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Wei Yan, Shifan Wang, Kaijian Xing, Sivacarendran Balendhran, Mike Tebyetekerwa, [Kenji Watanabe](https://orcid.org/0000-0003-3701-8119), [Takashi Taniguchi](https://orcid.org/0000-0002-1467-3105), Michael S. Fuhrer, Kenneth B. Crozier, James Bullock

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Electrostatically Induced Black Phosphorus Infrared PhotodiodesRESEARCH ARTICLEwww.afm-journal.deElectrostatically Induced Black Phosphorus InfraredPhotodiodesWei Yan, Shifan Wang, Kaijian Xing, Sivacarendran Balendhran, Mike Tebyetekerwa,Kenji Watanabe, Takashi Taniguchi, Michael S. Fuhrer, Kenneth B. Crozier,and James Bullock*Homojunctions are key elements in many mainstream electronic devices.However, conventional dopant-based “pn” homojunctions are not easilyachievable in new material families, such as the 2D materials. Several recent2D material studies have shown that lateral pn homojunctions can insteadbe electrostatically induced using back gates localized to either the source ordrain contacts. Here, a hBN-encapsulated black phosphorus dual-gate devicecontaining a lateral pn homojunction, whose orientation can be switchedvia application of back gate voltages, is demonstrated. Importantly, this studyextends the state-of-the-art for this architecture by characterizing the photore-sponse under infrared (𝝀 = 2.2 μm) illumination. It is shown that when biasedto form a homojunction, the device exhibits the photovoltaic effect, resultingin a specific detectivity of 8.5 × 108 cm Hz1/2 W−1 at 77 K under short-circuitconditions, and an open circuit photovoltage up to 175 mV at 77 K. Further, itis shown that the device can be operated in photoconductive mode, allowinga high responsivity of 0.55 A W−1. This device is thus highly reconfigurableas it can be switched between photovoltaic and photoconductive modesof operation to prioritize low noise and fast response or high responsivity.1. IntroductionBlack phosphorus (bP) has emerged as one of the most promis-ing van der Waals materials for infrared (IR) optoelectronicW. Yan, S. Wang, S. Balendhran, K. B. Crozier, J. BullockDepartment of Electrical and Electronic EngineeringThe University of MelbourneParkville, Victoria 3010, AustraliaE-mail: james.bullock@unimelb.edu.auK. Xing, M. S. FuhrerSchool of Physics and AstronomyMonash UniversityClayton, Victoria 3800, AustraliaM. TebyetekerwaSchool of Chemical EngineeringThe University of QueenslandBrisbane, Queensland 4072, AustraliaThe ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/adfm.202316000© 2024 The Authors. Advanced Functional Materials published byWiley-VCH GmbH. This is an open access article under the terms of theCreative Commons Attribution-NonCommercial License, which permitsuse, distribution and reproduction in any medium, provided the originalwork is properly cited and is not used for commercial purposes.DOI: 10.1002/adfm.202316000applications.[1–3] It exhibits a direct bandgapthat can be tuned by controlling its numberof layers (≈2 eV for monolayer, to ≈0.31 eVfor bulk),[2,4–6] by the application of strain(bulk bandgap of 0.22 eV for 1.21% ten-sile, 0.53 eV for 0.66% compressive),[7,8]by alloying with arsenic (bulk bandgap of0.269 eV for 91% As),[9] and by embed-ding within an electric field (bulk bandgapof 0.160 eV under 0.48 V nm−1 field).[10]Its direct bandgap, low Auger coefficient,and low surface recombination velocityresult in a high radiative efficiency,[11]which has already been exploited in IRlight emitting diodes.[7,12–14] It exhibits ahigh carrier mobility, allowing the demon-stration of detectors with GHz responsespeeds,[1,15] and anisotropic optical proper-ties, which have been used to detect lin-ear polarization.