Analyses for various doping structures of SOI-based optical phasemodulator using free carrier dispersion effectB.

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Pleasecitethisarticleinpressas:B.Mardiana,etal.,AnalysesforvariousdopingstructuresofSOI-basedopticalphasemodulatorusingfree carrierdispersioneffect,Optik-Int.J.LightElectronOpt.(2013),http://dx.doi.org/10.1016/j.ijleo.2013.09.050

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IJLEO-53972; No.ofPages4

Optikxxx (2013) xxx–xxx

ContentslistsavailableatScienceDirect

Optik

j o ur na l h o m e p a g e :w w w . e l s e v i e r . d e / i j l e o

Analyses

for

various

doping

structures

of

SOI-based

optical

phase

modulator

using

free

carrier

dispersion

effect

B.

Mardiana

a,b,∗

,

Sahbudin

Shaari

a

,

P.

Susthitha

Menon

a

,

H.

Hazura

a,b

,

A.R.

Hanim

a,b

,

N.

Arsad

a

,

H.

Abdullah

a

a_Institute_of_{Microengineering}_and_{Nanoelectronics,}_Universiti_Kebangsaan_Malaysia,₄₃₆₀₀_UKM_Bangi,_Malaysia

b_Faculty_of_Electronic_and_Computer_Engineering,_Universiti_Teknikal_Malaysia_Melaka_(UTeM),_Hang_Tuah_Jaya,₇₆₁₀₀_Durian_Tunggal,_Melaka,_Malaysia

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received30April2013 Accepted16September2013

Available online xxx

a

b

s

t

r

a

c

t

Thispaperhighlightsthestudyonvariousstructureofsilicon-on-insulator(SOI)opticalphase

modula-torsbasedonfreecarrierdispersioneffect.TheproposedmodulatorsemploytheforwardbiasedP-I-N

diodestructureintegratedinthewaveguideandwillbeworkingat1.55␮mopticaltelecommunications

wavelength.Threekindsofstructurearecomparedsystematicallywherethep+andn+dopingpositions

arevaried.ThemodelingandcharacterizationoftheSOIphasemodulatorswascarriedoutby3D

numer-icalsimulationpackage.Ourresultsshowthatthepositionofdopingregionshaveagreatinfluencesto

thedeviceperformance.Itwasdiscoveredthatthebeststructureinthisworkdemonstratedmodulation

efficiencyof0.015Vcmwithalengthof155␮m.

1. Introduction

Recently, silicon-on-insulator (SOI) based optical modulator have earned an evolving interest due to its significant role in theinter-chipsopticalinterconnect.Silicon-on-insulator(SOI) sub-strates are widely usedto fabricate optoelectronic devices due tothehighindexcontrastbetweenthesilicon coreandthe sil-icacladding[1].Furthermore,siliconhasproventobearelatively cheapermaterialcomparedtootherIII–Vsemiconductor materi-alsand suitableforintegratedphotonicsystem.Inaddition,SOI hassignificantadvantagesinwhichithasverylowbendingloss [2].Therefore,morecompactdevicecanbematerializedwiththe utilizationofSOIasthesubstratematerial[3].

Phasemodulatorareusedtochangethedatasignalfrom electri-caldomaintoopticaldomain.Themosteffectivewaytomodulate thesignalinsilicon-basedmodulatorisbyfreecarrierdispersion effect.Thismechanismisusedduetothefactsthatunstrainedpure crystalsilicondoesnothaveelectro-opticeffectsuchusPockels effect,KerreffectandFranz–Keldysheffect[4].Tocreatethefree carrierdispersioneffectinsiliconmodulator,theelectrical struc-turehasbeeneitheraP-I-Ndiodeininjectionmode,aPNdiode indepletionmodeorwiththeuseofMOScapacitor.Thecarrier injectionP-I-Ndiodestructureiswidelyimplementedaselectrical

∗_{Corresponding}_author_at:_Institute_of_{Microengineering}_and_{Nanoelectronics,}

UniversitiKebangsaanMalaysia,43600UKMBangi,Malaysia. E-mailaddress:[email protected](B.Mardiana).

structureofmodulatorduetoitshighefficiencyandsmallestsize [5].

