ARTICLEReceived16Nov2012|Accepted25Feb2013|Published27Mar2013Narrowbandphotodetectioninthenear-infraredwithaplasmon-inducedhotelectrondeviceAliSobhani1,2,
MarkW. Knight1,2, YuminWang1,2, BobZheng1,2, NicholasS. King1,2,
LisaV. Brown2,3,ZheyuFang1,2,4,5, PeterNordlander1,2,4&NaomiJ.
Halas1,2,4Ingratings,
incidentlightcancouplestronglytoplasmonspropagatingthroughperiodicallyspacedslitsinametal
lm, resultinginastrong,
resonantabsorptionwhosefrequencyisdeterminedbythenanostructureperiodicity.
Whenagratingispatternedonasiliconsub-strate,
theabsorptionresponsecanbecombinedwithplasmon-inducedhot
electronpho-tocurrent generation. This yields a photodetector with
a strongly resonant, narrowbandphotocurrent response in the
infrared, limited at low frequencies by the Schottky barrier,
notthe bandgap of silicon. Here we report a grating-based hot
electron device with signicantlylarger photocurrent
responsivitythanpreviouslyreportedantenna-basedgeometries.
Thegrating geometry also enables more than three times narrower
spectral response thanobservedfor nanoantenna-baseddevices.
Thisapproachopensupthepossibilityof plas-monic sensorswith direct
electrical readout,such as anon-chip surface plasmon
resonancedetectordrivenatasinglewavelength.DOI:10.1038/ncomms26421DepartmentofElectricalandComputerEngineering,
RiceUniversity, 6100Main, Houston, Texas77005,
USA.2LaboratoryforNanophotonics, RiceUniversity, 6100 Main,
Houston, Texas 77005, USA.3Department of Chemistry, Rice
University, 6100 Main, Houston, Texas 77005, USA.4Department
ofPhysicsandAstronomy, RiceUniversity, 6100Main, Houston,
Texas77005, USA.5School ofPhysics, StateKeyLabforMesoscopicPhysics,
PekingUniversity, Beijing100871, China.
CorrespondenceandrequestsformaterialsshouldbeaddressedtoN.J.H.
(email:[email protected]).NATURECOMMUNICATIONS | 4:1643 | DOI:
10.1038/ncomms2642 | www.nature.com/naturecommunications
1&2013MacmillanPublishersLimited.Allrightsreserved.Plasmonscoherent
oscillations of free electrons in metalssupport intense
electromagnetic eld concentration, andprovide the mechanism for
harvesting light from free spaceand conning it to nanoscale
volumes1,2. Interest in theproperties of surface plasmons has
recently expanded toencompass a broad range of technological
applications, suchas nonlinear optics3,4, metamaterials5,6, solar
cell energyharvesting7,8, surface-enhanced sensing and
spectroscopy9,10,and novel medical therapies11,12. These
technologies rely, tovarying degrees, on three key properties of
plasmonic resonances:high eld localization, resonant scattering and
heat generation innanoscale volumes.Another propertyof plasmons,
their abilitytogenerate hotelectrons viaplasmondecayat
highefciencies, has becomeatopicofrapidlyincreasinginterest1315.
Recentlyseveral groupshave reported tunable detection of light in
metalsemiconductordevices by exploiting plasmon-induced hot
electron generation1418.In these devices, the incident
electromagnetic radiation was
coupledintosurfaceplasmonsbymetallicnanoantennaspatternedontoasemiconductorsubstrate.
Thenonradiativedecayof theplasmonsresults in hot electrons that can
transfer across the Schottky barrierat the metalsemiconductor
interface andcanbe detectedas aphotocurrent16,18,19. This new type
of Schottky photodetectorenables a unique direct conversion of
light captured by theantenna into an electrical signal: the
tunability of thenanoantenna,
suchasitswavelengthandpolarizationdependence,enables a
straightforward tailoring of the responsivity properties ofthe
photodetector.The phenomenon of extraordinary optical transmission
(EOT)on periodic metallic hole/slit structures provides
anotherapproach to light harvesting that could be useful for
hotelectron-basedphotodetection.
Periodicslitsformagrating20,21,which compensates for the momentum
difference betweenpropagatingphotonsandplasmons,
andenablestheformationof surfaceplasmonwavescapableof
propagatingonthemetalsurface21. In addition to propagation along on
the upper surfaces,plasmons can also propagate through the slits of
the grating.
