keithley 2636B, a 254 nm semiconductor LED (Sensor Electronic Technology, Inc.) with filter modified by an Agilent source, and an optical power meter of Ophir (NOVA II header and PD300-UV probe). The spectral response was measured by LEDs or lasers and optical power meter of Ophir. The noise current of the devices was measured using an analysis meter (Stanford Research System, SR830). RESULTS AND DISCUSSION Figure 1a shows a schematic view of typical "sandwich" structure photodetector, whose corresponding optical photograph and scanning electron microscope (SEM) cross-sectional image are shown in Figure 1b and 1c, respectively (a packaged device and the surface SEM images are shown respectively in Figure S1 and S2, as seen in supporting information). In our photodetector, the 200 nm thick intrinsic n-type βGa2O3 and the heavily-doped p-type GaN (5 µm on sapphire) were used to construct a p-n heterojunction, here, β-Ga2O3 was an excellent SBUV photosensitive material due to its ultra-wide optical bandgap of ~5.2 eV (see in supporting information). As shown in Figure 1d, the XRD pattern of the sample shows a strong characteristic peak of (0002) plane of GaN substrate.19 The characteristic peaks of 19o and 59.1o shown in the inset correspond to (-201) and (-603) planes of β-phase Ga2O3, respectively.20 Raman analysis is shown in Figure 1e, the G, D and 2D modes of graphene register at 1589 cm-1, 1343 cm-1 and 2860 cm-1, respectively. In the low frequency region, the A1, E1, and E2 modes of GaN are located at 533 cm-1, 736 cm-1, 560 cm-1, and 570 cm1

, respectively.21 In the detailed section, the 201 cm-1 and 419 cm-1 characteristic

peaks with FWHM of 5 cm-1 come from β-phase Ga2O3.22,23 The atomic force microscope (AFM) images are shown in Figure S3. The elemental and crystalline characterizations of the material are shown in Figure 2. In Figure 2a, the electron diffraction and elemental analysis show a uniform elemental distribution of the Ga2O3 film. However, the C element isn’t being detected fully, due to the thin graphene. The high resolution transmission electron microscopy (HRTEM) image of graphene (in Figure 2b) indicates a high quality of the graphene. The graphene used as TCL is single layer. In figure 2c, the X-ray photoelectron spectroscopy (XPS) shows elemental component of C, Ga and O. All the above characterizations indicate that we have laid an important material


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basis for later fabrication of SBUV photodetector with high performance and largedetective-area. The dense β-Ga2O3 film is a prerequisite for constructing the device, and also a necessity for avoiding device breakdown caused by leakage current. We notice that the grain size has a great influence on the response speed of a photodetector. In this work, our β-Ga2O3 films have large grain size which allows the devices to possess fast respond speed. To investigate the photoresponse of the photodetector, photo-electronic measurements under SBUV illumination (Figure 3a) were conducted. It should be noted that Ohmic contacts are formed at Au/Ti/graphene and In/p-GaN, as verified by the linear I-V characteristic curves (see Figure S4 in supporting information). As seen in Figure 3b, the I-V curve under dark condition shows obvious rectifying effect, which can be attributed to the existing p-n junction between intrinsic n-type β-Ga2O3 and p-GaN. This specially designed structure is beneficial to obtain low dark current when the photodetector is under reverse bias, which resolves the problem of large background current in conventional large-area photodetector. Under a reverse bias of 3 V, the photodetector has an extremely low dark current density of 1.25 x 10-8 A/cm2. Benefitting from the β-Ga2O3/p-GaN heterojunction, the photodetector achieves a high light/dark rate of ~103 at -6 V. When illuminated under 254 nm light (the incident optical power density is 21.2 µW/cm2), the photocurrent increases exponentially under a reverse bias of -3.5 V, indicating multiple carrier generation in the photodetector. We assume such multiplication gain possibly comes from the carrier-carrier scattering in graphene, and the detailed discussion is shown in the last part. Responsivity (R), as an important parameter to characterize the photodetector performance, is defined as: R=∆I/PS, where ∆I is photocurrent, namely, Ip minus dark current Id, P is the incident optical power density, and S the light absorption area of photodetector.24 As it can be seen from Figure 3c, the responsivity of our photodetector slowly increases from 0 to -3.5 V, while sharply rises from -3.5 to -6 V, similar to the multiplication gain in avalanche photodiode. The responsivity ultimately reaches a maximum of about 12.8 A/W at -6V. This responsivity is much larger than that of the previous large-area SBUV photodetectors and UV photodetectors of other type (for example, ZnO and perovskite based photodetectors), as summarized in Table1.25,26,27 Our photodetector can also operate under zero bias and the corresponding discussions are given in supporting information, Figure S5



