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石墨烯介电常数模型与表面等离激元激发特性说明书及开题
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Light trapping and surface plasmonenhanced high-performance NIRphotodetectorLin-Bao Luo1,3, Long-Hui Zeng1,3, Chao Xie2,3, Yong-Qiang Yu2,3, Feng-Xia Liang2,3, Chun-Yan Wu1,3,Li Wang1,3& Ji-Gang Hu1,31School of Electronic Science and Applied Physics, Hefei University of Technology, Hefei, Anhui 230009, P. R. China,2School ofMaterials Science and Engineering, Hefei University of Technology, Hefei, Anhui 230009, P. R. China,3Anhui Provincial KeyLaboratory of Advanced Functional Materials and Devices, Hefei University of Technology, Hefei, Anhui 230009, P. R. China.Heterojunctions near infrared (NIR) photodetectors have attracted increasing research interests for theirwide-ranging applications in many areas such as military surveillance, target detection, and light vision. Ahigh-performance NIR light photodetector was fabricated by coating the methyl-group terminated Sinanowire array with plasmonic gold nanoparticles (AuNPs) decorated graphene film. Theoreticalsimulation based on finite element method (FEM) reveals that the AuNPsgraphene/CH3-SiNWs arraydevice is capable of trapping the incident NIR light into the SiNWs array through SPP excitation andcoupling in the AuNPs decorated graphene layer. What is more, the coupling and trapping of freelypropagating plane waves from free space into the nanostructures, and surface passivation contribute to thehigh on-off ratio as well.Silicon (Si) with an indirect narrow band-gap (,1.12 eV), has been central to numerous technologicalinnovations for decades and remains to be the irreplaceable key material for electronic industry1,2.Compared with their thin film and bulk counterparts, silicon nanowires (SiNWs) with large surface-to-volume ratio3, and superior transport property4, have shown intriguing potential as functional building block forfabricating various optoelectronic devices, such as field-effect transistors5, solar cell6, chemical and biologicsensors7,8, photodetectors (PDs)9, and so on. Among various SiNWs-based nano-devices, PDs assembled fromSinanostructureshavereceivedspecialresearchinterests10,11.Byvirtueoftheappropriateband-gapandenhancedoptical properties, SiNWs-based PDs are able to probe infrared light with high sensitivity and excellent photo-response, which can constitute the core component for large-scale applications in many areas such as militarysurveillance, target detection and tracking12. In order to further boost the device performance (e.g. on/off ratio,response speed) of SiNWs-based PDs, people are resorting to new device structures with suppressed carrierrecombination, enhanced light absorption and carrier transportation.Graphene, as a promising alternative to transparent electrode, has exhibited extraordinary properties such ashigh optical transmittance, high thermal conductivity, excellent electronic and mechanical properties, as well asoutstanding chemical/physical stability with tunable work function13. As a result, graphene has been widelyutilized in various photovoltaic devices14, PDs15, and light-emitting diodes (LEDs)16. Surface plasmons (SPs),the electromagnetic wave induced electrical charges collective oscillation at the surfaces of metals is the pillarstones of application in light trapping17,18. Plasmonic noble metal nanoparticles (e.g. Au, Ag NPs) decoratedgraphene emerges as an alternative, unique electrode material that displays a wide range of extraordinaryproperties19, including strong absorption of light in NIR and visible range and supporting the surface plasmonpolaritons(SPPs)ingraphene.Additionally,SPPsboundtothesurfaceofgrapheneexhibitsuniquefeaturessuchas high modal index, relatively low loss, and flexible tunability by electric field, and magnetic field20,21. Herein, weproposeasimplestrategytofabricatehigh-performance NIRPDsbycoatingSiNWsarraywithAuNPsdecoratedgraphene. The as-fabricated device exhibits obvious sensitivity to 850 nm light illumination with an on/off ratioof 106, the highest value compared with other Si based devices. Theoretical simulation based on finite elementmethod (FEM) reveals that the high performance can be ascribed to the excellent optical property of AuNPsgraphene/SiNWsarraywhichasahighlyefficientSPP-basedNIRlightcouplingsystem,iscapableoftrappingtheincident NIR light into the SiNWs array through SPP excitation and coupling in the AuNPs decorated grapheneOPENSUBJECT AREAS:NANOPHOTONICS ANDPLASMONICSSENSORS AND BIOSENSORSReceived22 November 2013Accepted13 January 2014Published28 January 2014Correspondence andrequests for materialsshould be addressed toL.