Inthispaper,theP-I-NSOIphasemodulatorsaredesignedin threestructureswithdifferentkindofdopingpositions.The three-dimensional(3D)semiconductorsimulationpackageSILVACOwas usedforthis purpose.In thispaper,the3Ddesignsareutilized becauseofthedesignsconsiderthedopingpositionsvariationsin theinz-axis.Therefore,thedispersioneffectoffreecarrierelectrons andholescanbeexaminedalongz-axis.

2. Theory

Themodelingof theopticalmodulator wasperformedusing acommercialnumericalsimulator3D-SILVACOwhichemploya drift-diffusionapproach[6].ThePoisson,carriercontinuityand cur-rentdensityequationsaresolvednumericallyinthreedimensions subjecttothedevice’sgeometryandboundaryconditionsimposed bythedevice’scontactsandbiasingconditions.Thebasicequation tobesolvedateachnodeduringasimulationtoolisgivenbythe Poissonequation:

∇

(∈

∇

)=−q[n−p+˙(N− A−N

+ D+N

− AA−N

+

DD)] (1) where istheelectrostaticpotential,εisepsilon,qisthe elec-troniccharge,nandpisthedensityofmovablecarriers,N_A−andN_D+ arerespectivelytheconcentrationofionizeddonorsandrecipients respectivelywhiletheN_AA− andN_DD+ istheconcentrationoftraps carrierthatservesastherecipientanddonorionized.

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2 B.Mardianaetal./Optikxxx (2013) xxx–xxx

and theprocess of carrier driftand diffusion. Continuity equa-tionfortheelectronandholecarriers describestherelationship betweentheseprocessesandisgivenby;

∂n

∂t =Gn−Rn+ 1

q

∇

· Jn (2)

∂p

∂t =Gp−Rp+ 1

q

∇

· Jp (3)

whereGnandGpwhichistherateofproductionofelectronsand

holesresulting fromexternal effects suchasopticalexcitations inhigh-energyphotons,whileJnandJparetheelectronandhole

currentdensity.RnandRpisthecarrierrecombinationrate.

Generally,currentflowoccursinthecombinedprocessof diffu-sionanddriftinthepresenceofanelectricfield.Currentdensityfor electronsandholes,JnandJpisgivenbythedrift-diffusionmodel

as;

_J_n₌_q_n_n_E₊_qD_n

∇

_n ₍₄₎

_J

p=qppE+qDp

∇

p (5) whereDnandDpisthediffusionconstantofelectronsandholes,

nandparetheelectronandholemobilityandEistheelectric

field.

Next,abriefexplanationofthephasemodulationmechanism oftheproposedmodulatorisgivenheretoexpresstheideaofthe proposeddeviceoperation.Phasemodulatorfabricationisdoneby incorporatingcomplementarymetal-oxidesemiconductor(CMOS) technology.Theelectronsandholeswillmovethroughthechannel asvoltageissupplied.Thecarrierconcentrationatinthewaveguide ismeasurebasedonthevalueofcarrierconcentration.The follow-ingequationexpressesthecalculationofrefractiveindexchange andfreecarrierabsorptionlossat1.55␮mwavelengthduetofree

carrierinjectionofthedevice[11]:thechangeofrefractiveindex, ncanbecalculated[7]:

n=−(8×10−22Ne+8.5×10−18(Nh)0.8) (6)

˛=8.5×10−18

Ne+6×10−18Nh (7)

whereNe isthechangeofthefreeelectronsconcentrationand

Nhisthechangeofthefreeholeconcentration.Then,the

propa-gatingopticalmode,niscalculatedfromtheequation[8]:

ϕ=2nL/0 (8)

whereL␲islengthoftheactiveregionofthemodulatorand0is opticalwavelength.