Thisphenomenonwasrstobservedformetalliclmswithperiodichole
arrays22. InEOT, plasmonic resonances enable far eldtransmission
through perforations that can be signicantlysmaller than the
minimum dimensions predicted usingdiffraction theory23.The same
plasmon modes that give rise to EOT also give rise tohot
electronsthroughnonradiativedecay. Bypatterningagold(Au) grating on
a semiconducting substrate with an appropriatelydesigned Schottky
barrier at the interface, the hot electronsgeneratedinthe metallic
structure shouldalsogive rise toaphotocurrent response. Just as in
the case of plasmonicnanoantennas, one would expect the optical
response of thegrating structure to be reected in the electric
current. Thisstrategyhas theadditional advantageof
alow-resistivityinputface (the nanostructuredmetal lm) that
shouldenhance thetransport properties of the device, including its
responsivity.Here we report the rst demonstration of a
plasmonicgrating-Schottky photodetector, which combines the
resonantnarrowband optical response of a grating structure with
hotelectron-enabled below bandgap photodetection. Device
measure-ments showed a drastic increase in the responsivity and
internalquantumefciency(IQE)relativetoearliernanoantenna-basedphotodetectors,
while simultaneously narrowing the spectralwidthof thephotodetector
responseandpreservingfree-spacecoupling andpolarizationselectivity.
The responsivity of thisdevice (0.6 mAW1at zero bias voltage) is
similar toresponsivities reported in the literature for propagating
plasmonsensors24. The IQEof this photodetector is B0.2%,
20timeslargerthanthepreviouslyreportedliteraturevalueof0.01%fornanoantenna-based
devices16. Unlike the previous reporteddevices mentioned, this
responsivitymaintains its narrowbandfull width at half
maximum(FWHM) of nominally 54 meV,narrower by a factor of more than
three compared withpreviously reported devices16,18. Using gratings
also permitsfacile linear tuning of the responsivity peak over a
broadAuTi layerAuSiWT1,300 1,400 1,500 1,600
1,70001002003004005006000.95 0.88 0.82 0.77 0.73Energy
(eV)Responsivity (nA mW1)Wavelength (nm)FWHM =54
meVDFigure1|Grating-Schottkyphotodetector.(a)Schematicofagoldgratingonann-typesiliconsubstratewitha2-nmTiadhesionlayer,
orientedtransversetothelaserpolarization.ForbettervisualizationthethicknessofTilayerisexaggerated.
Polarizationoftheincidentlaseranditskvectorare representedinyellow
andgreenarrows, respectively.(b)Scanning
electronmicroscopyimageofgoldgratingstructurewithgratingthickness(T)
200nm, interslitdistance(D) 950nmandslitwidth(W) 250nm.For
allstructuresthearraymeasured12 12 mm.The scalebaris1
mm.(c)Photocurrentresponsivitiesofgrating-basedphotodetectorsforthreedifferentgoldlayerthicknesses,
T 93 nm(black),
170nm(grey)and200nm(green),showingastrongintensitydependenceongratingthickness.ARTICLE
NATURECOMMUNICATIONS|DOI:10.1038/ncomms26422 NATURECOMMUNICATIONS |
4:1643 | DOI: 10.1038/ncomms2642 |
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regime in near-infrared (NIR), from1,295 nmto1,635 nm,
enablingtheuseofconventional
near-IRsourcesandalsointegrationwithotherdevices designedfor this
frequencyrange.ResultsDesign of the grating hot electron device.
Our fabricated devicesconsistofhigh-purityAu gratingswith
differentgratingperiodsona o1004n-type siliconwafer (500 mmthick,
110 Ocm),with a 2-nm thick titanium adhesion layer (Fig. 1a).
Although thisthintitaniumlayerweaklydampstheplasmonresponse25,
itiscrucial for bothdetermining the Schottky barrier height
andadhering the Au gratings to the silicon. We controlled
theplasmonic response of the grating structure by varying
threeindependent parameters: Au layer thickness (T), interslit
distance(D) and slit width (W) (Fig. 1a). To investigate the
dependence ofthedevicepropertiesonAulayerthickness,
wefabricatedthreedevices with T93 nm, 170 nmand 200 nm(Fig. 1b),
whilekeepingtheslitwidthandinterslitdistanceconstantat250 nmand 950
nm, respectively. Each grating covered a squaremeasuring 12 12
mm.We obtained the photocurrent responsivities for three
devicethicknesses (Fig. 1c). For all three Aulayer thicknesses,
thespectral response shows a single, narrowband responsivitypeak.