specifically. Spectral selectivity is another important parameter of photodetector. As shown in Figure 3d, our photodetector shows a sharp response cut-off at about ~280nm, indicating its excellent spectral selectivity for SBUV detection. More measurements of the photodetector detective performance are shown in Figure 4. Seeing from the I-V characteristics curves in Figure 4a, photocurrents significantly enhance with increasing optical power density. Even if it is illuminated under a low optical power density of 21.2 µw/cm2, the photodetector still has a much larger photocurrent than in the dark current, indicating its weak-light detection ability and high photo-responsivity. The photocurrent grows slowly when the bias is between 1 V and 3 V, which can be ascribed to the Schottky junction of Graphene/β-Ga2O3. The photocurrent is also observed to increase accordingly with growing optical power density (Figure S6), which is consistent with the situation of most current photodetectors. This phenomenon can be explained by the action of surface defects. Figure 4b shows the plotted photoresponsivity and external quantum efficiency (EQE) with changing optical power density. Under an optical power density of 21.2 µW/cm2, the photodetector achieves the maximum responsivity of 12.8 A/W. In addition, the EQE is calculated by equation: EQE=hcR/eλ, where h is Planck constant, c speed of light, e electron charge, and λ wavelength of the incident light.28 The corresponding EQE is given in Figure 4b with a maximum value of 6681%, demonstrating promising application of photodetector in weak light detection. The noise current (in) of photodetector is measured and shown in Figure S8. The noise equivalent power (NEP) can be calculated by: NEP=in/R. Detectivity, the ability to detect weak signal of the photodetector, is calculated by equation: D*= (SB)1/2/NEP, where Rλ is responsivity, and B bandwidth. Figure 4c shows the detectivity of our large-detective-area SBUV photodetector. When the bias voltage is larger than 5.5 V, the detectivity is up to 1013 Jones, which is comparable to the best performance of the previously reported photodetectors. Response speed reflects the response ability of a photodetector to high-frequency optical signal. Figure 4d shows a typical time-dependent current curve at a bias of 5 V when the photodetector is illuminated under switched 254 nm light with different power densities. The response rise time (tr) defines the current to increase from 10% to 90% of the maximum value when the light is turned on, and the decay time (td) is a reverse process. The extracted tr and td are 1.5 and 2 ms, respectively, revealing an ultra-fast response speed of our photodetector for pulsed light. This response speed


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can actually meet the practical requirement. As shown in Figure 4e, the time-resolved photocurrent curve is under continuous pulsed light, showing that our photodetector has fast response speed and excellent repeatability. Stability is another important parameter of photodetector for practical application. To examine the stability of our designed large-detective-area SBUV photodetector, long-time-stability measurement was conducted (Figure 4f). Although the photodetector has been exposed in air for one month, its photocurrent shows almost no decay, indicating high response stability. We believe that through further studies, our large-detective-area SBUV photodetector can be widely used in practical applications, which is actually difficult for the current nanostructured photodetectors. For the multiplication gain in our photodetector mentioned above, the explanation is given here. Usually, multiple carriers can be generated by impact ionization and carrier-carrier scattering. It is well known that the appearance of impact ionization requires an extremely high electrical field to obtain enough-energy for carriers, which differs from our present measurements.29 Therefore, we believe that the multiplication gain in our photodetector is maybe related to the carrier-carrier scattering. In graphene, carrier-carrier scattering can compete with the phonon emission owing to its gapless nature, thus the probability of carrier-carrier scattering is enhanced.30 More importantly, multiple hot carriers (such as hot electrons) can be generated through the carrier-carrier scattering with a single high-energy photon or electron.31 These multiple hot carriers participate in the formation of circuit current, resulting in multiplication gain in photodetectors.32,33 The electronic energy band diagram of the heterojunction is shown in Figure 5a where the β-Ga2O3/p-GaN heterojunction causes conduction band potential energy difference (∆Ec1) and valence band potential energy difference (∆Ev1), and Graphene/β-Ga2O3 heterojunction generates ∆Ec2. When the device is applied under positive bias voltage (p-GaN in positive), the electron (as discussed here) will be initially blocked by the potential barriers between β-Ga2O3/p-GaN and Graphene/βGa2O3 heterojunctions, and the forward current will be in cut-off state. As the forward voltage increases, the electrons pass through the decreased potential barriers, and the forward current is in open state. This working mechanism is consistent with the I-V characteristics of the device (see Figure 3b). During the whole forward bias process, no obvious hot carrier multiplication in graphene is observed. When the device is applied under reverse bias voltage, the β-Ga2O3/p-GaN PN junction is in cut-off state.



