DTIC ADA564598: Electrically Driven Photonic Crystal Nanocavity Devices

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1700 IEEEJOURNALOFSELECTEDTOPICSINQUANTUMELECTRONICS,VOL.18,NO.6,NOVEMBER/DECEMBER2012
Electrically Driven Photonic Crystal
Nanocavity Devices
GaryShambat,BryanEllis,JanPetykiewicz,MarieA.Mayer,ArkaMajumdar,TomasSarmiento,
JamesS.Harris,Jr.,Fellow,IEEE,EugeneE.Haller,andJelenaVucˇkovic´
Abstract—Interestinphotoniccrystalnanocavitiesisfueledby nanostructureshaveopenedthedoorfornovelphysicsandde-
advancesindeviceperformance,particularlyinthedevelopment vice applications. A tremendous amount of progress has been
oflow-thresholdlasersources.Effectiveelectricalcontrolofhigh-
made both in optimizing cavity properties such as the quality
performancephotoniccrystallasershasthusfarremainedelusive
(Q)-factor [1]–[3] and mode volume as well as in developing
duetothecomplexitiesassociatedwithcurrentinjectionintocav-
ities. A fabrication procedure for electrically pumping photonic interesting applications that are intrinsically enabled through
crystalmembranedevicesusingalateralp-i-njunctionhasbeende- a nanoscale dielectric form factor [4]–[6]. Early work in this
velopedandisdescribedinthisstudy.Wehavedemonstratedelec- field showed that localized defect modes can be created by
tricallypumpedlasinginourjunctionswithathresholdof181nA
perturbingtheperiodicityofaphotoniclattice,creatinghighly
at 50 K—the lowest threshold ever demonstrated in an electri-
customizablenanocavitieswithsimplecontrolovermodefield
cally pumped laser. At room temperature, we find that our de-
vices behave as single-mode light-emitting diodes (LEDs), which patterns,radiationprofiles,spectralpositioning,andphotonlife-
whendirectlymodulated,haveanultrafastelectricalresponseupto time[7],[8].ThefirstexperimentaldemonstrationofaPCcav-
10GHzcorrespondingtolessthan1fJ/bitenergyoperation—the ity laser began a wave of research in active photonic crystal
lowest for any optical transmitter. In addition, we have demon-
cavitydevices[9].PCnanocavitylasershaveadvancedremark-
strated electrical pumping of photonic crystal nanobeam LEDs,
ably and now represent the state-of-the art in low-threshold
andhavebuiltfibertapercoupledelectro-opticmodulators.Fiber-
coupledphotodetectorsbasedontwo-photonabsorptionarealso lasers[10].InsuchhighQ-factorandsmallmode-volumecav-
demonstratedaswellasmultiplyintegratedcomponentsthatcan ities the Purcell factor can be quite high, reducing the lasing
be independently electrically controlled. Thepresented electrical threshold and increasing the modulation rate [11]. Optically
injectionplatformisamajorstepforwardinprovidingpractical
pumpedPCnanocavitylasershavebeendemonstratedtohave
lowpowerandintegrabledevicesforon-chipphotonics.
thresholds of only a few nanowatts, high output powers, and
Index Terms—Cavity resonators, electro-optic modulation, modulation rates exceeding 100 GHz [12]–[14]. Furthermore,
lasers,light-emittingdiodes,modulation,photodetectors,photonic
they can operate in continuous wave mode at room tempera-
bandgapmaterials,quantumdots(QDs).
ture and can be efficiently coupled to passive waveguides for
optoelectronicintegratedcircuitapplications[15],[16].
Although sophisticated PC lasers and active devices have
been developed, their corresponding electrical control has
I. INTRODUCTION
laggedtremendously.Becauseofthechallengesassociatedwith
PHOTONICcrystal(PC)nanocavitieshavebeenthefocus electrical pumping of photonic crystal membranes, all of the
of intense research in recent years as these engineered aforementioned laser demonstrations relied on impractical op-
tical pumping. There has been one previous demonstration of
an electrically pumped photonic crystal laser using a vertical
ManuscriptreceivedDecember31,2011;revisedMarch20,2012;accepted
p-i-njunctiongrownwithinthesemiconductormembrane[17].
March28,2012. ThisworkwassupportedbytheInterconnectFocusCenter,
oneofthesixresearchcentersfundedundertheFocusCenterResearchProgram, A current post is used to inject carriers into the cavity region;
aSemiconductorResearchCorporationprogram,andbytheAirForceOffice however, the current post limits the Q-factor of the cavity, re-
ofScientificResearchMURIforComplexandRobustOn-chipNanophotonics
stricts the choice of the cavity design, and requires a compli-
(Dr. Gernot Pomrenke), under Grant FA9550-09-1-0704, and by the Direc-
tor, Office of Science, Office of Basic Energy Sciences, Materials Sciences cated fabrication procedure [18]. In addition, a high threshold
andEngineeringDivision,oftheU.S.DepartmentofEnergyunderContract current of 260 μA was observed, significantly higher than in
DE-AC02-05CH11231.TheworkofG.ShambatwassupportedbyaStanford
opticallypumpedPCdevicesandevenexceedingthatofgood
GraduateFellowshipandNationalScienceFoundation(GRPF).TheworkofB.
ElliswassupportedbyStanfordGraduateFellowship. vertical-cavity surface-emitting lasers (VCSELs) [19]. Other
G. Shambat, B. Ellis, J. Petykiewicz, A. Majumdar, T. Sarmiento, groups have used similar vertical p-i-n designs for electrical
J. S. Harris, and J. Vucˇkovic´ are with the Department of Electri-
pumpingofPClight-emittingdiodes(LEDs)buthavenotbeen
cal Engineering, Stanford University, Stanford, CA 94305 USA (e-mail:
[email protected]; [email protected]; [email protected]; arkam@ abletoresolvethecomplicationsassociatedwiththisinefficient
stanford.edu; [email protected]; [email protected]; jela@ injectionplatform[20],[21].Duetothelimitedcurrentspread-
stanford.edu).
ingabilityofthethinconductivelayers,mostoftheelectrolu-
M. A. Mayer and E. E. Haller are with the Materials Sciences Division,
LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720USA,andalso minescence(EL)isnotcoupledtothecavity,wastingelectrical
withtheUniversityofCalifornia,Berkeley,CA94720USA(e-mail:mamayer@ powerandheatingthedevice.
berkeley.edu;[email protected]).