[4,16] Preliminary demon-strations of larger area bP devices havealso been made using inkjet printing ofsolution dispersed bP inks,[17,18] although this remains an area ofdevelopment.Unlike other van der Waals materials, where research hasfocused mainly on the monolayer to the few-layer regime,K. WatanabeResearch Center for Electronic and Optical MaterialsNational Institute for Materials Science1-1 Namiki, Tsukuba 305-0044, JapanT. TaniguchiInternational Center for Materials NanoarchitectonicsNational Institute for Materials Science1-1 Namiki, Tsukuba 305-0044, JapanM. S. FuhrerAustralian Research Council Centre of Excellence in Future Low-EnergyElectronics Technologies (FLEET)Monash UniversityClayton, Victoria 3800, AustraliaK. B. CrozierSchool of PhysicsThe University of MelbourneParkville, Victoria 3010, AustraliaK. B. CrozierAustralian Research Council (ARC) Centre of Excellence forTransformative Meta-Optical Systems (TMOS)The University of MelbourneParkville, Victoria 3010, AustraliaAdv. Funct. Mater. 2024, 34, 2316000 2316000 (1 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbHhttp://www.afm-journal.demailto:james.bullock@unimelb.edu.auhttps://doi.org/10.1002/adfm.202316000http://creativecommons.org/licenses/by-nc/4.0/http://crossmark.crossref.org/dialog/?doi=10.1002%2Fadfm.202316000&domain=pdf&date_stamp=2024-05-07www.advancedsciencenews.com www.afm-journal.deFigure 1. a) Exploded-view schematic diagram of hBN/bP/hBN dual-gate structure with two Cr/Au back gates on a Si/SiO2 substrate. An example biasingconfiguration is shown by the black lines. b,c) Illustrative band diagrams of the device operating as a pn or np junction diode when a strong positivebias is applied to different back gates.much of the research on bP has been in its bulk form (i.e.,>≈8 layers).[6,19,20] This is especially true for demonstrationsof photodetectors/light emitters in the mid-wave IR (MWIR)region.[4,9,14,21–23] These have been formed predominantly bystacking bulk bP with other 2D materials, creating functional het-erojunctions, for example, with MoS2,[4,7,24] MoTe2,[25] WSe2,[26],and WS2.[27] An alternative approach to create junctions for lightcollection/emission is to instead electrostatically induce a homo-junction within the bP absorber/emitter.[28] This can be achievedusing localized rear gates under a bP channel, such as the dual-gate structure shown in Figure 1. In this structure, asymmetricpotentials can be applied to the two gates, resulting in the for-mation of a lateral pn homojunction. Figure 1a shows a deviceschematic with an example biasing configuration. By applyinga positive bias to back gate 1, while holding back gate 2 at zerobias, a lateral homojunction can be formed creating the banddiagram shown in Figure 1c. The strong positive bias invertsthe above channel region to n-type, from its typical p-type state.The direction of the junction can be reversed by switching thegate bias configuration as illustrated in Figure 1b. Several earlierstudies have demonstrated this, or similar, device architectureson 2D materials. These include monolayer and few-layer transi-tion metal dichalcogenide absorbers, such as MoS2, MoSe2, WS2,and WSe2,[29–33] as well as few-layer bP absorbers.[28] The efficacyof these electrostatically induced lateral pn homojunctions hasbeen demonstrated in photodiodes, photovoltaic cells, and light-emitting diodes.[34–40] Table 1 presents a comparison of recentstudies utilizing dual-gate structures, together with their accom-panying photodetector/photovoltaic figures-of-merit. At present,all these demonstrations have focused on visible wavelengthapplications.In this study, we extend this family of devices into the IRregion, demonstrating functionality to wavelengths as long as𝜆 = 2.2 μm. Importantly, to be able to detect infrared wave-lengths we utilize bulk bP at thicknesses of 10–20 nm, whichis thin enough to be modulated by back gates, while still main-taining its bulk bandgap (Eg ≈0.31 eV). This bP flake is encap-sulated by two hexagonal boron nitride (hBN) layers that pro-tect the bP flake from oxidation, with the bottom hBN layeralso serving as a rear gate dielectric.