With the increase of the reverse bias voltage, the electrons in the β-Ga2O3 obtain kinetic energy under the electric field. These high-energy electrons eventually excite additional electron-hole pairs in graphene. This process can be simply expressed as: e(β-Ga2O3)+Ke(Vr)→e(Graphene)+e’(Graphene)+h’(Graphene), where the electrons in Graphene/β-Ga2O3 heterojunction gain combined energy of potential energy ∆Ec2 and kinetic energy Ke(Vr) of reversed electric field, to create a lower-energy electron and an electron-hole pair. The kinetic energy Ke(Vr) can be expressed as: Ke(Vr)=∆Ec2+e(αVr), where α is the proportional constant between the potential energy and the applied voltage.34 The variable temperature I-V characteristic of the device is shown in Figure 5b. The reverse current increases significantly with the increase of temperature, which means that multiple carriers are produced by the hot carriers. With growing temperature, the incensement of reverse current is significantly larger than that of forward current. Moreover, when the temperature is above 390K, the reverse current is larger than the forward current though the two are put under the same voltage (8V). This indicates that only under the reverse bias, will the device produce a multiplier current, which is in consistentence with our mechanism explanation. Different from zener breakdown, through comparing the current-voltage curves of PD1 (with graphene) and PD2 (without graphene) (Figure S9), the reversed dark current of PD1 has an obviously exponential increment at about 3.5 V bias (see the inset in Figure S9), while PD2 presents a smooth dark-current curve, indicating inexistence of junction broken in β-Ga2O3/p-GaN heterojunction. The photocurrent of PD1 is also found to be enhanced when using graphene. These findings have confirmed the critical role of graphene in our photodetector, which is in agreement with the above discussions. Within the range of 310-370K, the reverse photocurrent increases significantly, while the forward photocurrent barely changes (Figure 5c), which could be attributed to the conduction current at forward PN junction. The reverse photocurrent also increases linearly with the rising light power density, which is in agreement with the experimental result measured at room temperature. In Figure 5d, exponential increment of the reversed photocurrent of PD1 appears at 3.5 V when the photodetector is illuminated under 254 nm light, but disappears under 365 nm light. The transmittance spectra of β-Ga2O3 and graphene are shown in Figure S7 in supporting information, revealing that the difference of the above two situations. In


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the β-Ga2O3 film, photo-carriers can only be excited when under 254 nm light. We reckon the above results are closely related to the gain achieved through hot carrier multiplication in graphene. CONCLUSION In summary, here we used large-size graphene as TCL, high quality β-Ga2O3 as PSL and p-GaN as BCL to fabricate a high sensitivity, fast-response-speed and largedetective-area (>24 mm2) SBUV photodetector successfully. Using rectifying effect of p-GaN/β-Ga2O3 diode, the photodetector obtained an extremely low dark current density of 1.25 x 10-8 A/cm2, and solved the problem of large background current in conventional large-area photodetector. Additionally, the photodetector behaves a responsivity up to 12.8 A/W, which is 1-3 orders of magnitude higher than that of the conventional large-area SBUV detectors. The high responsivity is due to its unique gain mechanism, in which the hot carrier multiplication in graphene generates additional photocurrent under SBUV illumination. The photodetector possesses a reliable stability and a fast response speed with a rise time of 1.5 ms, which is close to the actual detective requirements. In addition, its excellent spectral selectivity demonstrates that the photodetector is quite suitable for SBUV detection. All the above performances prove that the large-detective-area photodetector has great potential in SBUV detection. Besides, the designed strategy provides a reference for manufacturing large-active-area, high-sensitivity, and high-speed photodetector.