We have, therefore, devised a new method for electrical
Colorversionsofoneormoreofthefiguresinthispaperareavailableonline
athttp://ieeexplore.ieee.org. control of photonic crystal cavities employing a lateral p-i-n
DigitalObjectIdentifier10.1109/JSTQE.2012.2193666
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1. REPORT DATE 3. DATES COVERED
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Electrically Driven Photonic Crystal Nanocavity Devices
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Engineering,Stanford,CA,94305
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13. SUPPLEMENTARY NOTES
14. ABSTRACT
Interest in photonic crystal nanocavities is fueled by advances in device performance, particularly in the
development of low-threshold laser sources. Effective electrical control of highperformance photonic
crystal lasers has thus far remained elusive due to the complexities associated with current injection into
cavities. A fabrication procedure for electrically pumping photonic crystal membrane devices using a
lateral p-i-n junction has been developed and is described in this study.We have demonstrated electrically
pumped lasing in our junctions with a threshold of 181 nA at 50 K?the lowest threshold ever demonstrated
in an electrically pumped laser. At room temperature, we find that our devices behave as single-mode
light-emitting diodes (LEDs), which when directly modulated, have an ultrafast electrical response up to 10
GHz corresponding to less than 1 fJ/bit energy operation?the lowest for any optical transmitter. In
addition, we have demonstrated electrical pumping of photonic crystal nanobeam LEDs and have built
fiber taper coupled electro-optic modulators. Fibercoupled photodetectors based on two-photon absorption
are also demonstrated as well as multiply integrated components that can be independently electrically
controlled. The presented electrical injection platform is a major step forward in providing practical low
power and integrable devices for on-chip photonics.
15. SUBJECT TERMS
16. SECURITY CLASSIFICATION OF: 17. LIMITATION OF 18. NUMBER 19a. NAME OF
ABSTRACT OF PAGES RESPONSIBLE PERSON
a. REPORT b. ABSTRACT c. THIS PAGE Same as 11
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SHAMBATetal.:ELECTRICALLYDRIVENPHOTONICCRYSTALNANOCAVITYDEVICES 1701
Fig.1. (a)Schematicoftheelectricallydrivenphotoniccrystalcavitydevices.
Thep-typedopingregionisindicatedinred,andthen-typeregioningreen.
Thewidthoftheintrinsicregionisnarrowinthecentertodirectcurrentflow
tothecavity.Thedopingprofileistaperedtoensureproperelectricalinjection.
Atrenchisaddedtothesidesofthecavitytoreduceleakagecurrent.(b)Tilted
SEM of a fabricated laterally doped structure. A faint outline of the doping
regionsisvisible.
junction[shownschematicallyinFig.1(a)andvisualizedinthe
scanningelectronmicroscope(SEM)imageinFig.1(b)][22],
[23].Thegeometryinherentto2-Dphotoniccrystalmembranes
lendsitselfmoreeasilytolateralcurrentinjection(LCI)defined
by ion implanted p- and n-type regions. This device architec-
ture has a simple and flexible fabrication procedure with high
control over the current flow. In addition, arbitrary PC cavity
designs with high Q-factors can be used along with coupling
waveguides for efficient light extraction. Moreover, doping is
introduced only in desired areas, enabling efficient integration
ofactiveandpassivedevices(asopposedtoverticalp-i-njunc- Fig.2. Schematicillustrationofthefabricationprocess(a)startingmaterial
isa220-nmthickmembraneofGaAswithembeddedInAsQDs.(b)SiandBe
tions where the doped regions are defined during growth, and
ionsareimplantedthroughasiliconnitridemaskpatternedbyelectronbeam
in the entire wafer). While the most immediate application of lithography.(c)Implanteddopantsareactivatedbyannealingat850◦Cfor30s
our lateral junction is for nanocavity lasers and light-emitting withanitridecapwhichissubsequentlyremovedbydryetching.(d)Photonic
crystalpatternisdefinedinaresistbyelectronbeamlithographyandtransferred
diodes, the architecture is suitable for any number of applica-
intotheGaAsmembranebydryetching.(e)AlGaAssacrificiallayerisremoved
tionsneedingactive,integrablecontrolofphotoniccrystalcavi- withwetetching.(f)pandncontactsaredepositedbyphotolithographyand
ties.Finally,suchaplatformoffersanewdegreeoffreedomfor liftoff.
electrical control or tuning beyond that demonstrated in prior
devicesmanipulatedbyelectrostaticsorlocalfields[24],[25].
tions. It is possible to form lateral junctions in III-V materials
Our paper is organized as follows. Section II goes over the
during molecular beam epitaxy by incorporating appropriate
device fabrication details and simulated electrical behavior.
dopantionsduringmultipleregrowthsteps.However,thistech-
In Section III, we present our electrically driven nanocavity
niqueisverycomplicatedandrequiresadditionalelectron-beam
laser results at cryogenic temperatures. Section IV discusses
lithographysteps[29].Alternatively,ionimplantationhasbeen
room temperature operation and ultrafast modulation of our
explored as a method to form LCI edge-emitting GaAs lasers
single-modeLED.SectionVdescribesadditionalapplications
and more recently electroluminescent devices in InGaAsP PC
ofourlateraljunctioninnanobeamphotoniccrystalLEDsand
membranes[30],[31].Webuildoffoftheseworksandrefinea
fiber taper-coupled devices such as electro-optic modulators,
techniquetoformalateraljunctioninGaAswithhighprecision
photodetectors, and multiply integrated components. Finally,
andreproducibility.