[34] When biased as a pho-todiode, these devices achieve open circuit voltages higher thanhalf bP’s bulk bandgap, which is a promising demonstrationgiven their early stage of development. In addition, it is shownthat such devices can be easily switched between photodiodeand photoconductor modes of operation, enabling reconfigurabledetectors.2. Results and DiscussionAn optical micrograph of a representative hBN/bP/hBN dual-gatedevice on a Si/SiO2 substrate is presented in Figure 2a. A de-tailed account of the fabrication procedure for these devices isprovided in the Experimental Section and shown in Figure S2(Supporting Information). Briefly, the thin bP flake (10–20 nm)is encapsulated between two hBN layers (bottom layer 10–30 nm,top layer 10–60 nm), and transferred onto a set of two back gateCr/Au electrodes, using a poly(methyl methacrylate) (PMMA)dry transfer process. Prior to transfer, two vias are etched intothe top hBN layer and are filled with Au (≈25 nm) to form thesource and drain contacts. Finally, Cr/Au lines and wire bond-ing pads are formed, by electron beam lithography and elec-tron beam evaporation, at the abovementioned Au-filled vias. ThehBN/bP/hBN sandwich structure serves primarily to reduce oxi-dation/degradation of the bP layer, which is known to be unsta-ble in air.[2] To further explore the hBN/bP/hBN sandwich struc-ture, cross-sectional scanning transmission electron microscopy(STEM), and energy-dispersive X-ray spectroscopy (EDX) wasperformed. Results from these measurements are provided inFigure 2b,c. The STEM image in Figure 2b shows three layersseparated by thin amorphous interlayers. The layered nature ofthe bP and hBN can be clearly seen with interlayer spacings of5.3 and 3.3 Å, respectively, which agree well with those reportedin the literature.[41–43] The two thin (≈2 nm) interfacial layers, sit-uated between the hBN and bP layers, are most likely POx whichis commonly found on the surface of bP due to unintentionaloxidation of bP during fabrication. Figure 2c shows an image ofAdv. Funct. Mater. 2024, 34, 2316000 2316000 (2 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202316000 by National Institute For, Wiley Online Library on [19/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deTable 1. 2D material devices featuring electrostatically induced lateral pn homojunctions.Material Bandgap[eV]Excitationwavelength[nm]Meas. condition VOC [mV] ISC [nA] PCE [%] R[mA W−1]ResponsetimeRefs.MonolayerWSe2≈1.65 532 295 K, Vacuum 650 1 0.005 210 - [35]MonolayerWSe21.64 500–800 295 K, Vacuum 640 0.014 0.5 16 - [34]MonolayerWSe21.65 640 295 K, Vacuum 700 0.41 0.01 0.24 ≈10 ms [36]10-layerMoSe21.41 AM1.5 Unspecified 700–850 0.606 14 ≈477 - [37]4 nm WSe2 ≈1.2 530 295 K, Vacuum 400 30 - - - [38]9 nmMoTe21 450 Unspecifiedtemperature,Vacuum300 0.4 0.57 ≈34 - [39]3.6 nmMoTe21 658 295 K, Ambient 420 8 2.98 1500 - [40]“Few-layer”bP<1.31 640 295 K, Vacuum 50 1 <0.001 ≈0.26 ≈2 ms [28]12 nm bP ≈0.31 2200 77 K, Vacuum 175 32 0.04 7.92 ≈13 μs This work12 nm bP ≈0.31 2200 295 K, Vacuum 74 5.1 0.01 1.26 ≈10 μs This workthe location (top) and elemental profile (bottom) of an EDX linescan across these interfaces. These confirm the identity of hBNand bP layers as well as the POx interlayers. The POx interlayerswere likely formed during the exfoliation/transfer process, afterthat the hBN encapsulation prevented further oxidation until theTEM lamella was prepared ≈1 month later. Note that the mea-sured bP thickness (≈12 nm), and hBN bottom layer (≈15 nm)that serves as the gate dielectric, are typical for devices reportedin this study.To investigate the effectiveness of the hBN/bP/hBN back gates,the dependence of the drain current on the gate voltage ID–VG,is measured. Figure 3a,b shows the ID–VG behavior when vary-ing the voltage at back gate 1 and back gate 2, respectively (re-ferred to henceforth as VBG1 and VBG2). Measurements are takenin the dark, under a drain source voltage VDS of 10 mV (dashedlines) and 100 mV (solid lines), at both 295 K (red curves) andat 77 K (blue curves). The device shows p-type behavior aroundVG = 0 V, as expected for bP.