ASSOCIATED CONTENT Supporting information Photographic device; SEM and AFM images; Ohmic contact between graphene and Ti/Au electrodes as well as between p-GaN and In electrodes; Photodetector’s operation under zero bias; Transmitted spectra of β-Ga2O3 and graphene; The noise current of photodetector; Comparison between device with graphene and without graphene. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Wei Zheng: 0000-0003-4329-0469



Feng Huang: 0000-0002-4623-2216 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS We acknowledge financial support from the National Natural Science Foundation (No. 91333207, 61604178, U1505252 and 61427901), the Guangdong Natural Science Foundation (No. 2014A030310014), and Guangzhou Science and Technology Program（No.201607020036）.

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(17) Sun, L.; Chen, J.; Li, J.; Jiang, H., AlGaN solar-blind avalanche photodiodes with high multiplication gain. Appl. Phys. Lett. 2010, 97, 191103. (18) Zheng, W.; Lin, R.; Zhang, Z.; Liao, Q.; Liu, J.; Huang, F., An ultrafast-temporally-responsive flexible photodetector with high sensitivity based on high-crystallinity organic-inorganic perovskite nanoflake. Nanoscale 2017, 9, 12718-12726. (19) Balkas, C. M.; Basceri, C.; Davis, R. F., Synthesis and characterization of high purity, single phase GaN powder. Powder Diffr. 2013, 10, 266-268. (20) Ahman, J.; Svensson, G.; Albertsson, J., A Reinvestigation of [beta]-Gallium Oxide. Acta Crystallographica Section C 1996, 52, 1336-1338. (21) Rao, C. N. R.; Sood, A. K.; Subrahmanyam, K. S.; Govindaraj, A., Graphene: The New Two‐ Dimensional Nanomaterial. Angew. Chem., Int. Ed. 2009, 48, 7752-7777. (22) Zheng, W.; Li, F.; Li, G.; Liang, Y.; Ji, X.; Yang, F.; Zhang, Z.; Huang, F., Laser Tuning in van der Waals Crystals. ACS nano 2018, 12, 2001-2007. (23) Zheng, W.; Zheng, R.; Huang, F.; Wu, H.; Li, F., Raman tensor of AlN bulk single crystal. Photon. Res. 2015, 3, 38-43. (24) 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. (25) Ghosh, D.; Kapri, S.; Bhattacharyya, S., Phenomenal Ultraviolet Photoresponsivity and Detectivity of Graphene Dots Immobilized on Zinc Oxide Nanorods. ACS Appl. Mater. Interfaces 2016, 8, 35496-35504. (26) Shen, Y.; Yan, X.; Si, H.; Lin, P.; Liu, Y.; Sun, Y.; Zhang, Y., Improved Photoresponse Performance of Self-Powered ZnO/Spiro-MeOTAD Heterojunction Ultraviolet Photodetector by PiezoPhototronic Effect. ACS Appl. Mater. Interfaces 2016, 8, 6137-6143. (27) Fu, X.-W.; Liao, Z.-M.; Zhou, Y.-B.; Wu, H.-C.; Bie, Y.-Q.; Xu, J.; Yu, D.-P., Graphene/ZnO nanowire/graphene vertical structure based fast-response ultraviolet photodetector. Appl. Phys. Lett. 2012, 100, 223114. (28) Zheng, W.; Zhang, Z.; Lin, R.; Xu, K.; He, J.; Huang, F., High‐Crystalline 2D Layered PbI2 with Ultrasmooth Surface: Liquid‐Phase Synthesis and Application of High‐Speed Photon Detection. Adv. Electron. Mater. 