DTIC ADA527899: Design and Analysis of In0.53Ga0.47As/InP Symmetric Gain Optoelectronic Mixers

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Solid-StateElectronicsxxx(2010)xxx–xxx
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Solid-State Electronics
journal homepage: www.elsevier.com/locate/sse
Design and analysis of In Ga As/InP symmetric gain optoelectronic mixers
0.53 0.47
Wang Zhanga, Nuri W. Emanetoglua,*, Neal Bambhab, Justin R. Bickfordb
aElectricalandComputerEngineering,UniversityofMaine,USA
bSensorsandElectronDevicesDirectorate,ArmyResearchLaboratory,USA
a r t i c l e i n f o a b s t r a c t
Articlehistory: A symmetric gain optoelectronic mixer based on an indium gallium arsenide (In Ga As)/indium
0.53 0.47
Received14December2009 phosphide(InP)symmetricheterojunctionphototransistorstructureisbeinginvestigatedforchirped-
Receivedinrevisedform9June2010 AMlaserdetectionandranging(LADAR)systemsoperatinginthe‘‘eye-safe”1.55lmwavelengthrange.
Accepted6July2010
Signalprocessingofachirped-AMLADARsystemissimplifiedifthephotodetectorinthereceiverisused
Availableonlinexxxx
asanoptoelectronicmixer(OEM).AddinggaintotheOEMallowsthefollowingtransimpedanceampli-
Thereviewofthispaperwasarrangedby
fier’sgaintobereduced,increasingbandwidthandimprovingthesystem’snoiseperformance.Asym-
Prof.A.Zaslavsky
metric gain optoelectronic mixer based on a symmetric phototransistor structure using an indium
galliumarsenidenarrowbandgapbaseandindiumphosphideemitter/collectorlayersisproposed.The
Keywords:
devicesaresimulatedwiththeSynopsisTCADSentaurustools.Theeffectsofbase–emitterinterfacelay-
LADAR
Phototransistor ers,basethicknessandthedopingdensitiesofthebaseandemittersonthedeviceperformanceareinves-
Optoelectronicmixer tigated. AC and DC simulation results are compared with a device model. Improved responsivity and
lowerdarkcurrentarepredictedfortheoptimizedInGaAs/InPdeviceoverpreviouslyreporteddevices
withindiumaluminumarsenideemitter/collectorlayers.
(cid:2)2010ElsevierLtd.Allrightsreserved.
1.Introduction theresponsefrombackgroundlightaveragestozero.Anadditional
3dBsignalprocessinggainisalsoobtained.AstheOEMoutputis
Optoelectronic mixing devices mix a modulated optical signal thelowfrequencydifferencesignal,thegainofthefollowingtrans-
with a reference electrical signal to obtain an electrical low fre- impedance amplifier (TZA) can be increased, improving LADAR
quencydifferencesignal.Thesedevicesareparticularlyattractive performance. ARL has previously demonstrated chirped-AM LA-
forchirped-AMlaserdetectionandranging(LADAR)systems.The DAR systems with GaAs and InGaAs metal–semiconductor–metal
Army Research Laboratory (ARL) has been developing chirped- (MSM)SchottkyphotodetectorOEMsforoperationatthe800nm
AMLADARsystemsforapplicationssuchasreconnaissance,terrain [2,4]and1550nmwavelengths[5,6].Asymmetricphotodetector
mapping, force protection, facial recognition, robotic navigation withgainwouldimproveoverallsystemperformance,whilepre-
andweaponsfuzing[1].Inthissystem,theamplitudeofthelaser servingtheadvantagesofferedbyMSMOEMdevices.
beamismodulatedwithachirpedfrequencylocaloscillator(LO) We previously reported Symmetric Gain OptoElectronic Mix-
signal. The reflected laser beam is detected by a photodetector, ers (SG-OEMs) for 1.55lm operation based on a symmetric het-
whichconvertsitintoanelectricalsignal(RF).Thistimedelayed erojunction phototransistor using In Al As/In Ga As
0.52 0.48 0.53 0.47
RFwillbeatadifferentfrequencythantheinstantaneousLO,with heterostructures [7]. The emitter/collector was In Al As (In-
0.52 0.48
thedifferencefrequency(f )determinedbythedistancetotarget, AlAs), and the base was In Ga As (InGaAs), both of which
IF 0.53 0.47
the frequency bandwidth, DF, of the chirp, and its duration, T. are lattice matched to the InP substrate. Two-dimensional de-
Therefore, the RF signal is mixed with the IF signal to obtain the vice simulations, using the Synopsis TCAD Sentaurus suite, were
differencesignal,andthusdistanceinformation.Signalprocessing carried out at the University of Maine to design optoelectronic
ofachirped-AMLADARsystemissimplifiedifthephotodetectorin mixers with suitable optical gain and frequency bandwidth.