The width of these peaks, which have a FWHMof544 meV, is
independent of gratingthickness. For deviceswith a 200-nm thick Au
layer, the peak responsivity increasedbyafactorof
sevenrelativetothethinnest 93-nmgrating, atrend reproduced in the
calculated absorption spectra(Supplementary Fig. S1). This
signicant increase illustratesthe strong dependence of the
absorption on Au lm thickness,in a direct analogy with the known
dependence of EOT on lmthickness. Where EOT can enhance the
transmission oflight22,26,27, we can similarly exploit this effect
to amplifyplasmonicabsorptionatthemetalsemiconductorinterface28.The
reection and transmission spectra for these
threethicknessesareprovidedinSupplementaryFig. S2.The thickness
dependence of the absorption cross-section
ofgratingsisduetotheinteractionbetweenpropagatingsurfaceabSicAuabSiAuFigure2|Propagationofsurfaceplasmonsongratings.(a)Therearethreeformsofsurfaceplasmonpolaritons(SPPs)aroundeachgoldpitch:a,
SPPs oscillating at the top surface of gratings; b, SPPs
oscillating thoughthe slits and; c, SPPs oscillating at the bottom
surface of gratings at the
TiSiSchottkyinterface.(b)PlasmonicheatabsorptioncalculatedbyFDTD(shownwithalogarithmicscaleforclarity).
Mostofthehotelectrongenerationoccursatthebottomsurfaceofthegoldlayer.1,200
1,300 1,400 1,500 1,600 1,7008008509009501,0001,0501,100D:
Interslit distance (nm)Peak wavelength (nm)1,200 1,300 1,400 1,500
1,600 1,70001002003004005006001.03 0.95 0.88 0.82 0.77
0.73Responsivity (nA mW1) Energy (eV)Wavelength (nm)D = 800 nmD =
850 nmD = 900 nmD = 950 nmD = 1,000 nmD = 1,050 nmD = 1,100 nmW =
250 nmT = 200
nmFigure3|Controllingthedetectionwavelengthbytuningtheinterslitdistance.(a)Responsivitypeaksredshiftwithincreasinggratinginterslitdistance
(D). Changing D from 800 to 1,100nm tuned the responsivity
peakfrom1,290to1,635 nm(T 200nm, W250nm). Theredshiftinresponsivity
occurred with no change in FWHM. (b) FDTD (empty
circles)andexperimentaldata(lledcircles)fortheresponsivitypeakpositioninwavelength
showing a linear dependence on interslit distance (grey
dashedline). Images on the right show a representative top view
scanning electronmicroscopy image of the grating for each
corresponding responsivity
peak.Frombottomtotop,Dincreasesin50-nmincrements.NATURECOMMUNICATIONS|DOI:10.1038/ncomms2642
ARTICLENATURECOMMUNICATIONS | 4:1643 | DOI: 10.1038/ncomms2642 |
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3&2013MacmillanPublishersLimited.Allrightsreserved.plasmons on
the top and bottom surfaces of each Au pitch. InFig. 2a, we
schematically show the surface plasmonspropagatingontheupper
andlower surfaces of thegrating.These surface plasmons can
interfere destructively orconstructively with each other through
the slits in thegratings. This interference allows us to minimize
thedestructive interference by using a specic thickness,
whichmaximizes the amplitude of surface plasmon polariton decayand
hot electron generationproximal to the interfacialregion,
increasingthephotocurrent29.The other reason for the high
responsivity of grating-Schottky devices is the localizationof
absorptionlosses forgratings on silicon. The mean free path of the
hot electrons inAuis B35 nm30,31;
hotelectronsgeneratedintheAufurtherfromthe metalsemiconductor
interface tendtolose energythrough electronelectron scattering.
Once the energy of a hotelectron drops below the Schottky barrier
height, transmissionisunlikelytooccurandtheelectronwill
reectbackintotheAu without contributing to the photocurrent. To
minimizethese losses, the grating geometry has been designed
togeneratehot electrons primarilynear
theSchottkyinterface.Thislocalizedabsorptioncanbeseeninnite-differencetimedomain
(FDTD) simulations of a representative structurewith(T, D, W) (200,
950, 250) nm(olive curve, Fig.
1c).Calculatedabsorptionmapsattheresponsivitypeak(Fig. 2b,l 1,460
nm) show that almost all the absorption occurs at
themetalsemiconductorinterface.Photodetector tuning. For a given Au
layer thickness, the gratingresonance canbe tunedthroughthe
NIRspectral regime bycontrolling the interslit distance (D) (Fig.