2016, 2, 1600291. (29) Tielrooij, K. J.; Song, J. C. W.; Jensen, S. A.; Centeno, A.; Pesquera, A.; Zurutuza Elorza, A.; Bonn, M.; Levitov, L. S.; Koppens, F. H. L., Photoexcitation cascade and multiple hot-carrier generation in graphene. Nat. Phys. 2013, 9, 248-252. (30) Johannsen, J. C.; Ulstrup, S.; Cilento, F.; Crepaldi, A.; Zacchigna, M.; Cacho, C.; Turcu, I. C. E.; Springate, E.; Fromm, F.; Raidel, C.; Seyller, T.; Parmigiani, F.; Grioni, M.; Hofmann, P., Direct View of Hot Carrier Dynamics in Graphene. Phys. Rev. Lett. 2013, 111, 027403. (31) Winzer, T.; Knorr, A.; Malic, E., Carrier Multiplication in Graphene. Nano Lett. 2010, 10, 48394843. (32) Song, J. C. W.; Rudner, M. S.; Marcus, C. M.; Levitov, L. S., Hot Carrier Transport and Photocurrent Response in Graphene. Nano Lett. 2011, 11, 4688-4692. (33) Gabor, N. M.; Song, J. C. W.; Ma, Q.; Nair, N. L.; Taychatanapat, T.; Watanabe, K.; Taniguchi, T.; Levitov, L. S.; Jarillo-Herrero, P., Hot Carrier–Assisted Intrinsic Photoresponse in Graphene. Science 2011, 334, 648-652. (34) Barati, F.; Grossnickle, M.; Su, S.; Lake, R. K.; Aji, V.; Gabor, N. M., Hot carrier-enhanced interlayer electron–hole pair multiplication in 2D semiconductor heterostructure photocells. Nat. Nanotechnol. 2017, 12, 1134-1139. (35) Kong, W. Y.; Wu, G. A.; Wang, K. Y.; Zhang, T. F.; Zou, Y. F.; Wang, D. D.; Luo, L. B., Graphene‐β‐Ga2O3 Heterojunction for Highly Sensitive Deep UV Photodetector Application. Adv. Mater. 2016, 28, 10725-10731. (36) Parish, G.; Keller, S.; Kozodoy, P.; Ibbetson, J. P.; Marchand, H.; Fini, P. T.; Fleischer, S. B.; DenBaars, S. P.; Mishra, U. K.; Tarsa, E. J., High-performance (Al,Ga)N-based solar-blind ultraviolet p–i–n detectors on laterally epitaxially overgrown GaN. Appl. Phys. Lett. 1999, 75, 247-249. (37) Hou, Y. N.; Mei, Z. X.; Liu, Z. L.; Zhang, T. C.; Du, X. L., Mg0.55Zn0.45O solar-blind ultraviolet detector with high photoresponse performance and large internal gain. Appl. Phys. Lett. 2011, 98, 103506. (38) Tian, C.; Jiang, D.; Li, B.; Lin, J.; Zhao, Y.; Yuan, W.; Zhao, J.; Liang, Q.; Gao, S.; Hou, J.; Qin, J., Performance Enhancement of ZnO UV Photodetectors by Surface Plasmons. ACS Appl. Mater. Interfaces 2014, 6, 2162-2166.



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Figure 1. (a) Schematic presentation of a "sandwich" structure photodetector with large-detectivearea. (b) Optical photograph of the practical photodetector (c) Scanning electron microscope (SEM) cross-sectional image of the sample, showing 5 µm GaN and 200 nm β-Ga2O3. The enlarged image corresponding to white dotted square shows a dense morphology of the β-Ga2O3 film. (d) XRD pattern shows strong characteristic peak of (0002) plane of GaN substrate. In the inset, the characteristic peaks related to β-phase Ga2O3 respectively locates at 19o and 59.1o, corresponding to (-201) and (-603) planes. (e) Raman pattern of the sample shows typical modes of graphene locating at 1343 cm-1 (D), 1589 cm-1 (G), and 2860 cm-1 (2D). The bottom shows the A1 and E2 modes of GaN, and the enlarged part shows the characteristic peaks of β-Ga2O3 locating at 201 cm-1 and 419 cm-1.