[9] Regardless of which back gate isused, a small n-type branch is seen at gate biases above VG = 10 V,confirming that it is possible to use either VBG1 or VBG2 to elec-trostatically create an n-type region in the bP layer. Significantlybetter characteristics are obtained at low temperatures. For exam-ple, an increase in the on/off drain current ratio from ≈2 to ≈4orders of magnitude is measured when cooling from 295 to 77 K.This behavior is expected from a narrow bandgap material wherethermal generation can significantly increase the carrier densitywith temperature.[44] We note that earlier devices revealed that ap-plying back gate voltages of ≈30 V and above resulted in a highchance of breakdown for the bottom hBN layer.[45,46] As such, aback gate voltage limit of 26 V is used for this study.Figure 2. Images of the hBN/bP/hBN sandwich structure. a) Optical micrograph of a completed hBN/bP/hBN sandwich structure device. b) Cross-sectional STEM HAADF image of part hBN/bP/hBN sandwich structure showing the top-hBN, bP, and bottom-hBN layers. c) STEM HAADF (top) andEDX line scan of the Si, N, B, P, and O K edges.Adv. Funct. Mater. 2024, 34, 2316000 2316000 (3 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202316000 by National Institute For, Wiley Online Library on [19/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 3. Dark current–voltage characteristic of bP dual-gate device. a,b) ID–VG characteristics of bP dual-gate device modulating BG1 and BG2, respec-tively. These were measured in the dark at 295 K (red) and at 77 K (blue) under VDS = 10 mV and VDS = 100 mV bias. c) ID–VD characteristics of bPdual-gate device operating in the dark at 77 K, for VBG1 ranging from 0 to 26 V. d) ID–VD characteristics of bP dual-gate device operating in the dark at77 K, for VBG2 varying from 0 to 18 V.Based on the above results, a lateral pn homojunction ex-hibiting diode-like behavior should be achievable in either direc-tion with sufficiently positive VBG1 or VBG2. A family of ID–VDcurves measured at 77 K while varying VBG1 or VBG2 in isolation(i.e., whilst holding the other back gate at 0 V), is presented inFigure 3c,d, respectively. These two figures show IDVD behaviortransitioning from Ohmic to rectifying behavior when VBG1 orVBG2 is increased beyond 7 or 12 V, respectively. The Ohmic char-acteristics seen around VBG1 = VBG2 = 0 V, indicates the lack ofany significant potential barriers at the bP/Au contacts. The rec-tification ratio seen with strong positive VBG1 or VBG2 is similarto that measured on bP/MoS2 photodiodes,[4] and demonstratethat a lateral pn homojunction can be formed in either direc-tion. Analogous, but less pronounced, rectification behavior canbe seen when performing the same measurements at 295 K, asshown in Figure S3a (Supporting Information).Next, we test the ability of these lateral pn homojunctions tocollect photoexcited carriers generated with IR light. For thesetests the device photoactive area is 36 μm2, and a 𝜆 = 2.2 μmlaser is used to illuminate the device with a power density of≈11.2 Wcm−2. Figure 4a shows the ID–VD behavior under con-stant illumination, measured at 77 K. This plot‘s curves are mea-sured under increasingly high VBG2 (while keeping VBG1 = 0 V)showing the formation of an open circuit voltage (VOC) and shortcircuit current (ISC) under IR illumination, confirming its func-tionality as a photodiode at VBG2 above 10 V. It clearly shows theVOC increase with VBG2 reaching a maximum of ≈175 mV. Theprogression of the VOC and ISC with increasing VBG2 is providedin Figure 4b, showing a positive correlation with applied backgate bias. From these results, it is clear that a higher VBG2 in-creases the lateral pn homojunction built-in potential and pro-vides higher junction collection probability. The saturation cur-rent I0 of this device can also be extracted above VBG2 = 13 V. Acomparison of I0 extracted using multiple techniques (see Sup-porting Information for explanation of extraction methods) isprovided in Figure 4b, showing values ≈0.3 nA, which are com-parable to those measured on similar devices previously.