the receiver is used as an optoelectronic mixer (OEM) [2]. A DC Two symmetric gain OEM heterostructures were grown at ARL
biased phototransistor can be used as an optoelectronic mixer using molecular beam epitaxy, and prototype devices were fab-
[3].Alternatively,aphotodetectorwithasymmetricI–Vcharacter- ricated. Device diameters ranged from 18lm to 30lm. Etch
isticallowsdrivingtheOEMdirectlywiththelocaloscillatorsignal, steps during device fabrication revealed cracking defects in the
withoutaDCbias[2].Sensitivitytobackgroundlightisreduced,as thin films. Therefore, an investigation into an alternative device
structure with InP layers was initiated to improve lattice-match-
ing with the substrate and to reduce the defect density in the
* Correspondingauthor.Tel.:[phone].
thin films.
E-mailaddress:[email protected](N.W.Emanetoglu).
0038-1101/$-seefrontmatter(cid:2)2010ElsevierLtd.Allrightsreserved.
doi:10.1016/j.sse.2010.07.003
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(2010),doi:10.1016/j.sse.2010.07.003
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
Design and analysis of In0.53Ga0.47As/InP symmetric gain optoelectronic
5b. GRANT NUMBER
mixers
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13. SUPPLEMENTARY NOTES
14. ABSTRACT
A symmetric gain optoelectronic mixer based on an indium gallium arsenide (In0.53Ga0.47As)/indium
phosphide (InP) symmetric heterojunction phototransistor structure is being investigated for chirped- AM
laser detection and ranging (LADAR) systems operating in the ??eye-safe? 1.55 lm wavelength range.
Signal processing of a chirped-AM LADAR system is simplified if the photodetector in the receiver is used
as an optoelectronic mixer (OEM). Adding gain to the OEM allows the following transimpedance
amplifier?s gain to be reduced, increasing bandwidth and improving the system?s noise performance. A
symmetric gain optoelectronic mixer based on a symmetric phototransistor structure using an indium
gallium arsenide narrow bandgap base and indium phosphide emitter/collector layers is proposed. The
devices are simulated with the Synopsis TCAD Sentaurus tools. The effects of base?emitter interface layers
base thickness and the doping densities of the base and emitters on the device performance are
investigated. AC and DC simulation results are compared with a device model. Improved responsivity and
lower dark current are predicted for the optimized InGaAs/InP device over previously reported devices
with indium aluminum arsenide emitter/collector layers.
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 5
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Prescribed by ANSI Std Z39-18
2 W.Zhangetal./Solid-StateElectronicsxxx(2010)xxx–xxx
Inthiswork,InP/In Ga Asheterostructurebasedsymmet- Table1
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ricgainoptoelectronicmixersareinvestigatedthroughsimulation Listofmaterialparametersusedinthesimulations.