Section VI summarizes our results and provides future direc-
We first note the choice of III-V membrane and active
tionsforworkinthisarea.
gain material, here gallium arsenide and indium arsenide self-
assembledquantumdots(QDs).QDsarepreferredforthegain
II. FABRICATIONANDDESIGN
mediumbecausetheycanhaveexceptionallylowtransparency
As mentioned earlier, our solution to the PC cavity electri- carrier densities, allowing for reduced threshold lasing [15].
calinjectionproblemistousealateraljunctionformedbyion The low transparency carrier density also allows for relaxed
implantation [22]. LCI has become routine for silicon-based constraints on the carrier injection levels and corresponding
electro-optic ring modulators in recent years owing to mature doping levels. Furthermore, nonradiative recombination of the
device process knowledge [26]. More recently, a lateral junc- dotsthemselvesisimprovedversussimilarquantumwell(QW)
tioninSiphotoniccrystalshasbeendemonstratedforcompact systems[32].
modulators and detectors [27], [28]. On the other hand, far Fig. 2 shows a simplified schematic diagram of the lateral
lessattentionhasbeenputtowardIII-Vprocessedlateraljunc- junctionphotoniccrystalfabricationprocedure.First,alignment
1702 IEEEJOURNALOFSELECTEDTOPICSINQUANTUMELECTRONICS,VOL.18,NO.6,NOVEMBER/DECEMBER2012
marks were defined on the unpatterned wafer using electron
beam lithography and a thick layer of silicon nitride was de-
posited on the sample to serve as a mask for ion implantation
ofSi.Electronbeamlithographywasusedtopatternthen-type
dopingregionandSiionswereimplantedatanenergyanddose
such that the maximum of the doping density was 6 × 1017
cm−3 andthemaximumofthedopantdistributionwasnearthe
middleofthemembrane.Anothernitridemaskwasusedtode-
finethep-typedopingregion,formedbyBeionsandachieving
a doping density of 2.5 × 1019 cm−3. Next, a tensile strained
silicon nitride cap was deposited to prevent As out-diffusion
during the subsequent high temperature anneal. The samples Fig.3. (a)AFMtopographyimageofthefabricateddevicewithoutaphotonic
were then annealed in a rapid thermal annealer to activate the crystal.Thescalebaris5μm.(b)SCMimageoftheregionin(a).Thep-side
ofthedeviceisinthelowerleftcornerandthen-sideintheupperright.The
dopantsandremovealmostallofthelatticedamagecausedby
trenchisetchedatthedevicecenter,showingtheprecisionofthealignmentof
the ionimplantation. The photonic crystalpattern was defined thedopingregions.“SCMdata”areacombinationofthephaseandamplitude
usingelectronbeamlithographyandetchedintothemembrane. ofcapacitancedata,wherethestrengthofthesignalisdirectlyproportional
totheintensityofdopinginthelocalregionunderthetip[34].Thescalebar
Simultaneouswiththephotoniccrystal,trencheswereetchedto
is5μm.
thesidesofthecavityandallthewayaroundeachofthecon-
tacts;thiswasfoundtoreducetheleakagecurrenttoreasonable
levels.Finally,metalcontactsweredepositedinaliftoffprocess,
activated, and the photonic crystal membranes were undercut.
Forfullfabricationdetailssee[23].
SiandBeionswerechosenbecausetheyofferthebestcombi-
nationoflowlatticedamageandhighactivationefficiency[33].
In fact, our p- and n-type region carrier concentrations of 2.5
× 1019 cm−3 and 6 × 1017 cm−3 are close to the highest ex-
pectedvaluesforactivatedcarrierconcentrationsfromBeandSi
dopantsinGaAs[33].Inourformerstudy,weusedMgionsfor
thep-region;however,wefoundactivatedcarrierconcentrations
Fig.4. (a)TypicalI–Vcurveforalateraljunctionfabricatedwithoutproper
werelowerat3×1018 cm−3 [22].Boththephysicallayoutand isolationtrenches.Themagnitudeofthecurrentisexceptionallyhighbecause
carrier concentration of our devices were found through scan- ofthelargeleakagepathwaythroughtheentiretopGaAsmembrane.Theshape
oftheI–Vcurvedoesnotresembleanidealdiodeandismorecharacteristicof
ningcapacitancemicroscopy(SCM)[34],asseeninFig.3.The
linearresistivebehavior.(b)Diagramoftheleakagecurrent.Yellowsquaresare
imageshowsthatwehaveexcellentcontroloverthedopinglay- metalcontactsandblackrectanglesaretrenches.Mostofthecurrentisdirected
outwithanaccuracyofwithin30nmdeterminedbyoure-beam awayfromthecavitydeviceregionasseenbytheorangearrows.
alignmentprocedure. Bycomparing thepositionofthenitride
maskedgetothemeasureddopingedge,wecandeterminethe
distanceofdopantdiffusionand,thereforecontroltheintrinsic sionthattheQDcoreandwettinglayerintermix[36],[37].The
cavity region width. This parameter is very important for sev- effectofmaterialintermixingdecreasestheQDcarrierconfine-
eralreasons.Spatialoverlapofthecavitymodeprofilewiththe ment potential, which has implications for room temperature
heavily doped regions leads to free carrier absorption (FCA) operation,discussedinSectionIV.
and degradation of the Q-factor. In terms of realizing the best We found that trenches surrounding the metal contact pads
intrinsicQ-factor,thep-andn-regionswouldideallybespaced arevitalinordertoensurecurrentflowsthroughonlythecavity
out as far as possible. Counteracting this force is the fact that region.DuetolightbackgrounddopingofthetopGaAsmem-
carrierinjectionintotheintrinsicregiongoesdowndrastically brane,currentcanspreadlaterallythroughtheentiremembrane,
as the doping regions are spaced farther apart, eventually to a bypassingthecavityandresultinginpoorcurrent–voltagechar-
valuebelowtheinversionconditionfortheQDs.Thishappens acteristics. An example I–V plot of one of our former lateral
because the diffusion length for carriers in GaAs with a high diodedevicesfabricatedwithoutisolationtrenchesisshownin
nonradiativerecombinationrateisextremelyshort(200nmfor Fig.4.Thequasi-linearcurrentresponseisunrepresentativeof
electrons and 40 nm for holes) [35]. Therefore, a compromise a working diode and is more likely the result of the resistive
between FCA and carrier injection must be made for a set in- membrane. The measured current level is in the milliampere
trinsic region width. Finally, ion implantation degrades much range,whichisordersofmagnitudehigherthanwhatisexpected
oftheQDemissionevenafterthelatticeisannealedsoproper and observed from properly functioning diodes with isolation
alignmentoftheintrinsicregionwiththePCcavityiscritical. trenches.Asanalternativetothephysicalcutsimposedbythe
OurannealedQDsexhibitablueshiftbyabout80nmanda isolationtrenches,futuredevicescouldpotentiallyemployhy-
reductioninroomtemperatureemissionbyafactorof10(cryo- drogen implantation as a current aperture (as in VCSELs) in
genicemissionintensityisunchanged).Previousreportsonan- ordertopreservethemechanicalstructureofthesemiconductor
nealedInAsQDsshowedsimilarresultswiththelikelyconclu- membrane[38].