3a). Increasing D from800 nm to 1,100 nm, in 50-nm increments,
tuned the responsivitypeakbetween1,295 nmand1,635 nm. Forall
sevengratingsinFig. 3, the width W and thickness T were held
constant at 250 nmand200 nm, respectively. Wavelengthselectivitywas
preservedwhentuning the device over a broadspectral range,
withallresponsivity spectra exhibiting a peak FWHM of 536 meV.The
responsivity peaks exhibit a linear redshift with
increasinginterslitdistance, D,
whichmatchesthespectrasimulatedusingtheFDTDmethodverywell (Fig.
3b). Thecalculatedresponseswere based on experimentally measured
dimensions, and
theopticalconstantsofallmaterialsusedinourdeviceswerepara-meterized
using literature values with no adjustable parameters3234.
Wepresentedtheresponsivityspectrafor devices
withthesameinterslitdistancesasmentionedinFig. 3,
butdifferentslitwidths of 200 nm and 300 nm in Supplementary Fig.
S3.We can understand both the linear tunability of ourphotodetector
and the lineshape by examining the surfacecharge
distributiononaAugrating(Fig. 4). Arepresentativephotocurrent
spectrum, correspondingtoadevicewith(T, D,W) (200, 1,100, 250) nm,
shows twopeaks at 1,635 nmand1,285 nm, which we refer to as Mode i
and Mode ii, respectively(Fig. 4a). A calculated absorption
spectrum closely matches boththe experimental linewidths and the
relative peak amplitudes. Foreach of these modes we have calculated
the surface chargedensity, where eachblue andredregionat the
metalsiliconinterface represents an anti-node of charge
accumulation(Fig. 4b,c). For Modei, surfacechargeoscillations
atthemetalsiliconinterface have ve anti-nodes, while the higher
energymode, Modeii, has sixanti-nodes. Thenumber of anti-nodesdenes
the mode number of the standing surface plasmon. For agiven mode
number, increasingthe interslit distance, D, linearlyincreases the
wavelengthof the standing wave at the metalsemiconductorinterface
proportionately, and therefore increasesthe wavelength of the
responsivity peak with the samedependence.DiscussionOne of the
electrical limitations of these types of Schottkyphotodetectors is
the Schottky barrier height at the metalsemiconductorinterface.
ByttingtheIVcurveofourmetalsemiconductor structure tothe
IVcharacteristic of a typicalSchottky diode, we extracted a barrier
height of B0.5 eV, similarto reportedliterature values of
titaniumandsiliconSchottkybarrier heights35. Controlled doping of
the interfacial layer allowsengineeringof theSchottkybarrier36,
andcouldpotentiallybeused to extend the effective detection range
of these devices deepinto the infrared, orincrease
deviceresponsivitiesby raising theIQE. Visible-light detectors
could also be designed by switching tohigher-bandgapsemiconductors
thanSi, suchas TiO2.Device1,200 1,300 1,400 1,500 1,600
1,7000.00.20.40.60.81.01.03 0.95 0.88 0.82 0.77 0.73Energy
(eV)Photocurrent (a.u.)Wavelength (nm)Mode iMode iiMode iMode
iiCharge (a.u.)Charge
(a.u.)AuSiAuSi101101Figure4|Identicationofthetwo
modesinresponsivityspectra.(a)FDTD(black
curve)andexperimentalresponsivity(redcurve)spectraforthegratingstructurewithT
200nm, D1,100nmandW250nm.The
correspondingresponsivitycurvehastwomodes:
Modeiatthewavelengthof1,615nm and Mode ii at the wavelength of
1,285nm. (b) Surface charge plot at the metalsemiconductor
interface at Mode i. (c) Surface charge plot at
themetalsemiconductorinterfaceatModeii.ARTICLE
NATURECOMMUNICATIONS|DOI:10.1038/ncomms26424 NATURECOMMUNICATIONS |
4:1643 | DOI: 10.1038/ncomms2642 |
www.nature.com/naturecommunications&2013MacmillanPublishersLimited.Allrightsreserved.responsivitiescanalsobeimprovedbyincreasingtheSchottkycontact
area of the device, as shown recently by Scales et al.37.Insummary,
a narrowband, geometricallytunable
photode-tectorintheNIRregionhasbeendemonstratedthatcombinesthe
spectral properties of gratings withanelectrical
responsebasedonplasmon-inducedhot electrongeneration.