usingtheSynopsisTCADSentaurustools.Asymmetricgainopto- InGaAs InP InAlAs
electronic mixer for the chirped-AM LADAR applications should
Eg(eV) 0.718721 1.33587 1.48159
havehighresponsivitybelow5V(peakvoltagefor24dBmLOsig- v0(eV) 4.5472 4.4 4.2711
nal) and low dark current. In comparison with In0.52Al0.48As, InP er 13.9061 12.4 12.3948
choansdauscmtiaolnlebrabnadndogffaspetd.iTffheures,nccoemwpitahraInti0v.5e3Gsiam0.u47laAtsioannsdaaredicffaerrreiendt NNcv((ccmm(cid:3)(cid:3)33)) 27..55319076(cid:2)(cid:2)11001187 52..6036(cid:2)(cid:2)11001197 59..74811542(cid:2)(cid:2)11001178
outtoinvestigatetheadvantagesanddisadvantagesofInPbased
SG-OEMsincomparisonwithInAlAsbasedones.Thetwo-dimen-
sionalsimulationsareusedtodesignboththelayerstructurefor Alargeandsymmetricgain,lowleakagecurrent,andabreak-
growth and the horizontal structure for the photolithography downvoltagelargerthan5Varetheimportantspecificationsfor
masks. The simulations predict a higher responsivity for the InP anoptoelectronicmixerinLADARapplications.AnOEMwithlarge
based SG-OEMs, as well as a smaller dark current, hence lower gainwouldallowthefollowingtransimpedanceamplifier’sgainto
noisefloor.Highlydopedbase–emitterinterfacelayersareinvesti- bereduced,increasingtheTZAfrequencybandwidthandimprov-
gated for improving device performance, as previously predicted ing overall system performance. Fig. 2 compares the simulated
forInAlAsbasedSG-OEMs.Thebasewidthandbase–emitterdop- responsivity versus bias voltage of InP and In0.52Al0.48As based
ingdependenceofthedarkcurrentandresponsivityarealsoinves- SG-OEMswiththesamelayerthicknessanddopingprofile,while
tigatedtooptimizethedevicestructureforhighresponsivity,low theirdarkcurrentperformanceiscomparedinFig.3.Thesimula-
leakagecurrent,andalargebreakdownvoltage. tions predict that InP based SG-OEMs will have a lower dark
current, and be less susceptible to the Early effect and punch-
through breakdown. The InP/In Ga As based structure A de-
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vice has a base width of 800nm, base doping of 2.5(cid:2)1016cm(cid:3)3
2.Simulationandmodeling andcollector/emitterdopingdensityof5(cid:2)1015cm(cid:3)3(dopingpro-
file#2).Thisdevicehasaresponsivityof10.42A/Wat3V,whichis
2.1.DCsimulationandanalysis approximately two-thirds of its In0.52Al0.48As/In0.53Ga0.47As coun-
terpart, at the gain of a lower dark current, as can be seen in
Thetwobasicdevicestructures,withandwithouttheemitter- Fig. 3. The lower dark current in the InP/In0.53Ga0.47As structure
sidehighlydopedinterfacelayer,areshowninFig.1,forthecaseof isdueto:(i)atwo-dimensionalelectrongasaccumulationatthe
InP emitter/collector layers. Structure A has 10nm thick highly n++–N++ isotype heterojunction contact layer interface; and (ii) a
dopedInPinterfacelayersbetweenthebaseandemitter/collector larger Early effect in the base of the In0.52Al0.48As/In0.53Ga0.47As
layers,whilestructureBdoesnot.Thebaselayerthicknessandthe device, caused by a larger built-inpotential at the p-N anisotype
dopinglevelsofthebaseandemitterlayersarethemainparame- heterojunction.
tersvariedinthedevicesimulations.Thematerialparametersused Another important consideration is the effect of the highly
inthesimulationarelistedinTable1. dopedemitterside interface layerson device performance. Fig. 2
Fig.1. ThestructuresofthetwoInP/In0.53Ga0.47Asheterostructurebasedsymmetricgainoptoelectronicmixers.StructureAwithandBwithouttheemitter-sidehighlydoped
interfacelayers.
Pleasecitethisarticleinpressas:ZhangWetal.DesignandanalysisofIn Ga As/InPsymmetricgainoptoelectronicmixers.SolidStateElectron
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Table2
Dopingprofilesofthreesamplestructures.
Dopingprofile Dopingprofile2 Dopingprofile
1 3
Basedoping 1(cid:2)1016cm(cid:3)3 2.5(cid:2)1016cm(cid:3)3 5(cid:2)1016cm(cid:3)3
Emitter/collector 5(cid:2)1016cm(cid:3)3 5(cid:2)1015cm(cid:3)3 5(cid:2)1015cm(cid:3)3
doping
tremes, and #2 being the optimum doping profile. Fig. 4 shows
the DC responsivities of seven InP/In Ga As SG-OEMs based
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onstructureAwithdopingprofile#2,andbasethicknessesrang-
ing from 500nm to 1100nm. It should be noted that the device
showninFig.2hadthesamedopingprofile.SG-OEMresponsivity
decreaseswithincreasingbasethickness,asthetransistorgainde-
creasesfasterthantheincreaseintheabsorptionwithincreasing
base thickness in this range. To a first order, the responsivity, R,
isproportionalto:
Fig. 2. Responsivity versus bias voltage for InP/In0.53Ga0.47As and In0.52Al0.48As/ ð1(cid:3)e(cid:3)adÞ
In0.53Ga0.47Asbasedsymmetricphototransistorswiththesamelayerthicknessand R/ ð1Þ
doping profile. Both structure A and B are included for InP/In0.53Ga0.47As based d2
structures.