[28] Theformation of a VOC and ISC can also be replicated when usingVBG1 to form the lateral pn homojunction, as shown in Figure 4c.This plot compares typical ID–VD sweeps in the dark/under IRillumination for high VBG1 or VBG2 biases, showing that similarVOC and ISC are formed in both directions. The power conver-sion efficiency (PCE) under a wavelength 𝜆 = 2.2 μm can be ap-proximated to be ≈0.04% based on these measurements. Thisis a slight improvement over the efficiency observed in previ-ous studies on electrostatically induced bP homojunctions (seeFigure S4, Supporting Information for power–voltage curve).[28]A similar suite of measurements taken at 295 K, yielded lowerresults with a maximum VOC of 74 mV (see Figure S3b,c, Sup-porting Information). While this result is lower than that mea-sured at 77 K, likely due to the increase in the intrinsic carrierconcentration,[47] it is still the highest reported for this devicearchitecture to date (under similar illumination power). We notethat the formation of a VOC and ISC confirms the device is notAdv. Funct. Mater. 2024, 34, 2316000 2316000 (4 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202316000 by National Institute For, Wiley Online Library on [19/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deFigure 4. Current–voltage characteristic of bP dual-gate pn junction diode showing the photovoltaic effect under 𝜆 = 2.2 μm laser illumination. a) Backgate dependence of bP dual-gate pn junction showing photodiode like behavior with larger positive VBG2 biases. b) Top: The relationship between open-circuit voltage (VOC) and short-circuit current (ISC) against VBG2 at 77 K. Bottom: The relationship between extracted saturation current (I0) and VBG2.c) ID–VD characteristics under different VBG1 and VBG2 configurations (dark and 𝜆 = 2.2 μm laser illumination) at 77 K.detecting light as a photoconductor under these bias conditions.However, it can be used as a photoconductor under VBG1 = VBG2= 0 V biasing conditions, showing a decrease in resistance underillumination, as highlighted in Figure S5 (Supporting Informa-tion). In the photoconductive mode the responsivity of the dualgate device is also found to be highly dependent on illuminationintensity (see Figure S5b, Supporting Information), as is com-monly found for 2D material photoconductors.[48–50]To further standardize the performance of this device, pho-todetector signal-to-noise and speed figures-of-merit are alsomeasured under 𝜆 = 2.2 μm illumination. The specific detectiv-ity (D*) is calculated from measurements using the dark currentand/or saturation current to estimate noise (details in the Exper-imental section).[51–53] Figure 5a shows the relationship betweenD* and VBG2 (blue squares) or VBG1 (red circles) for a bP dual-gatedevice at 77 K (solid) and 295 K (hollow). As with the VOC andISC trend discussed above, the detectivity increases with VBG2 orVBG1. The highest D* of 8.5 × 108 cm Hz1/2 W−1 is obtained whenVBG1 = 26 V and VBG2 = 0 V at 77 K. As expected, the bP dual-gate device exhibits higher D* at 77 K than at 295 K. A similar,but lower, D* of ≈2.7 × 108 cm Hz1/2 W−1 is calculated for the de-vice in the photoconductive mode (i.e., when VBG1 = VBG2 = 0 V)under VDS = 350 mV. As expected, the photoconductive modeproduces a higher responsivity, reaching 0.55 A W−1 under VDS= 500 mV, which is offset by a higher noise current, but may stillbe useful in applications requiring high responsivity. It is likelythat this higher responsivity is partially attributable to the trap-induced photoconductive gain mechanism, which is commonlyfound in 2D material photoconductors.