Figure 2. (a) Interface SEM image of Graphene/Ga2O3, and corresponding elemental analysis. (b) HRTEM patterns of the graphene TCL, showing single layer and high quality. (c) XPS characterization of the Graphene/Ga2O3 heterojunction. The peak shift of C 1s could be derived from the interaction of graphene. The peaks of Ga 3d and O 1s show the existence of O vacancy, which causes intrinsic n-type conduction of the Ga2O3 film.


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Figure 3. (a) Measurement schematic of the large-detective-area SBUV photodetector. (b) I-V characteristics of the photodetector. When the reverse bias voltage is 3.5 V, the photocurrent exponentially increases, achieving a maximum value of 63.5 µA under reverse voltage of -6 V. (c) Responsivity as a function of the reverse bias voltage. (d) Spectral response of the photodetector shows a response cut-off at ~280 nm, revealing an excellent spectral selectivity for SBUV signal detection.



Figure 4. (a) I-V characteristics as a function of bias, varying with incident optical power density. (b) Responsivity and EQE of the photodetector (254 nm). (c) Detectivity of the photodetector varying with different bias voltage. (d) and (e) are typical time-dependent current curve of the photodetector, showing a fast response speed of tr=1.5 ms and td=2 ms. (f) Stability of the photodetector, long-time exposure under atmospheric environment (for 1 month) only causes an infinitesimal l photocurrent decay.


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Figure 5. (a) The electronic energy band diagram of the heterojunction shows conduction band potential energy difference (∆Ec1, ∆Ec2) and valence band potential energy difference (∆Ev1) in Graphene/β-Ga2O3 and β-Ga2O3/p-GaN heterojunction, respectively. Under reversed voltage, the electrons in the β-Ga2O3 obtain kinetic energy under the electric field, and eventually propel the graphene to excite additional electron-hole pairs. (b) The temperature-dependent I-V characteristic of the device. The reversed current increases significantly as temperature goes up, and its value is larger than forward current under 390K. (c) The reverse photocurrent under 310K and 370K in various light intensity. The reverse photocurrent under 370K is also significantly larger than that under 310K. (d) I-V characteristic of PD1 (with graphene) illuminated under different-wavelength lights. Exponential increment of the photocurrent appears at 3.5 V under 254 nm light, but disappears under 365 nm light, which is closely related to the gain obtained through hot carrier multiplication.



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Table 1. Comparison of figures in SBUV photodetectors. Dark Current Bias voltage R (A cm-1) (V) (A/W)

Material

Detective area (cm2)

Graphene/β-Ga2O3

0.24

1.25x10-8

6

β-Ga2O3

0.8

1.37x10-6

2D β-Ga2O3

2x10-6

β-Ga2O3

D* (Jones)

Rise time

Decay time

Ref.

12.8

1.3x1013

1.5 ms

2 ms

This work

20

39.3

5.92x1013

94.83 s

219.1 s

35

1.5x10-4

10

3.3

4.0x1012

30 ms

60 ms

16

1.96x10-2

1.58x10-6

10

-

-

0.62 s

8.97 s

17

AlGaN

3.1x10-4

7.88x10-9

0

0.0798

-

-

-

18

AlGaN

1x10-4

1x10-8

0

0.05

-

-

4.5 ns

36

MgZnO

3x10-5

2.3x10-7

130

0.022

-

-

500 ns

37

Pt/ZnO

3.75x10-4

2x10-5

3

1.306

-

-

-

38

ZnO

7.9x10-3

9.1x10-4

-5

101.2

0.44 s

0.59 s

39

CH3NH3PbCl3

0.01

4.15x10-5

15

0.0496

1.2x1010

24 ms

62 ms

40

CH3NH3PbBr3

-

-

3

0.1

7.1x1011

70 µs

150 µs

41


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Figure 1 126x79mm (300 x 300 DPI)



Figure 2 199x159mm (300 x 300 DPI)


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Figure 3 98x72mm (300 x 300 DPI)



Figure 4 99x46mm (300 x 300 DPI)


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Figure 5 144x105mm (300 x 300 DPI)



TOC Figure 83x35mm (300 x 300 DPI)


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Î²-Ga2O3 Heterojunction Deep

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