-B.L. () or J.-G.H.()SCIENTIFIC REPORTS | 4 : 3914 | DOI: 10.1038/srep039141layer.ThegeneralityoftheaboveresultsuggeststhatourNIRPDwillhave potential application in future optoelectronic devices.ResultsThe scheme in Fig. 1a illustrates the procedures to fabricate theAuNPsgraphene/SiNWs array NIRPD. The vertical SiNWs arraysynthesized via a Ag-assisted chemical etching method was methyl-terminated to reduce charge recombination at the surface of theSiNWs. The polymethylmethacrylate (PMMA)-supported multi-layer graphene film was then transferred onto the surface of theSiNWs array, followed by removal of the PMMA in acetone.Finally, AuNPs were deposited onto graphene by spin-coatingAuCl3solution. Fig. 1b shows the cross-sectional SEM image ofSiNWs array, from which well-aligned SiNWs with length of about28 , 30 mm were observed. According to the statistical distributionin Fig. 1c, the diameters of the SiNWs are in the range of 120220 nm, with an average value of ,170 nm. Raman spectrum ofthe graphene film is found to consist of two sharp peaks: i.e., 2D-band peak at ,2698 cm21and G-band peak at ,1581 cm21. Theintensity ratio of I2D5IG 1.02, and the presence of weak D-bandscattering at ,1350 cm21, suggest that the graphene is mainly com-posed of multilayer graphene with few defects (Fig. 1d)22,23. Fig. 1gdepicts a representative SEM image of multilayer graphene/SiNWsSchottkyjunctionwhichismodifiedbyAuNPswithdiametersintherange of 40100 nm. These AuNPs, uniformed dispersed on thegraphene film (Fig. 1e), are formed through the reduction of Au ionsby accepting electrons from graphene (c.f. Fig. 1f)24.In this study, the SiNWs array was intentionally functionalized bymonolayer methyl-group such that the surface recombination can beeffectivelysuppressed25. Fig.2ashowsthecurrent-voltage (I-V) curvesof a typical AuNPsgraphene/SiNWs array device at room temper-ature, from which one can see that the device exhibits obvious rec-tifying behavior due to the presence of Schottky barrier betweengraphene and SiNWs. Fig. 2b compares the current density voltage(J-V) curves of three samples with and without surface functionaliza-tion. It can be easily seen that, with the illumination of a NIR light(l 5 850 nm, 3 mW/cm2), the AuNPsgraphene/SiNWs arraystructure exhibits the best photovoltaic characteristics in terms ofopen circuit voltage (Voc) and short circuit current density (Jsc).Remarkably, such a photovoltaic effect can allow the efficient sensingof NIR light. As illustrated in Fig. 2c, when the light was turned onand off alternately at zero bias voltage, the AuNPsgraphene/SiNWsarray can be reversibly switched between low- and high-resistivitystates with a dark current of (Idark) 4.1 pA, and a photocurrent(Ilight) of 27 mA, yielding an Ilight/Idarkratio .106, the highest valueamong the three samples (see Fig. 2c and Fig. S1). As we will discusslater, this high on-off ratio could be partially attributed to theenhanced optical absorption of the NIRPD after decoration of theplasmonic AuNPs. Moreover, the increased sheet conductivity of thegraphene electrode after decoration with AuNPs can be also contrib-uted to the high on-off ratio as it can substantially reduce the seriesresistance in the circuit. Fig. 2d depicts the spectral response of threedevices, it is clear that the spectral response of these three devicesdisplays virtually similar spectral selectivity, with peak sensitivity ataround 950 nm.DiscussionThe photocurrent of the current NIRPD displays high dependenceon the intensity of excitation light. Fig. 3a plots the photocurrent ofAuNPsgraphene/SiNWs array NIRPD under light illuminationFigure 1|Devicefabrication andstructure characterization. (a)Schematicillustration ofthestep-wise process forfabrication ofthe NIRPD; (b)Cross-sectional SEM image of the SiNWs, the inset shows the magnified SEM image of the SiNWs