Thewave-lengthselectivity, lineartunabilityandhighresponsivityof
thedevice make this type of photodetector useful for bridging the
gapbetween optical and electrical systems. This unique type of
devicecould have numerous applications requiring compact,
wave-length-sensitive detection, such as chemical or
bio-sensing,imaging and ranging, and communications
systems.MethodsDevicefabrication. Our photodetectors were made by
planar fabrication. Thesubstrate was lightly doped n-type
siliconcoated with native oxide (110 Ocm,o1004 orientation). We
depositeda 400-nm layer of photoresist (ZEP 520A,Zeon Corp.) by
spin coating, and subsequently patterned using electron
beamlithography. The samples were developed for 60 s in a 3:1
solution of ZED-ND50(n-amyl acetate) to isopropanol. A reactiveion
etch was used to strip the nativeoxide, and the samples were
immediatelytransferred into an electron beamevaporator and coated
with a 2-nm layer of titanium followed by a Au layer(devices used
Au thicknesses of 93, 170 and 200 nm). Excess material was
removedby liftoff in dimethylacetamide at 60 C. The sample was then
washed with iso-propanol and dried with N2.Optical measurement
techniques. The photocurrent responsivities were obtainedusing a
custom photocurrent microscope. Each device was illuminated at
normalincidence by an ultrabroadband white-light laser (Fianium),
with frequencyselectivity achieved using an acousto-optical tunable
lter (Crystal Tech.). For eachfrequency, the photocurrentwas
measured by modulating the laser source with achopper and a lock-in
amplier (Signal Recovery, 7280DSP). The centre of eacharray was
illuminated with the laser spot focused to B3-mm FWHM,
whicheliminated contributions from the array edges (each grating
was 12 12 mm).Measurementswere taken with light polarized
perpendicular to the slits; long-itudinally polarizedlight does not
couple to grating modes38and gave nomeasureable
photocurrent.FDTDsimulations. FDTD simulations were done in two
dimensions using theexperimental dielectric function from Johnson
and Christy for the Au material. Thesize of each modelled grating
was 12 mm to match the fabricated devices. Theincident beam was an
s-polarized free-space electromagnetic wave with
transversepolarization of the electric eld to the orientation of
slits. The beam had a Gaussianprole with a FWHM of 3 mm, which was
approximately the same as the experi-mentally used spot size.
Absorption spectra were calculated by integrating the
totalabsorption within each simulated grating array.IQEcalculation.
IQE was calculatedaccording to IQE 100 (Ip/q)/(Pab/hn)
inpercentage, where Ip is the photocurrent, q is the electron
charge, Pab is the powerof absorbed light and hn is the photon
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(2004).AcknowledgementsWe thank Fangfang Wen, J. B. Lassiter,
Shaunak Mukherjee, Alexander Urban, AndreaSchlather and Surbhi Lal
for their insight and input. This research was
nanciallyNATURECOMMUNICATIONS|DOI:10.1038/ncomms2642
ARTICLENATURECOMMUNICATIONS | 4:1643 | DOI: 10.1038/ncomms2642 |
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5&2013MacmillanPublishersLimited.Allrightsreserved.supported by
National Security Science and Engineering Faculty Fellowship
programmeof the US Department of Defense grant N00244-09-1-0067,
Robert A. Welch Foundationgrants C-1220 and C-1222, Ofce of Naval
Research grant N00014-10-1-0989 and NSFMRI grant
ECCS-1040478.AuthorcontributionsA.S. prepared the samples, A.S.,
M.W.K. and B.Z. contributed to photocurrentmeasurements. L.V.B.
performed FTIR measurements. Y.W. did FDTD calculations. Allthe
authors contributed to revising the manuscript and Supplementary
Information, andparticipated in discussions about this
work.AdditionalinformationSupplementary Informationaccompanies this
paper at http://www.nature.com/naturecommunicationsCompeting
nancial interests: The authors declare no competing nancial
interests.Reprints and permission information is available online
at http://npg.nature.com/reprintsandpermissions/How to cite this
article: Sobhani, A. et al. Narrowband photodetection in the
near-infrared with a plasmon-induced hot electron device. Nat.
Commun. 4:1643 doi: 10.1038/ncomms2642 (2013).ARTICLE
NATURECOMMUNICATIONS|DOI:10.1038/ncomms26426 NATURECOMMUNICATIONS |
4:1643 | DOI: 10.1038/ncomms2642 |
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