SHAMBATetal.:ELECTRICALLYDRIVENPHOTONICCRYSTALNANOCAVITYDEVICES 1703
Fig. 5. (a) Simulated current density plot (in A/cm2) of the L3 photonic
crystalcavitydesignwitha400-nmintrinsicregionwidthanda5-μmouter Fig.6. (a)IRcameraimageofaroomtemperatureLED(seeSectionIV)with
mirrorspacing.(b)SimulatedI–Vplotsofthedevicewitha400-nmintrinsic onlyawide,5-μmintrinsicregionspacing.Thedopingisoutlinedinyellow
regionspacingandnoholes(green),a400-nmintrinsicregionspacingwith dashedlinesandtrenchesareshownbyblackrectangles.Theemission,while
holes(red),andadevicewithonlyonelarge5μmwideintrinsicregionwith weak,isstillvisibleatthen-typeregionboundary.(b)IRcameraimageofa
holes(blue).(c)Calculatedeletrondensitymap(incm−3)foradevicewith similarLEDbutwithatapereddopingprofileandwithcavityintrinsicregion
a 400-nm intrinsic region biased at 1.2 V. Inset shows a zoom-in of a hole separationof400nm.TheELismuchbrighteratthecavitycomparedtothen-
regionseveralperiodsawayfromthecavity.(d)Calculatedholedensitymap typeregionboundaryduetomoreefficientelectricalinjection.(c)Comparison
(incm−3)foradevicewitha400-nmintrinsicregionbiasedat1.2V.Inset oftheI–VcurvesforthewideintrinsicregionLED(bluetrace)andthenarrow
showsazoom-inofaholeregionseveralperiodsawayfromthecavity. intrinsicregionLED(redtrace).
simulationswerecarriedoutat300K,weexpectsimilardevice
2-D finite element Poisson simulations were carried out to
electricalperformanceatlowertemperatureswithslightlylower
predicttheexpectedelectricalbehaviorofourdevices.Previous
currentsandslightlyhighercarrierinjectionlevelsduetoslower
studiesofelectricaltransportthroughInPphotoniclatticeshave
nonradiativerecombination.
shownthatcurrentflowpersistseveninthepresenceofetched
To understand the effect of leakage current through the PC
holes [39]. The transport is modified by a geometric fill fac-
mirrors,wealsosimulatedeviceshavingawide5-μmintrinsic
torthataccountsforthereducedmembranecross-sectionalarea
region spacing and no taper profile for the doping. We see in
fromthelatticeholesandtheirrespectivedepletionzonescaused
Fig. 5(b) that a small residual current does flow, but the mag-
bytheFermilevelsurfacepinning.Thoughtheexactnatureof
nitude is lower by a factor of 10. In Fig. 6(a), an IR image of
surfacestatesrequiresadetailedanalysisoftrapenergylevels,
theELfromawideintrinsicregiondeviceshowsthatindeeda
weapproximatetrapsasacceptortypenearmidbandandobtain
smallamountofemissionisobservedandisconcentratedatthe
good agreement between simulation and experiment. To find
edge of the n-type doping region (this asymmetry in emission
the appropriate trap density to use in simulation, we measure
is due to the doping asymmetry and was confirmed via simu-
thematerialfreecarrierlifetimeusingatime-resolvedphotolu-
lation). Even for devices with a normal tapered doping profile
minescence setup and obtain a value of 6 ps (see Section IV).
andsubmicroncavityintrinsicregion,weobservefiniteleakage
Thisextremelyshorttimeofcarrierrelaxationisduetothenon-
from the PC mirror intrinsic region sections as evidenced by
radiative recombination from etched hole surfaces. From the
the IR emission at the n-type region boundary [see Fig. 6(b)].
nonradiativecarrierlifetime,webackoutasurfacetrapdensity
Simulations show that the mirror leakage current accounts for
of5×1013 cm−2,andsimulateourdevicetoproducethecur-
aboutone-thirdofthetotalcurrentthroughthemembrane.The
rentdensityandsteady-stateelectronandholedensityplotsin
magnitudeoftheELismuchsmalleratthemirrorboundaries
Fig.5.
compared to the cavity, however, as seen by the brightness of
For a bias of 1.2 V, Fig. 5(a) shows that the current flow
thesignal.ComparingtheI–Vcurvesforthesetwodeviceswe
is primarily through the L3 cavity region while minimal leak-
seethatthewideintrinsicregiondevicehasalowercurrentby
agecurrenttravelsthroughthewideintrinsicmirrors.Aunique
overanorderofmagnitude,aspredictedbysimulation.Toavoid
current crowding effect is also visible around the cavity edges
thePCmirrorleakageinfuturedevices,hydrogenimplantation
whichapproachesalargevalueof10kA/cm2.Thediodeattains
could again be used as a final current aperture, so long as the
microamplevelcurrentswhenbiasednear1V,andhasaseries
implantationdamageisspacedatleastafewmicronsawayfrom
resistance of roughly 1 kΩ, dominated by the air-hole modi-
thecavity.
fiedsheetresistanceatthisdopinglevel[seeFig.5(b)].Mean-
while the steady-state e/h carrier densities saturate at around
1016 cm−3 atthecavitycenter[seeFig.5(c)and(d)].Theinjec- III. ULTRALOWTHRESHOLDLASER
tionlevelisfarlowerthanthenearbydopinglevelsduetothe Inthissection,wepresentresultsonourelectricallypumped
highnonradiativerecombinationrate.Whiletheaforementioned quantumdotPClaser[23].Ourfirstattempttoproducesucha
1704 IEEEJOURNALOFSELECTEDTOPICSINQUANTUMELECTRONICS,VOL.18,NO.6,NOVEMBER/DECEMBER2012
curve,weobserveadominantexponentialcurrent(theredline)
correspondingtocurrentflowingthroughthecavityalongwith
a leakage component at low biases. We found that for this set
of devices the leakage was due to an incompletely removed
AlGaAs sacrificial layer and subsequent devices did not have
thisproblem(seeSectionIV).