[54,55]The D* is also investigated as a function of VDS when inthe photovoltaic mode (i.e., when VBG1 = 0 V, VBG2 = 18 V).As expected, Figure 5b shows that low D* values are obtainedin the forward bias region, increasing to a peak D* of≈7.2 × 108 cm Hz1/2 W−1 at zero bias, with only slightly lowerFigure 5. hBN/bP/hBN dual-gate device as a photodetector. a) The relationship between the specific detectivity D* of a bP dual-gate device at VDS = 0 Vand the magnitude of VBG1 (red) or VBG2 (blue) at 295 K (hollow) and 77 K (solid). b) D* of a bP dual-gate device, with fixed back gate condition (VBG1= 0 V, VBG2 = 18 V), under different VDS bias conditions at 77 K. c) Frequency response of hBN/bP/hBN dual-gate device under photovoltaic (top) andphotoconductive (bottom) modes of operation. d) Normalized photoresponse under IR illumination as a function of linear polarization angle.Adv. Funct. Mater. 2024, 34, 2316000 2316000 (5 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202316000 by National Institute For, Wiley Online Library on [19/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deD* obtained in the reverse bias region. It should be noted thatthe above D* values are upper limit estimates due to the useof calculated idealized noise rather than utilizing noise spectraldensity measurements. In reality, the noise is likely also affectedby non-fundamental noise mechanisms, such as flicker noise,which can be significantly higher than the calculated idealizednoise.[56] In addition, no attempt was made to optimize the opti-cal structure of the device. Light absorption enhancement strate-gies could be combined with this structure to greatly enhancethe bP layer absorption, such as integrating structures that sup-port surface plasmons,[57] matching the absorption wavelengthwith exciton resonances,[58] or by simply tuning the layer thick-nesses in the stack. The latter of these is explored in supportinginformation Figure S1 (Supporting Information) and shows thata greater than 10× improvement could be obtained by alteringthe hBN layer thicknesses in isolation.Next, the frequency response of the bP dual-gate device is in-vestigated by subjecting it to electrically modulated (1 kHz) 𝜆 =2.2 μm illumination at 77 K. The device’s response under differ-ent back gate biasing modes is tested. As shown in Figure 5c, un-der the photovoltaic mode (i.e., when VBG1 = 0 V, VBG2 = 18 V), therise and fall times of the device are less than 13 μs, limited in thiscase by our instrumentation. We expect that the actual responsetimes of the device in the photovoltaic mode, that is, had we notbeen limited by instrumentation, are much shorter.[23,24] Underthe photoconductor mode (i.e., when VBG1 = VBG2 = 0 V, VDS =100 mV), the rise and fall time are found to be 25 and 44.6 μs, re-spectively. The slower response speed of photoconductors is com-monly explained by a greater dependency on the lifetime of trapsin the bP channel.[53,55] For context to the above figures of merit, acompilation of results for 2D material detectors in the 1–3.5 μmIR region has been included in Table S1 (Supporting Informa-tion). It can be seen that speed and responsivity/detectivity re-sults obtained in the photoconductive and photodiode modes ofoperation fall within the range of reported values for 2D material-based photodetectors in the short-wave IR wavelength space. Fi-nally, the polarization dependence of a representative bP dual-gate device was measured using an unpolarized broadband IR il-lumination source with a wire grid polarizer. Figure 5d shows theexpected strong dependence of the photoresponse on the linearpolarization angle that is characteristic of bP’s anisotropic crystalstructure.