whereaistheabsorptioncoefficientanddisthethicknessofthe
baseregion,relatingresponsivitytothebasewidthdependenceof
transistorgain.Incontrast,increasingbasethicknessreducesbase
width modulation, i.e. the Early effect, and correspondingly in-
creases the base punch-through breakdown voltage. Narrow base
widthdevices(600nmandbelow)arepredictedtofailatbiasvolt-
agesbelow4Vduetopunch-throughbreakdown.Fig.5showsthe
DCresponsivityofthreeInP/In Ga AsSG-OEMsbasedonstruc-
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tureB,withbasethicknessof700nm,800nmand900nm.Except
fortheinterfacelayers,theyhavethesame#2dopingprofile.Sim-
ilar to the structure A devices, the responsivity decreases with
increasingbasethickness.Ingeneral,thestructureBdeviceshave
better responsivity, and lower susceptibility to the Early effect of
theircounterparts,andprovideabetterSG-OEMdevice.
Fig. 6 compares the responsivities of 800nm base InP/In
0.53-
Ga As SG-OEMs with the three doping profiles given in Table
0.47
1. Device A.1 has the largest responsivity below 5V, which is
13.42A/Wat1Vand38.81A/Wat2V,comparedto6.312/8.194
forA.2,and4.564/8.194forA.3.However,theabruptjumpinthe
responsivity at 3V is due to base punch-through breakdown,
whichmakesdeviceA.1unsuitableforpracticalapplications.This
Fig. 3. Dark current versus bias voltage for InP/In0.53Ga0.47As and In0.52Al0.48As/ low voltage base punch-through in A.1 is caused by the emitter/
In0.53Ga0.47Asbasedsymmetricphototransistorswiththesamelayerthicknessand
dopingprofile.BothstructuresAandBareincludedforInP/In0.53Ga0.47Asbased
structures.
comparestheresponsivityofstructureA(withtheinterfacelayers)
andstructureB(withouttheselayers)asafunctionofthebiasvolt-
age. Both structures have base thickness of 800nm and doping
densitiesof2.5(cid:2)1016cm(cid:3)3 in thebase and5(cid:2)1015cm(cid:3)3in the
emitter/collector layers. Structure B is predicted to have a larger
responsivitythanstructureAatlowbiasvoltages.Figs.2and3also
showthatstructureBislesssusceptibletotheEarlyeffect,andhas
lowerdarkcurrent.TheweakerEarlyeffectforstructureBisdueto
thelackofthehighlydopedemitter–baseinterfacelayer.Instruc-
tureA,thehighlydoped(10nm,1018cm(cid:3)3)emitterinterfacelay-
erscausemostofthedepletionregiontoextendintothebase.In
contrast,the depletion region is smaller in the base for structure
B,whichlackstheseinterfacelayers.
Basethicknessandthedopingdensitiesofthebaseandemitter/
collectorlayersarekeySG-OEMdesignparameters.Theseparam-
eters were systematically varied across a series of simulations to
investigatetheireffectsondeviceperformance.Table2liststhree
Fig. 4. Responsivity of structure A.2 as a function of base thickness. The base
doping profiles out of this data set, #1 and #3 being at the ex- thicknessesrangefrom600nmto1000nm.
Pleasecitethisarticleinpressas:ZhangWetal.DesignandanalysisofIn Ga As/InPsymmetricgainoptoelectronicmixers.SolidStateElectron
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4 W.Zhangetal./Solid-StateElectronicsxxx(2010)xxx–xxx
Fig. 5. Responsivity of structure B.2 as a function of base thickness. The base Fig.6. ResponsivityofstructureAasafunctionofthedopingprofilesgiveninTable
thicknessesrangefrom600nmto1000nm. 1.Basethicknessis800nm.
collectordopingdensitybeinglargerthanthebasedopingdensity, Table3
whichcausesthedepletionregiontoextendmostlyintothebase. Devicedimensionsusedinthesimulations.
DeviceB.2with800nmbase(Fig.5)hasasatisfactoryrespon- Parameter Size(lm)
sivity(14.88A/Wat3V)inthelowvoltagerangeanddoesnotsuf-
Innermesa 16
fer from punch-through breakdown below 5V, the operational
Outermesa(bottomcontact) 30
range of the devices. This responsivity value corresponds to Topcontactwidth 12
58,125lA/Wlm2 when normalized by area. In comparison, Ali- Topcontactmetalwidth 14
berti et al. reported a responsivity of 99lA/Wlm2 at 3V for an Bottomcontactwidth 2
InGaAsMSMoptoelectronicmixerwith1lmthickabsorptionre- Bottomcontactmetalwidth 5
gion[5].Thus,theproposedSG-OEMdevicesarepredictedtohave
approximately580timeshigherresponsivityperunitarea.