Fig.8(a)showsthecurrentin-lightoutpropertiesofourlaser
at 50 and 150 K. We observe a clear lasing threshold for tem-
peraturesbelow150K.Wefindthatthethresholdofourlaser
is181nAat50Kand287nAat150K(correspondingto208
and 296 nW of consumed power and threshold current densi-
ties of 26 and 42 A/cm2). To the best of our knowledge, this
is the lowest threshold ever demonstrated in any electrically
pumpedlaser.Itisthreeordersofmagnitudebetterthanthe260
μA threshold of previous QW PC cavity lasers and more than
Fig.7. (a)Opticalmicroscopeimageofacompletedeviceshowinglargemetal
anorder of magnitude better than thethresholds of metal-clad
contactpadsalongwiththeconnectingwirebonds.ThePCdiodeislocatedin
thecenterbetweenthecontactpads.(b)SEMimageofafabricatedlaserdiode lasersandmicropostlasers[17],[40],[41].Thelowthresholds
device.Thep-sideofthedeviceappearsonthetopoftheimage,andthen-side demonstratedintheselasersarearesultoftheoptimizedlateral
onthebottom.Thescalebaris10μm.(c)SEMimageofthephotoniccrystal
current pathway, where charge can be efficiently delivered to
cavity[zoom-inofthecentralregionof(b)].Thescalebaris300nm.
thecavitycenter.Fromthecollectedpowerdataaswellasthe
measuredlossinoursetup,weestimatethetotalpowerradiated
by the laser to be on the order of a few nanowatts well above
threshold.TheinsetinFig.8(a)showstheexperimentalfar-field
radiationpatternofthecavityabovethreshold,showingaclear
specklepattern.
Atlowvoltagesbeforethediodehasfullyturnedonweob-
serveleakagecurrentbypassingthecavitythroughresidualAl-
GaAsmaterialthatwasnotfullyetchedaway.Therefore,ifthe
AlGaAswasfullyremovedforthisdevice,thethresholdcould
be significantly lower. To find the potential threshold reduc-
tion,wefitthecurrentvoltagecharacteristicstoanidealdiode
Fig.8. (a)Experimentalcurrent-lightcharacteristicsforthelaserat50K(blue equation to determine the fraction of current flowing through
points)and150K(greenpoints).Theblacklinesarelinearfitstotheabove
the entire GaAs membrane [as shown in Fig. 8(b)]. The laser
thresholdoutputpowerofthelasers,whichareusedtofindthethresholds.The
insetisafarfieldradiationpatternofthelaseratacurrentof3μA.(b)I–Vplot thresholdafterthisleakagecurrentcorrectionisonly70nA.If
forthelaserdevice.Theinitialhumpisleakagecurrentnotflowingthroughthe wefurtherconsiderthatonly64%ofthemembranecurrentflows
cavity.Theredlineshowsthecurrentthatisflowingthroughthedevice.Inset
throughthejustthecavityregion,theultimatelaserthresholdat
showsthelasingspectrumabovethreshold.
50Kisonly45nA.
IV. ULTRAFASTSINGLE-MODELED
laser using 900 nm InAs QDs was unsuccessful even at cryo-
genic temperatures [22]. For these shallow confinement QDs, Room temperature operation of our nanocavity light source
thetransparencycarrierdensityisinexcessof1017 cm−3.With revealsinterestingphysicalpropertiesthatcanbeexploitedfor
previousactivateddopantlevelsinthe1018 range,theinjected ultrafastmodulation[42].Thoughourdevicesdonotlase,they
carrierconcentrationinthecavityismuchlowerthanthatneeded exhibit effectively single-mode LED behavior with QD emis-
forinversionandgainasperthediscussioninSectionII.There- sioncoupledtocavityresonancesandcanbedirectlymodulated
fore, it is not surprising that stimulated emission was not ob- atveryhighspeeds.Deviceswerefabricatedasdescribedpre-
servedforourformerdevices. viouslywiththeonlydifferencebeingalongerundercutstepto
Forourlasingstructure,weuseaGaAsmembranewiththree eliminate substrate leakage current. A fabricated diode is seen
layers of high density (300 dots/μm2) InAs QDs with peak in Fig. 9(a) with a corresponding I–V curve and output emis-
emissionintensitynear1300nm.ThesedeepconfinementQDs sionspectrumforaforwardbiasof10μA[seeFig.9(b)].The
haveamuchlowertransparencycarrierdensityof5×1014cm−3 current is slightly greater than at cryogenic temperatures be-
and should acheive inversion with our electrical scheme. The causeofincreasednonradiativerecombinationcurrentatroom
parameters of the cavity are chosen so that the fundamental temperature.Additionallysubstrateleakagecurrentisnolonger
cavitymodeisatawavelengthof1174nmatlowtemperature, observed because of the optimized undercut step. Bright and
within the ground state emission of the QDs. Fig. 7 shows clearly defined cavity modes peak well above the background
optical and electron microscope pictures of a fully fabricated QDemissionwiththefundamentalmodecenteredat1260nm
device. Fig. 8(b) is an I–V plot of our laser device. From the andhavingaQ-factorof1600.
SHAMBATetal.:ELECTRICALLYDRIVENPHOTONICCRYSTALNANOCAVITYDEVICES 1705
Fig.9. (a)Diodecurrent–voltageplotmeasuredforatypicalLEDdevice.
Leakagecurrentisminimizedforthissetofroomtemperaturedevices.The
insetshowsanSEMimageofafabricateddevice.(b)Spectrumofthecavity
foraforwardbiascurrentof10μA.Thefundamentalmodeforthisdeviceis
Fig.10. (a)TiltedSEMimageofananobeamstructure.Then-typedopingis
at1260nm.(c)Directmodulationresultsoftheroomtemperaturesingle-mode
seenasdarkergrayandthedopingregionsareoutlinedinwhitedashedlines.