[4,16,59]3. ConclusionIn this study, hBN/bP/hBN dual-gate devices are fabricated anddemonstrated as IR photodetectors. It is shown that applying suf-ficiently large bias to one of the two rear gates, while holding theother at zero bias, leads to the formation of a lateral pn junction.Under IR (𝜆 = 2.2 μm) illumination, this lateral pn junction ex-hibits the photovoltaic effect yielding a VOC as high as 175 and74 mV, at 77 and 295 K, respectively. These are the highest valuesreported for bP based dual-gate devices to date. When being usedto detect light, under zero source-drain voltage, a specific detec-tivity of 8.5 × 108 and 2 × 107 cm Hz1/2 W−1 is measured at 77and 295 K, respectively. By modulating the back gate voltage, thedual-gate structure also allows switching between photoconduc-tive and photovoltaic modes of operation. This allows a tradeoffbetween low noise/fast response (photovoltaic mode) and highresponsivity (photoconductive mode). This development extendsthe application of dual-gate van der Waals materials photodetec-tors into the IR wavelength space.4. Experimental SectionDevice Fabrication/Imaging: hBN/bP/hBN dual-gate devices were fab-ricated on Si/SiO2 substrates pre-patterned with Cr (5 nm) / Au (45 nm)back gates defined by electron beam lithography (EBL). The bottom hBNlayer was first transferred onto the pre-patterned substrate, positionedover a set of back dual-gates, using a poly(methyl methacrylate) (PMMA)based dry transfer process. In a separate process, a stack comprising thetop hBN layer and bP was prepared. Vias in the top hBN layer were pat-terned using standard UV photolithography and then etched with reac-tive ion etching (RIE) (Oxford Instrument PLASMALAB100 ICP380). Thisetching step involved subjecting patterned hBN flakes to SF6 plasma (IF-100, ICP-600) for 5 s to completely remove the unprotected hBN. Af-ter that, 25 nm of Au was deposited into the etched region of hBN viaelectron-beam evaporation. Then, bP flakes were exfoliated in a low oxy-gen/moisture environment and the top hBN layer with Au vias was trans-ferred on top. Subsequently, the hBN/bP heterostructure was transferredonto the bottom-hBN to fabricate the hBN/bP/hBN dual-gate structure.Finally, source and drain regions were defined by EBL, and Cr (10 nm)/Au(60 nm) contacts were deposited via electron beam evaporation. Followingthe lift-off of Cr/Au in the non-patterned areas, the completed devices weremounted and wire-bonded into a chip carrier for characterization. The bPbulk crystals, sourced from HQ graphene, have been recently measuredto have carrier concentrations between 6.5 × 1011 and 1.25 × 1013 cm−2and Hall mobilities between 529 and 1615 cm2 V−1s−1.[60]Transmission electron microscopy (TEM) and Scanning TEM (STEM)imaging were performed using Hitachi HF5000 Cs-corrected S/TEM sys-tem. The HF5000 is equipped with Gatan OneView camera for TEM imag-ing and Hitachi bright field (BF) and dark field (DF) detectors for STEMimaging. This microscope also had symmetrically opposed dual 100 mm2EDX detectors (Oxford) for high sensitivity elemental analysis. STEM im-ages were captured using an illumination angle of ≈32 mrad. The STEMcamera length was 37 cm. Collection angle (𝛽) was 40 < 𝛽 < 213 (for DFimaging) and 𝛽 < 3.3 (for BF imaging). Sample for TEM imaging was pre-pared using NX5000 FIB.Device Characterization: Dark current–voltage measurements of thehBN/bP/hBN dual-gate devices were measured using a Keysight B1500Asemiconductor device parameter analyzer. Light current–voltage measure-ments were conducted using a 𝜆 = 2.2 μm laser (CNI MDL-H-2200) asthe illumination source, and three source meters (Keithley 2425/2450).Rise/fall times were extracted by electrically modulating the laser illumi-nation and capturing the time domain photocurrent, after amplificationfrom a transimpedance amplifier (SR570), on