L=0/2n (9)

3. Methodology

Inthispaper,thestructureisbasedonaP-I-Nlateraloptical phasemodulatorfabricatedonsilicon-on-insulator(SOI)substrate [9].ThemodelsweredevelopedusingtheAthenaandDevedit mod-uleinSilvaco.Thefabricationprocessesbeginwiththeformation oftheSOI layer.Then, thesilicon layeris lightlydopedwith a backgroundconcentrationof1×1014_cm−3_._The_rib_waveguide_is formedusingtheetchingprocesswhereanoxidelayerisusedasa mask.Theribheightandwidthforthewaveguidestructure spec-ifiedtoobtainsinglemodebehavior.Theribstructureisdesigned tohave0.46␮minslabheight(h)and0.4␮minribwidth(W).

Theactiveareaofp-i-ndiodestructurewasfabricatedthroughthe ionimplantationprocesswhereboththep+ _and_n+ _region_were implantedwithboronandphosphorusrespectively.Thep+ _type regionwasdopedwithboronconcentrationof5×1019_cm−3_at_ion

Fig.1. Thefirststructurewithonepairofwellsnexttothewaveguiderib.

Fig.2. Thesecondstructurewithtwopairsofwellsnexttothewaveguiderib.

implantationenergyof10keVandannealingtemperatureof600◦_C.

Whilethen+ _type_region_was_doped_with_phosphorous concen-trationof5×1019_cm−3_at_ion_implantation_energy_of₃₀_keV_and annealingtemperatureof600◦_C._Finally,_the_{metallization}_process

wasdonetoformtheelectrodesofanodeandcathode.

ThreedifferentstructuresofSOIphasemodulatorweredesigned andstudiedinthiswork.Inthefirststructure,onlyonepairofp+ andn+_wells_existed_adjacent_to_the_waveguide_rib_as_shown_in Fig.1.Fig.2showsthesecondstructurewhere2pairsofwellswere placedattheregionnexttotheribwaveguide.Finallyinthethird structureasshowninFig.3,inadditiontothetwopairsofwells,an additionaltwopairsofdopedwellswereplacedonthewaveguide rib.

4. Resultsanddiscussion

Priortoelectricalcharacterization,theopticalanalysiswas car-ried out by using FDTD simulation to ensure the single mode behavioroftheproposeddevice.Fig.1showstheTE fundamen-talmodeprofileoftheproposeddeviceatwavelength1.55␮m.It

isproventhatthesinglemodebehaviorwasobtainedascalculated (Fig.4).

Theperformanceof thesiliconphase modulatorswere eval-uatedbyvaryingthepositionsofthen+ _and_p+_doping_regions. Theeffectofdopingregionspositionwillbeinvestigatedbased ontotalfreecarrierconcentration,refractiveindexchange,free carrierabsorptionloss,sizeandthemodulationefficiencyofthe modulators.

Fig.5 shows the carrier concentrationof free electrons and holesofthethreedifferentstructuresofmodulatorswhenspecific

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B.Mardianaetal./Optikxxx (2013) xxx–xxx 3

Fig.4. Totalfreecarrierconcentrationagainstappliedvoltage.

Fig.5. Totalfreecarrierconcentrationalongx-axis.

voltageisapplied.Themeasurement offreeelectronsandholes wastakenalongxaxisfromx=1.75tox=2.15whichistheareaof opticalmodepropagateinthedevice.Itisclearlyseenthat struc-ture1andstructure3gavehigherdensityoffreecarriercompareto structure2.Thishappenedduetointerdigitateddopingpositionof p+_dan_n+_region_in_structure₂_and_structure₃_cause_free_electrons andholestomoveinmanydirectionsinsteadofonedirectionas instructure1.Therefore,theexistingoffreeelectronsandholesin theopticalmodeguidingregionarereducing.Theresultsproofthat theconcentrationofexistingfreecarrierintheactiveregioncanbe manipulatedbythepositionofdopingregionn+_and_p+_._Thus,_the determinationofdopingpositionsareveryimportantindesigning theP-I-NSOIphasemodulatorbecauseitwillinfluencetheway howthefreeelectronsandholesmoveintheactiveregionofthe device.