LEDfortwodifferentpatternsequencesshowingultrafastoperation.
Thebeamisdeflecteddownbyasmallamountlikelyduetostrainfromthe
GaAs/AlGaAsinterface.(b)ELspectrumforananobeamdeviceataforward
biasof5μA.Thecavityfundamentalmodeisthesharppeakat1255nmandthe
backgroundQDemissionisthebroadspectrumbelow.TheQ-factorisfound
tobe2900.(c)NanobeamcavityemissiontakenwithanIRcamera.Anoutline
Timeresolvedphotoluminescencelifetimemeasurementsus- ofthecavityisseenbytheyellowlinesandthescalebaris5μm.Thecavity
emissionisbrightatthecenterasexpectedandthereisslightELscatteredout
ingaTi:Sapphirelaserasanopticalpumpwerestudiedtoexam-
atthenanobeamedges.
inetherecombinationratesinoursystem.Atroomtemperature,
thenominalQDlifetime(duetobothradiativeandnon-radiative V. ADDITIONALAPPLICATIONSOFLATERAL
components) is 100–300 ps, limited by bulk nonradiative re- JUNCTIONACTIVEDEVICES
combination[42].TheQDsinouractiveLEDdevicesundergo
arapidthermalannealingstepat850◦CwhichcausestheQD A. NanobeamPhotonicCrystalLED
ensembleemissiontoblueshiftby80nmanddecreaseininten- 1-Dnanobeamcavitieshaveemergedrecentlyasacompeting
sityby tenfold.Likely, theQDcore semiconductor intermixes technology to 2-D photonic crystal membrane technology. As
with the wetting layer surroundings during the anneal as seen lightisconfinedinthesecavitiesbydistributedBraggreflection
previously[36],[37].Thiseffectwouldinduceashallowercon- in only 1-D, they can have a smaller footprint and higher Q-
finingpotentialandhencepromoteamorerapidreemissionof factors than their 2-D counterparts. Recently researchers have
carriersfromtheQDs.ForourannealedPCdevices,wemeasure beenabletodemonstratehighQ-factorsin1-Dnanobeamcav-
a total QD lifetime of only 10 ps at 1100 nm, indicating rapid itiesinlow-indexmaterialssuchassilicondioxideandsilicon
escape of carriers from our modified QDs and subsequent fast nitride[45],[46].Theyhavealsobeenusedtodemonstratelow
nonradiativerecombinationfrometchedsurfaces[42]. threshold lasers, optomechanical crystals, and chemical sen-
WeperformdirectelectricalmodulationstudiesofourLEDs sors[47]–[49].Weshowthatinspiteoftheirnarrowcrosssec-
next to determine the time-resolved response of our devices. tions,nanobeamcavitiescanbeefficientlyelectricallypumped
Fig. 9(c) shows the single-mode output for two different bit byalateralp-i-njunction[35].
patternsfedtothediodethroughapulsepatterngenerator.The Fig.10(a)showsSEMimagesofafabricated,laterallydoped
diode replicates the voltage pulse patterns very well and has photoniccrystalnanobeam.Thestructureconsistsofthesame
pulse widths of 100 ps, limited by our pattern generator. Fast 220-nm GaAs membrane with InAs QDs but instead of a ta-
electricaldataaretherebymappedontothesinglemodecarrier pereddopingprofile,thedopingisuniformalongthebeamwith
of the nanocavity LED, which can be used as the light source a 400-nm intrinsic region gap. Beams have a five hole taper
foropticalinterconnecttransmission. cavity(latticeconstantfroma=322nmtoa=266nm)with
Our device is over an order of magnitude faster with three hole radius r = 0.22a and are 500–600 nm wide. Fig. 10(b)
orders of magnitude lower power consumption (here only is an EL spectrum of a representative nanobeam device when
2.5μW)comparedtopreviouslyshowndirectlymodulatedpho- biased to 5 μA. The EL IR image corresponding to Fig. 10(b)
toniccrystalsingle-modeLEDsatcryogenictemperatures[20]. isshowninFig.10(c).Together,theseresultsdemonstratethat
Thepowerconsumptionforthe10GHznonreturn-to-zerospeed thecurrentisefficientlydirectedbythelateralp-i-njunctionto
diodeinFig.9isonly2.5μW,indicatinganaverageenergyper the cavity region. We find that as the injection current of the
bit of only 0.25 fJ [42]. Power output for this particular LED cavity is increased a small amount of differential gain is ob-
is quite low in the tens to hundreds of picowatt owing to the served,indicatinglasinginthiscavitydesignispossible.These
fastnonradiativerecombination,butisstillwithintherangeof results represent a promising advance toward practical active
detectionforadvancedphotodiodetechnology[43],[44]. nanobeamdevicearchitectures.
1706 IEEEJOURNALOFSELECTEDTOPICSINQUANTUMELECTRONICS,VOL.18,NO.6,NOVEMBER/DECEMBER2012
Fig.11. (a)Schematicfortaper-basedmodulatorexperimentwithunderlying
opticalimageofafibertaperalignedtoacavity.PDisphotodiodeandOSA Fig.12. (a)Schematicforthefiber-coupledphotodetector.Thetaperisaligned
isopticalspectrumanalyzer.Thecavityregionisoutlinedintheredbox.The aspreviouslyandthephotocurrentismonitoredasalaserisfedintooneendof
scalebaris10μm.(b)100MHzmodulationofourdevice,detectedusinga thefiber.Thecavityregionisoutlinedintheredbox.Thescalebaris10μm.
p-i-nphotodetectorwithanRFamplifiercircuit. (b)PhotocurrentIVplotsforseveralinputlaserpowers.