2.2.ACsimulationanddevicemodeling
Two major contributors to the SG-OEM’s frequency response
will be base transit time and the junction capacitances of the
base-emitter/collectorjunctions.Asetofsmallsignalsimulations
were carried out to determine the total capacitance seen by the
LO signal driving the SG-OEM for both structures A and B, with
dopingprofile#2andbasethicknessesof600nmand800nm.Ta-
ble 3 lists the device dimensions used for these simulations. The
small signal superimposed on the DC bias has a frequency of
1MHz.Simulationswerecarriedoutfordarkconditions,andunder
anopticalilluminationof1mW/cm2.
Fig.7showstotalcapacitanceversusbiasvoltageofthetwode-
vicestructures.Thetotalcapacitanceseenattheterminalsforde-
vices A and B are about 7fF and 2.5fF at zero bias and dark
conditions, respectively, which decreases in all four devices as
the bias voltage increases. This is due to the decrease of reverse
biasedpnjunctioncapacitancewithincreasingbiasvoltage:
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
qN N e e Fig.7. TotalcapacitanceofstructureA.2andstructureB.2asafunctionofbase
C¼ A;B D;EC B EC ð2Þ thickness.
2ðN e þN e ÞðV þV Þ
A;B B D;EC EC bi R
whereN andN arethedopingdensityofbaseandemitter/col- andthusleavesthecapacitancealmostunchangedwiththevaria-
A,B D,EC
lector,respectively,e istherelativepermittivityoftheInGaAsbase tion in the base width. Also displayed in Fig. 7 are the simulated
B
ande thatoftheInPemitter/collector,V isthebuilt-inbarrier,V capacitancecurvesof800nmbasewidthdevices,underanoptical
EC bi R
isthebiasvoltageandqisunitcharge.Thepeakat4Vforthestruc- illumination of 1mW/cm2. Capacitance values shift upwards
ture A device with 600nm base width is due to punch-through approximately 0.3–0.5fF under illumination. Structure B devices
breakdown,consistentwithDCresponsivitysimulations.Thetotal will provide a better frequency performance, due to their lower
capacitancedecreasesslightlywithincreasingbasethickness.This overallcapacitance.
dependenceonthebasethicknessisweakenedbytheemitter-side Fig.8showstheequivalentcircuitmodelforthetwoterminal
highly doped interface layer for structure A, which reduces the symmetric gain optoelectronic mixer. The current source repre-
extensionofthedepletionregionintotheemitter/collectorlayers, sents the photocurrent (RPopt). The forward and reversed biased
Pleasecitethisarticleinpressas:ZhangWetal.DesignandanalysisofIn Ga As/InPsymmetricgainoptoelectronicmixers.SolidStateElectron
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W.Zhangetal./Solid-StateElectronicsxxx(2010)xxx–xxx 5
currentcrowdinginthenarrowingcontactlayerbecomesanissue,
thisresistanceispredictedtobeapproximately5.7kXlm,assum-
ing the contact layer is etched mid-way. This resistance will be
dependentonaccuratecontroloftheinnermesaetchstepinthe
devicefabricationprocess.
3.Conclusion
InP/In Ga AsbasedSG-OEMsareapromisingreplacement
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for In Al As/In Ga As SG-OEMs, due to reduced defect
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densitiesin the materials,and beingless susceptible to theEarly
effect and punch-through break down. Two-dimensional device
simulations were used to optimize device structure over base
width,baseandemitterdoping,andemitter/baseinterfacelayers.
Itwasdeterminedthathighlydopedinterfacelayerscausedanin-
creaseindarkcurrentanddevicecapacitanceandalsoloweredthe
base punch-through breakdown voltage. An optimized device
structurewasorderedformaterialgrowthanddevicefabrication,
tobepresentedinafuturepublication.
Fig.8. EquivalentcircuitmodelofanSG-OEMdevice.
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contactlayerafterthemesaetchstep.Athighcurrentlevels,when
Pleasecitethisarticleinpressas:ZhangWetal.DesignandanalysisofIn Ga As/InPsymmetricgainoptoelectronicmixers.SolidStateElectron
0.53 0.47
(2010),doi:10.1016/j.sse.2010.07.003