Fig.6shows therelationshipbetweendrivevoltageandthe refractiveindexchange(n)at1.55␮mwavelength.Ingeneral,

thetotalrefractiveindexchange(n)increasesastheapplied volt-ageishigher.Forinstanceinstructure1,byvaryingtheapplied voltagefrom0.7Vto1Vcausedalmost0.005changesinrefractive indexofthewaveguide.Thishappenedduetomoreinjectedfree holesandelectronsmovedfromthedopingregiontotheoptical guidanceareawhenmoreappliedvoltagewassupplied.Thus,this scenariocausestheincreaseinthefreecarrierdensityand result-ingimprovementofrefractiveindexchange(n)intheguidance regionofthedevice.Itisfoundthat,thestructure1hasthelargest effectontherefractiveindexchange(n),followedbythestructure 3andthestructure2.

Fig.6.Refractiveindexchangeforphasemodulatorswithvariouswaveguide struc-tures.

Fig.7. Freecarrierabsorptionlossforphasemodulatorswithvariouswaveguide structures.

Table1

Modulationefficiencyandlengthofmodulator.

Structure 1 2 3

Length(cm) 0.0155 0.0261 0.0205

ModulationEfficiency(Vcm) 0.013 0.023 0.018

Fig. 7 shows the comparison of the absorption lossesvalue withvariousstructureofSOIP-I-Nphasemodulators.Theresults indicated that, upon increasing the drive applied voltage, the absorptionlossincreasedgradually.Structure1producedhighest freeabsorptionlosscomparedtostructure2andstructure3.Even thoughstructure1isproventohavethehighestrefractiveindex change(n)intheprevioussectionbutthisstructureissuffering frombiggestabsorptionloss(˛).Therefore,inseekingabalanced responsebetweenthehighrefractiveindexchangeandlowfree carrierabsorptionloss,adesigntradeoffbetweenthetwoisneeded inordertoproduceagoodSOIphasemodulator.

Table1showsthecomparisonofestimatedlengthandthe mod-ulationefficiency.Themodulationefficiencyisaveryimportant parameterforcharacterizingthemodulatorperformanceandits valuecanbedeterminedbyoverlapping themodulationregion withtheopticalfield.Theoptimizedinteractivelengthofvarious structuresofphasemodulatorsinvestigatedinthisworkisobtain fromthevalueofrefractiveindexchange(n)inFig.6and calcu-latedfromEq.(9).Themodulationefficiencycanbepredictedby FigureofMerit(FoM)V␲L␲,whereV␲isthevoltagetoachievea␲

phaseshift.ThelowerthevalueofthisFoM,themoreefficientthe modulatoris.Resultshowsthatasmallestdevicecanberealized withstructure 1andresultingthebestmodulationefficiencyof 0.00018Vcm.Asacomparison,themodulationefficiencyof struc-ture1is50%betterthanstructure3%and82%betterthanstructure 2.

5. Conclusion

TheSOIphasemodulatorsbasedonP-I-Ndiodestructurehas beenmodeledusinga3DSILVACOsimulationpackage.Wehave analyzedtheperformanceofmodulatorswithvariousstructureof dopingpositions.Wehaveshownthatdopingpositionsofn+_and_p+ canbeusedtomanipulatethedensityoffreeelectronsandholesin adevice.Inseekingabalancedresponsebetweenthehighrefractive indexchangeandcarrierabsorptionloss,adesigntradeoffbetween thetwoisneededtobetakenintoconsiderationdependingonthe applicationofthemodulator.

Acknowledgments

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