B. Fiber-TaperCoupledElectro-OpticModulator
coupling wavelength is noted and a tunable laser matched to
Wefurthertesttheusefulnessofourlateralp-i-nstructureby
thecavityresonanceisfedintoonetaperend.Current–voltage
demonstrating an electro-optic modulator based out of a cav-
tracesarethentakenforvariousinputlaserpowers.Weseein
ity coupled to a fiber taper waveguide [50]. GaAs has a much
Fig.12(b)thatthedeviceindeedfunctionsasatwo-photonde-
stronger free carrier dispersion than does silicon and has fast
tector with photocurrent increasing proportionally to the input
carrierrecombinationatsurfaces.Togetherwiththesmallmode
laserpower.Byincorporatingthetaperlossaswellasthepower
volume of the cavity of 0.7(λ/n)3, over an order of magnitude
coupling ratio into the cavity, we estimate the responsivity to
smaller than typical microring resonators [26], these advan- bearound10−3 A/W.Photodetectionoccursevenwithourlow
tages enable ultralow power operation at potentially very high
Q-factorsof1000–2000,andisseenonlywhenthepumplaseris
speeds.Theelectricallycontacteddeviceswerefabricatedasdis-
withinthecavitybandwidth,confirmingresonantenhancement.
cussedearlierwithcavitiescenterednear1500nminapassive
The absolute responsivity could be further improved by using
GaAsmembrane.Fibertaperswerefabricatedasbeforehavinga
cavitieswithhigherQ-factors.
1μmdiameterandonlyafewdBofloss[51].
Modulation is achieved by first aligning the fiber taper to
the cavity [see Fig. 11(a)] such that the fundamental mode D. InterconnectedLateralJunctionDevices
resonance of the PC cavity appears as a dip in transmission
We conclude our results by discussing multiply intercon-
whenlightissentthroughthefiber.Fig.11(b)shows100MHz
nected lateral junction devices. Practical devices in future on-
operation using a p-i-n detector with an RF amplifier circuit,
chipnetworkswillneedtobemultiplexedathighdensity,requir-
proving high-speed operation by free carrier dispersion. The
ingaminimumfootprintforallcomponents:lasers,modulators,
ultimate switching speed for an injection-based diode is given
anddetectors.Forhighdataratestobepossible,wavelengthdi-
by the carrier lifetime (6 ps), suggesting an ultimate speed of
vision multiplexing is necessary, requiring many independent
upto100GHzhere.Wefindthatourswitchingenergyisonly
lightchannels.Arecentstudyillustratedthisconceptbymulti-
0.6fJ/bit[50],againconfirmingthelowpoweradvantageofour
plexingseveralindependentringresonatormodulatorsatdiffer-
nanocavity.
entwavelengthstoonecoupledwaveguide[54].Inourdevices,
we are able to reproduce this behavior with multiple cavities
C. Fiber-CoupledPhotodetector
embeddedinthesamePClattice.
Thefiber-tapercoupledtoanelectricallycontactedphotonic Forourstructures,wefabricateonelargePClatticewiththree
crystal can be used to demonstrate a cavity-enhanced two- uncoupledcavitiesspacedbysixormorelatticeperiodsinpas-
photon photodetector. Cavities have been previously used to siveGaAsmaterial[seeFig.13(a)–(c)].Thenumberoflinked
enhance the responsivity of photodetectors in both the linear cavitiescanbeincreasedfurtherbutforthepresentdemonstra-
andtwo-photon regimes [28],[52].Conceptually, acavitycan tionweusethree.Thelocalholeradiusandlatticeconstantcan
providealongereffectivepathlength(forlinearabsorption)as be tuned so that the cavity modes have identical or different
wellasanincreasedlocallightintensity[fortwo-photonabsorp- wavelengths.Anexamplewheredifferentwavelengths arede-
tion(TPA)].Ordinarily,GaAswillhavezerolinearabsorption sirablewouldbeforhavingmultiplexedsourcesormodulators
near 1.5 μm due to its larger bandgap; however its nonlinear fordifferentchannels.Identicalwavelengthsmightbeusefulfor
TPAcoefficientis10cmGW−1,whichisovertentimesgreater matching a source with a modulator, a source with a detector,
thantheTPAcoefficientofsilicon[53].Therefore,two-photon etc. The doping layout is such that large intrinsic regions sep-
absorption and subsequent photodetection may be possible in arateadjacentcavities[seeFig.13(c)].Thisisdonesothatan
GaAswithouttheneedforextremelyhighQ-factorcavities. appliedbiastoonesetofcontactsdoesnotproduceacrosstalk
Weperformmeasurements onourpreviously fabricated lat- current flow in a neighboring cavity. Finally, trenches are fab-
eraljunctionsat1500nm.Asbefore,thefibertaperisaligned ricatedtoisolateeachmetalcontactpadtoeliminatecrosstalk
andcoupledtotheL3cavity[seeFig.12(a)].Thetransmission [seeFig.13(a)].
SHAMBATetal.:ELECTRICALLYDRIVENPHOTONICCRYSTALNANOCAVITYDEVICES 1707
inordertoensurepropercarrierinjection(seeSectionII).The
secondlimitationtotheQ-factorwebelieveisduetosomesur-
facerougheningoftheGaAsmembranecausedbytherepeated
depositionandetchingofthenitridemasks.Asapointofrefer-
ence,wehaveobtainedQ-factorsinexcessof10000usingthis
samematerialforundopedGaAsphotoniccrystalcavities[56].
Anadditional challenge toroomtemperature lasingisthatthe
InAs QDs are strongly affected by the rapid thermal anneal at
850◦C.Theiremissionintensitygoesdownbyafactorof10at
roomtemperature,likelyduetoreducedcarrierconfinementin
thedots(seeSectionIV).Whileitmaybepossibletooptimize
thefabricationprocesstosimultaneouslyincreasetheQ-factor
Fig.13. (a)TiltedSEMpictureofafabricatedtriplecavitydevice.Thein-
andmaintaintheoriginalQDgain,itwillnotbetrivial.