array; (c) Statistical distribution of the diameters of SiNWs;(d) Raman spectrum of the multilayer graphene film, the inset shows a SEM image of the graphene film on the Si substrate; (e) SEM image of AuNPson graphene film, the inset shows TEM image of a AuNP; (f) The Au 4f spectrum of AuNPs on the graphene film; (g) Cross-sectional SEM imageof NIRPD./scientificreportsSCIENTIFIC REPORTS | 4 : 3914 | DOI: 10.1038/srep039142with different intensities. It is obvious that the photocurrent of thedevice increases gradually with increasing light intensity. Thisdependence of photocurrent on light intensity can be fitted by asimple power law: iphoto5 APh, where A is a constant for a certainwavelength, and the exponent h (0.5 , h , 1) determines the res-ponseofphotocurrenttolightintensity26.Byfittingthisequation,hisestimated to be 0.91. This nearly integer exponent suggests that thetrapstates havebeeneffectivelylessened whichishighlybeneficial tothe photosensing ability27. Next, the photoresponse of the NIRPD topulsed IR light (850 nm) was also investigated. As shown in Fig. 3d,for all switching frequencies (02200 Hz), our device can be revers-ibly switched between high and low conductance. Notably, even at2200 Hz, the relative balance only decreases by less than 10%, indi-cating great potential to monitor pulsed light with very high fre-quency. As shown in Fig. 3e, the response time is defined as thetime required for the photocurrent to increase from 10% to 90%,the recovery time is defined analogously. Careful analysis of curveleads to a small response/recovery time (tr/tf) of 73/96 ms in switch-ing frequency of 2200 Hz. This response speed is much quicker thanthat of similar PDs (c.f. Table 1). We believe the fast response speedcanbeattributedtothefollowingtwofactors:(1)Thebuilt-inelectricfield formed by the Schottky junction. During light detection, thephoto-excited electron-hole pairs can be quickly separated by theelectric field and then transferred to the electrodes, giving rise to fastresponse speed. (2) Effective surface passivation of SiNWs surface.The surface energy band bending caused by the -CH3modificationcan also facilitate the separation of photo-generated electron-holepairs in the radial direction of SiNWs.In order to clearly compare the performance of our device withexisting counterparts, the responsivity (R) and detectivity (D*), twokey metrics normally used to quantify the performance of photode-tector are calculated. For the present NIRPD, the responsivity anddetectivity were calculated to be 1.5 A/W and 2.54 3 1014, respect-ively (Please refer to the supporting information for the detailedcalculation of both parameters). Table 1 summarizes the device per-formanceofthepresentdeviceandotherPDswithsimilarstructures.Althoughthe responsivity does notshowany obvious advantage, thedetectivity of the present device shows the highest value, and theresponse/recovery time of our device is substantially reduced, incomparison with other Si based NIRPDs. What is more, the on/offratio is much higher than not only graphene/ZnO nanorods array28,graphene/Ge wafer29,graphene-PbS quantum dot(QDs)30,grapheneQDs arrays31, and graphene/CdSe nanobelt PDs32, but also Si basedphotodetector including graphene/Si substrate (104)2and pure gra-phene/SiNWs heterojunction (,103)33.To unveil the physics behind the high performance of NIRPD, westudied the optical properties of AuNPsgraphene/SiNWs arraystructure using FEM. For the sake of convenience, we only considerthegraphene/SiNWsatfirst.Undertheilluminationof850 nmlight,theequivalentrelativepermittivityofgraphenesheetcanbeobtainedbyKumbo formula as20.62451 0.0015i (seedetails in thesupport-inginformation),where thenegativerealpart andnear-zeroimagin-ary part of epsilon suggests that it behaves as a low-loss transparentmetal film. These unique electrical and optical characteristic enablethe dual-function for the graphene sheet, i.e., serving as both excel-lent transparent electrode and metallic thin film for SPP excitation.Fig. 4ah illustrate the electric field energy distribution of the gra-phene/SiNWs array with average period and diameter of 220 and140 nm, 240 and 160 nm, 260 and 180 nm, 280 and 200 nm. It isvisible that incident light can be efficiently coupled into the