an oscilloscope (TektronixTDS 3034B). A schematic of this set-up can be found in the previouswork, for example in Figure S9 (Supporting Information) of Ref. [61] Thelow/room temperature measurements mentioned above were performedin a cryostat with an IR transmissive zinc selenide window with/withoutliquid nitrogen cooling. External quantum efficiency was extracted fromthe light/dark current–voltage behavior using:[62]𝜂e =Rhcq𝜆(1)where R is the responsivity of the device, given by the photocurrent dividedby the incident power (Iph/P), 𝜆 is the light wavelength, q is the elementarycharge, h is the Planck constant, and c is the speed of light. Specific detec-tivity values were then calculated, based on the following equation:[53]D∗ =𝜂e𝜆qhc(2q (ID + 2I0)A)−1∕2(2)where A is the area of the device, ID is the dark current, and I0 is the satu-ration current.Adv. Funct. Mater. 2024, 34, 2316000 2316000 (6 of 8) © 2024 The Authors. Advanced Functional Materials published by Wiley-VCH GmbH 16163028, 2024, 32, Downloaded from https://onlinelibrary.wiley.com/doi/10.1002/adfm.202316000 by National Institute For, Wiley Online Library on [19/12/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.dewww.advancedsciencenews.com www.afm-journal.deThe polarization dependent photoresponse was measured using an un-polarized broadband IR illumination source (Thorlabs SLS 203L) togetherwith a wire grid polarizer (WP25M-UB), to create a beam with a single lin-ear polarization. This beam was mechanically chopped and directed ontoa bP dual gate device and the linear polarization was varied by rotatingthe wire grid polarizer. The photocurrent from the bP dual gate device wasamplified using a transimpedance pre-amplifier (SR570), then measuredby a lock-in amplifier (SR510).Supporting InformationSupporting Information is available from the Wiley Online Library or fromthe author.AcknowledgementsThis work was supported by the Australian Research Council(DE210101129 and DP210103428). K.X. and M.S.F. acknowledgessupport from the Australian Research Council (DP200101345). M.T.acknowledges support from the Centre for Microscopy and Microanalysis,UQ, for providing infrastructure for materials characterization and JosephFernando and Joseph Otte for assistance with S/TEM imaging and EDXanalysis. K.B.C. acknowledges the support from the Australian ResearchCouncil (ARC) the Centre of Excellence for Transformative Meta-OpticalSystems (CE200100010). J.B. acknowledges support from the MelbourneCentre for Nanofabrication (MCN) through their Technology Fellowshipprogram. This work was performed in part at the Melbourne Centre forNanofabrication (MCN) in the Victorian Node of the Australian NationalFabrication Facility (ANFF).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available from the cor-responding author upon reasonable request.Keywords2D material, black phosphorus, electrostatic doping, infrared photodiode,pn junctionReceived: December 14, 2023Revised: March 7, 2024Published online: May 7, 2024[1] L. Li, Y. Yu, G. J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X. H. Chen, Y.Zhang, Nat. Nanotechnol. 2014, 9, 372.[2] F. Xia, H. Wang, Y. Jia, Nat. Commun. 2014, 5, 4458.[3] N. Youngblood, C. Chen, S. J. Koester, M. Li, Nat. Photonics 2015, 9,247.[4] J. Bullock, M. Amani, J. Cho, Y.-Z. Chen, G. H. Ahn, V. Adinolfi, V. R.Shrestha, Y. Gao, K. B. Crozier, Y.-L. Chueh, A. Javey, Nat. Photonics2018, 12, 601.[5] M. Huang, M. Wang, C. Chen, Z. Ma, X. Li, J. Han, Y. 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See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons Licensehttp://www.advancedsciencenews.comhttp://www.afm-journal.de Electrostatically Induced Black Phosphorus Infrared Photodiodes 1. Introduction 2. Results and Discussion 3. Conclusion 4. Experimental Section Supporting Information Acknowledgements Conflict of Interest Data Availability Statement Keywords