dependentcontactpadsareseenalongwiththeisolationtrenches.(b)Zoom-in
SEMof(a).Thedopingprofileis partiallyvisible. (c)Top-downSEM ofa Alternative to the GaAs platform, indium phosphide with
devicewithdopingregionslabeledanddelineatedbydashedlines.Thescale embedded QWs might prove to be a better material system
baris2μm.(d)Transmissionspectrumforafibertapercoupledtoasetofthree
forachievingroomtemperaturelasing.QWshavesignificantly
cavities.Thefundamentalmodewasdesignedtobeatauniquewavelengthfor
eachcavity.Independenttuningofeachmodeispossible. more gain than do QDs and room temperature lasing in InP is
routinelyobservedevenforlowQcavities[9],[57],[58].QWs
have even been shown to have increased photoluminescence
Tocoupleallthreecavitiesweagainuseafibertaperwave-
intensity after rapid thermal annealing [59]. The main chal-
guide that is appropriately placed on all three cavities. When
lengewithtransferringthisplatformtoInPistherelativelyhigh
positioned properly, we obtain clear transmission signals for
transparancycarrierdensityneededforQWinversion(typically
each cavity [see Fig. 13(d)]. In this example, the three cavity around1018 cm−3 [58]).AsmentionedinSectionII,achieving
modes were fabricated to have unique wavelengths near 1500
highcarrierdensitiesatthecavitycenterislimitedbyactivated
nmformodulationstudies.Asaforwardbiasisappliedtoasin-
doping levels and nonradiative recombination. Implantation in
glepairofcontactpads,thecorrespondingcavitymodeisseen
InPshouldresultinsimilaractivationlevelsasforGaAs,though
to shift in accordance to the modulation properties discussed
the Si (n-dopant) would have a higher activation compared to
earlier. Meanwhile the other two cavity modes are unaffected
Be(p-dopant)[33].Atthesametime,nonradiativerecombina-
by the applied bias. This is repeated with the other two cavity
tion for InP has been shown to be over an order of magnitude
modes with the same result. Therefore, independent electrical
slowerduetothepositioningofsurfacetrapdefectswithinthe
controlwithoutcrosstalkcanbeachievedinacompactplatform.
bandgap[60],[61].OurpreliminaryPoissonsimulationsshow
Extensionsofthisdesigncaneasilybemadewithanynumber
thatitshouldbepossibletoattaininversioninQWsandlasingat
ofactivecomponentsfordenseoptoelectronicintegration.
roomtemperatureinproperlydesignedInPlateraljunctions.If
thiscanbedone,practicallow-thresholdlaserswithhighoutput
VI. DISCUSSIONANDOUTLOOK
powers and fast modulation rates could become a true reality
Wehavedemonstratedanovelplatformfortheelectricalcon- foron-chipphotonics.
trolofactivephotoniccrystalnanocavitydevices.Inparticular, The uniquely modified properties of our QDs at room tem-
we have achieved efficient electrical injection into 2-D mem- peraturehaveallowedustodemonstratea10-GHzsingle-mode
branesofGaAscontainingQDsforlaserandLEDapplications. LED with sub-fJ/bit energy consumption. While a traditional
Ourlaseroperatingat50Khasathresholdofonly181nA,the laseristypicallythedeviceofchoiceforanopticalsource,our
lowest of any electrically driven laser ever demonstrated. The single-mode LED has all the look and feel of a laser with its
lasersoperateupto150KbeforetransitioningtoLEDsdueto high bandwidth in a single channel and can be used for prac-
limitedQDgainandhighcavityloss. tical communications. The cavity allows light to be efficiently
The principalreason why wedonotobserve lasingatroom channeledintoawell-definedmodethatcanbefurtherextracted
temperature is that the Q-factor of our cavities is too low, and with a coupled waveguide. The narrow spectral linewidth also
hence the cavity loss is too great compared to the supplied allows for wavelength division multiplexing in densely inte-
QDgain.PreviousreportsonInAsquantumdotlasersshowed grated systems. Low output power remains an issue in these
that room temperature lasing in this material system is quite types of sources relying on nonradiative recombination alone.
challenging due to the low maximum gain provided by high A fast, single-mode LED that is also very efficient by having
density QDs, quoted as up to 5 cm−1 per layer [15], [55]. In a large Purcell enhancement would require sophisticated en-
ordertorealizelasingwithsuchlowgainmedia,thecavityloss gineering of the material system and careful optimization of
must be exceptionally small with Q-factors in excess of 104. cavityparameters[62].Wepointout,however,thateventhough
Our devices, on the other hand, have Q-factors ranging from our LEDs are low power, optical interconnect links could be
1000to2000,limitedbyseveralfabricationcomplexities. designed around such transmitters by using sensitive detector
Thefirstisthatthedopingregionspartiallyoverlapwiththe technology.
cavitymodefield,resultinginfreecarrierabsorption.Thisloss Finally, we have utilized our lateral p-i-n junction to
isunavoidablesincethep-andn-regionsmustbecloselyspaced demonstrate LEDs in 1-D photonic crystal nanobeams and
1708 IEEEJOURNALOFSELECTEDTOPICSINQUANTUMELECTRONICS,VOL.18,NO.6,NOVEMBER/DECEMBER2012
fiber-coupledmodulatorsanddetectors.Theformerworkisan [12] S.Strauf,K.Hennessy,M.T.Rakher,Y.S.Choi,A.Badolato,L.C.An-
encouragingresultinthatelectricalinjectioncanbesurprisingly dreani,E.L.Hu,P.M.Petroff,andD.Bouwmeester,“Self-tunedquantum
dotgaininphotoniccrystallasers,” Phys.Rev.Lett.,vol.96,2006.
efficientforsuchsmallstructures.Nanobeamshaveconducting
[13] S. Matsuo, A. Shinya, T. Kakitsuka, K. Nozaki, T. Segawa, T. Sato,
cross-sectionalareasofjustafewhundredsquarenanometers, Y. Kawaguchi, and M. Notomi, “High-speed ultracompact buried het-
yethighcurrentdensitiesandcarrierinjectionlevelssimilarto erostructurephotonic-crystallaserwith13fJofenergyconsumedperbit
transmitted,” Nat.Photon.,vol.4,pp.648–654,2010.
thoseof2-Dmembranesarepossible.
[14] H. Altug, D. Englund, and J. Vucˇkovic´, “Ultra-fast photonic crystal
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cavities. Design flexibility is inherent in our platform and nu-
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merous parameters can be easily changed to provide a custom AlAsoxide-GaAsdistributedBraggreflectors,” IEEEPhoton.Technol.
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