SiNWs.Suchalighttrappingeffectissoefficientthatthefieldintensityisveryhigh even in the NW, ,4 mm away from the Schottky contact, lead-ingtothehighenergyutilizationefficiency.Specifically,accordingtothe magnified electric field energy distribution (Fig. 4b, d, f, h), thestrongest electric field can be induced by the SiNWs with a diameter180 nm, which agrees with the average diameter of the as-etchedSiNWs. Moreover, waves with the large wave vector provided bythescatteringintheprocessofend-couplingcanexcitetheSPPwavesinthegraphenelayer,leadingtoapparentelectricfieldenhancement,which can greatly facilitate the improvement of electron transportFigure 2|OptoelectroniccharacteristicsoftheNIRPD.(a)I-VcurvesoftheNIRPDmeasuredatroomtemperature,theinsetshowsthedevicestructure;(b) I-V characteristics, and (c) photoresponse of three representative devices under 850 nm light illumination at Vbias5 0 V; (d) The correspondingspectral response, the sensitivity is defined by 100 3 Icurrent/Imax, where Imaxis the maximum photocurrent of AuNPsgraphene/CH3-SiNWs array, ataround 950 nm. To make the analysis more reliable, we kept the light power identical for all wavelengths during analysis./scientificreportsSCIENTIFIC REPORTS | 4 : 3914 | DOI: 10.1038/srep039143efficiency at the interface between the graphene sheet and SiNWs.This excellent optical property is in stark contrast to the case of bulkSi/graphene. Fig. 5a shows the poor field penetration depth andintensity in bulk Si layer. It can be observed that most of the lightwill be reflected at the incident interface, and no SPP waves aregenerated due to unmatched moment between incident light in freespace and SPP waves in graphene sheet34,35. As matter of fact, thistheoretical simulation result is in good agreement with the reducedreflectivity, as illustrated in Fig. 5b. The effective light trapping effectand remarkable SPP-induced field enhancement at top interfaceoffered by the graphene/SiNWs array structure are highly beneficialto the IR light detection as they can provide a long optical path andmoresurfaceareaforefficientlightharvesting,enhancinglightcoup-ling efficiency from free space to SiNWs, and suppress lightreflection36.Interestingly, the light trapping efficiency and localized field spa-tialdistributionofthegraphene/SiNWsarrayjunctioncanbefurtherenhanced through the introduction of the plasmonic AuNPs on thesurface of multilayer graphene layer. In this case, the AuNPsgra-phene/SiNWs array is virtually an SPP-based light coupling regimeinwhichtheAuNPscanactassub-wavelengthscatteringelementstocouple and trap freely propagating plane waves from free space intoanabsorbingsemiconductorstructurebyforwardscatteringthelightinto the nanowires17. Furthermore, SPPs excited at the AuNPs/gra-phene interface can efficiently trap and guide light into the SiNWs.Fig. 6ad show the simulated electric field energy distribution of theFigure 3|PhotoresponseoftheNIRPD.(a)PhotoresponseoftheAuNPsgraphene/CH3-SiNWsarrayNIRPDunderlightilluminationwithincreasingintensities from 0.81 to 174 mW/cm2; (b) The fitting of the relationship between the photocurrent and light intensity. The light wavelength was 850 nm;(c) Photoresponse of the NIRPD to pulsed IR light irradiation (850 nm) at different frequencies; (d) The relative balance (Imax2 Imin)/Imaxversusswitching frequency; (e) A single normalized cycle measured at 2200 Hz for estimating both response time (tr) and recovery time (tf).Table 1|Summary of the device performances of the present NIRPD and other photodetectors with similar device structuresDevice StructureResponsivity (A/W)Response/Recovery timeIlight/IdarkDetectivity (Jones)RefAuNPsgraphene/CH3-SiNWs1.573/96 ms,106,1014Our workgraphene/Si0.4351.2/3 ms,104,1082graphene/ZnO nanorods array1130.7/3.6 ms,103/28graphene/Ge wafer0.0523/108 ms,104,101029graphene-PbS QDs10710/10 ms/,101330graphene QDs arrays8.61/,101/31graphene/n-CdSe nanobelt8.770/137 ms,105/32graphene/pure SiNWs/20 ms,103/33/scientificreportsSCIENTIFIC REPORTS | 4 : 3914 | DOI: 10.1038/srep039144Figure 4|Theoreticalsimulation ofgraphene/SiNWs array structure. Simulatedelectric fieldenergy distribution forthe graphene/SiNWs withaverageperiod and diameter of (a) 220 and 140 nm, (c) 240 and 160 nm, (e) 260 and 180 nm and (g) 280 and 200 nm; (b), (d), (f) and (h) show the magnifiedelectric field energy distribution of one unit of graphene/SiNWs. The average value of the gap between two adjacent SiNWs is ,80 nm./scientificreportsSCIENTIFIC REPORTS | 4 : 3914 | DOI: 10.1038/srep039145devices with AuNPs of different diameters, from which one can seethat the strong coupling between the AuNPs and graphene sheetmakes the SPP localized field more concentrated in the area justbelow the AuNPs37. Notably, relatively strong localized filed can beobserved in the coupling system composed of graphene and AuNPswith diameter in the range of 6070 nm. Such a special feature cannot only cause increased field enhancement in graphene sheet, butalso lead to directional scattering and coupling of light into theFigure 5|OpticalpropertiesoftheplanarSiandH-SiNWs.(a)TheelectricfieldenergydistributionofbulkSiwithgraphenelayer.(b)Reflectionspectraof both planar Si and H-SiNWs array.Figure 6|Simulated electric fieldenergydistributionof theNIRPDs. Thediameters ofthe AuNPsare 40 nm(a), 50 nm (b), 60 nm(c)and70 nm (d),respectively. Note that the period and diameter of SiNWs are 260 and 180 nm, respectively. (e) The transmittance of graphene with and without Audecoration./scientificreportsSCIENTIFIC REPORTS | 4 : 3914 | DOI: 10.1038/srep039146SiNWs under the AuNPs. This improvement in light trapping effi-ciency and localized field enhancement agrees well with transmit-tance spectrum shown in Fig. 6e.Apart from the SPPs-induced light trapping effect, the surfacemodification of SiNWs contributes to the enhancement of deviceperformance as well. Fig. 7 illustrates the energy band diagrams ofthe graphene/SiNWs Schottky junction before and after surfacemodification. It is noted that surface modification can considerablyalter the surface electron affinity of SiNWs: The offset of surfaceelectron affinity for the CH3-SiNWs could be as high as 10.35 eV,while it is 20.12 eV for H-SiNWs without CH3-termination38.Therefore, the built-in electric field was greatly strengthened in theCH3-SiNWs device, as compared with the H-SiNWs device. On theother hand, the surface energy band bending caused by the -CH3modification can also facilitate the separation of photogeneratedelectron-hole pairs in the radial direction of SiNWs; the holes willaccumulate on the NW surface while electrons in the NW core,preventing thecarrierrecombination. Thirdly,passivation ofsurfacestates via Si-C on the SiNWs surface decreases the probability ofelectron-holecombinationandthusincreasesthecarriermobilities39.As a result, the lifetime of the minority carrier (holes) in the SiNWswill be prolonged, giving rise to increased photocurrent.In summary, high-performance NIRPD have been successfullyfabricated by coating the surface of graphene/SiNWs array withplasmonic AuNPs. The on/off ratio can reach as high as 106, thehighestvalueeverreported.Inaddition,theresponsitivityanddetec-tivityat zero bias voltage were estimated to be1.5 AW21,and 2.5231014cmHz1/2W21, respectively. Further device analysis shows thatthe NIRPD can work in a wide range of chopping frequencies, withhigh response speed (response/recovery time of 73/96 ms). Theore-tical simulation based on FEM reveals that the high sensitivity ismainlyattributedtolighttrappingandremarkableSPP-inducedfieldenhancement. Moreover, the coupling and trapping of freely prop-agatingplanewavesfromthelightintotheabsorbingsemiconductorstructure, and efficient surface passivation of the SiNWs also con-tribute to the observed high performance.MethodsPreparation and characterization of SiNWs array and graphene. The verticalSiNWs array in this work was prepared using a Ag-assisted chemical etchingapproach. Briefly, windows with size of 0.2 3 0.25 cm2were first defined on the SiO2(300 nm)/n-Siwafers(resistivity:110 Vcm)substratesbyadhesivetape,followedbydipping into a buffered oxide etch (BOE) solution until the underlying Si substrateswere exposed. The as-obtained substrate was then immersed into a mixed solution ofHF (4.8 M) and AgNO3(0.005 M) to coat AgNPs, which will act as catalyst duringetching in the etchant composed of HF (4.8 M) and H2O2(0.4 M). After etching for20 min, the as-obtained SiNWs array were dipped into a diluted HNO3and HFsolution to dissolve Ag and SiO2on the surface of SiNWs array, respectively. Methylgroup-terminated SiNWs array (denoted as CH3-SiNWs) was prepared following atwo-step chlorination/alkylation method25. The multilayer graphene film was grownon 20 mm thick Cu foil at 1000uC via a chemical-vapor deposition (CVD) method40.After growth, the graphene films were spin-coated with 5 wt.%polymethylmethacrylate(PMMA) in chlorobenzene, and thenthe underlying Cufoilwere removed in Marbles reagent solution (CuSO45HCl5H2O 510 g550 ml550 ml). The graphene film was rinsed in deionized water to remove theremaining ions. The multilayer graphene films were directly transferred onto SiO2/Sisubstrate and the PMMA was removed by acetone. The chemical composition of theAuNPs decorated graphene was analyzed by X-ray photoemission spectroscopy(XPS) which was performed on a VG ESCALAB 220i-XL analysis system equippedwith a monochromatic Al x-ray (1486.6 eV) source.Device construction and evaluation. To fabricate the AuNPs decorated graphene/SiNWs Schottky junction photodetector, 5/50 nm Ti/Au electrode, which served asthe electrical contact for graphene, was first deposited on the SiO2/Si substrate usingelectron-beam evaporator. Then the PMMA-supported multilayer graphene filmsweredirectlytransferredontothetopofSiNWs.Afterdryingat100uCfor10 min,theresidual PMMA on graphene film was removed by acetone. Indium-gallium (In-Ga)alloywasthenpastedontherearsideofSisubatratetoachieveohmiccontact.AuNPson graphene was obtained by spin-coating of AuCl3(10 mM in nitromethane) ontothe substrate at 2000 rpm for 1 min. The device characteristics of graphene/SiNWswere measured using a Keithley 4200 semiconductor characterization system. Todetermine thespectral responseandtimeresponse ofthe Schottkyjunctiondevices,ahome-built system composed of a light source (LE-SP-LS-XE), a monochromator(LE-SP-M300), an oscilloscope (Tektronix, TDS2012B), and an optical chopper (LE-oc120) was used.Theoretical simulation. The simulation of electric field energy density distributionwas performed by the finite element method (FEM). In our calculation, the incidentlightisp-polarizedwithawavelengthof850 nm,andthepermittivityofSiandAuare13.410 1 0.029i and 228.168 1 1.752i41,42, respectively. The equivalent relativepermittivity of graphene can be determined by the formula eG1zisGg0k0d, in whichthemultilayeredgrapheneistreatedasanultra-thinfilmwiththicknessof,1 nm,iisthe pure imaginary, g0and k0are the impedance and wave vector in air, respectively.TheconductivitysGisdeterminedbyKumboformula43,44,andcanbeevaluatedas45,46sGie2kBTp? h2(vzit1)fmckBTz2lnexp(mckBT)z1?g1wherev isthe lightangular frequency,Tthe temperature,ethe electron charge, ? hthereduced Plancks constant, kBthe Boltzmann constant, mcand t the chemistrypotential and the momentum relaxation time, respectively. Specifically, under l 5850 nm (1.459 eV), T 5 300 K, mc5 0.12 eV and t 5 0.5 ps, the equivalent relativepermittivityofgrapheneisestimatedtobe20.624510.0015i,suggestingametal-likebehavior and little optical loss.1. Sze,S.M.&Ng,K.K.Physicsandpropertiesofsemiconductors-areviewPhysicsof semiconductor devices 78 (John Wiley & Sons, Inc., New Jersey, 2006).2. 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Lett.94, 193101 (2009).Figure 7|Workingmechanism of theAuNPsgraphene/SiNWs array NIRPD.Energy banddiagrams ofbefore (a), andaftersurface modification (b);the WG/WSidenote the work functions of graphene/SiNWs, respectively, ECand EVare the conduction band minimum and valence bandmaximum of Si, respectively, EF, xSiand E0are the Fermi energy level, the electron affinity of Si, and the vacuum energy level, respectively./scientificreportsSCIENTIFIC REPORTS | 4 : 3914 | DOI: 10.1038/srep0391479. Zhang, A., Kim, H. K., Cheng, J. & Lo, Y. H. Ultrahigh responsivity visible andinfrared detection using silicon nanowire phototransistors. Nano Lett. 10,21172120 (2010).10.Yang, C.,Barrelet, C. J., Capasso, F. & Lieber, C.M. Single p-type/intrinsic/n-typesilicon nanowires as nanoscale avalanche photodetectors. Nano Lett. 6,29292934 (2006).11. Mulazimoglu, E. et al. Silicon nanowire network metal-semiconductor-metalphotodetector. Appl. Phys. Lett. 103, 083114 (2013).12. Rogalski, A. 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